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Phase formation and electrical properties of Bi2O3-based compounds in the Bi2O3-La2O3-MoO3 system
SOSI-14125; No of Pages 7 Solid State Ionics xxx (2016) xxx–xxx
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Phase formation and electrical properties of Bi2O3-based compounds in the Bi2O3-La2O3-MoO3 system☆ E.I. Orlova a, E.P. Kharitonova a, N.V. Gorshkov b, V.G. Goffman b, V.I. Voronkova a,⁎ a b
M.V. Lomonosov Moscow State University, Leninskie gory, 119991 Moscow, Russia Yuri Gagarin State Technical University of Saratov, 410054 Saratov, Russia
a r t i c l e
i n f o
Article history: Received 20 July 2016 Received in revised form 8 November 2016 Accepted 21 November 2016 Available online xxxx Keywords: Bi2O3 La2O3 MoO3 Polymorphism Oxygen conductivity Vogel–Fulcher–Tammann law
a b s t r a c t The ternary system Bi2O3–La2O3–MoO3 is shown to contain a wide range of (Bi2O3)x(La2O3)y(MoO3)z (x + y + z = 1) compounds isostructural with the cubic phase δ-Bi2O3, stable at room temperature: 0.65 ≤ x ≤ 0.93. In addition, the ternary system contains two tetragonal phases (β at x = 0.96 and β′ in the region 0.50 ≤ x ≤ 0.61, 0.14 ≤ y ≤ 0.21, and 0.22 ≤ z ≤ 0.31) and a rhombohedral phase in a narrow composition range (0.60 ≤ x ≤ 0.64) on the Bi2O3–La2MoO6 join. When heated to 1000–1100 °C, the rhombohedral and β′ tetragonal compounds undergo one phase transition, to the cubic phase with the δ-Bi2O3 structure. The samples with the βBi2O3 structure exhibit a more complex polymorphism. The electrical conductivity of the samples increases with Bi2O3 concentration. The conductivity of the cubic materials with x = 0.8 reaches 0.6 S/cm at 800 °C. The hightemperature (350–800 °C) conductivity of such samples follows the Vogel–Fulcher–Tammann law. At lower temperatures, their conductivity exhibits Arrhenius behavior with an activation energy near 0.8 eV. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Bi2O3-based oxide phases stand out among oxygen ion conductors in that they have high oxygen ion conductivity. These materials can be used in gas sensors, as solid electrolytes in fuel cells, and as gas separation membranes. The polymorphism and special conductive properties of bismuth oxide have been the subject of many reports and reviews [1–4]. The high conductivity of Bi2O3 was first mentioned by Takahashi et al. [5]. At room temperature, bismuth oxide has a monoclinic (α) structure (sp. gr. P21/c). On heating, this phase transforms into a cubic (δ) phase, which exists in a rather narrow temperature range: 730–825 °C. The δphase was shown to have space group Fm–3m [6,7]. In its fluorite structure, a quarter of the anion sites are vacant, being responsible for its high conductivity. On cooling, it transforms into the α-phase, passing through metastable phases: tetragonal (β) and cubic (γ). Replacing bismuth by a rareearth element (Y, Dy, or Er), Iwahara et al. [8] were able to stabilize the δ-phase. According to Watanabe [9], however, firing at a temperature on the order of 600 °C causes the stabilized phase to decompose.
☆ This work was presented during the 12th International Symposium on Systems with Fast Ionic Transport, Kaunas, Lithuania 03-07.07.2016. ⁎ Corresponding author at: Moscow State University, Leninskie gory 1, Moscow 119991, Russia. E-mail address: [email protected] (V.I. Voronkova).
The binary systems Bi2O3–Ln2O3 with large rare earths (La, Pr, and Nd) contain rhombohedral phases [1]. In addition to the rhombohedral compounds, the Bi2O3–Ln2O3 systems contain other phases with a cubic, tetragonal, orthorhombic, or monoclinic structure, depending on synthesis conditions [1,2,10]. As shown in a number of studies [11–13], tungsten and molybdenum also stabilize the δ-phase. The Bi2O3–MoO3 binary has been the subject of several studies [14–16]. It was shown to contain many phases. The tetragonal compound Bi 14 MoO24 stands out among them [17]. δ-Bi2O3 can be successfully stabilized by so-called codoping, e.g. with tungsten and a rare earth [18–22]. Watanabe and Sekita [18] investigated the ternary system Bi2O3–Er2O3–WO3 and showed that codoping prevented the compound from decomposing on heating. The formation and conductive properties of the δ-phase codoped with dysprosium and tungsten have been the subject of several studies [19–21]. The conductivity of such compounds reaches 0.1–1 S/cm at 800 °C. The effect of molybdenum on the stabilization of δ-Bi2O3 in the case of codoping has not yet been studied in sufficient detail. Kharitonova et al. [23,24] investigated the Nd2 MoO6 –Bi 2O 3 and Pr2 MoO6 –Bi 2O 3 joins in the ternary systems Bi2 O 3 –Nd2 O 3 –MoO3 and Bi2 O 3–Pr 2O 3 –MoO 3 and identified both cubic and tetragonal phases with high conductivity. In this paper, we describe the formation and electrical properties of conductive phases in the ternary system Bi2O3–La2O3–MoO3 (Bi2O3 codoped with lanthanum and molybdenum).
http://dx.doi.org/10.1016/j.ssi.2016.11.019 0167-2738/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: E.I. Orlova, et al., Phase formation and electrical properties of Bi2O3-based compounds in the Bi2O3-La2O3-MoO3 system, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.019
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2. Experimental
3. Results and discussion
Polycrystalline Bi2O3–La2O3–MoO3 samples were prepared by solidstate reactions in air, using appropriate mixtures of 99.9%-pure Bi2O3, La2O3, and MoO3. Before weighing, the La2O3 was calcined at 1000 °C for 1 h to remove water and carbon dioxide. The compositions of the samples were (Bi2O3)x(La2O3)y(MoO3)z, with x + y + z = 1. The starting reagents were weighed on a Sartorius E1200S balance with an accuracy of ± 0.001 g. The total weight of the resultant ceramics was 3.5 g. The initial composition of the samples was determined with an accuracy (Δx, Δy, and Δz) in the range ± 0.0001–0.005. The samples were fired in two steps, with intermediate grinding and pressing at 0.1 GPa. In preliminary syntheses of polycrystalline samples in the system under consideration, we tested firing in two to five steps at various temperatures. The duration of each step was varied from 12 h to a week. The results showed that two 12-h steps were sufficient to reach equilibrium. In the first step, the firing temperature was 750– 800 °C. The density of the ceramics was found to increase with firing temperature, so the firing temperature in the second step was chosen to be near the melting point of the material: 820–1300 °C, depending on the composition of the ceramics. For the samples containing N85 mol% Bi2O3 (x N 0.85), this temperature was 820–850 °C. Most of the samples containing 50–85 mol% Bi2O3 were prepared at temperatures from 900 to 1000 °C. The materials containing 30–60 mol% Bi2O3 and a large excess of La2O3 relative to MoO3 were prepared by firing between 1100 and 1300 °C. To avoid the formation of metastable Bi2O3based phases, all of the samples were cooled slowly. The heating and cooling rates in our syntheses were 5 K/min. To assess the effect of quenching, presynthesized samples were heated to a preset temperature (690–1050 °C), held there for 30 min, and then quenched in cold water. To ascertain whether the intended compositions were obtained, we evaluated bismuth and molybdenum volatility in our samples by thermogravimetry using a Netzsch STA 449C system. Heating the samples to their melting points was found to cause a weight change no N0.1%, indicating that none of their components vaporized. Thus, we are led to conclude that the final composition of the ceramics produced by firing below their melting points was near the intended composition: (Bi2O3)x(La2O3)y(MoO3)z with x + y + z = 1. The phase analysis of ground polycrystalline samples was performed by X-ray diffraction (XRD) on a DRON-2.0 diffractometer (Cu Kα radiation, 2θ = 20°–60°, 0.05° scan step). Their unit-cell parameters were evaluated by least squares fitting using the Powder 2.0 program. Ground SiO2 single crystals were used as an external standard. The density of some of the polycrystalline samples was determined by hydrostatic weighing in toluene. The samples were characterized by differential scanning calorimetry (DSC) in air using the Netzsch STA 449C (30–1150 °C, heating/ cooling rate of 10 K/min, platinum crucibles). To verify DSC data reproducibility, two or three heating/cooling cycles were performed for each sample. The conductivity of the samples was measured at 1 MHz between 30 and 850 °C by a two-probe technique using a Tesla BM 431E bridge. The measurements were made during heating and cooling at 10 K/min. In addition, (Bi 2 O 3 ) 0.80 (La 2 O 3 ) 0.1 (MoO 3 ) 0.1 and (Bi 2O 3 ) 0.61 (La 2 O3 ) 0.17 (MoO3 ) 0.22 samples were characterized by two-probe impedance spectroscopy at frequencies from 0.01 Hz to 1 MHz, temperatures from 50 to 700 °C, and an applied sinusoidal voltage of 0.1 V peak, using a Novocontrol Alpha AN impedance analyzer. In the conductivity measurements, we used platinum electrodes made by firing platinum paste at 800 °C for 15 min (the samples were heated and cooled at 5 K/min). In the case of the (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 sample, which was obtained as the α-phase, electrodes were produced by firing at 650 °C, i.e. below the temperature of the α → δ phase transition, to avoid the formation of the tetragonal metastable phase β- Bi2O3.
