NASICON to Scandium Wolframate Transition in Li M Hf (PO ) (M5cr, Fe): Structure and Ionic Conductivity

Solid State Ionics 112 (1998) 53±62

NASICON to scandium wolframate transition in

Li11xx M Hf22x (PO43 ) (M 5 Cr, Fe): structure and ionic conductivity

Enrique R. Losillaa,** , SebastianÂ Bruque a, , Miguel A.G. Aranda a , Laureano Moreno- Realab , E. Morin , M. Quarton b aDepartamento de QuõmicaÂÂ Inorganica, Cristalograf Âõa y Mineralogõa Â, Universidad de Malaga Â, Aptd. 59, 29071 Malaga Â, Spain bLaboratoire de Cristallochimie du Solide, UniversiteÂ Pierre et Marie Curie,4Place Jussieu, 75252 Paris Cedex 05, France Received 6 February 1998; received in revised form 5 June 1998; accepted 5 June 1998

Abstract

The Li11xx M Hf22x (PO43 ) (M 5 Cr, Fe, Bi) systems have been studied and single phases have been isolated for M 5 Cr and Fe. The samples have been characterized by X-ray powder diffraction, diffuse re¯ectance and impedance spectroscopy.

There is a reconstructive transition between rombohedral NASICON and orthorhombic Sc243 (WO ) -type structures as a function of x, at very low values, 0.2 and 0.1 for Cr and Fe, respectively. For the Cr series, a further subtle structural change has been observed for x values higher than 1.7. These phases have the Sc243 (WO ) -type framework, but the symmetry is orthorhombic Pcnb at low values of x and monoclinic P21 /n at high values. The structural changes are discussed on the basis of the sizes of the cavities left by the two frameworks and the lithium order/disorder in these voids. These materials are ionic conductors and their electrical behaviours are also discussed.  1998 Elsevier Science B.V. All rights reserved.

Keywords: NASICON-related materials; Scandium wolframate; Li ionic conductors

1. Introduction to four octahedra. A given stoichiometry may crystallize in several structure types such as Garnet [3],

NASICON [1,2], Langbeinite [4] and Sc243 (WO ) Since the discovery of fast Na1 ion transport [5], these being not simple distortions of each other. properties in the NASICON system [1,2], numerous The structure adopted by Ax M243 (PO ) seems to studies on related phosphates have been carried out. depend on the size of the A and M cations and the IV Phosphates with the general formula Ax M243 (PO ) value of x. For example, NaM 2 (PO 43 ) adopts a x2 III IV consist in a [M243 (PO ) ] framework built up by NASICON-type structure and K243 M M (PO ) has corner-sharing MO64 octahedra and PO tetrahedra. Langbeinite-type structure [6]. In such frameworks, each octahedron is surrounded Jouanneaux et al. [7] pointed out that the x2 with six tetrahedra and each tetrahedron is connected [M243 (PO ) ] framework with Sc 2 (WO 43 ) -type structure is more suitable for small A cations than a * Corresponding authors. NASICON-type structure. The Sc243 (WO ) -type

0167-2738/98/$ ± see front matter  1998 Elsevier Science B.V. All rights reserved. PII: S0167-2738(98)00207-0 54 E.R. Losilla et al. / Solid State Ionics 112 (1998) 53 ±62 structure (SW) may adopt two symmetries. The 2. Experimental section highest symmetry is orthorhombic, space group 2.1. Synthesis Pcan. Primitive monoclinic symmetry also occurs

(s.g. P21 /n) being pseudo-orthorhombic. A complete Li11xx M Hf22x (PO43 ) (M 5 Cr, Fe, Bi) composi- description of these structures (NASICON, SW, tions were prepared by conventional solid-state

