Yuuki Kitanaka

High-oxygen-pressure crystal growth of ferroelectric Bi4Ti3O12 single crystals

We have investigated the effects of high-oxygen-pressure crystal growth of ferroelectric Bi4Ti3O12 on the polarization properties along the a(b) axis. Domain observations by piezoresponse force microscope demonstrate that a small remanent polarization (Pr) for the crystals grown at 0.02MPa is attributed to the clamping of 90° domain walls by oxygen vacancies. The vacancy formation of Bi and O during crystal growth at high temperatures is suppressed at a higher oxygen pressure, leading to a larger Pr of 47μC∕cm2 for the crystals grown at 1MPa oxygen. High-oxygen-pressure sintering is proposed to be effective for obtaining Bi4Ti3O12-based devices with enhanced polarization properties.

Ferroelectric polarization and piezoelectric properties of layer-structured K0.5Bi4.5Ti4O15 single crystals

Single crystals of ferroelectric K0.5Bi4.5Ti4O15 with a Bi-layered structure were grown by the flux method, and the properties of polarization hysteresis, piezoelectric strain, and leakage current were investigated along the polar a axis at 25°C. K0.5Bi4.5Ti4O15 crystals exhibited a large remanent polarization of 31μC∕cm2, which suggests that K0.5Bi4.5Ti4O15 has the largest spontaneous polarization among Bi layer-structured ferroelectrics with four TiO6 layers in the perovskite blocks. Strain measurements showed that the piezoelectric strain constant was 31pm∕V.

Laser beam scanning microscope and piezoresponse force microscope studies on domain structured in 001-, 110-, and 111-oriented NaNbO3 films

NaNbO3 (NN) films were epitaxially grown on SrRuO3/(001), (110), and (111)SrTiO3 substrates, and these NN films were characterized by a laser beam scanning microscope and a piezoresponse force microscope. The 001-oriented NN film had antiferroelectric 90° domains with 100 and 010 polarization axes and 90° domain walls exhibiting piezoresponse. The piezoresponding domain walls would be induced by ferroelasticity. On the other hand, the 110- and 111-oriented NN films possessed 60° domains. The 60° domains of 110-oriented NN film were constructed by antiferroelectric 11¯0 domain and piezoresponding {101} and {011} domains. In the case of 111-oriented NN, three kinds of 60° domains (11¯0 and 01¯1, 01¯1 and 101¯, and 101¯ and 11¯0) were observed. The fine domains with piezoresponse were also observed in the mixed region with the three 60° domains. From the stress measurement, we found that the difference in the domain structure of 001-, 110-, and 111-oriented NN films depends not only on the orientation direct...

Defect control for polarization switching in BiFeO 3 single crystals

BiFeO₃ (BFO) single crystals were grown and the effects of Zn and Mn co-doping on the polarization and leakage current properties were investigated at 25°C for establishing materials design based on defect chemistry. Although Zn doping or Mn doping led to a deterioration in the properties, Zn-Mn co-doping led to a large remanent polarization (36 μC/cm²), a low coercive field (19 kV/cm), and a relatively low leakage current density (~10^-8 A/cm²). It is proposed that defect dipoles composed of Zn²⁺ and Mn⁴⁺ act as effective nucleation sites for ferroelectric domains during polarization switching in BFO crystals.

Crystal Growth and Characterization of (Bi0.5Na0.5)TiO3–BaTiO3 Single Crystals Obtained by a Top-Seeded Solution Growth Method under High-Pressure Oxygen Atmosphere

A single crystal of ferroelectric 0.88(Bi,Na)TiO3–0.12BaTiO3 (BNT–BT) solid solution with tetragonal P4mm structure was grown by a top-seeded solution growth (TSSG) method at a high oxygen pressure (PO2 ) of 0.9 MPa. The crystals exhibited a large remanent polarization (Pr) of 54 µC/cm2, which leads to a spontaneous polarization estimated to be 54 µC/cm2. The large Pr compared with that of crystals grown at PO2 = 0.1 MPa is suggested to originate from a low oxygen vacancy concentration. The high-PO2 TSSG method is demonstrated to be effective for obtaining large-sized, high-quality BNT–BT crystals.

