**4. Microstructural effects on conduction properties**

#### **4.1 Crystallization and phase diagram**

As expected from the previously reported results on NaYPSi, the crystallization of the superionic conducting N5-type phase took place, depending both on the contents of [R] and [P], at temperatures of 800 to 1000°C in most NaRPSi glasses of Er to Sm except for scandium and lanthanum NaRPSi glasses. The N5 single phase region was wider for NaRPSi of smaller R, but was limited at the [P]≈0 region. The effect of phosphorus substitution for Si is important in the crystallization of N5-type phase. The composition 7

Na3.9Y0.6P0.3Si2.7O9 a glass-ceramic of N5 single phase NaYPSi was easily obtained at a temperature higher than 900°C for only three hours. The composition Na3.75Y0.75Si3O9 (or

Fig. 8. Phase transformation rate (αv) of N3- to N5-type NaYPSi on the specimen

**4. Microstructural effects on conduction properties** 

**4.1 Crystallization and phase diagram** 

Fig. 8 shows the kinetic characteristics of phase transformation of the metastable phase of N3- to N5-type NaYPSi of specimen Na3.9Y0.6P0.3Si2.7O9 at various temperatures. The transition rates, αv, of the silicophosphate NaYPSi were much higher than those of the

The results shown were analyzed with the Avrami empirical equation, αv=1-exp(-*ktn*), where *k* is the rate constant, and *n* is a constant. The data on αv obtained at the initial and intermediate stages gave a linear relationship between ln(ln(1-αv)-1) and ln(*t*) with a correlation coefficient of more than 0.99. The Avrami parameter and rate constants obtained are summarized in Table 1. Based on the Arrhenius relationship (Fig. 9), *k*=*A*exp(-*E*v/*RT*) with *E*v as the activation energy and constants *A* and *R*, on those *k* values which increased with increasing temperature, we obtained an activation energy of 1.2×103 kJ/mol, suggesting that the phase transformation can be rather difficult to take place. An addition of phosphorus and the excess sodium seem effective to the promotion of the phase

As expected from the previously reported results on NaYPSi, the crystallization of the superionic conducting N5-type phase took place, depending both on the contents of [R] and [P], at temperatures of 800 to 1000°C in most NaRPSi glasses of Er to Sm except for scandium and lanthanum NaRPSi glasses. The N5 single phase region was wider for NaRPSi of smaller R, but was limited at the [P]≈0 region. The effect of phosphorus substitution for Si is important in the crystallization of N5-type phase. The composition 7

Na5YSi4O12) was inferior in the same meaning.

Na3.9Y0.6P0.3Si2.7O9.

transformation.

Na3.75Y0.75Si3O9 silicate material.

$$\text{Na}\_{3.9}\text{R}\_{0.6}\text{P}\_{0.3}\text{Si}\_{2.7}\text{O}\_{9}\tag{7}$$

was experimentally shown as the most appropriate composition for the crystallization of N5-type phase.


Table 1. Kinetic parameters of phase-transformation of N3- to N5-type NaYPSi of Na3.9R0.6Si2.7O9.

Fig. 9. Arrhenius-type plot of ln *k* with 1000/T of specimen Na3.9Y0.6Si2.7O9.

The relationship between the ionic radius of R3+ (*r*R) and the hexagonal lattice parameters of N5-type single phase is consistent with the previous report on Na5RSi4O12 (R=Sc-Sm) in the tendency that both lattice parameters increased with increasing *r*R. The elongation of these lattice axes is attributed to the octahedral coordination of R3+ with the O2- of SiO4- or PO4 tetrahedra of the 12-membered rings. The local structure around R3+ ions is to be further discussed below in relation to conduction properties. On the formation of N5-type single phase, the incorporation of excess sodium ions [4(3+3*x*-*y*)/3-5=(12*x*-4*y*-3)/3 in composition 3] and substitution of rare earth ions [1-4(1-x)/3=(4*x*-1)/3] must be accounted for in view of N5-type crystal structure.

Banks *et al*. have reported the values of σ300 as 5×10-3 to 1×10-2 S/cm for glass-ceramic Na5RSi4O12 (R=Er, Y, Gd, Sm), which are as low as those of the mixed phase NaRPSi specimens. The single phase N5-type glass-ceramic was not obtained in the present work. Based on the above crystallization analysis, their glass-ceramic specimens are reasonably

Preparation of Na<sup>+</sup>

Na3.9R0.6Si2.7O9.

Superionic Conductors by Crystallization of Glass 93

Table 2. Conduction properties of various NaRPSi glass-ceramics with composition

The complex impedances and admittances of the measured NaRPSi glass-ceramics consisted of two semicircles below 300°C. The two intercepting points on the real axis are interpreted as the resistance of crystallized grains (*R*G) and the total resistance of grains and remaining glassy grain boundaries (*R*GB). Shown in Fig. 10 are examples of the temperature dependence Arrhenius plots made on the basis of the calculated conductivity values of grains and grain boundaries of the glass-ceramic NaYPSi (Na3.9Y0.6P0.3Si2.7O9) and NaSmPSi (Na3.9Sm0.6P0.3Si2.7O9), in which the geometrical ratios of thickness to surface area for grains were also used for convenience for those of grain boundaries because of their undefinable shapes. Table 2 summarizes the measured conductivities (σ300) and the calculated activation energies (*E*a) assigned for grains of the glass-ceramics with composition 7 of Sc to La, regardless of whether their crystalline phases are N5-type or not. The conductivities, σ300, of single-phase NaRPSi specimens of Er to Sc range from 4×10-2 to 1×10-1 S/cm; in accordance the *E*a falls in the range of 23 to 27 kJ/mol. In contrast, the mixed phase NaRPSi of Sc and In showed much smaller σ300 of 3×10-3 with an *E*a of 35 to 40 kJ/mol, while non-NaRPSi glassceramics with unknown or mixed phases showed much lower conductivities of 1×10-5 to

The tendency of the conduction properties in single-phase NaRPSi specimens is consistent with the reported result measured on the corresponding polycrystalline Na5RSi4O12; σ increased with increasing *r*R. The previous works have proposed a mechanism that rare earth ions, octahedrally coordinated with the non-bridging oxide ions of the 12-membered rings of silica tetrahedra, work to expand the conduction paths for Na+ ions along the c-axis,

As *R*GB decreases rapidly with increasing temperature because of high (*E*a)GB to a comparable value with *R*G at 300°C (Fig. 10), the total conductivities (*R*G+*R*GB) are dominated by grain boundary conductivity. The grain size-dependence of σ300 is therefore explained by the decrease in the number of poorly conductive grain boundaries with increasing grain size.

which can explain the observed dependence of *E*a on *r*R in this work.