Garnet, Langbeinite, Bi0.5 Sb 1.5 (PO 4 ) 3 ) and the rela- reaction. Stoichiometric quantities of Li23 CO , HfO 2 , tionship between the unit cell symmetries and param- (NH42 ) HPO 4 and M 2 O 3 synthesized as described eters values has been already reported [7±10]. The below, were ground and heated in a Pt crucible to values of the unit cell parameters are sometimes very give the following overall reaction: similar and the way to distinguish between (11 x)/2Li CO 1 (22 x)HfO 1 (x/2)M O 1 NASICON and SW structures from powder diffrac- 23 2 23 tion data was smartly discussed [7]. →n 3(NH42 ) HPO 4Li 11xx M Hf22x (PO43 ) 1 (11 x)/ On the other hand, materials based on this stoi- ↑ ↑ ↑ chiometry (with Li cations) are promising candidates 2CO231 6NH 1 9/2H 2 O as solid electrolytes if the conductivity properties at room temperature are enhanced. In general, Cr23 O was obtained by thermal decomposition of

LiM243 (PO ) (M5 Ge, Ti, Sn, Zr and Hf) com- (NH4227 ) Cr O . Fe 23 O was prepared by calcination at pounds crystallize in the NASICON structure and 4008C of iron acetohydroxide. This precursor was they are moderate lithium ion conductors with quite prepared dissolving Fe(NO33 ) ?9H 2 O and 2 31 different lithium mobility for different M(IV) cations NH43 CH COO (OAc :Fe molar ratio of 2.7) in

[11±14]. An outstanding case is LiZr243 (PO ) as it water. NH3 aq. (25% w/w) was then added to obtain may crystallize in the NASICON or SW structures apH¯9. After reaction, the brown solid was sepa- depending upon the synthetic temperature [15]. The rated by centrifugation, washed with deionized water conductivity may be increased by partial substitution up to a pH¯7.5, and air-dried. Bi23 O (Panreac, 41 of M by trivalent cations as Al, Ga, In, Ti, Sc, Y, 99.5%), Li23 CO (Probus, 99.5%) and HfO 2 (Al- La, Cr, Fe [12,16,17]. The reason for this con- drich, 99.8%, microparticle size ,1 mm) were dried ductivity improvement is mainly due to a much at 2008C and used without further treatment; lower porosity of the pellets. Most works have been (NH42 ) HPO 4 (Panreac, 98%) was used as supplied. carried out in the zirconium and titanium systems The starting compounds were thoroughly mixed and there are very few studies on M 5 Sn and Hf. To and ground together with acetone in an agate mortar our knowledge, Aono et al. have carried out the only for one hour and heated at 0.58C?min21 to 4008C study about electrical properties and crystal structure and left at that temperature for one day to release of solid electrolytes based on LiHf243 (PO ) [18]. gases (NH32 , H O and CO 2 ). To avoid or minimize These authors suggested a NASICON-type structure the formation of by-products (LiMP27 O with M5Cr, for Li11xx M Hf22x (PO43 ) (M 5 Cr, Fe, Sc, In, Lu, Y) Fe and unreacted M23 O or HfO 2 ), the samples were series, although the samples were poorly character- heated at 0.58C?min21 with intermediate regrindings ized and sometimes multiphases. (with acetone for 30 min every 1508C) up to the ®nal

LiHf243 (PO ) crystallizes in the NASICON struc- temperatures which depend upon the M metal. Tmax ture and it undergoes a topotactic and reversible for Bi, Fe and Cr were 750, 950 and 11008C, phase transition at low temperature that was char- respectively. These temperatures were maintained for acterized by variable temperature neutron powder 2 to 4 days until no changes were observed in the diffraction [19]. In this work, we have continued our X-ray powder patterns. studies on Li-containing NASICON materials. We have extended the study to the Li11xx M Hf22x (PO43 ) 2.2. X-ray powder diffraction characterization (M5 Cr, Fe, Bi) systems. The samples have been characterized by X-ray powder diffraction, diffuse X-ray powder diffraction patterns were recorded at re¯ectance and impedance spectroscopy. room temperature on a Siemens D5000 automated E.R. Losilla et al. / Solid State Ionics 112 (1998) 53 ±62 55 diffractometer using graphite-monochromated CuKa Rigaku Thermo¯ex TG 8110 apparatus from room radiation. The XRD patterns (128#2u #558) were temperature to 11008C at a heating rate of 108C/min autoindexed using the TREOR90 program [20]. A with calcined Al23 O as standard reference. high resolution synchrotron powder pattern for The diffuse±re¯ectance spectra, UV±VIS, were

