Microscopic calculations of displacive (Elpasolite family) and order-disorder (Potassium selenate family) structural phase transitions

The critical temperatures and thermodynamic characteristics for the displacive phase transition O ⁷_h → O ⁵_4h in crystals of the elpasolite family Rb₂ K BF₆ (B = Sc, In, Lu) and for the successive phase transitions O ⁴_6h → D ¹⁶_2h → I → C ^2h/5 (I is the incommensurate phase) in crystals of the selenate potassium family have been calculated. The calculations were carried out using the effective Hamiltonian method. The Hamiltonian parameters for the elpasolite-like crystals were determined from ab initio calculations, and those for the crystals of the selenate potassium family were found using a small number of fitting parameters.