Spectroscopic studies of charge density wave bronzes and fullerene polymers

This dissertation presents several spectroscopic studies of two different types of novel low-dimensional solids: charge density wave (CDW) bronzes and fullerene polymers.; The optical properties of quasi-two-dimensional (Q2D) CDW bronzes are interesting and relatively unexplored with respect to those of the one-dimensional CDW materials. The structural basis of (PO₂)₄(WO ₃)_2m (m = 2–14) and Mo₄O₁₁ is similar, with several corner-shared MO ₆ (M = W, Mo) octahedral layers connected by one AO₄ (A = P, Mo) tetrahedral layer, resulting in Q2D structures and CDW instabilities. The optical properties of (PO₂)₄(WO ₃)_2m (m = 2, 4, 6, 7), η- and γ-Mo₄O₁₁ are studied by both transmittance and polarized reflectance measurements as a function of temperature or magnetic field over a wide frequency range. All these Q2D CDW materials display anisotropic optical properties. In particular, the interlayer infrared spectra of (PO₂)₄(WO₃)_{2 m} (m = 4, 6, 7) show strong vibration modes with weak electronic background, characteristic of a non-metallic response. In contrast, the ab-plane optical conductivity is dominated by free carrier response with screened phonons. The reflectance spectra of both (PO₂)₄(WO₃)_{2 m} and Mo₄O₁₁ display anisotropic intra- t_2g W d → d electronic excitations (∼12000 cm−1) and isotropic O p → W d (∼32000 cm −1), consistent with the band structures of these materials. The polarized optical reflectance of the CDW superconductor (PO₂) ₄(WO₃)_2m ( m = 7) displays a clear suppression of the optical conductivity along the b-axis below 140 K, indicating partial CDW gap formation. Similarities and differences between the optical response of the m = 7 compound and other CDW superconductors are discussed.; High-resolution far-infrared transmittance spectra of the two-dimensional tetragonal and one-dimensional orthorhombic high-temperature/high-pressure C₆₀ polymers are presented, to probe the effects of the symmetry reduction, C₆₀ ball distortion, and the defects on these materials. The vibrational features and their mode parentage are assigned according to quantum molecular dynamics calculations. The results show that many of the polymer vibrational modes are derived from more than one I_h C₆₀ parent symmetry, confirming that a weak perturbation model is inadequate for these covalently bonded C₆₀ balls. In addition, detailed comparison between the far-infrared vibrational features of the HTHP and RbC₆₀ orthorhombic polymers shows that the striking spectral differences are due to charge effects on the C₆₀ ball distortion rather than electron-phonon coupling in RbC₆₀.