Is there a wurtzite-modification of lithium bromide?: Studies on the system LiBr/LiI

Deposition of mixtures of LiBr/LiI (ratio: LiBr/LiI = 3: 1, 2:1, 1:1 1:2 1:3, 1:4) and of pure Lil and LiBr from the gas phase onto a sapphire substrate at - 196 degreesC in a high vacuum chamber were investigated by means of temperature-dependent X-ray diffraction. Below 0 degreesC Lil crystallizes in the hexagonal Wurtzite-modification (beta-Lil) with a = 451.4(l) und c = 731.1(2) pm, which transforms into the cubic rock salt modification (alpha-Lil, a = 602.57(3) pm) by heating up to room temperature. Co-depositions of LiBr/Lil formed solid LiBri-I-x(x) solutions that also crystallize in the Wurtzite-modification, below room temperature. Compared to beta-Lil.. these solid solutions are more stable and transform into the cubic phase at the significantly higher temperature of 80degreesC, Thelattice constants of LiBrI-I-x(x) with x approximate to 0.7 are a = 445.48(7), c = 719. 1 (1) pm and with x approximate to 0.4 are a 431.50(5), c = 691.7(1) pm, The hexagonal phase LiBr1-xIx is observed for the complete series of mixed crystals with 0.25 less than or equal to x less than or equal to 0.8. Both cubic phases, alpha-Lil and LiBr, show solubilities of up to ca. 10 % of the respective other compound. In case of pure LiBr only the cubic modification (a = 551.54(2) pm, 25 degreesC) was observed in the complete temperature range (- 196 degreesC to 25 degreesC).