J. MATER. CHEM., 1994, 4( 5), 703-705 Bi,W, -xCuxOG-x(0.7<x 60.8): A New Oxide-ion Conductort Vandana Sharma, Ashok K. Shukla and Jagannatha Gopalakrishnan Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-5600 72, lndia Anion-deficient Aurivillius phases of the general formula, Bi,W, -,CU,O~-~, possessing orthorhombic/tetragonal Bi,WO,-like structures, have been synthesized by quenching the oxide melts. The tetragonal phase stabilized for the compositions 0.7 <x <0.8 is a good oxide-ion conductor in the temperature range 500-900 K, the x =0.7 composition exhibiting the highest conductivity in the series. Solid oxide-ion electrolytes have applications in sensors, oxygen pumps and in high-temperature electrolyser-fuel-cell hybrid systems.All the solid electrolytes known, however, exhibit low oxide-ion conductivities below 1100 K, limiting their application. Therefore, there is a need for fast oxide-ion conductors that are operative at temperatures around 700 K.',2 Recently, efforts have been directed towards developing such materials. Abraham et aL3 discovered that Bi4V20 which belongs to the Aurivillius layered perovskite family, exhibits high oxide-ion conductivity in its high-temperature tetragonal phase (Tr=850 K). Subsequently,e it has been possible to stabilize the tetragonal phase at room temperature by substituting appropriate amounts of copper, titanium or niobium for vanadium in Bi4V2011; all the tetra- gonal phases stabilized by substitution exhibit high oxide-ion conductivities in the temperature range 500-900 K, the highest oxide-ion conductivity observed to date in this family of oxides being ca.1x lop4Q-' cm-' at 700 K for the composi- tion Bi,V .8Ti0.2010.9.6 Here, we report the synthesis of a new series of oxide-ion conductors, Bi,W, -,Cu,06 -,,, derived from the Aurivillius phase Bi2W06 by substitution of Cu" for Wvl. The highest oxide-ion conductivity of ca. 1 x R-' cm-' at 700 K is exhibited by the composition Bi,Wo,3Cuo,,04,6; this value is about an order of magnitude higher than the conductivities of copper/titanium-substituted Bi4V2OI1 reported previously.6 The significance of this work lies in that, unlike Bi4V2011= Bi,VO,,, no.,,the parent Aurivillius phase, Bi2W06, of the present series is a stoichiometric compound without oxygen vacancies. Accordingly, the present study opens up the pos- sibility of 'engineering' new oxide-ion electrolytes starting from other Aurivillius phases7 of the general formula (Bi202)(An- 'Bn03"+ ') by appropriate chemical substitution. Experimental Various members of the Bi2Wl -,CU,O~-~, family were syn- thesized by melting stoichiometric mixtures of appropriate amounts of the oxides followed by quenching the melt.The melting temperature varied between 1320 and 1110 K. The compositions with higher copper content melted at lower temperatures. The products were characterized by X-ray powder diffraction (JEOL JDX-8P diffractometer) using Cu-Ka radiation.Infrared (IR) spectra of the oxides were recorded on a Briiker IFS 113 V FT-IR spectrometer at room temperature in a KBr matrix. Samples were compressed into pellets of diameter 9 mm and thickness 1.5 mm. The pellets were sintered at 873 K in air for 24 h. Impedance data on the sintered pellets of ca. 95% theoretical density coated with platinum paint were obtained at various partial pressures of TContribution no. 1007 from the Solid State and Structural Chemistry Unit. oxygen in the temperature range 500-900K, employing a 4194-A Hewlett-Packard impedance-gain phase analyser in the frequency range 100 Hz-15 MHz interfaced to an IBM-PC. Samples were equilibrated at constant temperature for ca. 