Crystal Structure and Microwave dielectric Properties of Ba(Zn1/3Ta2/3)O3–(Sr,Ba)(Ga1/2Ta1/2)O3 Ceramics

The microwave dielectric properties and crystal structure of Ba(Zn_1/3Ta_2/3)O₃– (Sr,Ba)(Ga_1/2Ta_1/2)O₃ ceramics were investigated in the present study. The Q value of Ba(Zn_1/3Ta_2/3)O₃ was improved by adding 5 mol% Sr(Ga_1/2Ta_1/2)O₃. The maximum Q value of Q × f = 162000 GHz was obtained at 0.95Ba(Zn_1/3Ta_2/3)O₃. 0.05Sr(Ga_1/2Ta_1/2)O₃. For this composition, a lattice super structure caused by hexagonal ordering was observed. A further improvement in the Q value was attained when some Sr was replaced with Ba, and 0.95Ba(Zn_1/3Ta_2/3)O₃· 0.05(Sr_0.25Ba_0.75)(Ga_1/2Ta_1/2)O₃ exhibited a maximum Q value such that Q × f = 210000 GHz. Despite the increased Q value with the replacement of Sr by Ba, the c/a value, which indicates the degree of lattice distortion, remained constant near 3/2. The Q value thus improved without lattice distortion in the system Ba(Zn_1/3Ta_2/3)O₃-(Sr,Ba)(Ga_1/2Ta_1/2)O₃, whereas the improvement of Q value increased with lattice distortion in the solid solution system with Ba(Zn_1/3Ta_2/3)O₃ as an end member.     

Raman Spectroscopic Study of Gallium-Doped Ba(Zn1/3Ta2/3)O3

Resonators of Ba(Zn_1/3Ta_2/3)O₃, sintered between 1450° and 1600°C, are characterized by Raman spectroscopy, X-ray diffraction, and scanning electron microscopy. The quality factors of the resonators are found to depend on sintering temperature, and at 1600°C there is evidence of Zn loss from the surface. The frequency of the A_1g Raman mode changes from 800.9 cm⁻¹ for a sample with Q = 80000 (2 GHz), to 794.5 cm⁻¹ when Q = 44000 (2 GHz). Changes in the position of this and other Raman modes are thought to be due to distortions of the oxygen octahedra, brought about by Zn loss. The presence of a BaTa₂O₆ phase at the surface is confirmed by XRD and SEM.     

Pb(Zn1/3Ta2/3)O3–PbTiO3 Derived from Mechanical Activation

Nanocrystalline Pb(Zn_1/3Ta_2/3)O₃ (PZTa) of perovskite structure, which cannot be synthesized by either the conventional solid-state reaction of mixed oxides or wet chemistry routes, has been successfully synthesized via a mechanical activation route. The effects of PbTiO₃ (PT) doping in PZTa on the phase formation, thermal stability of the nanocrystalline perovskite phase, and dielectric properties of the resulting PZTa–PT are studied. PZTa doped with a low PT content exhibited a diffuse phase transition, while those with high PT contents demonstrated an expected sharp phase transition at the Curie temperature.     

Dielectric Properties of Ceramics in Ba (Co1/3 Nb2/3)O3–Ba(Zn1/3Nb2/3)O3 Solid Solution

The dielectric properties of the Ba (Co_1/3 Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ system were determined. Ba (Co_1/3 Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ has a complex perovskite structure, a high dielectric constant, a low dielectric loss, and a low temperature coefficient of the resonant frequency. A solid-solution ceramic with 0.7Ba (Co_1/3 Nb_2/3)O₃·0.3 Ba(Zn_1/3Nb_2/3)O₃ has a dielectric constant of K=33.5, Q=11000 at 6.5 GHz, and a temperature coefficient of the resonant frequency of τ_f=0 ppm/°C. The temperature coefficient of resonant frequency can be varied by changing the composition. The Q values of the ceramics can be increased by annealing in a nitrogen atmosphere. These ceramics can be used for resonant elements and stabilized oscillators.     

