Cr VI →Cr V Transition in Uranyl Chromium Compounds: Synthesis and High‐temperature X‐ray Diffraction Study of Cs 2 [(UO 2 ) 2 (CrO 4 ) 3 ]

Single crystals of Cs 2 [(UO 2 ) 2 (CrO 4 ) 3 ] ( 1 ) were obtained by hydrothermal synthesis. The structure of 1 is based upon 2‐dimensional uranyl chromate layers with the composition [(UO 2 ) 2 (CrO 4 ) 3 ] 2– . The cesium atoms are located in the interlayer under “bulbs” centered at the Cr(2)O 4 tetrahedra. The thermal behavior of 1 was studied in air. The diffraction maxima of 1 gradually disappear at approximately 450 °C. At this temperature, peaks of Cs 2 [(UO 2 ) 2 (Cr 2 O 8 )] ( 2 ) appear indicating reduction of Cr 6+ to Cr 5+ . Thermal expansion of 1 and 2 is anisotropic. Thermal expansion of the crystal structure of 1 in the [001] direction is greater than that in the other directions, while thermal expansion in the ab plane is uniform. Thermal expansion of 2 is strongly anisotropic as well with the higher expansion along the direction approximately perpendicular to the layers (along the a axis). The high‐temperature study demonstrates that stable uranyl compounds with Cr 5+ may form not only from hydrothermal aqueous solutions, but also as a result of high‐temperature solid‐state reactions.