Extreme condition high temperature and high pressure studies of the K–U–Mo–O system

Abstract

Herein the first examples of alkali earth uranyl molybdates synthesised using extreme conditions of high temperature and high pressure (HT/HP) methods, namely K₂[UO₂(Mo₂O₇)₂], K₂[(UO₂)₂(Mo(VI)₄Mo(IV)(OH)₂)O₁₆], K₃[(UO₂)₆(OH)₂(MoO₄)₆(MoO₃OH)] and K₅[(UO₂)₁₀MoO₅O₁₁OH]·H₂O, are described and characterised. K₂[UO₂(Mo₂O₇)₂] forms a monoclinic 2D layered structure in space group P2₁/c that consists of interlinking Mo₂O₇ dimers that link isolated UO₂²⁺ moieties forming [UO₂(Mo₂O₇)₂]²⁻ layers which are separated by K⁺ cations. K₂[(UO₂)₂(Mo(VI)₄Mo(IV)(OH)₂)O₁₆] forms a disordered triclinic 3D framework structure in space group P. The structure consists of isolated UO₂²⁺ moieties connected in a layered configuration via Mo(VI)O₆ polyhedra of which the layers are bridged by Mo(IV)O₆ polyhedra that are partially positionally disordered by charge balancing K⁺ and bridging Mo⁴⁺ cations. K₃[(UO₂)₆(OH)₂(MoO₄)₆(MoO₃OH)] adopts a disordered orthorhombic 3D framework structure in space group Pbcm consisting of small channels and large cavities built upon corner sharing MoO₄ and UO₂²⁺ moieties that respectively encapsulate ordered and disordered K⁺ cations. K₅[(UO₂)₁₀MoO₅O₁₁OH]·H₂O forms a triclinic 3D framework structure in space group P consisting of interlinking UO₆, UO₇ and MoO₅ polyhedra which utilise cation–cation interactions between UO₂²⁺ moieties to create infinite channels parallel to the [001] direction which contain partially disordered K⁺ cations and H₂O molecules. A combination of single crystal X-ray diffraction, bond valence sums calculations and scanning electron microscopy with energy dispersive X-ray spectroscopic measurements was used to characterise all obtained samples in this investigation. The structures uncovered in this investigation are discussed systematically in detail with other members of the broader A⁺–U–Mo–O system from the literature where the relationship between the degree of pressure applied and U/Mo ratio used during synthesis on the ability to obtain high dimensional structures via condensation and oligomerization of polyhedra is identified and discussed in detail.