.__ f!!!id 2s SOLID zyxwvuts ‘8% STATE ELSEVIER Solid State Ionics 100 (1997) 127-134 IONIC3 Ionic conductivity and structural characterization of Na,.,Nb,.,Zr,.~(PO,), with NASICON-type structure Carla Verissimo”, Francisco M.S. Garridol,“, Oswald0 L. A1vesa, Paloma Calleb, Ana Martinez-Ju&rez”, Juan E. Iglesias”, Jo& M. Rojo”‘” ‘Laboratorio de Quimica do Estado Solido; Imstituto de Quimica, UNICAMP, CP 6154, 13081 Campinas, Sao Paula, Brazil bDepartamento de Quhica-Fisica Aplicada, Facultad de Ciencias, UAM, 28049 Madrid, Spain ‘Institute Ciencia de Mat&ales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain Received 18 February 1997; accepted 25 April 1997 zyxwvutsrqponmlkjihgfedcbaZYXWVUT Abstract The NASICON-type Na,,,Nb,.,Zr,.,(PO,), was prepared by solid state reaction of Nb,O, and the precursor yNaHZr(PO,), at 700°C. The EPR spectra showed a signal with a g factor of 1.984 assigned to Nb (IV) species in octahedral oxygen environments. The X-ray powder diffraction pattern obtained with monochromatic radiation was indexed on the basis of a rhombohedral cell, the hexagonal parameters being uH = 8.8061(2) and cH = 22.7638(7) A. The Nat ion conduction was measured by the complex impedance method (frequency range: O.l-lo5 Hz; temperature range: 20-500°C) on four pellets previously sintered at 450, 750, 900 and 1000°C. The conductivity data are discussed in relation to the sintering temperature. An activation energy of 0.60 eV for the movement of Na’ ions in the NASICON framework has been found. Keywords: NASICON; Ionic conductivity; Sodium niobium; Zirconium 1. Introduction It is known that materials with NASICON-type structure are in general good ion conductors due to the presence of channels in which alkali ions can move easily [ 11. The crystal structure, firstly reported for the composition NaZr,(PO,),, consists of a framework in which Zr,(PO,), units are linked to each other [2,3]. These units are built up of two ZrO, octahedra and three PO, tetrahedra sharing comers; *Corresponding author. Tel.: +34 1 334 9000; fax: +34 1 372 0623; e-mail: immrm90@pinarl.csic.es ‘Permanent address: Instituto de Ouimica: Universidade Federal do Rio de Janeiro, CP 68563, 21941, RI, Brazil. 0167-2738/97/$17.00 0 1997 Elsevier PZI SO167-2738(97)00307-X Science B.V. All rights reserved. phosphate then, every oxygen belongs simultaneously to a tetrahedron and an octahedron. The Na+ ions occupy a site with an octahedral oxygen environment between two Zr2(P0,), units at the intersection of three conduction channels (Ml site). There is, however, another available site for sodium, the M2 site which is surrounded by 8-10 oxygens at each bend of the conduction channels. Both sites are arranged in an alternating manner along the conduction channels. The NASICON structure lends itself to a large number of substitutions. Thus, P5+ can be partially replaced by Si4+ in the tetrahedral environments [4-111, Zr4+ can be substituted by either tetravalent (Ge, Ti, Hf, Sn, ...) or trivalent (Y, SC, Cr, ...) cations 128 C. Verissimo et al. I Solid State Ionics 100 (1997) 127-134 the manufacturer, and checked with NBS Si standard (a = 5.430940 A for the above wavelength). Peak positions were obtained by the peak finding routine of the diffractometer software package, but incompletely resolved multiplets and shoulders were remeasured by hand. EPR spectra were recorded at 100 K temperature, using a Varian E-12 spectrometer with 100 kHz field modulation and operating at 9.1 GHz frequency. The powder samples were placed in a cylindrical quartz tube of 3 mm inner diameter, the weight of the sample was in all cases 30 mg. The microwave power level was kept at 2 mW. The scan range