Journal. Ordering-Induced Microstructures and Microwave Dielectric Properties of the Ba(Mg 1/3 Nb 2/3 )O 3 BaZrO 3 System

Transcription

1 Journal J. Am. Ceram. Soc., 81 [3] (1998) Ordering-Induced Microstructures and Microwave Dielectric Properties of the Ba(Mg 1/3 Nb 2/3 )O 3 BaZrO 3 System Mehmet A. Akbas* and Peter K. Davies* Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania The structures of compositions across the Ba(Mg 1/3 Nb 2/3 )- O 3 BaZrO 3 (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 Pm31, additions of 5 15 mol% BZ stabilize a cubic (Fm3m), 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 randomlayer 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 is obtained for a 5 mol% substitution. I. Introduction THE proliferation of commercial wireless technologies, such as cellular phones and global positioning systems, has placed increasing demands on the performance of dielectric resonators in the microwave frequency range. To be effective, these microwave ceramics must combine a high relative permittivity ( r > 25) with a low dielectric loss (Q > 5000, where Q 1/tan ) and a near-zero temperature coefficient of resonant frequency ( f ). These criteria are satisfied in systems such as BaTi 4 O 9,Ba 2 Ti 9 O 20, Sn-ZrTiO 4,Ba 6 3x RE 8+2x Ti 18 O 54 (where RE is a rare-earth element), and Ba(Zn 1/3 Ta 2/3 )O Of the commercialized microwave ceramics, the so-called super Q mixed-metal perovskites have the lowest losses (Q > at 10 GHz) and are widely used in high-frequency applications. 6 The highest Q values have been reported for 1:2 mixed-metal tantalate perovskites, such as Ba(Zn 1/3 Ta 2/3 )- O 3 (BZT) and Ba(Mg 1/3 Ta 2/3 )O 3 (BMT). In these systems, the B-site cations are stoichiometrically ordered in an hexagonal unit cell (Pm31), and the Ta 5+ and Zn(Mg) 2+ cation layers have a {Zn(Mg) 2+ Ta 5+ Ta 5+ } repeat sequence along the {111} crystallographic planes of the parent cubic cell (Fig. 1). The cation P. P. Phule contributing editor Manuscript No Received February 26, 1997; approved June 16, Supported in part by the MRSEC Program of the National Science Foundation under Award No. DMR and by Grant No. DMR (Ceramics Division). Support from the electron microscopy facility at the National Science Foundation, through the MRSEC program, is also acknowledged. Presented at the 98th Annual Meeting of the American Ceramic Society, Indianapolis, IN, April 14 17, *Member, American Ceramic Society. Fig. 1. Schematic illustration of 1:2 cation ordering in Ba(Mg 1/3 Nb 2/3 ) O 3 ; the upper-left illustration shows two of the four possible 111 directions for the ordering of magnesium and niobium, and the lowerright illustration shows the arrangement of magnesium and niobium for one of the orientational variants. Oxygen anions have been omitted for clarity. layers are separated by a close-packed anion layer, which is displaced toward the smaller pentavalent cations. Excellent reviews of the ordered perovskite structures can be found elsewhere Previous research has shown that the microwave-loss properties of 1:2 perovskite ceramics are very sensitive to the B-site cation ordering, and the Q value improves as the degree of order increases. 6,11 These improvements can be induced by annealing the ceramics at high temperatures for long soak times; for example, the Q f value of BZT increases from 6000 to after heating at 1350 C for 120 h. 11 The beneficial effect of the increase in order has been attributed to the reduction in the volume of lossy intragrain domain boundaries. 12,13 Tamura et al. 4 showed that the microwave-loss properties of BZT can also be enhanced through the substitution of small concentrations (<4 mol%) of BaZrO 3 (BZ). The addition of BZ dramatically reduces the sintering time required to access a high-q state, and a Q f value of has been obtained for BZT 4 mol% BZ after sintering for 4hat1500 C. 4 With the elimination of the need for an extended high-temperature treatment, this substitution enables these systems to be commercialized 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, which seemed to contradict the observations of the beneficial effect of cation order on dielectric loss in pure BZT and BMT. These apparent contradictions were resolved in recent investigations of the system using transmission electron microscopy (TEM) and X-ray diffractometry (XRD). 12,13 At low levels of substitution ( 2.15 mol% BZ), the solid solutions retained the structure of the BZT end member; however, the size of the 1:2 ordered domains decreased systematically as the amount of BZ 670

