Cesium polytungstates with blue-tint-tunable near-infrared absorption.

Revisiting Wöhler's method (1824), Cs-doped tungsten bronzes were synthesized by reducing Cs-polytungstate at high temperature, and were pulverized into nanoparticles for determining their optical properties. The high-temperature reduced CsWO crystals absorbed strongly in the near-infrared, providing an improved luminous transparency with a less-bluish tint than normal CsWO synthesized in a reductive atmosphere. The high-temperature reduction caused an orthorhombic-to-hexagonal phase transformation and a nonmetal-metal transition, which was monitored by spectrophotometry, X-ray diffraction, and X-ray photoelectron spectroscopy measurements, assisted by a first-principles analysis using a DFT+U method. The high-temperature reduction of CsWO is concluded to decrease the number of W deficiencies and produce oxygen vacancies, releasing both free and trapped electrons into the conduction band and thereby activating the near-infrared absorption. The comparatively narrow bandgap of CsWO was identified as the origin of the less-bluish tint of the produced Cs tungsten bronzes.