Reduced WO3 single crystals were investigated by conductive and piezoresponse force microscopy. The reduced crystals showed a piezoelectric active surface layer with a noncentrosymmetric tetragonal structure which was different from the bulk of the crystal. The domain walls carry a high current while the bulk remained insulating. Twin-related interfaces were atomically thin, the upper bound being less than 10nm. The reduced surface layer remained in the piezoelectric state even after several weeks, while the current in the twin boundaries was reduced by re-oxidization. This layer showed a significant piezoelectric activity with a piezoelectric coefficient of about 7.9 pm/V.

Nanoscale Properties of Thin Twin Walls and Surface Layers in Piezoelectric WO3−X. Y.Kim, M.Alexe, E.K.H.Salje: Applied Physics Letters, 2010, 96[3], 032904