**4.3 Structure and conduction properties of grain boundaries** 

**4.2 Conduction properties of crystalline grains** 

1×10-4 S/cm with an *E*a of 55 to 58 kJ/mol.

considered to suffer from phase inhomogeneity brought about by insufficient annealing. The formation of N5-type structure from the precursor glasses is a matter of crystallization kinetics, since single-phase N5 has been synthesized in single crystal or polycrystalline form based on the composition of N5. It is noted here that the precursor phases identified were N3- or N9-type. Both N3 and N9 are considered to form iso-structural with Ca3Al2O6 to be comprised of the skeleton structure of 6-membered SiO4-tetrahedra rings. It is generally known that phosphorus pentoxide acts as nucleating agent in the formation of glassceramics. It is therefore presumed at present that the substitution of an asymmetric PO4 tetrahedron has the weakening effect on the bonding of the skeleton structure of 6 membered SiO4-tetrahedra rings, resulting in the tendency to form the stable 12-membered structure.

Fig. 10. The Arrhenius plots of the conductivities of grains (G), grain boundaries (GB) and the total bulk (T) of the glass-ceramic Na3.9Y0.6P0.3Si2.7O9 (A) and Na3.9P0.3Sm0.6Si2.7O9 (B).

considered to suffer from phase inhomogeneity brought about by insufficient annealing. The formation of N5-type structure from the precursor glasses is a matter of crystallization kinetics, since single-phase N5 has been synthesized in single crystal or polycrystalline form based on the composition of N5. It is noted here that the precursor phases identified were N3- or N9-type. Both N3 and N9 are considered to form iso-structural with Ca3Al2O6 to be comprised of the skeleton structure of 6-membered SiO4-tetrahedra rings. It is generally known that phosphorus pentoxide acts as nucleating agent in the formation of glassceramics. It is therefore presumed at present that the substitution of an asymmetric PO4 tetrahedron has the weakening effect on the bonding of the skeleton structure of 6 membered SiO4-tetrahedra rings, resulting in the tendency to form the stable 12-membered

Fig. 10. The Arrhenius plots of the conductivities of grains (G), grain boundaries (GB) and the total bulk (T) of the glass-ceramic Na3.9Y0.6P0.3Si2.7O9 (A) and Na3.9P0.3Sm0.6Si2.7O9 (B).

structure.


Table 2. Conduction properties of various NaRPSi glass-ceramics with composition Na3.9R0.6Si2.7O9.

#### **4.2 Conduction properties of crystalline grains**

The complex impedances and admittances of the measured NaRPSi glass-ceramics consisted of two semicircles below 300°C. The two intercepting points on the real axis are interpreted as the resistance of crystallized grains (*R*G) and the total resistance of grains and remaining glassy grain boundaries (*R*GB). Shown in Fig. 10 are examples of the temperature dependence Arrhenius plots made on the basis of the calculated conductivity values of grains and grain boundaries of the glass-ceramic NaYPSi (Na3.9Y0.6P0.3Si2.7O9) and NaSmPSi (Na3.9Sm0.6P0.3Si2.7O9), in which the geometrical ratios of thickness to surface area for grains were also used for convenience for those of grain boundaries because of their undefinable shapes. Table 2 summarizes the measured conductivities (σ300) and the calculated activation energies (*E*a) assigned for grains of the glass-ceramics with composition 7 of Sc to La, regardless of whether their crystalline phases are N5-type or not. The conductivities, σ300, of single-phase NaRPSi specimens of Er to Sc range from 4×10-2 to 1×10-1 S/cm; in accordance the *E*a falls in the range of 23 to 27 kJ/mol. In contrast, the mixed phase NaRPSi of Sc and In showed much smaller σ300 of 3×10-3 with an *E*a of 35 to 40 kJ/mol, while non-NaRPSi glassceramics with unknown or mixed phases showed much lower conductivities of 1×10-5 to 1×10-4 S/cm with an *E*a of 55 to 58 kJ/mol.

The tendency of the conduction properties in single-phase NaRPSi specimens is consistent with the reported result measured on the corresponding polycrystalline Na5RSi4O12; σ increased with increasing *r*R. The previous works have proposed a mechanism that rare earth ions, octahedrally coordinated with the non-bridging oxide ions of the 12-membered rings of silica tetrahedra, work to expand the conduction paths for Na+ ions along the c-axis, which can explain the observed dependence of *E*a on *r*R in this work.

#### **4.3 Structure and conduction properties of grain boundaries**

As *R*GB decreases rapidly with increasing temperature because of high (*E*a)GB to a comparable value with *R*G at 300°C (Fig. 10), the total conductivities (*R*G+*R*GB) are dominated by grain boundary conductivity. The grain size-dependence of σ300 is therefore explained by the decrease in the number of poorly conductive grain boundaries with increasing grain size.

Preparation of Na<sup>+</sup>

type glass-ceramic NaSmPSi.

of the presented specimens were very small.

Superionic Conductors by Crystallization of Glass 95

most conductive; however, NaSmPSi with the largest Sm3+ ions was less conductive than NaYPSi with medium Y3+ ions. In the present study, the N5-type NaSmPSi ionic conductors were prepared by crystallization of glasses. The optimum conditions for crystallization were discussed with reference to the conduction properties and the preparation of crack free N5-

Fig. 11. Program of temperature and time for the production of NaSmPSi glass-ceramics.

