The Low and High Temperature Crystal Structures of [Mg(H 2 O) 6 ] X Br 3 Double Salts ( X = Rb, Cs

The crystal structures of the alkali double salts [Mg(H 2 O) 6 ] X Br 3 ( 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(H 2 O) 6 ](NH 4 )Br 3 ( C 2/ c ; Z = 4; a = 9.64128(6) Å, b = 9.86531(5) Å, c = 13.78613(9) Å, β = 90.0875(5)° for [Mg(H 2 O) 6 ]RbBr 3 ; a = 9.82304(7) Å, b = 9.98043(6) Å, c = 14.0100(1) Å, β = 90.1430(4)° for [Mg(H 2 O) 6 ]CsBr 3 ). At a temperature of T = 358 K, [Mg(H 2 O) 6 ]RbBr 3 undergoes a reversible phase transition towards a cubic perovskite type of structure with the [Mg(H 2 O) 6 ] 2+ octahedron in the cuboctahedral cavity exhibiting 4‐fold disorder ( $Pm{\bar 3}m$ ; a = 6.94198(1) Å at T = 458 K). In case of [Mg(H 2 O) 6 ]CsBr 3 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.