Structural Effects on the Electronic Properties of TlCu 7– δ S 4 Investigated by Diffraction‐Resistance Measurements

The design and construction of a specialised sample cell for the simultaneous investigation of electron‐transport and structural properties by powder neutron diffraction in the temperature range1.5 ≤ T /K ≤ 300 is described. This cell has been used to investigate the structural origin of anomalies in the transport properties of the pseudo‐one‐dimensional ternary sulfide TlCu 7– δ S 4 ( δ = 0.15). The data reveal an anomaly in ρ ( T ) observed at 255 K, which is associated with a phase change involving a change in symmetry from I 4/ m to P 4/ n on cooling. This corresponds to a doubling of the crystallographic c axis and is evidenced by the splitting of a strong reflection at d = 2.28 Å and the appearance of additional peaks in the low‐temperature diffraction data. A lower temperature ρ ( T ) anomaly at 205 K is related to subtle structural distortions involving chains of edge‐shared Cu 4 tetrahedra.