New perovskite materials of the La2-xSrxCoTiO6 series
Mercedes Yuste,a Juan Carlos Perez-Flores,*a Julio Romero de Paz,b Ma Teresa Azcondo,a
Flaviano Garcıa-Alvaradoa and Ulises Amadora
Received 3rd February 2011, Accepted 13th May 2011DOI: 10.1039/c1dt10196j
Substitution of La3+ by Sr2+ in the double perovskite La2CoTiO6 yields materials of the La2-xSrxCoTiO6
series showing a significant amount of trivalent cobalt ions when prepared at ambient atmosphere. Theas-prepared compounds can be reduced in severe conditions retaining the perovskite structure whileinducing the formation of a large amount of oxygen vacancies. The limit of aliovalent substitution inthis series was found to extend up to x = 1. For substitution of La3+ up to 15% cobalt and titanium areordered, though the order is progressively lost as x increases; for x ≥ 0.30 no ordering is observed asevidenced by magnetic measurements. The ability of these materials to present either cobalt ions in amixed oxidation state or large amounts of anion vacancies depending on the atmosphere makes theminteresting to be further investigated regarding their electrical and electrochemical properties, andhence, their usefulness in some electrochemical devices.
1. Introduction
In recent years perovskite-like compounds have been extensivelyinvolved in the development of new materials potentially usefulfor technological applications. Their stability in oxidising and re-ducing atmospheres at high temperatures, together with their highelectronic and ionic conductivity, makes perovskites promisingmaterials to be used as components (electrodes, electrolytes andinterconnectors) of several electrochemical devices. For electrolyteapplication ionic conductivity should dominate over electronicconductivity whereas for interconnectors and electrodes the con-trary is needed; good electrodes should also show an adequatecatalytic activity.1,2 Some perovskite-like oxides may display theseproperties since they simultaneously present cations with mixedoxidation states and anionic (oxygen) vacancies.3–7
Among the perovskites, those called “double perovskites” withgeneral formula A2B¢B¢¢O6 present a huge variety of compositionsand properties, related to the capability of the simple perovskitestructure to accommodate different transition metal cations B¢ andB¢¢ of different sizes and electronic structures.4 Besides, partialsubstitutions of cations in the A site open new possibilities,including the induction of anionic vacancies through aliovalentsubstitution.
The use of cobalt in B-positions usually results in high electronicconductivity, due to both its ability to present mixed oxidationstates and the important covalent character of the Co–O bonds.The presence of Co2+ and Co3+ in perovskite-like cobaltites will
aUniversidad San Pablo CEU, Dpto. Quımica, Urb. Monteprıncipe, E-28668,Boadilla del Monte (Madrid), Spain. E-mail: [email protected]; Fax: 34 913510496; Tel: 34 91 3724715bUniversidad Complutense de Madrid, CAI Tecnicas Fısicas, Ciudad Uni-versitaria s/n, E-28040, Madrid, Spain
allow oxygen-deficient oxides to be obtained under reducingconditions (by reduction of Co3+ to Co2+), giving rise to theformation of anionic vacancies and oxygen-ion mobility andinducing oxygen conduction.8–10
In the framework of our research on materials for solid-state fuel cells (SOFCs), in this paper we explore the dopingof La2CoTiO6
11,12 with Sr, as a simple procedure to obtain newmaterials in the La2-xSrxCoTiO6 series. A range of solid solutions(i.e. the values of x giving single-phase samples), a preliminarystructure characterisation performed by X-ray diffraction (XRD)and a detailed study of the effect of the aliovalent substitutionon the cations’ oxidation states determined by chemical titrationand thermo-gravimetric analysis (TGA), are presented. Magneticsusceptibility measurements were used to asses cation ordering andto help in the determination of the actual compounds’ symmetry.Electric and electrochemical characterisation are ongoing and willbe published in due course.
2. Experimental
2.1. Samples
Samples of compositions La2-xSrxCoTiO6 (0 £ x £ 1) were preparedby a modified Peccini method. About 10 g of each sample wasprepared by dissolving stoichiometric amounts of high purityCo(CH3COO)2·4H2O (Aldrich, 99.99%), La2O3 (Aldrich, 99.9%)and SrCO3 (Aldrich 99.9%) in ca. 20 ml of concentrated hotnitric acid (Panreac 66%); ca. 50 ml of distillate water was added.Under heating and vigorous stirring, citric acid was added in amolar ratio of citric acid to metal ions of 3 : 1. Then titanium wasadded as anatase (Aldrich, purity 99.9%) to obtain a homogeneoussuspension, since TiO2 is not soluble. When the solution wasconcentrated to half of its initial volume, 3 ml of diethyleneglycol
was added to promote polymerisation. When a solid resin isformed it is allowed to cool down to room temperature and thenmilled in an agate mortar; the obtained powder was burned at 1073K to remove organic matter. After milling and homogenisation theresulting powder was heated at 1773 K for 24 h and cooled down (ata rate of 2 K min-1) to room temperature to obtain well-crystallisedmaterials.
