ALTHOUGH pyrochlores are widely used in active and passive

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1 J. Am. Ceram. Soc., 83 [1] (2000) Crystal Chemistry and Dielectric Properties of Chemically Substituted (Bi 1.5 Zn 1.0 Nb 1.5 )O 7 and Bi 2 (Zn 2/3 Nb 4/3 )O 7 Pyrochlores Matjaz Valant*, and Peter K. Davies* Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania The effect of the chemical substitution of Ti, Zr, and Gd cations on the structure and properties of bismuth-based pyrochlores was investigated for (Bi 1.5 Zn 1.0 Nb 1.5 )O 7 and Bi 2 (Zn 2/3 Nb 4/3 )O 7. Broad ranges of solid solutions based on cubic (Bi 1.5 Zn 1.0 Nb 1.5 )O 7 were observed for Ti and Zr cations: (Bi 1.5 Zn 1 (x/3) Nb 1.5 (2x/3) Zr x )O 7 (0.0 < x < 1.5) and (Bi 1.5 Zn 1 (x/3) Nb 1.5 (2x/3) Ti x )O 7 (0.0 < x < 2.25). For Gd cations, the range of solid solution was quite limited in (Bi 1.5 x Gd x Zn 1.0 Nb 1.5 )O 7 (0.0 < x < 0.2); however, the range of solid solution is more extensive in the system that contains Ti and Gd cations ((Bi 1.5 x Gd x Zn 0.5 Ti 1.5 Nb 0.5 )O 7 (0.0 < x < 0.6)). The stability fields of the solid solutions could be interpreted by using the ratio of the ionic sizes of the A- and B-site ions in the pyrochlore structure (R A /R B ). The substituted phases exhibited permittivities ( ) in the range of and relatively low dielectric losses (tan < ) at 1 MHz. The temperature coefficients of permittivity ( ) varied in accordance with the R A /R B value and could be tuned from 88 ppm/k to 1300 ppm/k. For the orthorhombic Bi 2 (Zn 2/3 Nb 4/3 )O 7 pyrochlore, the ranges of solid solution were very limited and the dielectric properties remained similar to those of the undoped phase ( 90, 150 ppm/k). I. Introduction ALTHOUGH pyrochlores are widely used in active and passive electronic applications, such as switching elements, thermistors, thick-film resistors, and materials for screen printing, they have been recognized only recently as potential candidates for temperature-stable, low-loss, high-permittivity dielectric applications. Members of a family of bismuth-based pyrochlores are particularly interesting; these families include Bi 2 (Zn 2/3 Nb 4/3 )O 7 with a high relative permittivity ( ) in the range of , dielectric losses (tan ) as low as at a frequency of 1 MHz, and a temperature coefficient of permittivity ( ) that can be negative or positive. 1 5 In addition to these promising properties, these systems can be sintered at very low temperatures (950 C), which makes them viable as cofired dielectric components. In this paper, we explore the response of the structure and dielectric properties of two bismuth zinc niobate pyrochlores Bi 2 (Zn 2/3 - Nb 4/3 )O 7 and (Bi 1.5 Zn 1.0 Nb 1.5 )O 7 to a series of cation substitutions and attempt to correlate the changes in chemistry to. C. Randall contributing editor Manuscript No Received October 1, 1998; approved May 19, Supported by the National Foundation, under Grant Nos. DMR and , and the National Science Foundation (NSF) Division of International Programs, under Grant No. INT The MRSEC Shared Experimental Facilities also were used through the NSF, under Grant No. DMR *Member, American Ceramic Society. On leave from Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia. Stoichiometric oxide pyrochlores can be described by the general formula A 2 B 2 O 7. Most bismuth-based pyrochlores adopt a cubic crystal structure with a space group Fd3m; 6 however, in some rare cases, these pyrochlores do crystallize in a noncubic symmetry. Investigations of a series of antimony-based pyrochlores in the Bi 2 O 3 Sb 2 O 5 MO systems (M Cd, Zn) revealed the existence of (M 2 2x Bi 3x Sb 2 x )O 7 solid solutions with values of 20 and modest dielectric losses (tan ). 5 These studies also demonstrated that could be tuned through appropriate doping. Studies of the cation distributions in the bismuth zinc antimonates have revealed that the Zn cations occupy both the A- and B-sites of the pyrochlore structure, and, for example, the x 0.5 compositions can be represented by the structural formula (Bi 1.5 Zn 0.5 )(Sb 1.5 Zn 0.5 )O 7. 3 Investigations of the closely related niobate and tantalate systems led to the discovery of a new family of bismuth-based pyrochlores Bi 2 (M M )O 7 and Bi 2 (M 2/3 M 4/3 )O 7 (where M Zn, Mg, Ni, Sc, In, or Cu, and M Nb or Ta) with significantly higher permittivities. 