was 4000 G. Complex impedance measurements were carried out with a 1174 Solar&on frequency response analyzer connected to a 1186 Solartron electrochemical interface. The powder sample was pelletized under a pressure of 3000 Kg cm-‘. Then the pellets (of about 6 mm diameter and 1 mm thickness) were sintered in air at 450, 750, 900 and 1000°C; the time spent at each temperature was 6 h. Gold electrodes were deposited on the two faces of the pellets by vacuum evaporation. Platinum electrodes were obtained in some particular case from a platinum paint (Engelhard 6082). The frequency range used was O.l10’ Hz. The electrical measurements were carried 2. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Experimental out in heating and cooling runs with the pellet under The precursor y-NaHZr(PO,), . nH,O (n detera nitrogen flow. The density of the pellets was determined by mined by TG measurements) was prepared following the procedure described elsewhere [46]. Stoichioimmersion in ethanol or water according to the metric amounts of the precursor and Nb,O, were Archimedes method. thoroughly mixed in an agate mortar, and then calcined at increasing temperature up to 700% where the time spent was 20 h. 3. Results and discussion X-ray powder diffraction patterns were obtained in a Shimadzu XD3A diffractometer with Cu Ko 3.1. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO Sy nthesis radiation and a Ni filter. For indexing purposes a Philips X’Pert-MPD diffractometer was used. The Na,.,Nb,.,Zr,.,(PO,), was obtained by solid state powder data from 12” to 90” in 28 were collected by and the precursor y-NaHreaction of Nb,O, using strictly monochromatic CuKol 1 radiation (h = Zr(PO,), . nH,O. The latter compound shows a layer 1.5405981 A) selected by means of an incident structure in which the negative charge of the layers is beam, curved crystal Ge( 111) monochromator, of the compensated by Naf and H + ions placed in the symmetric Johansson type. The diffractometer radius interlayer space. The reaction between the dehywas 230 mm. Divergence slit of l”, receiving slit of drated reagents can be expressed as follows: 0.025”, and Soller slit of 1” axial divergence were 30 NaHZr(PO,), + 3 Nb,O, used. The diffractometer was previously aligned by -+ 20 Na,.,Nb,.,Zr,,(PO,), + 15 H,O. the use of a pressed powder Si pellet furnished by in the octahedral environments [12-171, and Na+ can be also substituted by other ions [18-331 such as Li+, Ag+‘, Cu+, . .. All these substitutions have led to stoichiometric compounds and solid solutions with different content in alkali ions. Recently, NASICON compounds with octahedral cations in two different oxidation states have been prepared [34-401. In fact, the alkali ions empty Nb,(PO,), and V,(PO,) compositions show niobium and vanadium in the oxidation states IV and V These compounds have been used as host materials for reductive insertion of lithium and sodium, the content in alkali ions being Other controlled by the reacting conditions. NASICON compounds containing only niobium in the oxidation state V have also been prepared [41451. The aim of this work is to study the ionic conductivity of Na,,,Nb,,,Zr,,,(PO,), with Zr and Nb as octahedral cations. This material has been prepared by solid state reaction of Nb,O, with a precursor in which the Na, Zr, and P contents are in the adequate stoichiometry. The new NASICON compound has been characterized by X-ray diffraction and EPR spectroscopy. C. Verissimo 129 et al. I Solid State Ioaics 100 (1997) 127-134 The reaction is complete at 700°C as deduced from X-ray diffraction data, i.e. no X-ray peaks corresponding to the reagents were found. This temperature (700°C) is lower than