3.1. Formation of Bi2O3-based compounds in the ternary system Bi2O3La2O3-MoO3 Fig. 1 shows powder XRD patterns of ground polycrystalline (Bi2O3)x(La2O3)y(MoO3)z samples. Fig. 2 indicates the compositions of the samples in the ternary system Bi2O3 –La 2O3 –MoO 3 and presents XRD results. A slight increase in La and Mo dopant concentrations allows the tetragonal phase with the β-Bi2O3 structure to be stabilized at room temperature ((Bi 2O 3 ) 0.96 (La 2 O 3) 0.02 (MoO 3 ) 0.02 sample, Fig. 1a). Higher doping levels lead to the formation of a cubic phase isostructural with the high-temperature cubic phase δBi 2O 3 (in the composition region 0.65 ≤ x ≤ 0.93, 0.03 ≤ y ≤ 0.175, and 0.015 ≤ z ≤ 0.22; Fig. 1b). Further changes in composition and a decrease in bismuth content to 0.50 ≤ x ≤ 0.61 (0.14 ≤ y ≤ 0.21 and 0.22 ≤ z ≤ 0.31) lead to distortion of the cubic (δ-Bi 2O 3 ) structure and the formation of another tetragonal phase (β′-phase) (Fig. 1c). The present results on the formation of the cubic phase (δ) and two tetragonal phases (β and β′) are similar in many respects to previously reported data for the Nd2MoO6–Bi2O3 join in the corresponding ternary system [23], but in the La system the stability fields of the δ- and β′phases are displaced from the Bi2O3-La2MoO6 join to higher MoO3 concentrations (Fig. 2). Another distinction from the Nd system is the formation of a rhombohedral phase on the Bi2O3-La2MoO6 join in a narrow range of Bi2O3 concentrations: 0.60 ≤ x ≤ 0.64 (Fig. 1d). The strongest reflections in the XRD patterns in Fig. 1d correspond to Bi2La4O9 [27], but according to the present XRD data Bi2La4O9 does not form solid solutions with any of the observed rhombohedral compounds. Fig. 3 shows the composition dependence of the unit-cell parameter for the cubic phase. The cubic cell parameter and volume increase sharply with decreasing bismuth oxide concentration and increasing codoping level. At a constant Bi2O3 concentration, the unit-cell parameter decreases with increasing lanthanum concentration. Note that the unit-cell parameter of the cubic phase was also observed to increase with decreasing bismuth oxide content in the neodymium system [23], but in the lanthanum system the increase is much greater. Given that the ionic radius of trivalent bismuth exceeds those of lanthanum, neodymium, and molybdenum, this behavior of the unit-cell parameter cannot be understood in terms of cation geometry and seems to be caused by changes in oxygen stoichiometry in response to changes in the composition of the samples. The unit-cell parameters of the tetragonal and rhombohedral compounds are presented in Table 1. Like in a previous study [23], for the convenience of comparison with the unit-cell parameters of the δ- and β′phases we used the same setting as Sillen [6,28] in calculating the unitcell parameters of (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 (β-Bi2O3 phase). Like in the Nd system, the tetragonal phases β and β′ differ in unit-cell geometry (a N c in the β′-phase and a/2 b c in the β-phase). Note that the tetragonal distortion of the structure of the β′-phase increases with decreasing bismuth concentration and increasing molybdenum content, i.e. as the composition moves farther away from the stability region of the cubic phase. Like in the case of the cubic materials, the unit-cell volume increases with codoping level. The unit-cell parameters of the rhombohedral compounds vary little with composition (Table 1). Comparison of the X-ray density, dx, and the density determined by hydrostatic weighing, dmeas, allowed us to evaluate the relative density of the ceramics, which was found to be high, on the order of 97–98%, for the cubic (δ) and tetragonal (β′) materials. For example, the cubic material (Bi2O3)0.9(La2O3)0.05(MoO3)0.05 had dx = 8.35 g/cm3 and dmeas = 8.18 ± 0.02 g/cm3. 3.2. Polymorphism The DSC heating curve of the tetragonal material (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 (β-Bi2O3 phase) has two endothermic
Please cite this article as: E.I. Orlova, et al., Phase formation and electrical properties of Bi2O3-based compounds in the Bi2O3-La2O3-MoO3 system, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.019
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Fig. 1. X-ray powder diffraction patterns of polycrystalline (Bi2O3)x(La2O3)y(MoO3)z samples: (a) (1) undoped Bi2O3 (monoclinic structure, α-phase) and (2) (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 (tetragonal structure, β-phase); (b) cubic compounds with the δ-Bi2O3 structure: (3) (Bi2O3)0.93(La2O3)0.055(MoO3)0.015, (4) (Bi2O3)0.91 (La2O3)0.045(MoO3)0.045, (5) (Bi2O3)0.90(La2O3)0.05(MoO3)0.05, (6) (Bi2O3)0.90(La2O3)0.07(MoO3)0.03, (7) (Bi2O3)0.85(La2O3)0.075(MoO3)0.075, (8) (Bi2O3)0.80(La2O3)0.03(MoO3)0.17, (9) (Bi2O3)0.80(La2O3)0.06(MoO3)0.14, (10) (Bi2O3)0.80(La2O3)0.08(MoO3)0.12, (11) (Bi2O3)0.80(La2O3)0.1(MoO3)0.1, (12) (Bi2O3)0.80(La2O3)0.11(MoO3)0.09, (13) (Bi2O3)0.75(La2O3)0.04 (MoO3)0.21, (14) (Bi2O3)0.75(La2O3)0.06(MoO3)0.19, (15) (Bi2O3)0.75(La2O3)0.08(MoO3)0.17, (16) (Bi2O3)0.75(La2O3)0.10(MoO3)0.15, (17) (Bi2O3)0.75(La2O3)0.13(MoO3)0.12; (18) (Bi2O3)0.70(La2O3)0.11(MoO3)0.19, (19) (Bi2O3)0.70(La2O3)0.13(MoO3)0.17, (20) (Bi2O3)0.70(La2O3)0.15(MoO3)0.15, (21) (Bi2O3)0.65(La2O3)0.13(MoO3)0.22, (22) (Bi2O3)0.65 (La2O3)0.15(MoO3)0.20, (23) (Bi2O3)0.65(La2O3)0.175(MoO3)0.175; (c) tetragonal compounds (β′-phase): (24) (Bi2O3)0.61(La2O3)0.15(MoO3)0.24, (25) (Bi2O3)0.61(La2O3)0.17(MoO3)0.22, (26) (Bi2O3)0.58(La2O3)0.19(MoO3)0.23, (27) (Bi2O3)0.55(La2O3)0.14(MoO3)0.31, (28) (Bi2O3)0.55(La2O3)0.18(MoO3)0.27, (29) (Bi2O3)0.50(La2O3)0.21(MoO3)0.29; (d) rhombohedral materials: (30) (Bi2O3)0.60(La2O3)0.2(MoO3)0.2, (31) (Bi2O3)0.61(La2O3)0.195(MoO3)0.195, (32) (Bi2O3)0.62(La2O3)0.19(MoO3)0.19, (33) (Bi2O3)0.63(La2O3)0.185(MoO3)0.185, (34) (Bi2O3)0.64(La2O3)0.18 (MoO3)0.18.
