Acta Chim. Slov. 2004, 57, 551-558.
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Short Communication
THE FORMATION AND STABILITY OF A BODY-CENTERED-CUBIC y-Bi203 SOLID SOLUTION IN THE Bi203-ZnO SYSTEM
Špela Kunej, Jyoti P. Guha, and Danilo Suvorov
Jožef Štefan Institute, Jamova 39, 1000 Ljubljana, Slovenia Received 22-J 2-2003
Abstract
Minor additions of ZnO to Bi203 result in the transformation of the face-centered-cubic crystal structure of the high-temperature polymorph of Bi203 to a body-centered-cubic solid solution (y-Bi203). In the binaiy system Bi203-ZnO, the b.c.c.-solid solution exists in the Bi203-rich part of this system and extends to a composition of 1.5 mol% ZnO. Beyond the solid-solubility limit, the solid solution coexists with a binary compound Bi38Zn058. On cooling below 700 °C, the y-Bi203 solid solution transforms to the low-temperature monoclinic polymorph a-Bi203 and Bi38Zn058.
Key words: y-Bi203, solid solution, Bi203-ZnO system
Introduction
Bismuth oxide, Bi203 is known to occur in several polymorphic forms with different crystal structures. The room-temperature polymorph, commonly known as a-Bi203, has a monoclinic structure that transforms into a high-temperature polymorph with a face-centered-cubic structure, referred to as 5-Bi203. The stability of these polymorphic forms of Bi203 is known to depend on various conditions such as temperature, thermal treatment and chemical doping, etc.1'2 Thus, minor doping of Bi203 with various oxides results in the formation of a body-centered-cubic solid solution (b.c.c), referred to as y-Bi203. The b.c.c. phase of Bi203 has the space group /23 and ao^10A, and is known to possess a sillenite-type crystal structure.2'3 For pure Bi203 this sillenite crystal structure is metastable, but under controlled cooling the crystal structure can be stabilized with minor additions of several cations, such as Si4+, Ge4+, Ti4+, etc.3'4'5 In general, compounds with a sillenite structure are represented by the stoichiometric composition Bi12MO20, where M is a cation of the additive oxide or a combination of oxide cations.2'6 However, in a recent publication by Valant and Suvorov,7 the sillenite composition has been corrected to Bii2(Bi4/5-„xMn+5x)Oi9.2+„x. The sillenite phases display a variety of physical properties, and can, as result, be used in a variety of
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electro-optic, acoustic and piezotechnics applications.8'9'10 Recently, sillenites have been used as dielectric materials in microwave-electronic tehnology.13 The major advantage of these materials compared to other microwave dielectric materials is their low sintering temperature (?850 °C), which allows the application of these materials in low-temperature cofired ceramic (LTCC) technology. The low sintering temperature also means that silver, with a melting point of 960 °C, can be used as an electrode material for these dielectrics. One of the problems accompanying the firing of LTCCs is the formation of several umvanted phases during sintering.14 In microwave applications of LTCC in the form of tapes, it is necessary to minimize the number of phases in order to simplify the system. When co-firing an LTCC module that is composed of more than one type of ceramic layer, ali the different layers must be compatible in terms of properties such as sintering parameters, thermal expansion coefficients and a chemical compatibility between ali the ceramic materials in the system, as well as between ali the materials present in the LTCC and the electrode. Thus, a basic understanding of the compatibility relations between the various phases that form during firing is essential.
Previous studies of the Bi203-ZnO system have reported somewhat contradictory results regarding the formation and stability of the binary compounds that occur in this system. The Bi203-ZnO phase diagram reported by Levin and Roth15 shows a binary compound with a chemical composition close to 6Bi203:ZnO. This compound, which has a body-centered-cubic structure, was found to melt congruently at approximately 800 °C. Furthermore, this phase diagram shows the presence of a eutectic between Bi203 and the 6Bi203:ZnO compound at a composition close to 8 mol% ZnO, with a melting point of 750 °C. Safronov et al.16 reported a phase diagram of the system Bi203-ZnO in which a binary compound with the chemical composition Bi48Zn073 (24Bi203:ZnO) was shown to occur. In contrast to the study reported by Levin and Roth,15 this compound was found to melt incongruently at 750 °C. In the Safronov phase diagram a binary eutectic between Bi48Zn073 and ZnO was shown to occur at a composition close to 14 mol% ZnO with a melting point of 740 °C. However, no solid solution was shown to exist at the Bi203-end of the Bi203-ZnO system. Bruton et al.17 prepared single crystals from the Bi203-ZnO system using the top-seed solution technique and reported the existence of a compound with the chemical composition Bii6Zn025 (8Bi203:ZnO). The
Š. Kunej, J. P. Guha, D. Suvorov: The Formation and Stability of a Body-Centered-Cubic ?-Bi203 Solid...
