Hydrothermal synthesis and crystal structure ofα-LiZnAsO4

Abstract

Single crystals of lithium zinc arsenate,α-LiZnAsO₄ , suitable for structure determination were prepared from an aqueous solution of H₃AsO₄ , Zn(NO₃)₂ and LiOH under hydrothermal conditions, at 240 °C. α-LiZnAsO₄ has a fully ordered phenacite structure built from a three-dimensional framework of cornersharing AsO₄ , ZnO₄ and LiO₄ tetrahedra. The tetrahedra form nets of six-rings surrounded by six four-rings in the direction of the c-axis. α-LiZnAsO₄ crystallises in the trigonal space group R3 (no. 146), a=14.009(2), c=9.386(2) Å, Z=18 and D_c=3.958 g cm^–3 . The structure was refined to: wR(F²)=0.130, R(F)=0.050 andS=0.982, for F²>2sigma;(F²). Synchrotron radiation, λ=0.9540 Å, was used for single crystal diffraction giving 917 unique reflections. Solid state ⁷Li MAS NMR showed a broadened Li signal implying more than one inequivalent Li (δ 0.21, reference: 1.00 mol dm^–3 LiCl). Linear thermal expansion coefficients in the temperature range 25–800 °C were determined using time resolved in situ synchrotron X-ray powder diffraction. A weak reversible, possibly second order, thermal event was observed at 682(1) °C using differential scanning calorimetry.