Stabilizing metastable ferroelectric tungsten trioxide phase at room temperature via solvothermal synthesis and millisecond pulsed laser irradiation

Epsilon tungsten trioxide (ε-WO₃) has drawn much attention for its unique gas sensing and ferroelectric properties. However, the strong metastability of ε-phase makes it extremely difficult to stabilize the phase at room temperature. For a long time, it was believed that the ε-phase can only be stabilized by rapid solidification processing. In this study, ε-WO₃ was stabilized by employing solvothermal synthesis and subsequent annealing, with the help of Cr- and Ti-dopants. Structural characterization using XRD, Raman and TEM analysis revealed that the as-annealed powders are a mixture of ε-WO₃ and γ-WO₃ and the fraction of ε-WO₃ is directly correlated with the Cr- and Ti-doping levels. Rietveld refinement suggests that the maximum obtainable fractions of ε-phase in as-annealed WO₃ are ∼68 % for Cr-doping and ∼87 % for Ti-doping. The growth mechanism is that solvothermal synthesis produced Cr- and Ti-doped W₁₈O₄₉ and subsequent annealing resulted in phase transition from W₁₈O₄₉ to ε-WO₃. The XPS analysis reveals the phase stabilization mechanism to involve the interstitial sites of WO₃ being occupied by Cr³⁺ and Ti⁴⁺ dopants, resulting in structure distortions. The fraction of ε-WO₃ can be further improved to almost 100 % by introducing kinetic constraints using pulsed laser irradiation to rapidly melt and solidify the WO₃ (in several milliseconds). A viable solvothermal synthesis route for ε-WO₃ and the feasibility to produce ε-phase via additive manufacturing are illustrated in this work.