Refinement of the structures of the layer silicates mcusi₄o₁₀ (m = ca, sr, ba) by rietveld analysis of neutron powder diffraction data

The MCuSi₄O₁₀ (M = Ca, Sr, Ba) compounds possess the gillespite (BaFeSi₄O₁₀) structure, which is tetragonal, P4/ncc, Z = 4, Data sets of 2365 step intensities were recorded for the title compounds with λ = 1.400 Å at T 295 K. For M = Ca, a = 7.3017(3) Å, c = 15.1303(6) Å, V_cell = 806.66(9) ų, R_exp = 0.0453, R_wp = 0.0515, for 487 reflections. For M = Sr, a = 7.3707(2) Å, c = 15.5904(6) Å, V_cell = 846.9(1) ų, R_exp = 0.0433, R_wp = 0.0489, for 521 reflections. For M = Ba, a = 7.4409(3) Å, c = 16.1367(8) Å, V_cell = 893.4(1) ų, R_exp = 0.0466, R_wp = 0.0522, for 546 reflections. The gillespite structure contains an unbranched single silicate layer, which can be viewed as a tesselation of corner-linked Si₄O₁₀ rings. The Cu in M CuSi₄O₁₀ is in square-planar coordination with the nonbridging oxygens of the corrugations of the silicate layer. The alkaline-earth atom, in distorted cubic coordination, joins together adjacent [CuSi₄O₁₀]^2- slabs. Increasing the size of the alkaline-earth atom increases the cell dimensions anisotropically (i.e., Δc > Δa) due to the inflexibility of the silicate anion. Room-pressure experiments show no evidence for oxygen intercalation or polytypism. The lattice mismatch between the [CuSi₄O₁₀]^2- slabs of the gillespite structure type and the square-planar CuO₂ layers of the superconducting cuprates is small, which suggests possible applications of these materials as exotic thin-film substrates for the high T_c cuprates or other novel structures having cuprate and silicate layers.