Direct evidence of polar ferroelastic domain boundaries in semiconductor BiVO4

Abstract

Ferroelastic domain boundaries in semiconductor bismuth vanadate, BiVO4, are examined using second harmonic generation (SHG) microscopy. Although the bulk is centrosymmetric, domain boundaries produce homogeneous SH signals. The polarization dependences of SH intensities exhibite strong anisotropy compatible with the polar symmetry m. The present results are compared with the experimental results of other ferroelastics we have observed so far. Unlike other ferroelastic materials, the directions of the SH maxima are in the same direction for all domain boundaries. Domain boundaries in ferroics are known to exhibit physical properties that do not exist in the bulk. Typical examples are domain boundaries with much higher conductivity than the bulk 1-6 , even being superconducting 7 , charged domain boundaries 8-15 , and polar domain boundaries 16-22. These phenomena have been directly measured by state-of-the-art techniques, such as aberration-corrected transmission microscopy, electron holography, atomic force microscopy etc. The fundamental significance of these phenomena lies in the possible development of new technologies where the domain boundary is the main element of the device. Racetrack memories 23 and logics for electronic circuits 24 are prominent examples of applications, and some of them have been already been achieved by using magnetic domain boundaries. Ferroelastic domain boundaries have several advantages for such future devices 25. Since the width of a ferroelastic domain boundary is much thinner than any magnetic boundaries, a high-density memory device can be realized. In particular, the existence of polar properties at a ferroelastic domain boundary is one of the most promising candidates, because the localized polarization

Domain-wall engineering and topological defects in ferroelectric and ferroelastic materials

“…So far, the point group symmetry has mainly been probed by SHG [71][72][73][74][75] , performing a polarisation analysis of the incident and emitted light fields. This approach has produced remarkable results, starting with the experimental confirmation of the non-centrosymmetric symmetry of strain-compatible ferroelastic walls [71][72][73][74][75] , but also observations that are at odds with the classical theoretical approaches. In calcium titanate (CaTiO3), domain walls were found that have, according to the SHG analysis, a polar axis deviating from the predicted possible directions (FIG.…”

“…Recent experimental works on polar domain walls have concentrated on demonstrating the loss of inversion symmetry 34,[71][72][73][74][75] , quantifying domain wall polarisation by structural studies 81 , and looking for evidence for characteristic electric or dielectric signatures of this domain wall polarisation 11,33,34,82,83 .…”

Ferroelastic Twinning in Minerals: A Source of Trace Elements, Conductivity, and Unexpected Piezoelectricity

“…Another useful method is phonon spectroscopy [75,76]. Polar twin walls were also confirmed in other minerals, such as palmierite [31,54] and clinobisvanite BiVO 4 [77,78]. Very high densities of twin walls, such as in tweed structures, lead to an overall SHG background that proves that the mineral contains an extremely high density of walls that is hard to see using other techniques [79].…”

Symmetry and polarity of antiphase boundaries in PbZrO$_3$

“…The double peak is caused by a nucleus at the APB's center. the non-polar APBs, compare with (10) and (11).…”

“…Domain walls (DWs) in ferroic materials became recently of increasing interest due to the substantial improvement of experimental techniques allowing their observation [3][4][5][6] and unveiling their potential for applications [7][8][9] . The tensor properties of DWs and in particular of antiphase boundaries (APBs) in perovskites were studied by several authors [1,6,[10][11][12][13]. The occurrence of polarization inside the DWs in otherwise non-polar samples was predicted on the basis of symmetry analysis using the layer-group method [14,15] and also described by the phenomenological Landau-Ginzburg approach [1,[16][17][18][19][20].…”