High-Resolution Transmission Electron Microscopy Observations on Antiphase Boundaries in Barium Lanthanum Magnesium Niobate

Atomic structural observations on the antiphase boundaries (APBs) in the complex barium lanthanum magnesium niobate perovskite compound Ba_0.7La_0.3(Mg_0.43Nb_0.57)O₃ (BLMN), which has a 1:1 chemical ordering of B-site cations, were conducted using high-resolution transmission electron microscopy. Using APB contrast, the curved APB was determined to have a ledged structure, with a terrace that was composed of the (111) plane at an atomic level. In APBs with finite widths, microfacets on the (111) planes also were observed.     

Microstructural Characteristics of Strontium Magnesium Niobate

Microstructural studies were conducted on the domain boundaries in Sr(Mg_1/3Nb_2/3)O₃ (SMN) complex perovskite compound using X-ray diffractometry and transmission electron microscopy. Both the 1:2 chemical ordering of B-site cations and the tilting of oxygen octahedra were involved in SMN. SMN had a 1:2 ordered monoclinic unit cell, which was distorted by the antiphase tilting of oxygen octahedra. Two types of domain boundaries were found: the antiphase boundaries (APBs), which are not confined crystallographically, and the ferroelastic domain boundaries, which were parallel to the crystallographic planes. SMN had the superlattice reflections of type ±⅙[111] and ±½[111] in the electron diffraction patterns, which cannot be indexed in terms of the 1:2 ordered trigonal phase with only a hexagonal unit cell. The presence of the ferroelastic domains that contained both the 1:2 ordered and the antiphase tilting had been verified by a high-resolution transmission electron microscopy lattice image. The structure of SMN was well explained by a model proposed by other researchers. The formation of the 1:2 ordered domains preceded the ferroelastic domains. Normally, the growth of the ferroelastic domain is not affected by APBs, but it is interrupted by them when the driving force for growth is insufficient, resulting in the stoppage of the domains at APBs.     

Effect of Barium Titanate on Microstructural Evolution and Properties of Lead Zinc Magnesium Niobate Ceramics

BaTiO₃ (BT) addition vastly alters the dielectric behavior of lead zinc magnesium niobate (PZMN) relaxor ferroelectrics. Heat treatment, including the sintering process, influences the dielectric properties through changes in the distribution of heterogeneous compositions and in the extent of diffused phase transition. With more than 15 mol% BaTiO₃ addition, two cubic phases composed of Ba-rich and Ti-rich PZMN substances exist. The destruction of the BaTiO₃ phase and reconstruction of Ba-rich and Ti-rich regions are initiated during heat treatment, and the extent of destruction and reconstruction determines the dielectric properties of the PZMN ceramics. The BT-rich PZMN phase, which occurs with higher BaTiO₃ addition, plays a less important role in the present investigation.     

Microstructural Changes in Lanthanum-Doped Barium Magnesium Niobate

Observations of microstructural changes in (Ba_0.95La_0.05)-(Mg_0.35Nb_0.65)O₃ and (Ba_0.925La_0.075)(Mg_0.36Nb_0.64)O₃ (BLMN) were carried out using high-resolution transmission electron microscopy (HRTEM) and synchrotron powder X-ray diffractometry (XRD). In both samples, not only 1:1 and 1:2 ordered domains coexisted in a single grain, but also the intermediate phase, whose structure had a superlattice modulation of 1.42 nm, which was equivalent to 6 times the unit cube of disordered perovskite found on the nanoscale. The ordered 1:2 domains gradually transformed to 1:1 ordered structure through the formation of an intermediate superlattice structure that comprised 6 × 6 × 6 cubic unit cells with different chemical orderings of B-site ions in B-site lattices. Also, the features of thin plates could be detected by XRD patterns and HRTEM. When the thicknesses were very thin, about several atomic distances, stacking faults occurred on (111) planes. However, when their thicknesses were >50 nm, the thin plates existed as a transition phase with their own structure. They were coherent with the matrix and continuously decomposed into the matrix phase by the lateral migration of the interfaces.     

Infrared Spectroscopic Investigations in Ordered Barium Magnesium Niobate Ceramics

Highly ordered Ba(Mg_1/3Nb_2/3)O₃ ceramics from hydrothermal powders were investigated for the first time using infrared spectroscopy. The experimental data were analyzed in view of the 16 predicted modes of the trigonal structure and adjusted by a four–parameter semiquantum model. The obtained phonon parameters allowed us to calculate the real part of the dielectric permittivity and losses in all infrared regions, and also to estimate the quality factor ( Q ) for this material in the microwave region. The values obtained for the dielectric permittivity (ɛ'= 19) and Q (12 800 at 10 GHz) showed that hydrothermal Ba(Mg_1/3Nb_2/3)O₃ ceramics are good materials for passive components in microwave circuits.     

