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 translation-engleiche symmetry reduction of index 2. The lost reflections and rotations are retained as twin operations.