Structural, vibrational and thermal expansion properties of Sc2W4O15

Structural and mechanical properties of nanocrystalline Ga2O3 films made by pulsed laser deposition onto transparent quartz substrates

This work reports on the correlation between structure, surface/interface morphology and mechanical properties of pulsed laser deposited (PLD) β-Ga2O3 films on transparent quartz substrates. By varying the deposition temperature in the range of 25 °C–700 °C, ∼200 nm thick Ga2O3 films with variable microstructure and amorphous-to-nanocrystalline nature were produced onto quartz substrates by PLD. The Ga2O3 films deposited at room temperature were amorphous; nanocrystalline Ga2O3 films were realized at 700 °C. The interface microstructure is characterized with a typical nano-columnar morphology while the surface exhibits the uniform granular morphology. Corroborating with structure and surface/interface morphology, and with increasing deposition temperature, tunable mechanical properties were seen in PLD Ga2O3 films. At 700 °C, for nanocrystalline Ga2O3 films, the dense grain packing reduces the elastic modulus Er while improving the hardness. The improved crystallinity at elevated temperatures coupled with nanocrystallinity, the β-phase stabilization is accounted for the observed enhancement in the mechanical properties of PLD Ga2O3 films. The structure-morphology-mechanical property correlation in nanocrystalline PLD β-Ga2O3 films deposited on quartz substrates is discussed in detail.

Interplay between Solubility Limit, Structure, and Optical Properties of Tungsten-Doped Ga2O3 Compounds Synthesized by a Two-Step Calcination Process

This work unfolds the fundamental mechanisms and demonstrates the tunable optical properties derived via chemical composition tailoring in tungsten (W)-doped gallium oxide (Ga₂O₃) compounds. On the basis of the detailed investigation, the solubility limits of tungsten (W⁶⁺) ion and associated effects on the crystal structure, morphology, and optical properties of W-doped Ga₂O₃ (Ga_{2-2 x}W _xO₃, 0.00 ≤ x ≤ 0.25, GWO) compounds are reported. GWO materials were synthesized via a conventional solid-state reaction route, where a two-step calcination is adopted to produce materials with a high structural and chemical quality. X-ray diffraction analyses of sintered GWO compounds reveal the formation of a solid solution of GWO compounds at lower concentrations W ( x ≤ 0.10), while unreacted WO₃ secondary phase formation occurs at higher concentrations ( x>0.10). Insolubility of W at higher concentrations ( x ≥ 0.15) is attributed to the difference in formation enthalpies of respective oxides, i.e., Ga₂O₃ and WO₃. GWO compounds exhibit an interesting trend in morphology evolution as a function of W content. While intrinsic Ga₂O₃ exhibits rod-shaped morphology, W-doped Ga₂O₃ compounds exhibit nearly spherical-shaped grain morphology. Increasing W content ( x ≥ 0.10) induces morphology transformation from spherical to faceted grains with different facets (square and hexagonal). Relatively larger grain sizes in GWO compounds might be attributed to vacancy assisted enhanced mass transport due to W incorporation and/or WO₃ induced liquid phase sintering. Our findings demonstrate a substantial red shift in band gap ( E_g), which is evident from the optical absorption spectra, enabling the wide spectral selectivity of GWO compounds. W 5d orbitals induced sp- d exchange interaction between valence band and conduction band electrons accounts for the substantial red shift in E_g of GWO compounds. Also, with increasing W, E_g decreases linearly, obeying Vegard law up to x = 0.15 and, at this point, an abrupt E_g drop prevails. The nonlinearity ( bowing effect) behavior in E_g beyond x = 0.15 is due to insolubility of W at higher concentrations. The fundamental scientific understanding of the interdependence of synthetic conditions, structure, chemistry, and band gap could be useful to optimize GWO materials for optical, optoelectronic, and photocatalytic device applications.