The crystal structure of a new nonstoichiometric phase Cs1-xLu 3F 10-x (x ⋍ 0.25)

A high-temperature phase with the formula Cs_1-xLu ₃F _10-x (x ⋍ 0.25) has been characterized during the investigation of the CsFLuF ₃ system. This phase crystallizes in the monoclinic system with unit-cell dimensions a = 13.764(5) Å, b = 7.947(1) Å, c = 4.299(2)Å, β = 90.04(5)° and space group Cm (No. 8), Z = 2. The structure was solved by conventional Patterson and Fourier methods and refined by full-matrix least-squares techniques to a conventional R of 0.053 ( R_w = 0.079) for 2038 independent reflections recorded on an automatic four-circle diffractometer. The structure may be regarded as built up of (Lu ₃F ₁₀) ^- layers that may be described as corner- and edge-shared LuF ₇ pentagonal bipyramids. These layers run parallel to the (001) plane. The structure extends along the third direction by cornersharing involving axial vertices of the pentagonal bipyramids. This three-dimensional framework delimits tunnels running parallel to the c direction where the Cs ⁺ ions lie. The partial occupancies of both the Cs site and one out of the seven independent fluorine sites results in the nonstoichiometry.