2.2 Experimental techniques
Purity and structure of the samples were determined by pow-der X-ray diffraction (XRD) on a Bruker D8 high-resolutiondiffractometer, using monochromatic CuKa1 (l = 1.5406 A)radiation obtained with a germanium primary monochromator,and equipped with a solid-state rapid LynxEye detector. Themeasured angular range, the step size and counting times wereselected to ensure enough resolution; the profile fits of theXRD data (Le Bail method) were performed using the FullProfprogram.13
analyser for energy dispersive spectroscopy (EDS).The cobalt oxidation state (and hence the oxygen content
assuming charge neutrality) of La2-xSrxCoTiO6 samples wasdetermined by titration using potassium dichromate. For eachanalysis five parallel experiments were carried out. The titrationmethod is based on dissolution of the sample in acidic solutionand subsequent reduction of the trivalent cobalt species with anappropriate reducing agent (divalent iron). To prevent oxidationof the reducing agent by atmospheric oxygen, the acidic solutionused as a solvent and titration medium was freed from dissolvedoxygen before each experiment by Ar bubbling. Also, it wasimportant to perform the titrations in an air-tight cell under anAr atmosphere. Typically, ca. 50 mg of the sample studied and anexcess of (NH4)2Fe(SO4)2·6H2O (Mohr salt) (ca. 120 to 180 mg)were dissolved in 20 ml of 12 M HCl solution at room temperature.Afterwards, 75 ml of distilled water and 3 ml of phosphoric acidwere added. This results in reduction of the present trivalentcobalt species of the sample to Co2+ ions and the formation of thestoichiometric amount of trivalent iron. The remaining Fe2+ ionsare then titrated using a 5 ¥ 10-3 M solution of K2Cr2O7 (typicallybetween 14 and 20 ml are needed). As an indicator, two drops of a0.01 M solution of tris(5,6-dimethyl-1,10-phenanthroline) iron(II)sulphate were used; the end-point was detected visually as thesolution changes from red to yellow-green, the end-point is clearlyobserved since phosphoric acid forms a colourless complex withtrivalent iron. This method gives a reproducibility of ±0.3 whenthe amount of Co3+ is expressed as percentage of total cobalt.
The cobalt oxidation state was furthermore determined inde-pendently by heating the samples under a reducing atmosphereto fully convert Co3+ to Co2+. For this purpose thermogravimetricanalysis was carried out by using a D200 Cahn Balance. Typicallyca. 70 mg of the sample were weighed to a precision of ±0.0005 mgat a total reduced pressure of 400 mb containing 60% He and 40%H2. Afterwards the sample was heated up to 1173 K at a rate of5 K min-1. In these conditions a pO2 of about 10-28 atm. is reached.
Magnetic measurements were performed using a SQUID mag-netometer (Quantum Design, model MPMS-XL). The temper-ature dependence of magnetization (M) was measured in the
temperature range 2–300 K at an applied magnetic field (H) of 0.1T upon heating samples under zero-field-cooled conditions from 2K (previously cooled at H = 0 T). The experimental molar magneticsusceptibility c (M/H) was calculated on the basis of the samplemass (~100 mg) and the molecular weight. The room-temperature(RT) effective magnetic moment (meff) was determined for eachsample as follows: magnetization was measured at 300 K as afunction of the applied magnetic field giving a linear dependencedescribed by the well-known equation M = cH correspondingto a paramagnetic behaviour; then, meff was determined fromthe slope, i.e. the magnetic susceptibility, using the conventionaldefinition meff = (8cT)
12 . Measurements of a second batch of
samples confirmed a good reproducibility of the values of themeff in all cases.
3. Results and discussion
3.1. Solid solution and crystal structure
As determined by XRD La2-xSrxCoTiO6 forms a solid solutionwith a compositional range 0 £ x £ 1. All of the samplesprepared were confirmed to be single-phase by EDS, the chemicalcompositions being close to the nominal ones within the techniqueerror. Even more, the distribution of all the constituting elementswas confirmed to be homogeneous along the samples. As anexample Fig. 1 shows the back-scattered electron (BSE) imageof the end-member of the series, LaSrCoTiO6, together with thecorresponding element-distribution maps. Since the contrast inBSE image is sensitive to the average composition (light elementsgive dark contrasts, and vice-versa) Fig. 1a already suggests ahomogeneous composition (dark zones in this figure correspondto holes in the pellet surface due to poor sinterization). This isconfirmed by the element-distribution maps shown in Fig. 1b to1f. It seems that some over-concentration of Sr and Co could beobserved in the grain boundaries, but most likely this is an artefactdue to the surface morphology since no segregation is observedfor any of the other elements.
