Local structure in perovskite relaxor ferroelectrics: high-resolution Nb 3QMAS NMR

Solid solutions of (1'-x)Pb(Mg(1/3)Nb(2/3))O3xPb(Sc(1/2)Nb(1/2))O3 (PMN/PSN) have been investigated using high-resolution 93Nb 3-quantum magic-angle spinning nuclear magnetic resonance experiments (3QMAS NMR). In previous MAS NMR investigations, the local B-cation ordering in these relaxor ferroelectric solid solutions was quantitatively determined. However, in conventional one-dimensional MAS spectra the effects of chemical shifts and quadrupole interaction are convoluted; this, in addition to the insufficient resolution, precludes reliable extraction of the values of isotropic chemical shift and quadrupole coupling product. In the current 3QMAS investigation, 93Nb spectra are presented for concentrations x=0, 0.1, 0.2, 0.6, 0.7, and 0.9 at high magnetic field (19.6 T) and fast sample spinning speed (35.7 kHz). Seven narrow peaks and two broad components are observed. The unique high-resolution of the two-dimensional 3QMAS spectra enables unambiguous and consistent assignments of spectral intensities to the specific 28 nearest B-site neighbor (nBn) configurations, (NMg, NSc, NNb) where each number ranges from 0 to 6 and their sum is 6. It is now possible to isolate the isotropic chemical shift and quadrupole coupling product and separately determine their values for most of the 28 nBn configurations. The isotropic chemical shift depends linearly on the number of Mg2+ cations in the configuration; delta iso CS=(13.7 +/- 0.1)NMg-970 +/- 0.4 ppm, regardless of the ratio NSc/NNb. For the seven Nb5+-deficient configurations (NMg, 6-NMg, 0) and the pure niobium configuration (0, 0, 6), the quadrupole coupling products (and hence the electric field gradients) are small (PQ approximately 6-12 MHz) and for the remaining configurations containing small, ferroelectric active Nb5+ ions, the quadrupole coupling products are significantly larger (PQ approximately 40 MHz), indicating larger electric field gradients.     

Local structure in perovskite relaxor ferroelectrics: high-resolution 93Nb 3QMAS NMR

Solid solutions of (1'-x)Pb(Mg(1/3)Nb(2/3))O3xPb(Sc(1/2)Nb(1/2))O3 (PMN/PSN) have been investigated using high-resolution 93Nb 3-quantum magic-angle spinning nuclear magnetic resonance experiments (3QMAS NMR). In previous MAS NMR investigations, the local B-cation ordering in these relaxor ferroelectric solid solutions was quantitatively determined. However, in conventional one-dimensional MAS spectra the effects of chemical shifts and quadrupole interaction are convoluted; this, in addition to the insufficient resolution, precludes reliable extraction of the values of isotropic chemical shift and quadrupole coupling product. In the current 3QMAS investigation, 93Nb spectra are presented for concentrations x=0, 0.1, 0.2, 0.6, 0.7, and 0.9 at high magnetic field (19.6 T) and fast sample spinning speed (35.7 kHz). Seven narrow peaks and two broad components are observed. The unique high-resolution of the two-dimensional 3QMAS spectra enables unambiguous and consistent assignments of spectral intensities to the specific 28 nearest B-site neighbor (nBn) configurations, (NMg, NSc, NNb) where each number ranges from 0 to 6 and their sum is 6. It is now possible to isolate the isotropic chemical shift and quadrupole coupling product and separately determine their values for most of the 28 nBn configurations. The isotropic chemical shift depends linearly on the number of Mg2+ cations in the configuration; delta iso CS=(13.7 +/- 0.1)NMg-970 +/- 0.4 ppm, regardless of the ratio NSc/NNb. For the seven Nb5+-deficient configurations (NMg, 6-NMg, 0) and the pure niobium configuration (0, 0, 6), the quadrupole coupling products (and hence the electric field gradients) are small (PQ approximately 6-12 MHz) and for the remaining configurations containing small, ferroelectric active Nb5+ ions, the quadrupole coupling products are significantly larger (PQ approximately 40 MHz), indicating larger electric field gradients.     

Calcium substituting B-site in relaxor ferroelectrics with perovskite structure probed by chemical ordering

Chemical ordering of Ca²⁺ doped 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ ceramics were investigated by dielectric spectra, TEM diffraction and A_1g mode in Raman spectra. It is found degree of relaxor behavior increases first, and then decreases. It conflicts the prediction Ca²⁺ substitutes for A-site ion Pb²⁺ according to crystal chemistry theory. In this letter, a new mechanism that Ca²⁺ substitutes for B-site ions has been proposed, which satisfactorily explained change of chemical ordering. It exhibits strong evidence doped ions with larger ionic radius (Ca²⁺) are quite possibly substitute much smaller ones (Nb⁵⁺ or Ti⁴⁺) in B-site rather than all substitute larger A-site ion in relaxor ferroelectrics.     

