acknowledge Electron Microscopy Laboratory in Peking University for the use of aberration corrected transmission electron microscope. J.L. and J.Z. conceived and supervised the project; J.Z., G.K. and Y.Z. grew the crystals and did SEM; B.H. carried out AFM and optic microscopy studies with assistances of E.E., J.Y., and S.X.; J.L. carried out crystallographic analysis of 7 ferroic domains; S.C. and P.G. carried out and analyzed HRTEM; Q.L. and H.W. carried out and analyzed synchrotron XRD; A.X., J.Z. and G.K. carried out and analyzed EBSD. J.L. wrote the manuscript, and all authors participated in the revision.

Piezoresponse force microscopy (PFM) is a powerful tool widely used to characterize piezoelectricity and ferroelectricity at the nanoscale. However, it is necessary to distinguish microscopic mechanisms between piezoelectricity and non-piezoelectric contributions measured by PFM. In this work, we systematically investigate the first and second harmonic apparent piezoresponses of silicon wafer in both vertical and lateral modes, and we show that it exhibits apparent electromechanical response that is quadratic to the applied electric field, possibly arising from ionic electrochemical dipoles induced by the charged probe. As a result, the electromechanical response measured is dominated by the second harmonic response in vertical mode, and its polarity can be switched by the DC voltage with evolving coercive field and maximum amplitude, in sharp contrast with typical ferroelectric materials we used as control. The ionic activity in silicon is also confirmed by scanning thermo-ionic microscopy (STIM) measurement, and this work points toward a set of methods to distinguish true piezoelectricity from the apparent ones.