Relaxor Behavior of the 0.9BaTiO3–0.1La(Mg1/2Ti1/2)O3 Solid Solution

Low-frequency dielectric response of air- and oxygen-sintered ceramics with the composition 0.9BaTiO₃–0.1La(Mg_1/2Ti_1/2) O₃ (0.9BT–0.1LMT) has been studied in the temperature range of 12–550 K. In comparison with pure BT, in 0.9BT–0.1LMT the dielectric permittivity maximum is shifted by almost 300 K toward lower temperatures. Both real and imaginary parts of dielectric permittivity of the solid solution, in the range 12–150 K, show a strong frequency-dependent behavior, which is typical of relaxors. On the basis of the model of exponential cluster size distribution and the Cole–Cole equation, the degree of interaction between the polar clusters was estimated. It was shown that the oxygen vacancies arising during sintering at high temperatures did not affect noticeably the relaxor properties of the material. The role of heterovalent La³⁺/Ba²⁺ and Mg²⁺/Ti⁴⁺ substitutions in the relaxor behavior formation is discussed.     

Contribution of Structure to Temperature Dependence of Resonant Frequency in the (1 −x)La(Zn1/2Ti1/2)O3·xATiO3 (A = Ca, Sr) System

The crystal structure and microwave dielectric properties of the (1 − x ) La(Zn_1/2Ti_1/2)O₃· x SrTiO₃ and (1 − x )La(Zn_1/2Ti_1/2)O₃· x CaTiO₃ system were investigated. X-ray powder diffraction showed that cation ordering disappeared at x > 0.3 for both systems. However, infrared spectra demonstrated that short-range cation ordering could exist at x = 0.4. Permittivity and the temperature coefficient of the resonant frequency (τ_f) of both systems exhibited nonmonotonic variations with composition. Both systems exhibited a τ_f of zero at the same composition of x = 0.5 although the τ_f of SrTiO₃ was about two times larger than that of CaTiO₃. The behavior of the permittivity and τ_f were described by the tilting of oxygen octahedra and cation ordering. The relation between τ_f and cation ordering of La(Zn_1/2Ti_1/2)O₃ was discussed in conjunction with the experimental results on metal halides. It is suggested that cation ordering induced a negative τ_f and suppressed the increase of permittivity for compositions between x = 0 to x = 0.5 for (1 − x )La(Zn_1/2Ti_1/2)O₃· x SrTiO₃ and (1 − x )La(Zn_1/2Ti_1/2)O₃· x CaTiO₃ systems.     

Elastic Properties and Sound Wave Velocity of PZT Transducers Doped with Ta and La

Lead zirconate-titanate (Pb(Zr_0.54Ti_0.46)O₃) was doped with varied concentrations of Ta⁵⁺ and La³⁺ and sintered at 1200°C for 2 h. Dense ceramic materials were obtained when the cation stoichiometry was adjusted to allow for charge compensation of the donor ions by Pb vacancies. Bulk density, capacitance at 1 KHz, grain size, piezoelectric charge, and elastic compliance were determined experimentally. Sound wave velocity, inductance, and capacitance near resonance were calculated.     

Direct Compositional Analysis of Ordered Domains in Pure and La-Doped Pb(Mg1/3Nb2/3)O3 by Analytical Electron Microscopy Techniques

The ordered domain structures in Pb(Mg_1/3Nb_2/3)O₃(PMN) and Pb_{1– x} La _x (Mg_1/3Nb_2/3)O₃ are identified using high-resolution transmission electron microscopy (HRTEM) and nanobeam diffractometry (NBD). The chemical compositions in the ordered domains and in the disordered matrices are also acquired using energy-dispersive spectroscopy (EDS). The best matching computer-simulated HRTEM image has a Mg²⁺/Nb²⁺ ratio of return ½. There is no obvious chemical composition difference between the ordered domains and the disordered matrices. The number of the normalized total positive valence electrons remains almost constant in the ordered domains and in the disordered matrices for all the samples. The reason for the growth of the ordered domains in La-doped PMN also is discussed.     