2 March 1998 Ordering Induced Microstructures and Microwave Dielectric Properties of the BMN BZ System 671 increased. The high Q values of these ceramics were attributed to the stabilization of the ordering-induced domain boundaries by the partial segregation of zirconium. Those studies also revealed that when the level of substitution is increased to 4 mol%, the system adopts a cubic 1:1 ordered cell with a doubled perovskite repeat (Fig. 2). This structure was subsequently shown to be stable up to 25 mol% BZ, and a fully disordered cation arrangement was only formed for substitution levels >35 mol%. 14 The positions of the phase boundaries in the BZT BZ system coincided with compositions previously shown to exhibit abrupt changes in the variation of Q with BZ content. 4,14 Because of their very high Q values, most studies of the microwave properties of the 1:2 family of perovskites have focused on tantalate compositions such as BZT and BMT. 4,6,13 Although the dielectric properties of the corresponding niobate systems have been examined, their losses are somewhat higher than their tantalate counterparts (e.g., for BMN, Q 5600 at 10.5 GHz). 15 Compared to the tantalates, the niobates are cheaper to manufacture and have a higher relative permittivity. Therefore, it may be of interest to explore whether their dielectric properties can also be optimized via chemical substitution. In this paper, we examine the effect of the substitution of BZ on the microstructure and microwave properties of BMN and compare the behavior of this system to its tantalate counterpart. II. Experimental Methods Ceramics in the (1 x)bmn (x)bz system were prepared via the mixed-oxide method, using high-purity (>99.9%) BaCO 3, MgO, Nb 2 O 5, and ZrO 2 as the starting materials. Stoichiometric proportions of these powders were weighed and mixed in an agate mortar for 10 min. The mixture was precalcined at 1100 C for 12 h and milled in acetone for 4 h with Y 2 O 3 -stabilized ZrO 2 balls. The slurry was dried and calcined at 1375 C for 12 h. The resultant calcine was ball milled for another 12 h and uniaxially pressed at 100 MPa into pellets that were 8 mm in diameter and several millimeters thick. For the dielectric measurements, the samples were isostatically pressed at 600 MPa before sintering. The final sintering was conducted at 1640 C for 10 h in air using a MoSi 3 muffle furnace with a heating and cooling rate of 300 C/h. In all cases, the density was >94% of the theoretical value. The relative permittivity of the ceramics was measured in the GHz range using the parallel-plate method, combined with a network analyzer and computer. 16,17 The accuracy of this measurement is better than ±0.25%. Measurements of the dielectric loss (Q) and the temperature coefficient of resonant frequency ( f ) were made at microwave frequencies using cavity methods. 18 The f values refer to the C range, according to Fig. 2. Schematic representation of the doubled perovskite cell of the cubic Fm3m), 1:1 ordered, A( 1/2 1/2 )O 3 structure. f f f o T, where f is the change in frequency, f o the reference frequency at 25 C, and T the temperature. XRD (Model Geigerflex D-max-B diffractometer, Rigaku, Tokyo, Japan) was used to determine the phase purity, and the lattice parameters were calculated via the least-squares method using data collected with an internal silicon standard. The microstructures of the ceramics were characterized using a TEM microscope (Model 400 EM, Philips Electronic Instruments, Mahwah, NJ) that was operated at 120 kv. Thin foils for the TEM investigations were prepared by mechanically grinding the ceramic pellets to a thickness of m and polishing both sides with 5, 1, and 0.25 m Al 2 O 3 powder. The samples were mounted on copper aperture grids, and a final thinning was conducted via argon-ion milling (Model Model 600 Dual Ion Mill, Gatan, Pleasanton, CA) at 6 kv and 0.6 ma, followed by a carbon coating, to prevent charging. III. Results and Discussion (1) Phase Stability: XRD and Electron Diffraction Studies Figure 3 shows the 110 zone-axis electron diffraction pattern of the (1 x)bmn (x)bz ceramics as a function of x. The 1:2 order in pure BMN (x 0) is characterized by