Structure refinement, physical properties and electronic structure of new electrochemically copper intercalated group Vb ditellurides Cu_xMTe_2 (M = V, Nb, Ta)

摘要

The layered group Vb ditellurides MTe_2 (M = V, Nb, Ta) were intercalated with Cu (a) electrochemically and (b) via an autoclave route, forming new compounds of the compositions Cu_xVTe_2 (x = 0.85-0.92), Cu_xNbTe_2 (x = 0.78-1.00) and Cu_xTaTe_2 (x = 0.79-0.95). While for Cu_xNbTe_2 a composition of x = 0.69 was reported by Tremel et al. previously, no Cu intercalation compounds of type Cu_xVTe_2 and Cu_xTaTe_2 are known, to date. In the isotypic compounds, Cu is tetrahedrally coordinated by Te, and is placed between corrugated MTe_2-layers. The crystal structures for three selected compositions were refined by Rietveld analysis (Cu_(0.85)VTe_2: P2i/m, a = 6.6713(3) k,b = 3.8689(3) A, c = 7.0693(5) A, beta = 107.116(9) deg; CuNbTe_2: P2_1/m,a = 6.7074(2) k,b = 3.8127(2) A, c = 7.2592(3) A, beta= 107.011(4) deg; Cu_(0.89)TaTe_2: P2_1/m, a = 6.64992(8) k,b = 3.72451(6) A, c = 7.31271(6) A, beta = 107.2191(8) deg). All intercalated phases show metallic conductivity. Cu_(0.87)VTe2 and CuNbTe2 exhibit Pauli paramagnetism. In Cu_(0.82)TaTe_2, the diamagnetic part of the susceptibility dominates over the Pauli contribution, leading to overall diamagnetic behaviour. Calculations on basis of refined structural data were performed using the tight binding linearised muffin tin orbital within the atomic sphere approximation method (TB-LMTO-ASA). Even though shorter Cu-Cu distances (2.48-2.55 A) than in elemental Cu (2.56 A) are present in all three compounds, calculated electron localisation function plots (ELF) indicate solely covalent bonding interactions between M and one type of Te.