Reaction Kinetics of Perovskite Phase Formation in Lead Zinc Magnesium Niobate Ceramics

The reaction kinetics of perovskite Pb[(Zn,Mg)_1/3Nb_2/3]O₃─SrTiO₃ (PZMN–ST) phase formation vary with the phase condition of the starting materials. Basically, the reaction is controlled by a diffusion process with either a fixed number of nuclei or a constant nucleation rate. When the Pb₃Nb₂O₈ (P₃N₂) phase is used as the starting material, the reaction belongs to the former. The P₃N₂ phase acts as a nucleation center and fixes the number of nuclei for perovskite phase formation. If PbO and Nb₂O₅ instead of P₃N₂ are used as starting materials, the nucleation center will be the pyrochlore phase Pb₂Nb₂O₇ or P₃N₂ and is continuously created by the reaction of residual PbO with the formed pyrochlore Pb₃Nb₄O₁₃ phase. In the final stage of PZMN formation, the reaction is mainly through diffusion of ZnO into PMN or Zn-containing PMN. The importance of the pyrochlore phase to perovskite PZMN phase formation is consequently asserted.     

The Role of Excess Magnesium Oxide or Lead Oxide in Determining the Microstructure and Properties of Lead Magnesium Niobate

Near-phase pure perovskite lead magnesium niobate (PMN) with MgO or PbO additives was produced by reacting PbO with MgNb₂O₆ at 800°C and sintering at 1200°C. Dense ceramics were characterized by scanning electron microscopy, X-ray diffraction, and dielectric measurements. The microstructural studies showed that excess MgO exists as micrometer spherical particles either in the grain boundary as a discrete particle or in the perovskite grain as an inclusion. The pyrochlore phase exists in large isolated grains in the microstructure. The 10 mol% MgO excess composition had a peak dielectric constant of 19 500 at 100 Hz, which suggests very "clean" or uninhibiting grain boundaries. The excess addition of PbO did not improve the yield of perovskite PMN phase and decreased the dielectric constant. PMN grain boundaries are the dominant path of fracture. This paper, to a certain degree, explores the chemistry and characteristics of these grain boundaries.     

Dielectric and Structural Characteristics in Barium Lanthanum Magnesium Niobate

The dielectric properties and their related microstructural characteristics in solid solutions of (1 — x )Ba(Mg_1/3Nb_2/3)O₃ (BMN)— x La(Mg_2/3Nb_1/3)O₃ (LMN) (BLMN) were investigated by measuring the relative permittivity (ɛ_r), Q value, and temperature coefficient of resonator frequency (τ_f), and by observing the microstructure using transmission electron microscopy. The trend of variation of the temperature coefficient of the dielectric permittivity (τ_ɛ) was the same for our solid solutions as that reported by Reaney et al . When the tolerance factor ( t ) was >1.01 in BLMN with composition x = 0 to 1.0, where the tilting of oxygen octahedra was not involved, the components of the microstructure included a disordered and transition phase as well 1:1 and 1:2 ordered phases. In the region where 1.01 < t < 0.96 with x = 0.2 to 0.7, the 1:1 order, the disorder, and the phase due to the antiphase tilting of oxygen octahedra were present. Finally, in the region where t < 0.96 with x = 0.7 to 1.0, the microstructure of BLMN was the same as that of the pure LMN, including the 1:1 order and the antiphase, inphase tilting of oxygen octahedra, and the antiparallel shift of A-site cations.     

Microstructure and Dielectric Properties of Barium Strontium Magnesium Niobate

Dielectric properties and their related microstructural characteristics in solid solutions of (1 – x )Ba(Mg_1/3Nb_2/3)O₃– x Sr(Mg_1/3Nb_2/3)O₃ (BMN–SMN, or BSMN) were investigated by measuring the relative permittivity (ɛ_r), Q values, and temperature coefficient of resonator frequency (τ_f), and by observing microstructure using transmission electron microscopy. When the tolerance factor ( t ) was >0.99 in BSMN with composition 0 < x < 0.5, where the tilting of oxygen octahedra was not involved, the microstructure included only 1:2 ordered phase. In the region where 0.99 > t > 0.97 with 0.7 < x < 1.0, the phase due to the antiphase tilting of oxygen octahedral, the disordered phase, and the 1:2 ordered phase were also present. In a few of the grains, core–shell-type structures, whose main components were dislocations and stacking faults, were found in the solid solution of BSMN.     

