Crystal structure modulations in η-Cu5Sn4

Abstract

Abstract The high temperature modification of η-Cu6Sn5 (reassigned η-Cu5Sn4) was by means of electron diffraction found to exhibit structural modulations. The diffraction patterns may be indexed using the basic NiAs-cell and a modulation vector q ≈ (δ a */24, δ a */24, c */2), 8 ≤ δ ≤ 9. Models for the crystal structures of the two commensurate modulations δ = 8 and δ = 9 are suggested. Both superstructures are caused by the ordered occupancy of Cu in the trigonal bipyramidal sites. The model given for η 6-Cu5Sn4 (corresponding to δ = 8, found above 350° C) has the space group C2 and cell parameters a = 12.60 Å, b = 7.27 Å and c = 10.20 Å and β = 90.0°. η 8-Cu5Sn4, corresponding to δ = 9, has the unit cell (in P21/c) a = c = 9.83 Å, b = 7.27 Å and β = 62.5°.