Phase transition-crystal structure relations in ferroelectric Bi2.5Na1.5Nb3O12 compound by molecular dynamics simulation

Crystal structure-phase transition relations in the ferroelectric Bi2.5Na1.5Nb3O12 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 deg. C. The excess mixing enthalpy of the calculated compound increased from 0 to 14.2 kJ/mol in the temperature range of 27-410 deg. C, whereas the values of the compound varied from -9.6 to 0 in the temperature range of 585-800 deg. C. Thus, it was recognized that the thermodynamic stability of the compound obtained at the temperatures higher than 585 deg. C, because the excess mixing enthalpy of the compound at the temperatures higher than 585 deg. 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.