45 min prior to each set of impedance measurements.Results and Discussion Typical X-ray diffraction patterns of copper-substituted bis- muth tungstates, Bi,W1-,Cu,O,-,, (O<x<0.8), are shown in Fig. 1 and the lattice parameters derived from the XRD patterns are listed in Table 1. Solid solutions of Bi,W,-,Cu,O,-,, exist for the range O<x<O.8. Phases up to x=0.65 adopt the orthorhombic Bi2WO6 structure, while those in the narrow range 0.7dxGO.8 stabilize with the tetragonal structure. We see that the c parameter and the unit-cell volume change abruptly at x =0.7 when the structure changes from orthorhombic to tetragonal (Table 1). A similar volume change also occurs across the orthorhombic-tetragonal transition in Bi4V2011.3 Since X-ray diffraction studies reflect only gross structural features, we recorded the IR spectra in the region 1000-500 cm to examine the local structural changes (d1 / A A* J I I 1 I IJ 0 20 30 40 50 60 2Bldeg rees Fig. 1 XRD patterns of (a) Bi2W0.3C~0.704.6r(b) Bi2W0.7C~0,305.4, Bi2W06 and (dl Bi2W0.2Cu0.804.4 J.MATER. CHEM., 1994, VOL. 4 Table 1 Synthesis conditions and cell parameters of copper-substituted bismuth tungstates synthesis crystal sample temperature/K system 1073 orthorhombic 1073 orthorhombic 1323 orthorhombic 1223 orthorhombic 1173 orthorhombic 1153 orthorhombic 1140 orthorhombic 1123 tetragonal 1110 tetragonal 1110 tetragonal accompanying the phase transformation. The IR spectra for typical copper-substituted bismuth tungstates along with the spectrum of the parent Bi,WO, are shown in Fig.2.The spectrum of Bi,WO, is similar to that reported by Bode et a[.,*showing absorption bands at 822, 743, 696, 596 and 548 cm-'. The spectra of Bi2W1-,Cu,O,~,, (O<xG0.65) orthorhombic phases are similar to the Bi,WO, spectrum. However, the spectrum of the tetragonal Bi2W0.3C~0,704.6 is completely different, revealing the absence of the features found for Bi,W06, and Bi2W0.7C~0.305.4. This indicates that the local structure around the W/Cu atoms in the tetragonal Bi2W0,3C~0.704,6is significantly different from that of the orthorhombic Bi,WO, and Bi,Wo.7Cuo.305~4. Indeed, the spectral features of Bi2W0.3C~0.704.6 are typical of fast-ion conducting materials such as CSHSO,.~~'~ Electrical conductivity values for various specimens of copper-substituted bismuth tungstates were obtained from their complex impedance plots.Typical impedance data for Bi,Wo,3Cuo.704,, are shown in Fig. 3. The bulk resistance of the sample is taken as the intercept of the high-frequency arc on the real Z axis, while the low-frequency intercept of the combined arcs amounts to the total pellet resistance compris- ing the bulk resistance and the resistance due to the grain boundaries. For example, the bulk and grain boundary resist- ances for a sample of Bi2Wo~3Cuo,704.6 at 650 K are approxi- mately equal (1.4 ka), amounting to a total pellet resistance ~~ 1000 900 800 700 600 500 wavenum berkm-' Fig.2 IR spectra of (a) Bi2W0,,,Cuo.,04.6, (b) Bi2W0.7C~0.305.4 and (C) Bi2WO6 a/A b/A C/A volume/A3 cell 5.47(5) 5.48 5.43(6) 5.47 16.42(7) 16.38 -492 5.49 5.47 16.33 490 5.48 5.46 16.35 489 5.49 5.47 16.34 49 1 5.48 5.45 16.39 490 5.48 5.45 16.37 489 3.95b - 16.66 518 3.95b - 16.72 522 3.95b - 16.75 522 r I IVI I I 4, 0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 Z'/kQ Fig. 3 Impedance data for B~2Wo,3Cuo.,04,6 at 650 K in the frequency range 100 Hz-15 MHz. (a) lo6Hz, (b) lo4Hz. of 2.8 kQ, as shown in Fig. 3. The conductivity data for Bi2Wl -,CU,O,-~, as calculated from the bulk resistances of the respective samples is shown in Fig. 4. The temperature dependence of the conductivity of the x =0.7 member at 1atm oxygen partial pressure, together with the variation in conduc- tivity of the series with x at 850 K and po, = 1 atm are given in this figure.We see that there is an abrupt (about an order of magnitude) increase in the conductivity across the orthorhombic-, tetragonal phase transformation at x=0.7. r I 1 I I I I) 1.2 1.4 1.6 1.8 2.0 2.2 1O~WT Fig. 4 Temperature dependence of oxide-ion conductivity for Bi2w0.3cu0.704.6 "t PO2 atm (-A-)and Bi4V1.8Ti0.2010.9 at PO2 atm (-.