The Low and High Temperature Crystal Structures of [Mg(H2O)6]XBr3 Double Salts (X = Rb,Cs

The crystal structures of the alkali double salts [Mg(H₂O)₆]XBr₃ (X = Rb⁺, Cs⁺) were analyzed in dependence on temperature from laboratory and synchrotron X‐ray powder diffraction data. At room temperature, both compounds are isostructural to [Mg(H₂O)₆](NH₄)Br₃ (C2/c; Z = 4; a = 9.64128(6) Å, b = 9.86531(5) Å, c = 13.78613(9) Å, β = 90.0875(5)° for [Mg(H₂O)₆]RbBr₃; a = 9.82304(7) Å, b = 9.98043(6) Å, c = 14.0100(1) Å, β = 90.1430(4)° for [Mg(H₂O)₆]CsBr₃). At a temperature of T = 358 K, [Mg(H₂O)₆]RbBr₃ undergoes a reversible phase transition towards a cubic perovskite type of structure with the [Mg(H₂O)₆]²⁺ octahedron in the cuboctahedral cavity exhibiting 4‐fold disorder ( ; a = 6.94198(1) Å at T = 458 K). In case of [Mg(H₂O)₆]CsBr₃ the lattice parameters in dependence on temperature show a distinct kink at T = 340 K, but no symmetry breaking phase transition occurs before decomposition starts. The dominant role of hydrogen bonding with respect to the stability of the crystal structures is discussed.