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

The system LiOH-Zn(NO3)(2)-H3AsO4-H2O was investigated using hydrothermal synthesis in the temperature range 20-240 degrees C. In addition to alpha-LiZnAsO4, ZnHAsO4. H2O and Zn-9(AsO4)(6). 4H(2)O a microporous lithium zinc orthoarsenate LiZnAsO4. H2O can be prepared in neutral to slightly acidic solution at low temperature. The crystal structure of LiZnAsO4. H2O can be viewed as an ABW type framework of AsO4 and ZnO4 tetrahedra forming 8-ring channels in the c direction where the extra framework lithium cations and water molecules are placed. The Li cations are forming tetrahedra by coordination to three framework oxygens and the water oxygen atom. Rietveld refinement was performed on powder diffraction data using synchrotron X-rays, lambda=1.3808(1)Angstrom, R-p=0.0458, wR(p)=0.063. Crystal data of the orthorhombic LiZnAsO4. H2O: a=10.8472(9), b=8.2903(5) and c=5.1575(4) Angstrom, V=463.80(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 alpha-LiZnAsO4 by dry hearing was investigated using in situ synchrotron radiation powder diffraction, [lambda=1.0989(1) Angstrom] thermogravimetry and differential scanning calorimetry giving an endothermic enthalpy change of Delta H=36(7) kJ mol(-1), peak value at 218(2)degrees C. (C) 1998 Elsevier Science B.V. All rights reserved.