ISSN 00360236, Russian Journal of Inorganic Chemistry, 2015, Vol. 60, No. 5, pp. 619–625. © Pleiades Publishing, Ltd., 2015. Original Russian Text © Yu.F. Kargin, S.N. Ivicheva, V.V. Volkov, 2015, published in Zhurnal Neorganicheskoi Khimii, 2015, Vol. 60, No. 5, pp. 691–697.
PHYSICOCHEMICAL ANALYSIS OF INORGANIC SYSTEMS
Phase Relations in the Bi2O3–TiO2 System Yu. F. Kargin, S. N. Ivicheva, and V. V. Volkov Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninskii pr. 49, Moscow, 119991 Russia email:
[email protected] Received December 1, 2014
Abstract—Experimental data are presented and available published information is generalized on phase rela tions in the Bi2O3–TiO2 system. The phase diagrams in stable equilibrium and metastable states are con structed. The homogeneity range of the phase with a sillenite Bi12TiO20 structure is refined, and so is the tem perature range of the existence of the metastable and equilibrium states of the phase with a pyrochlore Bi2Ti2O7 structure. DOI: 10.1134/S0036023615050083
The Bi2O3–TiO2 system attracts the attention of researchers owing to the diversity of physical (dielec tric, piezoelectric, photorefractive, photocatalytic, ferroelectric) properties of compounds in this system [1–9]. In the literature, there are contradictory data on the numbers and compositions of compounds in the Bi2O3–TiO2 system [10–26]. According to Skanavi [10], there are ten bismuth titanates, which are char acterized by low conductivity and high permittivity. In the following works, these data were not confirmed, and, by the present time, four bismuth titanates have been conclusively established to exist: Bi2Ti4O11, Bi2Ti2O7, Bi4Ti3O12, and Bi12TiO20. Whereas the compo sitions of the compounds Bi2Ti4O11 and Bi4Ti3O12 are doubtless, there are different opinions in the literature on bismuth titanates of the compositions Bi2Ti2O7 and Bi12TiO20. For example, according to Belyaev et al. [11], the composition of intermediate phases within the concentration range adjacent to the ordinate Bi2O3 is 12Bi2O3 : 1TiO2 (Bi24TiO38), and within the concen tration range between the compounds Bi2Ti4O11 and Bi4Ti3O12, it is 1Bi2O3 : 3TiO2 (Bi2Ti3O9), whereas Shi mada et al. reported Bi2Ti2O7 [12]. Levin and Roth studied in detail the effect of group I–VIII element oxides on the polymorphism of bismuth oxide and described the phase with a sillenite structure as having the Bi12TiO20 composition and, presumably, the con gruent type of melting [13]. The first most comprehensive investigation of phase equilibria in the Bi2O3–TiO2 system was made by Speranskaya et al. in 1965 by differential thermal analysis and Xray powder diffraction analysis [14]. According to the phase diagram [14], in the Bi2O3– TiO2 system, not only the compound with a sillenite structure occurs, to which the composition Bi8TiO14 (4 : 1) was assigned, but also bismuth titanates Bi4Ti3O12 (2 : 3) and Bi2Ti4O11 (1 : 4) form. All the
compounds melt incongruently at 1148, 1483, and 1553 K, respectively. Morrison, using the horizontal directional crystal lization method, proposed two alternative melting dia grams with the compound Bi12TiO20 melting either congruently or incongruently. Brutton performed a detailed study of phase equilibria within the concentration range from 2 to 22 mol % TiO2 by a thermobalance technique [16]. The liquidus tem perature was determined from the change of the weight of a seed crystal (or a platinum wire) immersed in a solution of a given composition. At a temperature above the liquidus temperature, the seed crystal weight decreased (there was dissolution), whereas at a tem perature below the liquidus temperature, the seed crystal weight increased (there was crystallization). The liquidus temperature was taken to be the temper ature at which the seed crystal weight remained con stant, i.e., the solid phase was in equilibrium with the liquid phase. The temperature of incongruent melting of the compound with a sillenite structure of the com position Bi12TiO20 is 1146 K (as given by Speranskaya et al. [14]), and the eutectic between Bi2O3 and Bi12TiO20 is at 2 mol % TiO2 and melts at 1068 K. Nonetheless, Maier et al. for the phase with a sille nite structure indicated a wide range of solid solutions (9.1–18.2 mol % TiO2) [17], but Zhou and Park showed that the homogeneity range of Bi12TiO20 on the side of bismuth oxide does not exceed 0.5 mol % (Fig. 1a) [18]. The dependence of the unit cell param eter of the phase with a sillenite structure on the com position of the solid solution (table) was obtained by Volkov [18] using single crystals grown at compositions corresponding to various points of the crystallization branch (4, 6, 8, 10, and 12 mol % TiO2). For compar ison, the table presents similar results obtained by Miyazawa and Tabata [22].
