Translational Boundaries as Incipient Ferrielectric Domains in Antiferroelectric PbZrO_{3}.

In the archetypal antiferroelectric PbZrO_{3}, antiparallel electric dipoles cancel each other, resulting in zero spontaneous polarization at the macroscopic level. Yet in actual hysteresis loops, the cancellation is rarely perfect and some remnant polarization is often observed, suggesting the metastability of polar phases in this material. In this work, using aberration-corrected scanning transmission electron microscopy methods on a PbZrO_{3} single crystal, we uncover the coexistence of the common antiferroelectric phase and a ferrielectric phase featuring an electric dipole pattern of ↓↑↓. This dipole arrangement, predicted by Aramberri et al. to be the ground state of PbZrO_{3} at 0 K, appears at room temperature in the form of translational boundaries. The dual nature of the ferrielectric phase, both a distinct phase and a translational boundary structure, places important symmetry constraints on its growth. These are overcome by sideways motion of the boundaries, which aggregate to form arbitrarily wide stripe domains of the polar phase embedded within the antiferroelectric matrix.

A Comparative Study of Eu3+-Doped Sillenites: Bi12SiO20 (BSO) and Bi12GeO20 (BGO).

The spectroscopic properties of Eu3+-doped Bi12SiO20 (BSO) were investigated and compared with that of Eu3+-doped Bi12GeO20 (BGO). The emission properties and the absorption spectra have been measured at 10 K as well as at 300 K (room temperature). Luminescence was detected due to the direct excitation of the 5D0 level of Eu3+, as well as through the excitation of the 5D1 level. The Judd-Ofelt theoretical framework was used to compute the radiative lifetimes (τ) and the omega parameters (Ωλ). The electric dipole transition probabilities, asymmetry ratios (R), along with the branching ratios (ß) were also determined based on the obtained experimental data. The strongest detected luminescence belongs to the 5D0 → 7F0 transition observed at 578 nm, similar to the BGO sillenite. Reasons for the major presence of the 5D0 → 7F0 emission, theoretically forbidden by the Judd-Ofelt Theory, were investigated and compared with that of the BGO sillenite. Obtained results showed that the strong 5D0 → 7F0 line is also present in Eu:BSO, indicating that this is a feature of the entire sillenite family and not just Eu:BGO.

Ultrahigh Piezoelectric Strains in PbZr1-xTixO3 Single Crystals with Controlled Ti Content Close to the Tricritical Point.

Intensive investigations of PbZr1-xTixO3 (PZT) materials with the ABO3 perovskite structure are connected with their extraordinary piezoelectric properties. Especially well known are PZT ceramics at the Morphotropic Phase Boundary (MPB), with x~0.48, whose applications are the most numerous among ferroelectrics. These piezoelectric properties are often obtained by doping with various ions at the B sites. Interestingly, we have found similar properties for undoped PZT single crystals with low Ti content, for which we have confirmed the existence of the tricritical point near x~0.06. For a PbZr0.95 ± 0.01Ti0.05∓ 0.01O3 crystal, we describe the ultrahigh strain, dielectric, optical and piezoelectric properties. We interpret the ultrahigh strain observed in the region of the antiferroelectric-ferroelectric transition as an inverse piezoelectric effect generated by the coexistence of domains of different symmetries. The complex domain coexistence was confirmed by determining optical indicatrix orientations in domains. The piezoelectric coefficient in this region reached an extremely high value of 5000 pm/V. We also verified that the properties of the PZT single crystals from the region near the tricritical point are incredibly susceptible to a slight deviation in the Ti content.

Long-Term Isothermal Phase Transformation in Lead Zirconate.

Lead zirconate PbZrO3 has been the subject of research interest for several dozen years. Recently, even its antiferroelectric properties have started to be questioned, and many researchers still deal with the so-called intermediate phase below Curie temperature (TC), whose existence is not fully understood. It turns out that PbZrO3 doped with Nb exhibits below TC phases with complex domain structures. One of them undergoes self-organization taking place at a constant temperature, and transforms, after several minutes, into a lower phase. This isothermal transition was investigated through dielectric, pyroelectric current and Raman scattering measurements. Discontinuities accompanied it in the permittivity and pyroelectric current. The obtained Raman spectra proved that those discontinuities are strictly linked with the isothermal transition between two intermediate phases. The ordering process in lead sublattice stimulated by thermal fluctuations is discussed as a driving force for this peculiar phenomenon.

Vibrational and optical studies of a nonlinear optical crystal, Cs2Bi2O(Ge2O7).

Single crystals of a non-centrosymmetric and polar Cs2Bi2O(Ge2O7) compound were grown from melt. This material was characterized by polarized Raman scattering at room temperature. The comparison to the polycrystalline IR and Raman spectra has been made and assignment of all observed modes to the respective vibrations and their symmetry has been proposed. Our results show that this crystal is promising SRS-active nonlinear optical material for up- and down-Raman laser-frequency converters with the most intense lasing line at 481 cm-1. Optical studies show that Cs2Bi2O(Ge2O7) exhibits intense absorption bands with peaks maxima at 260 and 293 nm (77 K) typical for 1S0→1,3P1 transitions of Bi3+ ions, which are parity-allowed due to spin-orbit coupling as well as unexpected band centered at 360 nm that was assigned to intervalence charge transfer transition.

