Dimensionality Effects on the Optical Properties of (PO2)4(WO3)2m (m = 2, 4, 6, 7)

Abstract

We report the 300 K polarized reflectance spectra and calculated electronic band structures of a series monophosphate tungsten bronzes, (PO2)4(WO3)2m (m = 2, 4, 6, 7). These materials have several layers of corner-sharing WO6 octahedra separated by one PO4 layer, leading to an octahedral layer thickness that is “tunable” with m. In the optical regime, the spectra of the m = 2, 4, 6, and 7 materials display an anisotropic electronic excitation, originating from the W intra-t2g d → d transition. The intensity and frequency of the intra-t2g d → d excitation vary with the octahedral layer thickness. Several vibrational modes along the interlayer direction of the m = 4, 6, and 7 compounds change with m as well. These results are consistent with the lattice becoming softer with increasing m. The low-frequency electrodynamics of the monophosphate tungsten bronzes shows a gap or pseudogap feature in the infrared region, demonstrating a ubiquitous bound-carrier response in these tungsten bronzes.