Insights into Structural Diversity and Morphotropic Evolution in A₄Th(WO₄)₄ (A=Li, Na, K, Rb and Cs) Family

Abstract

A series of four new thorium tungstates, A4Th(WO4)4 (A = Li, K, Rb, Cs), were synthesized using high‐temperature solid‐state method. Structural studies of the materials reveal distinct structural changes with underlying similarities influenced by the nature of A‐site cations. Li4Th(WO4)4 exhibits a two‐dimensional corrugated sheet structure stabilized by Li+ cations, crystallizing in P space group. K4Th(WO4)4 crystallizes in I41/a space group and exhibits a three‐dimensional framework consisting of ThO8 antiprisms and WO4 tetrahedra. Rb4Th(WO4)4 forms in C2/c space group, showcasing a framework with two‐dimensional sheet structure. Cs4Th(WO4)4 is isostructural to Rb4Th(WO4)4 but displays increased interlayer spacing due to the larger ionic radius of Cs. Raman spectroscopy confirms the structural data, revealing distinct vibrational modes that correlate with tungsten coordination changes from WO6 in Li4Th(WO4)4 to WO4 in the Cs4Th(WO4)4. This study demonstrates how variation of alkali metals affects the structural properties of thorium tungstates series, showing a morphotropic evolution as a factor of the ionic radii of the alkali metals.