superlattice reflections 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 the unit cell and the formation of a twin-related 1:2 ordered domain structure. The addition of BZ stabilizes a face-centered cubic (Fm3m), 1:1 ordered, Ba( 1/2 1/2 )O 3 -type structure, which is identified by the F-spots at (h ± 1 2, k ± 1 2, l ± 1 2) inthe electron diffraction patterns of samples with x 0.05 and 0.10 (marked by arrows in Figs. 3(B) and (C)). For small concentrations of BZ (0 < x 0.05), the samples reside in a two-phase field and the patterns contain reflections from the 1:2 and 1:1 ordered phases (Fig. 3(B)). Single-phase 1:1 order is observed at higher BZ contents (e.g., x 0.1 (Fig. 3(C))), and a disordered cation distribution is stabilized at compositions with x > 0.25 (Fig. 3(D)). Analyses of these samples via XRD confirmed the results of the electron diffraction study. The changes in the cation ordering were readily reflected by the change in position and intensity of the superlattice reflections, which are indicated in Fig. 4. The additional reflections from the 1:2 ordering in pure BMN are replaced by those which correspond to the 1:1 ordering for x 0.1 and 0.15, whereas the sample with x 0.25 could be indexed in terms of a single-phase disordered perovskite. The destabilization of the 1:2 ordering in BMN and the transformation to a 1:1 ordered structure is identical to the phase behavior of the BZT BZ and BMT BZ systems. 14,19 However, the niobate system shows small differences in the range of stability of the 1:1 ordered phase, which, in contrast to the tantalates, has not been formed at x This difference could result from the impact of the cooling history on the ordering in BMN BZ, which became evident during the studies 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, shows the microstructure of a typical grain in a single-phase 1:2 ordered 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 four equivalent {111} crystal planes and produces twin-related interfaces that lie along the 111 directions. Antiphase boundaries (APB) are also a general feature of the microstructure (marked by arrows in Fig. 5) and are mostly located in the vicinity of the domain boundaries. The crystallization of the hexagonal 1:2 ordered structure

3 672 Journal of the American Ceramic Society Akbas and Davies Vol. 81, No. 3 Fig. 3. [110] electron diffraction patterns of BZ-substituted BMN (the amount of BZ is (A) 0 mol% (x 0.0), (B) 5 mol% (x 0.05), (C) 10 mol% (x 0.1), and (D) 35 mol% (x 0.35)). can occur through the formation of equivalent orientational variants, with the c-axis of each variant lying along one of the four 111 directions of the parent subcell 13 (Fig. 1). For any given 110 zone axis, contrast differences that are associated with the ordering are only visible for the two variants that have their c-axes oriented perpendicular to the beam direction; this is consistent with the absence of any contrast in 50% of the dark-field image in Fig. 5. (B) 1:1 Ordered BMN BZ: The diffraction studies indicated that the phase relations in BMN BZ are very similar to those that have been determined previously for the BZT BZ and BMT BZ systems. 14,19 However, the microstructural features of the 1:1 ordered phases in the niobates and tantalates were found to be very different. The structure of the 1:1 ordered samples of BMN BZ was examined via dark-field microscopy using the ( 3 2, 3 2, 3 2) supercell reflection and was compared to the corresponding phases in the BMT BZ system. The most-detailed investigations were conducted on 1:1 ordered samples of BMN and BMT that each contained 10 mol% BZ. The synthesis conditions for the BMT BZ specimen, which was prepared as part of a related study of that system, 19 were identical 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 contain 1:1 ordered domains that are 4 5 nm in size and surrounded by a disordered matrix. More than 50% of the sample is disordered. The microstructure of the niobate is markedly different from that of its tantalate counterpart (BMT 10% BZ), which is fully ordered after experiencing the same thermal processing treatment (Fig. 6(B)). In addition to the absence of any disordered second phase, the microstructure of the tantalate also contains much-larger 1:1 domains ( 200 nm), which are separated by APBs. The reasons for these differences will be discussed in the following section.

