Crystal structure of pseudobrookite-type Mg₅Nb₄O₁₅ from 293 to 1117 K

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Single crystals of a magnesium niobium oxide Mg₅Nb₄O₁₅ were prepared at 1673 K. This compound has an orthorhombic pseudobrookite-type structure (space group Cmcm). The single-crystal structure refinement was carried out in the temperature range from 293 to 1173 K. The final R-factors were R = 0.025-0.038 and R_w = 0.031-0.044. The lattice parameters were a = 3.806(3), b = 10.057(3) and c = 10.260(4) Å at 293 K and a = 3.810(6), b = 10.188(5) and c = 10.398(5) Å at 1117 K. The linear thermal expansion coefficients were α_a = 1.28 × 10^-6, α_b = 15.8 × 10^-6 and α_c = 16.3 × 10^-6 K^-1. Thermal expansion anisotropy in the bond distances and angles is consistent with the thermal expansion anisotropy in the lattice parameters.