Structural properties, thermodynamic stability and reaction pathways for solid-state synthesis of Bi₂WO₆ polymorphs

This study presents a density functional theory (DFT) investigation into the structural, electronic, and optical properties, thermodynamic stability, phase competition, and reaction pathways of Bi₂WO₆, a notable compound within the Aurivillius family of oxides. We examine three distinct polymorphs of Bi₂WO₆: the low-temperature orthorhombic phase (P1), the intermediate-temperature orthorhombic phase (P2), and the high-temperature monoclinic phase (P3). Electronic structure analysis indicates band gaps of 2.339 eV (P1), 2.312 eV (P2), and 2.128 eV (P3), with the valence band primarily composed of O 2p states and the conduction band of Bi 6p and W 5d states. Optical properties, including the dielectric function and absorption spectra, show distinct behaviors for each phase, particularly P3. Elastic and phonon property analyses confirm the mechanical and dynamical stability of all three phases, with the P1 phase exhibiting the highest bulk modulus and stiffness among the polymorphs. Our effective mass calculations suggest that the P3 phase may have better charge carrier mobility compared to the P1 and P2 phases. Structural optimizations reveal marginal differences in total energy among these phases, suggesting their potential coexistence or easy phase transitions under varying conditions. Detailed Gibbs free energy calculations confirm that the P1 phase is the most stable at low temperatures, in agreement with experimental data in the literature. We also construct a chemical reaction network to explore feasible reaction pathways for the solid-state synthesis of Bi₂WO₆ from Bi₂O₃ and WO₃ precursors, identifying several low-cost reaction pathways, including both direct and multi-step routes involving intermediates such as Bi₁₄WO₂₄ and Bi₂W₂O₉.