Electrocrystallization and Characterization of Polymorphic Forms of Barium Metaplumbate Ngee S. Chong,# Nian T. Suen,† Ta L. Chou,‡ and Horng Y. Tang*,† Department of Chemistry, Middle Tennessee State UniVersity, Murfreesboro, Tennessee 37132, Department of Applied Chemistry, National Chi Nan UniVersity, Puli, Taiwan, and National Synchrotron Radiation Research Center, Hsinchu, Taiwan
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 5 1779–1782
ReceiVed January 4, 2008; ReVised Manuscript ReceiVed February 3, 2008
ABSTRACT: The conducting oxide of BaPbO3 crystallized in KOH melt at various pH2O and voltage Ev values are systematically studied by the electrochemical method. For crystals prepared in a low pH2O environment, the electrolysis product displays the Pb3O4 phase, whereas the single phase of BaPbO3 can be produced under high pH2O condition. The grown phases are found to depend on the concentration of superoxide [O2-] and peroxide [O22-] intermediates in the melt. By adjusting the concentration of [O2-]/[O22-] intermediates via careful control of the pH2O and atmosphere of the electrochemical cell, the formation temperature of BaPbO3 can be reduced to as low as 200 °C, which is significantly lower than the temperature used in other synthetic techniques. The BaPbO3 crystals with cubic, tetragonal, and orthorhombic structures are grown at 450, 340, and 260 °C, respectively. The symmetry of crystalline BaPbO3 structure prepared under these temperatures can be sustained at ambient conditions to allow a detailed study of the distorted perovskite structure. Introduction The applications of materials in the BaPbO3 perovskite family are of interest to researchers by virtue of their superconductivity and anticorrosion properties.1–4 Potential applications of these materials as catalysts, infrared detectors, and anticorrosion electrodes of the lead/acid batteries have been studied.5,6 When BaPbO3 materials are doped with Sb or Bi, they become superconductors as in BaPb0.75Sb0.25O3 with a Tc of 3.5 K and BaPb0.75Bi0.25O3 with a Tc of 12 K.1,7 The perovskite oxide with a generalized structural formula of ABO3 consists of a framework of the corner-sharing BO6 units of octahedra with the A cation being surrounded at eight corners. In the distorted condition, the BO6 octahedra tilt to reduce packing strength from the nonideal size of the A cation. For the BaPbO3 compound, the PbO6 octahedra are slightly distorted at room temperature due to the large radius of the Ba2+ cation.8,9 The influence of the asymmetrical structural feature on the electronic band and the superconductivity has been studied for a long time.10–12 Until now, experimental studies of the degree of displacement from the centrosymmetric point at room temperature are still being pursued.13,14 The major variation of structural and physical analysis may result from a high temperature sample preparation process. Most of the measurements were performed with powdery samples prepared by solid-state sintering. The BaPbO3 single crystals were grown by a high temperature flux method at high temperature.15 Phase transition of powdery samples occurs during the cooling process with structural transformation from cubic to tetragonal and then to the space group with lower symmetry.2 Sample inhomogeneity and microtwinning defects resulting from the high temperature preparation may complicate the interpretation of high-resolution diffraction analysis.13,14 In this work, we have developed an electrochemical method to overcome the sample preparation problems mentioned above * To whom correspondence should be addressed. Present address: No. 1 University Rd., Department of Applied Chemistry, National Chi-Nan University, Puli, Nantou, Taiwan. Telephone: +886-49-2910-960. Fax: +886-49-2917-956. E-mail:
[email protected]. # Middle Tennessee State University. † National Chi Nan University. ‡ National Synchrotron Radiation Research Center.