Low-Temperature Route to Lead Magnesium Niobate

Lead magnesium sniobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), was prepared by a sol-gel technique using alkoxide precursors. The hydrolysis-condensation mechanism leads to a translucent gel which is dried under hypercritical conditions to avoid collapse of the porous texture. After drying, the aerogel exhibits a B-deficient pyrochlore structure which progressively inserts magnesium oxide when the temperature increases. Near 700°C, this pyrochlore phase completely transforms into the PMN-perovskite phase. Above 1000°C, the loss of lead oxide leads to the destabilization of the PMN-perovskite with formation of an A-deficient pyrochlore phase.     

Diffuse Phase Transition of Tantalum-Bearing Strontium Barium Niobate

The dielectric constants, ε' and ε", of tantalum-bearing strontium barium niobate (SBNT) ceramics were measured as functions of temperature and frequency. The phase transition of SBNT became ambiguous by adding Ta and the apparent Curie temperature determined from the maximum of ε' had a linear relation with the axial ratio c/a. A dielectric loss peak was observed in the phase transition region of SBNT, and its peak frequency increased with temperature. The loss peak was attributed to dielectric relaxation by the jump between two states with different polarizing directions as a preliminary phenomenon of the phase transition. A model based on the local variation from one unit cell to the next rather than the macroscopic compositional fluctuation in the crystal was proposed for the mechanism of diffuse phase transition.     

Rapid Sintering of Stoichiometric Zinc-Modified Lead Magnesium Niobate

A rapid thermal-processing technique was used to densify zinc-modified lead magnesium niobate (PMZN30) of the form Pb(Mg_0.7Zn_0.3)_1/3Nb_2/3O₃. This dielectric composition showed relaxor behavior with a maximum permittivity near room temperature. Rapid densification was observed at temperature. Rapid densification was observed at temperatures of 1100°C and above, whereas lower temperatures showed a significant reduction of densification and electrical properties. A sintering schedule of 4 min at 1100°C with no prevention of PbO volatilization resulted in densification on the order of 90% of theoretical density and a weak field dielectric constant in excess of 14 000. Perovskite grain size was consistently in the range of 1 μm or less. The formation of pyrochlore on sample surfaces showed a direct relation to time and temperature.     

Preparation of Strontium Barium Niobate by Sol–Gel Method

Crack-free SBN (Sr_xBa_1-xNb₂O₆) thin films have been prepared by a sol–gel method with metal alkoxides. A homogeneous and stable precursor solution was obtained from Sr and Ba metal and Nb(OEt)₅ in ethanol with a key additive of ethoxyethanol. SBN (where x = 0.5) powder crystallized to orthorhombic phase at 700°C, and then transformed completely to tetragonal phase at 1200°C. The formation of tetragonal SBN was observed on sapphire (R) substrates at 700°C, whereas the tetragonal phase began to appear in the powders at 1000°C. SBN films with highly preferred orientation were successfully synthesized on MgO (100) substrates at 670°C.     

Fabrication of Optically Transparent and Electrooptic Strontium Barium Niobate Ceramics

The tetragonal tungsten bronze (TTB) type of Sr _x Ba_{1− x} ^m Nb₂O₆ (SBN) (0.5 ≤ x ≤ 0.75) was synthesized by a mixed-oxide route. Using the two-step densification process of pressureless sintering followed by oxygen HIPing, transparent ceramics were successfully fabricated. Microstructural, dielectric, optical, and electrooptic properties were investigated. All single-phase TTB-type SBN showed the characteristics of relaxor ferroelectrics. The extinction coefficients of the sample with the highest transparency were calculated from transmission spectra to be 30.4, 5.8, and 2.4 cm⁻¹ at 600, 1450, and 2000 nm, respectively. The linear electrooptic coefficients for SBN55, r ₃₃ and r ₁₃, were 46 × 10⁻¹² and 21 × 10⁻¹² m/V at 633 nm, respectively.     

