BANDS, BONDS, AND CHARGE-DENSITY WAVES IN THE NBSE3 FAMILY OF COMPOUNDS

Through detailed examination of the crystal structure and bonding conditions in the trichalcogenides, it has been possible to gain a deeper understanding of the band structure of these complex materials, and with this of their unusual electronic behavior. The character of the observed semimetallicity in TaSe3 and NbSe3 is much clarified. Each material in the group-V family is seen to achieve a very individually tailored band structure, enfolding the various structural and energy-level adjustments. The nonuniformity and the molecularization of these structures contrasts strongly with the dichalcogenide behavior. The structural, if not the electronic, dimensionality of the group-V trichalcogenides remains closer to two than to one. It is found that the periodic structural distortions developed by NbSe3 are probably more suited to a metal-metal bonding description than to the traditional Fermi surface determined instability of a charge-density wave. The field-induced sliding of these distortions can then be described in terms of cooperative bond flipping through a discommensurate superlattice. We examine in crystal chemical terms why this motion is so unusually weakly pinned to defects and impurities in NbSe3. In fact, the investigation shows that it will be very hard to find a material better suited to the realization of this phenomenon. © 1979 The American Physical Society.