619
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KARGIN et al. T, °C 1250 1200 1150 1100 L 1050 1000 950 900 L + δ* 850 L+6:1 800 750 δ*Bi2O3 + 6 : 1 730 700 650 αBi O + 6 : 1 2 3 600
(а)
L + βCTB 873 6 : 1 + βCTB 670 6 : 1 + αCTB
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 mol % Bi2O3 TiO2 T, °C
(b)
1100 1000
L
900
L + δ*
L + βCTB 835
800 700 600 500 α
δ* + βCTB δ*
650 δ* + αCTB
520
α + αCTB
400 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 Bi2O3 TiO2 mol % Fig. 1. Phase diagrams of the Bi2O3–TiO2 system within the concentration range 0–35 mol % TiO2 in (a) stable equilibrium and (b) metastable states [27].
As the point the composition at which is the closest to the stoichiometric composition of Bi12TiO20 was the composition point of a single crystal (the unit bcc cell parameter а = 1.0169 nm) produced by Mar’in under hydrothermal conditions [19]. For the phase with a sil lenite structure (space group I23), two crystalchemi cal models of the formation of a solid solution were proposed: Bi12Ti1 – х[VTi]xO20 – δ and Bi12Ti1 – хBixO20 – δ [3, 18]. For Bi12Ti1 – х[VTi]xO20 – δ, the changes in the composition of the phase and in the unit cell parame ter were attributed to the formation of vacancies at tet rahedral positions of the bcc cell (according to Sarin et al. [20]), the occupancy of the position 2а by tita
nium atoms is q(Ti) = 0.9, and the occupancy by oxy gen atoms is q(О3) = 0.95. Volkov [18] assumed that the variablecomposition phase Bi12Ti1 – хBixO20 – δ can be represented as a solid solution of two isostructural phases: the metastable cubic γmodification of Bi2O3 (а = 1.0264 nm) and Bi12TiO20 (а = 1.0169 nm). The possibility of the for mation of a solid solution between the metastable γBi2O3 phase and the compound Bi12TiO20 was noted by Schrimm et al. [21] on the basis of the linear change in the unit cell parameter of samples cooled from 1073 K. Miyazawa and Tabata [22] based on the data on the change in the unit cell parameter of Bi12TiO20 crystals
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grown at compositions corresponding to various points of the crystallization branch (table) assumed the retro grade solubility of bismuth oxide with the homogeneity range 13.85–14.3 mol % TiO2. However, according to Volkov [18], the minimal titanium oxide content of the variablecomposition phase Bi12TixO20 – δ is reached at the eutectic temperature and is ≈13.5 mol % TiO2. Note that the wide homogeneity ranges (≈12.5–15.5 mol % for Bi12SiO20 and Bi12GeO20 and 9.1–18.2 mol % TiO2 for Bi12TiO20) indicated by Maier et al. [17] and Lomonov [23] contradict to available literature data. The metastable equilibrium in the Bi2O3–TiO2 sys tem within the concentration range 0–30 mol % TiO2 (Fig. 1b), which is reached in crystallization from melt superheated above the temperatures of structural rear rangements of melt, is characterized by the absence of a compound with a sillenite structure and by the for mation of a solid solution based on δBi2O3 [27]. According to Kargin et al. [28] and Endrzheevskaya [29], the interaction of components in the bismuth containing oxide systems Bi2O3–GeO2, Bi2O3–SiO2, etc. in the solid phase also occurs with the formation of metastable compounds Bi2GeO5, Bi2SiO5, etc. and δ and γmodifications of bismuth oxides as interme diate compounds. By Xray powder diffraction analy sis and Xray microanalysis, Kargin [30] showed that the chemical and phase compositions of the surface of single crystals of Bi12SiO20, Bi12GeO20, Bi12TiO20, and Bi4M3O12 (M = Si, Ge, Ti) change in annealing in a vacuum. Because, in a vacuum, the volatility of bis muth oxide is much higher than that of the second component, the surface of crystals of Bi12SiO20, Bi12GeO20, and Bi12TiO20 during annealing is being enriched with