The order/disorder phase transition of hypophosphorous acid H3PO2

Abstract

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 ${Z}^{\prime }=\frac{1}{2}$ ), 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 ${Z}^{\prime }=1$ ), 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.