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

Mehmet A. Akbas* and Peter K. Davies*

Department of Materials Science and Engineering, University of Pennsylvania,Philadelphia, Pennsylvania 19104–6272

The structures of compositions across the Ba(Mg1/3Nb2/3)-O3–BaZrO3 (BMN–BZ) system have been examined usingX-ray diffractometry and transmission electron micros-copy, and their dielectric properties have been character-ized in the microwave range. Although pure BMN adopts a1:2 ordered structure, of space groupPm31, additions of5–15 mol% BZ stabilize a cubic (Fm3m), 1:1 ordered phasewith a doubled perovskite repeat. At higher levels of sub-stitution (>25 mol% BZ), the B-site cations are disordered.After normal sintering, the niobates in the 1:1 phase fieldare comprised of nanometer-sized ordered domains thatare dispersed in a disordered matrix. However, by reducingthe cooling rate to 10°C/h, a fully ordered microstructure isformed with domain sizes >100 nm in size. The structure ofthe 1:1 phases has been interpreted using a ‘‘random-layer’’ model, in which one site is occupied by niobium, andthe second is occupied by a random distribution of theremaining cations. The addition of small concentrations ofBZ produces a 100% improvement in the dielectric-lossproperties of BMN, and a Qzf value of 82000 is obtained fora 5 mol% substitution.

I. Introduction

THE proliferation of commercial wireless technologies, suchas cellular phones and global positioning systems, has

placed increasing demands on the performance of dielectricresonators in the microwave frequency range. To be effective,these ‘‘microwave ceramics’’ must combine a high relativepermittivity («8r > 25) with a low dielectric loss (Q > 5000,whereQ 4 1/tand) and a near-zero temperature coefficient ofresonant frequency (tf). These criteria are satisfied in systemssuch as BaTi4O9, Ba2Ti9O20, Sn-ZrTiO4, Ba6−3xRE8+2xTi18O54(where RE is a rare-earth element), and Ba(Zn1/3Ta2/3)O3.1–7

Of the commercialized microwave ceramics, the so-called‘‘super Q’’ mixed-metal perovskites have the lowest losses(Q > 12000 at 10 GHz) and are widely used in high-frequencyapplications.6 The highestQ values have been reported for‘‘1:2’’ mixed-metal tantalate perovskites, such as Ba(Zn1/3Ta2/3)-O3 (BZT) and Ba(Mg1/3Ta2/3)O3 (BMT). In these systems, theB-site cations are stoichiometrically ordered in an hexagonalunit cell (Pm31), and the Ta5+ and Zn(Mg)2+ cation layers havea {Zn(Mg)2+Ta5+Ta5+} repeat sequence along the {111} crys-tallographic planes of the parent cubic cell (Fig. 1). The cation

layers are separated by a close-packed anion layer, which isdisplaced toward the smaller pentavalent cations. Excellent re-views of the ordered perovskite structures can be found else-where.8–10

Previous research has shown that the microwave-loss prop-erties of 1:2 perovskite ceramics are very sensitive to the B-sitecation ordering, and theQ value improves as the degree oforder increases.6,11 These improvements can be induced byannealing the ceramics at high temperatures for long soaktimes; for example, theQzf value of BZT increases from∼6000to 168000 after heating at 1350°C for 120 h.11 The beneficialeffect of the increase in order has been attributed to the reduc-tion in the volume of lossy intragrain domain boundaries.12,13

Tamuraet al.4 showed that the microwave-loss properties ofBZT can also be enhanced through the substitution of smallconcentrations (<4 mol%) of BaZrO3 (BZ). The addition of BZdramatically reduces the sintering time required to access ahigh-Q state, and aQzf value of 105000 has been obtained forBZT–4 mol% BZ after sintering for 4 h at1500°C.4 With theelimination of the need for an extended high-temperature treat-ment, this substitution enables these systems to be commer-cialized for a wide range of applications.

