The phase transitions of ferroelastic K2SeBr6: A calorimetric and x‐ray diffraction study

Guinier x‐ray powder diffraction data in the temperature range from T=10 to 290 K, and heat capacity measurements from T=40 to 280 K indicate three phases for K2SeBr6. The high‐temperature cubic structure [K2PtCl6 ‐type, space group Fm3m; a=10.4211(7) A at T=290 K] transforms at 237.8±0.3 K with ΔStr=(1.36±0.02) J K−1 mol−1 to a tetragonal phase with a Rb2TeI6 ‐type structure [space group P4/mnc; a=7.313(1) A, c=10.474(2) A at T=220 K]. A second phase transition at 215±1 K [ΔStr=(0.54±0.01) J K−1 mol−1 ] results in a monoclinic phase with a K2TeBr6 ‐type structure [space group P21/n; a=7.265(3) A, b=7.265(3) A, c=10.438(4) A, β=89.66(5)° at T=80 K]. Both transitions can be interpreted in terms of softening of rotary phonons at the X edge of the Brillouin zone. From the thermal parameters of a single crystal x‐ray structure determination at T=293 K, this low‐temperature instability can be predicted. The results agree with comparable data of ferroelastic K2TeBr6 and refute some earlier speculations concern...