Structure and Domain Formation in WO₃ Thin Films Under Epitaxial Strain

Tungsten oxides have been a topic of research interest for their gas sensing, catalytic and electrochromic capabilities. Recently, it was found that epitaxial thin films of tungsten trioxide (WO₃) grown on a substrate of yttrium aluminate (YAlO₃) may even exhibit piezoelectricity localized to the monoclinic twin walls as a result of strain gradients at those walls.[1,2,5] A full understanding of the structure and electronic properties of these films is, however, not yet available.
We have grown epitaxial films of WO₃ on (001)- and (110)-oriented YAlO₃ as well as (100)-oriented SrTiO₃ by pulsed laser deposition to investigate the effect of strain on their structural and electronic properties. We have probed the structure and domain formation in these films by large-volume reciprocal space mapping using x-ray diffraction (XRD) and atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM) combined with advanced real-space image analysis. We further characterized their local electronic properties by piezoresponse force microscopy (PFM) and conductive atomic force microscopy (c-AFM).
XRD has revealed that WO₃ films grown on YAlO₃, i.e. under conditions of low epitaxial strain, assume a triclinically distorted monoclinic phase which has not been reported in the literature and may be responsible for the previously observed piezoelectric response of these films. Furthermore, we find that the formation of twin domain patterns is highly sensitive to small changes to the imposed epitaxial strain: the two orientations of YAlO₃ result in entirely different domain patterns.
On (110)-oriented YAlO₃, we find a stripe-like domain pattern reminiscent of that found in traditional ferroelectrics like PbTiO₃, whereas on (001)-oriented YAlO₃, we find a wavy domain pattern in which the domain walls do not follow crystallographic planes. The latter observation suggests that the domain pattern in this case is not strongly influenced by strain matching to the substrate, which may be related
to the concept of domain glasses described by Salje et al.[4] In-situ heating XRD shows that the film exhibits a two-step phase transition to a single-domain tetragonal symmetry around 600^oC. PFM shows that the domains in the room temperature phase are polar and associated with a local change of electrical conductivity measured by c-AFM, forming a nanoscale network of regions with higher and lower conductivity. In situ heating shows that the polar domains remain unchanged up to a temperature of at least 250^oC.
Our epitaxial thin films of WO₃ are structurally and functionally different from known bulk phases of the same material as well as from previously reported epitaxial films.The rich phase space reflects the presence of competing interactions and the large degree of complexity of this material. Their characteristics make them attractive candidates for application in oxide electronics, neuromorphic computing[3] and catalysis.

[1] J. T. Eckstein et al. J. Appl. Phys., 131(21), 2022.
[2] Y. Kim et al. Appl. Phys. Lett., 96(3):3, 2010.
[3] G. Lu et al. Frontiers in Materials, 11, 2024.
[4] E. K. H. Salje et al. Phys. Status Solidi B, 251(10):2061–2066, 2014.
[5] S. Yun et al. Nat. Commun., 11(1):4898, 2020.