The first investigations of the structure of the high-Q BZT–BZ ceramics indicated that the cations were disordered, whichseemed to contradict the observations of the beneficial effect ofcation order on dielectric loss in pure BZT and BMT. Theseapparent contradictions were resolved in recent investigationsof the system using transmission electron microscopy (TEM)and X-ray diffractometry (XRD).12,13 At low levels of substi-tution (#2.15 mol% BZ), the solid solutions retained the struc-ture of the BZT end member; however, the size of the 1:2ordered domains decreased systematically as the amount of BZ

P. P. Phule—contributing editor

Manuscript No. 191176. Received February 26, 1997; approved June 16, 1997.Supported in part by the MRSEC Program of the National Science Foundation

under Award No. DMR 96-32598 and by Grant No. DMR 94-21184 (CeramicsDivision). Support from the electron microscopy facility at the National ScienceFoundation, through the MRSEC program, is also acknowledged.

Presented at the 98th Annual Meeting of the American Ceramic Society, Indian-apolis, IN, April 14–17, 1996.

*Member, American Ceramic Society.

Fig. 1. Schematic illustration of 1:2 cation ordering in Ba(Mg1/3Nb2/3)O3; the upper-left illustration shows two of the four possible⟨111⟩directions for the ordering of magnesium and niobium, and the lower-right illustration shows the arrangement of magnesium and niobiumfor one of the orientational variants. Oxygen anions have been omittedfor clarity.

J. Am. Ceram. Soc., 81 [3] 670–76 (1998)Journal

670

increased. The highQ values of these ceramics were attributedto the stabilization of the ordering-induced domain boundariesby the partial segregation of zirconium. Those studies alsorevealed that when the level of substitution is increased to∼4mol%, the system adopts a cubic ‘‘1:1’’ ordered cell with adoubled perovskite repeat (Fig. 2). This structure was subse-quently shown to be stable up to∼25 mol% BZ, and a fullydisordered cation arrangement was only formed for substitutionlevels >35 mol%.14 The positions of the phase boundaries inthe BZT–BZ system coincided with compositions previouslyshown to exhibit abrupt changes in the variation ofQ with BZcontent.4,14

Because of their very highQ values, most studies of themicrowave properties of the 1:2 family of perovskites havefocused on tantalate compositions such as BZT and BMT.4,6,13

Although the dielectric properties of the corresponding niobatesystems have been examined, their losses are somewhat higherthan their tantalate counterparts (e.g., for BMN,Q 4 5600 at10.5 GHz).15 Compared to the tantalates, the niobates arecheaper to manufacture and have a higher relative permittivity.Therefore, it may be of interest to explore whether their di-electric properties can also be optimized via chemical substi-tution. In this paper, we examine the effect of the substitutionof BZ on the microstructure and microwave properties of BMNand compare the behavior of this system to its tantalate coun-terpart.

II. Experimental Methods

Ceramics in the (1 −x)BMN–(x)BZ system were preparedvia the mixed-oxide method, using high-purity (>99.9%)BaCO3, MgO, Nb2O5, and ZrO2 as the starting materials. Stoi-chiometric proportions of these powders were weighed andmixed in an agate mortar for 10 min. The mixture was precal-cined at 1100°C for 12 h and milled in acetone for 4 h withY2O3-stabilized ZrO2 balls. The slurry was dried and calcinedat 1375°C for 12 h. The resultant calcine was ball milled foranother 12 h and uniaxially pressed at 100 MPa into pellets thatwere 8 mm in diameter and several millimeters thick. For thedielectric measurements, the samples were isostatically pressedat 600 MPa before sintering. The final sintering was conductedat 1640°C for 10 h in air using a MoSi3 muffle furnace with aheating and cooling rate of 300°C/h. In all cases, the densitywas >94% of the theoretical value.