4 March 1998 Ordering Induced Microstructures and Microwave Dielectric Properties of the BMN BZ System 673 Fig. 5. Dark-field TEM micrograph of pure BMN collected using the (200) reflection; an electron diffraction pattern is shown in the inset. Fig. 4. XRD patterns of (1 x)bmn (x)bz. The microstructures that are observed in the 1:1 ordered BMN BZ phases are very similar to those reported for leadbased perovskite relaxor ferroelectrics such as Pb(Mg 1/3 Nb 2/3 )- O 3 (PMN) and Pb(Mg 1/3 Ta 2/3 )O 3 (PMT). 20,21 For 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, the and sites in the 1:1 ordered Pb( 1/2 1/2 )O 3 domains are exclusively occupied by Mg 2+ and Nb 5+, respectively. To conserve overall electroneutrality, the net negative charge of the domains is assumed to be compensated by a positively charged, Nb 5+ -rich disordered matrix that surrounds the ordered domains. If this model is correct, any growth of the nanosized ordered domains will be severely inhibited by the associated space charge. The primary support for this model for the PMN family of relaxors originates from the absence of any domain coarsening with extended thermal treatment. 20,24 (3) Domain Growth in BMN BZ To explore the validity of a space-charge model, we attempted to modify the microstructures of the 1:1 ordered BMN BZ samples using different thermal treatments. To promote the growth of the ordered domains, it is clearly necessary to use conditions that reside in the region of thermodynamic stability of the 1:1 phase and yet allow significant cation diffusion. For the BMN BZ system, the range of temperature that satisfied both conditions was quite narrow and was initially identified using slow-cooling experiments, to examine the largest-possible temperature interval. In these experiments, the samples were heated to the sintering temperature (1640 C) and cooled 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 7 compares the dark-field TEM images collected from BMN 10% BZ after normal sintering (Fig. 7(A)) and after slow cooling (Fig. 7(B)). The slow-cooled samples are essentially fully ordered, with domain sizes of >100 nm. The large increase in the degree of order and domain size is also evident in the X-ray patterns collected from samples that were cooled at different rates (Fig. 8). The peaks associated with the 1:1 order in the pattern of the slow-cooled sample are sharper and have a significantly higher relative intensity. From these results, it is clear that a space-charge model cannot be accepted as a valid explanation for the 1:1 order in BMN BZ ceramics. Instead, the experimental data favor a random-layer model, which was previously proposed to explain the 1:1 order in the BZT BZ and BMT BZ systems. 13,19,25 In this model, for the ordered Ba( 1/2 1/2 )O 3 structure, the positions are assumed to be exclusively occupied by Nb 5+, and the sites are assumed to be occupied by a random mixture of Mg 2+,Zr 4+, and the remaining Nb 5+ cations (Fig. 2). For the random-layer structure, the 1:1 ordered (1 x)bmn (x)bz solid solutions can be represented as Ba[(Mg (2 z)/3 Nb (1 2z)/3 Zr z ) 1/2 Nb 1/2 ]O 3, where z 2x. In this case, the 1:1 ordered domains are charged balanced and the overall 1:2 cation stoichiometry is preserved in all regions of the samples. (4) High-Temperature Phase Equilibria Although the random-layer model readily explains the coarsening of the domains during slow cooling, a satisfactory model must also be able to rationalize the formation of the nanodomain + disordered matrix two-phase assemblage that is observed in the as-sintered samples (Fig. 7(A)). The previous explanations for the existence of this type of microstructure in 1:2 mixed-metal perovskites have been based on the physical limitations that are implemented by the atomistic model, i.e., the space charge associated with the domain matrix charge imbalance. By examining the stability of the 1:1 structures at