Band offsets in heterojunctions formed by oxides with cubic perovskite structure

A number of recent discoveries on heterostructures formed by oxides suggest the emergence of a new direction in microelectronics, the oxide electronics. In the present work, band offsets in nine heterojunctions formed by titanates, zirconates, and niobates with the cubic perovskite structure are calculated from first principles. The effect of strain in contacting oxides on their energy structure; the GW corrections to the band edge positions resulting from many-body effects; and the conduction band edge splitting resulting from spinorbit coupling are consistently taken into account. It is shown that the neglect of the many-body effects can cause errors in the determination of the band offsets, reaching 0.36 eV. The fundamental inapplicability of the transitivity rule often used to determine the band offsets in heterojunctions by comparing the band offsets in a pair of heterojunctions formed by the components of the heterojunction under study with a third common component is demonstrated. The cause of the inapplicability is explained.     

Coupled-optical-phonon-mode electron scattering in perovskite oxides

Conductivity and Hall effect measurements were made on KTa_1?xNb_xO₃(x= 0.00,0.04,0.11 and 0.16) in the intermediate temperature range (77–300 K). It was found that the temperatures dependence of the Hall mobility obeyed a semiempirical relationship of Wemple et al. in this temperature range. The main scattering mechanism is proposed to be longitudinal optical phonons which are coupled to the “soft” transverse optical phonons associated with ferroelectricity.     

Resistance switching memory in perovskite oxides

The resistance switching behavior has recently attracted great attentions for its application as resistive random access memories (RRAMs) due to a variety of advantages such as simple structure, high-density, high-speed and low-power. As a leading storage media, the transition metal perovskite oxide owns the strong correlation of electrons and the stable crystal structure, which brings out multifunctionality such as ferroelectric, multiferroic, superconductor, and colossal magnetoresistance/electroresistance effect, etc. The existence of rich electronic phases, metal–insulator transition and the nonstoichiometric oxygen in perovskite oxide provides good platforms to insight into the resistive switching mechanisms.     

Relaxation processes in the paraelectric phase of ceramic oxides with a perovskite structure

Solid solutions of barium, strontium, and lead titanate ceramics with a perovskite structure are studied using impedance spectroscopy in the frequency range 10²–10⁶ Hz at temperatures from 20 to 450°C. The experimental data represented by the dispersion of the electric modulus are compared with the results of calculations performed for different mechanisms of the non-Debye relaxation in terms of the Havrilyak-Negami empirical formulas and the Jonscher universal dielectric response with the aim of determining a more adequate approach. The activation energy of relaxation processes in the paraelectric phase of the samples under investigation are calculated.     

New ferromagnetic oxides derived from the perovskite structure prepared under high pressures

Abstract

Using a simple approach of ferromagnetic interactions through a 180° superexchange in a perovskite lattice a composition has been set-tip : La₂MnIrO₆. A band-structure calculation having confirmed such a simple approach, the compound was prepared and characterized. Structural and magnetic studies have confirmed the ferromagnetic behaviour of such an oxide. This methodology is now used to define a new class of ferromagnetic oxides.     

Spin and orbital excitations in perovskite Mn oxides

An effective Hamiltonian of perovskite type LaMnO₃ is derived by taking into account the degeneracy ofe _g orbitals, Hund coupling and strong Coulomb interactions betweend-electrons in Mn ions. Spin and orbital excitations are calculated by using the effective Hamiltonian.     

Universal octahedral-site distortion in orthorhombic perovskite oxides

Lattice parameters of the orthorhombic perovskites RMO3 (R=rare earth, M=Ti, V, ..., Ni, and Ga) have been simulated based on the ionic M-O bond length and rigid MO6/2 octahedra. Comparison with experimental data shows that the long-standing lattice-parameter anomaly generally found for the larger R3+ ions in these families is caused by a structural feature that is not revealed by the geometric tolerance factor widely used for the perovskites.     

Probabilistic models of a local structure of perovskite solid solutions

The magnitudes of the structure amplitudes (|F(h, k, l)|) of the X-ray diffraction reflections measured for a PbIn_1/2Nb_1/2O₃ single crystal are compared with those calculated with the use of a sequence of probability-theoretical models constructed for the structure of the PbIn_1/2Nb_1/2O₃ compound. It is demonstrated that there is an optimum accuracy of the probabilistic models above which the R factor and the zeroth electron density synthesis deteriorate. A hypothesis is put forward that the optimum accuracy of the model is determined by the error in the measurement of the experimental values used in the comparison with theory.     