Acta Chim. Slov. 2004, 57, 551-558.
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compound was found to melt congruently at 760 °C and form two eutectics: one at a composition close to 9 mol% and the other at 12 mol% ZnO, with melting points of 745 °C and 740 °C, respectively. However, Wang and Morris18, who have investigated several compositions in this system, observed the formation of a compound with a Bi203/ZnO molar ratio of 20:1. Subsequent crystal-structure studies reported by several workers4'6'19 have shown the existence of a binary compound in the system Bi203-ZnO with the chemical formula Bi38Zn058 (19Bi203:lZnO). In contrast to the previously reported Bi203-ZnO phase diagrams, we have observed the formation of a body-centered-cubic solid solution in the Bi203-rich area of this system. The results reported in this study primarily deal with the nature and formation of this solid solution and its compatibility with the binary compounds that occur in this system.
Experimental
The samples were prepared using the solid-state reaction technique and high-purity (>99.99%) powders of Bi203 and ZnO. Before weighing, the powders were dried at 400 °C for several hours to remove any moisture present in the materials. Several specimens with the general composition (l-x)Bi203:xZnO, where x = 0-0.1, were prepared from the Bi203 and ZnO powders. The powders were weighed with an accuracy of ±0.0002, mixed in an agate mortar with acetone, and then pressed into pellets. The pellets were placed in an A1203 crucible over an Au-foil and calcinated at 700-730 °C for 4-5 hours with intermediate cooling, crushing, mixing and pressing to achieve homogeneity. Some of the calcinated specimens were heat-treated at 700-780 °C. The phases present in the calcinated and heat-treated specimens were determined by X-ray powder diffraction (XRD) (ENDEAVOR D4, Bruker axs) using CuKa radiation at a scanning rate of 0.0272s and 29 = 10-70°. The microstructures and individual phases of the heat-treated specimens were examined using scanning electron microscopy (SEM) (Jeol JXA840A, Japan) combined with energy-dispersive spectroscopy (EDS) (Tracor Northern, Model NORAN Series II, USA). Prior to the microstructure examination, the specimens were first ground, then polished with 1 um diamond paste, and fmally etched in a HN03-HF-water solution.
Š. Kunej, J. P. Guha, D. Suvorov: The Formation and Stability of a Body-Centered-Cubic y-Bi203 Solid...
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Results and discussion
The Bi203-ZnO binary phase diagram proposed by Safronov et al.16 (Figure 1) shows a compositional area located between Bi203 and Bi48Zn073 below 740 °C, in which the low-temperature monoclinic a-Bi203 coexists with Bi48Zn073. However, no solid solution is shown in the Bi203-rich area of this phase diagram.
Figure 1. Equilibrium phase diagram of the Bi203-ZnO system (Safronov et al.16).
In our study, an effort was made to stabilize the high-temperature f.c.c-based crystal structure of the 5-Bi203 polymporph with minor additions of ZnO. The XRD results indicated that a small amount ZnO enters into the f.c.c. 5-Bi203 lattice to form a solid solution with a b.c.c-based, y-Bi203 crystal structure. The XRD patterns of several samples containing various proportions of Bi203 and ZnO that were heat-treated at 700-750 °C are shown in Figure 2. The solid-solubility limit of ZnO in Bi203, as evident from the XRD and SEM analyses of several compositions, extends to a composition of 1.5 mol% ZnO.
An SEM micrograph of a sample containing 1.5 mol% ZnO, vvhich was fired at 750 °C and quenched to room temperature, is shovvn in Figure 3. The microstructure shovvs a near-single-phase y-Bi203 solid solution with a small amount of Bi38Zn058 at the grain boundaries.
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Figure 2. XRD patterns of a series of binaiy compositions containing: A) compound Bi38Zn058, B) 3.5 mol% ZnO, C) 2 mol% ZnO; heat-treated at 740 °C and quenched to room temperature, and D) 1.5 mol% ZnO; heat-treated at 750 °C and quenched to room temperature, E) 2 mol% ZnO; heat-treated at 740 °C and slowly cooled (y = y-Bi203,ss; s = Bi38Zn058; a = a-Bi203).
