The order/disorder phase transition of hypophosphorous acid H₃PO₂

Abstract

Hypophosphorous acid, H₃PO₂ is dimorphic with a phase transition in the 200-225 K range. The H₃PO₂ molecules are connected by hydrogen bonding to infinite chains extending in the [100] direction. In the high-temperature phase (P2₁2₁2, Z′=1/2), the hydrogen bonds are disordered about a two-fold rotation axis. On cooling below the phase transition temperature, the hydrogen bonds become ordered, resulting in a symmetry reduction of the klassengleiche type of index 2. In the low-temperature phase (P2₁2₁2₁, Z′=1), the c parameter is doubled with respect to the high-temperature phase. The hydrogen-bonding topology of the high- and low-temperature phases are double-infinite directed and undirected linear graphs, respectively.