Polarization rotation of field-induced phase transformations was observed by means of a polarizing microscope on a ͑001͒-cut single crystal Pb(Mg 1/3 Nb 2/3 ) 0.76 Ti 0.24 O 3 ͑PMNT24%͒ at room temperature with an electric field applied along the ͓001͔ direction. A hysteresis loop was also measured for comparison. As the electric field increases, polarizations of rhombohedral R domains rotate toward the ͓001͔ tetragonal T 001 phase through M A -type monoclinic distortions, i.e., R→M A →T 001 . The crystal cannot entirely reach the T 001 phase as the field approaches 44 kV/cm. In addition, spotlike domains that perhaps correspond to M B -type monoclinic or triclinic phases were also observed. This field-induced phase transformation is strongly irreversible.A morphotropic phase boundary ͑MPB͒ between rhombohedral (R) and tetragonal (T) phases originally was proposed in relaxor-based ferroelectric ͑FE͒ crystals Pb(Mg 1/3 Nb 2/3 ) 1Ϫx Ti x O 3 (PMNTx) and Pb(Zn 1/3 Nb 2/3 ) 1Ϫx Ti x O 3 (PZNTx) for 0.28рxр0.36 and x Ͻ0.15, respectively. 1,2 However, monoclinic (M ) and orthorhombic (O) phases have been reported in both PMNT and PZNT systems, which depend strongly on Ti concentration, temperature range, strength of the external electric (E) field, and crystallographic orientation. 3-13 Compositional heterogeneities ͑or spatial phase segregation͒ are also commonly observed in these materials, especially for compounds near the MPB.An almost certainly monoclinic phase was induced by the E field applied along ͗001͘ in a PZNT8% single crystal, as observed with in situ x-ray diffraction. 4 A metastable orthorhombic FE phase ͑between R and T phases͒ was proposed in PZNT8% crystals oriented along ͗001͘, ͗110͘, and ͗111͘ from E-field-dependent polarization results. 5 By synchrotron x-ray diffraction with the E field applied along the ͓001͔ direction, various polarization rotation paths of R-M A -M C -T and R-M A -T were proposed in PZNT8% and PZNT4.5% crystals, respectively. 6 An O phase was observed in PZNTx for xϭ8%, xϭ9%, and 10% after a prior E-field application along ͓001͔. 7 With a prior E field poling along a ͑011͒-cut PMNT33% crystal, an extra O phase between R and T phases was claimed from the dielectric result. 8 From polarization results, an E-field-induced transformation from ͗111͘ R to ͗110͘ O phase through M B -type M distortion, i.e., R-M B -O, was proposed on a ͑110͒-cut PMNT30% crystal. 9 Based on synchrotron results, an M A -type M phase was observed in a ͑001͒-cut PMNT35% single crystal previously poled under an E field of 43 kV/cm. 10 However, unpoled and weakly poled PMNT35% samples exhibit an average R symmetry. 10 Irreversible field-induced phase transformations through M A -type monoclinic distortions, i.e., R→M A →T→M A →R 111 were observed in a ͑111͒-cut PMNT33% single crystal. 11 An M C -type M phase has so far been reported in PMNT only for powder and ceramic samples. [12][13][14] It is important to note that the effect of M A -type polarization rotation will be largely cancelled in randomly oriente...

Photovoltaic (PV) effects, power-conversion efficiencies, and structures have been systematically measured in (Bi 1Àx Ca x)FeO 3Àd ceramics for x ¼ 0.05, 0.10, and 0.15. The heterostructures of indium tin oxide (ITO) film/(Bi 1Àx Ca x)FeO 3Àd ceramics/Au film exhibit significant PV effects under illumination of k ¼ 405 nm. The maximum power-conversion efficiency in the ITO/(Bi 0.90 Ca 0.10)FeO 2.95 (BFO10C)/Au can reach 0.0072%, which is larger than 0.0025% observed in the graphene/polycrystalline BFO/Pt films [Zang et al., Appl. Phys. Lett. 99, 132904 (2011)]. A theoretical model based on optically excited current in the depletion region between ITO film and Ca-doped BFO ceramics is used to describe the I-V characteristic, open-circuit voltage, and short-circuit current density as a function of illumination intensity. This work suggests that the Ca-substitution can reduce the rhombohedral distortion and stabilize the single-phase structure. V

A sequence of field-induced phase transformations was observed by means of a polarizing microscope on a (111)-cut single crystal Pb(Mg1/3Nb2/3)0.67Ti0.33O3 at room temperature with an electric field applied along the [111] direction. As electric field increases, polarizations of the rhombohedral R domains whose polar directions are not along [111] rotate first toward a tetragonal T phase associated with 90° domain walls. Then the crystal gradually reaches total optical extinction for all polarizer angles at 12 kV/cm, indicating a single [111] rhombohedral R111 domain with polarization aligned with the electric field. These field-induced phase transformations are irreversible and proceed most likely through MA-type monoclinic distortions, i.e., R→MA→T→MA→R111.

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A monoclinic phase is evidenced between rhombohedral and cubic phases in a ͗111͘-cut single crystal (PbMg 1/3 Nb 2/3 O 3 ) 0.67 (PbTiO 3 ) 0.33 ͑PMN-33% PT͒ from observation of domain structures. Near 360 K the structure begins to distort from the rhombohedral toward the tetragonal phase through monoclinic domains ͑probably M A type but perhaps M B type͒. However, the present ͗111͘-cut crystal seems to disfavor the tetragonal phase and persists in the monoclinic phase up to Tϳ420 K, where the cubic phase begins to develop. Temperature-dependent orientations of optical indicatrices of domains indicate polarization rotations within the monoclinic planes. In addition, a previously electric-field-cooled process enhances a long-range transformation.