Anamalous-scattering contrast study of the mixed-valence charge-density-wave conductor NbSe₃

The valence-contrast diffraction method is used at the Nb K edge in the analysis of the mixed-valence solid NbSe₃. The NbSe₃ structure consists of three parallel chains of prismatically coordinated Nb atoms and undergoes two low-temperature Peierls-type phase transitions that lead to modulated structures. Previous analysis of synchrotron-radiation single-crystal data has shown that the transition that occurs on cooling at 145 K (leading to the q₁-type modulation) involves displacements of Nb_III atoms as well as of some of the coordinated Se atoms, while the second transition (the q₂-type modulation), at 59 K, involves displacements of Nb_I atoms and an Se atom of column II [van Smaalen, de Boer, Meetsma, Graafsma, Sheu, Darovskikh, Coppens & Levy (1992). Phys. Rev. B, 45, 3103-3106]. The present room-temperature study, performed at seven different wavelengths in the vicinity of the Nb absorption edge, shows the Nb_II-atom edge to be shifted to higher energies by about 5 eV relative to those of the other Nb atoms. This indicates a smaller population of the Nb_II-atom conduction band, in agreement with band structure calculations but not with conclusions derived from nuclear magnetic resonance measurements.