Atomic Resolution Transmission Electron Microscopy of the Microstructure of Ordered Ba(Cd1/3Ta2/3)O3 Perovskite Ceramics

Microstructures of ordered Ba(Cd_1/3Ta_2/3)O₃ perovskite dielectric ceramics with and without a boron additive have been observed by atomic resolution transmission electron microscopy (TEM). The selected area electron diffraction and lattice image show a well-ordered structure with hexagonal symmetry (lattice constants of a ∼5.8 Å and c ∼7.1 Å) in the ordered Ba(Cd_1/3Ta_2/3)O₃ with a boron additive, which is similar to those in ordered Ba(Zn_1/3Ta_2/3)O₃ and Ba(Mg_1/3Ta_2/3)O₃ ceramics. Ordered domains with a twin crystallographic relationship and high-density domain interfaces induced by ordering were observed in the ordered Ba(Cd_1/3Ta_2/3)O₃ without a boron additive sintered at a relatively high temperature. Atomic resolution TEM further revealed the conservative twin boundaries along (001) and (110) planes and non-conservative antiphase boundaries with a projected displacement vector of the type [001] in the ordered Ba(Cd_1/3Ta_2/3)O₃ without a boron additive. Finally, the energetics of different domain interfaces are discussed with the interfacial structures in ordered Ba(Cd_1/3Ta_2/3)O₃ ceramics revealed by an electron microscope.     

Structure and Dielectric Properties of the Ba(Mg1/3Nb2/3)O3-La(Mg2/3Nb1/3)O3 System

A complete range of perovskite solid solutions can be formed in the (1 − x )Ba(Mg_1/3Nb_2/3)O₃- x La(Mg_2/3Nb_1/3)O₃ (BMN-LMN) pseudobinary system. While pure BMN adopts a 1:2 cation ordered structure, 1:1 ordered phases are stabilized for 0.05 ≤ x ≤ 1.0. Dark-field TEM images indicate that the La-doped solid solutions are comprised of large 1:1 ordered domains and no evidence was found for a phase-separated structure. This observation coupled with the systematic variations in the intensities of the supercell reflections supports a charge-balanced "random-site" model for the 1:1 ordering. The substitution of La also induces a transformation from a negative to positive temperature coefficient of capacitance in the region 0.25 ≤ x ≤ 0.5.     

Modeling the Dielectric Response and Relaxation Spectra of Relaxor Ferroelectrics

A multi-Debye relaxation model that is based upon the Boltzmann superposition principle has been used to determine the frequency-dispersion behavior of Pb(Mg_1/3-Nb_2/3)O₃ (PMN) and Pb_0.88La_0.08(Zr_0.65Ti_0.35)O₃ (PLZT 8/65/35) relaxor ferroelectrics. For PMN, relaxation times on the order of seconds contribute to the dispersive character of the dielectric properties. A defect-relaxation mechanism that involves Mg cation hopping in a structure that is composed of interpenetrating 〈111〉 chains of ordered Pb(Mg_1/2Nb_1/2)O₃ is proposed for PMN. Motion of the Mg cations results in destruction and reconstruction of these ordered chains, which contributes to the dispersive character of PMN.     

Effect of BaWO4 on the Microwave Dielectric Properties of Ba(Mg1/3Ta2/3)O3 Ceramics

Physical and microwave dielectric properties of complex perovskite Ba(Mg_1/3Ta_2/3)O₃ ceramics have been investigated as a function of the amount of BaWO₄ in the temperature range from 20° to 80°C at 10.5 GHz. Up to 0.05 mol BaWO₄ addition, the lattice constant ratio ( c/a ), ordering parameter, apparent density, and unloaded Q all increase, due to the increase in the substitution of Ta⁵⁺ ions of Ba(Mg_1/3Ta_2/3)O₃ by W⁶⁺ ions from the melted BaWO₄ at above 1430°C. With further addition of BaWO_4, the unloaded Q decreases, due to an increase of the BaWO₄ phase. The temperature coefficient of resonant frequency (TCF) can be controlled by the volume mixture rule of Ba(Mg_1/3Ta_2/3)O₃ and BaWO₄. When 0.09 mol BaWO₄ is added, TCF becomes 0 ppm/°C.     

