26 Whilst classical high temperature synthesis methods typically lead to the thermodynamically stable form, on the nanoscale by a judicious choice of the reaction conditions, it is possible to selectively obtain the less stable form. 23–25 Similarly, sodium ternary rare earth fluorides are known to form two polymorphs: a cubic phase and a hexagonal phase. REF 3 with RE 3+ ions that have a size in between are dimorphic with the orthorhombic form being the room temperature stable phase and the tysonite structure being the high temperature phase. For the lighter REF 3 (RE = La–Nd) with larger ionic radius the trigonal tysonite type of structure (LaF 3, P c1) is the stable form, whilst for the heavier REF 3 with smaller RE 3+ (Dy–Lu, Y, YF 3 type) and the orthorhombic β-YF 3 ( Pnma) is the preferred structure. Their relative stability depends on the rare earth ionic radius and temperature. 14–22 For rare-earth trifluorides, REF 3, two polymorphs are known. 13 Based on these criteria, binary and ternary fluorides appear to be effective host materials for RE 3+ ion doping. ![]() 1 The main criteria in the choice of a suitable host matrix are a low phonon energy, a high refractive index and easy incorporation of dopants together with chemical and thermal stability. 11,12 A judicious choice of the host and dopant ion/ion pairs is a precondition to obtain the desired optical materials. 1–10 With respect to their narrow emission bands, large Stokes shifts, weak autofluorescence and decay times typically in the range of milliseconds, RE-doped materials in certain respects are superior to organic dyes and semiconductors. Introduction Over the last decade rare-earth (RE) ion doped materials have drawn tremendous attention in the field of photonic and biophotonic applications. The optical properties of the obtained materials promises use for various optoelectronic applications. For cubic NaREF 4 a tensile strain is observed, whilst for the hexagonal and trigonal binary fluoride a compressive strain is observed. ![]() The lattice strain changes with the rare-earth fluoride phase. The growth mechanism behind this morphology change is explained from atomistic origin using electron microscope studies. For rare earth ions with a larger radius than that of La 3+ (1.216 Å), instead of ternary fluorides, binary fluorides REF 3 in the trigonal modification is obtained. ![]() For rare-earth cations with smaller ionic radii (below 1.075 Å), cubic NaREF 4 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 NaREF 4 with a nanorod-like morphology is observed. 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.
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