that normally reported (lOOO-1200°C) when the reagents were Na,CO,, Zfl,, and Nb,O, [35,36,40,42], suggesting that the reaction is favoured by the presence of the precursor. The Na, Zr and P contents of the precursor are kept in the final product as deduced from chemical analysis data. Then, the synthesis procedure has led to a new NASICON material with a 10% deficit in octahedral cation and a sodium content higher than that reported for other Nb-NASICON compounds [35,36,40]. WH,PO,),, 3.2. Structural characterization Electron spin resonance measurements were performed at 100 K on the sample as prepared and after calcination at 750 and 1000°C. In all cases the EPR spectrum showed a unique low-intensity signal characterized by a g factor of 1.984 (Fig. 1). The g value is close to that reported for niobium IV in Nb,(PO,), (g = 1.893) [47] indicating that Nb has been incorporated into the NASICON framework. In addition, a portion of the octahedral Nb is in the oxidation state IV [48]. The signal intensity de3600 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED 3000 3200 3400 creases by ca. 70% as the calcination temperature increases to 1000°C. This decrease is explained by Ma g ne tic fie ld (G ) the oxidation of Nb(IV) to Nb(V) which progresses Fig. 1. EPR spectra of the sample as prepared (top) and calcined in the presence of oxygen with rising temperature. at 1000°C for 6 h (bottom). The spectra were recorded at 100 K. The X-ray powder diffraction pattern of the preSpectrometer settings: receiver gain, 8X 103; modulation anpared NASICON is shown in Fig. 2. The peak plitude, 1 G, time constant, 0.1 s. 4000 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 0 L 40 Bragg Fig. 2. X-ray powder diffraction pattern 60 80 Ang le (de g re e s) of Na,_,Nb, 3Zr,,5(P04)3 obtained with monochromatic Cu Kcl, radiation. 130 C. Verissimo et al. I Solid State Ionics 100 (1997) 127-134 positions were used in an automatic indexing proTable 1 cedure TREOR90 [49]. Least squares refinement XRD powder pattern with 66 unambiguously indexed reflections yielded R3c, a, = 8.8061(2) A, the hexagonal lattice parameters: uu = 8.8061(2) and cu = 22.7638(7) A. The Miller indices (Table 1) of 01 2 13.97 all reflections were consistent with space group R~c, 10 4 19.45 which is the usual one in rhombohedral NASICON1 1 0 20.15 type compounds. The figures of merit [50,51] of this 1 1 3 23.34 indexing are M,, =31 and F,,=42(0.006587,73), 0 2 4 28.15 with all 75 observed lines indexed. 1 1 6 31.09 21 1 31.25 The lattice parameters of our compound are 0 1 8 33.59 compared with those of related materials in Table 2. 21 4 34.86 1: can be seen that the average Nb-0 distance (1.99 3 0 0 35.28 A) [35,36], shorter than the average Zr-0 distance 2 0 8 39.48 (2.07 A) [2], makes the overall cell dimensions 2 2 0 40.96 1 1 9 41.12 shorter for the Nb-NASICON compounds. The slight 1 0 10 41.36 deficiency in octahedral cations should operate in the 21 7 41.85 same way. However, the presence of excess sodium 0 3 6 42.78 lengthens the un parameter (see Nb,P,O,,, 2 43.48 3 1 and NaNbZrP,O,,; and see 1 2 8 44.72 Na,.W,P,O,,, Na,.,Sc,.,Zr,.,Si,.,P,.,O,,, and Na,Zr,Si,PO,,) while the cu parameter is not appreciably affected in the compounds having only niobium as octahedral cation (Nb,P,O,, and Na,.,Nb,P,O,,). The same effect is observed for the Na,ZrP,O,, composition [52] where half the octahedral zirconium positions are occupied by sodium, and all the sites for sodium, Ml and M2, are occupied. According to these observations we can expect for our compound (Na,.SNbo.3Zr,.SP,0,,) a small decrease in the cell parameters with respect to NaZr2P30,2, due to the substitution for Zr by Nb as well as