Phase transition-crystal structure relations in ferroelectric Bi_(2.5)Na_(1.5)Nb_3O_(12) compound by molecular dynamics simulation

摘要

Crystal structure-phase transition relations in the ferroelectric Bi_(2.5)Na_(1.5)Nb_3O_(12) compound were investigated by using molecular dynamics simulation method. The lattice parameters, a and b, varied with the increasing temperature, and these lattice parameters of the calculated compound indicated the similar value at the temperatures higher than 610 ℃. The excess mixing enthalpy of the calculated compound increased from 0 to 14.2 kJ/mol in the temperature range of 27-410℃, whereas the values of the compound varied from -9.6 to 0 in the temperature range of 585-800℃. Thus, it was recognized that the thermodynamic stability of the compound obtained at the temperatures higher than 585 ℃, because the excess mixing enthalpy of the compound at the temperatures higher than 585 ℃ indicated negative values. From these results, it was considered that the phase transition from orthorhombic crystal structure to tetragonal crystal structure may be related to the variations in thermodynamic stability.