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

A new polymorph of lithium zinc phosphate hydrate, beta-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; beta-LiZnPO4. H2O is apparently the only member of the zeolite ABW group of materials crystallising in space group P2(1)ab. It has a doubled b axis compared to a known phase of LiZnPO4. H2O. The P-31 MAS NMR spectrum showed two signals; delta 6.94(2) and 7.88(2). The known phase of LiZnPO4. H2O transforms into beta-LiZnPO4. H2O after treatment in 0.9 M LiNO3 (aq) at 80 degrees C and 10 kbar for 20 h. A mechanism for the pressure-induced displacive phase transition is proposed. Dry or hydrothermal heating of beta-LiZnPO4. H2O at >100 degrees C gave delta(I)-LiZnPO4. The enthalpy change for the dehydration with a maximum at 219(2) degrees C was determined to be Delta H = 43(6) kJ mol(-1), using differential scanning calorimetry.