5 674 Journal of the American Ceramic Society Akbas and Davies Vol. 81, No. 3 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 0.1) collected using the ( 3 2, 3 2, 3 2) supercell reflections ((A) sintered at 1640 C for 10 h and cooled at a rate of 300 C/h, and (B) sintered at 1640 C for 10 h and cooled to 1000 C at a rate of 10 C/h). high temperature, we have found that the formation of the nanodomains in BMN BZ can be explained using conventional phase equilibria. To examine the phase stabilities at elevated temperatures, a series of BMN BZ compositions were air quenched from temperatures as high as 1640 C and examined using XRD. Figure 8 compares the pattern collected from a sample of BMN 10% BZ that was quenched from 1640 C to those obtained after slow cooling (10 C/h) and normal sintering (300 C/h). The very weak and diffuse supercell reflections are barely discernible in the air-quenched samples. We concluded that any ordering that is present in this sample occurred during the quench and, at 1640 C, the sample resides in a disordered perovskite phase field. Samples quenched from 1600 and 1550 C were disordered and ordered, respectively, and the boundary between 1:1 order and complete disorder for BMN 10% BZ was estimated to lie between these temperatures. Quenching experiments that were performed on samples with different compositions showed that the temperature of the order disorder transformation decreased as the BZ concentration increased. The lowering of the transition temperature at higher BZ contents probably explains why the range of homogeneity of the 1:1 phase observed in the as-sintered samples was narrower than that found in the corresponding tantalate systems. Although this is the first direct observation of an order disorder transition for the 1:1 phases in these systems, these transitions have been reported previously for some 1:2 ordered niobate end members, e.g., Ba(Ni 1/3 Nb 2/3 )O 3, Ba(Co 1/3 Nb 2/3 )O 3, and Ba(Zn 1/3 Nb 2/3 )O Recognizing the existence of this transformation at temperatures below those used in sintering, the variation of the microstructures of 1:1 BMN BZ with thermal history can be readily explained with the aid of the schematic phase diagram that is shown in Fig. 9. Although the general features of the phase diagram are based on our experimental data, quantification of the exact reaction types, temperatures, and compositions requires additional work. Similarly, the nature of the transformation reactions between the different ordered phases (i.e., first order versus second order) has not been characterized. Because the sintering temperature of BMN 10% BZ resides within the stability field of a disordered perovskite structure, the transformation to the 1:1 ordered state and, therefore, the growth of the ordered domains can only occur during the cooling cycle. Rapid cooling can almost completely preserve the metastable disordered phase; the slower cooling in the standard sintering procedure results in the nucleation and partial growth of 1:1 nanodomains and a metastable two-phase assemblage, whereas slow cooling induces complete order and a larger domain size. A similar approach can be used to explain the differences in the microstructures of the tantalates and niobates. A series of quenching experiments conducted on BMT 10% BZ revealed that the degree of order was independent of the cooling rate. 19

6 March 1998 Ordering Induced Microstructures and Microwave Dielectric Properties of the BMN BZ System 675 Composition, x Table I. Dielectric Loss Properties of the (1 x)bmn (x)bz System Dielectric loss, Q Microwave frequency, f (GHz) Q f Fig. 8. XRD patterns of BMN 10 mol% BZ (slow cooled from 1640 C at a rate of 10 C/h (pattern A ), as-sintered (pattern B ), and air quenched from 1640 C (pattern C )). Apparently, the disordering temperatures for the tantalates are significantly higher than the niobates, and the 1:1 ordered phases are stable at the sintering temperature. Therefore, each of the different stages in the processing of the tantalate ceramics (calcination and sintering) are conducted in the region of stability of 1:1 order (see Fig. 9). In this case, there is sufficient time for significant domain growth and achievement of a fully ordered microstructure. (5) Microwave Dielectric Properties of (1 x)bmn (x)bz The dielectric properties of the as-sintered BMN BZ ceramics were measured at microwave frequencies. At 10 GHz, the relative permittivities of the as-sintered BMN BZ ceramics were in the range of 31 34; the value of f increases from a value of 21 for x 0 to a value of 60 for x The values obtained for the losses are listed in Table I and are plotted as a function of composition in Fig. 10. For pure BMN, the measured Q f value (38000) is in good agreement with the previously reported value. 