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Narrow bandgap ferroelectrics are receiving a renewed interest for photovoltaic applications aiming at exploiting new functionalities arising from bulk photovoltaic effect (BPE). We report on the photovoltaic response of vertical capacitors of ferroelectric hexagonal LuMnO3 films sandwiched between semitransparent top electrodes (Pt, Co, Ti) and a common bottom (Pt) bottom electrode. Our results show that the presence of electrodes, other than their optical transmittance, crucially determines the imprint in the ferroelectric layer and ultimately the sensitivity of short circuit current density (Jsc) to the ferroelectric polarization direction. The use of ultrathin (7 nm) Pt top electrodes allowed to obtain a large Jsc (up to 100 mA/cm2) and an open circuit voltage of Voc ≈ 0.52 V, with a responsivity of 2 × 10−3 A/W. Yet, polarization back-switching due to imprint largely washes out the dependence of Jsc on the direction of the polarization and thus, at first sight, Jsc seemingly appears to be ruled by conventional photovoltaic response. However, a pioneering analysis of the light-polarization dependent photosensitivity, allowed to disentangle for the first time, a genuine contribution of BPE from a ubiquitous Fresnel-like contribution arising from interfaced optical media., Financial support from the Spanish Ministry of Science, Innovation and Universities, through the “Severo Ochoa” Programme for Centers of Excellence in R&D (FUNFUTURE CEX2019-000917-S), MAT2017-85232-R (AEI/FEDER, EU) and PID2019-107727RB-I00 (MINECO/FEDER, EU) projects, and from Generalitat de Catalunya (2017 SGR 1377) is acknowledged. I. F. acknowledges RyC Contract RYC-2017-22531. Project supported by a 2020 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation. ICTS-CNME at UCM is acknowledged for offering access to STEM microscopy and expertise. A. Q. acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities through the Juan de la Cierva formación contract (FJC2019-039780-I). Y. S. and H. T. are financially supported by China Scholarship Council (CSC) with No. 201806410010 and No. 201906050014, respectively. The work of Y. S. and H. T. has been done as a part of their Ph.D. program in Materials Science at Universitat Autònoma de Barcelona., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Short-circuit photocurrent due to bulk photovoltaic effect displays an oscillatory dependence on the polarization state of light. Here, we explore how the ferroelectric polarization direction in h-LuMnO3 crystals affects the oscillating short-circuit photocurrent. It is shown that after prepoling the crystal at saturation, at remanence, the direction and amplitude of photocurrent oscillations are no longer dictated by prepoling voltage but are largely modulated by polarization back-switching, here ruled by the imprint field. Thus, the light polarization dependence of photocurrent is also ruled by the imprint field. The impact of these effects on the determination of the Glass coefficients of the material is discussed., Financial support from the Spanish Ministry of Science and Innovation (10.13039/501100011033), through the Severo Ochoa FUNFUTURE (No. CEX2019-000917-S); the TED2021-130453B-C21 (AEI/FEDER, EU), PID2020-118479RB-I00 (AEI/FEDER, EU), and PID2019-107727RB-I00 (AEI/FEDER, EU) projects; and from CSIC through the i-LINK (No. LINKA20338) program is acknowledged. Project supported by a 2020 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation. Y.S. is financially supported by China Scholarship Council (CSC) through No. 201806410010. The work of Y.S. has been done as part of her Ph.D. program in Materials Science at Universitat Autònoma de Barcelona., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Hexagonal manganites, such as h-LuMnO3, are ferroelectric with its polar axis along the hexagonal axis and have a narrow electronic bandgap (≈ 1.5 eV). Using Pt electrodes, h LuMnO3 single crystals display a strong rectification, characteristic of a Schottky diode, and a large photoresponse. It is found that the short circuit photocurrent Jsc along the polar axis is modulated (up to 25 %) by the direction of the ferroelectric polarization P, leading to a short circuit photocurrent loop that mimics the ferroelectric polarization, and by polarization back switching. However, a non-switchable Jsc persists. Diffusion photocurrent is shown to dominate current-in-plane measurements and contributes to the non-switchable Jsc. This observation illustrates the dramatic role of the large optical absorption in hexagonal manganites. The accompanying optical dichroism shall challenge disentangling a genuine bulk photovoltaic response in h-LuMnO3 single crystal contributing to the non-switchable Jsc. Epitaxial thin films may offer a suitable alternative., Financial support from the Spanish Ministry of Science, Innovation and Universities, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (FUNFUTURE CEX2019-000917-S), MAT2017-85232-R (AEI/FEDER, EU), and PID2019-107727RB-I00 (MINECO/FEDER, EU) projects and from Generalitat de Catalunya (No. 2017 SGR 1377) is acknowledged. I.F. acknowledges RyC Contract No. RYC-2017-22531. Project supported by a 2020 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation. Y.S is financially supported by China Scholarship Council (CSC) with No. 201806410010. The work of Y.S. has been done as a part of her Ph.D. program in Materials Science at Universitat Autònoma de Barcelona. A. Schankler and A. M. Rappe are acknowledged for the critical reading of the manuscript.

