The phase transition of rubidium hydrogen carbonate, RbHCO3.

Rubidium hydrogen carbonate, RbHCO3, features an order/disorder phase transition at TC = 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 [HCO3]22- pairs of hydrogen carbonate groups connected by strong hydrogen bonding. The [HCO3]22- pairs are connected by Rb+ cations into a three-dimensional network. In HT-RbHCO3, the hydrogen atom is disordered. In LT-RbHCO3, 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.