1 The two members of this family that have received the most attention (Bi 1.5 Zn 1.0 Nb 1.5 )O 7 and Bi 2 (Zn 2/3 Nb 4/3 )O 7 were shown to adopt a cubic and an orthorhombically distorted stoichiometric A 2 B 2 O 7 pyrochlore structure, respectively. 2,7 The distortion in Bi 2 (Zn 2/3 Nb 4/3 )O 7 was suggested to result from the long-range coupling of the 6s 2 lone-pair electrons of Bi 3 cations on the A-site position. 2 For cubic (Bi 1.5 Zn 1.0 Nb 1.5 )O 7, the distribution of the cations on the A- and B-site positions has not been determined experimentally. However, the most likely distribution, as suggested by Wang et al., 2 is (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7. This distribution also agrees with the results for the antimonates and will be adopted here. Cubic (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7 has 170, tan , and 400 ppm/k; for Bi 2 (Zn 2/3 Nb 4/3 )O 7, 90, tan , and 150 ppm/k. The much-larger permittivity of the cubic zinc-rich pyrochlore is unusual, given that the expected dielectric polarizability per volume unit for Bi 2 (Zn 2/3 Nb 4/3 )O 7, which is calculated using the empirical ion polarizabilities that were derived by Shannon, 8 is significantly higher than that for (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7. This unexpected result suggests that the orthorhombic distortion also reduces the overall ionic polarizability. In this paper, we report on our attempts to modify the value of for (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7 and Bi 2 (Zn 2/3 Nb 4/3 )O 7 by substituting selected cation substituents into the pyrochlore structure. Because a near-zero value is critical for several applications, one of our main goals is to establish a correlation between the changes in composition and the values of the pyrochlore solid solutions. In particular, we examine the potential incorporation of tetravalent cations, such as Ti 4 and Zr 4, onto the B-sites and larger trivalent cations, such as Gd 3, onto the A-sites. The effect of the substitutions on the dielectric properties is correlated to the relative sizes of the A- and B-site occupants. II. Experimental Procedures Syntheses of the bismuth zinc niobate-based pyrochlores were conducted using solid-state reaction techniques. Stoichiometric 147

2 148 Journal of the American Ceramic Society Valant and Davies Vol. 83, No. 1 mixtures of dried reagent-grade oxides were homogenized and prereacted at a temperature of 650 C for 2 h. During the prereaction, Bi 2 O 3 reacts to form less-volatile compounds and permits additional firing at higher temperatures without significant losses of bismuth. The prereacted powders were calcined at temperatures of C with an intermediate grinding to achieve equilibrium. Then, the powders were pressed uniaxially at 150 MPa into pellets and sintered at temperatures of C. Higher sintering temperatures (up to 1250 C) were required for the gadolinium-containing compositions. The density of each sample was 95% of the theoretical value. The progress of the reactions was monitored via powder X-ray diffractometry (XRD), using a diffractometer (Model Geigerflex, D-Max-B, Rigaku Co., Tokyo, Japan) with CuK radiation. Lattice parameters were calculated via least-squares refinement of data collected over a 2 scanning range of at a scan rate of /min. Silver powder was used as an internal standard. Microstructural and energy-dispersive X-ray (EDX) analyses of the ceramics were conducted using a scanning electron microscopy (SEM) system (Model 6400, JEOL, Tokyo, Japan) that was equipped with a spectrometer (Oxford Instruments, Oxford, U.K.). The dielectric measurements were made at frequencies in the range of 1 khz to 1 MHz on silver-plated pellets using a high-precision impedance capacitance resistance (LCR) meter (Model HP 4824A, Hewlett Packard, Palo Alto, CA). The temperature dependence of the dielectric properties was measured over a temperature range of 50 C to 130 C by placing the samples in an environmental chamber (Model Delta 9023, Delta Design, San Diego, CA) and monitoring the temperature with a copper constantan (T-type) thermocouple. III. Results (1) Zr 4 