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.