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  1. The prospect of creating ferroelectric or high permittivity nanomaterials provides motivation for investigating complex transition metal oxides of the form Ba(Ti, MV)O3, where M = Nb or Ta. Solid state processing typically produces mixtures of crystalline phases, rarely beyond minimally doped Nb/Ta. Using a modified sol-gel method, we prepared single phase nanocrystals of Ba(Ti, M)O3. Compositional and elemental analysis puts the empirical formulas close to BaTi0.5Nb0.5O3−δ and BaTi0.5Ta0.5O3−δ. For both materials, a reversible temperature dependent phase transition (non-centrosymmetric to symmetric) is observed in the Raman spectrum in the region 533–583 K (260–310 °C); for Ba(Ti, Nb)O3, the onset is at 543 K (270 °C); and for Ba(Ti, Ta)O3, the onset is at 533 K (260 °C), which are comparable with 390–393 K (117–120 °C) for bulk BaTiO3. The crystal structure was resolved by examination of the powder x-ray diffraction and atomic pair distribution function (PDF) analysis of synchrotron total scattering data. It was postulated whether the structure adopted at the nanoscale was single or double perovskite. Double perovskites (A2B′B″O6) are characterized by the type and extent of cation ordering, which gives rise to higher symmetry crystal structures. PDF analysis was used to examine all likely candidate structures and to look for evidence of higher symmetry. The feasible phase space that evolves includes the ordered double perovskite structure Ba2(Ti, MV)O6 (M = Nb, Ta) Fm-3m, a disordered cubic structure, as a suitable high temperature analog, Ba(Ti, MV)O3Pm-3m, and an orthorhombic Ba(Ti, MV)O3Amm2, a room temperature structure that presents an unusually high level of lattice displacement, possibly due to octahedral tilting, and indication of a highly polarized crystal.

     
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    Free, publicly-accessible full text available April 7, 2025
  2. The bulk photovoltaic effect (BPE) has drawn considerable attention due to its ability to generate photovoltages above the bandgap and reports of highly enhanced photovoltaic current when using nanoscale absorbers or nanoscale electrodes, which, however, do not lend themselves to practical, scalable implementation. Herein, it is shown that a strikingly high BPE photoresponse can be achieved in an ordinary thin‐film configuration merely by tuning fundamental ferroelectric properties. Nonmonotonic dependence of the responsivity (RSC) on the ferroelectric polarization is observed and at the optimal value of the film polarization, a more than three orders of magnitude increase in theRSCfrom the bulk BaTiO3value is obtained, reachingRSCclose to 10−2 A W−1, the highest value reported to date for the archetypical ferroelectric BaTiO3films. Results challenge the applicability of standard first‐principles‐based descriptions of BPE to thin films and the inherent weakness of BPE in ferroelectric thin films.

     
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  3. null (Ed.)
  4. The quantum phenomenon of shift photovoltaic current was predicted decades ago, but this effect was never observed directly because shift and ballistic currents coexist. The atomic-scale relaxation time of shift, along with the absence of a photo-Hall behavior, has made decisive measurement of shift elusive. Here, we report a facile, direct-current, steady-state method for unambiguous determination of shift by means of the simultaneous measurements of linear and circular bulk photovoltaic currents under magnetic field, in a sillenite piezoelectric crystal. Comparison with theoretical predictions permits estimation of the signature length scale for shift. Remarkably, shift and ballistic photovoltaic currents under monochromatic illumination simultaneously flow in opposite directions. Disentangling the shift and ballistic contributions opens the way for quantitative, fundamental insight into and practical understanding of these radically different photovoltaic current mechanisms and their relationship. 
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  5. Abstract

    Ferroelectric materials, which exhibit switchable polarization, are potential candidates for photovoltaic applications owing to their intriguing charge carrier separation mechanism associated with polarization and breaking of inversion symmetry. To overcome the low photocurrent of ferroelectrics, extensive efforts have focused on reducing their bandgaps to increase the optical absorption of the solar spectrum and thus the power conversion efficiency. Here, a new avenue of enhancing photovoltaic performance via engineering the polarization across a morphotropic phase boundary (MPB) is reported. Tetragonal compositions in the vicinity of the MPB in a PbTiO3‐Bi(Ni1/2Ti1/2)O3solid solution are shown to generate up to 3.6 kV cm−1photoinduced electric field and 6.2 µA cm−2short‐circuit photocurrent, multiple times higher than its pseudocubic counterpart under the same illumination conditions with excellent polarization retention. This enhancement allows the investigation of the correlation between the polarization switching and photovoltaic switching, which enables a controllable multistate photocurrent. Combined with a bandgap of 2.2 eV, this material exhibits a sizable photoresponse over a broad spectral range. These findings provide a new approach to improve the photovoltaic performance of ferroelectric materials and can expand their potential applications in optoelectronic devices.

     
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  6. Abstract

    Domain walls separating regions of ferroelectric material with polarization oriented in different directions are crucial for applications of ferroelectrics. Rational design of ferroelectric materials requires the development of a theory describing how compositional and environmental changes affect domain walls. To model domain wall systems, a discrete microscopic Landau–Ginzburg–Devonshire (dmLGD) approach with A‐ and B‐site cation displacements serving as order parameters is developed. Application of dmLGD to the classic BaTiO3, KNbO3,and PbTiO3ferroelectrics shows that A–B cation repulsion is the key interaction that couples the polarization in neighboring unit cells of the material. dmLGD decomposition of the total energy of the system into the contributions of the individual cations and their interactions enables the prediction of different properties for a wide range of ferroelectric perovskites based on the results obtained for BaTiO3, KNbO3,and PbTiO3only. It is found that the information necessary to estimate the structure and energy of domain‐wall “defects” can be extracted from single‐domain 5‐atom first‐principles calculations, and that “defect‐like” domain walls offer a simple model system that sheds light on the relative stabilities of the ferroelectric, antiferroelectric, and paraelectric bulk phases. The dmLGD approach provides a general theoretical framework for understanding and designing ferroelectric perovskite oxides.

     
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