Recent research on alkali-doped tungsten bronzes has revealed their potential solar control properties in the form of nanoparticles which have attracted more attention owing to their strong near-infrared absorption and high luminous transmission. Considering the intimate relationship between the absorption mechanism and conduction electrons, broad knowledge on the effects of the electron sources, such as doped alkali ions and oxygen defects, on the crystal structure is paramount. Generally, alkali tungsten bronzes consisting of corner-shared WO6 octahedra form hexagonal structure when incorporated with large ions such as cesium. This confirms the possibility of associated structural changes and oxygen defects which require a quantitative evaluation to clarify possible planar defects.
To this note, a joint effort by Ms. Mika Okada and Mr. Hideaki Fukuyama from Ohkuchi Electronics Company together with Mr. Katsushi Ono, Dr. Satoshi Yoshio and Dr. Kenji Adachi from Sumitomo Metal Mining Company in Japan hoped to settle this matter by investigating the role of varied amounts of oxygen vacancies and alkali dopant on the composition and structural changes of industrial cesium-doped hexagonal tungsten bronzes. The work is now published in the Journal of The American Ceramic Society.
Various methods including X-ray diffraction, X-ray photoelectron spectroscopy, and Raman Spectroscopy were used in the analysis. Generally, the authors noted the existence of plenty of oxygen defects in the cesium-hexagonal tungsten bronze crystallized in a reductive atmosphere. This virtually poses a question if the Magneli designation of MxWO3 may not be appropriate at least in certain synthesis methods. As such, variances were observed in the lattice parameters of the cesium-hexagonal tungsten bronze. The variance occurred on a single line on the space and it depended on the number of oxygen defects and cesium dopants only. This implied the possible influence of the emitted electrons on the structural change.
To determine the origin of the structural changes, a coordination modification of the W-O octahedral dimensional was carried out. It was confirmed to be a destabilization of the pseudo-Jahn-Teller distortion as a result of the donated electrons. This was supported by the linear change of structural deviations observed towards regular WO6 octahedra induced by the increase in the oxygen defects and cesium content. When comparing the effects of the electrons on the dimensional change and lattice parameters, electrons emitted from the oxygen vacancies recorded lower magnitude effects as compared to that emitted from cesium ions. As such, the authors concluded that the electrons emitted from the oxygen vacancies should be localized in the cesium-hexagonal tungsten bronze as supported by theoretical calculations provided separately. It also led to a striking result in the companion work that those vacancy-derived localized electrons, not the Cs-derived electrons, were the main contributor to the low-energy optical absorption in tungsten bronzes.
In summary, the research team successfully investigated the effects of the varying amounts of oxygen vacancy and alkali dopants on the composition and structural changes of the cesium-hexagonal tungsten bronze. Based on the experimental results, the study has been identified by the Advances in Engineering selection team as a key contributor to the development of advanced alkali-doped hexagonal tungsten bronze materials for numerous industrial applications.
Okada, M., Ono, K., Yoshio, S., Fukuyama, H., & Adachi, K. (2019). Oxygen vacancies and pseudo Jahn-Teller destabilization in cesium-doped hexagonal tungsten bronzes. Journal of The American Ceramic Society, 102(9), 5386-5400.
Yoshio, S., Okada, M., Adachi, K. (2018). Destabilization of Pseudo Jahn-Teller Distortion in Cesium-doped Hexagonal Tungsten Bronzes. Journal of Applied Physics, 124, 063109.
Machida, K., Okada, M., Adachi, K. (2019). Excitations of free and localized electrons at nearby energies in reduced cesium tungsten bronze nanocrystals. Journal of Applied Physics, 125, 103103.