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.     

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.     

Point defects in ferroelectrics with perovskite structure

First-principle calculations of the geometry and electronic structure of lattice defects (substituents, vacancies, and binary complexes) in perovskites are considered. A supercell method and modifications of a cluster method for simulating point lattice defects in insulators are discussed. Experimental data on dielectric relaxation in magnesium- and manganese-doped strontium titanate and possible mechanisms of the formation of reorientable polar defects are considered.     

核壳结构对弛豫铁电体介电行为的影响

介电弥散和介电隔离率的温度非线性关系是弛豫铁电体的主要特征. 通过对掺杂成分以线性梯度递减的核壳结构进行热力学函数分析, 认为核壳结构能够在低温区保持较高的介电常数, 但不能导致介电隔离率与温度的非线性关系. 通过对不同浓度掺杂的铁电体扩散相变的比较, 认为掺杂浓度会影响晶粒掺杂成分的不均匀性, 在较宽的分布条件下会导致介电隔离率与温度的非线性关系. 因而在介电常数的峰值温度区域, 顺电相与铁电相的晶粒共存. 温度变化会影响两相比例及铁电畴的变化, 从而导致弛豫铁电体的介电弥散性. 核壳结构会增大介电弥散性. 铁电陶瓷的掺杂物种类、掺杂物浓度和烧结温度均会影响核壳结构的成分不均匀性和介电弥散性.     

93Nb NMR spin echo spectroscopy in single crystal NbSe3

We report electric field induced phase displacements of the charge density wave (CDW) in a single crystal of NbSe3 using 93Nb NMR spin-echo spectroscopy. CDW polarizations in the pinned state induced by unipolar and bipolar pulses are linear and reversible up to at least E = (0.96)ET. The polarizations have a broad distribution extending up to phase angles of order 60 degrees for electric fields close to threshold. No evidence for polarizations in excess of a CDW wavelength or for a divergence in polarization near ET are observed. The results are consistent with elastic depinning models, provided that the critical regime expected in large systems is not observable.     

Slow polarization processes in relaxor ferroelectrics

The general features of the slow polarization kinetics in relaxor ferroelectrics, observed and studied by us in the specific example of crystals of barium-strontium niobate (SBN) solid solutions of various compositions and of lead magnesium niobate (PMN), are considered. The dielectric hysteresis loops and spectra of the polarization relaxation time distribution in quasi-static and static electric fields were found to reveal characteristic anomalies related to a random internal electric field in the bulk of a relaxor ferroelectric. Such a field caused by structural disorder accounts for the anomalously broad spectrum of potential barriers for domain walls. The part played by free charge carriers in the formation of giant barriers is demonstrated. The paper presents some quantitative data characterizing the specific features of the structure and physical properties of the relaxors.     

Relationship between local structure and relaxor behavior in perovskite oxides

Despite intensive investigations over the past five decades, the microscopic origins of the fascinating dielectric properties of ABO3 relaxor ferroelectrics are currently poorly understood. Here, we show that the frequency dispersion that is the hallmark of relaxor behavior is quantitatively related to the crystal chemical characteristics of the solid solution. Density functional theory is used in conjunction with experimental determination of cation arrangement to identify the 0 K structural motifs. These are then used to parametrize a simple phenomenological Landau theory that predicts the universal dependence of frequency dispersion on the solid solution cation arrangement and off-center cation displacements.     

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.     

Anomalies of polarization processes in relaxor ferroelectrics

The anomalous polarization of relaxor ferroelectrics (relaxors), such as solid solutions of barium-strontium niobate (SBN) and lead magnesium niobate (PMN), is studied in ac low-frequency (up to 10^?4 Hz) and dc electric fields. The results of studying dielectric hysteresis loops, polarization relaxation, the coercive-field distribution spectrum, nonlinear polarization for various scales of inhomogeneities, and polarization in a photosensitive SBN relaxor subjected to illumination are presented. All of the anomalous polarization properties of relaxors substantially distinguish them from ordinary homogeneous ferroelectrics and serve as signs and a measure of structural disorder. This article presents a brief review of our recent studies and some new results obtained as a result of their analysis.     

93Nb NMR chemical shift scale for niobia systems

⁹³Nb solid-state NMR spectra of a series of inorganic niobates with Nb in different oxygen coordination environments were measured. For all studied compounds the chemical shielding and quadrupole tensor parameters were determined using conventional and ultrahigh field NMR facilities, ultrahigh speed MAS, DQ STMAS, solid-echo and computer modeling. It has been demonstrated that the ⁹³Nb isotropic chemical shift is sensitive to the coordination number of Nb sites. For the first time the ⁹³Nb NMR chemical shift scale for NbO_x polyhedra in solid materials has been proposed: for four-coordinated Nb sites, the isotropic shifts occur from −650 to −950 ppm; five-coordinated Nb sites have the isotropic shifts in the range of –900 to –980 ppm; for six-coordinated Nb sites the isotropic shifts vary from −900 to −1360 ppm; the shifts from −1200 to −1600 ppm are typical for seven-coordinated Nb sites; for eight-coordinated Nb sites the shifts are higher than −1400 ppm. The possible correlation between the value of the isotropic chemical shift and the ionic character of the NbO_x–MO_y polyhedra association has been suggested. The magnitude of the ⁹³Nb quadrupole coupling constant depends on the local symmetry of Nb sites and may vary from hundreds of kHz to hundreds of MHz.     