peaks, which correspond to the structural phase transitions at 628 and 715 °C (Fig. 4, sample 1). XRD data for the sample quenched from 690 °C demonstrate that the phase existing between 628 and 715 °C is isostructural with α-Bi2O3 (Fig. 5). The DSC curve obtained during heating of the quenched sample to 800 °C and subsequent cooling (Fig. 4, sample 2) shows one phase transition at 715 °C during heating and 520 °C during cooling, following which the sample again becomes
Fig. 2. Stability regions of cubic, tetragonal, and rhombohedral Bi2O3-based compounds in the ternary system Bi2O3-La2O3-MoO3. The solid triangles represent the reported compositions of some compounds in the Bi2O3–La2O3 and Bi2O3–MoO3 binaries [1,2,14,15,25,26].
tetragonal and isostructural with β-Bi2O3. Thus, the anomaly at 628 °C in the DSC heating curve can be interpreted as due to the β → α (tetragonal to monoclinic) phase transition. The high-temperature phase transition, at 715 °C, seems to be the α → δ (monoclinic to cubic) phase transition, as supported by the behavior of the electrical conductivity characteristic of this phase transition (see below). The exothermic peak at 520 °C in the DSC cooling curve is due to the δ → β (high-temperature cubic to tetragonal) phase transition. Note that quenching from 800 °C (Fig. 5) did not stabilize the cubic phase: according to XRD data, the material remained tetragonal. The rhombohedral and β′ tetragonal materials are similar in polymorphism: they undergo a single phase transition in the range 1000– 1100 °C (Fig. 6). XRD data for the samples quenched from the stability
Fig. 3. Composition dependences of unit-cell parameters for cubic (Bi2O3)x(La2O3)y (MoO3)z samples.
Please cite this article as: E.I. Orlova, et al., Phase formation and electrical properties of Bi2O3-based compounds in the Bi2O3-La2O3-MoO3 system, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.019
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Table 1 Unit cell parameters of tetragonal and rhombohedral samples in Bi2O3-La2O3-MoO3 ternary system. Composition
Symmetry of room temperature phase
Unit cell parameters, Å
(Bi2O3)0.96(La2O3)0.02(MoO3)0.02
β, tetragonal
(Bi2O3)0.61(La2O3)0.15(MoO3)0.24
β′, tetragonal
(Bi2O3)0.61(La2O3)0.17(MoO3)0.22
β′, tetragonal
(Bi2O3)0.58(La2O3)0.19(MoO3)0.23
β′, tetragonal
(Bi2O3)0.55(La2O3)0.14(MoO3)0.31
β′, tetragonal
(Bi2O3)0.55(La2O3)0.18(MoO3)0.27
β′, tetragonal
(Bi2O3)0.50(La2O3)0.21(MoO3)0.29
β′, tetragonal
(Bi2O3)0.64(La2O3)0.18(MoO3)0.18
Rhombohedral⁎
(Bi2O3)0.63(La2O3)0.185(MoO3)0.185
Rhombohedral
(Bi2O3)0.62(La2O3)0.19(MoO3)0.19
Rhombohedral
(Bi2O3)0.61(La2O3)0.195(MoO3)0.195
Rhombohedral
(Bi2O3)0.60(La2O3)0.20(MoO3)0.20
Rhombohedral
a = 2 × 5.499(5), c = 5.678(4) a = 5.6888(7), c = 5.602(1) a = 5.692(2), c = 5.607(2) a = 5.704(1), c = 5.588(1) a = 5.741(1), c = 5.520(2) a = 5.7176(5), c = 5.5603(7) a = 5.742(3), c = 5.536(4) a = 31.94(3), c = 19.73(3) a = 31.92(3), c = 19.72(2) a = 31.96(3), c = 19.72(2) a = 31.92(3), c = 19.75(2) a = 31.95(4), c = 19.77(3)
⁎ Unit cell parameters of rhombohedral compounds are indicated in hexagonal setting.
range of the high-temperature phase indicate that it is isostructural with the cubic phase δ-Bi2O3. According to the DSC results, most of the samples with the cubic structure undergo no phase transitions during multiple heating–cooling cycles in the range 30–1000 °C (Fig. 7), but not all of the cubic materials are thermally stable. (Bi2O3)0.93(La2O3)0.055(MoO3)0.015 undergoes two phase transitions during heating: cubic to monoclinic (δ → α) phase transition at 610 °C and monoclinic to cubic (α → δ) phase transition near 700 °C. During cooling, no monoclinic phase is formed and the sample retains its cubic structure down to room temperature. (Bi2O3)0.90(La2O3)0.05(MoO3)0.05 was also found to have an instability range during heating in the range 750–800 °C, represented by a broad endothermic peak in its DSC curve. (Bi2O3)0.90(La2O3)0.07(MoO3)0.03, (Bi2O3)0.85(La2O3)0.075(MoO3)0.075, and the other samples containing less Bi are stable and undergo no phase transitions.
Fig. 4. DSC heating–cooling curves of two polycrystalline (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 samples. Sample 1, isostructural with β-Bi2O3 at room temperature, was slowly cooled from 820 °C. Sample 2, isostructural with α-Bi2O3 at room temperature, was quenched from 690 °C.
Fig. 5. X-ray powder diffraction patterns of ceramic (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 samples: (1) sample slowly cooled from 820 °C to room temperature, (2) sample quenched from 690 °C, and (3) sample quenched from 820 °C. The main reflections from the monoclinic (α) and tetragonal (β) phases are marked by + and *, respectively.
3.3. Electrical conductivity Fig. 8 presents Arrhenius plots of conductivity for the (Bi2O3)x(La2O3)y(MoO3)z samples with the cubic, β′ tetragonal, and rhombohedral structures. The data were obtained by dynamic measurements at 1 MHz. Fig. 9 shows the bulk conductivity evaluated from impedance spectra. In general, complex impedance plots consist of two semicircles representing bulk and electrode conductivities. It can be seen that the conductivity data obtained by the two techniques differ little. The conductivity of the samples gradually increases with Bi2O3 concentration, independent of the symmetry of their structure. The tetragonal material (Bi2O3)0.50(La2O3)0.21(MoO3)0.29 had the lowest conductivity (0.05 S/cm at 800 °C). The highest conductivity (0.6 S/cm at 800 °C) was offered by the cubic material (Bi2O3)0.8(La2O3)0.1(MoO3)0.1. Over the entire temperature range studied, the conductivity of the samples with 0.5 ≤ x ≤ 0.7 follows the Arrhenius law, Ea σ ¼ σ 0 exp kT with an activation energy Ea near 0.7–0.8 eV (Table 2). In the temperature range studied (30–850 °C), the conductivity curves of the above
Fig. 6. DSC heating curves of tetragonal and rhombohedral (Bi2O3)x(La2O3)y(MoO3)z: (1) (Bi2O3)0.50(La2O3)0.21(MoO3)0.29, (2) (Bi2O3)0.55(La2O3)0.18(MoO3)0.27, (3) (Bi2O3)0.58(La2O3)0.19(MoO3)0.23, (4) (Bi2O3)0.61(La2O3)0.17(MoO3)0.22, (5) (Bi2O3)0.61(La2O3)0.15(MoO3)0.24, (6) (Bi2O3)0.61(La2O3)0.195(MoO3)0.195.