Hexagonal manganites, such as h-LuMnO3, are ferroelectric and have a narrow electronic band gap of ≈1.5eV. Here we report on the photoresponse of h-LuMnO3 single crystals. It is found that the short circuit photocurrent density (Jsc) and the open circuit voltage (Voc) are dependent on the direction of the polarization plane of a linearly polarized impinging light. Its angular dependence indicates the contribution of bulk photovoltaic effect to the short circuit photocurrent. It is also observed that a switchable drift photocurrent, originating from the depoling field of the ferroelectric and thus tunable (, Financial support from the Spanish Ministry of Science, Innovation and Universities, through the “Severo Ochoa” Programme for Centers of Excellence in R&D (FUNFUTURE CEX2019-000917-S), PID2020-118479RBI00 (AEI/FEDER, EU) (AEI/FEDER, EU), and PID2019- 107727RB-I00 (AEI/FEDER, EU) projects, and from Generalitat de Catalunya (2017 SGR 1377) is acknowledged. I.F. acknowledges RyC Contract RYC-2017-22531. Project supported by a 2020 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation. The theoretical component of this work (A.M.S. and A.M.R.) was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DE-FG02-07ER46431. Computational support was provided by the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy, Office of Science User Facility located at Lawrence Berkeley National Laboratory, operated under Contract No. DE-AC02- 05CH11231.The experimental and theoretical contributions of Y.S. are financially supported by China Scholarship Council (CSC), respectively with No. 201806410010. The work of Y.S. has been done as a part of her Ph.D. program in Materials Science at Universitat Autònoma de Barcelona., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

The orientation characteristics of the defect dipoles formed by acceptor ions ( Cu Nb ″ ′ ) and oxygen vacancies ( V O • • ) in CuO-doped (K 0.48 Na 0.52 ) 0.96 Li 0.04 Nb 0.805 Ta 0.075 Sb 0.12 O 3 piezoceramics were investigated. The ferroelectric and piezoelectric properties of the ceramics were obviously affected by the defect dipoles which are oriented along with the domains when poled under a dc electrical field of 3.5 kV/mm at the temperature near T C . The poled ceramics with 1.0–3.0 mol% CuO displayed strong asymmetric P - E loops and a large internal bias field ( E i ), 3.89–4.57 kV/cm, when polarized at the temperature near T C . The enhanced piezoelectric coefficient d 33 , 186–192 pC/N, was obtained for the ceramics poled near T C when 0.02 ≤ x ≤ 0.03. The ceramics with oriented defect dipoles showed a relatively good thermal stability of d 33 until 180 °C.

The Mn-doped (K0.48Na0.52)NbO3 (KNN) ceramics were annealed in air at 600–800 °C for 16 h and the effects of annealing on the structure as well as the ferroelectric and piezoelectric properties were investigated. The 3 mol% Mn-doped ceramics sintered at 1030–1080 °C were found to be melted after being annealed at 800 °C, and the melting temperature decreased with the increase of Mn content. The TC and TO–T of the ceramics with x ≤ 0.04 shifted significantly to higher temperature after annealing. The annealed ceramics doped with 1–4 mol% Mn possessed a large internal bias field (Ei) of 5.08–6.6 kV/cm, and larger d33 and Qm. The d33 of the 1 mol% Mn-doped ceramics increased from 109 to 124 pC/N after annealed at 600 °C, and the largest Qm, 145, was obtained when the 2 mol% Mn-doped ceramics were annealed at 700 °C.

The effects of lithium on the structure and electrical properties of (K0.48Na0.52)1-xLixNbO3-0.015CuO lead-free piezoelectric ceramics were investigated. The addition of Li led to a decrease in the orthorhombic–tetragonal phase transition temperature (TO-T) of the ceramics, and the coexistence of the orthorhombic and tetragonal phases was formed in the ceramics with 0.05 ≤ x ≤ 0.08 at room temperature. The secondary phase K3Li2Nb5O15 was detected when x ≥ 0.03, showing that the lithium solubility limit in KNN–Cu-xLi ceramics was lower than that in Cu-free systems. The relatively large leakage current in the ceramics with high Li content was induced by the defects derived from the formation of K3Li2Nb5O15, which led to the creation of the A-site vacancies and the oxygen vacancies because of the deficiency of A-site ions. The internal bias field (Ei) and the ferroelectric and piezoelectric properties of the ceramics were affected obviously by the increase of the defect concentration.