**2. Materials**

84 Advances in Crystallization Processes

 Na4(3+3*<sup>x</sup>*-*<sup>y</sup>*)/3Y4(1-*x*)/3P4*y*/3Si4(3-*y*)/3O12 (3) In relation to previous works, formula 2 was employed in this work, and formula 3 is referred to in the results. The trivalent ions employed here for R3+ were Sc3+, In3+, Er3+, Gd3+, Sm3+, Eu3+, Nd3+ and La3+ as well as Y3+. These results are to be interpreted in terms of the

+1/3

Y

Na(3) Na(4)

Na(1), Na(2)

Na(5), Na(6)

effect of the rare earth ions on the crystallization of N5-type phase in glasses.


Fig. 1. Crystal Structure of Na5YSi4O12. Projection of the Na5YSi4O12 Structure on (100).

also be detailed based on the kinetic results.

system.

In the course of the fundamental studies on glass-ceramic Na3+3*<sup>x</sup>*-*<sup>y</sup>*R1-*x*P*y*Si3-*y*O9, we have interestingly found the crystallization of those N3- and Na9YSi6O18 (N9)-type phases as the precursors in the glasses. These are the analogues to the silicates N3 and N9 and therefore are the same members of the family of Na24-3*x*Y*x*Si12O36 as N5. Although we had also successfully synthesized those materials by the solid-state reactions of powders with the above composition of various sets of the parameters *x* and *y*, the metastability of those precursor phases had not been noticed in the synthesis. It has been observed that such precursor phases were transformed to the Na+-superionic conducting phase on specimens with appropriate sets of *x* and *y*. The present paper will deal with the thermodynamic and kinetic study on the phase transformation of metastable phases to the stable phase with Na+ superionic conductivity. The superiority of our present materials to the other silicate N5 will

The microstructure of a glass-ceramic, including neck growth among grains as well as grain size, is generally affected by the crystallization process. As the above mentioned devices utilize dc conduction properties of Na+-superionic conductors, another aim was to study the microstructural effects on the conduction properties of a whole glass-ceramic. Special attention was paid to the analysis of grain boundary properties using the Na2O-Y2O3-P2O5- SiO2 system. For the analysis of grain boundary properties, as will be discussed below, composition dependences of the conductivity of sodium silico-phosphate glasses containing Y2O3 were also studied in the Na2O-Y2O3-P2O5-SiO2 system. For convenience, the present materials are abbreviated as NaRPSi taken from the initials of the Na2O-R2O3-P2O5-SiO2

## **2.1 Preparation of glasses and glass-ceramics**

Precursor glasses were prepared from reagent-grade oxides of anhydrous Na2CO3, R2O3 (R=Y, Sc, In, Er, Gd, Sm, Eu, Nd, La), NH4H2PO4 and SiO2; the mechanically mixed powders according to formula 2 or appropriate compositions shown below were melted at 1350°C for 1 h after calcinations at 900°C for 1 h. The melts were quickly poured into a cylindrical graphite, then annealed at 500°C for 3 h, giving NaRPSi glasses. The composition parameters studied were in the range of 0.2<x<0.6 and 0<y<0.5 of formula 2. As shown below, grain boundary conduction properties are discussed in relation to the properties of glasses. For the evaluation of the composition dependence of conductivity in Na+ conducting glasses, various sodium yttrium silico-phosphate glass specimens with different atomic ratios of [Na]/[P+Si] and [Na]/[Y] were also prepared.

Crystallization was carried out according to the previous report; bulk glasses were heated with an increasing rate of 75°C/h to a temperature above ca. 50°C of the glass transition point, which had been determined in advance by differential thermal analysis (DTA). This pretreatment was done in order to obtain homogeneous nucleation. After the annealing for 1 h, specimens were heated at temperatures of 800 to 1100°C, depending on the composition, for 0.5 to 72 h, thereafter slowly cooled in a furnace with a decreasing rate of 150°C/h to room temperature. These quenched glasses or glass-ceramic specimens were polished down with 0.5 μm diamond paste, thereafter subjected to the conductivity measurements.

