The modulated structure of niobium tetratelluride NbTe4

Abstract

Abstract 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 libration 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 discommensurations. 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 described as a redox wave.