A new polymorph of LiZnPO4·H2O; synthesis, crystal structure and thermal transformation

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Abstract
A new polymorph of lithium zinc phosphate hydrate, β-LiZnPO4·H2O, has been obtained by hydrothermal synthesis, and its crystal structure and thermal transformation investigated. The crystal structure can be viewed as a framework structure built from PO4 and ZnO4 tetrahedra with LiO4 tetrahedra and water molecules placed in eight-ring channels; β-LiZnPO4·H2O is apparently the only member of the zeolite ABW group of materials crystallising in space group P21ab. It has a doubled b axis compared to a known phase of LiZnPO4·H2O. The 31P MAS NMR spectrum showed two signals; δ 6.94(2) and 7.88(2). The known phase of LiZnPO4·H2O transforms into β-LiZnPO4·H2O after treatment in 0.9 M LiNO3 (aq) at 80 °C and 10 kbar for 20 h. A mechanism for the pressure-induced displacive phase transition is proposed. Dry or hydrothermal heating of β-LiZnPO4·H2O at >100 °C gave δI-LiZnPO4. The enthalpy change for the dehydration with a maximum at 219(2) °C was determined to be ΔH = 43(6) kJ mol–1, using differential scanning calorimetry.

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