193 Acta Chim. Slov. 1999, 46(2), pp. 193-202 PHASE EQULIBRIA IN THE SYSTEM BaO-TiO2-Gd2O3 D. Kolar, S. D. Škapin and D. Suvorov Jožef Stefan Institute, Department of Ceramics, Jamova 39, SI-1001 Ljubljana, Slovenia (Received 23.4.1999) Abstract The phase diagram BaO-TiO2-Gd2O3 has been determined for an isothermal subsolidus section at 1300°C. In contrast with other BaO-TiO2-R2O3 systems (R = La, Nd), it contains only 2 previously reported ternary compounds, Ba6Gd2Ti4O17 and Ba6-xGd8+2/3xTi18O54 solid solution. The solid solution range has been determined as 1.1 < x < 1.65. Introduction Ceramics based on the ternary BaO-TiO2-R2O3 system (R = rare earth) are extensively used for manufacturing passive electronic components. Of particular interest are compositions in the vicinity of BaTiO3. BaTiO3 slightly doped with rare earth oxides (up to a concentration of around 0.3 - 0.5 at %) is semiconductive with a large PTCR effect, and is used for the preparation of a wide variety of switching, heating and regulating devices. With higher amounts of rare earths, BaTiO3 exhibits high electrical conductivity and is used for manufacturing highly temperature stable ceramic capacitors ("NPO" type). Ceramics in the vicinity of BaO:R2O3:TiO2 = 1:1:4 are used in the manufacture of high permittivity microwave resonators. Dedicated to the memory of Prof. Dr. Jože Šiftar 194 The electrical properties of BaO-TiO2-R2O3 ceramics strongly depend on their crystal structure, stoichiometry, grain size and phase composition. Therefore, to optimize their properties and assure reproducibility, phase equilibrium data are of great importance. In the present work, equilibrium phases in the BaO-TiO2-Gd2O3 were studied by microanalysis of sintered specimens in combination with electron microscopy and X-ray diffractometry. From the results, the subsolidus phase diagram at 1300° was constructed. Literature survey The phase equilibrium diagram Gd2O3-TiO2 was, according to the authors' knowledge, not described. Two compounds are known to exist: Gd2TiO5 (Due to the Gd2O3 : TiO2 ratio 1 : 1 abbreviated to "GT") [1] and Gd2Ti2O7 ("GT2") [2]. In the absence of firm data on the Gd2O3-TiO2 system, some conclusions may be drawn from the expected similarity with the other rare earth oxide - TiO2 systems. In the La2O3-TiO2 system, 5 compounds have been confirmed: La4Ti9O24 ("L2T9"), La2Ti2O7 ("LT2"), La4Ti3O12 ("L2T3") and La2TiO5 ("LT") [3-4]. The system BaO-TiO2 has been extensively investigated and high temperature equilibrium diagrams have been reported by several groups [5-9]. Although details vary, it is generally accepted that five stable binary compounds exist: Ba2TiO4 ("B2T"), Ba6Ti17O40 ("B6T17"), Ba4Ti13O30 ("B4T13") BaTi4O9 ("BT4") and Ba2Ti9O20 ("B2T9"). The compounds B6T17, B4Ti13 and BT4 decompose to liquid and solid phases at approximately 1350°C, 1365°C and 1446°C, respectively. B2T9 decomposes peritectoidaly into BT4 and TiO2 at 1420°C [9]. The phase equilibrium diagram BaO-Gd2O3 was not published, according to the authors' knowledge. The compound BaGd2O4, "BG" is registered in the XRD 195 powder file [10]. The compound is reported to be stable up to 1860°C. A similar rare-earth system, BaO-La2O3 has been reported in [11], with only the BaLa2O4 ("BL") compound melting incongruently at 1845°C. In the same article, another family of rare-earth titanates with composition Ba3R4O9 is briefly mentioned. The Ba3Sm4O9 was reported to exist above 1550°C. Ternary system BaO-Gd2O3-TiO2 has not been recorded. In similar ternary systems with other rare earth oxides, several ternary compounds are known. The data with references are collected in recently published phase diagrams, determined in the authors' laboratory [12,13]. Of particular industrial importance for microwave applications is the compound BaR2Ti4O12 ("114") with solid solubility range, expressed by the formula Ba6-xR8+2/3xTi18O54. The solid solubility range varies with the rare earth element, being within 0 < x < 3 (14). For the Gd 114 compound, reported data are in disagreement over the composition. Stoichiometry of x = 0 [14], 0 Ł x Ł 1.5 [15] and x = 1.5 [16,17] was reported. Other ternary compounds, confirmed in the La-based system, include Ba La2Ti3O10 ("113") Ba La2Ti2O8 ("112") and Ba2La4Ti5O18 ("225"). Recently, a new Ba-rich compound was identified in BaO-R2O3-TiO2 systems, with composition (54-55) BaO . 