X-Ray Crystallographic Analysis of the Charge-Density Wave Modulated Phases in the NbTe₄-TaTe₄ System

Abstract

NbTe₄ and TaTe₄ belong to the important structural family CuAl₂ (tl20) characterized by a tetragonal antiprismatic coordination of the minority component. The basic structures were first described by Selte and Kjekshus [1] and Selte and Bjerkelund [2]. In these structures the Te atoms form 1-D infinite columns of stacked tetragonal antiprisms along [001] which are centered by the metal atoms (Figure 1). The arrangement of the antiprismatic columns in the tetragonal a ₁ a ₂-plane yields short Te-Te contacts, forming Te₂ pairs with covalent bonds of about 2.92 Å. Thus, following the usual chemical arguments MTe₄ (M=Nb,Ta) is a ditelluride of M(IV), i.e. M⁴⁺(Te ^2-₂ )₂. The structure is strongly related to that of the mineral patronite V(S₂)₂ [3,4] where, in addition, covalently bonded V₂ pairs are formed along the columns of the quasi- tetragonal antiprisms. The formation of M₂ pairs is very common in the chemistry of M⁴⁺ compounds with a d¹ electron configuration. Seite and Kjekshus [1] also observed weak satellite reflections which indicated a twofold superstructure along [110] and a threefold superstructure along [001]. However, they did not account for these additional reflections in their structure determination. Thus, the reported structure is really an average structure (Table I).