Complex structure and Mössbauer effect observed in the course of phase transitions in PbZr0.72Sn0.28O3 single crystal.

The Mössbauer effect was used to study the structural transitions in a PbZr0.72Sn0.28O3 single crystal. Two kinds of quadrupole splittings were registered and connected with two different environments of the Sn ion occupying the center of SnO6 octahedra. What is responsible for the existence of these two environments is the disorder in tilts of oxygen octahedra and antiparallel shifts of pairs of lead ions. Both disorders decide on structural transitions which the PbZr0.72Sn0.28O3 single crystal undergoes. The two kinds of quadruple splittings that have been observed do not disappear at temperatures far above phase transitions. This indicates that the structure of the paraelectric phase is locally non-centrosymmetric and confirms pre-transitional effects previously reported for Sn doped PbZrO3 single crystals.

Crystal growth, IR specular reflectance and polarized Raman studies of LiNa5Mo9O30 polar single crystal.

A non-centrosymmetric, polar LiNa5Mo9O30 single crystal was grown using Kyropoulos technique and characterized by polarized Raman scattering and polarized infrared specular reflection spectroscopy at room temperature. The comparison to the polycrystalline spectra has been made and assignment of all observed modes to the respective vibrations and their symmetry has been proposed. Based on the four parameter model the infrared reflection spectra were analyzed and the LO-TO splitting has been determined for the observed modes. Stronger anharmonicity of bands assigned to stretching vibrations of the Mo-O⋯Mo bridges has been evidenced. Our results show that this crystal is attractive and promising SRS-active nonlinear optical material for up- and down-Raman laser-frequency converters with the most intense lasing lines at 947 and 884 cm-1. Dielectric permittivity studies in the frequency domain exhibit two anomalies that are assigned to thermally activated relaxation and conductivity processes. The observed increase in electrical conductivity above 400 K is dominated by ionic contribution and change of the conductivity mechanism.

Enhanced 1.5 µm emission of Er3+-doped multifunctional Bi2ZnOB2O6 microcrystals.

The efficiency of the 1.5 µm emission associated with the 4I13/2 → 4I15/2 transition of Er3+ ions of a series of Er3+ and Yb3+/Er3+-doped Bi2ZnOB2O6 microcrystalline powders was investigated. Bi2ZnOB2O6 is an excellent nonlinear optical material as well as a good host matrix for luminescent rare-earth ions. The investigated powders were synthesized by means of the modified Pechini method and their orthorhombic structure with Pba2 space group were confirmed by XRD measurements. The vibrational properties of Bi2ZnOB2O6:Yb3+/Er3+ were studied using µ-Raman spectroscopy. The low phonon energy of the Bi2ZnOB2O6 matrix allows effective phonon assisted energy transfer between rare-earth ions and/or multiphonon relaxation processes of rare-earth ions. It was revealed that the intensity of the 1.5 µm emission under 980 nm excitation increased with increasing Er3+ concentration (from 0.5 to 3.0 at%) for Bi2ZnOB2O6:Er3+, while for co-doped Bi2ZnOB2O6:Yb3+/Er3+ systems a significant increase in this emission was observed for the optimal Yb3+/Er3+ concentration (1.5/0.5 at%). Moreover, the intensity of the 1.5 µm emission decreases with increasing temperature for all investigated samples. Additionally, Bi2ZnOB2O6:Yb3+/Er3+ powders exhibit effective up-conversion luminescence in the visible range under 980 nm excitation. In the up-conversion spectra of Bi2ZnOB2O6:Yb3+/Er3+ powders, the bands corresponding to green and red emission of Er3+ ions (2H11/2 → 4I15/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions, respectively), as well as the bands at about 487 nm (blue emission) associated with second harmonic generation produced by the Bi2ZnOB2O6 matrix were detected. The results indicate a potential for application of Bi2ZnOB2O6:Yb3+/Er3+ powders as effective multifunctional new-generation photonic materials.

Phase transitions and interrelated instabilities in PbHfO3 single crystals.

PbHfO(3) is investigated theoretically and experimentally with respect to possible precursor effects starting in the paraelectric phase far above the cubic to tetragonal phase transition temperature. The theoretical modeling within the polarizability model predicts a giant softness of the system with spatially large polar and antiferrodistortive domain formation which compete with each other. These predictions are substantiated by the experiments, where the softness and the precursor effects are confirmed by birefringence, dielectric permittivity measurements and elastic properties by Brillouin scattering. The intermediate phase is found to have the polar nature confirmed by P-E hysteresis loop measurements, which is another manifestation of the competition between interrelated instabilities, namely a polar one and an antiferroelectric one.

Precursor dynamics, incipient ferroelectricity and huge anharmonicity in antiferroelectric lead zirconate PbZrO3.

To better understand the phase transition mechanism of PbZrO3 (PZO), the lattice dynamics of this antiferroelectric compound are investigated within the polarizability model, with emphasis on the cubic to orthorhombic phase transition. Similarly to ferroelectric phase transitions in ABO3 perovskites, polar dynamical clusters develop and grow in size upon approaching T(C) from the high temperature side and never form a homogeneous state. Simultaneously, elastic anomalies set in and compete with polar cluster dynamics. These unusual dynamics are responsible for precursor effects that drive the PZO lattice towards an incipient ferroelectric state. Comparison of the model calculations with the temperature dependences of elastic coefficients measured on PZO single crystals reveals a striking similarity.