Samples were prepared according to the chemical formula mentioned above of Na3+3*<sup>x</sup>*-*<sup>y</sup>*Sm1 *<sup>x</sup>*P*y*Si3-*y*O9. The temperatures employed for nucleation and crystallization of glass specimens were also determined by the results of DTA analysis. Fig. 11 shows the program of temperature and time for the production of glass-ceramic NaSmPSi employed in the present work. The N5-type NaSmPSi ionic conductors were successfully produced by crystallization of glasses. Although the glass samples heated by the program pattern (A) shown in Fig. 11 broke during crystallization and the glass-ceramic NaSmPSi obtained by the pattern (B) was difficult to prevent from cracking during crystallization, most of the NaSmPSi compounds by the pattern (C) were obtained as crack free bulky glass-ceramics (the glass samples broke during crystallization when heating time for crystallization was over 5 h). Fig. 12 shows the phase-composition diagram of samples crystallized at 900°C by the pattern (C). The crystallization of the N5 single-phase glass-ceramic NaSmPSi was dependent strongly on the concentrations of both [R] and [P] (or *x* and *y* in the composition parameters) and the temperature for crystallization of glass specimens. Fig. 13 shows SEM photograph of microstructure of specimen with the Na3.9Sm0.6P0.3Si2.7O9 composition heated at 900°C by the pattern (C). The grain size of the specimen was about 3-5 μm. The state of grain growth is promoted with increase of heating temperature and heating time for crystallization. Although grain growth may cause high conductivity, it was difficult to prevent the sample heated for a long time from cracking during crystallization. Studies are underway to produce a crack free sample. Conduction properties were measured by the ac two-probe method on cylindrical glass-ceramics of typically 15 mm in diameter and 2 mm in thickness with an LF impedance analyzer. Electrodes were prepared by sputtering of gold on polished surfaces. The applied ac field ranged from 5 to 10 MHz in frequency. The temperature dependence of the conductivity was measured in a similar way at several temperatures ranging from room temperature to 350°C. Table 3 summarizes the conduction properties of the N5-type glass-ceramic NaSmPSi specimens. It was found that NaSmPSi containing the largest Sm3+ ions was less conductive than NaYPSi with medium Y3+ ions as the grain sizes

The conduction properties of grain boundaries were strongly dependent on the annealing conditions, although those of the grains were little changed by annealing temperature and time. Glass-ceramics are generally composites consisting of crystallized grains and small amounts of residual glass (<1%). To compare the properties of grain boundaries with those of glasses, the conduction properties of sodium yttrium silicophosphate glasses with various compositions were measured. Unlike glass-ceramics the impedance loci of glasses were comprised of one arc, which indicates that there is no polarization arising from microstructural inhomogeneity. Based on the intercepting points on the horizontal axis, the composition dependence of conduction properties of σ300 and *E*a were evaluated. The value of σ300 ranged from 1×10-4 to 5×10-3 S/cm and *E*a increased from 53 to 67 kJ/mol with [Na] or [Na]/[Y]. These results are also in good agreement with those reported for the glasses in the Na2O-Y2O3-SiO2 system. The values of (*E*a)GB of the specimens annealed below 950°C for shorter times correspond to those in the range of glasses, strongly suggesting that their grain boundaries are a glassy matrix. The above mentioned dependence of (*E*a)GB on [Na2O] is explained by the well-known tendency that the conduction properties of glasses are improved by increasing [Na2O], which provides the increase of carrier Na+ ions. The ratio of [Na]/[Y] is also an important parameter for the conduction properties, showing an effect on the conduction properties similar to [Na2O].

In order to identify the structure of the grain boundaries of the specimen (Na3.9Y0.6P0.3Si2.7O9) annealed at 800°C for 0.5 h, TEM analysis was performed both on grains and grain boundaries. The results show clear electron diffraction on grains, while not on grain boundaries. This fact confirms that the grain boundaries are amorphous. Compositional analyses were also performed, however, [Na] was difficult to determine because of the evaporation by electron ablation. It was also observed that the glassy phase was condensed at triple points enclosed by grains, and that neck growth among the grains was well developed. Thus, it is reasonable to consider that the grain boundaries annealed at lower temperatures are amorphous, while those annealed at higher temperatures for longer periods of time are poorly conductive crystalline compounds in the specimens.

#### **5. Recent research on conductive improvement and structural control of glass-ceramic Na+ superionic conductors**

#### **5.1 Preparation of crack free Na5YSi4O12-type glass-ceramics containing the largest Sm3+ ions: Crystallization condition and ionic conductivity**

Glass-ceramics of the phosphorus containing N5-type Na+ superionic conductors have been developed by crystallization of glasses with the composition formula 2. The R elements have a significant effect on the crystallization of glasses, as well as on the conduction properties. To date, polycrystalline N5-type NaRPSi has been obtained with Sc, Y, Gd or Sm as the R element. The ionic radius of R (sixfold oxygen coordinated R) has been expected to have a significant effect on the crystallization of the phase. The reported results on the silicate ceramics show that the conductivity of the N5-type NaRPSi increases with increasing ionic radius of R, giving the order NaSmPSi>NaGdPSi>NaYPSi>NaScPSi. It can be expected that NaSmPSi is the most conductive. However, this order was not always true in glass-ceramics. Although most of the NaRPSi compounds were obtained as crack free bulky glass-ceramics (15 mm in diameter, 5 mm in thickness), NaSmPSi was difficult to prevent from cracking during crystallization. It was found that crack free NaGdPSi with larger Gd3+ ions was the

The conduction properties of grain boundaries were strongly dependent on the annealing conditions, although those of the grains were little changed by annealing temperature and time. Glass-ceramics are generally composites consisting of crystallized grains and small amounts of residual glass (<1%). To compare the properties of grain boundaries with those of glasses, the conduction properties of sodium yttrium silicophosphate glasses with various compositions were measured. Unlike glass-ceramics the impedance loci of glasses were comprised of one arc, which indicates that there is no polarization arising from microstructural inhomogeneity. Based on the intercepting points on the horizontal axis, the composition dependence of conduction properties of σ300 and *E*a were evaluated. The value of σ300 ranged from 1×10-4 to 5×10-3 S/cm and *E*a increased from 53 to 67 kJ/mol with [Na] or [Na]/[Y]. These results are also in good agreement with those reported for the glasses in the Na2O-Y2O3-SiO2 system. The values of (*E*a)GB of the specimens annealed below 950°C for shorter times correspond to those in the range of glasses, strongly suggesting that their grain boundaries are a glassy matrix. The above mentioned dependence of (*E*a)GB on [Na2O] is explained by the well-known tendency that the conduction properties of glasses are improved by increasing [Na2O], which provides the increase of carrier Na+ ions. The ratio of [Na]/[Y] is also an important parameter for the conduction properties, showing an effect on

In order to identify the structure of the grain boundaries of the specimen (Na3.9Y0.6P0.3Si2.7O9) annealed at 800°C for 0.5 h, TEM analysis was performed both on grains and grain boundaries. The results show clear electron diffraction on grains, while not on grain boundaries. This fact confirms that the grain boundaries are amorphous. Compositional analyses were also performed, however, [Na] was difficult to determine because of the evaporation by electron ablation. It was also observed that the glassy phase was condensed at triple points enclosed by grains, and that neck growth among the grains was well developed. Thus, it is reasonable to consider that the grain boundaries annealed at lower temperatures are amorphous, while those annealed at higher temperatures for longer

periods of time are poorly conductive crystalline compounds in the specimens.