The relative permittivity of the ceramics was measured in theGHz range using the parallel-plate method, combined with anetwork analyzer and computer.16,17The accuracy of this mea-surement is better than ±0.25%. Measurements of the dielectricloss (Q) and the temperature coefficient of resonant frequency(tf) were made at microwave frequencies using cavity meth-ods.18 The tf values refer to the 25°–60°C range, according to

tf 4 DfzfozDT, where Df is the change in frequency,fo thereference frequency at 25°C, andDT the temperature.

XRD (Model Geigerflex D-max-B diffractometer, Rigaku,Tokyo, Japan) was used to determine the phase purity, and thelattice parameters were calculated via the least-squares methodusing data collected with an internal silicon standard. The mi-crostructures of the ceramics were characterized using a TEMmicroscope (Model 400 EM, Philips Electronic Instruments,Mahwah, NJ) that was operated at 120 kV. Thin foils for theTEM investigations were prepared by mechanically grindingthe ceramic pellets to a thickness of∼20–30mm and polishingboth sides with 5, 1, and 0.25mm Al2O3 powder. The sampleswere mounted on copper aperture grids, and a final thinningwas conducted via argon-ion milling (Model Model 600 DualIon Mill, Gatan, Pleasanton, CA) at 6 kV and 0.6 mA, followedby a carbon coating, to prevent charging.

III. Results and Discussion

(1) Phase Stability: XRD and Electron Diffraction StudiesFigure 3 shows the 110 zone-axis electron diffraction pattern

of the (1 −x)BMN–(x)BZ ceramics as a function ofx. The 1:2order in pure BMN (x 4 0) is characterized by superlatticereflections located at (h ±1⁄3, k ±1⁄3, l ±1⁄3) along either of the⟨111⟩ crystallographic directions (marked by arrows in Fig.3(A)). The presence of these spots indicates a tripling of theunit cell and the formation of a twin-related 1:2 ordered domainstructure. The addition of BZ stabilizes a face-centered cubic(Fm3m), 1:1 ordered, ‘‘Ba(b81/2b91/2)O3’’-type structure, whichis identified by the F-spots at (h ±1⁄2, k ±1⁄2, l ±1⁄2) in theelectron diffraction patterns of samples withx 4 0.05 and 0.10(marked by arrows in Figs. 3(B) and (C)). For small concen-trations of BZ (0 <x # 0.05), the samples reside in a two-phasefield and the patterns contain reflections from the 1:2 and 1:1ordered phases (Fig. 3(B)). Single-phase 1:1 order is observedat higher BZ contents (e.g.,x 4 0.1 (Fig. 3(C))), and a disor-dered cation distribution is stabilized at compositions withx >0.25 (Fig. 3(D)).

Analyses of these samples via XRD confirmed the results ofthe electron diffraction study. The changes in the cation order-ing were readily reflected by the change in position and inten-sity of the superlattice reflections, which are indicated in Fig.4. The additional reflections from the 1:2 ordering in pureBMN are replaced by those which correspond to the 1:1 or-dering forx 4 0.1 and 0.15, whereas the sample withx 4 0.25could be indexed in terms of a single-phase disordered perov-skite.

The destabilization of the 1:2 ordering in BMN and thetransformation to a 1:1 ordered structure is identical to thephase behavior of the BZT–BZ and BMT–BZ systems.14,19

However, the niobate system shows small differences in therange of stability of the 1:1 ordered phase, which, in contrast tothe tantalates, has not been formed atx 4 0.25. This differencecould result from the impact of the cooling history on theordering in BMN–BZ, which became evident during the stud-ies of the microstructural features of the BMN–BZ and BMT–BZ systems.

(2) Ordering-Induced Microstructures(A) 1:2 Ordered BMN: The dark-field TEM image in Fig.

5, which has been collected using the (200) reflection, showsthe microstructure of a typical grain in a single-phase 1:2 or-dered sample of BMN. Each grain contains large (>100 nm)ordered domains that seem to nucleate at the grain boundaries.The growth of the domains can occur along any one of the fourequivalent {111} crystal planes and produces twin-related in-terfaces that lie along the⟨111⟩ directions. Antiphase bound-aries (APB) are also a general feature of the microstructure(marked by arrows in Fig. 5) and are mostly located in thevicinity of the domain boundaries.

