Hydrothermal synthesis, crystal structure refinement and thermal transformation of LiZnAsO4.H2O

The system LiOH-Zn(NO3)(2)-H3AsO4-H2O was investigated using hydrothermal s ynthesis in the temperature range 20-240 degrees C. In addition to alpha-Li ZnAsO4, ZnHAsO4. H2O and Zn-9(AsO4)(6). 4H(2)O a microporous lithium zinc o rthoarsenate LiZnAsO4. H2O can be prepared in neutral to slightly acidic so lution at low temperature. The crystal structure of LiZnAsO4. H2O can be vi ewed as an ABW type framework of AsO4 and ZnO4 tetrahedra forming 8-ring ch annels in the c direction where the extra framework lithium cations and wat er molecules are placed. The Li cations are forming tetrahedra by coordinat ion to three framework oxygens and the water oxygen atom. Rietveld refineme nt was performed on powder diffraction data using synchrotron X-rays, lambd a=1.3808(1)Angstrom, R-p=0.0458, wR(p)=0.063. Crystal data of the orthorhom bic LiZnAsO4. H2O: a=10.8472(9), b=8.2903(5) and c=5.1575(4) Angstrom, V=46 3.80(6) Angstrom(3), Pna2(1) (No.33), Z=4, M=229.255 g mol(-1), D-c=3.328 g cm(-3). The thermal expansion of LiZnAsO4. H2O and dehydration to form alp ha-LiZnAsO4 by dry hearing was investigated using in situ synchrotron radia tion powder diffraction, [lambda=1.0989(1) Angstrom] thermogravimetry and d ifferential scanning calorimetry giving an endothermic enthalpy change of D elta H=36(7) kJ mol(-1), peak value at 218(2)degrees C. (C) 1998 Elsevier S cience B.V. All rights reserved.