 **superionic conductors** 

**Sm3+ ions: Crystallization condition and ionic conductivity** 

**5. Recent research on conductive improvement and structural control of** 

**5.1 Preparation of crack free Na5YSi4O12-type glass-ceramics containing the largest** 

Glass-ceramics of the phosphorus containing N5-type Na+ superionic conductors have been developed by crystallization of glasses with the composition formula 2. The R elements have a significant effect on the crystallization of glasses, as well as on the conduction properties. To date, polycrystalline N5-type NaRPSi has been obtained with Sc, Y, Gd or Sm as the R element. The ionic radius of R (sixfold oxygen coordinated R) has been expected to have a significant effect on the crystallization of the phase. The reported results on the silicate ceramics show that the conductivity of the N5-type NaRPSi increases with increasing ionic radius of R, giving the order NaSmPSi>NaGdPSi>NaYPSi>NaScPSi. It can be expected that NaSmPSi is the most conductive. However, this order was not always true in glass-ceramics. Although most of the NaRPSi compounds were obtained as crack free bulky glass-ceramics (15 mm in diameter, 5 mm in thickness), NaSmPSi was difficult to prevent from cracking during crystallization. It was found that crack free NaGdPSi with larger Gd3+ ions was the

the conduction properties similar to [Na2O].

**glass-ceramic Na+**

most conductive; however, NaSmPSi with the largest Sm3+ ions was less conductive than NaYPSi with medium Y3+ ions. In the present study, the N5-type NaSmPSi ionic conductors were prepared by crystallization of glasses. The optimum conditions for crystallization were discussed with reference to the conduction properties and the preparation of crack free N5 type glass-ceramic NaSmPSi.

Fig. 11. Program of temperature and time for the production of NaSmPSi glass-ceramics.

Samples were prepared according to the chemical formula mentioned above of Na3+3*<sup>x</sup>*-*<sup>y</sup>*Sm1 *<sup>x</sup>*P*y*Si3-*y*O9. The temperatures employed for nucleation and crystallization of glass specimens were also determined by the results of DTA analysis. Fig. 11 shows the program of temperature and time for the production of glass-ceramic NaSmPSi employed in the present work. The N5-type NaSmPSi ionic conductors were successfully produced by crystallization of glasses. Although the glass samples heated by the program pattern (A) shown in Fig. 11 broke during crystallization and the glass-ceramic NaSmPSi obtained by the pattern (B) was difficult to prevent from cracking during crystallization, most of the NaSmPSi compounds by the pattern (C) were obtained as crack free bulky glass-ceramics (the glass samples broke during crystallization when heating time for crystallization was over 5 h). Fig. 12 shows the phase-composition diagram of samples crystallized at 900°C by the pattern (C). The crystallization of the N5 single-phase glass-ceramic NaSmPSi was dependent strongly on the concentrations of both [R] and [P] (or *x* and *y* in the composition parameters) and the temperature for crystallization of glass specimens. Fig. 13 shows SEM photograph of microstructure of specimen with the Na3.9Sm0.6P0.3Si2.7O9 composition heated at 900°C by the pattern (C). The grain size of the specimen was about 3-5 μm. The state of grain growth is promoted with increase of heating temperature and heating time for crystallization. Although grain growth may cause high conductivity, it was difficult to prevent the sample heated for a long time from cracking during crystallization. Studies are underway to produce a crack free sample. Conduction properties were measured by the ac two-probe method on cylindrical glass-ceramics of typically 15 mm in diameter and 2 mm in thickness with an LF impedance analyzer. Electrodes were prepared by sputtering of gold on polished surfaces. The applied ac field ranged from 5 to 10 MHz in frequency. The temperature dependence of the conductivity was measured in a similar way at several temperatures ranging from room temperature to 350°C. Table 3 summarizes the conduction properties of the N5-type glass-ceramic NaSmPSi specimens. It was found that NaSmPSi containing the largest Sm3+ ions was less conductive than NaYPSi with medium Y3+ ions as the grain sizes of the presented specimens were very small.

Preparation of Na<sup>+</sup>

Superionic Conductors by Crystallization of Glass 97

**5.2.1 Ionic conductivities of Nasicon-type glass-ceramic superionic conductors in the** 

Our phosphorus containing compositions have been confirmed superior to the mother composition of Na5RSi4O12, especially in the production of the single-phase glass-ceramics. Considering the inference, our main work has recently been focused on the synthesis of various glass-ceramics with single-phase Na5RSi4O12. In the present study, the glassceramics of the titanium-, germanium- or tellurium-containing Na5RSi4O12-type (R=Y) Na+ superionic conductors (N5YXS) from the glasses with the composition Na3+3xY1-xXySi3-yO9 (X=Ti; NYTiS, Ge; NYGeS, X=Te; NYTeS) were prepared, and the effects of X elements on the separation of the phase and the microstructural effects on the conduction properties of

The glass-ceramics have been obtained under the appropriate sets of the parameters x and y of the composition formula Na3+3xY1-xXySi3-yO9 ranging in x=0.1∼0.55 and y=0.1∼0.45. The precursor glasses were made by melting stoichiometric mixtures of reagent-grade powders of anhydrous Na2CO3, Y2O3, (TiO2, GeO2 or TeO2) and SiO2 at 1300∼1400°C for 1 h, followed by annealing for several hours at an optimum temperature. The N5YXS ionic conductors were successfully produced by crystallization of glasses. Figs. 14, 15 and 16 show the diagrams of phase-composition-crystallization temperature of NYTiS, NYGeS and NYTeS glass-ceramics, respectively. The phase formed was dependent on composition and crystallization temperature. N5YTiS, N5YGeS and N5YTeS are obtained as a stable phase at high-temperatures. The crystallization of N5 single phase is strongly dependent both on the contents of yttrium and (titanium, germanium or tellurium) ions (or the values x and y correspond to the composition parameters in Na3+3xY1-xXySi3-yO9). N3 and N9 phases can be crystallized as the hightemperature stable phases at the regions of higher [Y] and rather lower [Y], respectively. The combination of x and y was most varied in N5YGeS and more limited in the order of N5YTeS>N5YTiS. Table 4 summarizes the conduction properties of the N5 glass-ceramics with Na3.6Y0.8Ti0.2Si2.8O9, Na4.2Y0.6Ge0.3Si2.7O9 and Na4.2Y0.6Te0.3Si2.7O9 compositions. Their conductivities and activation energies are of the order of 10-2 S/cm at 300°C and of 15 to 24 kJ/mol, respectively. The conductivity decreases giving the order N5YGeS>N5YTeS>N5YTiS. It