Cation Substitution No evidence for the existence of oxygen vacancies was observed in previous investigations of (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7 and Bi 2 (Zn 2/3 Nb 4/3 )O 7 ; therefore, in this work, a stoichiometric A 2 B 2 O 7 formalism for the pyrochlores was assumed. Implementing the electroneutrality principle, two possible mechanisms for the incorporation of Zr 4 ions into the crystal structure of the cubic (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7 can be presented: (i) 2Zr 4 3 Nb 5 Bi 3, and (ii) 3Zr 4 3 Zn 2 2Nb 5. The validity of both substitution mechanisms was investigated by preparing appropriate stoichiometric mixtures and examining the synthesis products. For mechanism (i), the substitution of one Nb 5 cation and one Bi 3 cation for two Zr 4 cations results in a starting composition with the stoichiometry of (Bi 1.5 (x/2) ZnNb 1.5 (x/2) Zr x )O 7. A mixture with x 0.6 reached an equilibrium state (as gauged by the absence of any changes in the X-ray patterns) after reaction at C. XRD and SEM examination revealed that this sample mainly consisted of a zirconium-doped (Bi 1.5 Zn 0.5 )- (Zn 0.5 Nb 1.5 )O 7 matrix phase; however, significant amounts of ZnO and a ZrO 2 Nb 2 O 5 solid solution also were detected (Fig. 1). The presence of these secondary phases, coupled with the observation of a significant bismuth deficiency in the pyrochlore matrix, was considered to be sufficient evidence to exclude this mechanism as a valid substitution scheme. Mixtures that were prepared using scheme (ii) (with a stoichiometry of (Bi 1.5 Zn 1 (x/3) Nb 1.5 (2x/3) Zr x )O 7 ) yielded an almostsingle-phase product after reaction at C (Fig. 1). Although the X-ray patterns contained weak additional peaks from small amounts of secondary phases of ZnO and a ZrO 2 Nb 2 O 5 solid solution (Fig. 2), SEM analyses indicated that the composition of the matrix phase was essentially identical to that of the starting oxide mixture. The amounts of secondary phases increased at higher values of x (1.0 and 1.5), although, again, EDS analysis showed a steady increase in the concentration of Zr cations in the pyrochlore matrix. The lattice parameters of the pyrochlore phase also showed a systematic increase, relative to x (Fig. 3). By examining the path of the reaction, the appearance of the secondary phases in the (Bi 1.5 Zn 1 (x/3) Nb 1.5 (2x/3) Zr x )O 7 solid Fig. 1. XRD patterns of starting compositions of (Bi 1.5 (x/2) ZnNb 1.5 (x/2) - Zr x )O 7 (x 0.6) (pattern a) ) and (Bi 1.5 Zn 1 (x/3) Nb 1.5 (x/2) Zr x)o7 (x 0.6) (pattern b) ) after firing at 1000 C for 2 h ( A denotes ZrO 2 (ss), and B denotes ZnO). solutions was determined to result from kinetic rather than thermodynamic factors. During the prereaction at 650 C, Bi 2 O 3 preferentially reacts with Nb 2 O 5 to form less-volatile bismuth niobates; as the concentration of Zr cations increases (and the amount of Nb cations decreases), the ratio of Bi 2 O 3 to Nb 2 O 5 shifts toward higher bismuth concentrations and, in agreement with the Bi 2 O 3 Nb 2 O 5 phase diagram, 9 the prereaction produces a phase assemblage that is comprised of Bi 5 Nb 3 O 15 and Bi 2 O 3. The ZnO Bi 2 O 3 and ZrO 2 Bi 2 O 3 binary phase diagrams 10 show that the formation of binary phases of Bi 2 O 3 with ZnO or ZrO 2 is not possible at this temperature; therefore, some of the total bismuth content must remain in the form of unreacted and volatile Bi 2 O 3. During calcination ( 1000 C), Bi 5 Nb 3 O 15 and Bi 2 O 3 react with ZnO and ZrO 2 to form (Bi 1.5 Zn 1 (x/3) Nb 1.5 (2x/3) Zr x )O 7 solid solutions. However, the kinetics of the reaction are not fast enough to prevent some partial evaporation of Bi 2 O 3. The resultant excess of the other oxides is responsible for the appearance of minor secondary phases. Additional support for this mechanism was obtained by introducing excess Bi 2 O 3 into the reaction mixture, to compensate for the volatilization (Fig. 4). The introduction of a 15% excess of Bi 2 O 3 almost completely prevents the formation of secondary phases in compositions with x 1.0. However, traces of Fig. 2. XRD patterns of powders with (Bi 1.5 Zn 1 (x/3) Nb 1.5 (x/2) Zr x )O 7 (x 0 1.5) after firing at 1000 C for 2 h ( A denotes ZrO 2 (ss), and B denotes ZnO).

