The mercury chromates Hg6Cr2O9 and Hg6Cr2O10 -: Preparation and crystal structures, and thermal behaviour of Hg6Cr2O9

The basic mercury(l) chromate(VI), Hg6Cr2O9 (= 2Hg(2)CrO(4) (.) Hg2O), has been obtained under hydrothermal conditions (200 degrees C, 5 days) in the form of orange needles as a by-product from reacting elemental mercury and K2Cr2O7. Hydrothermal treatment of microcrystalline Hg6Cr2O9 in demineralised water at 200 degrees C for 3 days led to crystal growth of red crystals of the basic mercury(I, II) chromate(VI), Hg6Cr2O10 (= 2Hg(2)CrO(4) (.) 2HgO). The crystal structures were solved and refined from single crystal X-ray data sets. Hg6Cr2O9: space group P2(1)2(1)2(1), Z = 4, a = 7.3573(12), b = 8.0336(13), c = 20.281(3)angstrom, 3492 structure factors, 109 parameters, R[F-2 > 2 sigma(F-2)] = 0.0371, wR(F-2 all) = 0.0517; Hg6Cr2O10: space group Pca2(1), Z = 4, a = 11.4745(15), b = 9.4359(12), c = 10.3517(14)angstrom, 3249 structure factors, 114 parameters, R[F-2 > 2 sigma(F-2)] = 0.0398, wR(F-2 all) = 0.0625. Both crystal structures are made up of an intricate mercury-oxygen network, subdivided into single building blocks [O-Hg-Hg-O] for the mercurous compound, and [O-Hg-Hg-O] and [O-Hg-O] for the mixed-valent compound. Hg6Cr2O9 contains three different Hg-2(2+) dumbbells, whereas Hg6Cr2O10 contains two different Hg-2(2+) dumbbells and two H cations. The Hgl-Hgl distances are characteristic and range between 2.5031(15) and 2.5286(9)angstrom. All Hg-2(2+) groups exhibit an unsymmetrical oxygen environment. The oxygen coordination of the H cations is nearly linear with two tightly bonded O atoms at distances around 2.07 angstrom. For both structures, the chromate(VI) anions reside in the vacancies of the Hg-O network and deviate only slightly from the ideal tetrahedral geometry with average Cr-O distances of ca. 1.66 angstrom. Upon heating at temperatures above 385 degrees C, Hg6Cr2O9 decomposes in a four-step mechanism with Cr2O3 as the end-product at temperatures above 620 degrees C. (c) 2006 Elsevier Inc. All rights reserved.