and have prepared stable BaPbO3 crystals with cubic, tetragonal, and lower symmetry structures. In contrast to the solid-state reaction, the low temperature electrochemical deposition was performed with a completely different synthetic strategy. The crystallization of the deposited oxide was achieved by an electrochemical system via a potentiostatic or galvanic mode. The electro-crystallization process was successfully applied in the synthesis of oxide compounds with mixed valence state.16–19 However, mechanistic study of the electrocrystallization process is rare due to the complex inter-relationships of controlled potential, temperature, electrolyte concentration, and water concentration (pH2O) in the mixed valence system. The synthesis of BaPbO3 and the Pourbaix diagram of lead in KOH melt were relatively simple based on the experimental results. The cyclic voltammetry (CV) experiments performed under different temperatures and pH2O conditions and the relationships of these factors to the growth mechanism were investigated. The concentration ratio of [O2-]/ [O22-] intermediates in KOH melt was found to be the determining factor for BaPbO3 phase formation. By regulating the atmosphere of the cell used for electrodeposition and adjusting the pH2O value to control the concentration of [O2-]/ [O22-] intermediates, BaPbO3 crystals can be successfully produced in the temperature range from 450 to 200 °C. The crystals obtained at elevated temperatures above and below the phase transition temperature were found to preserve their structures at ambient room temperature. The BaPbO3 grown at 450 °C displayed a cubic symmetry with space group Pm3jm. Crystals grown at 340 °C had a tetragonal structure with I4/ mcm space group. For experiments performed at 260 °C or below, an orthorhombic structure with reduced symmetry Ibmm was identified for the crystals collected. Experimental Procedures The details of the electrochemical cell design used to control atmosphere were previously described in the literature.16 A threeelectrode EG&G 273A potentiostatic system was used for the CV studies. The pseudoreference electrode was a 1 mm platinum rod, the working electrode was 1 mm platinum wire, and the counter electrode was a coiled platinum wire. In a typical deposition, 100 g of KOH
10.1021/cg800009b CCC: $40.75 2008 American Chemical Society Published on Web 04/22/2008
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Figure 1. Pourbaix diagram of alkali hydroxide melt.21 (Merck) containing about 2 wt% of H2O was placed in a 80 cm3 glassy carbon crucible and mixed with 5.62 g of Ba(OH)2 · 8H2O and 4.23 g of PbO. The mixture was melted and the electrochemical deposition was conducted at a temperature ranging from 200 to 450 °C. The lowered melting point of the melt was due to the high water content of commercial KOH. Crystal growth experiments were performed in a constant current mode with current density J ) 10 mA/cm2. Morphological examination of the crystals was carried out using a JEOL model 5200 scanning electron microscope (SEM). Nondestructive microelemental analysis was performed utilizing the Link energy dispersive X-ray (EDX) microanalysis system with a semiquantitative analysis program. X-ray diffraction patterns of these powders were taken utilizing a Shimadzu XRD-7000 diffractometer. The space groups of crystals were identified by the GSAS program.
Results and Discussion The electrochemical preparation of the BaPbO3 crystals can be represented as the following equation: KOH(l)solvent
Ba(OH)2 · 8H2O + 2Pb(II)O 98 BaPb(IV)O3 + electrolysis
Pb + 9H2O
(1)
The electro-crystallization of barium metaplumbate can be viewed as a lead oxide electro-polymerization process resulting in three-dimensional cross-linking with the incorporation of Ba2+ cations. That is, the electron transfer and cross-linking of lead oxide complex were initiated under the influence of an electric field with the inclusion of Ba2+ ions from the melt as the crystal growth progresses. The previous results of molten salt electrolysis show that the product properties are dependent on the pH2O of KOH melt.16–19 The pH2O of fused hydroxides has rarely been studied by quantitative method because of the experimental difficulties in controlling or measuring the pH2O value accurately. Instead, the qualitative description of the acid–base behavior of fused hydroxides is commonly given by the equilibrium constant KOH- ) [H2O][O2-] for the reaction of 2OH- f H2O + O2-. By using the analogy for the autoprotolysis of water, the pKOH- )11.5 ( 0.7 had been determined by Tremillon and used to describe the acid–base behavior of the melt.20 On the basis of the definition of pKOH-, the Pourbaix diagram of fused hydroxide in Figure 1 can be constructed and designated as having an acidic melt if the pH2O value is less than 5.75.21 In this work, the KOH melt released water slowly at 260 °C and maintained its acidic condition for about 2-3 days. To reach the basic condition, most of the dissolved water in KOH melt can be readily removed by heating up the melt to 450 °C. By selecting the proper temperature of the KOH melt, electrochemical studies and crystal growth can be readily performed in the acidic (260 °C) or basic (450 °C) environment without difficulty.
Figure 2. The cyclic voltammograms of PbO dissolved in (a) acidic (b) basic KOH melt. The flat line without symbol indicates the background current of KOH melt taken in acidic condition. Two different scans of an extended voltage range (2) and a narrower voltage range (b) are shown. Insets: the electrochemical stripping peaks in acidic and basic environment.
