The Local Electronic and Crystal Structure of Transition Metal and Group III A-Vii a Oxides Probed by X-Ray Absorption Spectroscopy.

The d-orbitals of transition metals and the p -orbitals of the Group III A-VII A elements dominate the local electronic and structural properties in compounds of these materials. These important orbitals being, partially -filled and close to the Fermi level (E_ {F}) in energy, can be probed by X-ray absorption spectroscopy (XAS) which involves electronic transitions from atomic core-levels to empty states above the Fermi level. Specifically, the orbital electron/hole count, and the distribution in energy of these states can be addressed with XAS. In this work we develop XAS methods to probe local electronic and crystal structure and apply them to materials of current fundamental interest. In one part of this work we present XAS studies (L_1- and L_{2,3 }-edges) of a wide range of oxides involving all of the 4d-row transition metals (T). In view of the d-character of the final states at the L_ {2,3}-edges, XAS can provide a direct probe of the number and energy distribution of the T-d -states above E_{F}. We correlate the d-orbital occupancies with area of the intense 2p to 4d related "White Line" features at the L_{2,3}-edges of the transition metals in these compounds. Here the L _1-edge spectra are used to estimate the background continuum onset underlying the L_ {2,3}-edges and thereby to extract the d-orbital features. In addition we study the energy distribution of the d-states in a range of Mo-based oxides via L_{2,3}-edge XAS. The distinctive crystalline electric field splittings in the octahedral and tetrahedral ligand fields are observed in the L _{2,3}-edge WL-features. In an another part of this work, using K-edges of Cu and Ni in rm La_{2-x}Sr _{x}TO_4 (T = Cu, Ni) systems we investigate the changes in the local electronic and crystal structure and the nature of doped holes into the system upon Sr substitution. Our polarization dependent XAS results from single crystals of rm La _{2-x}Sr_{x}NiO_4 clarify the previous ambiguities in the spectral feature identification of Ni-K edge spectra. The splittings of the Ni 4p-levels (4p_pi - 4p_sigma) are shown to correlate with the anisotropies in the Ni-O distances in the crystal. The collapse of the pi-sigma ^littings, mainly moving of the 4p_ pi states to higher energies, causes an overestimation in the absorption edge shift and consequently the Ni-valence upon Sr substitution. Finally, we utilize the p-final state of the L_1- and K-edge transitions to investigate, for the first time, the p-level occupancies of Group III A-VII A elements in a wide range of compounds and make use of the systematics we obtain from these relatively simple structured oxides in more complex materials of interest. rm IBi_2Sr_2CaCu_2O_ {y}, for example, is a high-T _{c} material showing interesting anisotropic normal state resistivity. In investigating the anisotropies in the local electronic structure we benefit from the previously described transition metal oxide studies (Cu, Ni K-edges) and the Group III A-VII A element studies (I L_1-edge). The results show that the intercalated I layers extract electronic charge from the adjacent Bi-O layers.