SYNTHESIS AND Na<SUP>+</SUP> CONDUCTION PROPERTIES OF FERRO-PHOSPHOSILICATE GLASS-CERAMICS WITH THE COMPOSITION Na<SUB>5-x</SUB>FeP<SUB>x</SUB>Si<SUB>4-x</SUB>O<SUB>12</SUB>

Our group has developed Na + -superionic conductive Narpsio (typically represented as a general formula of Na3+3x-yR1-xPy Si3-yO9, R = rare earth) glass-ceramics, as analogues of Na5RSi4O12 (N5)-type Na + -superionic conductors, and it was revealed that these showed excellent ionic conductivity up to ~10 -1 S/cm, which is comparable to those of the conventional ceramic Na + -conductor such as -and -aluminas and NASICONs.The R elements may have significant effects on the conduction properties as well as the crystallization of glasses.However, rare earth elements have problems in terms of the amount of resources and cost.Therefore, by replacing the R of the N5-type structure with Fe, the synthesis of inexpensive N5type glass-ceramics of the rare earth-free is expected.In this study, N5-type Narpsio conductors were prepared from glasses with a composition of Na5-xFePxSi4-xO12 (x = 0-0.2) by investigating their thermal crystallization conditions, and their ionic conductivities were evaluated.The conductivity in the grains of the glass-ceramics crystallized for 2 h increased slightly with the addition of P, and the conductivity was decreased by the longer thermal crystallization, probably due to the formation of impurity phases.However, it was found that the obtained rare earth-free N5-type glass-ceramics exhibited excellent conductivity on the order of 10 -3 S cm -1 .