Fig. 14. The diagrams of phase-composition of NYTiS glass-ceramics heated at 900°C (a) and

1000°C (b) for 5 h. ● Na5RSi4O12 (N5) ▼ Na9RSi6O18 (N9) ◎ N5+N9 ◇ N3+N9

**5.2 Composition control of silico-phosphate glass-ceramics** 

is considered that this order corresponds to the N5 single phase region.

**system Na2O-Y2O3-XO2-SiO2 (X=Ti, Ge, Te)** 

glass-ceramics were discussed.

Fig. 12. The diagram of phase-composition of NaSmPSi glass-ceramics crystallized at 900ºC. Na5YSi4O12 (N5) type, Na3YSi3O9 (N3) type, Na9RSi6O18 (N9) type N5+N3, N5+N9, N3+N9

Fig. 13. SEM photograph of the specimen with Na3.9Sm0.6P0.3Si2.7O9 composition heated at 900ºC by the pattern (C).


Table 3. Conduction properties of N5-type NaSmPSi glass-ceramics. Heat-treatment: 900ºC, 5 h

Fig. 12. The diagram of phase-composition of NaSmPSi glass-ceramics crystallized at 900ºC.

Fig. 13. SEM photograph of the specimen with Na3.9Sm0.6P0.3Si2.7O9 composition heated at

Table 3. Conduction properties of N5-type NaSmPSi glass-ceramics.

Na5YSi4O12 (N5) type, Na3YSi3O9 (N3) type, Na9RSi6O18 (N9) type

N5+N3, N5+N9, N3+N9

900ºC by the pattern (C).

Heat-treatment: 900ºC, 5 h

#### **5.2 Composition control of silico-phosphate glass-ceramics**

#### **5.2.1 Ionic conductivities of Nasicon-type glass-ceramic superionic conductors in the system Na2O-Y2O3-XO2-SiO2 (X=Ti, Ge, Te)**

Our phosphorus containing compositions have been confirmed superior to the mother composition of Na5RSi4O12, especially in the production of the single-phase glass-ceramics. Considering the inference, our main work has recently been focused on the synthesis of various glass-ceramics with single-phase Na5RSi4O12. In the present study, the glassceramics of the titanium-, germanium- or tellurium-containing Na5RSi4O12-type (R=Y) Na+ superionic conductors (N5YXS) from the glasses with the composition Na3+3xY1-xXySi3-yO9 (X=Ti; NYTiS, Ge; NYGeS, X=Te; NYTeS) were prepared, and the effects of X elements on the separation of the phase and the microstructural effects on the conduction properties of glass-ceramics were discussed.

The glass-ceramics have been obtained under the appropriate sets of the parameters x and y of the composition formula Na3+3xY1-xXySi3-yO9 ranging in x=0.1∼0.55 and y=0.1∼0.45. The precursor glasses were made by melting stoichiometric mixtures of reagent-grade powders of anhydrous Na2CO3, Y2O3, (TiO2, GeO2 or TeO2) and SiO2 at 1300∼1400°C for 1 h, followed by annealing for several hours at an optimum temperature. The N5YXS ionic conductors were successfully produced by crystallization of glasses. Figs. 14, 15 and 16 show the diagrams of phase-composition-crystallization temperature of NYTiS, NYGeS and NYTeS glass-ceramics, respectively. The phase formed was dependent on composition and crystallization temperature. N5YTiS, N5YGeS and N5YTeS are obtained as a stable phase at high-temperatures. The crystallization of N5 single phase is strongly dependent both on the contents of yttrium and (titanium, germanium or tellurium) ions (or the values x and y correspond to the composition parameters in Na3+3xY1-xXySi3-yO9). N3 and N9 phases can be crystallized as the hightemperature stable phases at the regions of higher [Y] and rather lower [Y], respectively. The combination of x and y was most varied in N5YGeS and more limited in the order of N5YTeS>N5YTiS. Table 4 summarizes the conduction properties of the N5 glass-ceramics with Na3.6Y0.8Ti0.2Si2.8O9, Na4.2Y0.6Ge0.3Si2.7O9 and Na4.2Y0.6Te0.3Si2.7O9 compositions. Their conductivities and activation energies are of the order of 10-2 S/cm at 300°C and of 15 to 24 kJ/mol, respectively. The conductivity decreases giving the order N5YGeS>N5YTeS>N5YTiS. It is considered that this order corresponds to the N5 single phase region.

Fig. 14. The diagrams of phase-composition of NYTiS glass-ceramics heated at 900°C (a) and 1000°C (b) for 5 h. ● Na5RSi4O12 (N5) ▼ Na9RSi6O18 (N9) ◎ N5+N9 ◇ N3+N9

Preparation of Na<sup>+</sup>

**type glass-ceramics** 

glass-ceramics were discussed.

