CRYSTAL STRUCTURE AND HIGH-TEMPERATURE BEHAVIOR OF SYNTHETIC MOURITE, (UO2)Mo5O14(OH)4(H2O)2

Abstract

A synthetic analog of mourite (SM), (UO2)Mo5O14(OH)4(H2O)2, has been hydrothermally synthesized at 220 ºС and characterized using single-crystal X-ray diffraction, single-crystal and powder X-ray diffraction studies at non-ambient temperatures, X-ray photoelectron spectroscopy, infrared spectroscopy, thermal, and chemical analyses. SM is monoclinic, P2/c, a = 9.9063(6), b = 7.1756(4), c = 12.2105(7) Å, β = 102.496(6)°, V = 847.41(9) Å3; the crystal structure has been refined to R1 = 0.043. The chemical composition of the SM is (the Mo2O5:MoO3 ratio obtained from X-ray photoelectron spectroscopy, H2O by stoichiometry; wt.%): Mo2O5 = 4.61, MoO3 = 61.06, UO3 = 26.95, H2O = 6.76, total 99.38. The empirical formula calculated on the basis of 22 oxygen atoms per formula unit with MoV + MoVI = 5 is (UVI1.03O2)[(MoVI4.63MoV0.37)Σ5.00O13.81(OH)0.19] (OH)4(H2O)2. The crystal structure of SM contains UO8, Mo1O6, Mo2O5(H2O), and Mo3O4(OH)2 polyhedra that share vertices and edges to form layers linked by hydrogen bonds only. SM is stable up to 250 ± 10 ºС. Upon heating, continuous dehydration occurs between 160–250 ºС until the formation of amorphous products; crystallization above 450 ºС produces UO2MoO4, MoO3, and UMo10O32. Below 250 ºС, thermal expansion of the compound is strongly anisotropic, with the maximal direction perpendicular to the plane of the layers.