Phase Transition in Cs2KMnF6: Crystal Structures of Low- and High-Temperature Modifications

Abstract

Crystalline Cs2KMnF6, when prepared below 500°C, adopts a tetragonal elpasolite structure type. Differential scanning calorimetric investigations indicated that Cs2KMnF6 undergoes a phase transition from the low-temperature tetragonal phase (LT) to a high-temperature phase (HT) at about 530°C. Single crystals of the new HT phase could be obtained by annealing a crystalline LT specimen at 600°C followed by rapid quenching to room temperature. In the present study the structures of both phases have been studied by single-crystal X-ray diffraction techniques. The LT phase has the tetragonal space group symmetry I4/mmm, with unit-cell parameters a=6.319(1) (a·2=8.936) and c=9.257(2) Å, and Z=2. The HT phase has the cubic symmetry Fm3m, with the cell parameter a=9.067 Å and Z=4. Structural models of the LT and HT phases have been refined vs collected single-crystal X-ray reflection data to R values of 0.034 and 0.022, respectively. The uneven Mn–F bond distance distribution in the LT form, four bonds of 1.860(6) two of 2.034(9) Å, are typical for an octahedrally coordinated high-spin Mn3+ ion affected by Jahn–Teller effects. Due to symmetry constraints, all six octahedral Mn–F bonds in the HT form are equal to 1.931(5) Å. However, the mean square atomic displacement parameters of the fluorine atoms increases significantly from about 0.022 Å2 for the LT phase to 0.042 Å2 for the HT phase. The increased displacement parameters indicate that the phase transition from the LT to the HT form is associated with a directional disorder of the Jahn–Teller distortions around the Mn3+ ions.