Superionic Conductors by Crystallization of Glass 99

**5.2.2 Effect of substitution of Si with V and Mo on ionic conductivity of Na5YSi4O12-**

Glass-ceramics of the vanadium- or molybdenum-containing N5-type Na+-superionic conductors were prepared by crystallization of glasses with the compositions Na3+3x-yY1 xVySi3-yO9 (NYVS) or Na3+3x-2yY1-xMoySi3-yO9 (NYMS), and the effects of V or Mo elements on the separation of the phase and the microstructural effects on the conduction properties of

The glass-ceramics have been obtained under the appropriate sets of the parameters x and y of the composition formulas Na3+3x-yY1-xVySi3-yO9 or Na3+3x-2yY1-xMoySi3-yO9 ranging in x=0.3∼0.5 and y=0.1∼0.4. The precursor glasses were made by melting stoichiometric mixtures of reagent-grade powders of anhydrous Na2CO3, Y2O3, V2O5, MoO3 and SiO2 at 1400°C for 1 h, followed by annealing for several hours at an optimum temperature. Shown in Fig. 17 are the diagrams of phase-composition-crystallization temperature of the glassceramic specimens with the Na3.9Y0.6V0.3Si2.7O9 (A) and Na3.7Y0.7Mo0.1Si2.9O9 (B) compositions. N5-type NYVS and NYMS are obtained as a stable phase at high-temperatures. The crystallization of N5 single phase is strongly dependent both on the contents of yttrium and (vanadium or molybdenum) ions (or the values x and y correspond to the composition parameters in Na3+3x-yY1-xVySi3-yO9 or Na3+3x-2yY1-xMoySi3-yO9). N3 and N9 phases can be

Fig. 17. The diagrams of phase-composition-crystallization temperature of NYVS

● Na5RSi4O12 (N5) □ Na3RSi3O9 (N3) △ Na9RSi6O18 (N9) □ N5+N3 △ N5+N9

(a) and NYMS (b) glass-ceramics crystallized at 800~1100°C.

Fig. 15. The diagrams of phase-composition of NYGeS glass-ceramics heated at 900°C (a) and 1000°C (b) for 5 h. ● Na5RSi4O12 (N5) ▼ Na9RSi6O18 (N9) 〇 N5+N3 ◎ N5+N9

Fig. 16. The diagrams of phase-composition of NYTeS glass-ceramics heated at 900°C (a) and 1000°C (b) for 5 h. ● Na5RSi4O12 (N5) ▼ Na9RSi6O18 (N9) ◎ N5+N9


Table 4. Conduction properties of the N5 glass-ceramics with Na3.6Y0.8Ti0.2Si2.8O9, Na4.2Y0.6Ge0.3Si2.7O9 and Na4.2Y0.6Te0.3Si2.7O9 compositions.

Fig. 15. The diagrams of phase-composition of NYGeS glass-ceramics heated at 900°C (a) and 1000°C (b) for 5 h. ● Na5RSi4O12 (N5) ▼ Na9RSi6O18 (N9) 〇 N5+N3 ◎ N5+N9

Fig. 16. The diagrams of phase-composition of NYTeS glass-ceramics heated at 900°C

(a) and 1000°C (b) for 5 h. ● Na5RSi4O12 (N5) ▼ Na9RSi6O18 (N9) ◎ N5+N9

Table 4. Conduction properties of the N5 glass-ceramics with Na3.6Y0.8Ti0.2Si2.8O9,

Na4.2Y0.6Ge0.3Si2.7O9 and Na4.2Y0.6Te0.3Si2.7O9 compositions.

#### **5.2.2 Effect of substitution of Si with V and Mo on ionic conductivity of Na5YSi4O12 type glass-ceramics**

Glass-ceramics of the vanadium- or molybdenum-containing N5-type Na+-superionic conductors were prepared by crystallization of glasses with the compositions Na3+3x-yY1 xVySi3-yO9 (NYVS) or Na3+3x-2yY1-xMoySi3-yO9 (NYMS), and the effects of V or Mo elements on the separation of the phase and the microstructural effects on the conduction properties of glass-ceramics were discussed.

The glass-ceramics have been obtained under the appropriate sets of the parameters x and y of the composition formulas Na3+3x-yY1-xVySi3-yO9 or Na3+3x-2yY1-xMoySi3-yO9 ranging in x=0.3∼0.5 and y=0.1∼0.4. The precursor glasses were made by melting stoichiometric mixtures of reagent-grade powders of anhydrous Na2CO3, Y2O3, V2O5, MoO3 and SiO2 at 1400°C for 1 h, followed by annealing for several hours at an optimum temperature. Shown in Fig. 17 are the diagrams of phase-composition-crystallization temperature of the glassceramic specimens with the Na3.9Y0.6V0.3Si2.7O9 (A) and Na3.7Y0.7Mo0.1Si2.9O9 (B) compositions. N5-type NYVS and NYMS are obtained as a stable phase at high-temperatures. The crystallization of N5 single phase is strongly dependent both on the contents of yttrium and (vanadium or molybdenum) ions (or the values x and y correspond to the composition parameters in Na3+3x-yY1-xVySi3-yO9 or Na3+3x-2yY1-xMoySi3-yO9). N3 and N9 phases can be

Fig. 17. The diagrams of phase-composition-crystallization temperature of NYVS (a) and NYMS (b) glass-ceramics crystallized at 800~1100°C. ● Na5RSi4O12 (N5) □ Na3RSi3O9 (N3) △ Na9RSi6O18 (N9) □ N5+N3 △ N5+N9

Preparation of Na<sup>+</sup>

field.

condition that does not apply the voltage.

microstructure and the conduction properties.

Superionic Conductors by Crystallization of Glass 101

electric field. The conditions for bias crystallization are discussed with respect to the

The precursor glasses were made by melting stoichiometric mixtures of reagent-grade powders of anhydrous Na2CO3, Y2O3, SiO2 and NH4H2PO4 at 1350°C for 1h, followed by annealing for several hours at an optimum temperature. The annealed specimens were heated up to 900°C in an electric field for bias crystallization. The thermostable heating holder was produced in order to do the crystallization in a direct current electric field. This holder is made of alumina and platinum. Glass samples (5 mm × 5 mm × 8 mm) were held between the platinum plates, and crystallized in the electrical field using 1 V/mm. The thermal treatment

The microstructure was investigated with SEM. The grain length of the cross section which is parallel with the electric field direction was 10~15 nm, and it was proven to be smaller than the 15~30 nm grain length of the cross section which is perpendicular to the direction and the specimen crystallized by the conventional method. It was possible to control shape

Fig. 18. Current profile in relation to temperature during crystallization process in electric

Due to the bias field an electric current in relation to temperature was measured during crystallization process. Fig. 18 shows current profile in relation to temperature during crystallization process in the electric field. The largest observed current was 250 μA. The current profile exhibits three peaks at about 600°C, 700°C and 850°C. These temperatures correspond to those of nucleation, phase transition from N3-phase to N5-phase, and crystallization of glass specimens determined by DTA analysis, respectively. An electric current in relation to temperature was measured newly by applying the bias voltage only in two limited temperature range, because two main peaks were observed in Fig. 18. One range is from right before of the first main peak (511°C to 652°C), and another range is from right before of the second main peak (790°C to 865°C). The resulting current profile is shown in Fig. 19. It was found that the mass transfer in the specimen is being generated even in the

was the same as that used in conventional crystallization in nonelectric field.

