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

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 angstrom), cubic NaREF4 with a spherical morphology is obtained, whilst for rare-earth cations with radii between 1.08 angstrom and 1.13 angstrom, 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 angstrom), 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.