the deficiency in octahedral cations, and also an increase in the an parameter due to the presence of Na, with the result that the lattice parameters must be very similar to those of NaZr,P,O,,. This is indeed the case when the parameters for Na,.SNb,,,Zr,,,P,O1, are compared with those reported for NaZr,P,O,, in [2]. The values reported in [2] are probably more reliable than those of Refs. [3] and [15], since the last were measured with a flat sample diffractometer, and the former with a Guinier-Hagg focusing camera and strictly monochromatic radiation; in fact the comparison can be only made after the slight differences between the wavelength values used in each experiment are allowed for. Following the same line of thought, the un parameter of our compound ought to be slightly longer and the cn parameter quite longer 1 0 3 2 0 0 40 2 1 1 3 3 4 2 1 0 0 1 1 2 2 3 0 3 4 1 1 2 3 3 51 1 5 3 2 1 2 0 4 1 3 1 1 2 1 3 4 1 4 3 4 0 3 1 5 3 0 1 2 4 2 3 3 1 4 10 5 6 12 2 4 10 7 12 8 4 0 5 3 14 8 10 6 14 8 11 4 0 10 15 14 4 10 6 1 13 4 45.73 46.42 47.35 47.72 47.92 48.37 50.44 51.09 51.50 52.48 53.97 54.85 55.14 56.29 56.57 57.95 58.20 59.62 60.72 62.00 62.24 62.77 63.05 63.32 63.61 64.94 65.90 66.92 67.46 68.49 68.58 69.65 70.68 of rhombohedral Na,,,Nb,,,Zr,.,(PO,),. cH = 22.7638(7) A lOOZ/I,,, 13.98 19.44 20.16 23.34 28.15 31.09 31.25 33.60 34.86 35.28 39.49 40.96 41.13 41.36 41.84 42.77 43.49 44.73 45.71 46.43 47.35 47.73 47.93 48.35 50.45 51.08 51.50 52.48 53.97 54.84 55.14 56.27 56.56 57.95 58.20 59.62 60.72 62.01 62.24 62.77 63.05 63.30 63.60 64.94 65.90 66.92 67.47 68.47 68.59 69.66 70.68 6.3356 4.5610 4.4030 3.8083 3.1678 2.8742 2.8596 2.6660 2.5714 2.5421 2.2805 2.2015 2.1932 2.1813 2.1571 2.1119 2.0795 2.0250 1.9826 1.9546 1.9182 1.9042 1.8970 1.8804 1.8078 1.7865 1.7731 1.7422 1.6975 1.6723 1.6642 1.6331 1.6256 1.5902 1.5839 1.5495 1.5240 1.4957 1.4904 1.4792 1.4733 1.4677 1.4616 1.4348 1.4162 1.3971 1.3872 1.3688 1.3672 1.3488 1.3317 6.3301 4.5615 4.4013 3.8083 3.1670 2.8739 2.8600 2.6653 2.5713 2.5420 2.2802 2.2017 2.1927 2.1810 2.1574 2.1126 2.0791 2.0245 1.983 1 1.9542 1.9185 1.9040 1.8963 1.8810 1.8074 1.7865 1.7732 1.7423 1.6975 1.6727 1.6642 1.6337 1.6257 1.5901 1.5838 1.5495 1.5240 1.4955 1.4905 1.4790 1.4732 1.4680 1.4617 1.4348 1.4162 1.3970 1.3871 1.3691 1.3671 1.3488 1.3317 18 73 87 100 56 87 22” 3 22 38 6 5b 5b 3b 3 10 2 18 12 2’ 30 5” 5 <l 20 5 <l 9 11 22 <I” 3 5 4 11 13 6 2 2 6 2 6 4 2d 3d <l 6 131 zyxwvut C. Verissimo et al. I Solid State Ionics 100 (1997) 127-134 Table 1 (continued) h k 3 d caic d ohs 5 71.93 71.92 1.3116 1.3118 1 14 73.39 73.39 1.2891 1.2891 4 8 73.61 73.61 1.2857 1.2857 2” 0 74.61 74.60 1.2710 1.2711 18 75.05 75.04 1.2647 1.2647 1 15 1 42 06 than the respective parameters of NaNbZrP,O ,2, which happens to be the case as shown in Table 2. loOIlZ,,, 3.3. Ionic conductivity The impedance measurements were carried out on four pellets previously sintered at 450, 750, 900, and 2d 51 7 75.21 75.20 1.2623 1.2625 1000°C. They are called hereafter P4.50, P750, P900, 76.36 76.36 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1.2462 1.2461 1 34 2 and PIOOO, respectively. The density of the pellets 1.2377 r 3 2 13 76.98 e-l 77.01 1.2373 1 was determined by the Archimedes method and 77.02 1.2372 L 0 4 14 compared with that deduced from X-ray diffraction 2’ 43 4 77.97 77.97 1.2244 1.2244 data (3.155 g cm-3). The relative density of the 78.22 78.21 1.2212 1.2213 25 0 3 pellets changed from 74% for P450 to 98% for 78.48 78.48 1.2177 2 4 10 1.2177 3 PlOOO. r52 3 79.42 1.2057 79.43 1.2056 1 The impedance plots (imaginary -Z” vs. real Z’) e-l 79.46 1.2052 LO6 6 recorded at different temperatures for the P450 pellet 2 3 14 