Li1.8 Fe 0.8 Hf 1.2 (PO 4 ) 3 was collected on the diffrac- obtained on a Shimadzu UV-3100 spectrophotometer tometer of BM16 line of ESRF (Grenoble, France). using an integrating sphere coated with BaSO4 and The sample, loaded in a borosilicate glass capillary the same substance as reference blank. f 50.5 mm, was rotated during data collection (l5 0.39989(2) A).Ê Data from the nine detectors were 2.5. Ionic conductivity characterization normalized and summed up to 0.0038 step size with local software. The powder pattern was re®ned by Pellets of 10 mm of diameter and a thickness of the Rietveld method [21] with the PC version of 1±2 mm were prepared for conductivity measure- GSAS [22] that has a pseudo-Voigt peak shape ments, by pressing ®ne powder at 200 MPa at room function [23] with the asymmetry correction included temperature. The pellets were sintered at 950± [24]. 11008Cfor2hinorder to increase their mechanical strength. Electrodes were made by coating opposite 2.3. Chemical analysis pellet faces with platinum lac and dried by heating at 2008C. Conductivity was determined by a.c. impe- The M/Hf, M/P and Hf/P molar ratios were dance measurements from 20 Hz to 1 MHz using a checked by Analytical Electron Microscopy (AEM) Hewlett-Packard 4284A impedance analyzer at 208C using a Philips CM 200 Supertwin-DX4 with an intervals on a heating cycle from 100 to 6008C in dry electron probe microanalyzer Edax (Si±Li detector). argon. The detector system has an ultra-thin window resulting in a resolution of 149 eV. Samples for the electron microscopy study were prepared as follows: 3. Results and discussion a small amount was ground in an agate mortar and dispersed in absolute ethanol, several drops of the Under the above reported synthetic conditions, resultant suspension were deposited onto a carbon Li11xx Bi Hf22x (PO43 ) series are multiphases. Four ®lm supported on a nylon grid. HfP27 O and phases can be detected in these powder patterns: LiMP27 O (M5Cr, Fe) were used as standards for the Li42 P O 7 , LiPO 3 , BiPO 4 and HfO 2 . As direct ceramic AEM study. The results in several selected mi- synthesis was unsuccessful, two other synthetic crocrystals were in agreement with the nominal approaches were tested: (i) Low-temperature ion stoichiometries. To determine if lithium was lost in exchange reaction between A11xx Bi Hf22x (PO43 ) the synthetic conditions, the samples (¯50 mg) were (A5Na, K) with LiNO3 at 2008C. (ii) Using an dissolved in aqueous 40% w/w HF acid at ¯608C excess of lithium salts (LiPO323 , Li CO and LiCl) at and the lithium contents were determined by emis- the synthetic temperature, ¯7508C. All synthetic sion atomic spectroscopy in an air/acetylene ¯ame. attempts were fruitless as multiphases were invariab-

K was added, K/Li ratio: 1000/1, to avoid lithium ly obtained. Moreover, A11xx Bi Hf22x (PO43 ) (A5 ionization. The experimental bulk lithium content of Na, K) starting phases were not attained as single the samples were in the range 95±98% of the phase, but a mixture of AHf243 (PO ) (A5Na, K) and theoretical lithium amount deduced from the nominal BiPO4 was obtained. Only for x51 and A5K, chemical formula. K243 BiHf(PO ) , could a single phase be obtained. This material has a Langbeinite-type structure and its 2.4. Thermal analysis and diffuse re¯ectance study study is out of the scope of this work. Although

Li11xx Bi Hf22x (PO43 ) system seems to be not stable, Thermogravimetric and differential thermal analy- Berul et al. [25] have reported the synthesis of ses (TG±DTA) was performed for all materials on a Li32 Bi (PO 43 ) . 56 E.R. Losilla et al. / Solid State Ionics 112 (1998) 53 ±62