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.     

Ba(Zn1/3Ta2/3)O3 Ceramics with Low Dielectric Loss at Microwave Frequencies

The dielectric properties at microwave frequencies of Ba(Zn_1/3Ta_2/3)O₃ ceramics prepared by sintering were investigated. These ceramics had lower density but higher loss quality than ceramics hot-pressed at 1400°C. Loss quality was greatly improved by prolonged sintering. The Q of the ceramics measured by the dielectric resonator method was 14 000 at 12 GHz. The ceramics were investigated by X-ray diffraction analysis. It was found that Q improvement corresponds with increased Zn and Ta ordered structures in the ceramics.     

Ordering-Induced Microstructures and Microwave Dielectric Properties of the Ba(Mg1/3Nb2/3)O3–BaZrO3 System

The structures of compositions across the Ba(Mg_1/3Nb_2/3)-O₃-BaZrO₃ (BMN–BZ) system have been examined using X-ray diffractometry and transmission electron microscopy, and their dielectric properties have been characterized in the microwave range. Although pure BMN adopts a 1:2 ordered structure, of space group Pm     1, additions of 5–15 mol% BZ stabilize a cubic ( Fm     m ), 1:1 ordered phase with a doubled perovskite repeat. At higher levels of substitution (>25 mol% BZ), the B-site cations are disordered. After normal sintering, the niobates in the 1:1 phase field are comprised of nanometer-sized ordered domains that are dispersed in a disordered matrix. However, by reducing the cooling rate to 10°C/h, a fully ordered microstructure is formed with domain sizes >100 nm in size. The structure of the 1:1 phases has been interpreted using a "random-layer" model, in which one site is occupied by niobium, and the second is occupied by a random distribution of the remaining cations. The addition of small concentrations of BZ produces a 100% improvement in the dielectric-loss properties of BMN, and a Q · f value of 82000 is obtained for a 5 mol% substitution.     

Ordered Structure and Dielectric Properties of Lanthanum-Substituted Ba(Mg1/3Ta2/3)O3

Both 1:2 and 1:1 ordered structures form in the perovskite solid solutions of La-substituted BMT Ba_{1− x} La _x (Mg_{(1+ x )/3}Ta_{(2− x )/3})O₃, sintered at 1600°C. The 1:2 ordered structure exists in the composition range 0.0 ≤ x ≤ 0.12, while that of 1:1 ordered structure exists in a wider composition range 0.04 ≤ x ≤ 1.0. Two ordered phases coexist in 0.04 ≤ x ≤ 0.12. High-resolution micrographs indicate that 1:2 and 1:1 ordered domains coexist in one grain. The ordering parameter of 1:2 phase decreases with x , yet that of 1:1 phase increases with x . Both increase with soak time. Variations in ordering are discussed in terms of cation occupancy and crystal chemistry. The quality factor increases with x , reaches a maximum, then decreases with x . The dielectric constant increases with x first, and levels off.     

Order-Disorder Phase Formation in the Complex Perovskite Compounds Ba(Ni1/3Nb2/3)O3 and Ba(Zn1/3Nb2/3)O3

The order-disorder phase formation of the complex perovskite compounds Ba(Ni_1/3Nb_2/3)O₃ (BNN) and Ba(Zn_1/3-Nb_2/3)O₃ (BZN) was investigated using X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and energy-dispersive spectroscopy. The BNN and BZN samples were sintered over a temperature range of 1200° to 1500°C in air for 2 h. X-ray diffraction and transmission electron microscopy showed that these compounds exhibited a 1:2 ordering on the B-site within a narrow temperature range. When BNN and BZN were sintered above 1400° and 1350°C, respectively, a liquid phase formed in the grain boundary which was accompanied by disordering. The composition of the liquid phase resembled that of pyrochlore, with a small amount of nickel for BNN or zinc for BZN. The disordering with the formation of the liquid phase was attributed to the increase in defect concentration.     

