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.
Highlights
2.1 Crystal growthSolids with –H ⋯O ii d (O–H)⋯O hydrogen bonds involving acidic OH groups often undergo order/disorder solid-state phase transitions, where the hydrogen bond is disordered at high temperatures while ordered at low temperatures
H3PO2 exists as the tetrahedral HOP(O)H2 tautomer, where two H atoms are connected to P and the third H atom to O
Since the phase transition from HT to LT is of the klassengleiche type, i.e. the point symmetry is retained, there are no additional orientation states in the LT phase
Summary
Solids with O–H⋯O hydrogen bonds involving acidic OH groups often undergo order/disorder solid-state phase transitions, where the hydrogen bond is disordered at high temperatures while ordered at low temperatures. The most well studied example of these transitions is the potassium dihydrogen phosphate (KDP) family of compounds, which are paraelectric above and ferroelectric below the phase transition temperature [1]. These order/disorder phase transitions can lead to complex crystallographic behavior. We have recently described such a phase transition leading to threefold twinning [2]. The ordered phases are incommensurately modulated [3]. In this communication we report on the phase transition of hypophosphorous acid H3PO2, which.
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