Nearly Identical but Not Isotypic: Influence of Lanthanide Contraction on Cs₂NaLn(PS₄)₂ (Ln = La–Nd, Sm, and Gd–Ho)

The effect of lanthanide contraction often results in topological and symmetry changes in compounds with the same compositions as a function of lanthanide cation size. Here we report on the first example of a lanthanide thiophosphate exhibiting a change in the lanthanide cation environment without any topological or symmetry change. A series of new lanthanide thiophosphates with mixed alkali cations were obtained via a flux crystal growth technique using a CsI flux. The obtained compounds Cs₂NaLn­(PS₄)₂ (Ln = La–Nd, Sm, and Gd–Ho) were grown as large single crystals (∼0.1–1 mm³) and characterized using single-crystal X-ray diffraction and magnetic susceptibility measurements. As we moved across the series, the structural studies revealed a change in the lanthanide coordination environment depending on the identity of the lanthanide. Although all compounds in the Cs₂NaLn­(PS₄)₂ series crystallize in the same space group and have the same Wyckoff atom positions, a slight change in size between Sm³⁺ and Gd³⁺ causes a subtle change in coordination number from 9 (for Ln = La–Sm) to 8 (for Ln = Gd–Ho), resulting in two distinct but virtually identical structure types. Ab initio calculations were performed, and the observed experimental trend was corroborated computationally. Magnetic measurements performed on the Cs₂NaLn­(PS₄)₂ (Ln = Ce, Pr, Nd, Gd, and Tb) compounds revealed paramagnetic behavior.