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, Vcell = 806.66(9) Å3, Rexp = 0.0453, Rwp = 0.0515, for 487 reflections. For M = Sr, a = 7.3707(2) Å, c = 15.5904(6) Å, Vcell = 846.9(1) Å3, Rexp = 0.0433, Rwp = 0.0489, for 521 reflections. For M = Ba, a = 7.4409(3) Å, c = 16.1367(8) Å, Vcell = 893.4(1) Å3, Rexp = 0.0466, Rwp = 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 Tc cuprates or other novel structures having cuprate and silicate layers. © 1993 Academic Press, Inc.