Abstract
Abstract The crystal structures of methylammonium and dimethylammonium dihydrogenphosphite (MA⋅H2PO3, I2/a and DMA⋅H2PO3, P 2 1 / c $P{2}_{1}/c$ ) are built of infinite chains of hydrogen bonded H 2 P O 3 − ${\mathrm{H}}_{\mathrm{2}}\mathrm{P}{\mathrm{O}}_{\mathrm{3}}^{-}$ anions. The chains are connected by the ammonium cations via hydrogen bonding to di- (DMA⋅H2PO3) and triperiodic (MA⋅H2PO3) networks. Tetramethylammonium dihydrogenphosphite monohydrate (TMA⋅H2PO3⋅H2O) features temperature dependent dimorphism. The crystal structure of the high-temperature (HT, cubic P213) and low-temperature (LT, orthorhombic P212121) phases were determined at 150 and 100 K, respectively. The hydrogen bonding network in the HT phase is disordered, with H 2 P O 3 − ${\mathrm{H}}_{\mathrm{2}}\mathrm{P}{\mathrm{O}}_{\mathrm{3}}^{-}$ and H2O being located on a threefold axis and is ordered in the LT phase. On cooling, the point symmetry is reduced by an index of 3. The lost symmetry is retained as twin operations, leading to threefold twinning by pseudo-merohedry. The hydrogen-bonding networks of the HT and LT phases can be represented by undirected and directed quotient graphs, respectively.
Highlights
In our current line of research, we synthesized the 1:1 mono, di, tri- and tetramethylammonium salts of phosphorous acid H3PIIIO3 as precursors to fluorine containing PIII compounds in reactions of the typeIn contrast, the trimethylammonium salt remained an oil even at 255 K
In this work we present the order-disorder phase transition of TMA⋅H2PO3⋅H2O
Note that the standard uncertainty on the P–O1 distance of DMA⋅H2PO3 is distinctly larger owing to positional disorder of the O1 atom
Summary
In our current line of research, we synthesized the 1:1 mono-, di-, tri- and tetramethylammonium salts of phosphorous acid H3PIIIO3 as precursors to fluorine containing PIII compounds in reactions of the type. In contrast to MA⋅H2PO3 and DMA⋅H2PO3, whose structures were trivially solved and refined, crystals of TMA⋅H2PO3⋅H2O were threefold twins of orthorhombic individuals with a pseudo-cubic symmetry. Ordering of hydrogen bonding is one of the most important causes of order-disorder phase transitions in the solid state, the most well studied example probably being the potassium dihydrogen phosphate family of compounds [1]. The members of this family are ferroelectric below and paraelectric above the phase transition temperature. In this work we present the order-disorder phase transition of TMA⋅H2PO3⋅H2O It will be analyzed in the light of symmetry reduction and the topology of the hydrogen bonding network. The hydrogen bonding networks MA⋅H2PO3 and DMA⋅H2PO3 are discussed briefly
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