3.1. Li11xx M Hf22x(PO43)(M5Cr, Fe) nominal diffraction pattern of this sample was further studied stoichiometries by ®tting the data with GSAS program by the Le Bail method (no structural model is needed). The

Li11xx M Hf22x (PO43 ) (M5Cr, Fe) crystalline Rietveld re®ned monoclinic unit cell parameters samples are stable at high temperatures, and thermal (space group P21 /n) are also reported in Table 1. studies did not show any mass loss between room For the Li11xx Fe Hf22x (PO43 ) series, the x50.1 temperature and 11008C. DTA data did not show any composition is a biphase. This sample contains clear structural transition on heating. mainly NASICON material with a small amount of Chromium samples can be obtained at higher SW-type compound. For 0.3#x#1.5, the composi- temperatures (11008C) than those for iron samples tions are single phases with orthorhombic symmetry (9508C) due to the higher thermal stability. Melting similar to that of chromium compounds (Table 2). temperatures of the solid solutions were determined For compositions higher than 1.5, the samples melted approximately from the visual appearance of a little at the synthetic temperatures and it was not possible amount of sample heated isothermally in 208C steps. to obtain single phases. Lower synthetic tempera- It could be determined that the melting points of the tures (to avoid fusion) make a thermal treatment for

Cr±Hf series take place above 12008C whereas for several weeks necessary, and then the LiFeP27 O Fe±Hf, series occur around 10008C. phase appears. So, it can be concluded that Fe±Hf A detailed study of X-ray diffraction patterns of solid solution is more dif®cult to prepare than the

Li11xx Cr Hf22x (PO43 ) , shows that the solid solution Cr±Hf one. is not continuous in the overall compositional range. Lattice parameters as a function of composition At the lowest composition (x50.1), all re¯ections for both series are shown in Fig. 1 (unit cell edges) can be indexed in a LiHf243 (PO ) -type cell and Fig. 2 (volumes). The unit cell parameters a, b, c (NASICON). For x50.3 the sample is biphasic as a and the cell volume decrease linearly with x follow- mixture of NASICON and SW-type phases is ob- ing the Vegard's law. This diminution is more tained. Rich chromium compositions (x$0.5) form a pronounced in the Cr±Hf series than in the Fe±Hf orthorhombic solid solution that crystallizes in the series, as can be seen in the slope of the lines. These SW-type structure. X-ray powder diffraction patterns results are consistent with the values of the effective of single phase samples at room temperature have ionic radius, as CrIII (r50.615 A)Ê and Fe III (r5 been autoindexed giving orthorhombic solutions 0.645 A)Ê radii are smaller than that of HfIV (r50.71 (space group, Pcnb, Table 1). For the x51.7 materi- A)Ê [26]. al, there is splitting of selected diffraction peaks Electronic spectra for MIII -Hf materials (Fig. 3) indicating a monoclinic symmetry. The powder present the characteristic bands of the octahedral

Table 1

Lattice parameters for Li11xx Cr Hf22x (PO43 ) series ÊÊÊÊ3 ab xa(A) b (A) c (A) V (A ) MF20 20 0.1 8.810(1) 21.960(3) 1476.08 34 29 0.3 ± ± ± ± ± ± 0.5 12.234(3) 8.812(3) 8.719(4) 940.01 20 35 0.7 12.197(2) 8.771(1) 8.681(3) 928.73 31 40 1.0 12.153(4) 8.727(3) 8.637(4) 916.04 17 23 1.3 12.092(1) 8.668(1) 8.602(1) 901.55 41 55 1.5 12.061(1) 8.644(1) 8.590(2) 895.57 23 27 1.7c 11.9625(5) 8.5915(3) 8.5127(4) 874.9(1) ± ± 2.0d 11.8802(2) 8.5483(1) 8.4570(1) 858.8(1) ± ± a Ref. [32]. b Ref. [33]. c Monoclinic sample, b 590.284(3)8. d Monoclinic sample, b 590.420(1)8. E.R. Losilla et al. / Solid State Ionics 112 (1998) 53 ±62 57