Polarization-based adjustable memory behavior in relaxor ferroelectrics

The irreversible decay of the spontaneous polarization above the phase-transition temperature is a limiting factor in any application of ferroelectric crystals. Here we show that electric fields applied at high temperatures induce a preferred direction in the crystal which is stable even after repeated heating and cooling through the phase transition. This preference in direction leads to a reorientation of domains in the ferroelectric phase. We use pyroelectric measurements to show that the directional preference originates from internal charge carriers interacting with domain walls.     

钙钛矿型氧化物非常规铁电研究进展

钙钛矿型氧化物因具有丰富的磁性、铁电、力学和光学等诸多功能属性,在电子信息通信材料器件领域中有广阔的应用前景.在各种物理性质之中,铁电极化因其产生机制多样,并能与磁性和晶格应变相互耦合形成多铁性等特点,近十多年来一直被作为凝聚态物理研究的国际热点问题.与以自发极化作为初级序参量的常规铁电材料不同,非常规铁电材料中的铁电极化是被其他的序参量诱导而产生的.本综述围绕无机钙钛矿型氧化物非常规铁电体的研究进展进行了总结.回顾了该体系经典唯象理论和原子尺度的微观模型,有序排列的人工钙钛矿超晶格型结构,以及稀土正铁氧体单晶的反铁磁畴壁结构中非常规铁电的极化强度大小及其诱导机制,为系统理解非常规铁电提供了理论途径.     

The polarizability model for ferroelectricity in perovskite oxides

This article reviews the polarizability model and its applications to ferroelectric perovskite oxides. The motivation for the introduction of the model is discussed and nonlinear oxygen ion polarizability effects and their lattice dynamical implementation outlined. While a large part of this work is dedicated to results obtained within the self-consistent-phonon approximation, nonlinear solutions of the model are also handled, which are of interest to the physics of relaxor ferroelectrics, domain wall motions, and incommensurate phase transitions. The main emphasis is to compare the results of the model with experimental data and to predict novel phenomena.     

Metal-insulator transition in perovskite oxides: A low-temperature perspective

A review is presented of recent experimental results of low temperature studies of composition driven metal-insulator transition in perovskite oxides of the ABO₃ class. The evolution of physical properties like conductivity, tunnelling, density of states and magnetoconductivity has been studied at low temperatures (T < 10 K) because composition is varied so that the sample goes from the metallic state to the critical region through a weakly localized region. The results show an interesting interplay of disorder and correlation effects. Special attention has been paid to the critical region which is marked by very low conductivity and dσ/dT>0. In this region the following important observations emerge. (1) It is possible to have a metallic state [σ(T = 0) = σ ₀ ≠ 0] with σ ₀/σ _Mott ? 1 and dσ/dT > 0. (2) At T < 2 K the conductivity follows a power law σ ～T^ν , where the exponent can be related to the finite frequency response of a zero temperature phase transition. (3) The Coulomb interaction plays a major role and evidence from tunnelling experiments suggests that a gap in the density of states at the Fermi level opens up continuously as the critical region is approached from the metallic side. (4) The magnetoconductivity is relatively smaller in the metallic and the weakly localized region (except the hole-doped LaMnO₃ and related systems) but becomes very large at the critical region.     

Ferroelectric relaxor behavior in hafnium doped barium-titanate ceramic

Temperature and frequency dependence of the real (ε′) and imaginary (ε″) parts of the dielectric permitivity of cubic Ba(Ti_0.7Hf_0.3)O₃ ceramic has been studied in the temperature range of 100 K to 350 K at the frequencies 0.1 kHz, 1 kHz, 10 kHz, 100 kHz for the first time. Diffuse phase transition and frequency dispersion is observed in the permittivity-vs-temperature plots. This has been attributed to the occurrence of relaxor ferroelectric behavior. The observed relaxor behavior has been quantitatively characterized based on phenomenological parameters. A comparison with the Zr doped BaTiO₃ has also been presented. For Hf doped samples transmission electron microscopy (TEM) characterization do show the presence of highly disordered microstructure at length scales of few tens of nano-meters.     

Ultrafast photoelectric effects and high-sensitive photovoltages in perovskite oxides and heterojunctions

Perovskite oxides and heterojunctions have attracted much attention due to their multifunctional properties of electricity and optics and magnetic as well as the very good chemical and thermal stability. In this brief review, we describe the novel progress of researches in the optical characteristic, including ultrafast photoelectric effects of picosecond order in perovskite oxide single crystals, thin-films and heterojunctions, high-sensitive photovoltages, the enhanced transient lateral photovoltages in perovskite oxide thin-films and heterojunctions, and the high-sensitive ultraviolet (UV) photodetectors based on perovskite oxides. The recent advances present in this paper not only could stimulate theoretical studies on the mechanism but also would open up the possibilities in the developments of optoelectronic devices based on perovskite oxides and heterojunctions.