3 January 2000 Crystal Chemistry and Dielectric Properties of (Bi 1.5 Zn 1.0 Nb 1.5 )O 7 and Bi 2 (Zn 2/3 Nb 4/3 )O 7 Pyrochlores 149 Fig. 3. Cell volumes of zirconium-doped (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7 and Bi 2 (Zn 2/3 Nb 4/3 )O 7, as a function of the concentration of Zr cations. residual Bi 2 O 3 can be found on the grain boundaries of the sintered ceramics (Fig. 5), and this residual Bi 2 O 3 causes substantial deterioration of the dielectric properties. The dielectric properties of (Bi 1.5 Zn 1 (x/3) Nb 1.5 (2x/3) Zr x )O 7 solid solutions with x 0, 0.2, 0.6, and 1.0 are shown in Fig. 6. While the permittivity decreases from 152 for x 0to 101 for x 1.0, the dielectric losses increase with x and attain a value of tan for x 1.0. The increase in the value of tan may be due to the secondary phases that are present in samples of higher-x compositions. The temperature coefficient of the permittivity shows a large increase with x, from an initial value of 470 ppm/k for x 0to 88 ppm/k for x 1.0. We also attempted to substitute Zr cations into the orthorhombic Bi 2 (Zn 2/3 Nb 4/3 )O 7 structure. The range of solid-solution formation for Bi 2 (Zn (2/3) (x/3) Nb (4/3) (2x/3) Zr x )O 7 was very limited (x 0.2) and multiple phase assemblages were formed for higher-x compositions. Within this compositional range, the cell parameters showed almost no change and the dielectric properties were very similar to those of the parent compound (see Fig. 6). (2) Ti 4 Cation Substitution The same mechanism of substitution that was observed for Zr 4 cations (i.e., Zn 2 2Nb 5 3M 4 ) was used to form a series of Ti 4 solid solutions with (Bi 1.5 Zn 1 (x/3) Nb 1.5 (2x/3) Ti x )O 7.In this case, single-phase cubic pyrochlores could be formed over a wider range of x values (x ). The absence of any minor secondary phase formation that originates from the volatilization of bismuth can be understood in terms of the different reaction path for the titanium-containing ceramics. The higher reactivity of TiO 2 compared to ZrO 2 ensures that, during the prereaction at a temperature of 650 C, Bi 2 O 3 reacts with both Nb 2 O 5 and TiO 2, 11 and the absence of any free Bi 2 O 3 suppresses the loss of bismuth at higher temperatures. Detailed 1 analyses of the chemistry and dielectric properties of (Bi 1.5 Zn 1 (x/3) Nb 1.5 (2x/3) Ti x )O 7 will be reported elsewhere; 12 however, the basic findings that are necessary for this study will be reviewed here. In accordance with the ionic radii of the different cations that are involved in the substitution (r(ti 4 ) Å VI, 1 3r(Zn 2 ) 2 3r(Nb 5 ) 0.67 Å), 13 the lattice parameters of the solid solutions decrease across the entire range of x. However, a discontinuity in this variation is apparent at x 1.5 (see Fig. 7). The discontinuity is probably due to a change in the substitution mechanism at higher values of x. Because of their relatively small size, the Ti cations are expected to initially replace the Zn 2 and Nb 5 cations that are located on the B-sites of the pyrochlore Throughout this paper, the superscripted Roman numerals denote the coordination number of the ion. For example, VI denotes a coordination number of six. Fig. 4. XRD patterns of (Bi 1.5 Zn 1 (x/3) Nb 1.5 (x/2) Zr x )O 7 (x 1.0) with various amounts of excess Bi 2 O 3 (none (0%) (pattern a) ), 5% (pattern b) ), and 15% (pattern c) )) after firing at 1000 C for 2 h ( A denotes ZrO 2 (ss), and B denotes ZnO). Fig. 5. SEM images of the Bi 2 O 3 grain-boundary phases in (Bi 1.5 Zn 1 (x/3) - Nb 1.5 (x/2) Zr x )O 7 (x 1.0) with 15% excess Bi 2 O 3, after sintering at 1090 C for 5 h.