Table 2

Lattice parameters for Li11xx Fe Hf22x (PO43 ) series ÊÊÊÊ3 xa(A) b (A) c (A) V (A ) MF20 20 0.1 ± ± ± ± ± ± 0.3 12.269(4) 8.856(3) 8.754(2) 951.17 21 29 0.5 12.248(2) 8.827(1) 8.736(2) 944.47 20 33 0.7 12.225(1) 8.799(1) 8.713(2) 937.28 39 66 1.0 12.186(1) 8.753(1) 8.676(2) 925.43 50 87 1.3 12.144(2) 8.709(2) 8.644(3) 914.23 34 57 1.5 12.111(3) 8.682(3) 8.628(3) 907.21 17 21

Fig. 1. Variation of lattice parameters vs. composition for Fig. 2. Variation of the cell volume vs. composition for Li M Hf (PO ) (M5Cr, Fe) series. m ♦ 11xx 22x 43 Li11xx M Hf22x (PO43 ) series, M5Cr ( ), Fe ( ). Cell volume of d LiHf243 (PO ) from reference [19] ( ). Extrapolated volumes (x5 environment of chromium/iron ions. The 3d3 con- 0) of Cr (n) and Fe (.) samples for a hypothetical SW structure. ®guration of Cr31 has a4 F fundamental state with the ®rst4 P excited state. It can be predicted three licities. According to the high spin d5 con®guration, 44→ 6 spin allowed transitions: n12g2g2: A (F) T (F), n : the fundamental term is A1g (S) and the observed 44→ 44→ 64→ A2g (F) T 1g (F), n 3: A 2g (F) T 1g (P) which ap- transitions correspond to: n11g1g2: A (S) T (G), n : pear at 670, 468 and 300 nm respectively. These 64→ 644→ A1g (S) T 2g (G), n 3: A 1g (S) A 1g (G), E 2g (G) bands have been detected in all materials, although which appear at 700, 515 and 428 nm respectively. n3 band appears occasionally overlapped with the Due to the higher oxidant power of the Fe(III) in typical charger transfer band of LiHf243 (PO ) . There comparison with Hf(IV), L→M charger transfer are some sharp peaks in the n1 band due to well bands are found near the visible region. So, around known Ruby lines, which are spin forbidden transi- 200±400 nm, the charger transfer bands O→Hf (261 4 → tions from A2g (F) fundamental state to the doublet nm) and O Fe (322 nm) are situated. 22 2 states [ T1g (G), E g (G)] of the free ion G state.

When chromium content increases the intensities of 3.2. Crystal structure of Li11xx M Hf22x(PO43)(M5 the band rise too. Cr, Fe) solid solutions Fe±Hf materials show electronic spectra with bands of very low intensity corresponding to forbid- The crystal structure of Li1.8 Fe 0.8 Hf 1.2 (PO 4 ) 3 has den transition between states with different multip- been recently re®ned from synchrotron powder data 58 E.R. Losilla et al. / Solid State Ionics 112 (1998) 53 ±62

with the Hf/Fe ratio. Structural parameters for

Li1.8 Fe 0.8 Hf 1.2 (PO 4 ) 3 and a list of bond distances were given in Tables 1 and 2 of reference [27]. Wang et al. [29] have shown a similar structural IV III behavior for Li11x Ti22xx Ti (PO43 ) (0#x#2) related series where changes in the crystal structures as

a function of x were observed. LiTi243 (PO ) is rhombohedral with the NASICON structure but

Li1.78 Ti 2 (PO 4 ) 3 and Li 2.72 Ti 2 (PO 4 ) 3 are orthorhombic with SW-related structures. The maximum value for x, to maintain the NASICON structure, can be estimated as 0.40 from Fig. 2 of reference [29]. In

the related system, Li11x Ti22xx Sc (PO43 ) , x50.4 was the limit for the existence of the NASICON structure

[12]. For Li11x Ti22xx Cr (PO43 ) , x50.8 was the limit for the NASICON solid solution [30]. Although in these two works, the impurity phases were not identi®ed, the presence of related stoichiometry with SW structure and higher lithium content is very likely. Our research does not support the work of Aono et al. [18] where it was suggested that a NASICON rhombohedral structure for the whole