#### **2.2 Measurements and characterization**

Ionic conductivities were measured by the complex impedance method on cylindrical glasses or glass-ceramics of typically 15 mm in diameter and 2 mm in thickness. 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. The complex impedance or admittance loci of glass and glass-ceramics were analyzed by an equivalent circuit (Fig. 2), which was experimentally found to comprise one and two semicircles in NaRPSi glasses and glass-ceramics, respectively. The two intercepting points on the real axis are interpreted as the resistance of crystallized grains (*R*G(c)) and the total resistance of grains and remaining glassy grain boundaries (*R*GB(g)). Assume the complex admittance diagram shown in Fig. 3, where the parameters L1 and L2 are set here as the radii of the two arcs 1 and 2. Those parameters are related to one another as the following:

$$\mathcal{L}\_{1\lhd} \mathcal{1} / \left( R\_{\text{G}(\text{c})} + R\_{\text{G}\text{B}(\text{g})} \right) \tag{4}$$

and

$$\mathcal{L}\_{2^{\text{sc}}}(1/R\_{\text{G}(\text{c})}) - 1/\left(R\_{\text{G}(\text{c})} + R\_{\text{GR}(\text{g})}\right) \tag{5}$$

Then,

$$\mathcal{L}\_2/\mathcal{L}\_1 = \mathcal{R}\_{\text{GIR}(\emptyset)} / R\_{\text{G}(\emptyset)}\tag{6}$$

Therefore, in an ideal grass-ceramic where residual glass would have negligible influence on the total, arc 2 would be much smaller than arc 1, since L2/L1→0.

Preparation of Na<sup>+</sup>

G(c) and GB(g).

0

''

Superionic Conductors by Crystallization of Glass 87

G(c) GB(g) 1

**ARC 2**

**L2**

G(c) 1

*R*

Fig. 3. An idealized diagram of complex admittance for glass-ceramics, in which arc 1 (ARC 1) and arc 2 (ARC 2) are related to the crystallized grains (G(c)) and remaining glasses (GB(g)). L1, L2, *R*G(c), and *R*GB(g) are, respectively, the radii of arcs 1 and 2, the resistances of

**ARC 1**

*R*+ *RYY*

**L1**

'

Fig. 4. Composition dependence of precursor (*pp*) and high temperature-stable phases (*sp*) of glass-ceramic NaRPSi on P-Y (a), Y-Na (b) and P-Na (c) maps, where precursor phases N3 and N9 are shown with *circles* and *squares*, respectively. High temperature-stable phases are shown in such a way that solid marks means that N5-NaRSi is the stable, while *open marks* indicate that the precursor phases are also stable even at high temperatures. Mixed phases are also shown: *open circle* pp = sp = N3; *filled circle* pp = N3, sp = N5; *open square* pp = sp =

N9; *filled circle* pp = N9, sp = N5; *open split square* pp = N9, sp = N9 + N5

Crystalline phases of glass-ceramic specimens were identified by X-ray diffraction (XRD) method. The lattice parameters of the N5-type hexagonal unit cell were calculated by a leastsquares method using the XRD peaks of (054), (044), (134), (440) and (024). Glass-ceramics of Y3+-contained NaRPSi were subjected to scanning (SEM) and transmission electron microscope (TEM) for microstructural analysis. Electron diffraction and compositional analyses were also performed to characterize the structure of the grain boundary.

For the description of a specific NaRPSi, R of the term will be replaced, respectively, with Y, Sc, In, Er, Gd, Sm, Eu, Nd and La as NaYPSi, NaScPSi, NaInPSi, NaErPSi, NaGdPSi, NaSmPSi, NaEuPSi, NaNdPSi and NaLaPSi for Y2O3, Sc2O3, In2O3, Er2O3, Gd2O3, Sm2O3., Eu2O3, Nd2O3 and La2O3.