80.59 80.61 4 1.1910 1.1909 are shown in Fig. 3 (top). At low temperatures 25 6 83.01 83.00 1.1625 1.1625 4 (150°C) an arc and a spike are observed. When the 34 8 84.35 84.36 1.1473 1.1472 2 pellet is heated at higher temperatures (270°C) the 1 5 11 84.82 84.82 1.1422 1.1421 1 16 4 85.07 85.07 1.1394 1.1394 1 arc disappears and the spike is developed. A similar 0 3 18 85.74 85.74 1.1323 1.1322 2 behaviour was found for the other pellets. The 0 1 20 86.36 86.35 1.1257 1.1258 2 spikes, which show capacitances in the range I-10 86.78 86.76 1.1214 1.1216 1 4 15 <l FF, are ascribed to the blocking effect of Na+ ions at 5 0 14 87.65 87.63 1.1126 2 1.1124 the electrode surfaces. The capacitance associated 88.51 88.50 1.1038 3 2 16 1.1039 <l 88.82 88.84 2’ 1.1008 44 0 1.1005 with the arcs was, in all cases, in the range 5-10 pF, 4 3 10 89.08 89.09 1.0981 5 1.0982 which is the order of magnitude usually found for grain-interior response [53]. However, the resistance a Shoulder of the previous reflection. b Partially resolved triplet. deduced from the intersection of the arcs with the ’ Shoulder of the following reflection. real axis changed with the previous sintering treatd Partially resolved doublet. ment, the resistance at a given temperature being e Unresolved doublet. lower for higher sintering temperature. 0 0 2-5 Table 2 Lattice parameters of title and related compounds Compound aH (A) cn (A) v (A’) Temp. Reference Na,.F, 3Zr,.5P30,2 8.8061(2) 22.7638(7) 1528.8( 1) R.T. This work Na&P,O,, 8.8043(2) 22.7585(9) 1527.8( 1) R.T. t21 same 8.815( 1) 22.746(7) 1530.5(g) R.T. same 8.8103(3) 22.763(2) 1530.2(3) R.T. Na,.,Sc,,Z*,,Si,.,P,.,O,, Na,Zr,Si,PO,, 8.9834(l) 22.8658(5) 1598.1(l) R.T. 9.029” 22.974” 1622.0 520 K [31 Cl51 [lOI 141 t71 [351 [361 1401 ~52.1 same 9.074(2) 23.057(4) 1644.1(S) 623 K NfJ*P,O,, 8.6974(7) 22.123(2) 1449.3(3) R.T. 8.7362(9) 22.093(2) 1460.2(4) R.T. 8.776(2) 22.43( 1) 1496.1(2) R.T. 9.217” 22.39” 1647.3 473 K NaGb,PA NaNbZrP,0, Na,ZrP,O ,2 7 ._ a Estimated errors not reported in original references. 132 C. Verissimo et al. I Solid State Ionics 100 (1997) 127-134 200 400 I+ I I :” ,6 oiii: 0 Z’ (ohm) 6x106 2’ (ohm) 4 b (L1 -2 s 4 2 5 -4- z? .!z ‘; .E 0 _6_ zyxwvutsrqponmlkjihgfedcbaZYXWVUTS 1 1000/l lw,J U-W Fig. 3. Top: Impedance plots (imaginary -Z” vs. real Z’) obtained at two temperatures for the P450 pellet. Selected frequencies (in hertz) are marked. Bottom: Normalized imaginary modulus vs. frequency in a semi-log scale at three temperatures. The imaginary part of the electric modulus (M”) vs. frequency (log,& is shown for the P450 pellet in Fig. 3 (bottom). At 120°C an asymmetric peak is observed. This peak shifts towards high frequency when the pellet is heated at higher temperatures (150 and 210°C). A similar behaviour was observed for the other pellets. From the impedance plots we have determined the overall resistance of the pellet at each temperature, then the conductivity values have been calculated as usual. The frequency corresponding to the maxima of the M” peaks has been measured at each temperature for the four pellets. The plots of conductivity (log,,aT) and frequency (log,& as functions of inverse temperature (1000/T) are given in Fig. 4. The experimental data are well fitted to two Arrhenius expressions, VT= u,exp(-E&kT) and f = foexp(-E,lkT), where o-~ and& are pre-exponential factors, EcT and Ef are activation energies, and k is the Boltzmann constant. The parameters yielding the best fits together with the conductivity measured at 200°C are outlined in Table 3. The conductivity values are lower than those reported for the best NASICON materials [54,55]. 