Crystal structures, electrical properties, and electron energy-loss spectroscopy of the sodium and potassium tetragonal tungsten bronzes

The tungsten bronzes (M_xWO₃) are non-stoichiometric metal oxides which have attracted interest for their potential applications in plasmonics. Although the cubic structures of the sodium tungsten bronzes (Na_xWO₃) have been extensively studied, reports into the tetragonal-II structures (TII-) of any tungsten bronze are comparatively rare. In this work, TII-Na_xWO₃ and TII-K_xWO₃ were prepared by a furnace-assisted method, and characterised by X-ray and neutron powder diffraction, selected-area electron diffraction and electron energy-loss spectroscopy (EELS). A structural determination of TII-NaxWO3 was performed in the space group <i>>I4∕m</i>, in a √2 ✕ √2 ✕ 2 supercell of the P4∕mbm TII-K_xWO₃ structure. Density functional theory is then used to calculate the valence electronic structure and optical properties to support the EELS measurements. Similar to the cubic structures, the conduction band of TII- is composed of hybridised O 2p and W 5d states, which is filled by electrons donated from the inserted Na or K. The O sites which are in-plane with the W sites are found to have highly localised bands, resulting in low-energy interband transitions in the a∕b direction of the unit cell. In contrast, purely free-electron behaviour is observed in the c-direction below 2 eV. High-quality plasmon resonances are thus only observed in the c-direction, with energy and quality similar to those of the cubic structures. These anisotropic optical properties make TII-Na_xWO₃ and TII-K_xWO₃ interesting materials for further study as potential plasmonic materials.