Na+-fast ionic conducting glass-ceramics of silicophosphates

Abstract

The Na+-fast ionic conducting glass-ceramics with Na5YSi4O12 (N5)-type structure were successfully synthesized using the composition formula of Na3+3x-y R1-x P y Si3-y O9 for a variety of rare earth ions, R, under the appropriate composition parameters. In the crystallization of N5-type glass-ceramics, its relatives (Na3YSi3O9 (N3)- and Na9YSi6O18 (N9)-type glass-ceramics) structurally belonging to the family of Na24−3x Y x Si12O36 were found to crystallize as the precursor phase at low temperatures. In order to produce N5 single phase glass-ceramics, the concentration of both phosphorus and rare earth was found important. The meaning of the composition was evaluated by thermodynamic and kinetic studies on the phase transformation of metastable N3 or N9 phases to stable N5 phase with Na+-fast ionic conductivity. The possible combinations of x and y became more limited for the crystallization of the fast ionic conducting phase as the ionic radius of R increased, while the Na+ conduction properties were more enhanced in the glass-ceramics of larger R. These results are discussed in view of the structure and the conduction mechanism. Also studied were the microstructural effects on the conduction properties, which were dependent upon the heating conditions of crystallization. These effects were understood in relation to the grain boundary conduction properties as well as the transmission electron microstructural morphology of grain boundaries.