Hiroko Yokota

The missing boundary in the phase diagram of PbZr1−xTixO3

PbZr(1-x)Ti(x)O3 (PZT) is one of the most important and widely used piezoelectric materials. The study of its local and average structures is of fundamental importance in understanding the origin of its high-performance piezoelectricity. Pair distribution function analysis and Rietveld refinement have been carried out to study both the short- and long-range order in the Zr-rich rhombohedral region of the PZT phase diagram. The nature of the monoclinic phase across the Zr-rich and morphotropic phase boundary area of PZT is clarified. Evidence is found that long-range average rhombohedral and both long- and short-range monoclinic regions coexist at all compositions. In addition, a boundary between a monoclinic (M(A)) structure and another monoclinic (M(B)) structure has been found. The general advantage of a particular monoclinic distortion (M(A)) for high piezoactivity is discussed from a spatial structural model of susceptibility to stress and electric field, which is applicable across the wide field of perovskite materials science.

The not so simple cubic structure of PbZr1 − xTixO3 (PZT): complex local structural effects in perovskites

High-resolution neutron diffraction on the important piezoelectric lead zirconate titanate (PZT) has found that oxygen disorder exists well into the cubic phase. This unexpected result shows that within this phase there persists a remnant of the tilted oxygen octahedra present within the room-temperature ferroelectric phase. The result is that the cubic phase, far from having a simple crystal structure, exhibits a more complex local structure than had hitherto been thought.

Li Concentration Dependence of Dielectric Responses of Quantum Relaxor K1-xLixTaO3

Single crystals of K1-xLixTaO3 (KLT) with different Li concentrations are grown by the self-flux method. Li concentrations in the crystals are determined by second-harmonic-generation microscope observations. Complex dielectric constants of KLT are measured in the temperature range from 20 K to room temperature and the frequency range from 5 Hz to 1 MHz. The results are analyzed using the Cole–Cole plot and the Vogel–Fulcher law. For x>4.4%, two semicircles of the Cole–Cole plot appear in a specific temperature region. These two relaxation processes can be explained by the different types of dynamics of Li dipole moments. The activation energy Ea and the characteristic frequency ν0 are determined from the fitting of the two models. It is found that Ea does not show strong Li concentration dependence. The origin of the phenomenon is discussed using a polar nanoregion picture.

Optical Second Harmonic Generation Microscopy as a Tool of Material Diagnosis

The second harmonic generation microscope (SHGM) constructs images of intensity distributions of SH waves produced by the interaction of fundamental waves with a polar material. We have developed this nonlinear optical microscope in order to make possible nondestructive, three-dimensional (3D) observations of various kinds of inorganic and organic materials. The SHGM can disclose also inverted domain structures of antiparallel spontaneous polarizations using the interference with the reference SH waves. The observation principle and several applications to structural characterizations of LiNbO3 and LiTaO3 quasi-phase matching devices, domain structure analyses of a relaxor/ferroelectric solid solution Pb(Zn1/3Nb2/3)O3-9%PbTiO3 at the morphotropic phase boundary, development of order parameter in a quantum paraelectric relaxor Li-doped KTaO3, and antiphase polar domain structures of muscle fibers and myofibrils are surveyed by stressing the high effectiveness of the SHGM as a tool of material diagnosis.

Examination of ferroelectric and magnetic properties of hexagonal ErFeO3 thin films

Hexagonal ErFeO3 (h-ErFeO3) thin films were fabricated by a pulsed laser deposition method. X-ray diffraction experiments confirmed that single-phase h-ErFeO3 thin films were successfully obtained and were c axis-oriented. Ferroelectricity was confirmed by D–E hysteresis loop measurement at room temperature and the magnitude of spontaneous polarization was estimated to be 200 nC/cm2. A weak ferromagnetism appeared below 120 K and dielectric constants started to exhibit magnetic field dependence. These observations demonstrate the coexistence of ferroelectricity and weak ferromagnetism in h-ErFeO3 thin films.

