CRYSTAL GROWTH & DESIGN
Electrochemical Single-Crystal Growth of Nonstoichiometric Terbium Oxide Toshiyuki Masui, Shinya Isota, Shinji Tamura, and Nobuhito Imanaka* Department of Applied Chemistry, Faculty of Engineering, Osaka UniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
2008 VOL. 8, NO. 3 1035–1038
ReceiVed February 21, 2007; ReVised Manuscript ReceiVed September 7, 2007
ABSTRACT: Single crystals of some nonstoichiometric terbium oxide phases such as Tb16O30 (TbO1.875; π-phase), Tb24O44 (TbO1.833; β2-phase), and Tb11O20 (TbO1.818; δ-phase) were artificially grown by dc electrolysis of a Tb3+ ion conducting Tb2(MoO4)3 solid electrolyte. Selective growth of a specific phase has been realized by controlling the oxygen partial pressure, the electrolysis temperature, and the voltage applied during the electrolysis. The formation of high-quality single crystals has been evidenced by the selectedarea electron diffraction analysis. These nonstoichiometric phases of fluorite-related rare earth oxides have been difficult to grow in a single crystal form because they are usually obtained as mixtures of some RnO2n-2m (n ) 7, 9, 11, 12, 16, 19, 24, 29, 39, 40, 48, 62, and 88; m ) 1, 2, 3, 4, 6, and 8) phases. However, our dc electrolysis method can be simply applied to grow a specific nonstoichiometric phase in a single crystal form selectively at moderate temperatures at 800 or 900 °C, which had been unrealistic by the conventional growth processes via solidification of the melt at high temperatures above 2300 °C.
1. Introduction Rare earth ions usually have a trivalent state and most of the rare earth oxides are sesquioxides (R2O3; R ) rare earth metals).1 However, the occurrence of a homologous series of nonstoichiometric phases have been reported in the fluoriterelated rare earth oxides with the general formula RnO2n-2m (n ) 7, 9, 11, 12, 16, 19, 24, 29, 39, 40, 48, 62, and 88; m ) 1, 2, 3, 4, 6, and 8) for R ) Ce, Tb, and Pr.2–5 These nonstoichiometric phases have well-defined chemical compositions and ordered structures. Among these nonstoichiometric oxides, crystal structures of Ce7O12,6 Pr7O12,7 Pr9O16,8 Pr40O72,9 Pr24O44,10 Tb7O12, and Tb11O2011 have been identified by neutron powder diffraction, and other phases have been characterized by high-resolution transmission electron microscopy only for the polycrystalline samples. Although single crystals of rare earth sesquioxides can be easily grown by a conventional melt process, for example, the Verneuil method,12 it is a hard task to obtain single crystals of the nonstoichiometric oxides. In particular, there are few references on the crystal growth of the intermediate nonstoichiometric terbium oxide, and it still has been considerably difficult. For example, anodic electrocrystallization from alkaline hydroxide melts containing TbCl3 was effective to obtain oxygen-deficient terbium oxide, but the composition could not be identified completely.13 Hydrothermal growth was effective to grow Tb11O20 crystals, but smaller TbO2 crystals were deposited on their surface as byproduct.14 Therefore, it is significant to find a novel method to grow pure single crystals of such nonstoichiometric phases simply. We elucidate in this paper, on the contrary, that it is possible to grow intermediate oxide in a single crystal form by our original electrochemical method. This method utilizes a unique characteristic of solid electrolytes that only one ion species can migrate in the solid. This feature advantage is applicable for the single-crystal growth of nonstoichiometric oxides as well as high melting oxides by the dc electrolysis of trivalent cation conducting solid electrolytes, M2(MoO4)3 (M ) trivalent * Corresponding author. Tel: +81-6-6879-7352. Fax: +81-6-6879-7354. E-mail address:
[email protected].
Figure 1. Schematic illustration of the TbOx single-crystal growth by the dc electrolysis method.
cation),15,16 at low temperatures between 750 and 1000 °C. Actually, we have succeeded in the single-crystal growth of nonstoichiometric Tb16O3017 and Tb24O44,18 intermediate tetragonal δ-Al2O3,19–21 and some refractory oxides of M2O3 (M ) Sc, Y, and In).22–25 However, the electrolysis conditions have not been fully optimized for simplifying the growth process. In this study, therefore, we strictly optimized the growing conditions to achieve controllable growth of the different intermediate terbium oxide phases in a single-crystal form by a simple adjustment of the electrolysis conditions, for example, oxygen partial pressure, temperature, and applied voltage.