4 150 Journal of the American Ceramic Society Valant and Davies Vol. 83, No. 1 Fig. 6. Permittivity ( ) and the temperature coefficient of permittivity ( ), as a function of x in (Bi 1.5 Zn 1 (x/3) Nb 1.5 (x/2) Zr x )O 7 and Bi 2 (Zn 2/3 (x/3) - Nb 4/3 (2x/3) Zr x )O 7. structure. This mode of substitution can continue up to x 1.5, where the Zn cations on the B-site positions are completely substituted by Ti cations and the composition of the solid solution can be represented as (Bi 1.5 Zn 0.5 )(Ti 1.5 Nb 0.5 )O 7. Additional Ti substitution can only occur through the replacement of the Zn cations on the A-sites and the remaining Nb cations on the B-sites. Our studies indicate that this region of solid solubility extends to the x 2.25 end member, (Bi 1.5 Zn 0.25 Ti 0.25 )Ti 2 O 7. Figure 8 shows the dielectric properties for titanium-doped pyrochlores. As expected, the substitution of Ti cations induces significant changes in the dielectric properties. These variations are nonlinear across the system, and a discontinuity is observed at x 1.5, which is similar to the lattice-parameter behavior. For 0.0 x 1.5, systematically increases up to a maximum value of 200 for (Bi 1.5 Zn 0.5 )(Ti 1.5 Nb 0.5 )O 7, the dielectric losses remain low (tan ), and becomes increasingly negative, with the lowest value ( 1300 ppm/k) being observed for the x 1.5 composition. For 1.5 x 2.25, these trends are reversed decreases and increases. For the orthorhombic Bi 2 (Zn 2/3 Nb 4/3 )O 7 system, single-phase titanium solid solutions with Bi 2 (Zn 2/3 (x/3) Nb 4/3 (2x/3) Ti x )O 7 could be produced only for compositions with x 0.6. The lattice parameters show a very small decrease in this range (Fig. 7). During sintering, the single-phase orthorhombic solid solutions undergo partial decomposition at temperatures that are appropriate for effective densification ( 1050 C). SEM and XRD examination of the samples indicate that the decomposition products are the previously described cubic pyrochlore solid solutions, Bi 5 Nb 3 O 15, and Bi 2 O 3 (Fig. 9). This result would indicate a decomposition reaction of the type Bi 2 (Zn (2/3) x/3) Nb (4/3) 2x/3) Ti x )O (Bi 1.5Zn 0.5 )(Zn 0.5 x/3) Nb 1.5 2x/3) Ti x )O Bi 5Nb 3 O Bi 2O 3 The formation of additional phases during sintering influences the dielectric properties of such ceramics. We attempted to avoid decomposition by sintering at lower temperatures; however, the densities were 90% of the theoretical density. A compromise was found at a temperature of 1030 C, where the samples were dense enough ( 93%) for dielectric characterization and only contained small amounts of the decomposition products (see Fig. 9). Aside from an increase in the dielectric loss (up to tan ), which is probably associated with the partial decomposition, the permittivities ( 83 and 82) and temperature coefficients ( 144 and 186 ppm/k) for the x 0.2 and x 0.6 Fig. 7. Cell volumes of (Bi 1.5 Zn 1 (x/3) Nb 1.5 (x/2) Ti x )O 7 and Bi 2 (Zn 2/3 (x/3) Nb 4/3 (2x/3) Ti x )O 7, as a function of x.

5 January 2000 Crystal Chemistry and Dielectric Properties of (Bi 1.5 Zn 1.0 Nb 1.5 )O 7 and Bi 2 (Zn 2/3 Nb 4/3 )O 7 Pyrochlores 151 Fig. 8. Permittivity ( ) and the temperature coefficient of permittivity ( ), as a function of x in (Bi 1.5 Zn 1 (x/3) Nb 1.5 (x/2) Ti x )O 7 and Bi 2 (Zn 2/3 (x/3) - Nb 4/3 (2x/3) Ti x )O 7. compositions, respectively, were very similar to the x 0 end member (Fig. 8). (3) Gd 3 Cation Substitution The ionic radius of the Gd 3 cation (0.938 Å VI, Å VIII )is slightly smaller than that of the Bi 3 cation (1.17 Å VIII ) but is much larger than either the Zn 2 cation (0.74 Å VI, 0.90 Å VIII )or the Nb 5 cation (0.64 Å VI, 0.74 Å VIII ). 13 Therefore, from the standpoint of crystal chemistry, the most likely mode of substitution of Gd cations into the bismuth-based pyrochlores is through its replacement of Bi 3 cations on the A-site positions. A series of substituted solid solutions based on the cubic bismuth-based pyrochlore end member were prepared with (Bi 1.5 x Gd x Zn 0.5 )- (Zn 0.5 Nb 1.5 )O 7 ; however, single-phase samples could be prepared only for compositions with x 0.2. We also explored the effect of doping gadolinium on the Ti-substituted phase, (Bi 1.5 Zn 0.5 )- (Ti 1.5 Nb 0.5 )O 7. In this case, (Bi 1.5 x Gd x Zn 0.5 )(Ti 1.5 Nb 0.5 )O 7 solid solutions could be prepared at x 0.6. In both series of solid solutions, the cell volume decreased as x increased, which is consistent with the relative ionic sizes of the Gd 3 and Bi 3 cations. The dielectric properties of gadolinium-doped bismuth-based pyrochlores are reported in Table I. The trends in the dielectric properties are the same for both (Bi 1.5 x Gd x Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7 