Please cite this article as: E.I. Orlova, et al., Phase formation and electrical properties of Bi2O3-based compounds in the Bi2O3-La2O3-MoO3 system, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.019
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Fig. 7. DSC (a) heating and (b) cooling curves of cubic (Bi2O3)x(La2O3)y(MoO3)z: (1) (Bi2O3)0.93(La2O3)0.055(MoO3)0.015, (2) (Bi2O3)0.90(La2O3)0.05(MoO3)0.05, (3) (Bi2O3)0.90(La2O3)0.07(MoO3)0.03, (4) (Bi2O3)0.85(La2O3)0.075(MoO3)0.075, (5) (Bi2O3)0.80(La2O3)0.06(MoO3)0.14, (6) (Bi2O3)0.80(La2O3)0.1(MoO3)0.1, (7) (Bi2O3)0.75(La2O3)0.08(MoO3)0.17, (8) (Bi2O3)0.75(La2O3)0.10(MoO3)0.15, (9) (Bi2O3)0.70(La2O3)0.11(MoO3)0.19, (10) (Bi2O3)0.70(La2O3)0.15(MoO3)0.15, (11) (Bi2O3)0.65(La2O3)0.13(MoO3)0.22, (12) (Bi2O3)0.65 (La2O3)0.175(MoO3)0.175.
samples do not have any anomalies attributable to decomposition or structural phase transitions, in agreement with DSC data (Figs. 6, 7). The temperature dependence of conductivity for the cubic materials (Bi2O3)0.80(La2O3)0.10(MoO3)0.10 and (Bi2O3)0.80(La2O3)0.06(MoO3)0.14, which have the highest conductivity, consists of two distinct portions. At low temperatures (below 350–360 °C), their conductivity shows Arrhenius behavior with an activation energy near 0.8 eV. Above 350 °C, we observe a deviation from the Arrhenius law, and the conductivity is well represented by the Vogel–Fulcher–Tammann (VFT) law (Fig. 10), which is often used to describe the conductivity of polymers and glass-forming systems [29,30]: σ0 −B σ ¼ pffiffiffi exp kðT−T 0 Þ : T
listed in Table 2. We used the VFT equation to describe the present conductivity data by analogy with stabilized cubic molybdates based on the oxygen ion conductor La 2 Mo2 O 9, where such an approach was successfully used previously [31–34]. In the case of the above molybdates, the presence of two temperature ranges differing in conduction mechanism was accounted for by the existence of two cubic phases: high-temperature (β) and low-temperature (β ms), which were similar in symmetry but differed in oxygen order. The transition from Arrhenius behavior to VFT behavior was interpreted in terms of dynamic oxygen disorder in the high-temperature phase. A similar effect seems to occur in our case. It is worth noting that deviations from the Arrhenius law at high temperatures for stabilized Bi2O3-based phases were also observed previously [4,18,19], but the effect has not received proper attention.
The experimentally determined constants σ0, T0, and B in the VFT equation and activation energy Ea in the Arrhenius equation are
Fig. 8. Arrhenius plots of 1-MHz conductivity measured during heating for cubic, β′ tetragonal, and rhombohedral (Bi2O3)x(La2O3)y(MoO3)z.
Fig. 9. Bulk conductivity of (Bi2O3)0.80(La2O3)0.1(MoO3)0.1 (cubic) and (Bi2O3)0.61(La2O3)0.17(MoO3)0.22 (tetragonal, β′). Inset: impedance spectra of these samples at 250 and 300 °C.
Please cite this article as: E.I. Orlova, et al., Phase formation and electrical properties of Bi2O3-based compounds in the Bi2O3-La2O3-MoO3 system, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.019
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Table 2 The activation energy Ea (Arrhenius law) and parameters of Vogel-Fulcher-Tammann (VFT) equation calculated for conductivity of (Bi2O3)x(La2O3)y(MoO3)z samples. Composition
Conductivity behavior
Calculated parameters
(Bi2O3)0.80(La2O3)0.10(MoO3)0.10
T b 350 °C, Arrhenius T N 350 °C, VFT
Ea = 0.84 ± 0.01 eV σ0 = 120±9S/cmK1/2;
(Bi2O3)0.80(La2O3)0.06(MoO3)0.14
(Bi2O3)0.70(La2O3)0.13(MoO3)0.17 (Bi2O3)0.70(La2O3)0.15(MoO3)0.15 (Bi2O3)0.61(La2O3)0.195(MoO3)0.195 (Bi2O3)0.61(La2O3)0.15(MoO3)0.24 (Bi2O3)0.50(La2O3)0.21(MoO3)0.29
T b 360 °C, Arrhenius T N 360 °C, VFT
Arrhenius Arrhenius Arrhenius Arrhenius Arrhenius
B = 0.100 ± 0.006 eV; T0 = 460 ± 10 K Ea = 0.80 ± 0.01 eV σ0 = 215±9S/cmK1/2; B = 0.155 ± 0.004 eV; T0 = 394 ± 6 K Ea = 0.74 ± 0.01 eV Ea = 0.65 ± 0.01 eV Ea = 0.71 ± 0.01 eV Ea = 0.78 ± 0.01 eV Ea = 0.74 ± 0.01 eV
Fig. 11 shows Arrhenius plots of conductivity for the (Bi2O3)0.96 (La2O3)0.02(MoO3)0.02 ceramic that was slowly cooled from 820 °C and was isostructural with the tetragonal phase β-Bi2O3 at room temperature (Fig. 11, sample 1) and for the sample that was quenched from 690 °C and was isostructural with the monoclinic phase α-Bi2O3 at room temperature (Fig. 11, sample 2). The anomalies in the conductivity of both samples correspond to the thermal events observed in their DSC curves (Fig. 4). The conductivity curve of the sample isostructural with β-Bi2O3 at room temperature shows two phase transitions during heating, β → α (630 °C) and α → δ (710 °C), each accompanied by a sharp increase in conductivity by half an order of magnitude. During heating of the sample isostructural with the α-phase at room temperature, we observe one jump in conductivity, by two orders of magnitude, accompanying the α → δ (monoclinic to cubic) phase transition, typical of the polymorphism and conductivity of undoped bismuth oxide [5]. The conductivity curves of the two (Bi2O3)0.96 (La2O3)0.02(MoO3)0.02 samples during cooling show a single (δ → β) phase transition, at 510 °C, accompanied by a sharp drop in conductivity by two orders of magnitude. Note the large thermal hysteresis of the phase transitions. A similar polymorphism, with two phase transitions during heating and one during cooling was reported previously [22]
Fig. 11. Arrhenius plots of 1-MHz conductivity measured during heating and cooling for two ceramic (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 samples. Sample 1, isostructural with βBi2O3 at room temperature, was slowly cooled from 820 °C. Sample 2, isostructural with α-Bi2O3 at room temperature, was quenched from 690 °C.
for Bi14W1–xLaxO24–3x/2 obtained by codoping Bi2O3 with small amounts of lanthanum and tungsten. The conductivity of the high-temperature phase (δ-Bi2O3) of (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 slightly exceeds that of the stabilized cubic materials obtained in this study and approaches 1 S/cm at 800 °C. In the temperature stability ranges of the α- and β-phases, the conductivity of (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 approaches that of the tetragonal materials containing 50–60 mol% Bi2O3. 4. Conclusions Polycrystalline (Bi2O3)x(La2O3)y(MoO3)z (x + y + z = 1) samples have been prepared by solid-state reactions in air and the formation of Bi2O3-based compounds in the ternary system Bi2O3–La2O3–MoO3 has been studied. XRD data demonstrate that, in the range 0.65 ≤ x ≤ 0.93 at room temperature, the system contains compounds isostructural with the high-temperature, cubic phase δ-Bi2O3. According to DSC data, the cubic solid solution (Bi2O3)0.90(La2O3)0.07(MoO3)0.03 and the samples with 0.65 ≤ x ≤ 0.85 undergo no phase transitions in multiple heating–cooling cycles. The compounds richer in bismuth and molybdenum are unstable when heated. In addition to the cubic phase, the ternary system Bi2O3-La2O3-MoO3 contains tetragonal (β and β′) and rhombohedral phases. Over the entire temperature range studied, the β′ tetragonal, rhombohedral, and cubic materials have high conductivity, which reaches 0.05–0.6 S/cm (at 800 °C) and increases with bismuth content. The conductivity of most of the samples with x ≤ 0.7 exhibits Arrhenius behavior throughout the temperature range studied, with an activation energy from 0.7 to 0.8 eV. The (Bi 2 O 3 )0.80 (La 2 O3 ) 0.06 (MoO3 ) 0.14 and (Bi 2O 3 )0.80 (La 2 O 3) 0.10 (MoO 3 )0.10 samples, which have the highest conductivity, follow the Arrhenius law only at low temperatures (30–350 °C). Their high-temperature conductivity follows the Vogel–Fulcher–Tammann law, which seems to be evidence of considerable oxygen ion conductivity. Acknowledgments
Fig. 10. Arrhenius plots of conductivity for polycrystalline (Bi2O3)0.80(La2O3)0.10(MoO3)0.10. The dotted and solid lines represent fits to the Arrhenius and VFT laws.
This work was supported by the Russian Foundation for Basic Research, project no. 15-02-03492 a.