Microstructure and Microwave Dielectric Properties of Ba(Zn1/3Ta2/3)O3 Ceramics with ZrO2 Addition

The effect of ZrO₂ on crystallographic order, microstructure, and microwave dielectric properties of Ba(Zn_1/3Ta_2/3)O₃ (BZT) ceramics was investigated. A small amount of ZrO₂ disturbed the 1:2 cation ordering. The average grain size of the BZT significantly increased with the addition of ZrO₂, which was attributed to liquid-phase formation. The relative density increased with the addition of a small amount of ZrO₂, but it decreased when the ZrO₂ content was increased. Variation of the dielectric constant with ZrO₂ addition ranged between 27 and 30, and the temperature coefficient of resonant frequency increased abruptly as the ZrO₂ amount exceeded 2.0 mol%. The Q value of the BZT significantly improved with the addition of ZrO₂, which could be explained by the increased relative density and grain size. The maximum Q × f value achieved in this investigation was ∼164 000 GHz for the BZT with 2.0 mol% ZrO₂ sintered at 1550°C for 10 h.     

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.     

Dielectric-State Analysis in Solid-Solution Pb(Yb1/2Ta1/2)O3–Pb(Lu1/2Nb1/2)O3

The structure and temperature dependence of complex lead perovskite dielectrics were investigated for the system (1 − x )Pb(Yb_1/2Ta_1/2)O₃– x Pb(Lu_1/2Nb_1/2)O₃. Superlattice reflections for the compositions 0.8 < x < 1.0 were observed by X-ray diffractometry, and the temperature-composition dielectric-state diagram was determined. In the present study, the disordered middle composition, with 0.2 < x < 0.8, showed a diffuse paraelectric–ferroelectric phase transition, whereas the ordered end compositions, with 0 ≤ x < 0.2 and 0.8 < x ≤ 1.0, revealed successive sharp paraelectric–antiferroelectric and weak antiferroelectric–ferroelectric phase transitions. The dielectric state was confirmed by examining the variation of polarization ( P ) with electric field ( E ).     

Microscopic Calculation of Dielectric Loss at Microwave Frequencies for Complex Perovskite Ba(Zn1/3Ta2/3)O3

The dielectric loss tangent at microwave frequencies for the complex perovskite Ba(Zn_1/3Ta_2/3)O₃ was calculated with respect to the degree of structural disorder on B sites. Starting out from the equations of ion motion, dielectric loss was expressed in terms of the pair-correlation functions corresponding to the ordering of Zn and Ta ions on B sites. The characteristic length included in the pair-correlation functions corresponds to the average size of the region containing disorder in ion arrangements on B sites; thus the relation between the structural disorder on the B site and the dielectric loss tangent at microwave frequencies was clarified theoretically. The numerical results show that the microwave loss tangent values change their power from – 3 to – 6 with increasing degree of order on the B site, which agrees well with the experimental observations. Results obtained here confirm the physical origin of the microwave loss of complex perovskite Ba(Zn_1/3Ta_2/3)O₃.     

Perovskite Formation and Dielectric Characteristics of Pb(Zn1/3Ta2/3)O3 with PbTiO3 Substitution

Perovskite developments in the Pb(Zn_1/3Ta_2/3)O₃–PbTiO₃ system were explored. Formation yields and lattice parameters of the perovskite were determined from X-ray diffractometry results. Weak-field low-frequency dielectric properties of the system ceramics were investigated, followed by microstructure examination. Perovskite started to develop in Pb(Zn_1/3Ta_2/3)O₃ after the introduction of 30 mol% PbTiO₃, whereas complete stabilization was accomplished at 60% substitution. Dielectric relaxation behavior was not substantial across the entire composition range, whereas phase transition modes changed from diffuse to sharp with increased PbTiO₃ fraction.     