The crystallization of the hexagonal 1:2 ordered structureFig. 2. Schematic representation of the doubled perovskite cell of thecubic Fm3m), 1:1 ordered, A(b81/2b91/2)O3 structure.

March 1998 Ordering Induced Microstructures and Microwave Dielectric Properties of the BMN–BZ System 671

can occur through the formation of equivalent orientationalvariants, with thec-axis of each variant lying along one of thefour ⟨111⟩ directions of the parent subcell13 (Fig. 1). For anygiven 110 zone axis, contrast differences that are associatedwith the ordering are only visible for the two variants that havetheir c-axes oriented perpendicular to the beam direction; thisis consistent with the absence of any contrast in∼50% of thedark-field image in Fig. 5.

(B) 1:1 Ordered BMN–BZ: The diffraction studies indi-cated that the phase relations in BMN–BZ are very similar tothose that have been determined previously for the BZT–BZand BMT–BZ systems.14,19 However, the microstructural fea-tures of the 1:1 ordered phases in the niobates and tantalateswere found to be very different. The structure of the 1:1 or-dered samples of BMN–BZ was examined via dark-field mi-croscopy using the (3⁄2,3⁄2,3⁄2) supercell reflection and was com-pared to the corresponding phases in the BMT–BZ system. The

most-detailed investigations were conducted on 1:1 orderedsamples of BMN and BMT that each contained 10 mol% BZ.The synthesis conditions for the BMT–BZ specimen, whichwas prepared as part of a related study of that system,19 wereidentical to those used for BMN–BZ.

Figure 6(A) shows the microstructure of 1:1 ordered BMN–10% BZ after normal sintering at 1640°C. The grains contain1:1 ordered domains that are 4–5 nm in size and surrounded bya disordered matrix. More than 50% of the sample is disor-dered. The microstructure of the niobate is markedly differentfrom that of its tantalate counterpart (BMT–10% BZ), which isfully ordered after experiencing the same thermal processingtreatment (Fig. 6(B)). In addition to the absence of any disor-dered second phase, the microstructure of the tantalate alsocontains much-larger 1:1 domains (∼200 nm), which are sepa-rated by APBs. The reasons for these differences will be dis-cussed in the following section.

Fig. 3. [110] electron diffraction patterns of BZ-substituted BMN (the amount of BZ is (A) 0 mol% (x 4 0.0), (B) 5 mol% (x 4 0.05), (C) 10mol% (x 4 0.1), and (D) 35 mol% (x 4 0.35)).

672 Journal of the American Ceramic Society—Akbas and Davies Vol. 81, No. 3

The microstructures that are observed in the 1:1 orderedBMN–BZ phases are very similar to those reported for lead-based perovskite relaxor ferroelectrics such as Pb(Mg1/3Nb2/3)-O3 (PMN) and Pb(Mg1/3Ta2/3)O3 (PMT).20,21For the relaxors,the formation of a phase-separated ‘‘1:1 ordered nanodomain +disordered matrix’’ structure has been interpreted using a‘‘space-charge’’ model.22,23 According to this model, theb8and b9 sites in the 1:1 ordered Pb(b81/2b91/2)O3 domains areexclusively occupied by Mg2+ and Nb5+, respectively. To con-serve overall electroneutrality, the net negative charge of thedomains is assumed to be compensated by a positively charged,Nb5+-rich disordered matrix that surrounds the ordered do-mains. If this model is correct, any growth of the nanosizedordered domains will be severely inhibited by the associatedspace charge. The primary support for this model for the PMNfamily of relaxors originates from the absence of any domaincoarsening with extended thermal treatment.20,24

