The system LiOH-Zn(NO 3 ) 2 -H 3 AsO 4 -H 2 O was investigated using hydrothermal synthesis in the temperature range 20-240°C. In addition to a-LiZnAsO 4 , ZnHAsO 4 • H 2 O and Zn 9 (AsO 4 ) 6 • 4H 2 O a microporous lithium zinc orthoarsenate LiZnAsO 4 • H 2 O can be prepared in neutral to slightly acidic solution at low temperature. The crystal structure of LiZnAsO 4 • H 2 O can be viewed as an ABW type framework of AsO 4 and ZnO 4 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, l=1.3808(1) A ˚, R p =0.0458, wR p =0.063. Crystal data of the orthorhombic LiZnAsO 4 • H 2 O: a=10.8472(9), b=8.2903(5) and c=5.1575(4) A ˚, V=463.80(6) A ˚3, Pna2 1 (No. 33), Z=4, M=229.255 g mol-1, D c =3.328 g cm-3. The thermal expansion of LiZnAsO 4
• H 2 O and dehydration to form a-LiZnAsO 4 by dry heating was investigated using in situ synchrotron radiation powder diffraction, [l=1.0989(1) A ˚] thermogravimetry and differential scanning calorimetry giving an endothermic enthalpy change of DH=36(7) kJ mol-1, peak value at 218(2)°C.