Polymorphism in LiDy(WO4)2: Phase tunable synthesis, neutron diffraction, and symmetry-driven upconversion red emission

Abstract

Tunable polymorphic structures to achieve novel properties are of great concern for energy-related applications. Herein, we demonstrate temperature-driven irreversible structural phase transition in LiDy(WO4)2 (β-LiDyW and α-LiDyW). A facile sol-gel method has been employed to achieve phase pure crystalline polymorphs at relatively lower temperatures and time. LiDy(WO4)2 crystallizes in a monoclinic wolframite-type structure (space group, P21/n, No = 14) at ambient temperature and a Scheelite-like tetragonal structure (space group, I41/a, No = 88) upon heating at high temperature. Crystal structure analysis shows that Li+ and Dy3+ occupy the distinct site, and W forms a distorted WO6 octahedron in the β-LiDyW phase. In contrast, Li+ and Dy3+ are statistically distributed on a dodecahedral S4 site sharing a similar crystallographic lattice site, and W forms a WO4 tetrahedron in the α-LiDyW phase. The Wolframite to Scheelite (β → α) transformation is due to the increase in the crystal symmetry on the heating function where distorted WO6 transformed to free WO4 going from β → α phase and coordination of W+6 is lowered. Density functional theory calculations at 0 K revealed that the β-LiDyW is energetically more favorable than α-LiDyW by 337.3 meV per formula unit. The co-substitution of the Yb3+−Er3+ pair in LiDy(WO4)2 lattice displays a concentration-dependent upconversion red emission when excited with 980 nm. The monoclinic phase of LiDy0.7Yb0.2Er0.1(WO4)2 shows an intense red emission at 656 nm due to inherent lower crystal symmetry. Distorted WO6 and chemically induced lattice distortions in β-LiDy0.7Yb0.2Er0.1(WO4)2 would have strongly influenced the coordination environment of the Er3+ to achieve red emission. This study presents the structural relationship among the tunable crystallographic phases of LiDy(WO4)2 with the observed red emission induced by Yb3+−Er3 upon 980 nm irradiation.