Novel Doping Mechanism for Very-High-Permittivity Barium Titanate Ceramics

Barium titanate (BaTiO₃) can be doped with La³⁺ ions via partial substitution for Ba²⁺ ions; charge balance is maintained by the creation of Ti⁴⁺ vacancies. Samples processed in an atmosphere of 1 bar O₂ and a temperature of 1350°C are insulating and free from electronic defects associated with either O₂ loss or reduction of Ti⁴⁺ to Ti³⁺. The Curie temperature ( T _c) decreases approximately linearly as the lanthanum content increases and, at the same time, an increase in the permittivity (ɛ') maximum at T _c occurs. For the composition Ba_{1- x} La _x Ti_{1- x /4}O₃, where x = 0.05, ɛ' has a maximum value of 19000 at 18°C, compared with a typical value of 10000 at 130°C in undoped BaTiO₃ ceramics. This value is the highest value reported for A-site-doped BaTiO₃ and is linked to the mechanism of combined A-site doping and Ti-vacancy creation.     

Microwave Dielectric Properties of (1−x)(Na1/2Nd1/2)TiO3−xLa(Mg1/2Ti1/2)O3 via Ordering and Far IR Reflectivity Spectra

Microwave dielectric properties of (1− x )(Na_1/2Nd_1/2)TiO₃(NNT)− x La(Mg_1/2Ti_1/2)O₃(LMT) system have been investigated with focus on structural ordering and far IR reflectivity spectra. A single perovskite phase was found to exist with various superlattice reflections over the entire compositional range. 1:1 ordering observed in the (111) reflection of X-ray diffraction patterns was found to progress significantly in the specimens above x =0.8. The ordering characteristics that presumably related to B-site Mg/Ti ordering were assumed to affect favorably the quality factor at microwave frequencies. The increase of the quality factor also could be explained in terms of the decrease of lattice anharmonicity with LMT by analysis of far IR reflectivity spectra. As expected from the values of pure NNT and LMT, the dielectric constant and the temperature coefficient of frequency tended to gradually decrease with increasing LMT content.     

Ordering Structure and Dielectric Properties of Undoped and La/Na-Doped Pb(Mg1/3Nb2/3)O3

Transmission electron microscopy was used to study the ordered domain structures in undoped and La/Na-doped Pb(Mg_1/3Nb_2/3)O₃ (PMN), where the compositions of the doped samples were specifically chosen so as to elucidate the ordering mechanism. The results showed that the Mg²⁺ ions and Nb⁵⁺ ions are short-range ordered on the B-site sublattice in undoped PMN, with a domain size of 2 to 5 nm. This short-range ordering gives rise to B-site composition fluctuations occurring on a nanometer scale, and it is this compositional inhomogeneity which is believed to be responsible for the diffuse phase transition behavior. Donor doping with La₂O₃ can compensate for the local charge imbalance resulting from the short-range order and thus enhances the degree of ordering. Acceptor doping with Na₂O, however, increases the charge effect, and hence ordering is suppressed. The effect of Na doping and La doping on the dielectric properties of PMN is also discussed.     

Cation Ordering and Domain Boundaries in Ca[(Mg1/3Ta2/3)1−xTix]O3 Microwave Dielectric Ceramics

Cation ordering and domain boundaries in perovskite Ca[(Mg_1/3Ta_2/3)_{1− x} Ti _x ]O₃ ( x =0.1, 0.2, 0.3) microwave dielectric ceramics were investigated by high-resolution transmission electron microscopy (HRTEM) and Rietveld analysis. The variation of ordering structure with Ti substitution was revealed together with the formation mechanism of ordering domains. When x =0.1, the ceramics were composed of 1:2 and 1:1 ordered domains and a disordered matrix. The 1:2 cation ordering could still exist until x =0.2 but the 1:1 ordering disappeared. Neither 1:2 nor 1:1 cation ordering could exist at x =0.3. The space charge model was used to explain the cation ordering change from 1:2 to 1:1 and then to disorder. A comparison between the space charge model and random layer model was also conducted. HRTEM observations showed an antiphase boundary inclined to the (111) _c plane with a projected displacement vector in the 〈001〉 _c direction and ferroelastic domain boundaries parallel to the 〈100〉 _c direction.     

