The structural effects of alkaline- and rare-earth element incorporation into thorium molybdates

Abstract

Four novel thorium molybdates containing alkaline-earth or rare-earth metals were isolated from high-temperature solid-state synthesis. The incorporation of divalent and trivalent cations into the thorium molybdate system results in complex structural topologies. A₂MgTh₃(MoO₄)₈ (A = K, Rb) which crystallizes in the space group C2/c is the first instance among the thorium molybdate family that incorporates alkaline-earth metals. Its crystal structure is based on complex channels composed of ThO₈ square antiprisms and MoO₄ tetrahedra arranged in a corner-sharing manner. K₂SrTh₂(MoO₄)₆, as the first thorium polymolybdate compound, is constructed from ThO₈ square antiprisms and Mo₄O₁₆ tetramers. The Mo₄O₁₆ tetramers, lying in the (001) plane, are built from four edge-sharing MoO₆ octahedra. Nd₂Th₃(MoO₄)₉ is the first thorium molybdate containing rare-earth cations. The resemblance of Nd₂Th₃(MoO₄)₉ with hexagonal-ThMo₂O₈ reveals its potential as a host for different trivalent transuranium elements. Raman spectra analysis shows that the different Mo polyhedral geometries (MoO₄ tetrahedra and MoO₆ octahedra) have significant effects on the vibrational bands of these compounds. The thermal behavior and stability of the newly obtained materials have been studied.