In order to clarify the role of superionic phase transition temperature in M3D(XO4)(2), we have investigated the relation between a phase transition temperature T-c(D) and a lattice strain a(0). It was found that the square of a(0) is proportional to T-c(D). This result indicates that the lattice strain is significantly important to determine the appearance of superionic conductivity. Furthermore, it was also found that the minimum kinetic energy Delta K in deuterated materials is smaller than that in hydrated materials. Considering that Delta K means the breaking energy of hydrogen bond, it is deduced that the lowering of the phase transition temperature in deuterated materials is caused by the decrease of Delta K which corresponds to the breaking energy of hydrogen bond. (C) 2012 Elsevier By. All rights reserved.