Phase Equilibria in the System Chromium(II) Fluoride-Chromium (III) Fluoride

Abstract

An investigation of the CrF/sub 2/-CrF/sub 3/ system established the existence of a single chromium(II,III) fluoride covering the solid solution range CrF/sub 2.40/ to CrF/sub 2.45/. There are two invariant points, the eutectic at 831 plus or minus 5 deg containing 14 mole% CrF/sub 3/ and the peritectic at 997 plus or minus 5 deg containing 29 mole% CrF/sub 3/. CrF/sub 3/ forms a solid solution containing 10 mole% CrF/sub 2/. Melting points of the components CrF/sub 2/ and CrF/sub 3/, both previously reported to be 1100 deg , were determined to be 894 plus or minus 6 deg and 1404 plus or minus 20 deg , respectively. Because classical procedures yielded mixed products, new methods of synthesizing CrF/sub 2/ were sought. Five new procedures are reported: disproportionation of CrF/sub 3/ at 1100 deg , pyrolysis of (NH/sub 4/)/sub 3/CrF/ sub 6/ reduction of CrF/sub 3/ with Cr/sup o/ at 1100 deg , reduction of CrCl/sub 3/ with molten SnF/sub 2/, and oxidation of Cr/sup 0/ with a molten fluoride of a more noble metal, SnF/sub 2/, PbF/sub 2/, CuF/sub 2/, CdF/sub 2/, or BiF/sub 3/. (NH/sub 4/)/sub 2/CrF/sub 6/ for the preparation of CrF/sub 3/ and CrF/sub 2/ wasmore » synthesized by the reaction of molten NH/sub 4/HF/sub 2/ with CrF/sub 3/. 3H/sub 2/O, CrO/sub 3/, or (NH/sub 4/ )/sub 2/Cr/sub 2/O/sub 7/. The compound chromium(II,III) fluoride, previously unreported. was encountered when the disproportionation of CrF/sub 3/ was incomplete, when CrF/sub 3/ was partially reduced by hydrogen, and when CrF/sub 2/ was oxidized by HF or SnF/sub 2/. The compound forms green, translucent crystals that are biaxial (-) with refractive indices N/sub alpha / = 1.518 and N/sub gamma / = 1.534. (auth)« less