10 R2O3 [34-35] TiO2 (18). The compound is registered in the JCPDS file as Ba12Gd4.67Ti8O35 [19]. Compounds with similar composition BaO:R2O3:TiO2 = 6:1:4 were also reported by Chen at al [20]. Experimental procedure Samples were prepared by solid state reaction in air from high purity Gd2O3, TiO2, BaTiO3 and BaCO3. Prior to weighing, the weight loss of Gd2O3 was checked by ignition at 1300°C. Weighed batches were wet mixed in acetone using an agate mortar and pestle. The dried mixtures were pelleted and reacted on Pt foil at 1300°C for approximately 20 hours and rapidly cooled. To ensure the attainment of equilibrium, the sintered samples were crushed, repressed and fired several times. The prepared samples were examined by X-ray powder diffractometry (XRD). Polished 196 surfaces of the pellets were examined by optical and scanning electron microscopy, and quantitative analysis was performed by energy dispersive X-ray analysis (EDS). Results and Discussion Results of heating experiments on BaO-TiO2-Gd2O3 compositions are given in Tables 1-3; Only those results in which the samples were deemed to have reached equilibrium have been included. Table 1: Results of heating experiments in the binary Gd 2O3-TiO2 system at 1300°C Composition, mol % Phases detected by XRD Gd2O3 TiO2 33.3 66.6 18.2 81.8 40.0 60.0 60.0 40.0 50.0 50.0 Key: Abbreviations are explained in table 2 The system TiO2-Gd2O3 As noted in the literature survey, 5 compounds are known to exist in the similar rare-earth containing system TiO2-La2O3. To verify the possible existence of analogous compounds in the TiO2 - Gd2O3 system, samples listed in table 1 were prepared and submitted for XRD. Only two previously known compounds could be detected, Gd2Ti2O7 and Gd2TiO5. Results are consistent with [1]. GT2 GT2, T GT, GT2 GT, G GT 197 Table 2: Results of heating experiments in the system BaO-TiO2-Gd2O3 (1300°C, 40 h) Composition, mol % Phases detected (XRD, EDS) Sample BaO GCI2O3 TĚO2 1 70.0 15.0 15.0 B2T, BG, B 2 53.0 30.0 17.0 G, BG, B2T 3 53.0 17.0 30.0 G, 614, B2T 4 57.0 5.0 38.0 BT, B2T, 614 5 25.0 45.0 30.0 G, GT, 614 6 35.0 20.0 45.0 614, GT, BT 7 20.0 17.5 62.5 GT2, BT, 114 8 20.0 20.0 60.0 GT2, BT 9 20.0 25.0 55.0 GT, GT2, BT 10 32.5 6.0 61.5 BT, B6Ti7, 114 11 44.0 3.0 53.0 BT, 114 12 7.0 7.0 86.0 T, GT2, B2T9 13 16.5 5.0 78.5 GT2, BT4, B2T9 14 12.0 13.0 75.0 BT4, GT2 15 15.0 15.0 70.0 GT2, BT4, 114 16 18.0 5.0 77.0 GT2, 114, BT4 17 20.0 5.0 75.0 BT4, B4T13, 114 18 22.0 5.0 73.0 B4T13, B6Ti7, 114 Key: B : BaO G: Gd2O3 BT : BaTi03 BG: BaGe2C>4 B2T9: Ba2Ti902o GT: GCI2TĚO5 B6Ti7: Ba6Tii704o GT2: GCI2TĚ2O7 BT4: BaTi409 114: BaGd2Ti40i2 (Ba6-xGd8+2/3XTĚ8054Ss) B4T13: Ba4Tii303o 614: BaeGd2Ti40i7 T : TiO2 198 Table 3: Results of heating experiments of compositions corresponding to Ba6-xGd8+2/3xTiO18 solid solution Composition, mol % BaO Gd2O3 TiO2 x Phases detected (XRD, EDS) 19.9 15.1 65.0 0.5 114, BT 18.3 15.8 65.9 1.0 114, BT 17.6 16.2 66.2 1.2 114 17.3 16.3 66.4 1.3 114 17.0 16.5 66.5 1.4 114 16.7 16.7 66.6 1.5 114 16.4 16.8 66.8 1.6 114 16.0 17.0 67.0 1.7 114, GT2 15.0 17.5 67.5 2.0 114, GT2, BT4 Key: Abbreviations are explained in table 2 The ternary system BaO - Gd2O3-TiO2 The results listed in table 2 were used to construct the phase diagram at 1300°C shown in Fig. 1. Two ternary compounds were confirmed: Ba6Gd2Ti4O ("614") and Ba4.5Gd9Ti18O54 ("114"). The family of rare earth 114 compounds are known to form extensive solid solutions, represented by the formula Ba6-xR8+2/3xTi18O54, where the range of x varies with rare earth. To determine the extension of x in the case of the Gd 114 compound, a series of compositions with 0.5 < x < 2.0 were examined. After prolonged firing at 1300°C the samples were analysed by SEM and XRD. Results, given in table 3, confirmed a single phase structure in the composition region 1.2 < x < 1.6. Representative microstructures are shown on Figs. 2-6. Microstructures of compositions with x = 1.0 and x = 1.7 exhibit the presence of second phases, BaTiO3 and GdTi2O7, respectively. The microstructure of the composition with x = 1.2 exhibits a single-phase solid solution. These data are in disagreement with published results [14-17], indicating that the solid solubility range strongly depends on processing conditions. 