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.     

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.     

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.     

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.     

Structural Studies of Ordering in the (Pb1-xBax)(Mg1/3Nb2/3)O3 Crystalline Solution Series

The ordered structures of the (Pb_{1- x} Ba _x )(Mg_1/3Nb_2/3)O₃crystalline solution series were investigated by selected area electron diffraction (SAED) and high-resolution electron microscopy (HREM). At low Ba contents (e.g., x < 0.40), the ordered structure was found to be isostructural with Pb(Mg_1/3Nb_2/3)O₃, with a doubled unit cell characterized by 1/3{111} superlattice reflections. At higher Ba contents (e.g., x > 0.60), the ordered structure was characterized by 1/3{111} superlattice reflections. For intermediate Ba contents (e.g., x - 0.60), diffuse scattering along the {111} between diffuse 1/2{111} and 1/3{111} reflections was observed. The ordering is attributed to the distribution of the B-site cations between multiple sublattices. Strong fluctuations in the B-site cation ratio between ordered and disordered regions are believed not to exist; however, the possibility of weak fluctuations is consistent with the observed lattice images.     

Novel Route to Lead-Based Ferroelectric Compounds via Tetragonal Lead(II) Oxide Intermediates

Single-phase perovskite lead-based ferroelectric powders—Pb(Mg_1/3Nb_2/3)O₃(PMN) or 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃(0.9PMN–0.1PT)—were prepared using Mg(NO₃)₂, instead of MgO or MgCO₃, via a mixed-oxide method and one-step calcination. The reaction proceeded via the formation of 3Pb(NO₃)₂·7PbO, Pb(OH)₂, tetragonal PbO, and then 2PbO–Nb₂O₅(P₂N) for PMN or 3PbO–Nb₂O₅(P₃N) for PMN–PT; a mixture of PMN and Pb₂(Mg _x Nb_1.33)O_{5.33+ x} then formed, followed finally by the formation of single-phase PMN or 0.9PMN–0.1PT. Such prepared powder showed excellent room-temperature dielectric constants—13800 for PMN or 22600 for 0.9PMN–0.1PT—by sintering at a temperature of 900°C for 2 h.     

Cation Ordering Transformations in the Ba(Zn1/3Nb2/3)O3-La(Zn2/3Nb1/3)O3 System

Single-phase perovskites were formed in the (1−_x)Ba(Zn_1/3Nb_2/3)O₃-( _x )La(Zn_2/3Nb_1/3)O₃ system for compositions with 0.0≤ x ≤0.6. Although the stability of the trigonal "1:2" ordered structure of the Ba(Zn_1/3Nb_2/3)O₃ end member is very limited (0.0≤ x ≤0.05), low levels of lanthanum induce a transformation to a cubic, "1:1" ordered structure that has a broad range of homogeneity (0.05≤ x ≤0.6). Samples with x > 0.6 were comprised of La₃NbO₇, ZnO, and a perovskite with x = 0.6. The cubic 1:1 phases were fully ordered and no evidence was found for a compositionally segregated microstructure. These observations could not be reconciled in terms of a "space-charge" model; rather, they supported a charge-balanced, "random-site" structure for the 1:1 cation-ordered Ba(β_1/2'β_1/2")O₃ phases.     

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.     

Synthesis of 0.4Pb(Mg1/3Nb2/3)O3–0.3Pb(Mg1/2W1/2)O3–0.3PbTiO3 Ceramics by Metal Alkoxide Method, and Its Dielectric Properties

0.4Pb(Mg_1/3Nb_2/3)O₃–0.3Pb(Mg_1/2W_1/2)O₃–0.3PbTiO₃+ x MgO ( x = 0 to 0.04) were prepared by a metal alkoxide method. The percent of perovskite phase of the calcined powders increased with increased calcination temperatures. About 89% of perovskite phase was obtained at 1050°C. The dielectric constant of the pellets fired at 1100°C was increased by the addition of 10 wt% excess Mg(OC₂H₅)₂ and had a maximum value of 7532 at 1 kHz.     

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.     