and (Bi 1.5 x Gd x Zn 0.5 )(Ti 1.5 Nb 0.5 )O 7 solid solutions. For (Bi 1.5 x Gd x Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7 with x 0.2, the value of decreased to 116, whereas the value of increased to 350 ppm/k. For (Bi 1.5 x Gd x Zn 0.5 )(Ti 1.5 Nb 0.5 )O 7, the value of decreased from an initial value of 200 to 143 and 80 for x 0.2 and x 0.6, respectively, and the value of decreased from 1300 ppm/k to 560 and 480 ppm/k for x 0.2 and x 0.6, respectively. In all cases, tan at 1 MHz. Attempts to synthesize the orthorhombic substituted (Bi 2 x Gd x )(Zn 2/3 Nb 4/3 )O 7 solid solutions yielded only singlephase samples for x 0.2. The lattice parameters were essentially unchanged, and the dielectric properties ( 80, tan , and 240 ppm/k) also were similar to those of an undoped sample. IV. Discussion Our attempts to prepare chemically substituted forms of the cubic (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7 and orthorhombic Bi 2 (Zn 2/3 Nb 4/3 )- O 7 pyrochlores revealed large differences in their chemistry and properties. Although extensive ranges of solid solution could be prepared for the cubic phase, the solid solubilities in the orthorhombic compound were very limited. We begin this discussion by Table I. Dielectric Properties of Gadolinium-Doped (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7, (Bi 1.5 Zn 0.5 )(Ti 1.5 Nb 0.5 )O 7, and Bi 2 (Zn 2/3 Nb 4/3 )O 7 Gadolinium content, x Permittivity, Dielectric loss, tan Temperature coefficient of permittivity, (ppm/k) (Bi 1.5 x Gd x Zn 0.5 )(Zn 0.5 Nb 1.5 )O (Bi 1.5 x Gd x Zn 0.5 )(Ti 1.5 Nb 0.5 )O Fig. 9. Powder XRD patterns of Bi 2 (Zn 2/3 (x/3) Nb 4/3 (2x/3) Ti x )O 7 (x 0.6) after firing at 970 and 1030 C for 2 h ( E denotes -Bi 2 O 3, F denotes (Bi 1.5 Zn 1 (x/3) Nb 1.5 (x/2) Ti x )O 7, and G denotes Bi 5 Nb 3 O 15 ). (Bi 2 x Gd x )(Zn 2/3 Nb 4/3 )O

6 152 Journal of the American Ceramic Society Valant and Davies Vol. 83, No. 1 considering the ranges of solid-solution formation and then attempt to correlate the changes in their dielectric properties to the crystal chemistry of the pyrochlore structure. The substitution of Zr 4 and Ti 4 ions into cubic (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7 yielded broad ranges of solid-solution formation, accompanied by significant changes in the lattice parameters. In contrast, the solid solubility of the Gd 3 cation was quite limited. To understand these differences, it is useful to consider the ratio of the A- and B-site ionic radii (R A /R B ), which determines the stability field for the formation of a pyrochlore structure. Subramanian et al. 6 have shown that, for A 3 2 B 4 2 O 7 - type systems, pyrochlores typically are stabilized for 1.46 R A /R B 1.80, whereas for A 2 2 B 5 2 O 7 -type systems, the range is somewhat larger (1.4 R A /R B 2.2). For the parent (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7 end member, R A /R B The substitution of Ti cations, which, from size considerations, should preferentially partition to the B-site positions, increases the value of R A /R B,uptoR A /R B for (Bi 1.5 Zn 0.5 )(Ti 1.5 Nb 0.5 )O 7. However, the phase studies indicate that solid solutions of (Bi 1.5 Zn 1 (x/3) Nb 1.5 (2x/3) Ti x )O 7 can be formed up to x The change in the variation of the lattice parameters for compositions with x 1.5, and consideration of the overall chemistry, clearly indicate that, at x , the site distribution of the cations in the solid solution must change. One possible mode of substitution in this range would involve the continued introduction of Ti cations onto the B-sites to replace the Nb cations and partial substitution onto the A-site positions for the Zn cations (i.e., formation of solid solutions with (Bi 1.5 Zn 0.5 (y/3) Ti y/3 )(Ti 1.5 (2y/3) - Nb 0.5 (2y/3) )O 7, where 0.0 y 0.75). An alternate mechanism, in which the Ti cations remain partitioned on the B-sites and the Nb cations are partially relocated on the A-site positions, also is possible, up to x 2.0. From our limited structural data, it is impossible to discriminate between either one or a combination of the mechanisms; however, in either case, the mechanism of substitution is clearly different from that for the x 1.5 compositions. Both mechanisms involve a decrease in the average size of the cations on the A-site position; therefore, the value of R A /R B for the 1.5 x 2.25 solid solutions must decrease (e.g., R A /R B for (Bi 1.5 Zn 0.25 Nb 0.25 )Ti 2 O 7 ). For all the titaniumcontaining solid solutions, the value of R A /R B remains inside the stability-field limits that were identified by Subramanian et al. 6 The value of R A /R B is more difficult to determine in the (Bi 1.5 Zn 1 (x/3) Nb 1.5 (2x/3) Zr x )O 7 solid solutions, because the similarity in the size of the Zr cation (0.72 Å VI, 0.84 Å VIII ) and the Zn cation (0.74 Å VI, 0.90 Å VIII ) will almost certainly lead to the distribution of Zr cations over the A- and B-site positions. Therefore, the value of R A /R B will lie between those for complete B-site substitution and those for the substitution of Zn cations on the A-site and Nb cations on the B-site. In both cases, R A /R B decreases as x increases and attains values of 1.602, and 1.580, respectively, for x 1.0. The decrease in the value of R A /R B is much larger for the Gd 3 -substituted system, where, for x 0.6, R A /R B Therefore, at low dopant concentrations, R A /R B reaches the stability-field limit and, as a result, the solid-solubility range is narrow. A broader solid-solubility range is observed for the (Bi 1.5 x Gd x Zn 0.5 )(Ti 1.5 Nb 0.5 )O 7 pyrochlores where the parent (Bi 1.5 Zn 0.5 )(Ti 1.5 Nb 0.5 )O 7 compound has a higher R A /R B value (1.796), compared to that of (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7 (1.658), which is consistent with the previously described rationale. Therefore, a critical R A /R B value is attained at a higher concentration of Gd 3 cations (x 0.6, R A /R B 1.739). For orthorhombic Bi 2 (Zn 2/3 Nb 4/3 )O 7, the solid solubilities are very limited for all dopants. The highest solid solubility is exhibited by the titanium-containing solid solutions (up to x 0.6). Even for that system, the cell parameters are relatively insensitive to the cation substitution. The reduced solid solubilities in the orthorhombic phase probably can be explained in terms of the coupling among the Bi 6s 2 lone-pair electrons. 2 This coupling has been suggested to be responsible for the orthorhombic distortion of the parent structure and seems to completely suppress the flexibility of the structure in accommodating a significant number of substituents. The substituted cubic (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7 -based pyrochlores exhibit permittivities in the range of , which may make these pyrochlores useful for applications as dielectric electronic components. The dielectric losses of all the single-phase samples, measured at a frequency of 1 MHz, also are quite small (tan ). In addition to promoting changes in, the chemical substituents have a pronounced effect on. For efficient tuning of, an understanding of the relations between the changes in composition and the values of the pyrochlore solid solutions is critical. Figures 3, 6, 7, and 8 show that the large changes in the value of that are induced by the substitutions cannot be simply correlated to the magnitude of or to the accompanying changes in the cell volumes of the solid solutions. However, we have determined that a trend can be established by considering the change in the value of R A /R B for the pyrochlore solid solutions. Figure 10 shows that the values of all the systems decrease as the value of R A /R B increases. For example, in titanium-containing solid solutions with x 1.5, the increase in R A /R B that originates from the substitution of Ti cations on the B-site positions is accompanied by a large decrease in (for R A /R B 1.658, 470 ppm/k; for R A /R B 1.796, 1300 ppm/k). At higher concentrations of Ti cations (1.5 x 2.25), where the Zn cations must be removed from the A-site positions, the value of R A /R B decreases and adopts less-negative values. The same correlation is shown by the zirconium- and gadolinium-substituted (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7 systems (Fig. 10). The correlation between the structural tolerance factor (R A / R B ) and the temperature coefficient in the pyrochlores is very similar to that established previously for perovskite and tungsten bronze-based dielectric oxides. 14,15 In those systems, the correlation originates from the associated tilting of the oxygen octahedra. Similar property structure correlations perhaps are expected because the pyrochlore structure is comprised of a related cornershared octahedron network. The structure of cubic pyrochlores is flexible, so it can adapt to dopants of different sizes, and the variation in R A /R B can produce associated changes in the dielectric properties. The orthorhombic Bi 2 (Zn 2/3 Nb 4/3 )O 7 phase is apparently not as accommodating, and it does not seem possible to induce significant alterations in its dielectric properties via the formation of a solid solution. V. Conclusions Extensive ranges of solid-solution formation were discovered in Ti- and Zr-substituted (Bi 1.5 Zn 1.0 Nb 1.5 )O 7. The resultant solid Fig. 10. Correlation of the R A /R B ratio and for the cubic bismuth-based pyrochlore solid solutions.