Please cite this article as: E.I. Orlova, et al., Phase formation and electrical properties of Bi2O3-based compounds in the Bi2O3-La2O3-MoO3 system, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.019
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Please cite this article as: E.I. Orlova, et al., Phase formation and electrical properties of Bi2O3-based compounds in the Bi2O3-La2O3-MoO3 system, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.019
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Phase formation and electrical properties of Bi2O3-based compounds in the Bi2O3-La2O3-MoO3 system☆ E.I. Orlova a, E.P. Kharitonova a, N.V. Gorshkov b, V.G. Goffman b, V.I. Voronkova a,⁎ a b
M.V. Lomonosov Moscow State University, Leninskie gory, 119991 Moscow, Russia Yuri Gagarin State Technical University of Saratov, 410054 Saratov, Russia
a r t i c l e
i n f o
Article history: Received 20 July 2016 Received in revised form 8 November 2016 Accepted 21 November 2016 Available online xxxx Keywords: Bi2O3 La2O3 MoO3 Polymorphism Oxygen conductivity Vogel–Fulcher–Tammann law
a b s t r a c t The ternary system Bi2O3–La2O3–MoO3 is shown to contain a wide range of (Bi2O3)x(La2O3)y(MoO3)z (x + y + z = 1) compounds isostructural with the cubic phase δ-Bi2O3, stable at room temperature: 0.65 ≤ x ≤ 0.93. In addition, the ternary system contains two tetragonal phases (β at x = 0.96 and β′ in the region 0.50 ≤ x ≤ 0.61, 0.14 ≤ y ≤ 0.21, and 0.22 ≤ z ≤ 0.31) and a rhombohedral phase in a narrow composition range (0.60 ≤ x ≤ 0.64) on the Bi2O3–La2MoO6 join. When heated to 1000–1100 °C, the rhombohedral and β′ tetragonal compounds undergo one phase transition, to the cubic phase with the δ-Bi2O3 structure. The samples with the βBi2O3 structure exhibit a more complex polymorphism. The electrical conductivity of the samples increases with Bi2O3 concentration. The conductivity of the cubic materials with x = 0.8 reaches 0.6 S/cm at 800 °C. The hightemperature (350–800 °C) conductivity of such samples follows the Vogel–Fulcher–Tammann law. At lower temperatures, their conductivity exhibits Arrhenius behavior with an activation energy near 0.8 eV. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Bi2O3-based oxide phases stand out among oxygen ion conductors in that they have high oxygen ion conductivity. These materials can be used in gas sensors, as solid electrolytes in fuel cells, and as gas separation membranes. The polymorphism and special conductive properties of bismuth oxide have been the subject of many reports and reviews [1–4]. The high conductivity of Bi2O3 was first mentioned by Takahashi et al. [5]. At room temperature, bismuth oxide has a monoclinic (α) structure (sp. gr. P21/c). On heating, this phase transforms into a cubic (δ) phase, which exists in a rather narrow temperature range: 730–825 °C. The δphase was shown to have space group Fm–3m [6,7]. In its fluorite structure, a quarter of the anion sites are vacant, being responsible for its high conductivity. On cooling, it transforms into the α-phase, passing through metastable phases: tetragonal (β) and cubic (γ). Replacing bismuth by a rareearth element (Y, Dy, or Er), Iwahara et al. [8] were able to stabilize the δ-phase. According to Watanabe [9], however, firing at a temperature on the order of 600 °C causes the stabilized phase to decompose.
☆ This work was presented during the 12th International Symposium on Systems with Fast Ionic Transport, Kaunas, Lithuania 03-07.07.2016. ⁎ Corresponding author at: Moscow State University, Leninskie gory 1, Moscow 119991, Russia. E-mail address: [email protected] (V.I. Voronkova).
The binary systems Bi2O3–Ln2O3 with large rare earths (La, Pr, and Nd) contain rhombohedral phases [1]. In addition to the rhombohedral compounds, the Bi2O3–Ln2O3 systems contain other phases with a cubic, tetragonal, orthorhombic, or monoclinic structure, depending on synthesis conditions [1,2,10]. As shown in a number of studies [11–13], tungsten and molybdenum also stabilize the δ-phase. The Bi2O3–MoO3 binary has been the subject of several studies [14–16]. It was shown to contain many phases. The tetragonal compound Bi 14 MoO24 stands out among them [17]. δ-Bi2O3 can be successfully stabilized by so-called codoping, e.g. with tungsten and a rare earth [18–22]. Watanabe and Sekita [18] investigated the ternary system Bi2O3–Er2O3–WO3 and showed that codoping prevented the compound from decomposing on heating. The formation and conductive properties of the δ-phase codoped with dysprosium and tungsten have been the subject of several studies [19–21]. The conductivity of such compounds reaches 0.1–1 S/cm at 800 °C. The effect of molybdenum on the stabilization of δ-Bi2O3 in the case of codoping has not yet been studied in sufficient detail. Kharitonova et al. [23,24] investigated the Nd2 MoO6 –Bi 2O 3 and Pr2 MoO6 –Bi 2O 3 joins in the ternary systems Bi2 O 3 –Nd2 O 3 –MoO3 and Bi2 O 3–Pr 2O 3 –MoO 3 and identified both cubic and tetragonal phases with high conductivity. In this paper, we describe the formation and electrical properties of conductive phases in the ternary system Bi2O3–La2O3–MoO3 (Bi2O3 codoped with lanthanum and molybdenum).
http://dx.doi.org/10.1016/j.ssi.2016.11.019 0167-2738/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: E.I. Orlova, et al., Phase formation and electrical properties of Bi2O3-based compounds in the Bi2O3-La2O3-MoO3 system, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.019
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2. Experimental
3. Results and discussion
Polycrystalline Bi2O3–La2O3–MoO3 samples were prepared by solidstate reactions in air, using appropriate mixtures of 99.9%-pure Bi2O3, La2O3, and MoO3. Before weighing, the La2O3 was calcined at 1000 °C for 1 h to remove water and carbon dioxide. The compositions of the samples were (Bi2O3)x(La2O3)y(MoO3)z, with x + y + z = 1. The starting reagents were weighed on a Sartorius E1200S balance with an accuracy of ± 0.001 g. The total weight of the resultant ceramics was 3.5 g. The initial composition of the samples was determined with an accuracy (Δx, Δy, and Δz) in the range ± 0.0001–0.005. The samples were fired in two steps, with intermediate grinding and pressing at 0.1 GPa. In preliminary syntheses of polycrystalline samples in the system under consideration, we tested firing in two to five steps at various temperatures. The duration of each step was varied from 12 h to a week. The results showed that two 12-h steps were sufficient to reach equilibrium. In the first step, the firing temperature was 750– 800 °C. The density of the ceramics was found to increase with firing temperature, so the firing temperature in the second step was chosen to be near the melting point of the material: 820–1300 °C, depending on the composition of the ceramics. For the samples containing N85 mol% Bi2O3 (x N 0.85), this temperature was 820–850 °C. Most of the samples containing 50–85 mol% Bi2O3 were prepared at temperatures from 900 to 1000 °C. The materials containing 30–60 mol% Bi2O3 and a large excess of La2O3 relative to MoO3 were prepared by firing between 1100 and 1300 °C. To avoid the formation of metastable Bi2O3based phases, all of the samples were cooled slowly. The heating and cooling rates in our syntheses were 5 K/min. To assess the effect of quenching, presynthesized samples were heated to a preset temperature (690–1050 °C), held there for 30 min, and then quenched in cold water. To ascertain whether the intended compositions were obtained, we evaluated bismuth and molybdenum volatility in our samples by thermogravimetry using a Netzsch STA 449C system. Heating the samples to their melting points was found to cause a weight change no N0.1%, indicating that none of their components vaporized. Thus, we are led to conclude that the final composition of the ceramics produced by firing below their melting points was near the intended composition: (Bi2O3)x(La2O3)y(MoO3)z with x + y + z = 1. The phase analysis of ground polycrystalline samples was performed by X-ray diffraction (XRD) on a DRON-2.0 diffractometer (Cu Kα radiation, 2θ = 20°–60°, 0.05° scan step). Their unit-cell parameters were evaluated by least squares fitting using the Powder 2.0 program. Ground SiO2 single crystals were used as an external standard. The density of some of the polycrystalline samples was determined by hydrostatic weighing in toluene. The samples were characterized by differential scanning calorimetry (DSC) in air using the Netzsch STA 449C (30–1150 °C, heating/ cooling rate of 10 K/min, platinum crucibles). To verify DSC data reproducibility, two or three heating/cooling cycles were performed for each sample. The conductivity of the samples was measured at 1 MHz between 30 and 850 °C by a two-probe technique using a Tesla BM 431E bridge. The measurements were made during heating and cooling at 10 K/min. In addition, (Bi 2 O 3 ) 0.80 (La 2 O 3 ) 0.1 (MoO 3 ) 0.1 and (Bi 2O 3 ) 0.61 (La 2 O3 ) 0.17 (MoO3 ) 0.22 samples were characterized by two-probe impedance spectroscopy at frequencies from 0.01 Hz to 1 MHz, temperatures from 50 to 700 °C, and an applied sinusoidal voltage of 0.1 V peak, using a Novocontrol Alpha AN impedance analyzer. In the conductivity measurements, we used platinum electrodes made by firing platinum paste at 800 °C for 15 min (the samples were heated and cooled at 5 K/min). In the case of the (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 sample, which was obtained as the α-phase, electrodes were produced by firing at 650 °C, i.e. below the temperature of the α → δ phase transition, to avoid the formation of the tetragonal metastable phase β- Bi2O3.