Figure 3. SEM backscattered micrograph of a sample with 1.5 mol% ZnO, heat-treated at 750 °C and quenched to room temperature (?ss = ?-Bi2O3,ss and BZ = Bi38ZnO58).
It is evident from these XRD patterns (Figures 2a-d) that the b.c.c-based ?-Bi2O3 solid solution has a similar crystal structure to that of the compound Bi38ZnO58. As a
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result, it is difficult to distinguish the solid solution from the compound by an XRD analysis of the heat-treated samples. However, the microstructure examination of several samples clearly shows the existence of two different phases. An SEM micrograph of a sample containing 2 mol% of ZnO that was fired at 740 °C and quenched to room temperature is shown in Figure 4. The microstructure shows two phases: i.e., the ?-Bi203 solid solution and Bi38Zn058.
Figure 4. SEM micrograph of a sample with 2 mol% ZnO, heat-treated at 740 °C and quenched to room temperature (?ss = ?-Bi2O3,ss and BZ = Bi38ZnO58).
Figure 5. XRD patterns of Bi38ZnO58 at temperature a) 740 °C and b) 755 °C(? = ?-Bi2O3,ss and s = Bi38ZnO58).
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The XRD patterns of Bi38Zn058, which was obtained after firing at 740-755 °C, are shown in Figure 5. It was found from these XRD patterns that the Bi38Zn058 compound melts incongruently at 755 °C to form the y-Bi203 solid solution and a liquid. This result supports the earlier literature data.5'16
The XRD patterns of several samples indicated that the y-Bi203 solid solution is not stable when cooled to room temperature. Thus, the samples that were fired at 750 °C and then slowly cooled to room temperature revealed the presence of a-Bi203 and Bi38Zn058. The XRD pattern of a sample containing 2 mol% ZnO is shown in Figure 2e. The evidence obtained in our study indicated that on cooling to room temperature the b.c.c-based y-Bi203 transforms to the low-temperature monoclinic a-Bi203 and Bi38Zn058. The various results obtained in our study seem to differ significantly from those proposed by Safronov et al.16 for the Bi203-ZnO binary phase diagram (Figure 1). First, the formation of the y-Bi203 solid solution at the Bi203-end is not shown in the phase diagram. Second, a two-phase-field area between the y-Bi203 solid solution and the Bi38Zn058 compound occurs in this system. Unlike the previously reported melting of this compound,15'17 Bi38Zn058 melts incongruently at 755 °C. Thus, it is apparent from the various results obtained in this study that a revision of the Bi203-ZnO phase diagram must be undertaken to take into account these new fmdings.
Conclusions
Minor additions of ZnO to the f.c.c Bi203 (5-Bi203) result in the formation of a b.c.c-based solid solution (y-Bi203) at the Bi203-rich end of the Bi203-ZnO system. The solid solubility is limited to the composition 1.5 mol% ZnO. The Bi38Zn058 compound melts incongruently at 755 °C to form the y-Bi203 solid solution and a liquid. The y-Bi203 solid solution is stable at high temperatures, and on cooling to room temperature it transforms to the low-temperature monoclinic a-Bi203 and Bi38Zn058.
References
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7.  M. Valant, D. Suvorov, Chem. Mater. 2002, 14, 3471-3476.
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Povzetek
Dodatek ZnO k visoko temperaturni modifikaciji Bi2O3, ki ima ploskovno centrirano celico, omogoèa nastanek ?-Bi2O3 trdne raztopine s telesno centrirano celico. Ugotovili smo, da trdna raztopna ?-Bi2O3 obstaja do sestave 1.5 mol% ZnO v binarnem sistemu Bi2O3-ZnO. Pri presegu maksimalne trdne topnosti pa trdna raztopina soobstaja z binarno spojino Bi38ZnO58. Pri ohlajanju ?-Bi2O3 trdne raztopine, nastopi pri temperaturi pod 700 °C transformacija, pri kateri iz trdne raztopine nastaneta nizko temperaturna modifikacija Bi2O3 z monoklinsko celico in spojina Bi38ZnO58.
Š. Kunej, J. P. Guha, D. Suvorov: The Formation and Stability of a Body-Centered-Cubic ?-Bi203 Solid...