The mercury chromates Hg 6Cr 2O 9 and Hg 6Cr 2O 10—Preparation and crystal structures, and thermal behaviour of Hg 6Cr 2O 9

Abstract

The basic mercury(I) chromate(VI), Hg 6Cr 2O 9 (=2Hg 2CrO 4·Hg 2O), 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 K 2Cr 2O 7. Hydrothermal treatment of microcrystalline Hg 6Cr 2O 9 in demineralised water at 200 °C for 3 days led to crystal growth of red crystals of the basic mercury(I, II) chromate(VI), Hg 6Cr 2O 10 (=2Hg 2CrO 4·2HgO). The crystal structures were solved and refined from single crystal X-ray data sets. Hg 6Cr 2O 9: space group P2 12 12 1, Z = 4 , a = 7.3573 ( 12 ) , b = 8.0336 ( 13 ) , c = 20.281 ( 3 ) Å , 3492 structure factors, 109 parameters, R [ F 2 > 2 σ ( F 2 ) ] = 0.0371 , w R( F 2 all)=0.0517; Hg 6Cr 2O 10: space group Pca2 1, Z = 4 , a = 11.4745 ( 15 ) , b = 9.4359 ( 12 ) , c = 10.3517 ( 14 ) Å , 3249 structure factors, 114 parameters, R [ F 2 > 2 σ ( F 2 ) ] = 0.0398 , w R( 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. Hg 6Cr 2O 9 contains three different Hg 2 2+ dumbbells, whereas Hg 6Cr 2O 10 contains two different Hg 2 2+ dumbbells and two Hg 2+ cations. The Hg I–Hg I distances are characteristic and range between 2.5031(15) and 2.5286(9) Å. All Hg 2 2+ groups exhibit an unsymmetrical oxygen environment. The oxygen coordination of the Hg 2+ 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, Hg 6Cr 2O 9 decomposes in a four-step mechanism with Cr 2O 3 as the end-product at temperatures above 620 °C.