Charge Density Wave Phase Transitions and Microstructures in the TaTe4 — NbTe4 System

Abstract

Charge density waves (CDW) and their accompanying periodic lattice distortions are a unique and fascinating feature of low-dimensional systems. Although first predicted by Peierls [1] several decades earlier, it was only in the 1970’s that quasi-one- and quasi-two-dimensional metals were first synthesized and CDW-related phenomena observed. Since that time, CDW have been identified in an extremely diverse collection of materials including organic charge transfer salts such as TTF-TCNQ [2], numerous oxides such as the blue bronzes K0.3MoO3 and Rb0.3MoO3 and the Magneli phase η-Mo4O11 [3], metallic elements like α-U [4] and Cr [5], and martensitic precursor phases occurring in certain intermetallic alloys [6,7]. However, many of the CDW systems that have attracted the most interest to date are found among the transition metal chalcogenides. These compounds possess both a high degree of structural anisotropy and the partially filled energy bands necessary for CDW formation. Over the past two decades, CDW have been reported in various transition metal dichalcogenides (MX2) [8,9], trichalcogenides (MX3) [10–12], tetrachalcogenides (MX4) [13], triniobium tetrachalcogenides Nb3X4 [14–16] and numerous related compounds (here M refers to the transition metal elements V, Ti, Zr, Nb, Ta or Hf while X refers to the chalcogenides S, Se and Te). In many of these systems, temperature or compositional variations lead to transformations between distinct CDW phases accompanied by the formation and interaction of novel structural defects such as discommensurations and CDW domain boundaries. Using transmission electron microscopy (TEM), an extremely diverse range of microstructural phenomena has been observed in the transition metal chalcogenides and a complete survey of the field is beyond the scope of this article.