3.1. Formation of Bi2O3-based compounds in the ternary system Bi2O3La2O3-MoO3 Fig. 1 shows powder XRD patterns of ground polycrystalline (Bi2O3)x(La2O3)y(MoO3)z samples. Fig. 2 indicates the compositions of the samples in the ternary system Bi2O3 –La 2O3 –MoO 3 and presents XRD results. A slight increase in La and Mo dopant concentrations allows the tetragonal phase with the β-Bi2O3 structure to be stabilized at room temperature ((Bi 2O 3 ) 0.96 (La 2 O 3) 0.02 (MoO 3 ) 0.02 sample, Fig. 1a). Higher doping levels lead to the formation of a cubic phase isostructural with the high-temperature cubic phase δBi 2O 3 (in the composition region 0.65 ≤ x ≤ 0.93, 0.03 ≤ y ≤ 0.175, and 0.015 ≤ z ≤ 0.22; Fig. 1b). Further changes in composition and a decrease in bismuth content to 0.50 ≤ x ≤ 0.61 (0.14 ≤ y ≤ 0.21 and 0.22 ≤ z ≤ 0.31) lead to distortion of the cubic (δ-Bi 2O 3 ) structure and the formation of another tetragonal phase (β′-phase) (Fig. 1c). The present results on the formation of the cubic phase (δ) and two tetragonal phases (β and β′) are similar in many respects to previously reported data for the Nd2MoO6–Bi2O3 join in the corresponding ternary system [23], but in the La system the stability fields of the δ- and β′phases are displaced from the Bi2O3-La2MoO6 join to higher MoO3 concentrations (Fig. 2). Another distinction from the Nd system is the formation of a rhombohedral phase on the Bi2O3-La2MoO6 join in a narrow range of Bi2O3 concentrations: 0.60 ≤ x ≤ 0.64 (Fig. 1d). The strongest reflections in the XRD patterns in Fig. 1d correspond to Bi2La4O9 [27], but according to the present XRD data Bi2La4O9 does not form solid solutions with any of the observed rhombohedral compounds. Fig. 3 shows the composition dependence of the unit-cell parameter for the cubic phase. The cubic cell parameter and volume increase sharply with decreasing bismuth oxide concentration and increasing codoping level. At a constant Bi2O3 concentration, the unit-cell parameter decreases with increasing lanthanum concentration. Note that the unit-cell parameter of the cubic phase was also observed to increase with decreasing bismuth oxide content in the neodymium system [23], but in the lanthanum system the increase is much greater. Given that the ionic radius of trivalent bismuth exceeds those of lanthanum, neodymium, and molybdenum, this behavior of the unit-cell parameter cannot be understood in terms of cation geometry and seems to be caused by changes in oxygen stoichiometry in response to changes in the composition of the samples. The unit-cell parameters of the tetragonal and rhombohedral compounds are presented in Table 1. Like in a previous study [23], for the convenience of comparison with the unit-cell parameters of the δ- and β′phases we used the same setting as Sillen [6,28] in calculating the unitcell parameters of (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 (β-Bi2O3 phase). Like in the Nd system, the tetragonal phases β and β′ differ in unit-cell geometry (a N c in the β′-phase and a/2 b c in the β-phase). Note that the tetragonal distortion of the structure of the β′-phase increases with decreasing bismuth concentration and increasing molybdenum content, i.e. as the composition moves farther away from the stability region of the cubic phase. Like in the case of the cubic materials, the unit-cell volume increases with codoping level. The unit-cell parameters of the rhombohedral compounds vary little with composition (Table 1). Comparison of the X-ray density, dx, and the density determined by hydrostatic weighing, dmeas, allowed us to evaluate the relative density of the ceramics, which was found to be high, on the order of 97–98%, for the cubic (δ) and tetragonal (β′) materials. For example, the cubic material (Bi2O3)0.9(La2O3)0.05(MoO3)0.05 had dx = 8.35 g/cm3 and dmeas = 8.18 ± 0.02 g/cm3. 3.2. Polymorphism The DSC heating curve of the tetragonal material (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 (β-Bi2O3 phase) has two endothermic
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Fig. 1. X-ray powder diffraction patterns of polycrystalline (Bi2O3)x(La2O3)y(MoO3)z samples: (a) (1) undoped Bi2O3 (monoclinic structure, α-phase) and (2) (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 (tetragonal structure, β-phase); (b) cubic compounds with the δ-Bi2O3 structure: (3) (Bi2O3)0.93(La2O3)0.055(MoO3)0.015, (4) (Bi2O3)0.91 (La2O3)0.045(MoO3)0.045, (5) (Bi2O3)0.90(La2O3)0.05(MoO3)0.05, (6) (Bi2O3)0.90(La2O3)0.07(MoO3)0.03, (7) (Bi2O3)0.85(La2O3)0.075(MoO3)0.075, (8) (Bi2O3)0.80(La2O3)0.03(MoO3)0.17, (9) (Bi2O3)0.80(La2O3)0.06(MoO3)0.14, (10) (Bi2O3)0.80(La2O3)0.08(MoO3)0.12, (11) (Bi2O3)0.80(La2O3)0.1(MoO3)0.1, (12) (Bi2O3)0.80(La2O3)0.11(MoO3)0.09, (13) (Bi2O3)0.75(La2O3)0.04 (MoO3)0.21, (14) (Bi2O3)0.75(La2O3)0.06(MoO3)0.19, (15) (Bi2O3)0.75(La2O3)0.08(MoO3)0.17, (16) (Bi2O3)0.75(La2O3)0.10(MoO3)0.15, (17) (Bi2O3)0.75(La2O3)0.13(MoO3)0.12; (18) (Bi2O3)0.70(La2O3)0.11(MoO3)0.19, (19) (Bi2O3)0.70(La2O3)0.13(MoO3)0.17, (20) (Bi2O3)0.70(La2O3)0.15(MoO3)0.15, (21) (Bi2O3)0.65(La2O3)0.13(MoO3)0.22, (22) (Bi2O3)0.65 (La2O3)0.15(MoO3)0.20, (23) (Bi2O3)0.65(La2O3)0.175(MoO3)0.175; (c) tetragonal compounds (β′-phase): (24) (Bi2O3)0.61(La2O3)0.15(MoO3)0.24, (25) (Bi2O3)0.61(La2O3)0.17(MoO3)0.22, (26) (Bi2O3)0.58(La2O3)0.19(MoO3)0.23, (27) (Bi2O3)0.55(La2O3)0.14(MoO3)0.31, (28) (Bi2O3)0.55(La2O3)0.18(MoO3)0.27, (29) (Bi2O3)0.50(La2O3)0.21(MoO3)0.29; (d) rhombohedral materials: (30) (Bi2O3)0.60(La2O3)0.2(MoO3)0.2, (31) (Bi2O3)0.61(La2O3)0.195(MoO3)0.195, (32) (Bi2O3)0.62(La2O3)0.19(MoO3)0.19, (33) (Bi2O3)0.63(La2O3)0.185(MoO3)0.185, (34) (Bi2O3)0.64(La2O3)0.18 (MoO3)0.18.