Field cooled and zero field cooled 207Pb NMR and the local structure of relaxor PbMg1/3Nb2/3O3

An isotropic 207Pb NMR spectrum corresponding to the glassy matrix with spherical shell type Pb shifts from the cubic sites, as well as an anisotropic spectral component corresponding to polar nanoclusters with a Pb shift parallel to the [111] direction, have been observed in a PbMg(1/3)Nb(2/3)O3 (PMN) single crystal. This represents a microscopic confirmation of the model of relaxors first proposed by Burns and Dacol. A sudden increase in the intensity of the anisotropic cluster line is seen for electric fields larger than the critical field around 210 K. This demonstrates the occurrence of an orientational percolation type transition to the field-induced ferroelectric phase with about 50% of the Pb nuclei still remaining in the spherical glass matrix. A similar though smaller increase of the intensity of this line is also seen in the zero field cooled data, demonstrating that relaxor PMN is an incipient ferroelectric.     

Ab initio calculations of Born charges in ferroelectrics with a perovskite structure

A method has been proposed for calculating Born effective charges in compounds with a cubic perovskite structure. The method is based on the first-principles calculation of individual contributions from the short-range interaction and the intercell dipole-dipole interaction to the Born tensor Z _ii (s) for crystalline dielectrics. It has been shown that the contribution from the short-range interaction Z _ii ^sr (s) to the Born tensor components can be derived from ab initio calculations performed for polyatomic clusters. The results of the calculations of the short-range interactions Z _ii ^sr (s) for the cubic phases of the BaTiO₃, SrTiO₃, CaTiO₃, PbTiO₃, BaZrO₃, PbZrO₃, KNbO₃, and KTaO₃ compounds with the use of the electronic structure calculations within the Hartree-Fock approximation and the density functional theory are presented. For the BaTiO₃, SrTiO₃, CaTiO₃, PbTiO₃, KNbO₃, and KTaO₃ compounds, the components of the complete Born tensor have been also calculated. The obtained values of Z _ii (s) are in good agreement with the results of the calculations in terms of the linear response theory and the Berry phase approach.     

The lineshape of inelastic neutron scattering in relaxor ferroelectrics

We show that a microscopic reason for the steep drop of the optical phonon branch into an acoustic one (the so-called waterfall effect) in relaxor ferroelectrics may be the coupling of phonons with defects and impurities of different kinds, which is always present in relaxors. Namely, we do not specify the type of impurities but rather represent them as an ensemble of so-called two-level systems (TLS). This approach makes it possible to trace the evolution of the “waterfall” with temperature and the TLS concentration. To facilitate the planning of experiments on inelastic neutron scattering, we present a modification of the so-called Latin hypercube sampling method, which, based on some significance criteria, allows one to perform measurements that are highly significant for elucidating the physical nature of, e.g., phonon dispersion laws in relaxor ferroelectrics.     

Anisotropic dielectric function in polar nanoregions of relaxor ferroelectrics

The Letter suggests treating the infrared reflectivity spectra of single crystal perovskite relaxors as fine-grained ferroelectric ceramics: locally frozen polarization makes the dielectric function strongly anisotropic in the phonon frequency range and the random orientation of the polarization at nanoscopic scale requires taking into account the inhomogeneous depolarization field. Employing a simple effective medium approximation (the Bruggeman symmetrical formula) turns out to be sufficient for reproducing all principal features of room temperature reflectivity of Pb(Mg(1/3)Nb(2/3))O3. One of the reflectivity bands is identified as a geometrical resonance entirely related to the nanoscale polarization inhomogeneity. The approach provides a general guide for systematic determination of the polar mode frequencies split by the inhomogeneous polarization at the nanometer scale.     

弛豫铁电体弥散相变的玻璃化特性研究

根据弛豫铁电材料在相变区域的介电弥散行为和玻璃化 液体材料在过冷状态下黏度与温度的行为所共同满足的Vogel-Fulcher 函数关系, 分析了施主替代钛酸钡系列陶瓷的缺陷补偿原理, 通过引入玻璃化液体的构型熵概念, 研究了弛豫铁电材料中钛阳离子缺陷作用势的温度关系, 得到了如下结论: 施主掺杂含量的增加导致了无序度的增加, 钛离子缺陷浓度的增大和平均极性区域尺寸的减小; 在构型熵满足Vogel-Fulcher 函数关系的条件下, 温度越低, 钛离子缺陷作用的范围越大, 极化区域也越大. 缺陷作用的范围随温度的变化导致了弛豫铁电材料的弥散性. 温度下降到一定程度, 冻结效应发生, 介电弥散现象消失.