1:1 Ordered Domain Growth in Pb(Mg1/3Ta2/3)O3-La(Mg2/3Ta1/3)O3 Relaxor Ferroelectric Perovskites

Single-phase perovskite solid solutions are formed across the entire (1 - x )Pb(Mg_1/3Ta_2/3)O₃- x La(Mg_2/3Ta_1/3)O₃ (PLMT) pseudobinary system. Although as-sintered (1300°C, 3 h) samples with x lessthan equal to 0.1 adopt a phase-separated, "PMN-type" microstructure comprising small (2-3 nm) ordered domains dispersed in a disordered matrix, extensive domain growth and complete order can be induced by extended thermal annealing (1300°C, 24 h). These observations, and the alterations in the thermal stability and domain size across the system, can be interpreted using a charge-balanced random site model for the cation order. No evidence is found to support the space-charge models currently used to describe the cation ordering and properties of the corresponding niobate systems.     

Defect Structure and Phase Relations of Highly Lanthanum-Doped Barium Titanate

Incorporation of La³⁺ into the BaTiO₃ lattice was studied on samples of various composition, using quantitative WDS microanalysis (EPMA) in combination with scanning electron microscopy (SEM) and X-ray powder diffractometry (XRPD). Direct determination of solid-solution formulas by microanalysis supports the structure model of a solid solution with La³⁺ on Ba²⁺ sites and a deficient Ti⁴⁺ sublattice, independent of the starting composition. Solid solution extends on the tie line, which points from BaTiO₃ to La₄Ti₃O₁₂, to the composition of approximately Ba_0.70- La_0.38Ti_0.925( V "")_0.075O₃. On the basis of these results, the BaTiO₃-rich part of the BaO-La₂O₃-TiO₂ phase diagram was constructed.     

Effect of La/K A-site Substitutions on the Ordering of Ba(Zn1/3Ta2/3)O3

The 1/3 <111>-type ordering of Ba(Zn_1/3Ta_2/3)O₃ (BZT) can be transformed to 1/2 <111>–type ordering by substituting the La³⁺ cation into the A site. The 1/3 <111> ordering in BZT is shown to be reduced, discontinued, and then replaced by 1/2 <111> ordering, using X–ray diffraction, electron microscopy, and Raman spectroscopy. On the other hand, potassium–substituted BZT only displays a reduction in the degree of ordering.     

Synthesis of Micron-Scale Platelet BaTiO3

Micron-scale platelet barium titanate was synthesized using a twostep molten salt and topochemical technique. Plate-like BaBi₄Ti₄O₁₅ was first synthesized as a precursor by molten salt synthesis. Then, Bi³⁺ in the precursor was replaced by Ba²⁺ and formed perovskite-structure BaTiO₃ through a topochemical reaction. The BaTiO₃ single crystals have an average size of 5–10 μm and a thickness of 0.5 μm. The purpose of this article is to control the particle shape with desired structure. High aspect ratio BaTiO₃ platelets are suitable templates to obtain textured ceramics (especially Pb(Mg_1/3Nb_2/3)O₃–32.5PbTiO₃) by the templated grain growth process.     

Direct Imaging of Atomic Ordering in Undoped and La-Doped Pb(Mg1/3Nb2/3)O3

In this paper, we report the first direct observations of local ordering in undoped and La-doped Pb(Mg_1/3Nb_2/3)O₃ (PMN) on an atomic scale by high-resolution (∼1.26 Å) Z -contrast imaging. The 1:1 ordering occurs by a variation in the occupancy of Mg and Nb cations between B^I and B^II sublattices. In ordered regions, the B^II sublattice is dominantly occupied by Nb cations, but a small Mg occupancy cannot be excluded. The Mg cations were found to dominantly occupy the B^I sublattice. Within the B^I sublattice, the local Mg/Nb ratio was found to vary among the various B^I sublattices. The results show that the random-site model, rather than the space-charge model, is a better structural model for the 1:1 ordering observed in PMN. A random distribution of Mg and Nb cations on the B^I sublattice within 1:1 ordered regions is believed to be responsible for the relaxor behavior of mixed B-site cation relaxors, such as PMN.     

