Crystal structure and paramagnetic behaviour of

Abstract

The crystal structure of the lowest-temperature ε-phase of WO 3 has been determined as a function of temperature by neutron powder diffraction. The diffraction measurements show that there are no additional phases at low temperatures. Magnetic measurements suggest that no superconductivity exists at temperatures as low as 9 × 10 −5 K, but instead, an increase of the magnetic susceptibility according to a paramagnetic Curie law was found. This observation indicates that unpaired carriers persist in the sample after cooling to very low temperatures.

Structural and optical properties of visible active photocatalytic WO3 thin films prepared by reactive dc magnetron sputtering

“…Finally, it is apparent that the intensity of the peak corresponding to the (020) planes decreases relative to peaks from diffraction planes of (002) and (200) with increasing P tot . In the range 2h 5 33-35°, the highest intensity occurs at P tot 5 1.3 Pa, and the diffractogram resembles the e-WO 3 phase, which has an analogous XRD pattern, and a dominant peak at 33.34°, corresponding to the (À112) reflection, 29 again supporting significant e-WO 3 phase mixing at low P tot . With increasing P tot , the relative intensity of the (À112) reflection decreases compared to the adjacent peak from diffraction planes of (202) at 34.08, which eventually dominates the XRD spectrum at P tot 5 4.0 Pa, and corresponds to reflection in either the d-WO 3 or the c-WO 3 phase, which is reported to occur at 34.08°and 34.17°, respectively.…”

“…Indeed, both phases have been reported to coexist at room temperature and are sensitive to the way the samples are prepared, e.g., T s , T a , and P tot . [46][47][48] However, the (002) diffraction angle in e-WO 3 at 23.19°is close to the corresponding reflections in the c-WO 3 and d-WO 3 phases, 29 which occur at 23.12°, 46 and the experimental value of the peak at 23.15°may indicate mixing of the low temperature monoclinic e-WO 3 phase at low P tot . Supporting this argument is the observation of the peak at 24.25°, .…”

“…Nano-WO 3 is readily reduced, and nonstoichiometry is reported to involve structural changes. [27][28][29] Substoichiometry gives rise to polarons, which change the optical properties and increase the absorption in the near-infrared (NIR) spectral region. 29,30 For small values of substoichiometry, x, crystals of WO 3Àx are reported to undergo a structural phase transition from the room temperature d phase to the low temperature e phase at T ; 250 K. 31 The d phase shows metallic behavior, and the e phase has insulating properties for the electron/hole carriers.…”

“…[27][28][29] Substoichiometry gives rise to polarons, which change the optical properties and increase the absorption in the near-infrared (NIR) spectral region. 29,30 For small values of substoichiometry, x, crystals of WO 3Àx are reported to undergo a structural phase transition from the room temperature d phase to the low temperature e phase at T ; 250 K. 31 The d phase shows metallic behavior, and the e phase has insulating properties for the electron/hole carriers. 32 Reported values for the electronic band gap of nano-WO 3 vary considerably from E g 5 2.6 to 3.25 eV, 33,34 and this is probably due to a combination of one or several of the effects discussed above.…”

Density-functional study of charge doping inWO3

“…Excess electrons in the ǫ (lowtemperature monoclinic) phase have been shown experimentally to self-trap, forming polarons 26 . We found no localisation of charge or deformation (polaron formation) in any of our calculations.…”