silicon, germanium, or titanium oxide, respectively. The change in the phase composition of the surface of single crystals of Bi12SiO20, Bi12GeO20, and Bi12TiO20 in the course of their annealing in a vac uum can be described by the following schemes: Bi12SiO20 → Bi2SiO5 + Bi 2O3g → Bi4Si3O12 + Bi 2O3g → SiO2 + Bi 2O3g , Bi12GeO20 → Bi2GeO5 + Bi 2O3g → Bi4Ge3O12 + Bi 2O3g → Bi2Ge3O9 + Bi 2O3g → GeO2 + Bi 2O3g , Bi12TiO20 → Bi4Ti3O12 + Bi 2O3g → Bi2Ti4O11 + Bi 2O3g → TiO2 + Bi 2O3g , where the superscript g refers to the gaseous state of bismuth oxide. The sequence of the formation of phases on the surface of single crystals of Bi12SiO20, Bi12GeO20, and Bi12TiO20 in their heat treatment in a vacuum is deter mined by the change in the ratio Bi2O3 : MO2 in a series of compounds (both stable and metastable) RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
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Dependence of the unit cell parameter of the phase with a sillenite structure on the composition of the solid solution for single crystals grown at compositions corresponding to various points of the crystallization branch Composition at point of crystalli zation branch, mol % TiO2
Unit cell parameter a, nm
4
1.01760
6
1.01744
4%—1.017612, 5%—1.017594 6.5%—1.017482
8
1.01739
8%—1.017415,
10
1.01704
9%—1.017385 10%—1.017411
14.3
1.0169
Data [19]
Data [22]
forming in the Bi2O3–MO2 binary systems (M = Si, Ge, Ti) and is opposite to the sequence of the forma tion of phases under the conditions of the solidphase interaction of the initial components of these systems [28, 29]. The change in the composition of the surface layer can occur until the complete evaporation of bis muth oxide and the formation of a layer of the high melting oxide MO2. As noted above, in the Bi2O3–TiO2 system within the concentration range 50–100 mol % TiO2, accord ing to Speranskaya et al. [14], there are only Bi4Ti3O12 and Bi2Ti4O11. Twenty seven years after, based on the differential thermal analysis and Xray powder diffrac tion analysis data, Masuda et al. [24] established that, along with the two compounds, Bi2Ti2O7 also exists in the Bi2O3–TiO2 system, and in the phase diagram of this system, each of the three phases (2 : 3, 1 : 2, and 1 : 4) is stable within the temperature range from room tem perature to its in congruent melting temperature (1200, 1210, and 1240°C, respectively). For the phase with a sillenite structure, Masuda et al. [24] presented two ordinates corresponding to the compositions Bi12TiO20 and Bi8TiO14 with one peritectic horizontal at 865°C, which is incorrect from the standpoint of Gibbs’ phase rule. LopezMartinez et al. [25] pro vided the phase diagram of the Bi2O3–TiO2 system taking into account thermodynamic calculation results, which agree with the published experimental data [24]. Later, EsquivelElizonado et al. [26] pre sented the phase diagram combining the published experimental results [24] with corrections made for data taking into account the thermal instability of the phase Bi2Ti2O7. It was found [31–33] that Bi2Ti2O7 is an unstable phase because, while heating above 480– 650°С, it decomposes into Bi4Ti3O12 and Bi2Ti4O11 according to Xray powder diffraction data. Esquivel Elizonado et al. [26] assumed that Bi2Ti2O7 undergoes a phase transition and detected a diffuse exothermic (emphasis added) effect in the differential thermal Vol. 60
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KARGIN et al. DTA, μW/mg
TG, % exo
454 102 100
0.3 3 [1]
1
0.2
546
98
[1] 1195 1260 0.1 [2]
260
96 4
0
1230
94 2
[1]
92
–0.1 –0.2
90
–0.3 200
400
600
T, °C
800
1000
1200
Fig. 2. Differential thermal analysis and thermogravimetry curves of Bi2Ti2O7 samples: (1, 2) the initial Xray amorphous sample and (3, 4) and singlephase crystalline sample after annealing at 500°C.
