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

The basic mercury(I) chromate(VI), Hg6Cr2O9 (=2Hg2CrO4·Hg2O), has been obtained under hydrothermal conditions (200 °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 °C for 3 days led to crystal growth of red crystals of the basic mercury(I, II) chromate(VI), Hg6Cr2O10 (=2Hg2CrO4·2HgO). The crystal structures were solved and refined from single crystal X-ray data sets. Hg6Cr2O9: space group P212121, image, image, image, image, 3492 structure factors, 109 parameters, image, wR(F2 all)=0.0517; Hg6Cr2O10: space group Pca21, image, image, image, image, 3249 structure factors, 114 parameters, image, wR(F2 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 Hg22+ dumbbells, whereas Hg6Cr2O10 contains two different Hg22+ dumbbells and two Hg2+ cations. The HgI–HgI distances are characteristic and range between 2.5031(15) and 2.5286(9) Å. All Hg22+ groups exhibit an unsymmetrical oxygen environment. The oxygen coordination of the Hg2+ cations is nearly linear with two tightly bonded O atoms at distances around 2.07 Å. 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 Å. Upon heating at temperatures above 385 °C, Hg6Cr2O9 decomposes in a four-step mechanism with Cr2O3 as the end-product at temperatures above 620 °C.