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**4** 

*Japan* 

Toshinori Okura *Kogakuin University* 

 **Superionic Conductors** 

**by Crystallization of Glass** 

**Preparation of Na+**

The use of glass-making processing is favorable for the fabrication of Na+ conducting electrolyte tubes, which has been the key to the technological development of 1 MW Na/S secondary battery plants. However, the processing technique cannot be applied to wellknown β- and β″-aluminas (e. g., NaAl11O17 and NaAl5O8) and Nasicons (Na1+*x*Zr2P3-*x*Si*x*O12) because their high inclusion of Al2O3 or ZrO2 brings about the inhomogeneous melting or crystallization from glasses. Alternatively, Nasicon-like glass-ceramics were synthesized using the composition with lower content of ZrO2 (*m*Na2O·*x*ZrO2·*y*P2O5·(100-*m*-*x*-*y*)SiO2 [*m*=20, 30 mol%]), however, the conductivities (σ) attained were, at most, as high as σ300=2×10-2 S/cm at 300°C with the activation energies (*E*a) of ca. 30 kJ/mol. These low conductivities were attributed to the crystallization of the poorly conductive rhombohedral phase in these Nasicon-like materials. Na5YSi4O12 (N5), which comprises 12-(SiO4)4- tetrahedra membered skeleton structure (Fig. 1), is another Na+-superionic conductor with σ300=1×10-1 S/cm and *E*a=25 kJ/mol. A pioneering work on N5-type glass-ceramics has been performed by Banks *et al*. on the family of N5-type materials by substituting Y with Er, Gd or Sm. However, their results were not completely satisfactory because of the relatively lower conductivities of σ300<2×10-2 S/cm than the reported values of N5. This discrepancy may possibly have arisen from the occurrence of a less conductive metastable phase during

Contrary to the results of Banks *et al*., the present authors have produced glass-ceramics with σ300=1×10-1 S/cm and *E*a=20 kJ/mol, which were based on the phosphorus-containing N5-type materials discovered in the Na2O-Y2O3-P2O5-SiO2 system. These N5-type materials have been obtained, as well as Na3YSi3O9 (N3)-type materials, with the composition formula

 Na3+3*<sup>x</sup>*-*<sup>y</sup>*Y1-*x*P*y*Si3-*y*O9 (*x*<0.6, *y*<0.5) (1) With the aim of searching for more conductive glass-ceramic N5-type materials, the

 Na3+3*<sup>x</sup>*-*<sup>y</sup>*R1-*x*P*y*Si3-*y*O9 (2) for the synthesis of other kinds of rare earth N5-type glass-ceramics was studied first.

originally derived for N3-type solid solutions and expressed as follows,

verification of the validity of the generalized composition formula

Formula 2 is rewritten with formula 3 according to the formula N5.

**1. Introduction** 

crystallization, as is discussed below.