analysis curve within the range 612–729°С, which was assigned to a secondorder transition with a tempera ture of 670°C coinciding with the temperature of fer roelectric polymorphic transition of Bi4Ti3O12. Taking into account the differential thermal analysis data, EsquivelElizonado et al. [26] provided a “combined” phase diagram of the Bi2O3–TiO2 system, which com bined the published phase diagrams [14, 24, 25] with two proposed corrections: the line of polymorphic transition of Bi4Ti3O12 at 670°С was repeatedly extended to the phase Bi2Ti4O11 and the ordinate of the phase Bi2Ti2O7 has a discontinuity from 670 to 1200°С (to indicate the thermodynamic instability range). Obviously, these corrections are inconsistent with Gibbs’ phase rule. It is wrong to phase transformations of two phases by a single line; moreover, the assign ment of the phase transition of Bi2Ti2O7 to a second order transition is also incorrect because it has been unequivocally determined that Bi2Ti2O7 decomposes into two phases. Noteworthily, Speranskaya et al. [14] observed the Bi4Ti3O12 transformation at 670°C (exo thermic effect) in a twophase region to form Bi2Ti4O11 in the absence of a phase with a pyrochlore structure. The representation of the ordinate of the compound Bi2Ti2O7 by a dashed line (from 670 to 1200°С) is unfounded because the “thermodynamic instability” means the absence of a phase in equilibrium state. The phases existing at these temperatures in the subsolidus region between the compounds 2 : 3 and 1 : 4 are not shown in the proposed diagram [26]. Figures 2 and 3 present the results of our experi mental studies of the transformations of the phase with
a pyrochlore Bi2Ti2O7 structure at temperatures from room temperature to the melting point. Figure 2 pro vides the results of differential thermal analysis of the initial Xray amorphous sample obtained by precipita tion from a hydrated mixed bismuth titanium oxide sol based on polyhydric alcohols (curves 1, 2) and this sample after annealing at 500°C (curves 3, 4), which was singlephase pyrochlore (Fig. 3). It is seen that, while heating the amorphous sample, there is a broad endothermic effects within the range 50–150°C, which characterizes the water removal; there are also a number of exothermic effects with maxima at 200, 300, 454, and 546°C, which are due to the pyrolytic decomposition of organic and nitric components of the sol (mass spectral analysis of the gas phase detected С+, C3H8+, NO 2+ , and other organic radicals 42 to 46 in weight) and the pyrochlore Bi2Ti2O7 crystallization. The differential thermal analysis and thermogravime try curves of the crystalline sample (Fig. 2, curves 3, 4) exhibit neither thermal events, nor weight loss within the range 25–600°C; however, within the temperature range 600–1000°C, there is a broad exothermic effect. The endothermic effects with maxima at 1195, 1230, and 1260°C coincide for both samples and are related to the incongruent melting of Bi4Ti3O12, Bi2Ti2O7, and Bi2Ti4O11, respectively. The presence of three endot hermic effects indicates that the samples simulta neously contain three bismuth titanates forming as the result of the decomposition of Bi2Ti2O7 into Bi4Ti3O12 and Bi2Ti4O11 within the range 700–1000°C and sub sequent formation of Bi2Ti2O7 at temperatures above
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I
1150°C
1000°C
900°C
800°C Bi4Ti3O12 (350795) Bi2Ti4O11 (150325) 700°C Bi2Ti2O7 (320118)
600°C
500°C 10
20
30
40 2θ, deg
50
60
Fig. 3. Xray powder diffraction patterns of Bi2Ti2O7 samples annealed sequentially at 500, 600, 700, 800, 900, 1000, and 1150°C for 2 h.
