On the Thermal Dimorphy of the Strontium Perrhenate Sr[ReO₄]₂

Hygroscopic single crystals of a new hexagonal high‐temperature modification of Sr[ReO 4 ] 2 were prepared from a melt of Sr[ReO 4 ] 2 ⋅ H 2 O and SrCl 2 ⋅ 6 H 2 O. The structure analysis of the obtained crystals by X‐ray diffraction revealed that the title compound crystallizes in the ThCd[MoO 4 ] 3 ‐type structure with the hexagonal space group P 6 3 / m and the lattice parameters a =1023.81(7) pm and c =646.92(4) pm ( c / a =0.632) for Z =2 in its quenchable high‐temperature form. Two crystallographically independent Sr 2+ cations are coordinated by oxygen atoms forming either octahedra or tricapped trigonal prisms, whereas the Re 7+ cations are found in the centers of discrete tetrahedral meta ‐perrhenate units [ReO 4 ] − . Temperature‐dependent in‐situ PXRD studies of dry powder samples of Sr[ReO 4 ] 2 exhibited its thermal dimorphy with a phase‐transition temperature at 500–550 °C from literature‐known m‐Sr[ReO 4 ] 2 into the newly discovered h‐Sr[ReO 4 ] 2 (hexagonal).