Electrochemical Deposition of Pb3O4 and BaPbO3 in Acidic Melt. The fundamental Ev-pH2O phase diagram of the lead in NaOH melt has been reported by Eluard et al. and PbO was noted as the most stable oxide phase.22 The CV experiments of the KOH background and the Pb2+ in acidic KOH melt are shown in Figure 2a. Under the protection of N2 atmosphere, two electrochemical oxidation signals were observed at about E ) 320 mV and E ) 760 mV vs Pt pseudoreference electrode. The cyclic voltammogram scanned from 0 mV to 500 mV shows a nonreversible electrochemical behavior. Crystal growth performed by potentiostatic mode at 320 mV for 24 h resulted in the formation of orange color polycrystalline Pb3O4 at the Pt electrode. Despite the lack of the accuracy in the pH2O control, the results reveal that the Pb3O4 grown in acidic KOH melt had a higher average valence state than the PbO in acidic NaOH melt.22 It is presumed that the KOH melt had a more oxidative environment than the NaOH melt. The inset of Figure 2a reveals an enlarged electrochemical stripping peak associated with the second oxidation reaction. The stripping signal indicates the possibility of instant deposition right after the second oxidation reaction. A mixture of Pb3O4 and BaPbO3 deposits was obtained when the applied voltage of the electrochemical cell was maintained at 760 mV for 24 h. On the basis of Pourbaix diagram in Figure 1, the deposition in second oxidation step was likely to be related to the following two reactions: the dissociation reaction of the melt 2OH- f H2O + O2- and/or electrochemical oxidation reaction of 4OHf O2- + 2H2O + 3e-.20 In order to achieve the stable growth of BaPbO3, tetravalent lead ions were generated at the electrode/
Polymorphic Forms of Barium Metaplumbate
electrolyte diffusion double layer and reacted with O2- or O2that were concomitantly produced when a high voltage is applied. In the case of acidic conditions, the high water content of KOH melt may shift the equilibria of both reactions backward to yield OH- thereby inhibiting the stable growth of single phase BaPbO3 and producing a mixture of Pb3O4 and BaPbO3 instead. Electrochemical Crystallization of BaPbO3 in Basic Melt. The region of basic melt in the Pourbaix diagram can have relatively low oxidation potentials for the half-reactions of 2O2- f O2- + 3e- and 2O2- f O22- + 2e-.20,22 Furthermore, the dissociation reaction of the melt 2OH- f H2O + O2- is less prone to the reverse formation of OH- due to the presence of residual moisture compared to the acidic melt. The concentration of superoxide [O2-] and/or peroxide [O22-] at electrode surface, hereby designated as the [O2-]/[O22-] intermediates, could be increased while the electric field was applied. The cyclic voltammograms in Figure 2b show that the oxidation potential for the formation of BaPbO3 in basic melt indeed is lower than that in acidic condition. The irreversible electrochemical behavior implies the high reactivity of the generated O2- and O22- intermediates at electrode surface. The high concentration of [O2-]/[O22-] intermediates could act as oxidants that served to stabilize the Pb4+ at electrode/electrolyte interface. The electrochemical stripping peak can be clearly observed in the inset of Figure 2b at 400 mV. Crystal growth experiments performed by either potentiostatic control at 400 mV or galvanic control with current density J ) 10 mA/cm2 yielded a single phase of BaPbO3 crystals that was electrodeposited on the Pt working electrode. SEM/EDX analysis of the Ba/Pb mole ratio based on semiquantitative EDX analysis is close to 1. No detectable amount of potassium was observed in EDX measurements. As the positive potential or the current density was increased, a dendritic or powdery form of BaPbO3 deposit was observed due to the fast growth rate. The X-ray diffraction analysis of the grown BaPbO3 crystal in Figure 3a indicates a cubic structure with a space group of Pm3jm. The crystalline material has ideal perovskite structure with a ) 4.27(2) Å. This ideal BaPbO3 structure provides strong evidence that the tetravalent Pb4+ phase could be generated in basic melt as an electrodeposited layer at the electrode surface. The concentration of [O2-]/[O22-] intermediates generated from the electrochemical oxidation of 2O2- f O2- + 3e- and 2O2- f O22- + 2e- was related to the crystallization of BaPbO3 with the tetravalent state of lead. Electrochemical Crystallization of BaPbO3 in Oxygen Atmosphere. It is interesting to develop the crystal growth of BaPbO3 at low temperature that may have a different structure in contrast to compound grown at high temperature. Furthermore, low temperature deposition of the BaPbO3 layer on a wide variety of metal substrate can be easily accomplished with the electrochemical approach for anticorrosion application. The low temperature deposition attainable via the electrochemical approach is unique in the preparation of BaPbO3 compared to the significantly higher temperature of 600 to 950 °C encountered in RF-magnetron sputtering,23 solution-based spin coating,24 and solid-state reaction.2,25 The oxygen species present in the KOH acidic melt has been reported to be superoxide and peroxide ions that can be formed via the following reactions: 3O2(g) + 4OH- f 4O2- + 2H2O and O2- + e- f O22-.20,22 Since these reactions depend on the partial pressure of oxygen, the process of carrying out electrodeposition under an oxygen-rich atmosphere can be used to increase the concentration of [O2-]/[O22-] intermediates in acidic melt such that single phase BaPbO3 crystals had been
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Figure 3. Powder X-ray diffraction patterns of (a) cubic BaPbO3 crystals grown in 450 °C basic melt, (b) cubic, tetragonal, and orthorhombic crystals grown at different temperatures, (c) the (220) peaks on an expanded scale. The asterisks (*) indicate the diffraction peaks of the platinum electrode.
