Crystal structure of Rb₂Fe₅F₁₇

Abstract

Fine, whisker-like crystals of Rb₂Fe₅F₁₇ have been obtained from solid state stoichiometric mixtures of RbF and FeF₃. A structural model has been determined from single crystal diffractometer data in the Fmmm orthorhombic space group : $\text{a}\text{= 14.887(2)}\text{A}\text{̊}$, $\text{b}\text{= 25.814(3)}\text{A}\text{̊}$, $\text{c}\text{= 7.560(1)}\text{A}\text{̊}$; z = 8; ϱ_exp. = 3.529 g cm⁻³; ϱ_calc.=3.535 g cm⁻³; $\text{λ}{}_{\mathrm{<mtext>Co\; Ka</mtext>}}\text{=1.7903}\text{A}\text{̊}$; R = 0.068; R_ω = 0.096 for 175 reflections with $\text{I}\text{σ(}\text{I}\text{)}\text{> 6}$. The structure has been compared with that of HTB. Rb₂Fe₅F₁₇ exhibits layers of (Fe₅F₁₇)²ⁿ⁻_n composition containing FeF₆ corned-shared octahedra forming tunnels of hexagonal and triangular sections. Rubidium atoms are located between the layers and within the hexagonal tunnels with statistical occupancy. The structure has been roughly confirmed by TEM. The observed lowering in symmetry from Fmmm may be explained by partial ordering of rubidium and by possible bridging of (Fe₅F₁₇) sheets by FeF₆ octahedra.