Far-Infrared Reflection Spectra of Dielectric Ceramics for Microwave Applications

Far-infrared reflection spectra of dielectric ceramics, BaSm₂Ti₅O₁₄, BaTi₄O₉, and some pcrovskites such as Ba(Zn_1/3Nb_2/3)O₃, have been measured at room temperature using a Fourier transform infrared spectrometer in order to investigate the effect of the crystal structure on the dielectric properties. As for perovskites, Sr(Zn_1/3Nb_2/3)O₃ and Sr(Mg_1/3Nb_2/3)O₃, in which B site ions are ordered, were also measured. Reflectance data were analyzed by means of a factorized form of dielectric functions instead of the classical dispersion theory, and all of the spectra were well fitted. The values of dielectric constants and tan δ calculated from the reflectance data were in good agreement with resonant cavity measurements at 5 GHz. Furthermore, results of this study have shown that the main contribution to the microwave dielectric properties is caused by low-frequency optically active modes located at 50 to about 300 cm⁻¹, and for perovskite structures it is suggested that the ordering of B site ions is significant in obtaining low dielectric losses.     

Structure–Property Relations in xBaTiO3–(1−x)La(Mg1/2Ti1/2)O3 Solid Solutions

The microwave dielectric properties and microstructures of compounds in the solid solution series x BaTiO₃–(1− x )La(Mg_1/2Ti_1/2)O₃ (BTLMT) have been investigated. The structural phase transitions that occur as a function of x have been studied and are related to changes in the dielectric properties. For compounds where x ≤ 0.1, X-ray diffraction (XRD) showed evidence of 1:1 ordering between Mg and Ti cations. For x ≤ 0.3, XRD and electron diffraction revealed that compounds were tilted in both antiphase and in-phase. However, for 0.3 < x < 0.7, only antiphase tilting was present. The temperature coefficient of resonant frequency (τ_f) vs the relative permittivity (ɛ_r) was linear until x = 0.5 at which point in the solid solution the transition to a nontilted structure resulted in nonlinear behavior. τ_f values close to zero (−2 ppm/°C) were achieved at x = 0.5 (ɛ_r∼ 60), which had a quality factor ( Q · f _o) of 9600 GHz.     

Microstructural Observations in Barium Calcium Magnesium Niobate

Microstructural studies on (1 − x )Ba(Mg_1/3Nb_2/3)O₃– x Ca(Mg_1/3Nb_2/3)O₃ (BCMN) complex perovskite compounds, which are mixtures of Ba(Mg_1/3Nb_2/3)O₃ (BMN) and Ca(Mg_1/3Nb_2/3)O₃ (CMN), were conducted using scanning electron microscopy, transmission electron microscopy, and X-ray diffractometry. Pure BMN and CMN both have a 1:2 ordered structure, via the chemical ordering of B-site cations; however, the tilting of oxygen octahedra is involved in pure CMN, whose structure has a 1:2 ordered monoclinic unit cell that is characterized by (±1/6,±1/6,±1/6)-type superlattice reflections in electron diffraction patterns along the [110] zone axis that is based on a simple cubic perovskite. Studies of the morphologic differences have indicated two types of inhomogeneities in a mixture of the BCMN system: (i) a rather large-scale segregation (i.e., grain sizes of several micrometers), where the grains are separated compositionally as being barium-rich or calcium-rich, and (ii) fine-scale lamellar-type segregations 20 nm wide and 200 nm long. The segregation that is caused by Ba and Ca ions can be identified by the difference of superlattice modulations from high-resolution transmission electron microscopy lattice images.