3 zyxwvutsrqponmlkjihgfedcb 2 (K-l) Fig. 4. Plots of conductivity (open symbols) and frequency of the modulus peak (block symbols) vs. inverse temperature. Triangles, squares, circles, and diamonds correspond to the pellets P450, P750, P900, and PlOOO, respectively. The straight lines are best fits to the expressions UT= craexp(-E,,lkT) and f=f,exp(-Efl kT). The conductivity values reported for NaZr,(PO,), in Refs. [27] (+) and [59] (*) are also included. The increase in the overall conductivity for the pellet previously calcined at high temperature is concomitant with the decrease observed for the Nb(IV)-EPR signal. It points out that the electronic conductivity is negligible as compared with the ionic one. In addition, the increase in conductivity with sintering suggests some grain-boundary contribution in the impedance arcs. A change in EcT from 0.7440.01 to 0.61+0.01 eV for pellets sintered from 450 to 1000°C is also observed. The latter value coincides with the value of the Ef parameter obtained for the four pellets. Taking into account that the electric modulus is not affected by grain-boundary and electrode effects [56], the activation energy of 0.60 eV can be ascribed to the movement of Naf ions inside the grains. According to the model generally accepted for the NASICON structure in which alkali ions are moving along the conduction channels by hopping between the Ml and M2 sites [57,58], that energy is associated with the already mentioned hopping. Finally, the conductivity reported for the NaZr,(PO,), composition is compared with ours for Na,_,Nb,.,Zr,.,(PO,), in Fig. 4. Two values for NaZr,(PO,), have been plotted: one of them (+> was taken from [27] and corresponds to a pellet 133 C. Verissimo et al. I Solid State Ionics 100 (1997) 127-134 Table 3 Activation energies (E,, and I$), and pre-exponential factors (crOand&) for the fits of the Arrhenius plots in Fig. 4.Values of conductivity 200°C are also included. P450, P750, P900, and PlOOO are pellets previously sintered at 450, 750, 900, and lOOO”C, respectively log,,u, P450 P750 P900 PlOOO 0.74+0.01 0.66~0.01 0.64+0.01 0.61+0.01 (S cm-’ K) 4.41+0.03 4.3520.04 4.40t0.03 4.17?0.04 fl,,, 7.1 x 4.3 x 8.0~ 1.3x (S cm-‘) lo-’ 1o-6 1O-6 1o-5 at log,,.& (Hz) 0.62zO.02 0.61 kO.02 0.60t0.02 0.60?0.02 11.220.3 11.9-to.3 12.OkO.2 12.0?0.3 WI M.A. Subramanian, P.R. Rudolf, A. Clearfield, J. Solid State sintered at 1100°C for 6 h, the other (*) was taken Chem. 60 (1985) 172. from [59] in which the sintering temperature had not u31 C. Delmas, F. Cherkaoui, P. Hagenmuller, Mat. Res. Bull. 21 been reported. In both cases the conductivity values (1986) 469. for NaZr,(PO,),, even when the pellet was sintered u41 W. Wang, S. Wang, L. Rao, Z. Lu, X. Yi, Solid State Ionics 28-30 (1988) 424. at 1100°C are lower than those found for our Nbu51 J.L. Rodrigo, J. Alamo, Mat. Res. Bull. 26 (1991) 475. NASICON. Therefore, the substitution for Zr by Nb Cl61 Y. Saito, A. Kazuaki, T. Asai, H. Kageyama, 0. Nakamura, increases the mobility of Nat ions in the NASICON Solid State Ionics 58 (1992) 327. structure. This effect has also been observed in other t171 M.P. Carrasco, M.C. Guillem, J. Alamo, Solid State Ionics 63-65 (1993) 684. NASICON compounds with Li+ ions as charge U81 B.E. Taylor, A.D. English, T. Berzins, Mat. Res. Bull. 12 carriers [41]. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Acknowledgments Financial support by CICYT (project MAT950899) is gratefully acknowledged. We also thank FAPESP for the fellowship to C. Verissimo. References Ul J.B. Goodenough, H.Y-P. Hong, J.A. Kafalas, Mat. Res. Bull. 11 (1976) 203. t21 L. Hagman, P. Kierkegaard, Acta Chem. Stand. 22 (1968) 1822. [31 H.Y-P. Hong, Mat. Res. 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