First-Principles Investigation of Alkaline-Earth Metal Co-Doped Hexagonal WO₃ for Enhanced Near-Infrared Optical Response

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The structural, thermodynamic, electronic, and optoelectronic properties of co-doped hexagonal tungsten trioxide (h-WO₃) are investigated using Density Functional Theory (DFT) with the HSE06 hybrid functional and Density Functional Perturbation Theory (DFPT). To enhance near-infrared (NIR) response, beryllium is paired with alkaline-earth metals (Mg, Ca, Sr, and Ba) to form co-doped hexagonal tungsten bronze-like (h-HTB) configurations. Phonondispersion calculations confirm the dynamical stability of the co-doped systems, while formation-energy analysis indicates that the (Be, Ba) configuration is the most thermodynamically favorable, with a minimum formation energy of -2.68 eV. Electronicstructure calculations show that co-doping introduces impurity states within the pristine 3.3 eV band gap, leading to substantial band-gap narrowing across the dopant series. In particular, the (Be, Ba) system exhibits a reduced band gap of 0.9 eV, which is favorable for NIR absorption. Dielectric-function analysis further reveals a crossover in the real part of the dielectric function, ε₁(ω), together with a resonant feature in the imaginary part, ε₂(ω), near 0.9 eV, suggesting possible plasmonic behavior in the low-energy region. The calculated optical absorption spectra show strong NIR absorption with coefficients exceeding 10⁴ cm⁻¹ and trends comparable to those of Cs₀.₃₃WO₃ benchmark systems, while retaining visible-light transparency. Overall, the results identify (Be, Ba) co-doped h-WO₃ as a thermodynamically favorable and optically promising candidate for smart-window and photothermal applications.

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