2. Experimental Section Terbium molybdate, Tb2(MoO4)3, supplied for the dc electrolysis, was prepared by a solid-state reaction method. A stoichiometric amount of Tb4O7 (purity 99.9%) and MoO3 (purity 99.9%) was ground and mixed in a mortar. The mixture was calcined at 750 °C for 12 h in air. The resulting powder was made into pellets (10 mm in diameter and 0.8 mm in thickness) and heated at 1000 °C for 12 h in air. The resulting pellets were crushed and ground in a mortar again, and then the powder was re-pressed and sintered again at the same conditions to obtain the sample pellets for the electrolysis. The electrolysis of the Tb2(MoO4)3 solid was carried out by sandwiching the sample pellet between two ion-blocking Pt electrodes, as illustrated in Figure 1. In order to advance the electrolysis, the dc voltage applied should be higher than the decomposition voltage of Tb2(MoO4)3, which was determined by measuring the I-V relationship of the Tb2(MoO4)3 solid electrolyte shown in Figure 2. Since the
10.1021/cg070178y CCC: $40.75 2008 American Chemical Society Published on Web 02/20/2008
1036 Crystal Growth & Design, Vol. 8, No. 3, 2008
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Figure 2. The I-V relationship of the Tb2(MoO4)3 solid electrolyte. decomposition voltage was determined to be 1.8 V, the dc electrolysis was carried out at 3 and 11 V in a flow of N2-O2 mixed gas at 800 and 900 °C for a week under several different oxygen partial pressures. The Tb2(MoO4)3 decomposition is driven by applying dc voltage higher than the decomposition voltage of the Tb2(MoO4)3 solid, and Tb3+, MoO3, and gaseous O2 are produced according to the following half-cell reaction. 2Tb2(MoO4)3 f 4Tb3+ + 6MoO3 v + 3O2 v + 12e-
(1)
MoO3 produced at the anode readily sublimates during the electrolysis because it was conducted at 800 or 900 °C, which is above the MoO3 sublimation temperature (ca. 750 °C). Consequently, Tb3+ ions are successively produced from solid electrolyte at the anodic side and conducted through the Tb2(MoO4)3 bulk by ionic conduction from the anode to the cathode direction during the electrolysis. The Tb3+ ions are reduced to the metal state at the interface between the sample and the Pt cathode as soon as they arrive at the cathode. Tb3+ + 3e- f Tb (2) Since the dc electrolysis is carried out at elevated temperatures in the presence of oxygen, the Tb metal produced on the cathodic surface is immediately oxidized. 2Tb + xO2 f 2TbOx
(3)
Figure 3. X-ray powder diffraction patterns for the cathodic and the anodic surface of the Tb2(MoO4)3 solid electrolyzed at 3 V for a week under oxygen partial pressure of 0.10 MPa (a), 2.1 × 10-2 MPa (b), and 1 × 10-5 MPa (c).
3+
The Tb ions are steadily and gradually supplied from inside the Tb2(MoO4)3 electrolyte by the electrolysis; growth of Tb metal and the subsequent oxidation successively occur on the cathodic surface. As a result, terbium oxide single crystals of similar size are grown on the cathodic surface of the Tb2(MoO4)3 solid electrolyte. Since the oxygen content in the terbium oxide depends on the oxygen partial pressure, controllable growth of some different nonstoichiometric terbium oxides would be possible by adjusting the oxygen partial pressure. The samples after the dc electrolysis were characterized by X-ray powder diffraction using Cu KR radiation (XRD, Rigaku Multiflex). The XRD data were collected by a step scanning method in the 2θ range from 10° to 70° with a step width of 0.04°. The particles grown were characterized with a scanning electron microscope (SEM, Shimadzu SSX-550). The sample was sputter-coated with a platinum layer before SEM observation to avoid any possible surface charging effects.
