Defect chemistry in perovskite ferroelectrics—History, present status, and future prospects—

Abstract

Perovskite ferroelectrics such as BaTiO 3 and PbTiO 3 provide various functions arising from spontaneous polarization. Here, the history, present status, and future prospects of the defect chemistry for BaTiO 3 are described. Even for non-doped samples, unintentional impurities including Fe govern the defect chemistry and its related properties. By solving multiple nonlinear equations employing the thermodynamic data set reported, majority/minority defect-concentration diagrams are available not only in a high-temperature equilibrium state but also in a low-temperature quenched one. As an example, the defect diagrams of Ba(Ti,Ca)O 3 and Ba(Ti,Mn)O 3 are shown, and the roles of fixed valence (Ca) and variable-valence (Mn) acceptors are explained. I also introduce an example of the defect control in BaTiO 3 single crystals; an activation of visible-light photovoltaic effect, where two redox potentials derived from Fe 2+ and Fe 3+ act as scaffolds for generating electronhole pairs.

Bulk photovoltaic effect in ferroelectrics

“…gap-state engineering, can activate the visible-light PV response. The impact of chemical doping on the optical, [155][156][157][158][159][160] structural, 161,162) and electrical [163][164][165][166][167][168][169][170][171][172][173] properties of materials has been intensively studied by many researchers. Chemical doping of conventional ferroelectric oxides such as LiNbO 3 , 2,3,76) BaTiO 3 , 75,174,175) and Pb-based perovskites 85,87,176) effectively enhances the PV response.…”

Ferroelectric polarization of tetragonal BiFeO3—an approach from DFT calculations for BiFeO3–BaTiO3 superlattices—

“…Moreover, BiFeO 3 has a Curie temperature (T C ) of 830 °C, 47,48) which is much higher than that of PbTiO 3 (495 °C49) ) Barium titanate, BaTiO 3 , play an essential role for ceramic capacitor applications 50,51) and also for ferroelectricderived functions. [52][53][54][55][56] In tetragonal P4mm symmetry, Ti and Ba are displaced cooperatively along the polar c axis, leading to a P s of 26-29 μC cm −2 . 57,58) For (1−x) BiFeO 3 -x BaTiO 3 ceramics, structural analysis showed that an x increase changes a crystal symmetry from R3c rhombohedral to pseudo-cubic at ca.…”

Ferroelectric photovoltaic tensor in visible-light-active Fe-doped BaTiO3single crystals

“…9,10) Moreover, the polar lattice of BaTiO 3 gives rise to an unparalleled dielectric function that can be utilized for multilayer ceramic capacitors (MLCCs). 11,12) The widespread usage of MLCCs more or less relies on the defect chemistry [13][14][15][16][17][18][19][20][21] established for SrTiO 3 and BaTiO 3 . At present, intensive efforts [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] have been made to explore nanostructured materials and devices using BaTiO 3 -based materials, whereas the deep understanding of defects is far from complete for Pb(Zr, Ti)O 3 , [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] (Bi, Na)TiO 3 , …”

“…The well-established defect chemistry in BaTiO 3 17,19,98,99) enables us to develop a defect-induced function. Recently, we have proposed successive redox mediated PV effect that can activate visible-light response.…”

“…In (b), σ in the intermediate pO 2 region is extrapolated based on the relations of log n/log = -pO2 1/4 and log p/log = pO 2 1/4. It is considered that the total conductivity in this pO 2 region is dominated by oxide-ion conduction19) (not shown). The reduced sample was annealed at pO 2 900 °C = 10 −23 atm.…”