-), (B1203)0.8(Er203)0.2 (-*-), (zr02)0.9 (y203)0.1(-).The inset shows variation of the conductivity of Bi2W, -xCu,06-,, with x (O<x 60.80) at 850 K at po2= 1 atm. J. MATER. CHEM., 1994, VOL. 4 t A A 1o-~t-tti 1o-~ 0 10 20 -In Po2 Fig.5 Variation in conductivities of BiZW0,3Cu0,704,6(A) and Bi2Wo,,Cu,,,0,., (0)with oxygen partial pressure (Brouwer diagram) at (a) (c) 693 K and (b)(d)793 K. The conductivities for Bi2W0.3C~0,704.6 are between and 10-3 a-1 cm-' and the activation energy value is 0.35 eV in the temperature range 500-900 K. In Fig. 4 are also included the conductivity data for Zr02-Y203,11 Bi203-Er20312 and Bi4Vl.8Tio,2010.96for comparison. We see that the conductivity of Bi2Wo~3Cuo,704~6 is higher than those of Bi4V1.8Ti0.2010.9 and ZrO, -Y203 over the entire temperature range, while it is lower than the conductivity of Bi,03-Er203. The conduc- tivity of Bi2Wo~,Cuo~704,6 shows little variation with oxygen partial pressure (Fig.5), suggesting that the electrical conduc- tion is mainly ionic. Transport-number measurements are, however, required to establish the exact ionic and electronic components to the electrical conductivity. In Fig. 5 are also included the data on the dependence of conductivity on oxygen partial pressure for the orthorhombic Bi2W0.7C~0.305.4.The data do show the presence of an electronic component of the conductivity in this material at higher oxygen pressures. Our results in this respect are similar to those reported by Goodenough et all3 for Ba21n,0,. Goodenough et al. suggested that do cations, such as TiIV and NbV, which exhibit 'ferroelectric' displacement in octa- hedral oxygen coordination, and Cu" which occurs in four, five or six coordination readily in oxides, provide a low-energy barrier for oxide-ion migration, giving rise to the high oxide-ion conductivity of Bi4V201 substituted with Ti", NbV or Cu".An abrupt increase in the c parameter of the tetragonal phase of Bi2Wl -xCux06-2x (Table l), suggesting a cooperat- ive displacement of Wvl towards the apical oxygen. reveals that the Goodenough model is likely to be applicablc for the series reported in this paper. Determination of the crystal structures of the orthorhombic and tetragonal phases is required to establish details of the structure and mechanism of oxide-ion conduction of the Bi2Wl -xCux06-2x series. We thank Professor C. N. R. Rao, FRS for his continuing encouragement. We also thank Dr H. N. Vasan for his assistance in recording IR spectra.References 1 J. B. Goodenough and A. K. Shukla, in Solid State Ionic Devices, ed. B. V. R. Chowdhari and S. Radhakrishna, World Scientific, Singapore, 1988, pp. 573-604. 2 J. B. Goodenough, A. Manthiram, M. Paranthaman and Y. S. Zhen, Mater. Sci. Eng. B, 1992, 12, 357. 3 F. Abraham, M. F. Debreuille-Gresse, G. Mairesse and G. Nowogrocki, Solid State Ionics, 1988,28-30, 529. 4 F. Abraham, J. C. Boivin, G. Mairesse and G. Nowogrocki, Solid State Ionics, 1990,40-41,934. 5 T. Iharada, A. Hammouche, J. Fouletier, M. Kleitz, J. <'.Boivin and G. Mairesse, Solid State Ionics, 1991,48,257. 6 Vandana Sharma, A. K. Shukla and J. 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