199 BaO Ti02 GT- GT Gd203 mol% Fig. 1: Subsolidus phase diagram of the BaO-TiO2-Gd2O3 system in air at 1300°C. The ternary diagram should be self-explanatory. It is divided into a number of two-phase and three-phase regions. The tie-lines connecting the 114 solid solution with the various barium polytitanates were not drawn with certainty because the exact composition of the 114 solid solution in equilibrium with the corresponding polytitanate could not be determined. In contrast with other similar systems BaO-TiO2-R2O3 (R = La, Nd, Sm), the system BaO-TiO2-Gd2O3 does not include the tie-line Ba2Ti9O20 - 114 ss. Instead, analysis of samples No. 12, No. 13 and No. 16 confirmed the existence of compatibility triangles TiO2-Gd2Ti2O7-Ba2Ti9O20, Gd2T2O7-Ba2Ti9O20-BaTi4O9 and Gd2Ti2O7-BaTi4O9 - 114 ss, respectively. 200 Fig. 2: Microstructure of composition 44 m/o BaO, 3 m/o Gd2O3, 53 m/o TiO2 located on tie-line BT - BGT4 showing the two phase structure: A = BaTiO3, B = BaGd2Ti4O12ss. Fig. 3: Microstructure of composition Fig. 4: Microstructure of composition 20 m/o BaO, 5 m/o Gd2O3, 75 m/o TiO2 Ba6-xGd8+2/3xTi18O54 with x = 1.2. located in the compatibility triangle Single-phase structure. B4T13-114-BT4 showing three phase structure: A = BT4, B = B4Ti13, C =114. 201 Fig. 5: Micro structure of composition Fig. 6: Microstructure of composition Ba6-xGd8-2/3xTi18O54 with x = 1.0. Ba6-xGd8+2/3xTi18O54 with x = 2.0. Two-phase structure: A = BT, B = 114. Three-phase structure: A = GT2, B = 114, C = BT4. Conclusions A subsolidus ternary BaO-TiO2-Gd2O3 equilibrium diagram at 1300°C has been constructed. Nine stable previously reported binary compounds and 2 ternary compounds, Ba12Gd4.67Ti8O35 (possibly Ba6Gd2Ti4O17) and Ba6-xGd8+2/3xTi18O54 ("114" compound) have been confirmed. The solid solubility range of the 114 compound has been determined as 1.1 < x < 1.65. In contrast with other BaO-TiO2-R2O3 systems ( R = La, Nd), the tie line 114 - TiO2 does not exist in the Gd containing system. Instead, the tie lines Ba2Ti9O20-Gd2Ti2O7 and BaTi4O9-Gd2Ti2O7 were confirmed. Acknowledgments Financial support of the Ministry of Research and Technology of the Republic of Slovenia is gratefully acknowledged. The experimental help of D. Kop orcic is acknowledged. 202 References [I] L. G. Scherbakova, A. V. Kolesnikov, O. N. Breusov, Neorgan. Mat., 1979, 15, 2195-2201. [2] Powder Diffraction File, Card No. 23-0259, JCPDS - International Centre for Diffraction Data, Newtown Square, Pa. [3] J. B. McChesney and H. A. Sauer, J. Am. Ceram. Soc, 1962, 45, 416. [4] N. F. Fedorov, O. V. Melnikova, V. A. 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Frit, Ferroelectrics, 1992, 127, 35. [16] X. Jing, C. Zheng and A. West, Powder Diffraction File, Card No. PDF. 43- 233, JCPDS - International Centre for Diffraction Data, Newtown Square, Pa. [17] M. Valant, D. Suvorov and D. Kolar, Jpn. J. Appl. Phys., 1996, 35, 144-150 [18] C. Zheng and A. R. West, British Ceramic Proceedings, 1992, 49, 247-250. [19] Powder Diffraction File, Card No. 43-0422, JCPDS-International Centre for Diffraction Data, Newtown Square, Pa. [20] A. Chen, Yu Zhi, V. M. Ferreira, P. M. Vilarinho, J. P. Baptista, J. Mat. Sci. Letters, 1996, 15, 1313-1314. Povzetek Dolocen je bil ravnotežni fazni diagram sistema BaO-TiO2-Gd2O3 pri 1300°C. Sistem se razlikuje od sorodnih sistemov z drugimi oksidi redkih zemelj (La, Nd) v tem, da vsebuje le dve že opisani ternarni spojini, Ba6Gd2Ti4O17 in Ba6-xGd8+2/3xTi18O54 trdno raztopino. Obseg trdne raztopine pri 1300°C je 1.1 < x< 1.65.