Microstructure of Lanthanum Magnesium Niobate at Elevated Temperature

Microstructural studies of the complex perovskite compound La(Mg_2/3Nb_1/3)O₃ (LMN) were conducted using transmission electron microscopy (TEM) and X-ray diffractometry (XRD) at elevated temperatures. 1:1 chemical ordering of B-site cations and tilting of oxygen octahedra were observed in LMN. Three types of superlattice reflections, [1—2]{111}, [1—2]{110}, and [1—2]{100} were observed at room temperature and at 800°C in electron diffraction patterns. In the XRD experiments, the [1—2]{210} and [1—2]{300} extra peaks disappeared at temperatures >1200°C. However, the intensity of the superlattice [1—2]{111} peak did not change with increased temperature up to 1400°C. These results strongly indicated that the origin of superlattice reflection [1—2]{111} was different from that of the other superlattice reflections. It was mainly caused by the 1:1 chemical ordering of magnesium and niobium atoms. The TEM image observed at 800°C showed the ordered domain structures separated by the antiphase boundaries.     

Structural and Dielectric Characteristics in a Ca(Mg1/3Nb2/3)O3–CaZrO3 System

This study investigates the effect of CaZrO₃ (CZ) substitution on the evolution of an ordered structure in a Ca(Mg_1/3Nb_2/3)O₃ (CMN) system using Raman spectroscopy, X-ray diffractometry, and transmission electron microscopy. It indicates that a (1− x ) CMN−( x )CZ solid solution has the 1:2 and 1:1 ordered structure distorted by the antiphase, the inphase tilting of oxygen octahedra, and the antiparallel shift of A-site cation. A distinct correlation is noted between the transition of the ordered structure and microwave dielectric properties. The differences in ɛ_r and τ_f are attributed exclusively to the differences in the type of cation arrangement. The structure with the 1:2 ordering exhibits a lower relative permittivity and a more negative τ_f than the structure with the 1:1 ordering. The increased fraction of compressed Nb–O bond in the 1:2 ordered structure associated with a large NbO₆ octahedral distortion is correlated with a decrease in relative permittivity and change of τ_f toward more negative values. Simultaneously, the substitution of the Zr⁴⁺ ion causes a linear increase in polarizability, and it also results in an increase in the relative permittivity.     

Structural and Dielectric Characteristics of Barium Lanthanum Zinc Niobate

The dielectric properties and microstructural characteristics in solid solutions of Ba_{1− x} La _x [Zn_{(1+ x )/3}Nb_{(2− x )/3}]O₃ (BLZN) are investigated by measuring and observing these properties, respectively, by means of transmission electron microscopy and Raman spectroscopy. The 1:1 ordered structure of BLZN can be explained by the random-site model for the distribution of B-site cations. The decrease in the tolerance factor ( t ) by lanthanum substitution causes the tilting of oxygen octahedra. It appears that the onset of antiphase and inphase tilting causes the variation in the temperature coefficient of resonant frequency (τ_f). In the untilted region where t ≥1.01, the τ_f shows a linear increase with decreasing tolerance factor. The region of antiphase tilting, where 0.965≤ t <1.01, causes a rapid decrease in τ_f, including the reverse sign. The τ_f slowly increases, where t <0.965, which is due to the presence of inphase tilting of oxygen octahedra. The τ_f can be predicted by using the tolerance factor, and the near zero of τ_f can be obtained with lanthanum substitution in the solid solution of the BLZN system.     

Preparation and Spontaneous Polarization–Magnetization of a New Ceramic Ferroelectric–Ferromagnetic Composite

Ceramic composites with the composition of x PMZNT·(1 – x )NiCuZn have been prepared using a standard ceramic technique, in which x varies as 0, 0.1, 0.2, 0.4, 0.6, 0.9, and 1.0. PMZNT is the abbreviated form of 0.92Pb(Mg_1/3Nb_2/3)O₃·0.04Pb(Zn_1/3Nb_2/3)O₃·0.04PbTiO₃ (PMN-PZN-PT). NiCuZn is the abbreviated form of Ni_0.2Cu_0.2Zn_0.6Fe₂O₄. The presence of ferroelectric PMZNT phase and ferromagnetic NiCuZn ferrite phase has been confirmed using X-ray diffractometry. Ferroelectric hysteresis loops and magnetic hysteresis loops have been observed and studied. In polarization–electric-field curves, the remnant polarization and coercive fields display little asymmetric characterization because of the existence of the internal electric field. When the amount of NiCuZn ferrite phase increases, the coercive field increases. Meanwhile, the saturation magnetization decreases and the coercivity of the composites increases with the increase of phase fraction of PMZNT, because the interaction between magnetic grains (or magnetic connectivity) is weakened by the existence of nonmagnetic PMZNT phase distributed in the magnetic phases. Under an applied magnetic and electric field, the magnetization and polarization of the composites can be easily tuned. The sintered composites possess high density and fine-grained microstructure. The average grain size of NiCuZn ferrite grains is slightly larger than that of the PMZNT grains.     

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.     

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.     

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.