The modulated structure of niobium tetratelluride NbTe

The well known tetragonal NbTe4 structure was refined (R = 0.047; 186 independent reflections). In this average structure the Nb atoms in the centers of tetragonal Te8-antiprisms as well as the Te-atoms show strong anisotropic behaviour. This basic structure is modulated along the c-axis with an incommensurate modulation period of about μ = 3.20. The determination of the modulated structure is presented. At first, the determination was based on a commensurate threefold superstructure (μ' = 3.00), yielding four types of atomic shifts which belong to different but not independent periodic modulations of the atomic positions: a longitudinal deformation mode (1) of the Nb position along the c-axis; a transversal breathing mode (2) of the Te squares accompanied by both, a libra-tion mode (3) and a weak longitudinal mode (4) of the squares (R = 0.049; 1227 independent reflections including 904 satellite reflections). The Nb mode results in the formation of Nb3 groups and the breathing mode forms larger Te4 squares around short Nb —Nb distances and smaller ones between adjacent Nb3 groups. In a second step model calculations on chains of Nb atoms with a tenfold superstructure (μ' = 3.33) were performed, which show that most of the Nb positions remain virtually unchanged but substantiate the origin of the incommensurateness: Nb2 pairs, which are introduced into a chain of Nb3 groups as dis-commensurations. The result is discussed and a brief chemical Interpretation of the modulation is given. The formation of Nb3 groups indicates a lower mean oxidation state z(Nb) < + 4 and the modulation can be de-scribed as a redox wave. © 1985, R. Oldenbourg Verlag. All rights reserved.