Domain wall dynamics in tungsten trioxide: Evidence for polar domain walls

Domain walls have distinct properties from the bulk, and tailoring them to suit the needs for device applications is critical. Tungsten trioxide, ${\mathrm{WO}}_3$, is of great interest for device applications that make use of domain wall properties; it exhibits a phenomenologically rich sequence of phase transitions, virtually all of which are ferroelastic in character, resulting in many sets of domain walls at low temperatures, each with their own unique properties. Domain wall motion and its contribution to the piezoelectric response have been investigated in ${\mathrm{WO}}_3$ from 300 to 180 K using resonant ultrasound spectroscopy (RUS) and resonant piezoelectric spectroscopy (RPS), which showed that the $P{2}_1/n,P\overline{1}$, and $P{2}_1/c$ phases give a piezoelectric response despite the bulk being nominally centrosymmetric. Second harmonic generation (SHG) confirmed that polarity was strongest within the domain walls, and additional weak signals were found in the domains. Domain wall mobility was investigated in the $Pbcn,P{2}_1/n$, and $P{2}_1/c$ phases from 685 to 5 K. Domain walls in the $P{2}_1/n$ and $P\overline{1}$ were more mobile than those in the $Pbcn$ and $P{2}_1/c$ structures, and soon after the $P\overline{1}\to P{2}_1/c$ transition the walls become pinned at $\sim 140K$.