15 The value of Q increased by >100% with the substitution of small concentrations of BZ; for example, for x 0.05, the Q f value is When the concentration of zirconium was increased to x 0.1, the Q f value decreased to 55000; for x 0.35, the Q f value increased to The beneficial effect of low-level substitution on Q is similar to the reduced losses that were reported for 4-mol%-BZ-doped BZT, 4 although the magnitude of the increase for the BMN system is somewhat larger. For the BZT system, it has been suggested that, for low concentrations of BZ, which retain a 1:2 ordered structure, the zirconium cations preferentially segregate to the 1:2 domain boundaries and reduce their contribution to the loss. 12,13 It is possible that the large increase in Q for the BMN system results from a similar segregation phenomenon. However, the as-sintered x 0.05 sample actually resides in the 1:2 + 1:1 two-phase field. Although samples with lower concentrations of BZ have not been explored in this investigation, it would be of interest to study the dielectric properties of samples that reside in the narrow stability region of the 1:2 structure, to determine whether the Q f value for the x 0.05 sample represents a maximum for the system. The variation of Q at the higher concentrations of BZ is clearly quite unpredictable. In this region, the 1:1 and, ultimately, the disordered perovskite are stable. However, recall that the microstructural features of the as-sintered samples are far from equilibrium. Therefore, the dielectric properties in Fig. 10 represent the response of a metastable assemblage of 1:1 nanodomains, a disordered matrix, and the boundaries between them. It would be expected that the dielectric response of these compositions will be sensitive to changes in the cooling history of the ceramic. Indeed, it may be possible to tailor the microwave properties of the niobates by controlling their microstructure through alterations in the thermal treatment and cooling history. 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. 9. Schematic phase diagram showing the possible phase relations for the Ba(Mg 1/3 M 2/3 )O 3 BaZrO 3 systems (M Ta and Nb); an arbitrary temperature axis is used to highlight that, at the sintering temperature ( 1640 C), the niobates reside in a disordered phase field, while the tantalates reside in the 1:2 and 1:1 ordered phase fields. Fig. 10. Dielectric loss (Q f ) versus composition (x) for the (1 x)bmn (x)bz system.

7 676 Journal of the American Ceramic Society Akbas and Davies Vol. 81, No. 3 and disordered B-site cation distributions for 0.0 < x < 0.05, 0.05 < x < 0.25, and x > 0.25, respectively. The final microstructure of the niobates is very sensitive to the thermal history of the samples. For example, as-sintered samples with x 0.1 have a phase-separated microstructure that consists of metastable disordered and stable 1:1 ordered phases. A fully ordered structure that is comprised of domains >100 nm in size could be produced by reducing the cooling rate to 10 C/h. The sensitivity of the ordering to the cooling history can be attributed to an order disorder reaction that occurs in the temperature range of C. The structure of the 1:1 ordered domains can be interpreted, using a charge-balanced, randomlayer model. The substitution of small concentrations of BZ produces a significant increase in the dielectric loss Q, and a Q f value of is measured for samples where x Authors Note: During the review of this article, we discovered that the ordering-induced microstructures of the lead-based relaxor system Pb(Mg 1/3 Ta 2/3 )- O 3 PbZrO 3 can also be modified by high-temperature thermal treatment (see Ref. 29). Acknowledgments: The authors thank Trans-Tech, particularly Dr. Taki Negas and Dr. Steve Bell, for measuring the microwave dielectric properties. References 1 J. K. Plourde, D. F. Lin, H. M. O Bryan Jr., and J. Thompson Jr., Ba 2 Ti 9 O 20 as a Microwave Dielectric Resonator, J. Am. Ceram. Soc., 58 [9 10] (1975). 2 H. M. O Bryan Jr., J. Thomson Jr., and J. K. Plourde, A New BaO-TiO 2 Compound with Temperature Stable High Permittivity and Low Microwave Loss, J. Am. Ceram. Soc., 57 [10] (1947). 3 T. Negas, G. Yeager, S. Bell, and N. 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