(3) Domain Growth in BMN–BZTo explore the validity of a space-charge model, we at-

tempted to modify the microstructures of the 1:1 orderedBMN–BZ samples using different thermal treatments. To pro-mote the growth of the ordered domains, it is clearly necessaryto use conditions that reside in the region of thermodynamicstability of the 1:1 phase and yet allow significant cation dif-fusion. For the BMN–BZ system, the range of temperature thatsatisfied both conditions was quite narrow and was initiallyidentified using slow-cooling experiments, to examine the larg-est-possible temperature interval. In these experiments, thesamples were heated to the sintering temperature (1640°C) andcooled to 1000°C at a rate of 10°C/h instead of the faster rate(300°C/h) that was used in the normal sintering cycle. Figure 7compares the dark-field TEM images collected from BMN–

10% BZ after normal sintering (Fig. 7(A)) and after slow cool-ing (Fig. 7(B)). The slow-cooled samples are essentially fullyordered, with domain sizes of >100 nm. The large increase inthe degree of order and domain size is also evident in the X-raypatterns collected from samples that were cooled at differentrates (Fig. 8). The peaks associated with the 1:1 order in thepattern of the slow-cooled sample are sharper and have a sig-nificantly higher relative intensity.

From these results, it is clear that a space-charge modelcannot be accepted as a valid explanation for the 1:1 order inBMN–BZ ceramics. Instead, the experimental data favor a‘‘random-layer’’ model, which was previously proposed to ex-plain the 1:1 order in the BZT–BZ and BMT–BZ sys-tems.13,19,25In this model, for the ordered Ba(b81/2b91/2)O3 struc-ture, theb9 positions are assumed to be exclusively occupiedby Nb5+, and theb8 sites are assumed to be occupied by arandom mixture of Mg2+, Zr4+, and the remaining Nb5+ cations(Fig. 2). For the random-layer structure, the 1:1 ordered(1 − x)BMN–(x)BZ solid solutions can be represented asBa[(Mg(2−z)/3Nb(1−2z)/3Zrz)1/2Nb1/2]O3, wherez 4 2x. In thiscase, the 1:1 ordered domains are charged balanced and theoverall 1:2 cation stoichiometry is preserved in all regions ofthe samples.

(4) High-Temperature Phase EquilibriaAlthough the random-layer model readily explains the coars-

ening of the domains during slow cooling, a satisfactory modelmust also be able to rationalize the formation of the nanodo-main + disordered matrix two-phase assemblage that is ob-served in the as-sintered samples (Fig. 7(A)). The previousexplanations for the existence of this type of microstructure in1:2 mixed-metal perovskites have been based on the physicallimitations that are implemented by the atomistic model, i.e.,the space charge associated with the domain–matrix chargeimbalance. By examining the stability of the 1:1 structures at

Fig. 4. XRD patterns of (1 −x)BMN–(x)BZ.

Fig. 5. Dark-field TEM micrograph of pure BMN collected using the(200) reflection; an electron diffraction pattern is shown in the inset.

March 1998 Ordering Induced Microstructures and Microwave Dielectric Properties of the BMN–BZ System 673

high temperature, we have found that the formation of thenanodomains in BMN–BZ can be explained using conventionalphase equilibria.

To examine the phase stabilities at elevated temperatures, aseries of BMN–BZ compositions were air quenched from tem-peratures as high as 1640°C and examined using XRD. Figure8 compares the pattern collected from a sample of BMN–10%BZ that was quenched from 1640°C to those obtained afterslow cooling (10°C/h) and normal sintering (300°C/h). Thevery weak and diffuse supercell reflections are barely discern-ible in the air-quenched samples. We concluded that any or-dering that is present in this sample occurred during the quenchand, at 1640°C, the sample resides in a disordered perovskitephase field. Samples quenched from 1600° and 1550°C weredisordered and ordered, respectively, and the boundary be-tween 1:1 order and complete disorder for BMN–10% BZ wasestimated to lie between these temperatures. Quenching experi-ments that were performed on samples with different compo-sitions showed that the temperature of the order–disorder trans-formation decreased as the BZ concentration increased. Thelowering of the transition temperature at higher BZ contentsprobably explains why the range of homogeneity of the 1:1phase observed in the as-sintered samples was narrower thanthat found in the corresponding tantalate systems. Althoughthis is the first direct observation of an order–disorder transi-tion for the 1:1 phases in these systems, these transitions have

been reported previously for some 1:2 ordered niobate endmembers, e.g., Ba(Ni1/3Nb2/3)O3, Ba(Co1/3Nb2/3)O3, andBa(Zn1/3Nb2/3)O3.26–28