and orientation of crystal grain by the crystallization in the electrical field.

crystallized as the high-temperature stable phases at the regions of rather lower [Y] and higher [Y], respectively. The total conductivities and the activation energies are summarizes in Table 1. The total conductivities of the specimens (A) and (B) were 0.87×10-2 and 3.58×10-2 S/cm at 300°C, respectively, and the activation energies of those specimens were 38.1 and 21.8 kJ/mol, respectively. The combination of x and y was most varied in N5-type NYPS and more limited in N5-type NYVS and NYMS. The conductivity decreases giving the order NYPS>NYMS>NYVS. It is considered that this order corresponds to the N5 single phase region. We assume that the effect of the substitution of Si with V or Mo should be to bring about the difference of homogeneity in the N5 ring structure. The total and electronic conductivities and the Na+ ionic transport numbers of the specimen (A) determined by Wagner polarization method are summarizes in Table 5. The ionic transport numbers of the specimen (A) were nearly 0.9, while those of the specimen (B) were nearly 1. It is considered that about 10% of total conduction is electronic conduction (hopping conduction by transition metal vanadium) in the specimen (A). This result can explain following facts; the conductivity of the specimen (A) are lower than other N5 conductors.


Table 5. Total conductivities and activation energies of the glass-ceramic specimens Na3.9Y0.6V0.3Si2.7O9 (A) and Na3.7Y0.7Mo0.1Si2.9O9 (B).

#### **5.3 Structure and conduction properties of Na5YSi4O12-type glass-ceramics synthesized by bias crystallization of glass**

Glass-ceramics of the phosphorus containing N5-type Na+ superionic conductors were prepared by bias crystallization of glasses with the composition Na4.05Y0.55P0.3Si2.7O9 in an

crystallized as the high-temperature stable phases at the regions of rather lower [Y] and higher [Y], respectively. The total conductivities and the activation energies are summarizes in Table 1. The total conductivities of the specimens (A) and (B) were 0.87×10-2 and 3.58×10-2 S/cm at 300°C, respectively, and the activation energies of those specimens were 38.1 and 21.8 kJ/mol, respectively. The combination of x and y was most varied in N5-type NYPS and more limited in N5-type NYVS and NYMS. The conductivity decreases giving the order NYPS>NYMS>NYVS. It is considered that this order corresponds to the N5 single phase region. We assume that the effect of the substitution of Si with V or Mo should be to bring about the difference of homogeneity in the N5 ring structure. The total and electronic conductivities and the Na+ ionic transport numbers of the specimen (A) determined by Wagner polarization method are summarizes in Table 5. The ionic transport numbers of the specimen (A) were nearly 0.9, while those of the specimen (B) were nearly 1. It is considered that about 10% of total conduction is electronic conduction (hopping conduction by transition metal vanadium) in the specimen (A). This result can explain following facts; the

conductivity of the specimen (A) are lower than other N5 conductors.

Table 5. Total conductivities and activation energies of the glass-ceramic specimens

**5.3 Structure and conduction properties of Na5YSi4O12-type glass-ceramics** 

Glass-ceramics of the phosphorus containing N5-type Na+ superionic conductors were prepared by bias crystallization of glasses with the composition Na4.05Y0.55P0.3Si2.7O9 in an

Na3.9Y0.6V0.3Si2.7O9 (A) and Na3.7Y0.7Mo0.1Si2.9O9 (B).

**synthesized by bias crystallization of glass** 

electric field. The conditions for bias crystallization are discussed with respect to the microstructure and the conduction properties.

The precursor glasses were made by melting stoichiometric mixtures of reagent-grade powders of anhydrous Na2CO3, Y2O3, SiO2 and NH4H2PO4 at 1350°C for 1h, followed by annealing for several hours at an optimum temperature. The annealed specimens were heated up to 900°C in an electric field for bias crystallization. The thermostable heating holder was produced in order to do the crystallization in a direct current electric field. This holder is made of alumina and platinum. Glass samples (5 mm × 5 mm × 8 mm) were held between the platinum plates, and crystallized in the electrical field using 1 V/mm. The thermal treatment was the same as that used in conventional crystallization in nonelectric field.

The microstructure was investigated with SEM. The grain length of the cross section which is parallel with the electric field direction was 10~15 nm, and it was proven to be smaller than the 15~30 nm grain length of the cross section which is perpendicular to the direction and the specimen crystallized by the conventional method. It was possible to control shape and orientation of crystal grain by the crystallization in the electrical field.

Fig. 18. Current profile in relation to temperature during crystallization process in electric field.

Due to the bias field an electric current in relation to temperature was measured during crystallization process. Fig. 18 shows current profile in relation to temperature during crystallization process in the electric field. The largest observed current was 250 μA. The current profile exhibits three peaks at about 600°C, 700°C and 850°C. These temperatures correspond to those of nucleation, phase transition from N3-phase to N5-phase, and crystallization of glass specimens determined by DTA analysis, respectively. An electric current in relation to temperature was measured newly by applying the bias voltage only in two limited temperature range, because two main peaks were observed in Fig. 18. One range is from right before of the first main peak (511°C to 652°C), and another range is from right before of the second main peak (790°C to 865°C). The resulting current profile is shown in Fig. 19. It was found that the mass transfer in the specimen is being generated even in the condition that does not apply the voltage.

Preparation of Na<sup>+</sup>

parallel.

Superionic Conductors by Crystallization of Glass 103

Table 6. Conduction properties of the bias-crystallized NaRPSi glasses.

Fig. 21. Temperature dependence of conductivity of the bias crystallized NaRPSi glasses.

