First-principles study of phase transitions in KNO3.

We report a first-principles simulation study of phase transitions in KNO3, using our recently developed method for treating ionic molecular solids. With the interionic potentials calculated from ab initio electron charge densities of the ions, our structural static relaxation gave close fits to both the normal room-temperature (α-phase) and the ferroelectric (γ-phase) structures. Our supercell molecular-dynamics calculations closely simulated the transitions from the α phase and γ phase to the high-temperature disordered β phase, and successfully reproduced the abnormally large c-axis thermal expansion observed in experiment. Both transitions were found to be initiated by the rotations of the nitrate ions about their triad axes parallel to the c axis. Our lattice-dynamics calculations in the spirit of the quasiharmonic approximation revealed that these rotations are intimately connected to the large c-axis thermal expansion, and both of them derive from the strong anharmonicity in the system. ©1991 The American Physical Society. Used by permission. URL: http://link.aps.org/doi/10.1103/PhysRevB.44.7215 DOI: 10.1103/PhysRevB.44.7215 PHYSICAL REVIEW B VOLUME 44, NUMBER 14 1 OCTOBER 1991-11 First-principles study of phase transitions in KNO, H. M. Lu and J. R. Hardy Department of Physics and Center for Electro-Optics, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0111 (Received 21 December 1990; revised manuscript received 20 May 1991) We report a first-principles simulation study of phase transitions in KNO,, using our recently developed method for treating ionic molecular solids. With the interionic potentials calculated from ab initio electron charge densities of the ions, our structural static relaxation gave close fits to both the normal room-temperature (a-phase) and the ferroelectric (y-phase) structures. Our supercell moleculardynamics calculations closely simulated the transitions from the a phase and y phase to the hightemperature disordered /3 phase, and successfully reproduced the abnormally large c-axis thermal expansion observed in experiment. Both transitions were found to be initiated by the rotations of the nitrate ions about their triad axes parallel to the c axis. Our lattice-dynamics calculations in the spirit of the quasiharmonic approximation revealed that these rotations are intimately connected to the large c-axis thermal expansion, and both of them derive from the strong anharmonicity in the system.