Two Types of Domain Boundaries in Lanthanum Magnesium Niobate

Microstructural studies of the domain boundaries in the complex perovskite compound lanthanum magnesium niobate (La[Mg_2/3Nb_1/3]O₃, LMN) were conducted using transmission electron microscopy. Both the 1:1 chemical ordering of B-site cations and the tilting of oxygen octahedra affected the domain boundaries. Two types of domain boundaries were observed. In addition to the presence of antiphase boundaries, which were insensitive to the crystallographic planes, ferroelastic domain boundaries that were caused by the phase transition due to the tilting of oxygen octahedra also were present. In some grains, only one type of oxygen tilting was present, which resulted in a single domain in one grain. Two or three domains were observed in a grain where the walls were parallel to the {110} plane. Many domains also were observed in a grain that had boundaries whose linear characteristics were gradually reduced.     

Microstructure Characterizations in Calcium Magnesium Niobate

Microstructural studies on the domain boundaries in Ca(Mg_1/3CNb_2/3)O₃ (CMN) complex perovskite compound were conducted using X-ray diffractometry and transmission electron microscopy. The 1:2 chemical ordering of B-site cations and the tilting of oxygen octahedra were involved in the CMN microstructure, as inferred from the presence of two types of domain boundaries. One type was the antiphase boundaries (APBs), which did not lie on a specific set of crystallographic planes. These boundaries were caused by the chemical 1:2 ordering of B-site cations, magnesium and niobium. The other type was the ferroelastic domain boundaries, which were parallel to a certain crystallographic plane. Therefore, CMN had the 1:2 ordered monoclinic unit cell distorted by the antiphase or in-phase tilting of oxygen octahedra. CMN had the mixed phases rather than the homogeneous phase.     

Ordering Behavior of Lead Magnesium Niobate Ceramics with A-Site Substitution

The ordering behavior of lanthanum-modified lead magnesium niobate ceramics was investigated over a wide range of lanthanum content (from 0 to 50 at.%). The ordering of magnesium and niobium ions in the B site was examined by X-ray diffraction, and the superlattice reflection was associated with the A(B'_1/2B"_1/2)O₃-type partial ordering structure. Moreover, the ordering degree increased pronouncedly with respect to lanthanum content of the sample and almost reached unity at [La] = 50 at.%. Finally, a comparison of the ordering behavior was performed among several lead magnesium niobate (PMN) samples with different A-site substitutional ions. The mechanism used to enhance the ordering of PMN by A-site substitution is then described by the effect of high valence.     

Dielectric Properties of Lead Magnesium Niobate and Lead Iron Niobate Prepared by the Semiwet Hydroxide Route

Lead magnesium niobate (PMN) and lead iron niobate (PFN) were prepared by the semiwet hydroxide route, and their dielectric properties were measured in temperature ranges around their peak dielectric constants. The dielectric constant of PFN was much larger as compared with that of PMN sintered at the same temperature. The dielectric properties of PMN and PFN are compared and explained on the basis of their structure development.     

弛豫铁电陶瓷Ba(Mg,Nb)O_3的制备及性能研究

采用传统陶瓷烧结方法,在空气气氛下烧结制得Ba(Mg,Nb)O3(BMN)陶瓷,并分析了烧结温度对体系相组成及相结构的影响。X射线衍射(XRD)研究表明,随着烧结时间的增加,体系中钙钛矿相也相应增加。红外(FTIR)研究研究表明结构无序的BMN钙钛矿相中部分的区域出现了结构有序现象。通过X射线光电子能谱(XPS)可以发现,随着烧结温度的增加,体系中Nb2O5进入Ba(Mg1/3,Nb2/3)O3晶格。微波介电性能表明,随着烧结温度的增加,Ba(Mg,Nb)O3的rε和Qf值有逐渐增加的趋势。     

Diffusion of Sputtered Vanadium, Nickel, and Silver in Lead Magnesium Niobate Ceramic

The sputtering of nickel-vanadium (Ni-V) and silver (Ag) to form thin films on lead magnesium niobate (PMN) ceramic substrates has been studied. An empirical equation was derived which correlated the average Ni-V film thickness with three sputtering variables: input power, argon pressure, and sputtering time. The films were dense, smooth, and feature less. Upon annealing at 800°C in argon, both Ni and V diffused into PMN; but reverse diffusion from the ceramic into the metal film was not observed. Vanadium diffused at a faster rate than Ni, leaving pores in the film. The diffusion coefficient of Ni in PMN was determined to be 4.1 × 10⁻¹² cm²/s, whereas the diffusion coefficient of V was determined to be between 3.5 × 10⁻⁹ and 2.9 × 10⁻¹¹ cm²/s at 800°C. Sputtered Ag was also dense, smooth, and featureless. However, the diffusion of Ag at 800°C in PMN was much slower than that of either Ni or V.