Li11xx M Hf22x (PO43 ) (M5Cr, Fe, Sc, In, Lu, Y) series (0#x#0.4). For M5Cr and Fe, the existence limits for the NASICON solid solution is much narrower as previously described (x50.2 for Cr and

Fig. 3. Diffuse re¯ectance spectra of Li11xx M Hf22x (PO43 ) system: x50.1 for Fe). (a) x50; (b) M5Cr, x50.7; (c) M5Cr, x51.3; (d) M5Fe, Li1 Cr Hf2 (PO ) with x50.0 and 0.1, have x51.3. 1 xx2 x 43 3 volumes of 1487.54 and 1476.1 AÊ respectively, and crystallizes in the NASICON-type structure. To [27] using the SW structure of orthorhombic compare the unit cell volumes to those of the SW

Li32 Fe (PO 43 ) as starting model [28]. The SW struc- series, it is necessary to renormalize it to a same Z ture has been discussed in a number of previous value (34/6). The resulting volumes, 991.70 and 3 papers [5,7±10,28]. It is worthy to point out that the 984.1 AÊ , are much bigger than those obtained by 33 possibility of a monoclinic structure was tested as it extrapolation from Fig. 2, 961.9 AÊÊ and 957.8 A , has been reported in relation to monoclinic poly- assuming that this composition can crystallize with a morphs, b ¯90.28, for Li32 M (PO 43 ) (M5Sc, Fe). SW-type structure. This is a clue toward understand- The re®nements were unstable with a re®ned b angle ing the origin of the transition between structure of 89.992(5)8. Hence, the orthorhombic metric is types. ensured although we can not discuss the symmetry Based on our results, and those previously re- from powder diffraction data. The quality of the data ported, it can be suggested that the key parameter to was so good that it was possible to re®ne the understanding the structural change from NASICON lithiums positions and occupation factors. Three to SW, with increasing lithium content, is the size of possible Li sites exist in SW and a free re®nement of the cages in both structures. NASICON materials Li(3), at approximately (0.45 0.23 0.30), converged have two sites M1 and M2 of different sizes and to 6(4)%. Hence, it was assumed that the site was multiplicities. The M1 site (one per formula) is empty. Finally, the Li(1)/Li(2) ratio was re®ned coordinated by a trigonal antiprism of oxygens with constrained in such a way that the ®nal value agrees six Li±O bond distances, 2.46 AÊ for M5Hf (and E.R. Losilla et al. / Solid State Ionics 112 (1998) 53 ±62 59 slightly dependent upon the tetravalent metal cation) There is also an abrupt decrease in volume in and assuming that lithium is located at the center of Li11xx Cr Hf22x (PO43 ) series from x51.5 to 1.7 (Fig. the M1 cavity. The M2 site (three per formula) has a 2). This is a consequence of the symmetry change distorted 8-fold coordination with even larger dis- from orthorhombic Pcnb to monoclinic P21 /n (see tances to the neighbouring oxygens 232.41 A,Ê 23 Table 1). We have arbitrarily chosen this non-stan- 2.46 A,ÊÊ 232.68 A and 233.02 A Ê for M5Hf. These dard setting of the space group [60 to allow a direct bond distances are clearly too long for lithium to comparison between orthorhombic and monoclinic satisfy its valence. However, the SW structure has edges. This structural change is due to a lithium three effective sites for lithiums with smaller oxygen ordering/disordering process and it has been re- cages. In the orthorhombic SW ported previously as a function of composition and

Li1.8 Fe 0.8 Hf 1.2 (PO 4 ) 3 compound, the Li environment temperature. Ordered monoclinic structures undergo is four-fold coordinated with distances: Li(1)±O 23 phase transition(s) on heating to orthorhombic dis- 1.78 AÊÊ and 232.16 A, 3 and Li(2)±O 2.05 A, Ê 2.15 ordered structures [28,31]. This slight structural A,ÊÊ 232.28 A [27]. For materials with this change orthorhombic → monoclinic is non-recon-

