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

Abstract

This work unfolds the fundamental mechanisms and demonstrates the tunable optical properties derived via chemical composition tailoring in tungsten (W)-doped gallium oxide (Ga 2 O 3 ) compounds. On the basis of the detailed investigation, the solubility limits of tungsten (W 6+ ) ion and associated effects on the crystal structure, morphology, and optical properties of W-doped Ga 2 O 3 (Ga 2−2x W x O 3 , 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 WO 3 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., Ga 2 O 3 and WO 3 . GWO compounds exhibit an interesting trend in morphology evolution as a function of W content. While intrinsic Ga 2 O 3 exhibits rod-shaped morphology, W-doped Ga 2 O 3 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 WO 3 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 ef fect) 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.

Chemical composition tuning induced variable and enhanced dielectric properties of polycrystalline Ga2‐2xWxO3 ceramics

“…Finally, based on the observations made from previously reported structural details 42,67 and the present work on frequency and temperature dependent dielectric constant, dielectric loss, and ac electrical transport analysis, the effect of W-doping on the electrical conduction mechanism and dielectric properties of Ga 2 O 3 can be modelled simply from the microstructure and heterogeneity perspective. First of all, it must be emphasized that crystal structure, phase and microstructure analyses using X-ray diffraction (XRD), scanning electron microscopy (SEM) indicate that the W-doping induced changes are significant.…”

“…We adopted the previously established procedures and methods to synthesize all the GWO compounds. 42 Briefly, to prepare selected GWO composition, the precursors were weighed in stoichiometric proportions according to the calculations. An agate mortar is ideal for the quantity of powders which was used to pulverize the powder with acetone as wetting media.…”

“…The crystal structure, surface chemistry and chemical composition of all the synthesized GWO compounds are thoroughly established. 42…”

“…As reported elsewhere, the incorporation of W into Ga 2 O 3 results in microstructural changes. 42 Initially, the pristine Ga 2 O 3 exhibits a rod-shaped structure. The addition of small quantities of W into the system changes the morphology to spherical and as the W concentration increases the grains become faceted with square or hexagonal features.…”

“…38 In our previous research, we directed our efforts to elucidate the fundamental science of doped and alloyed Ga 2 O 3 materials, especially the design and development of materials with controlled chemical, physical and electronic properties. 3,[39][40][41][42] We paid utmost attention to the tailoring the optical band gap by customizing the deposition temperature and/or fabrication method for different cations in Ga 2 O 3 . Sustained efforts have proved a red shift in optical bandgap as reported for W-incorporated -Ga 2 O 3 39 and Ti-incorporated -Ga 2 O 3 40 polycrystalline thin films.…”