Two-Dimensional Uranyl Borates: From Conventional to Extreme Synthetic Conditions

A systematic investigation of uranyl borates under different synthetic conditions resulted in five new 2D compounds, namely, (H3O)[(UO2)(BO3)], Li[(UO2)(BO3)]center dot(H2O), alpha-K-4[(UO2)(5)(BO3)(2)O-4], beta-K-4[(UO2)(5)(BO3)(2)O-4] and K-2.5[(UO2)(5)(BO3)(2)O-2.5(OH)(1.5)]center dot(H2O)(2.5). (H3O)[(UO2)(BO3)] and Li[(UO2)(BO3)]center dot(H2O) were obtained from hydrothermal reactions at 220 degrees C using the same mineralizer. Both materials possess the uranophane sheet topology with different symmetry of the unit cells. In the structure of (H3O)[(UO2)(BO3)] and Li[(UO2)(BO3)]center dot(H2O), UO7 pentagonal bipyramids share edges and vertexes with four BO3 planar triangles. alpha-K-4[(UO2)(5)(BO3)(2)O-4] and beta-K-4[(UO2)(5)(BO3)(2)O-4] are polytypes. alpha-K-4[(UO2)(5)(BO3)(2)O-4] was synthesized from a high-temperature solid-state reaction under ambient pressure; whereas beta-K-4[(UO2)(5)(BO3)(2)O-4] was obtained from a high-temperature/high-pressure (HT/HP) reaction. Both structures have an identical anion topology, but beta-K-4[(UO2)(5)(BO3)(2)O-4] crystallizes in space group with higher symmetry. In K-2.5[(UO2)(5)(BO3)(2)O-2.5(OH)(1.5)]center dot(H2O)(2.5), which was obtained from a hydrothermal reaction, UO7 polyhedra share vertexes and edges with two independent BO3 triangles, forming the most complex uranyl borate layers among all five compounds. The different synthetic routes, novel topologies, thermal behavior and Raman spectra are discussed.