peaks, which correspond to the structural phase transitions at 628 and 715 °C (Fig. 4, sample 1). XRD data for the sample quenched from 690 °C demonstrate that the phase existing between 628 and 715 °C is isostructural with α-Bi2O3 (Fig. 5). The DSC curve obtained during heating of the quenched sample to 800 °C and subsequent cooling (Fig. 4, sample 2) shows one phase transition at 715 °C during heating and 520 °C during cooling, following which the sample again becomes
Fig. 2. Stability regions of cubic, tetragonal, and rhombohedral Bi2O3-based compounds in the ternary system Bi2O3-La2O3-MoO3. The solid triangles represent the reported compositions of some compounds in the Bi2O3–La2O3 and Bi2O3–MoO3 binaries [1,2,14,15,25,26].
tetragonal and isostructural with β-Bi2O3. Thus, the anomaly at 628 °C in the DSC heating curve can be interpreted as due to the β → α (tetragonal to monoclinic) phase transition. The high-temperature phase transition, at 715 °C, seems to be the α → δ (monoclinic to cubic) phase transition, as supported by the behavior of the electrical conductivity characteristic of this phase transition (see below). The exothermic peak at 520 °C in the DSC cooling curve is due to the δ → β (high-temperature cubic to tetragonal) phase transition. Note that quenching from 800 °C (Fig. 5) did not stabilize the cubic phase: according to XRD data, the material remained tetragonal. The rhombohedral and β′ tetragonal materials are similar in polymorphism: they undergo a single phase transition in the range 1000– 1100 °C (Fig. 6). XRD data for the samples quenched from the stability
Fig. 3. Composition dependences of unit-cell parameters for cubic (Bi2O3)x(La2O3)y (MoO3)z samples.
Please cite this article as: E.I. Orlova, et al., Phase formation and electrical properties of Bi2O3-based compounds in the Bi2O3-La2O3-MoO3 system, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.019
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Table 1 Unit cell parameters of tetragonal and rhombohedral samples in Bi2O3-La2O3-MoO3 ternary system. Composition
Symmetry of room temperature phase
Unit cell parameters, Å
(Bi2O3)0.96(La2O3)0.02(MoO3)0.02
β, tetragonal
(Bi2O3)0.61(La2O3)0.15(MoO3)0.24
β′, tetragonal
(Bi2O3)0.61(La2O3)0.17(MoO3)0.22
β′, tetragonal
(Bi2O3)0.58(La2O3)0.19(MoO3)0.23
β′, tetragonal
(Bi2O3)0.55(La2O3)0.14(MoO3)0.31
β′, tetragonal
(Bi2O3)0.55(La2O3)0.18(MoO3)0.27
β′, tetragonal
(Bi2O3)0.50(La2O3)0.21(MoO3)0.29
β′, tetragonal
(Bi2O3)0.64(La2O3)0.18(MoO3)0.18
Rhombohedral⁎
(Bi2O3)0.63(La2O3)0.185(MoO3)0.185
Rhombohedral
(Bi2O3)0.62(La2O3)0.19(MoO3)0.19
Rhombohedral
(Bi2O3)0.61(La2O3)0.195(MoO3)0.195
Rhombohedral
(Bi2O3)0.60(La2O3)0.20(MoO3)0.20
Rhombohedral
a = 2 × 5.499(5), c = 5.678(4) a = 5.6888(7), c = 5.602(1) a = 5.692(2), c = 5.607(2) a = 5.704(1), c = 5.588(1) a = 5.741(1), c = 5.520(2) a = 5.7176(5), c = 5.5603(7) a = 5.742(3), c = 5.536(4) a = 31.94(3), c = 19.73(3) a = 31.92(3), c = 19.72(2) a = 31.96(3), c = 19.72(2) a = 31.92(3), c = 19.75(2) a = 31.95(4), c = 19.77(3)
⁎ Unit cell parameters of rhombohedral compounds are indicated in hexagonal setting.
range of the high-temperature phase indicate that it is isostructural with the cubic phase δ-Bi2O3. According to the DSC results, most of the samples with the cubic structure undergo no phase transitions during multiple heating–cooling cycles in the range 30–1000 °C (Fig. 7), but not all of the cubic materials are thermally stable. (Bi2O3)0.93(La2O3)0.055(MoO3)0.015 undergoes two phase transitions during heating: cubic to monoclinic (δ → α) phase transition at 610 °C and monoclinic to cubic (α → δ) phase transition near 700 °C. During cooling, no monoclinic phase is formed and the sample retains its cubic structure down to room temperature. (Bi2O3)0.90(La2O3)0.05(MoO3)0.05 was also found to have an instability range during heating in the range 750–800 °C, represented by a broad endothermic peak in its DSC curve. (Bi2O3)0.90(La2O3)0.07(MoO3)0.03, (Bi2O3)0.85(La2O3)0.075(MoO3)0.075, and the other samples containing less Bi are stable and undergo no phase transitions.
Fig. 4. DSC heating–cooling curves of two polycrystalline (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 samples. Sample 1, isostructural with β-Bi2O3 at room temperature, was slowly cooled from 820 °C. Sample 2, isostructural with α-Bi2O3 at room temperature, was quenched from 690 °C.
Fig. 5. X-ray powder diffraction patterns of ceramic (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 samples: (1) sample slowly cooled from 820 °C to room temperature, (2) sample quenched from 690 °C, and (3) sample quenched from 820 °C. The main reflections from the monoclinic (α) and tetragonal (β) phases are marked by + and *, respectively.
3.3. Electrical conductivity Fig. 8 presents Arrhenius plots of conductivity for the (Bi2O3)x(La2O3)y(MoO3)z samples with the cubic, β′ tetragonal, and rhombohedral structures. The data were obtained by dynamic measurements at 1 MHz. Fig. 9 shows the bulk conductivity evaluated from impedance spectra. In general, complex impedance plots consist of two semicircles representing bulk and electrode conductivities. It can be seen that the conductivity data obtained by the two techniques differ little. The conductivity of the samples gradually increases with Bi2O3 concentration, independent of the symmetry of their structure. The tetragonal material (Bi2O3)0.50(La2O3)0.21(MoO3)0.29 had the lowest conductivity (0.05 S/cm at 800 °C). The highest conductivity (0.6 S/cm at 800 °C) was offered by the cubic material (Bi2O3)0.8(La2O3)0.1(MoO3)0.1. Over the entire temperature range studied, the conductivity of the samples with 0.5 ≤ x ≤ 0.7 follows the Arrhenius law, Ea σ ¼ σ 0 exp kT with an activation energy Ea near 0.7–0.8 eV (Table 2). In the temperature range studied (30–850 °C), the conductivity curves of the above
Fig. 6. DSC heating curves of tetragonal and rhombohedral (Bi2O3)x(La2O3)y(MoO3)z: (1) (Bi2O3)0.50(La2O3)0.21(MoO3)0.29, (2) (Bi2O3)0.55(La2O3)0.18(MoO3)0.27, (3) (Bi2O3)0.58(La2O3)0.19(MoO3)0.23, (4) (Bi2O3)0.61(La2O3)0.17(MoO3)0.22, (5) (Bi2O3)0.61(La2O3)0.15(MoO3)0.24, (6) (Bi2O3)0.61(La2O3)0.195(MoO3)0.195.
