Dicarboxylate mediated efficient morphology/phase tailoring of YPO4:Ln3+ crystals and investigation of down-/up-conversion luminescence

Abstract

The dicarboxylate anions of tartrate (C₄H₄O₆²⁻, Tar²⁻), malate (C₄H₄O₅²⁻, Mal²⁻), and succinate (C₄H₄O₄²⁻, Suc²⁻) were originally used as morphology/structure modifiers in the hydrothermal crystallization of YPO₄, and both hexagonal (h-) and tetragonal (t-) structured crystals with multiform morphologies of high uniformity were selectively synthesized. The materials were characterized by the combined techniques of XRD, FE-SEM, TEM, FT-IR, TG, and optical spectroscopy, and the mechanisms of phase/morphology evolution were expatiated. It was clearly shown that Tar²⁻ may induce an inverse phase transformation of YPO₄ from the thermodynamically stable tetragonal structure (nanoparticles) to the metastable hexagonal structure (microprisms), and a series of controlled experiments exhibited that the performance of Tar²⁻ was strongly affected by the experimental parameters of Tar²⁻/PO₄³⁻/Y³⁺ molar ratio, solution pH, hydrothermal temperature, and reaction time. In comparison with Tar²⁻, Mal²⁻ induced the direct crystallization of h-YPO₄ microprismatic crystals while Suc²⁻ restricted the 1-D growth of t-YPO₄ nanocrystals. For the samples calcined at 700 °C, the down-conversion photoluminescence of Eu³⁺ in h-YPO₄ and t-YPO₄ and the up-conversion luminescence of the (Yb/Er)³⁺ couple in t-YPO₄ were also comparatively studied and elucidated in detail.