The phase transition of rubidium hydrogen carbonate, RbHCO(3)

Rubidium hydrogen carbonate, RbHCO(3), features an order/disorder phase transition at T (C) = 245 K from the high-temperature (HT) disordered C2/m modification to the low-temperature (LT) C-1 modification. The crystal structures are characterized by [HCO(3)](2) (2−) pairs of hydrogen carbonate groups connected by strong hydrogen bonding. The [HCO(3)](2) (2−) pairs are connected by Rb(+) cations into a three-dimensional network. In HT-RbHCO(3), the hydrogen atom is disordered. In LT-RbHCO(3), ordering of the hydrogen atom leads to a translation­engleiche symmetry reduction of index 2. The lost reflections and rotations are retained as twin operations.