1000°C (Fig. 2), and also the peritectic nature of the pyrochlore melting Bi2Ti2O7 → L + Bi2Ti4O11. Our results agree with the known data on the decomposi tion of pyrochlore above 600°C [8, 31–33], i.e., indi cate its metastability within the temperature range from room temperature to 1000°C, and also with the data on the formation of Bi2Ti2O7 by solidphase syn thesis above 1000°C [34–37]. Note that, at low temperatures. the pyrochlore phase Bi2Ti2O7 in all the cited works was obtained as a fine powder by wet chemistry (coprecipitation, oxalate method, sol method). Thus, taking into account our experimental results, and also the above published data, phase relations in the Bi2O3–TiO2 system can be represented as the dia gram in Fig. 4. The range of the equilibrium existence RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
of bismuth titanate with a pyrochlore structure is within the temperature range 1000–1210°C. Probably, this phase is not strictly stoichiometric because the synthesis at high temperatures leads to the formation of cationdeficient samples [24, 25, 30]. For the sto ichiometric phase of pyrochlore Bi2Ti2O7, the unit cubic cell (space group F d 3 m ) parameter is a = 10,37949 Å [30]; cationdeficient (Bideficient) phases may differ in composition and unit cell param eter: for Bi1.74Ti2O6.62, a = 10.357 Å [24], and for Bi1.833Ti2O6.75, a = 10.354 Å [29], which indicates the possibility of the existence of a homogeneity range. At temperatures below 1000°C, the compound Bi2Ti2O7 is metastable, and its formation by precipita tion from solutions (or by the sol–gel method) is caused Vol. 60
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KARGIN et al. T, °C 1400 1350
L+1:4
1300
L + TiO2
1250
L+1:2
1200 1150
1195 L
1100 1050 1000 950
2:3+1:2 1:2+1:4 L+2:3 β2 : 3 + 1 : 4 873
L+6:1
900 850
1000
δBi2O3 + 6 : 1
800
αBi2O3 + 6 : 1
1260
1230
750 700
6 : 1 + β2 : 3
650
1 : 4 + TiO2 670
6 : 1 + α2 : 3
600 0 Bi2O3
10
20
30
40
50 60 70 80 90 mol % α2 : 3 + 1 : 4
100 TiO2
Fig. 4. Phase diagram of the Bi2O3–TiO2 system.
by the effect of the size factor on the stability of the phase, i.e. is due to the fine particle size (12–35 mm). ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research (project no. 130200662). REFERENCES 1. R. E. Newnham, R. W. Wolfe, and J. F. Dorrian, Mater. Res. Bull. 6, 1029 (1971). 2. H. Shulman, M. Testorf, D. Damjanovic, and N. Set ter, J. Am. Ceram. Soc. 79, 3124 (1996). 3. Yu. F. Kargin, V. I. Burkov, A. A. Mar’in, and A. V. Ego rysheva, Bi12MxO20 ± δ Crystals: Synthesis, Structure, and Properties (Azbuka, Moscow, 2005) [in Russian]. 4. H. Shulman, These No. 1646 (Ecole Polytechnique Federale de Lausanne, Lausanne, 1997), p. 1. 5. W. Wei, Y. Dai, and B. Huang, J. Phys. Chem. C 113, 5658 (2009). 6. W. F. Yao, H. Wang, X. H. Xu, et al., Appl. Catal. A. Gen. 259, 29 (2004). 7. J. Hou, Sh. Jiao, H. Zhu, and R. V. Kumar, J. Solid State Chem. 184, 154 (2011). 8. W. F. Su and Y. T. Lu, Mater. Chem. Phys. 80, 632 (2003). 9. H. Zhou and T.J. Park, J. Mater. Res. 21, 2941 (2006).
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Translated by V. Glyanchenko
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