Recognizing the existence of this transformation at tempera-tures below those used in sintering, the variation of the micro-structures of 1:1 BMN–BZ with thermal history can be readilyexplained with the aid of the schematic phase diagram that isshown in Fig. 9. Although the general features of the phasediagram are based on our experimental data, quantification ofthe exact reaction types, temperatures, and compositions re-quires additional work. Similarly, the nature of the transforma-tion reactions between the different ordered phases (i.e., firstorder versus second order) has not been characterized. Becausethe sintering temperature of BMN–10% BZ resides within thestability field of a disordered perovskite structure, the transfor-mation to the 1:1 ordered state and, therefore, the growth of theordered domains can only occur during the cooling cycle.Rapid cooling can almost completely preserve the metastabledisordered phase; the slower cooling in the standard sinteringprocedure results in the nucleation and partial growth of 1:1nanodomains and a metastable two-phase assemblage, whereasslow cooling induces complete order and a larger domain size.

A similar approach can be used to explain the differences inthe microstructures of the tantalates and niobates. A series ofquenching experiments conducted on BMT–10% BZ revealedthat the degree of order was independent of the cooling rate.19

Fig. 6. Dark-field TEM micrographs collected using the (3⁄2,3⁄2,3⁄2)supercell reflections of (A) BMN–10 mol BZ and (B) BMT–10 mol%BZ.

Fig. 7. Dark-field JEM micrographs of BMN–10 mol% BZ (x 40.1) collected using the (3⁄2,3⁄2,3⁄2) supercell reflections ((A) sintered at1640°C for 10 h and cooled at a rate of 300°C/h, and (B) sintered at1640°C for 10 h and cooled to 1000°C at a rate of 10°C/h).

674 Journal of the American Ceramic Society—Akbas and Davies Vol. 81, No. 3

Apparently, the disordering temperatures for the tantalates aresignificantly higher than the niobates, and the 1:1 orderedphases are stable at the sintering temperature. Therefore, eachof the different stages in the processing of the tantalate ceram-ics (calcination and sintering) are conducted in the region ofstability of 1:1 order (see Fig. 9). In this case, there is sufficienttime for significant domain growth and achievement of a fullyordered microstructure.

(5) Microwave Dielectric Properties of (1 − x)BMN–(x)BZThe dielectric properties of the as-sintered BMN–BZ ceram-

ics were measured at microwave frequencies. At 10 GHz, therelative permittivities of the as-sintered BMN–BZ ceramicswere in the range of 31–34; the value oftf increases from avalue of 21 forx 4 0 to a value of 60 forx 4 0.35. The valuesobtained for the losses are listed in Table I and are plotted asa function of composition in Fig. 10. For pure BMN, the mea-suredQzf value (38000) is in good agreement with the previ-ously reported value.15 The value ofQ increased by >100%with the substitution of small concentrations of BZ; for ex-ample, forx 4 0.05, theQzf value is 82000. When the con-

centration of zirconium was increased tox 4 0.1, theQzf valuedecreased to 55000; forx 4 0.35, theQzf value increased to77000.

