Hydrothermal synthesis and crystal structure of a new rubidium sodium niobium fluoride, RbNaNbF₇

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Single crystals of new rubidium sodium niobium fluoride, RbNaNbF₇, were prepared by a hydrothermal reaction using HF solution. This fluoride crystallized in the orthorhombic space group Cmcm (#63) with a = 6.7031(6), b = 10.9813(11) and c = 8.2969(8) Å (Z = 4) and the final R factors (all reflections) are R = 0.0364 and wR₂ = 0.0847. The crystal structure is formed by isolated polyhedra NbF₇ with quasi pentagonal bi-pyramid geometry, and Na atom also forms NaF₇ quasi pentagonal bi-pyramid polyhedron. Rb atom is coordinated by 8 fluorine atoms to form RbF₈ tetragonal prism polyhedron. The position of one fluorine atom splits statistically, which coordinates Nb and Na ions. The tantalum analogue, RbNaTaF₇ was the same crystal structure. This new rubidium sodium niobium fluoride exhibited photoluminescence properties by doping Mn⁴⁺ ion, which are similar to those of red phosphors. The emission intensity of RbNaNbF₇:Mn⁴⁺ spectrum was inferior to those of KNaMF₇:Mn⁴⁺ (M; Nb, Ta) on excitation at 455 nm.