Size of the rare-earth ions: a key factor in phase tuning and morphology control of binary and ternary rare-earth fluoride materials

Abstract

In an ionic liquid assisted solvothermal synthesis developed by us for the synthesis of rare-earth (RE) fluorides, it is possible to control the product formation by the choice of the rare earth ion. For rare-earth cations with smaller ionic radii (below 1.075 Å), cubic NaREF₄ with a spherical morphology is obtained, whilst for rare-earth cations with radii between 1.08 Å and 1.13 Å, the formation of hexagonal NaREF₄ with a nanorod-like morphology is observed. For rare earth ions with a larger radius than that of La³⁺ (1.216 Å), instead of ternary fluorides, binary fluorides REF₃ in the trigonal modification is obtained. The growth mechanism behind this morphology change is explained from atomistic origin using electron microscope studies. The lattice strain changes with the rare-earth fluoride phase. For cubic NaREF₄ a tensile strain is observed, whilst for the hexagonal and trigonal binary fluoride a compressive strain is observed. The optical properties of the obtained materials promises use for various optoelectronic applications.