Direct evidence of polar nature of ferroelastic twin boundaries in CaTiO3 obtained by second harmonic generation microscope

The polar nature of twin boundaries (TBs) of inherently nonpolar ferroelastic CaTiO3 is examined by the confocal second harmonic generation (SHG) microscope. Three-dimensional SHG images of samples with the pseudocubic (110) and (1-11) planes confirm that both crystallographically prominent TB (W plane) and nonprominent TB (W' plane) are SHG active and consequently polar. The directions of the electric polarization P-S are determined from the anisotropy of SHG intensities. In the W' plane, the SHG polar diagrams are well fitted using the point-group symmetry 2, which reveals that the P-S directions parallel to the twofold axis do not lie in theW ' plane but are tilted from it. In theW plane, the P-S direction lies in the mirror plane of the point-group symmetry m.