Polymorphism in photoluminescent KNdW₂O₈: Synthesis, neutron diffraction, and Raman study

Polymorphs of KNdW₂O₈ (α-KNdW₂O ₈ and β-KNdW₂O₈) phosphors were synthesized by an efficient solution combustion technique for the first time. The crystal structure of the polymorphs analyzed from Rietveld refinement of neutron diffraction data confirms that α-KNdW₂O₈ crystallizes in the tetragonal system (space group I4̄), and β-KNdW₂O₈ crystallizes in the monoclinic system (space group C2/m). The local structure of both polymorphs was elucidated using combined neutron pair distribution function (PDF) and Raman scattering techniques. Photoluminescence measurements of the polymorphs showed broadened emission line width and increased intensity for β-KNdW₂O ₈ in the visible region compared to α-KNdW₂O ₈. This phenomenon is attributed to the increased distortion in the coordination environment of the luminescing Nd³⁺ ion. Combined PDF, Rietveld, and Raman measurements reveal distortions of the WO₆ octahedra and NdO₈ polyhedra in β-KNdW₂O₈. This crystal structure-photoluminescence study suggests that this class of tungstates can be exploited for visible light emitting devices by tuning the crystal symmetry.