Halides of the alkaline metals

The radii ratios of hard sphere ions rK/rA can be calculated for the three important structure types of AB compounds (CsCl, NaCl, ZnS). Here the stability range for an octahedral coordination, i.e. of the rock salt type, is 0.414 ≤ r_K/r_A ≤ 0.732. LiI and LiBr with r_K/r_A = 0.35 and 0.39, respectively, are beyond this range, but both crystallize in the rock salt type. LiCl exhibits a ratio of r_K/r_A = 0.42 which is directly located on the border line. Therefore we are locking for applicable temperature and pressure conditions to obtain thermodynamic stable modifications of LiI, LiBr, and LiCl with lower coordination spheres. The existence of a metastable hexagonal modification of LiI (wurtzite type), lithium and iodine are tetrahedral coordinated, was confirmed by low-temperature depositions combined with Rietveld refinements.

Synthesis of β-LiBr (wurtzite type)

By applying the low-temperature-deposition method a new metastable modification of LiBr has been synthesized. The structure refinement via the Rietveld method supported the wurtzite structure type (P6₃mc, No. 186). The Li–Br distances for β-LiBr (2.52 and 2.53 Å) are 8% less than those for α-LiBr. The samples of LiBr transform during heating up to room temperature into the stable rock salt polymorph.

X-ray powder pattern of LiBr (deposition at: -50°C), measured at: -50°C (wurtzite- type), -30°C, -10°, 10°, and 25°C (rock salt polymorph); from bottom to top; tick marks β-LiBr.

Reference: Y. Liebold Ribeiro, D. Fischer, M. Jansen: Angew. Chem.120 (2008) 4500.

Synthesis of β-LiCl (wurtzite type)

The new metastable polymorph β­LiCl (wurtzite structure type) was synthesised by depositing LiCl vapour onto a cooled substrate (–30 to –60 °C) in a mixture with α­LiCl (rock salt structure type). When the samples are warmed up to room temperature, β­LiCl irreversibly transforms to α­LiCl. By Rietveld refinement the wurtzite type of structure (P6₃mc, No. 186) was suggested for the new hexagonal modification of LiCl with the Li–Cl distances of 2.32 and 2.34 Å. Moreover, the cell volume decreases as much as 16 % during the transition.

X-ray powder pattern of LiCl (deposition at: –50°C), measured at: –50°C (wurtzite- type, rock salt type), 0°C and 25°C (rock salt type); tick marks: α-LiBr; crystal structures of β–LiCl and α-LiCl.

Reference: A. Bach, D. Fischer, M. Jansen: Z. Anorg. Allg. Chem. 635 (2009) 2406-9.