Lix M243 (PO ) and high lithium content, SW structure structive as the framework of the materials remains is preferred because the lithiums can ful®l their unaltered and there are only minor atomic move- valence more ef®ciently. The limit for the solid ments to coordinate the ordered lithiums more solution depends on both the tetravalent and the ef®ciently. trivalent metals being usually between x50.2 and It has to be underlined that much care has to be x50.5. This behaviour is clearly seen in Fig. 2 as an taken to establish these compositional limits. The abrupt volume decrease with the change between the relationship between the cell parameters of NASICON and the SW-type structures. NASICON and SW structures (both can be derived

Fig. 4. A.C. impedance data for Li1.3 Fe 0.3 Hf 1.7 (PO 4 ) 3 presented in the complex impedance plane (a, c, d) and as impedance and modulus spectroscopic plots (b). 60 E.R. Losilla et al. / Solid State Ionics 112 (1998) 53 ±62 from a cubic cell with a¯12.5 A)Ê means that a in Arrhenius format in Fig. 5. Each data set shows a wrong cell can be derived if not all observables in similar temperature dependence, with an essentially the pattern (peaks and shoulders) are taken into linear region over the temperature range as would be account. Furthermore, the presence of low intensity expected from thermal analysis where no phase SW peaks can be misinterpreted as a monoclinic transitions were detected. distortion of the rhombohedral NASICON unit cell. Activation energies and conductivity values were

3.3. Electrical properties of Li11xx M Hf22x(PO43) (M5Cr, Fe) SW solid solutions

Typical a.c. impedance results for one composition, Li1.3 Fe 0.3 Hf 1.7 (PO 4 ) 3 , are shown in Fig. 4. These data are typical for all compositions. The results show that the pellet is an ionic conductor studied between blocking electrodes. The complex impedance plane representation at 1088C points out a distorted high frequency semicircle of capacitance 24 pF and a little low frequency spike (Fig. 4a). A spectroscopic plot for the same data of the imaginary parts of the impedance, Z0, and the complex electric modulus, M0, against log frequency (Fig. 4b) shows a well-de®ned single peak for the complex impedance. The M0 peak occurs at frequencies .107 Hz and is not fully seen in the spectra, however, the M0 plots show evidence of a shoulder peak at lower frequency at approximately the same position as the Z0 peak maximum. The capacitance value associated with Z0 maxima (C524 pF) is a typical value for a grain boundary phenomenon, so, the semicircle shown in Fig. 4a corresponds mainly to the grain boundary impedance. With increasing temperature, the pellet conductivity rises and the features of the a.c. response are displaced to higher frequencies. Complex impedance plane plots are shown for two higher temperatures in Fig. 4c±d. At the highest displayed temperature, the spike shows evidence of overlapping with an addi- tional semicircle; this semicircle may be associated with a reaction at the electrode±electrolyte interface or with a highly resistive grain boundary or surface layer. The value of the capacitance associated (6 nF) is typical of a double layer capacitance. So, it was not possible to accurately separate the bulk resistance values from the total resistance due to the poorly resolved nature of the semicircles. Thus, only total conductivities have been reported. Con- Fig. 5. Conductivity Arrhenius plots for Li11xx M Hf22x (PO43 ) : (a) ductivity data for various compositions are presented M5Cr, (b) M5Fe. E.R. Losilla et al. / Solid State Ionics 112 (1998) 53 ±62 61

Table 3

Conductivity data of Li11xx M Hf22x (PO43 ) M5Cr, Fe x Cr Fe

21 21 s3008Ca300(S cm ) Porosity (%) E (kJ/mol) s 8Ca(S cm ) Porosity (%) E (kJ/mol) 0.1 3.80.1024 46 46.31 ± ± ± 0.3 ± ± ± 1.16.1024 36 49.21 0.5 6.48.1025 45 55.96 1.30.1024 35 58.86 0.7 9.50.1025 44 55.00 1.50.1024 34 55.96 1.0 1.13.1024 34 55.96 3.41.1024 33 58.86 1.3 1.81.1024 26 51.14 3.94.1024 26 59.82 1.5 3.17.1024 24 44.38 3.92.1024 25 58.86 1.7 5.28.1024 24 53.07 ± ± ±

