Evidence for Deformation Modulated Structures in NbTe 4 and TaTe 4
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EVIDENCE FOR DEFORIATION MODULATED STRUCTURES IN NbTe
4
and TaTe 4
J. MAHY, G.A. WIEGERS,* J. VAN LANDUYT AND S. AIELINCKX University of Antwerp (RUCA), Groenenborgerlaan 171, B-2020 Antwerp, Belgium. *University of Groningen, Nijenborgh 16, 9747 AG Groningen, The Netherlands.
ABSTRACT X-ray and electron diffraction studies have shown that NbTe 4 and TaTe 4 exhibit a deformation modulated structure. The distortion can be described by a modulation vector which also has components Pernendicular to the chain axis (c-axis). The modulation is commensurate along the c-axis of the basic tetragonal structure in the case of TaTe 4 and incommensurate for NbTe 4 . Two main sets of satellite reflections could be distinguished in electron diffraction, which are in a first-second order relationship, as established by X-ray structure refinement. Additional reflections in the NbTe 4 electron diffraction oattern a'pear upon cooling down to liquid nitrogen temnerature, although no changes of the incommensurability with temperature were observed. Physical measurements showed a metallic behaviour for both comnounds with ptype dominance above 60K. No evidence was found for the occurrence of a Peierls-transition to an insulating or semiconducting state, nor did the temperature denendence of the electric and magnetic quantities show an anomalous behaviour. A deformation model is nrooosed in which nart of the transition-metal-atoms undergo displacements along the chain axis in order to form nairs and triolets. This distortion is associated with a relaxation of the chalcogen squares.
INTRODUCTION Over the past few years the metallic,
quasi one-dimensional transition-
metal 'olychalcogenides have become the subject of a number of investigations because of their peculiar physical characteristics. These characteristics were reminiscent of the effects observed in the well-known "one-dimensional" compounds KCP, TTF-TCNQ and related materials [1-31. Investigations by electron microscopy and diffraction on some of the trichalcogenides showed that the anomalies, observed in the temperature dependence of thermo-electric and magnetic quantities could be associated with the occurrence of new neriodicities in the crystal [4,51. These observations were interpreted in terms of electronic instabilities near the Fermi surface (charge density wave (CDW) mechanism). Other interpretations were based on the electronic contribution of the valence band [6] or electrostatic energy minimisation arguments [7].
EXPERIMENTAL
Crystals of NbTe 4 and TaTe 4 were grown starting from the elements as well as from the sintered compound, in evacuated and sealed silica tubes which were heated up to various temperatures between 550K and 900K and subsequently cooled down slowly (2K/h) to room temnerature. In all cases the 3 addition of traces of iodine (5mg/cm ) was found necessary to obtain crystal growth. However, transport reactions turned out to yield a relatively large quantity of metal-rich crystals, notably ditellurides. The ingots contained a number of types o
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