Synthesis of a New Modification of Lithium Chloride Confirming Theoretical Predictions

Abstract

AbstractIn accordance with prior calculations, the new polymorph β‐LiCl (wurtzite structure type) has been synthesised, by the low‐temperature atomic‐beam‐deposition (LT–ABD) technique, in a mixture with α‐LiCl (rock salt structure type) by depositing LiCl vapour (2 to 5.3 × 10–4 mbar) onto a cooled substrate (–30 to –60 °C). The maximum β‐LiCl fraction of 53 % was obtained using a sapphire (0001) substrate at –50 °C and 3.7 × 10–4 mbar LiCl vapour pressure. The proportion of the new polymorph contained in the bulk sample decreases as temperature or vapour pressure deviate from these values, until finally the rock salt type LiCl is found exclusively. When the samples are warmed up to room temperature, β‐LiCl irreversibly transforms to α‐LiCl. The X‐ray diffraction pattern of the two phase LiCl sample measured at –50 °C has been indexed and refined based on a hexagonal unit cell for β‐LiCl with the lattice constants a = 3.852(1) Å and c = 6.118(1) Å and a cubic unit cell for α‐LiCl with the lattice constant a = 5.0630(8) Å. By Rietveld refinement the wurtzite type ofstructure (P63mc, No. 186) was suggested for the new hexagonal modification of LiCl with the Li–Cl distances (2.32 and 2.34 Å) being 8 % smaller than those of α‐LiCl. Moreover, the cell volume decreases as much as 16 % during the transition from β‐LiCl to α‐LiCl. Both the shifts in bond lengths and volume correspond well with the situation encountered for LiBr and LiI. Besides the variation of LiCl vapour pressure and substrate temperature, also different substrate materials were employed for testing their influence on formation of the β‐LiCl polymorph.