Thin-Film Stabilization of a Ferroelectric Orthorhombic α‑Pr₂WO₆ Polymorph

An orthorhombic α-Pr₂WO₆ (PrWO) polymorph with a = 16.57(5) Å, b = 5.52(5) Å, and c = 8.73(1) Å, isostructural to α-La₂WO₆ and α-Nd₂WO₆, has been stabilized in the form of thin film by pulsed laser deposition on (001)-oriented SrTiO₃ substrates. Combining X-ray diffraction pole-figure measurements and transmission electron microscopy (TEM) analysis, the c-axis films gave evidence of the orientations [100]_PrWO || [110]_STO and [010]_PrWO || [110]_STO in the plane. Advanced φ-scans and reciprocal space mapping characterizations confirm the existence of the orthorhombic (Pm2₁n) structure in the film in place to the tetragonal one as also suggested. X-ray thermodiffraction measurements highlight the stability of this polymorph in thin film up to 900 °C at least. Optical measurements performed by spectroscopic ellipsometry reveal that the band gap in such 36 nm thick films (as confirmed by both X-ray reflectivity and TEM measurements) is 2.5 eV. Besides, the local piezoelectric hysteresis loops recorded by using the spectroscopic tool of piezoresponse force microscopy attest to the robustness of the piezoelectricity and ferroelectricity in these α-Pr₂WO₆ films. This study demonstrates the existence of a new lead-free ferroelectric material in the series of α-Ln₂WO₆ (lanthanide) tungstates, which can be considered as a promising candidate for applications in both nanoelectromechanical and energy-harvesting systems as well as for integrating optics.