The particle size distribution and the average particle size were determined by measuring the mean length of the shortest and the longest diameters in a particle for more than 150 particles on the SEM photograph. Selected area electron diffraction (SAED) measurement was performed with a transmission electron microscope (TEM, Hitachi H-800) equipped with a tilting device and operating at 200 kV. The camera length was 2.83 × 10-12 m (28.3 mÅ). The particles grown were rinsed out from the electrolyzed pellet with ethanol and then supported on an amorphous carbon film mounted on a copper TEM grid.
Table 1. Textual and Crystallographic Parameters of the TbOx Single Crystals temperature (°C)
voltage (V)
oxygen partial pressure (MPa)
phase and unit cell content
space group
a (Å)
b (Å)
c (Å)
900 900 900 900 900 900 800 800 800 800 800 800
11 11 11 3 3 3 11 11 11 3 3 3
0.10a 2.1 × 10-2b 1.0 × 10-5c 0.10a 2.1 × 10-2b 1.0 × 10-5c 0.10a 2.1 × 10-2b 1.0 × 10-5c 0.10a 2.1 × 10-2b 1.0 × 10-5c
π-Tb16O30 π-Tb16O30 β3-Tb48O88 π-Tb16O30 β2-Tb24O44 δ1-Tb11O20 π-Tb16O30 β3-Tb48O88 β3-Tb48O88 β3-Tb48O88 β2-Tb24O44 δ1-Tb11O20
monoclinic monoclinic Pn (monoclinic) monoclinic P1¯ (triclinic) P1¯ (triclinic) monoclinic Pn (monoclinic) Pn (monoclinic) Pn (monoclinic) P1¯ (triclinic) P1¯ (triclinic)
6.4 6.4 6.7 6.4 6.4 6.5 6.4 6.7 6.7 6.7 6.4 6.5
14.8 14.8 23.2 14.8 12.2 9.83 14.8 23.2 23.2 23.2 12.2 9.83
7.4 7.4 15.5 7.4 12.2 6.5 7.4 15.5 15.5 15.5 12.2 6.5
a
In pure O2. b In air. c In standard N2.
lattice parameters R (deg)
79.5 99.0
79.5 99.0
β (deg) 125.2 125.2 125.2 125.2 100.0 99.9 125.2 125.2 125.2 125.2 100.0 100.0
γ (deg)
69.6 95.9
69.6 95.9
average particle size (µm)
standard deviation (µm)
0.49 0.46 0.47 0.64 0.68 0.67 0.48 0.47 0.48 0.68 0.66 0.68
0.16 0.13 0.12 0.16 0.17 0.13 0.13 0.13 0.17 0.18 0.15 0.21
Single Crystals of Nonstoichiometric Terbium Oxide
Crystal Growth & Design, Vol. 8, No. 3, 2008 1037
Figure 4. SEM photographs and electron diffraction patterns of the cathodic surface of the Tb2(MoO4)3 pellets after electrolysis at 900 °C and 3 V for a week under oxygen partial pressure of 0.10 MPa (a), 2.1 × 10-2 MPa (b), and 1 × 10-5 MPa (c).
Figure 5. SEM photographs and electron diffraction patterns of the nonstoichiometric terbium oxide single crystals grown by the electrolysis for 1 week at 11 V and at 900 °C (a) and 800 °C (b) under the oxygen partial pressure of 2.1 × 10-2 MPa (air).
3. Results and Discussion Figure 3shows the representative X-ray powder diffraction patterns for the cathodic and the anodic surface of Tb2(MoO4)3 after the dc electrolysis at 900 °C and 3 V for a week. The oxygen partial pressure was adjusted to 0.10 MPa (in pure O2) (a), 2.1 × 10-2 MPa (in air) (b), and 1.0 × 10-5 MPa (in N2/ O2 ) 99.99:0.01, standard grade N2) (c). In all samples, additional diffraction peaks corresponding to the fluorite-related TbOx phases appeared on the cathodic surface after the electrolysis and no impurities other than TbOx and Tb2(MoO4)3 were detected, while only the peaks of Tb2(MoO4)3 were observed on the anodic surface even after the electrolysis.