successfully grown in acidic melt at 260 °C. The scanning electron micrograph of the BaPbO3 grown at 260 °C in oxygen atmosphere shows a [111] growth direction in Figure 4. The size of harvested products is in the range from 50 µm to 1 mm depending upon the deposition time. Although performing crystal growth at a low temperature is attractive, temperatures below 200 °C tend to result in powdery deposition, presumably due to low ion mobility in diffusion double layer that results in unstable growth. The use of high temperature leads to fast mass transport rate and produces crystals with a less stepped surface. Polymorphic Structures of the Electrocrystallized BaPbO3. The transition temperature of powdery BaPbO3 was observed by in situ synchrotron X-ray and neutron diffraction techniques. The primitive cubic structure undergoes a transition to the tetragonal form at about 400 °C and becomes tilted PbO6 octahedra if the temperature is below 300 °C.2 In the basic melt, the crystal growth was performed at 450 °C, which is above the transition temperature. The grown crystals were removed from the 450 °C melt to preserve the cubic structure. As shown by the X-ray diffraction pattern in Figure 3a, the space group Pm3jm can be preserved at room temperature. The X-ray diffraction data also indicate the possibility of growing poly-
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High concentrations of [O2-]/[O22-] oxidants promote the crystal growth of BaPbO3. The electrodeposition method facilitates crystal growth in a wide range of temperatures that can produce BaPbO3 crystals with different degrees of symmetry. Physical measurements associated with the structural displacement from the centrosymmetric point of BaPbO3 crystals can lead to a better understanding of the influence of asymmetrical structure on the electronic band and the superconductivity. Acknowledgment. This work is supported by the ROC National Science Council Grant NSC-96-2112-M-260-001MY3.
Figure 4. Scanning electron micrograph of BaPbO3 crystals grown from KOH melt.
morphic forms of BaPbO3 crystal. The crystals grown at 340 °C in an oxygen environment were found to have the I4/mcm space group as shown in Figure 3b,c and Supporting Information. Low temperature growth of BaPbO3 crystal at 260 °C or below leads to the Ibmm orthorhombic structure. The previous structural analysis via high resolution X-ray and neutron diffraction methods have yielded controversial results as to whether orthorhombic or monoclinic structure should be reported for the room temperature BaPbO3 compound.13,14 Powdery samples prepared at high temperature may be adversely affected by the high volatility of lead oxide or microtwinning defects during the cooling process. The low temperature electrodeposition process cannot be affected by vaporization problems and minimum microtwinning defects. In Figure 3c, the space group of crystals grown at 260 °C has reduced symmetry and is more likely to be orthorhombic as shown in Supporting Information. The use of high-resolution neutron and X-ray diffraction techniques can resolve the low temperature structure in more detail. Our preliminary results demonstrate the advantages of the electrochemical technique for growing cubic, tetragonal, and orthorhombic crystals at low temperature. The structures of powdery BaPbO3 at room temperature and liquid helium temperature were found to have same space group of Ibmm but with different tilting magnitudes.13 It would be interesting to investigate how the crystallographic unit cells and resistivities are affected in the cryogenic environment of the three polymorphs of BaPbO3 compounds. High resolution X-ray diffraction and measurement of the physical parameters of the polymorphic structures would promote a greater understanding of the correlation between crystallographic structure and electronic band characteristics. Conclusion The electrocrystallization of BaPbO3 was demonstrated, and the electrochemical mechanisms of crystal growth were presented. The pH2O and oxygen partial pressure are the primary factors affecting the formation of O2- and O22- intermediates.
Supporting Information Available: Indexed powder XRD patterns of the low temperature grown tetragonal and orthorhombic compounds. This material is available free of charge via the Internet at http:// pubs.acs.org.
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CG800009B