The beneficial effect of low-level substitution onQ is similarto the reduced losses that were reported for 4-mol%-BZ-dopedBZT,4 although the magnitude of the increase for the BMNsystem is somewhat larger. For the BZT system, it has beensuggested that, for low concentrations of BZ, which retain a 1:2ordered structure, the zirconium cations preferentially segre-gate to the 1:2 domain boundaries and reduce their contributionto the loss.12,13It is possible that the large increase inQ for theBMN system results from a similar segregation phenomenon.However, the as-sinteredx 4 0.05 sample actually resides inthe ‘‘1:2 + 1:1’’ two-phase field. Although samples with lowerconcentrations of BZ have not been explored in this investiga-tion, it would be of interest to study the dielectric properties ofsamples that reside in the narrow stability region of the 1:2structure, to determine whether theQzf value for thex 4 0.05sample represents a maximum for the system.

The variation ofQ at the higher concentrations of BZ isclearly quite unpredictable. In this region, the 1:1 and, ulti-mately, the disordered perovskite are stable. However, recallthat the microstructural features of the as-sintered samples arefar from equilibrium. Therefore, the dielectric properties in Fig.10 represent the response of a metastable assemblage of 1:1nanodomains, a disordered matrix, and the boundaries betweenthem. It would be expected that the dielectric response of thesecompositions will be sensitive to changes in the cooling historyof the ceramic. Indeed, it may be possible to tailor the micro-wave properties of the niobates by controlling their microstruc-ture through alterations in the thermal treatment and coolinghistory. These possibilities are currently being explored.

V. Conclusions

Investigations of the phase stability in the (1 −x)BMN–(x)BZ system have shown that the ceramics adopt a 1:2, 1:1,

Fig. 8. XRD patterns of BMN–10 mol% BZ (slow cooled from1640°C at a rate of 10°C/h (pattern ‘‘A’’), as-sintered (pattern ‘‘B’’),and air quenched from 1640°C (pattern ‘‘C’’)).

Fig. 9. Schematic phase diagram showing the possible phase rela-tions for the Ba(Mg1/3

2+ M2/35+ )O3–BaZrO3 systems (M4 Ta and Nb); an

arbitrary temperature axis is used to highlight that, at the sinteringtemperature (∼1640°C), the niobates reside in a disordered phase field,while the tantalates reside in the 1:2 and 1:1 ordered phase fields.

Table I. Dielectric Loss Properties of the(1 − x)BMN–(x)BZ System

Composition,xDielectricloss,Q

Microwavefrequency,

f (GHz) Q?f

0.0 3900 9.712 378800.05 7900 10.296 813400.1 5800 9.604 557000.15 6800 9.800 666400.25 8100 9.506 770000.35 8300 9.358 77680

Fig. 10. Dielectric loss (Qzf ) versus composition (x) for the(1 − x)BMN–(x)BZ system.

March 1998 Ordering Induced Microstructures and Microwave Dielectric Properties of the BMN–BZ System 675

and disordered B-site cation distributions for 0.0 <x < 0.05,0.05 < x < 0.25, andx > 0.25, respectively. The final micro-structure of the niobates is very sensitive to the thermal historyof the samples. For example, as-sintered samples withx 4 0.1have a phase-separated microstructure that consists of meta-stable disordered and stable 1:1 ordered phases. A fully orderedstructure that is comprised of domains >100 nm in size couldbe produced by reducing the cooling rate to 10°C/h. The sen-sitivity of the ordering to the cooling history can be attributedto an order–disorder reaction that occurs in the temperaturerange of 1550°–1600°C. The structure of the 1:1 ordered do-mains can be interpreted, using a charge-balanced, random-layer model. The substitution of small concentrations of BZproduces a significant increase in the dielectric lossQ, and aQzf value of 82000 is measured for samples wherex 4 0.05.

Authors’ Note: During the review of this article, we discovered that theordering-induced microstructures of the lead-based relaxor system Pb(Mg1/3Ta2/3)-O3–PbZrO3 can also be modified by high-temperature thermal treatment (seeRef. 29).

Acknowledgments: The authors thank Trans-Tech, particularly Dr.Taki Negas and Dr. Steve Bell, for measuring the microwave dielectric prop-erties.

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