Electronic Structure of Ba3Nb5O15 and Ba2SrNb5O15 Studied by Band Calculation and Photoemission Spectroscopy

We report results of band calculations and photoemission experiments for Ba3Nb5O15 and Ba2SrNb5O15 with tetragonal tungsten bronze structure. In the band calculations, the Nb 4d yz=zx bands dispersive along the c-axis accommodate 0.2 electrons per Nb both in Ba3Nb5O15 and Ba2SrNb5O15, which is consistent with the photoemission results. The anisotropy of the Nb 4d bands at the Fermi level provides the higher conductivity along the c-axis and the higher magnitude of the Seebeck coefficient along the a-axis. The calculated results are roughly consistent with the transport behaviors reported in the literatures. In particular, the magnitude of the Seebeck coefficient along the c-axis of Ba2SrNb5O15 is predicted to be smaller than that of Ba3Nb5O15 below 200K while they are comparable at higher temperatures. However, the magnitude of the Seebeck coefficient along the c-axis is considerably underestimated indicating that effective mass along the c-axis is selectively enhanced due to electronic correlation. The Nb 3d and 4d photoemission spectra suggest that the electronic correlation is enhanced in the surface region.