Flux crystal growth, structure, magnetic and optical properties of a family of alkali uranium(IV) phosphates

Abstract

A family of alkali uranium(IV) phosphates, AU2(PO4)3 (A = Li – Cs), was synthesized as single crystals by the reaction of US2 and (NH4)2HPO4 in the respective ACl (A = Li – Cs) flux contained in a sealed fused-silica tube at 850 °C, and as phase pure powders from a similar reaction using UF4 as the uranium source. AU2(PO4)3 (A = Li – Rb) crystallize in the NaTh2(PO4)3 (NTP) structure type with monoclinic space group C2/c and consist of a 3D structure that features a framework composed of edge- and corner-sharing polyhedra. The cesium analogue, CsU2(PO4)3, crystallizes in a different structure with space group P21/n that is related to the NaZr2(PO4)3 (NZP) structure type and consists of a framework composed of corner-sharing polyhedra. Two new alkali uranium phosphates, Li2U(PO4)2 and Cs4U4(P2O7)5, were also grown as single crystals at 700 °C. Li2U(PO4)2 was isolated in approximately 30% yield based on uranium. Li2U(PO4)2 crystallizes in space group P21/c exhibiting a layered structure while Cs4U4(P2O7)5, crystallizes in space group P21/n in a new structure type featuring a 3D framework. The magnetic susceptibilities and the field dependent magnetizations were measured for AU2(PO4)3 (A = Li, Na, K and Cs); all compounds exhibited negative Weiss temperatures with no obvious antiferromagnetic transition. Optical properties were measured by UV–vis and IR spectroscopy.