Polarization and Piezoelectric Properties of High Performance Bismuth Sodium Titanate Single Crystals Grown by High-Oxygen-Pressure Flux Method

(Bi0.5Na0.5)TiO3 (BNT) single crystals were grown by flux method at a high oxygen pressure (PO2 ) of 1 MPa, and their polarization and piezoelectric properties were investigated. BNT single crystals exhibited a saturated polarization hysteresis with remanent polarizations (Pr) of 31 µC/cm2 along [100]c, 44 µC/cm2 along [110]c and 54 µC/cm2 along [111]c. These results show that spontaneous polarization of BNT is approximately 55 µC/cm2 or larger. Strain measurements of rhombohedral BNT crystals suggest that the polarization reversal along [111]c is achieved by non-180° Ps rotation whereas that along [100]c is accomplished by 180° Ps switching.

Switchable diode-effect mechanism in ferroelectric BiFeO3 thin film capacitors

We investigate the mechanism of a switchable diode behavior observed in ferroelectric SrRuO3/BiFeO3 (BFO)/SrRuO3 capacitors. We experimentally demonstrate that the switchable diode effect observed in the capacitors is induced by the polarization reversal in the BFO film. The conductivity in an Ohmic region in different oxidation states provides direct evidence that electron hole acts as the majority carrier, delivering p-type conduction. Density functional theory (DFT) calculations show that the p-type conduction arises from an unoccupied gap state of Fe4+ in an FeO5 pyramid which is derived from Bi vacancy. Our experimental and DFT study leads to the conclusion that the switchable diode effect originates from an asymmetric band bending in the top and bottom depletion layers modulated by ferroelectric polarization and oxygen vacancies.

Synchrotron Radiation Study on Time-Resolved Tetragonal Lattice Strain of BaTiO3 under Electric Field

The dynamic response of an intrinsic lattice strain in a tetragonal BaTiO3 single crystal to an electric field is investigated. Time-resolved diffraction measurement using high-energy synchrotron radiation enables us to detect the time dependence of the small change in the tetragonality of BaTiO3 during polarization reversal and piezoelectric vibration after a step like electric field antiparallel to the spontaneous polarization is applied to the sample.

Cooperative effect of oxygen-vacancy-rich layer and ferroelectric polarization on photovoltaic properties in BiFeO3 thin film capacitors

Photovoltaic (PV) properties of ferroelectric SrRuO3/BiFeO3/SrRuO3 (SRO/BFO/SRO) epitaxial thin-film capacitors are investigated. The experimental results of the markedly reduced PV response caused by the doping of Mn provide evidence that the PV properties originate not from the bulk-derived PV effect but from an interfacial band bending. We show that the capacitors having a defective layer composed of oxygen vacancies at the SRO/BFO interface exhibit a markedly large photocurrent. Our study demonstrates that a cooperative effect between the interface charges arising from ferroelectric polarization and the oxygen-vacancy-rich layer enhances the PV response in capacitor form in the BFO system.

Giant photovoltaic effect of ferroelectric domain walls in perovskite single crystals.

The photovoltaic (PV) effect in polar materials offers great potential for light-energy conversion that generates a voltage beyond the bandgap limit of present semiconductor-based solar cells. Ferroelectrics have received renewed attention because of the ability to deliver a high voltage in the presence of ferroelastic domain walls (DWs). In recent years, there has been considerable debate over the impact of the DWs on the PV effects, owing to lack of information on the bulk PV tensor of host ferroelectrics. In this article, we provide the first direct evidence of an unusually large PV response induced by ferroelastic DWs—termed ‘DW’-PV effect. The precise estimation of the bulk PV tensor in single crystals of barium titanate enables us to quantify the giant PV effect driven by 90° DWs. We show that the DW-PV effect arises from an effective electric field consisting of a potential step and a local PV component in the 90° DW region. This work offers a starting point for further investigation into the DW-PV effect of alternative systems and opens a reliable route for enhancing the PV properties in ferroelectrics based on the engineering of domain structures in either bulk or thin-film form.