Critical Behavior of K1-xLixTaO3 with the Tri-Critical Concentration of Li under an Electric Field

Temperature dependences of the optical second harmonic generation in Li-doped KTaO 3 (KLT) with different Li concentrations x are measured to determine the coefficients in the Landau-Devonshire free energy function. The result specifies the tri-critical Li concentration x c . Temperature dependences of the dielectric constant of KLT with Li concentrations nearly located at x c are also measured under dc bias fields and the result suggests that these compounds lie in close vicinity of the line of critical end points. The tertiary phase diagram ( T , x , E ) of KLT is disclosed from the experiments.

Polar nature of stress-induced twin walls in ferroelastic CaTiO3

A compressive uniaxial mechanical stress is applied on ferroelastic CaTiO3 (CTO), and a change in the domain structure is observed under a polarization microscope and a second harmonic generation (SHG) microscope. New twin walls (TWs) appear perpendicular to the original TWs under stress. The SHG microscope observations and analyses confirm that this type of stress-induced TWs is polar, similar to the original TWs, and is crystallographically prominent with monoclinic symmetry m. A quantitative estimation of this stress-induced effect reveals that CTO is hard ferroelastic in the sense that the TW movement requires a large stress. A possible application of this phenomenon is discussed.

Local structures and temperature-driven polarization rotation in Zr-rich PbZr1-xTixO3

PbZr1-xTixO3, which has abundant structural variations in the corresponding physical properties, has been used in a large variety of applications. To understand the effect of the structure on its high-performance piezoelectric properties, its local and average structures are studied. Total scattering data have been obtained from high-energy synchrotron powder diffraction experiments at 20 K and 300 K. Using the reverse Monte Carlo method, information on cation displacements has been extracted from X-ray Pair Distribution Function data. This suggests that the local disorder of the B cations is mainly driven by thermal motion, while the local disorder of Pb is most likely caused by more complex factors, such as displacive disorder. Both rhombohedral and monoclinic local polarizations are observed in Zr-rich PZT, whose directions depend on temperature.

The appearance of weak ferromagnetism of hexagonal stabilized ErFeO 3 thin film

Hexagonal ErFeO3 thin films were fabricated by a pulsed laser deposition method. A high-field magnetization measurement and the Mössbauer spectroscopy under a magnetic field were conducted. It revealed that the magnetic moment of Fe ion lay almost in the ab plane of the thin film forming the 120° structure. The obtained spectrum was explained with the coexistence of two domains with the opposite cant direction of the net magnetic moments along the c axis. The increase of the quadrupole coupling constant with a magnetic field would suggest the displacement of Fe ion by a magnetic field.

Local Study on the Composition and Temperature Induced Structural Change of PZT

The crystal structure and its relationship with the high piezoelectric performance of PbZr1-xTixO3 (PZT) have been studied for years, especially near the morphotropic phase boundary (MPB). Now the bridging monoclinic Cm phase in the MPB separating the notgroup-subgroup-related R3c and P4mm has been widely accepted. However, in recent high-resolution powder diffraction experiments and Rietveld refinement analyses, the presence of the monoclinic Cm phase was found across the whole composition from the MPB to the rhombohedral region. Around the MPB, the structure seems to become more complicated, with a mixture of three phases[1]. It was also found that the local structure of PZT can be different from its average structure by examining the anisotropic displacement ellipsoids of both Pb and O: 1) the Pb atoms in the rhombohedral phase are displaced away from the threefold symmetry axes to create locally monoclinic symmetry that averages out to form an overall rhombohedral symmetry[2]; 2) at high temperature, the local effect of the low-temperature oxygen octahedra tilts remained even in the cubic phase[3]. In order to better study the local structures of PZT, we have carried out a series of high-resolution neutron diffraction experiments on both powder samples using Pair Distribution Function (PDF) analysis and single crystal samples using Diffuse Scattering (DS) analysis. The difference between the average and local structures is confirmed. The local distribution of the Pb displacements directions are plotted out for different compositions in PZT (x = 0.20, 0.30 and 0.40), which reveals on approaching MPB, the Pb atoms are tend to displace on one monoclinic mirror plane forming a macroscopic monoclinic phase. This is the first time that the local monoclinic short range order in Zr-rich PZT has been observed.