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Fig. 7. DSC (a) heating and (b) cooling curves of cubic (Bi2O3)x(La2O3)y(MoO3)z: (1) (Bi2O3)0.93(La2O3)0.055(MoO3)0.015, (2) (Bi2O3)0.90(La2O3)0.05(MoO3)0.05, (3) (Bi2O3)0.90(La2O3)0.07(MoO3)0.03, (4) (Bi2O3)0.85(La2O3)0.075(MoO3)0.075, (5) (Bi2O3)0.80(La2O3)0.06(MoO3)0.14, (6) (Bi2O3)0.80(La2O3)0.1(MoO3)0.1, (7) (Bi2O3)0.75(La2O3)0.08(MoO3)0.17, (8) (Bi2O3)0.75(La2O3)0.10(MoO3)0.15, (9) (Bi2O3)0.70(La2O3)0.11(MoO3)0.19, (10) (Bi2O3)0.70(La2O3)0.15(MoO3)0.15, (11) (Bi2O3)0.65(La2O3)0.13(MoO3)0.22, (12) (Bi2O3)0.65 (La2O3)0.175(MoO3)0.175.
samples do not have any anomalies attributable to decomposition or structural phase transitions, in agreement with DSC data (Figs. 6, 7). The temperature dependence of conductivity for the cubic materials (Bi2O3)0.80(La2O3)0.10(MoO3)0.10 and (Bi2O3)0.80(La2O3)0.06(MoO3)0.14, which have the highest conductivity, consists of two distinct portions. At low temperatures (below 350–360 °C), their conductivity shows Arrhenius behavior with an activation energy near 0.8 eV. Above 350 °C, we observe a deviation from the Arrhenius law, and the conductivity is well represented by the Vogel–Fulcher–Tammann (VFT) law (Fig. 10), which is often used to describe the conductivity of polymers and glass-forming systems [29,30]: σ0 −B σ ¼ pffiffiffi exp kðT−T 0 Þ : T
listed in Table 2. We used the VFT equation to describe the present conductivity data by analogy with stabilized cubic molybdates based on the oxygen ion conductor La 2 Mo2 O 9, where such an approach was successfully used previously [31–34]. In the case of the above molybdates, the presence of two temperature ranges differing in conduction mechanism was accounted for by the existence of two cubic phases: high-temperature (β) and low-temperature (β ms), which were similar in symmetry but differed in oxygen order. The transition from Arrhenius behavior to VFT behavior was interpreted in terms of dynamic oxygen disorder in the high-temperature phase. A similar effect seems to occur in our case. It is worth noting that deviations from the Arrhenius law at high temperatures for stabilized Bi2O3-based phases were also observed previously [4,18,19], but the effect has not received proper attention.
The experimentally determined constants σ0, T0, and B in the VFT equation and activation energy Ea in the Arrhenius equation are
Fig. 8. Arrhenius plots of 1-MHz conductivity measured during heating for cubic, β′ tetragonal, and rhombohedral (Bi2O3)x(La2O3)y(MoO3)z.
Fig. 9. Bulk conductivity of (Bi2O3)0.80(La2O3)0.1(MoO3)0.1 (cubic) and (Bi2O3)0.61(La2O3)0.17(MoO3)0.22 (tetragonal, β′). Inset: impedance spectra of these samples at 250 and 300 °C.
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Table 2 The activation energy Ea (Arrhenius law) and parameters of Vogel-Fulcher-Tammann (VFT) equation calculated for conductivity of (Bi2O3)x(La2O3)y(MoO3)z samples. Composition
Conductivity behavior
Calculated parameters
(Bi2O3)0.80(La2O3)0.10(MoO3)0.10
T b 350 °C, Arrhenius T N 350 °C, VFT
Ea = 0.84 ± 0.01 eV σ0 = 120±9S/cmK1/2;
(Bi2O3)0.80(La2O3)0.06(MoO3)0.14
(Bi2O3)0.70(La2O3)0.13(MoO3)0.17 (Bi2O3)0.70(La2O3)0.15(MoO3)0.15 (Bi2O3)0.61(La2O3)0.195(MoO3)0.195 (Bi2O3)0.61(La2O3)0.15(MoO3)0.24 (Bi2O3)0.50(La2O3)0.21(MoO3)0.29
T b 360 °C, Arrhenius T N 360 °C, VFT
Arrhenius Arrhenius Arrhenius Arrhenius Arrhenius
B = 0.100 ± 0.006 eV; T0 = 460 ± 10 K Ea = 0.80 ± 0.01 eV σ0 = 215±9S/cmK1/2; B = 0.155 ± 0.004 eV; T0 = 394 ± 6 K Ea = 0.74 ± 0.01 eV Ea = 0.65 ± 0.01 eV Ea = 0.71 ± 0.01 eV Ea = 0.78 ± 0.01 eV Ea = 0.74 ± 0.01 eV
Fig. 11 shows Arrhenius plots of conductivity for the (Bi2O3)0.96 (La2O3)0.02(MoO3)0.02 ceramic that was slowly cooled from 820 °C and was isostructural with the tetragonal phase β-Bi2O3 at room temperature (Fig. 11, sample 1) and for the sample that was quenched from 690 °C and was isostructural with the monoclinic phase α-Bi2O3 at room temperature (Fig. 11, sample 2). The anomalies in the conductivity of both samples correspond to the thermal events observed in their DSC curves (Fig. 4). The conductivity curve of the sample isostructural with β-Bi2O3 at room temperature shows two phase transitions during heating, β → α (630 °C) and α → δ (710 °C), each accompanied by a sharp increase in conductivity by half an order of magnitude. During heating of the sample isostructural with the α-phase at room temperature, we observe one jump in conductivity, by two orders of magnitude, accompanying the α → δ (monoclinic to cubic) phase transition, typical of the polymorphism and conductivity of undoped bismuth oxide [5]. The conductivity curves of the two (Bi2O3)0.96 (La2O3)0.02(MoO3)0.02 samples during cooling show a single (δ → β) phase transition, at 510 °C, accompanied by a sharp drop in conductivity by two orders of magnitude. Note the large thermal hysteresis of the phase transitions. A similar polymorphism, with two phase transitions during heating and one during cooling was reported previously [22]
Fig. 11. Arrhenius plots of 1-MHz conductivity measured during heating and cooling for two ceramic (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 samples. Sample 1, isostructural with βBi2O3 at room temperature, was slowly cooled from 820 °C. Sample 2, isostructural with α-Bi2O3 at room temperature, was quenched from 690 °C.
for Bi14W1–xLaxO24–3x/2 obtained by codoping Bi2O3 with small amounts of lanthanum and tungsten. The conductivity of the high-temperature phase (δ-Bi2O3) of (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 slightly exceeds that of the stabilized cubic materials obtained in this study and approaches 1 S/cm at 800 °C. In the temperature stability ranges of the α- and β-phases, the conductivity of (Bi2O3)0.96(La2O3)0.02(MoO3)0.02 approaches that of the tetragonal materials containing 50–60 mol% Bi2O3. 4. Conclusions Polycrystalline (Bi2O3)x(La2O3)y(MoO3)z (x + y + z = 1) samples have been prepared by solid-state reactions in air and the formation of Bi2O3-based compounds in the ternary system Bi2O3–La2O3–MoO3 has been studied. XRD data demonstrate that, in the range 0.65 ≤ x ≤ 0.93 at room temperature, the system contains compounds isostructural with the high-temperature, cubic phase δ-Bi2O3. According to DSC data, the cubic solid solution (Bi2O3)0.90(La2O3)0.07(MoO3)0.03 and the samples with 0.65 ≤ x ≤ 0.85 undergo no phase transitions in multiple heating–cooling cycles. The compounds richer in bismuth and molybdenum are unstable when heated. In addition to the cubic phase, the ternary system Bi2O3-La2O3-MoO3 contains tetragonal (β and β′) and rhombohedral phases. Over the entire temperature range studied, the β′ tetragonal, rhombohedral, and cubic materials have high conductivity, which reaches 0.05–0.6 S/cm (at 800 °C) and increases with bismuth content. The conductivity of most of the samples with x ≤ 0.7 exhibits Arrhenius behavior throughout the temperature range studied, with an activation energy from 0.7 to 0.8 eV. The (Bi 2 O 3 )0.80 (La 2 O3 ) 0.06 (MoO3 ) 0.14 and (Bi 2O 3 )0.80 (La 2 O 3) 0.10 (MoO 3 )0.10 samples, which have the highest conductivity, follow the Arrhenius law only at low temperatures (30–350 °C). Their high-temperature conductivity follows the Vogel–Fulcher–Tammann law, which seems to be evidence of considerable oxygen ion conductivity. Acknowledgments
Fig. 10. Arrhenius plots of conductivity for polycrystalline (Bi2O3)0.80(La2O3)0.10(MoO3)0.10. The dotted and solid lines represent fits to the Arrhenius and VFT laws.
This work was supported by the Russian Foundation for Basic Research, project no. 15-02-03492 a.
Please cite this article as: E.I. Orlova, et al., Phase formation and electrical properties of Bi2O3-based compounds in the Bi2O3-La2O3-MoO3 system, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.019
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Please cite this article as: E.I. Orlova, et al., Phase formation and electrical properties of Bi2O3-based compounds in the Bi2O3-La2O3-MoO3 system, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.019