Nanoscale domains and preferred cracking planes in Pb(Zn_1/3Nb_2/3)O₃–(6–7)% PbTiO₃ single crystals studied by piezoresponse force microscopy and fractography

This paper presents recent studies on surface and cross-sectional domain structures of Pb(Zn_1/3Nb_2/3)O₃–(6–7)% PbTiO₃ (PZN–PT) single crystals using piezoresponse force microscopy and three-point bending technique. The surface domain structures for the rhombohedral-based single crystals in (001) orientation are found to be influenced by polishing process, whereas the surface domains on the (011)-oriented single crystals are aligned along [011] direction, unaffected by the polishing process. On the other hand, the domain structures on the cross-sectional fracture surface show preferential alignment which agrees reasonably with the rhombohedral dipoles on the {100} and {110} planes. The differences between the surface and cross-sectional domain structures could be attributed to stress compensation between the surface strain effect and the minimization of elastic energy. In addition, both surface and cross-sectional surface demonstrate nanoscale domains, about 100–200 nm in size. Further fractography observation suggests that the preferred cracking planes for the PZN–PT single crystals are {110} and {100} planes. The {110} planes may be the slip planes along which material pile up is observed upon indentation loading. The pile up results in tensile hoop stress, producing radial cracks along the {100} cleavage planes. To accommodate the localized stress change, new ferroelastic domains by mechanical stress are then formed without interrupting the out-of-plane piezoelectric response. Since the material pile up is thought to cause enhanced toughness along {110} planes, the PZN–PT single crystal in [011]-poled orientation exhibits more superior piezoelectric properties compared to that of the [001]-poled counterpart.