The low-energy optical absorption mechanism of tungsten oxides and tungsten bronzes is unresolved, with the primary models reported involving free-electron and polaron excitations. Herein, a new mechanism is proposed, based on a systematic and detailed analysis of optical profiles in Cs-doped hexagonal tungsten bronze nanoparticles with varied amounts of oxygen vacancy (VO) and Cs dopant. The Drude–Lorentz analysis of absorption bands, incorporating a newly-developed Mie scattering integration method, has clarified the observed absorption profiles as consisting of three peaks of anisotropic plasmon and polaron exitations. The behavior of the deconvoluted components indicates that VO and Cs+ provide localized and delocalized electrons, respectively, both contributing to the collective plasmon resonance against external electromagnetic waves.

Structural and compositional changes of Cs‐doped hexagonal tungsten bronzes (HTB) with respect to variations in oxygen deficiency and alkali content have been investigated in detail through x‐ray diffraction Rietveld analysis, x‐ray photoelectron spectroscopy, and Raman spectroscopy. Cs‐HTB crystallized in a reductive atmosphere is evidenced to generally contain plenty of oxygen defects, and a general formula, CsxWO3−y (0.20 ≤ x ≤ 0.32, 0 < y ≤ 0.46), is proposed. Lattice parameters of Cs‐HTB are observed to vary according to the relation, c (Å) = −3.436a (Å) + 33.062. The coordinated modification of W–O octahedral dimensions suggests the origin of the structural change with increasing x and y to be a destabilization of the pseudo Jahn‐Teller distortion due to donated electrons. The dimensional change of lattice due to electrons emitted from oxygen defects is appraised only 1/18 as that due to electrons from doped alkali ions, suggesting that most electrons from oxygen defects should be localized in Cs‐HTB.

Nanoparticles of alkali-doped tungsten bronzes are an excellent near-infrared shielding material, but exhibit slight chromatic instabilities typically upon applications of strong ultra-violet light or heating in humid environment, which acts detrimentally to long-life commercial applications. Origin of the chromatic instabilities in cesium-doped tungsten bronze has been investigated, and it has been found that the coloration and bleaching processes comprised electronic exchanges which accelerate or depress the polaron excitation and the localized surface plasmon resonance. Coloration on UV illumination is evidenced by electron diffraction as due to the formation of HxWO3, which is considered to take place in the surface Cs-deficient WO3 region via the double charge injection mechanism. On the other hand, bleaching on heating in air and in humid environment is shown to accompany the extraction of Cs and electrons from Cs0.33WO3 by X-ray photoelectron spectroscopy and X-ray diffraction analysis and is concluded to be an oxidation of Cs0.33WO3 on the particle surface.

In Cs-doped hexagonal tungsten bronzes (Cs-HTBs), X-ray diffraction–Rietveld analysis has revealed that an increase in the alkali dopant and oxygen vacancies (VO) elongate the c-axis, contract the a-axis, and decrease the deviations of the W–O distance and W coordinates from those of a regular WO6 octahedron. These structural changes are interpreted as a destabilization of pseudo-Jahn–Teller (PJT) distortion by electron donation from Cs+ and VO. A dramatic difference is observed in the destabilization efficiency between the donated electrons from Cs+ and VO, suggesting that the former and latter electrons should be delocalized and localized, respectively. First-principles density functional theory calculations using optB86b-vdW functionals reproduced the behavior of c-axis elongation and a-axis contraction by Cs doping. The projected orbital density of states indicates that the Cs-derived electrons are donated to W-5dyz and W-5dzx orbitals to extend along the c-axis, whereas the VO-derived electrons are donated to W-5dxy and W-5dx2−y2 orbitals to strongly localize in the a–b plane. In HTBs, an anisotropic increase and decrease in the t2g* antibonding electrons from the doped alkali are concluded to induce the anisotropic structural change in PJT distortions.

Based on the metallurgical point of view, we aimed to design a new form of copper catalysts with high thermal stability and activity. Delafossite CuCrO 2 has been studied as a precursor for copper catalyst. The CuCrO 2 was reduced to fine dispersion of Cu and Cr 2 O 3 particles with porous structure by the treatment in H 2 at 600°C, which exhibited much higher activity and thermal stability for steam reforming of methanol (SRM) than those of the CuO and/or Cr 2 O 3 catalysts. Sintering of Cu particles was significantly suppressed even after H 2 reduction at 600°C. Moreover, the CuCrO 2 can be regenerated by calcination in air at 1,000°C where the activity is also restored completely even after sintering at high temperatures. Fine porous structure generated by the reduction of CuCrO 2 and immiscible interaction between Cu and Cr 2 O 3 are important in stabilizing of copper nanoparticles. Based on these findings, we propose that the CuCrO 2 is an effective precursor for a high performance copper catalyst.