Order-disorder phase transitions in KNO₂, CsNO₂, and TlNO₂ crystals: A molecular dynamics study

The order-disorder phase transitions of KNO₂, CsNO₂, and TlNO₂ have been studied using parameter-free molecular dynamics simulation. It is found that the phase transitions in nitrites investigated are driven by the rotations of the NO₂^- ions about different axes together with displacements of cations and anions. We successfully reproduce the high-temperature phases of these nitrites, i.e., the NaCl-like structure for KNO₂ and CsCl-like structure for Cs(Tl)NO₂. Based on the investigation of the radial distribution function of the cations and anions, we explain why KNO₂ and Cs(Tl)NO₂ form quite different low-temperature phases.