Determination of the Compositional Fluctuation in the Perovskite Ternary System PbZrO3–PbTiO3–Pb(Mg1/3Ta2/3)O3

A method was developed for the determination of a region of compositional fluctuation in the perovskite ternary solid-solution system PhZrO₃-PbTiO₃-Pb(Mg_1/3Ta_2/3)O₃. The compositional fluctuation in the ternary system extends two-dimensionally on the phase diagram. The method described in this paper utilized the fact that the compositional fluctuation region abuts on the curves of composition whose lattice spacing is the lower or higher fluctuation limit. The fluctuation limits of the lattice spacings were estimated from X-ray diffraction analysis.     

Preparation of Ba(Mg1/3Ta2/3)O3 Using Oxine

A solution containing Mg²⁺ and Ta⁵⁺ was added to an aqueous ammonia solution of oxine, resulting in a precipitate. After the precipitate was thermally decomposed and fired, it was mixed with BaCO₃ powder and fired again at high temperatures to obtain Ba(Mg_1/3Ta_2/3)O₃ (BMT). This method resulted in BMT formation at temperatures lower than those used in the conventional mixed-oxide method, and single-phase BMT formed directly at 1300°C without intermediates.     

Infrared Reflection of Ba(Mg1/3Ta2/3)O3 Ceramics

The dielectric function of the ordered Ba(Mg_1/3Ta_2/3)O₃ ceramics was investigated by the infrared reflectance spectra taken over the 50–4000-cm⁻¹ range. The detailed crystal structure of the specimen was examined by the Rietveld method. The space group of trigonal D _3d³ and the degree of long-range order of 0.97 for Mg and Ta atomic arrangement were confirmed. The reflectance spectra were analyzed on the basis of the four-parameter semiquantum model assuming 16 infrared active vibrational modes allowed for the related D _3d³ structure. The lowest-frequency optical mode was found at 60 cm⁻¹, which can be assumed to involve the motions of the heavy TaO₆ octahedra.     

Effects of Stacking Scheme on Microwave Dielectric Characteristics of Ca(Mg1/3Nb2/3)O3–Ba(Zn1/3Nb2/3)O3 Layered Dielectric Resonators

Ca(Mg_1/3Nb_2/3)O₃ (CMN) and Ba(Zn_1/3Nb_2/3)O₃ (BZN) ceramic disks were stacked with three stacking schemes, designated as CMN/BZN, CMN/BZN/CMN, and BZN/CMN/BZN, to yield layered dielectric resonators, and the microwave dielectric characteristics were evaluated with the TE_01δ mode. Both experiments and finite element analysis showed that the microwave dielectric characteristics of the layered resonator were determined not only by the volume fraction of BZN but also by the stacking scheme. For each stacking scheme, a good combination of microwave dielectric characteristics with an effective dielectric constant of 34.33–34.52, a Q × f value of 58 800–62 080 GHz, and a near-zero temperature coefficient of resonant frequency could be achieved by adjusting the volume fraction of BZN. The effects of the stacking scheme on the microwave dielectric characteristics of the temperature-stable layered resonator were discussed by combining finite element analysis and dielectric composite models.     

Perovskite Phase Formation in the Relaxor System [Pb(Fe1/2Nb1/2)O3]1-x–[Pb(Zn1/3Nb2/3)O3]x

Phase formation and dielectric properties of the compositions in the system [Pb(Fe_1/2Nb_1/2)O₃]₁_ _x –[Pb(Zn_1/3Nb_2/3)O₃] _x were investigated as possible materials for multilayer ceramic capacitors. The formation of the phase with perovskite structure and dielectric properties of ceramics at room temperature in the entire composition range are presented. The undesirable pyrochlore phase can be suppressed up to x = 0.6 by adopting calcination of B-site oxides, followed by reaction with PbO. Compositions in the single-phase range can be sintered at less than 1000°C.