Figure 4 shows the SEM photographs at the cathodic surface of the Tb2(MoO4)3 pellets electrolyzed at 3 V and 900 °C for a week under oxygen partial pressure of 0.10 MPa (a), 2.1 × 10-2 MPa (b), and 1 × 10-5 MPa (c). Well-defined polyhedral particles were observed on the cathodic surface, while no deposits were recognized on the anodic surface. In addition, the particle formation was not observed on another pellet of Tb2(MoO4)3 polycrystals subjected to the same conditions without the electrolysis. The crystal growth was carried out in various combinations of temperature, applied voltage, and oxygen partial pressure. The average particle size and standard deviation of all crystals are summarized in Table 1. The particles
1038 Crystal Growth & Design, Vol. 8, No. 3, 2008
grown at 11 V are smaller than those obtained at 3 V, but the size was independent of the oxygen partial pressures at the same dc voltage. In addition, it has been demonstrated that the crystallites monotonically grow with the electrolysis period.18,23,24 In order to identify whether each particle is exactly a singlecrystal form, the selected-area electron diffraction (SAED) patterns of the crystals were measured for all samples. The results obtained for the particles grown at 900 °C and 3 V at different oxygen partial pressures are also shown in Figure 4. The sharp diffraction spots indicate the superior crystallinity, and the energy-dispersive X-ray analyzer displayed no impurities in the particles. The net patterns for the samples consistently correspond to those of zone axis patterns of the π-Tb16O30, the β2-Tb24O44, and the δ-Tb11O20 phases,11,26,27 which are the intermediate nonstoichiometric oxides in the RnO2n-2m series of the fluorite-related phases. The diffraction spots obtained above explicitly elucidate that each particle of these nonstoichiometric oxides exists in a single-crystal form. These results clarify that the artificial single-crystal growth of such intermediate oxides can be realized for the first time by our simple dc electrolysis method, and the objective phase is intentionally prepared by controlling the oxygen partial pressure during the electrolysis at the same temperature and voltage. The phase and lattice parameters of all terbium oxide particles obtained by the present crystal growth method in a variety of conditions are collected in Table 1. The growth conditions and the nonstoichiometric phase formed are correlated, and there is regularity between them that higher oxidation state is formed at higher voltage and higher temperature. For example, the SEM photographs of the crystals grown in air at different temperatures and dc voltages are depicted in Figure 5. The SAED patterns in Figure 5a,b are assigned to π-Tb16O30 and β3-Tb24O44, respectively.27 By comparing the data in Figure 5a (π-Tb16O30) with that in Figure 4b (β2-Tb24O44), we find that higher oxidation state is obtained at higher dc voltage at the same temperature. On the other hand, higher oxidation state is formed at higher temperature at the same voltage, as ascertained by comparing the data in Figure 5a (π) with that in Figure 5b (β3). The reason for the formation of higher oxidation state has already been discussed in our previous papers17,18 from the viewpoint of the migration and supplying rate of the Tb3+ cations to the cathodic surface. It was attributed to the preferential nucleation of small particles at higher voltage and at higher temperature, which can be oxidized more easily to afford high oxidation state.18 Furthermore, it can be seen in Figure 4 and Table 1 that the oxidation state of nonstoichiometric terbium oxide strongly depends on the oxygen partial pressure. As a result, selective single-crystal growth of nonstoichiometric terbium oxides becomes possible by controlling temperature, voltage, and oxygen partial pressure in our dc electrolysis method. Under several conditions examined in this study, the simple adjustment of the oxygen partial pressure at lower voltage is relatively convenient for the selective singlecrystal growth of the nonstoichiometric terbium oxides.
Masui et al.
electrolysis of the Tb3+ ion conducting solid electrolyte, Tb2(MoO4)3. It can be concluded that it is possible to grow a specific nonstoichiometric phase in a single-crystalline form selectively by properly optimizing temperature, applied voltage, and oxygen partial pressure during the electrolysis. Acknowledgment. We would like to thank Dr. Takao Sakata and Prof. Dr. Hirotaro Mori (Osaka University) for their assistance with the SAED measurement. The present work was supported by a Grant-in-Aid for Scientific Research (No. 18655087) from the Japanese Ministry of Education, Science, Sports and Culture. This work was also partially supported by the Industrial Technology Research Grant Program in ’02 (project ID: 02A27004c) from the New Energy and Industrial Technology Development Organization (NEDO) based on funds provided by the Japanese Ministry of Economy, Trade and Industry (METI).
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4. Conclusions Selective single crystal growth of the nonstoichiometric terbium oxide phases such as π-Tb16O30, β-Tb24O44, and δ-Tb11O20 was successfully realized for the first time by the dc
(26) (27)
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CG070178Y