extracted from the Arrhenius plot and they are listed [7] A. Jouanneaux, A. Verbaere, Y. Piffard, A.N. Fitch, M. in Table 3. It is evident that conductivities increase Kinoshita, Eur. J. Solid State Inorg. Chem. 28 (1991) 683. 31 41 [8] Y. Piffard, A. Verbaere, M. Kinoshita, J. Solid State Chem. with the substitution of M for Hf in the SW 71 (1987) 121. solid solution. Conductivity values are slightly high- [9] S. Oyetola, A. Verbaere, D. Guyomard, Y. Piffard, J. Solid er for iron compounds than for chromium materials. State Chem. 77 (1988) 102. Activation energies are in the range 44.38 to 59.82 [10] V.B. Kalinin, A.M. Golubev, Inorg. Mater. 29 (1993) 581. kJ/mol and do not show a clear trend in the solid [11] J.M. Winand, A. Rulmont, P.J. Tarte, J. Solid State Chem. 93 solution probably due to the dominant effect of the (1991) 341. [12] M.A. Subramanian, R. Subramanian, A. Clear®eld, Solid grain boundary. For x50.1 (M5Cr), the conduc- State Ionics 18±19 (1986) 562. tivity is higher than those of other members of the [13] H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G. Adachi, series because the structure is NASICON instead of J. Electrochem. Soc. 140 (1993) 1827. SW. [14] J. Kuwano, N. Sato, M. Kato, K. Takano, Solid State Ionics 70±71 (1994) 332. [15] F. Sudreau, D. Petit, J.P. Boilot, J. Solid State Chem. 83 (1989) 78. Acknowledgements [16] H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G. Adachi, J. Electrochem. Soc. 137 (1990) 1023. The work in MalagaÂ was supported by the re- [17] K. Ado, Y. Saito, T. Asai, H. Kageyama, O. Nakamura, Solid search grant CICYT MAT97/326-C4-4. We thank State Ionics 53±56 (1992) 723. [18] H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G. Adachi, Prof. Anthony R. West (University of Aberdeen, Solid State Ionics 62 (1993) 309. Scotland) for fruitful comments. [19] E.R. Losilla, M.A.G. Aranda, M. Martinez-Lara, S. Bruque, Chem. Mater. 9 (1997) 1678. [20] P.E. Werner, M. Westdahl, L. Eriksson, J. Appl. Cryst. 18 (1985) 367. References [21] H.M. Rietveld, J. Appl. Cryst. 2 (1969) 65. [22] A.C. Larson, R.B. von Dreele, GSAS (Program version: PC, [1] J.B. Goodenough, H.Y.-P. Hong, J.A. Kafalas, Mater. Res. summer 96), Report No LA-UR-86-748 (1994) Los Alamos Bull. 11 (1976) 203. National Laboratory. [2] H.Y.-P. Hong, Mater. Res. Bull. 11 (1976) 173. [23] P. Thompson, D.E. Cox, J.B. Hastings, J. Appl. Cryst. 20 [3] W. Prandl, Z. Kristallogr. 123 (1966) 81. (1987) 79. [4] A. Zemann, J. Zemann, Acta Cryst. 10 (1957) 409. [24] L.W. Finger, D.E. Cox, A.P. Jephcoat, J. Appl. Cryst. 27 [5] S.C. Abrahams, J.L. Bernstein, J. Chem. Phys. 45 (1966) (1994) 892. 2745. [25] S.I. Berul, N.I. Grishina, Russian J. Inorg. Chem. 16 (1971) [6] A. Leclaire, A. Benmoussa, M.M. Borel, A. Grandin, B. 1674. Raveau, J. Solid State Chem. 78 (1989) 227. [26] R.D. Shannon, Acta Cryst. A32 (1976) 751. 62 E.R. Losilla et al. / Solid State Ionics 112 (1998) 53 ±62

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