7 January 2000 Crystal Chemistry and Dielectric Properties of (Bi 1.5 Zn 1.0 Nb 1.5 )O 7 and Bi 2 (Zn 2/3 Nb 4/3 )O 7 Pyrochlores 153 solutions have permittivities in the range of , temperature coefficients of permittivity in the range of 1300 ppm/k to 88 ppm/k, and low dielectric losses. The substitution of Gd cations into (Bi 1.5 Zn 1.0 Nb 1.5 )O 7 is more limited but can be extended by codoping with Ti cations. The ranges of solid-solution formation and the changes in the magnitude of the temperature coefficients are directly related to the structural tolerance factor, R A /R B. In contrast, the same substituents show limited solid solubility in orthorhombic Bi 2 (Zn 2/3 Nb 4/3 )O 7 and do not induce any significant changes in the dielectric properties. References 1 D. P. Cann, C. A. Randall, and T. R. Shrout, Investigation of the Dielectric Properties of Bismuth Pyrochlore, Solid State Commun., 7, (1996). 2 X. Wang, H. Wang, and X. Yao, Structure, Phase Transformations, and Dielectric Properties of Pyrochlores Containing Bismuth, J. Am. Ceram. Soc., 80 [10] (1997). 3 A. Mergen and W. E. Lee, Crystal Chemistry, Thermal Expansion and Dielectric Properties of (Bi 1.5 Zn 0.5 )(Sb 1.5 Zn 0.5 )O 7 Pyrochlore, Mater. Res. Bull., 32, (1997). 4 G. I. Golovshchikova, V. A. Isupov, A. G. Tutov, I. E. Myl nikova, P. A. Nikita, and O. I. Tulinova, Relaxational Character of the Dielectric Polarization in the Region of the Phase Change of New Compounds of the Pyrochlore Type, Sov. Phys. Solid State (Engl. Transl.), 14, (1973). 5 J. C. Champarnaud-Mesjard, M. Manier, B. Frit, and A. Tairi, Bismuth(III)- and Antimony(V)-Based Ceramics with Anion-Deficient Fluorite Structure, J. Alloys Compd., 188, (1992). 6 M. A. Subramanian, G. Aravamudan, and G. V. Subba Rao, Oxide Pyrochlores A Review, Prog. Solid. State Chem., 15, (1983). 7 D. Liu, Y. Liu, S.-Q. Huang, and X. Yao, Phase Structure and Dielectric Properties of Bi 2 O 3 ZnO Nb 2 O 5 -Based Dielectric Ceramics, J. Am. Ceram. Soc., 76 [8] (1993). 8 R. D. Shannon, Dielectric Polarizabilities of Ions in Oxides and Fluorides, J. Appl. Phys., 73 [1] (1993). 9 R. S. Roth and J. Waring, Phase Equilibrium Relations in the Binary System Bismuth Sesquioxide Niobium Pentoxide, J. Res. Natl. Bur. Stand., Sect. A, 66A [6] (1962). 10 E. M. Levin and R. S. Roth, Polymorphism of Bismuth Sesquioxide. II. Effect of Oxide Additions on the Polymorphism of Bi 2 O 3, J. Res. Natl. Bur. Stand., Sect. A, 68A [2] (1964). 11 E. I. Speranskaya, I. S. Rez, L. V. Kozlova, V. M. Skorikov, and I. Slavov, The System Bismuth Oxide Titanium Oxide, Russ. J. Inorg. Mater., 1 [2] (1965). 12 M. Valant and P. K. Davies, Synthesis and Dielectric Properties of Pyrochlore Solid Solutions in the Bi 2 O 3 ZnO Nb 2 O 5 TiO 2 System, J. Mater. Sci., 34 [22] (1999). 13 R. D. Shannon, Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides, Acta Crystallogr., Sect. A: Found. Crystallogr., A32, (1976). 14 E. L. Colla, I. M. Reaney, and N. Setter, Effect of Structural Changes in Complex Perovskites on the Temperature Coefficient of the Relative Permittivity, J. Appl. Phys., 74, (1993). 15 M. Valant, D. Suvorov, and C. J. Rawn, Intrinsic Reasons for the Variations in the Dielectric Properties of Ba 6 3x R 8 2x/3 Ti 18 O 54 (R La Gd) Solid Solutions, Jpn. J. Appl. Phys., Part I, 38 [5A] (1999).