Phase transition-crystal structure relations in ferroelectric Bi_2.5Na_1.5Nb₃O₁₂ compound by molecular dynamics simulation

Crystal structure-phase transition relations in the ferroelectric Bi_2.5Na_1.5Nb₃O₁₂ 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 °C. The excess mixing enthalpy of the calculated compound increased from 0 to 14.2 kJ/mol in the temperature range of 27-410 °C, whereas the values of the compound varied from -9.6 to 0 in the temperature range of 585-800 °C. Thus, it was recognized that the thermodynamic stability of the compound obtained at the temperatures higher than 585 °C, because the excess mixing enthalpy of the compound at the temperatures higher than 585 °C 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.