Fig. 20 shows the temperature dependence Arrhenius plots of the conductivities of various specimens. The complex admittances of the measured glass-ceramics consisted of two semicircles below 300°C. The two intercepting points on the real axis are interpreted as the resistance of crystallized grains (*R*G) and the total resistance of grains and remaining glassy grain boundaries (*R*GB). As *R*GB decreases rapidly with increasing temperature because of high (*E*a)GB to a comparable value with *R*G at 300°C, the total conductivities (*R*G+*R*GB) are dominated by grain boundary conductivity. The effect of the grain boundary is greatly seen on the appearance at lower temperatures. Table 6 summarizes the conduction properties obtained from Fig. 20. The cross sections which are parallel and perpendicular to the electric field direction showed the ionic conductivities of 0.0923 and 0.132 mS/cm at 300°C, respectively. It was found that the bias crystallized specimens were less conductive than that crystallized by the conventional method. Fig. 21 shows the temperature dependence of conductivity of the bias crystallized specimen. In the temperatures over 300°C, anisotropy in the conductivity was observed. It was also found that the cross section which is perpendicular to the electric field direction was more conductive than that in parallel with the direction. The microstructure and the electric conductivity of the NaRPSi glass-ceramics perpendicular to the electric field direction were significantly different from those in

Fig. 19. Current profile in relation to temperature measured by applying voltage in two limited temperature ranges.

Fig. 20. Temperature dependence Arrhenius plots of the conductivities of the bias crystallized NaRPSi glasses. ○ Conventional △ Parallel □ Perpendicular

Crystalline phases were identified on the sample after the crystallization in the electric field by the X-ray diffraction (XRD) method in order to consider the possibility of structural change by the movement of Na+ ion which is a carrier. In the several cut sections, no difference in the fundamental structure was observed. Judging from the patterns, the N5 single phase ionic conductors were successfully produced by bias crystallization of glasses.

Fig. 19. Current profile in relation to temperature measured by applying voltage in two

Fig. 20. Temperature dependence Arrhenius plots of the conductivities of the bias crystallized NaRPSi glasses. ○ Conventional △ Parallel □ Perpendicular

Crystalline phases were identified on the sample after the crystallization in the electric field by the X-ray diffraction (XRD) method in order to consider the possibility of structural change by the movement of Na+ ion which is a carrier. In the several cut sections, no difference in the fundamental structure was observed. Judging from the patterns, the N5 single phase ionic conductors were successfully produced by bias

limited temperature ranges.

crystallization of glasses.


Table 6. Conduction properties of the bias-crystallized NaRPSi glasses.

Fig. 21. Temperature dependence of conductivity of the bias crystallized NaRPSi glasses.

Fig. 20 shows the temperature dependence Arrhenius plots of the conductivities of various specimens. The complex admittances of the measured glass-ceramics consisted of two semicircles below 300°C. The two intercepting points on the real axis are interpreted as the resistance of crystallized grains (*R*G) and the total resistance of grains and remaining glassy grain boundaries (*R*GB). As *R*GB decreases rapidly with increasing temperature because of high (*E*a)GB to a comparable value with *R*G at 300°C, the total conductivities (*R*G+*R*GB) are dominated by grain boundary conductivity. The effect of the grain boundary is greatly seen on the appearance at lower temperatures. Table 6 summarizes the conduction properties obtained from Fig. 20. The cross sections which are parallel and perpendicular to the electric field direction showed the ionic conductivities of 0.0923 and 0.132 mS/cm at 300°C, respectively. It was found that the bias crystallized specimens were less conductive than that crystallized by the conventional method. Fig. 21 shows the temperature dependence of conductivity of the bias crystallized specimen. In the temperatures over 300°C, anisotropy in the conductivity was observed. It was also found that the cross section which is perpendicular to the electric field direction was more conductive than that in parallel with the direction. The microstructure and the electric conductivity of the NaRPSi glass-ceramics perpendicular to the electric field direction were significantly different from those in parallel.

Preparation of Na<sup>+</sup>

**7. Acknowledgment** 

**8. References** 

support and warm encouragement.

glasses. *Solid State Ionics*, 22, 257.

*Ceram. Intl.*, 7(2), 43-47.

*Mater. Res. Bull.*, 13, 757-761.

*Ceram. Soc. Jpn.*, 97, 1097-1103.

*Solid State Ionics*, 86-88, 511-516.

Na5YSi4O12. *Appl. Phys. Lett.*, 37, 934-936.

(1985). *J. Electrochem. Soc.*, 132, 1340-1345.

Wiley & Sons, Inc., New York, p. 368-369.

of compounds. *Solid State Ionics*, 6, 195-200.

Superionic Conductors by Crystallization of Glass 105

numbers of these glass-ceramics determined by Wagner polarization method were nearly 0.9 for the specimen (A) and 1 for the specimen (B) at 300°C, respectively. It is considered that about 10% of total conduction is electronic conduction in the specimen (A). This result can explain following facts; the conductivity of the specimen (A) are lower than other N5 conductors, and it is seen in the temperature dependence Arrhenius plots for the specimen

We have successfully produced the N5-type glass-ceramic conductors by bias crystallization of the glasses with the composition Na4.05Y0.55P0.3Si2.7O9 in an electric field. The microstructure and the conduction properties were dependent on the current direction in the process of crystallization. The cross sections which are parallel and perpendicular to the electric field direction showed the ionic conductivities of 0.0923 and 0.132 mS/cm at 300°C, respectively. The microstructure and the electric conductivity of the glass-ceramics perpendicular to the

I would like to thank Prof. Kimihiro Yamashita (Tokyo Medical and Dental University, Japan) and Professor emeritus Hideki Monma (Kogakuin University, Japan) for their

Alexander, M. G. (1987). Effect of modifier cations on Na+ conductivity in sodium silicate

Banks, E. & Kim. C. H. (1985). Ionic conductivity in glass and glass-ceramics of the Na3YSi3O9 and Na5YSi4O12 type materials. *J. Electrochem. Soc.*, 132, 2617-2621. Beyeler, H. U. & Himba, T. (1978). The sodium conductivity paths in the superionic

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Hong, H. Y-P.; Kafalas, J. A. & Bayard, M. (1978). High Na+-ion conductivity in Na5YSi4O12.

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Kingery, W. D.; Bowen, H. K. & D. R. Uhlmann. (1976). *Introduction to Ceramics, 2nd ed.*, John

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Okura, T.; Tanaka, M.; Kanzawa, H. & Sudoh, G. (1996). Synthesis and conduction

Okura, T.; Yamashita, K. & Umegaki, T. (1996). Na+-ion conduction properties of glass-

possible mechanism for the ionic conductivity in the Na5RESi4O12 (RE=Y, Sc) class

properties of Na+ superionic conductors of sodium samarium silicophosphates.

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conductors Na5RESi4O12. *Solid State Commun.*, 27, 641-643.

(A) that the lines drawn from the conductivity of grains are bending upwards.

electric field direction were significantly different from those in parallel.