The initial compound, x = 0, La2CoTiO6, is confirmed to showa monoclinic symmetry with space group P21/n (#14) with aunit cell
√2ap ¥
√2ap ¥ 2ap where ap refers to the cell of the
cubic simple perovskite, as previously reported.11,12 The XRDpatterns for other members of the series with increasing levelsof strontium substitution can be also assigned to monoclinicsymmetry. However, since the monoclinic angle, (b), is in all casesvery close to 90◦, higher symmetry (namely orthorhombic S.G.Pnma, #62) cannot be discarded. It must be stressed that the reasonfor the lowering in symmetry of double perovskites, A2B¢B¢¢O6 suchas the title compounds, is mainly due to the ordering of Ti and Coin the B¢ and B¢¢ positions of the structure. In the orthorhombicmodel there is only one crystallographic site 4b (0 0 1
2) for B cations,
i.e. the symmetry does not allow for an ordered arrangement of theB cations. On the contrary, in the monoclinic model the 4b site inPnma can be split into two independent crystallographic sites, 2d( 1
20 0) and 2c ( 1
20 1
2), which allow an ordered arrangement of the B-
cations. Since the X-ray scattering factors of Co and Ti are almostidentical, XRD is not suited for determining the actual symmetryof these perovskites. Fortunately, magnetic measurements help usto determine the existence of order for Co and Ti ions, and hencethe actual symmetry of these oxides. Thus, on the basis of magnetic
Fig. 1 (a) Back-scattered electrons (BSE) image taken at a magnification of ¥1000 and element-distribution maps of as prepared LaSrCoTiO6 sample:(b) La, (c) Sr, (d) O, (e) Co and (f) Ti.
measurements one can assume that La2-xSrxCoTiO6 compoundspresent monoclinic symmetry for 0 £ x < 0.30 whereas oxides with0.30 £ x < 1.00 have orthorhombic symmetry.
The cell parameters of both the as-prepared and reduced seriesare given in Tables 1 and 2, respectively (see below). Fig. 2aand 2b show the graphic results of the fitting of XRD data tothe monoclinic or orthorhombic cells presented in Table 1 forLa2CoTiO6 and La1.50Sr0.50CoTiO6, respectively.
Interestingly, the unit cell volume decreases as x increasesalong the as-prepared La2-xSrxCoTiO6 series, as shown in Fig. 3.This seems not to be in agreement with what is expected onthe basis of ionic sizes: since the radius of Sr2+ (XIIr = 1.44 A)is larger than that of La3+ (XIIr = 1.36 A) for the same co-ordination14 one would expect the unit cell to expand as lanthanumis replaced by strontium. Even more, for x values high enough,Co3+ and Ti4+ are randomly distributed among the B¢ and B¢¢ sites
Table 1 Unit cell parameters for as-prepared La2-xSrxCoTiO6 obtained from XRD data
Fig. 2 Graphic results of the fitting of XRD data for (a) La2CoTiO6
to a monoclinic cell, and (b) La1.50Sr0.50CoTiO6 to an orthorhombic cell(see Table 1). Experimental data (circles), calculated pattern (continuousline) and their difference (continuous line at the bottom) are plotted; thepositions of the Bragg reflections are indicated by vertical bars.
Fig. 3 Unit cell volume as a function of the strontium content, x, for theas-prepared La2-xSrxCoTiO6 and reduced series La2-xSrxCoTiO6-d . Linesare guides for the eye.
(see magnetic properties below), thus extra repulsions betweentetravalent titanium cations appear in the structure due to Ti4+
located at antisite B-positions; this should result in the volumeincreasing with x.15 Therefore, other mechanisms must operate inthe materials to overcome these effects. The most obvious ones areschematised in eqn (1) and (2).
xSrO (La2CoTiO6) → xSr¢La + xCo∑
Co (1)
xSrO (La2CoTiO6) → xSr¢La + x/2 V ∑∑
O (2)
According to eqn (1) Co2+ ions oxidise into Co3+ to compensatefor the loss of positive charge induced by substituting trivalentLa3+ ions by divalent Sr2+ ones. Another charge-compensation
mechanism is described in eqn (2): the creation of anion vacanciestends to equal the loss of positive charge due to aliovalentreplacement. Obviously these two mechanisms can simultaneouslyoperate in a given material. As Co3+ ions are smaller than Co2+
the effect of cobalt oxidation on the unit cell volume is evident.However, how the creation of oxygen vacancies results in a volumecontraction is not so evident. One could think that the creationof oxygen-vacancies to compensate the substitution of La3+ bySr2+ should produce an expansion of the unit cell. Indeed, a lossof Madelung energy is induced in the material upon doping, dueto both the lowering of positive charge in the A-sites and thepresence of anionic vacancies (loss of negative charges). As a result,the bonding energy decreases and the unit cell should expand.However, very often the simple ionic model does not work in realmaterials and more elaborate approaches are needed to explainthe effect of the creation of oxygen vacancies.16–18
As will be discussed below, magnetic measurements, chemicaltitration and thermogravimetric analyses demonstrate that oxida-tion of Co2+ to Co3+, (eqn (1)) is the main charge-compensatingmechanism, being the reason for the volume contraction alongthe series La2-xSrxCoTiO6 as x increases. Interestingly, whichof the charge-compensating mechanisms actually operates in agiven material is related to the nature of the atoms present; forinstance, in similar double-perovskites La2-xSrxNiTiO6 we foundthat substitution of La3+ by Sr2+ results in the creation of a largeamount of oxygen vacancies and only a small fraction of nickel isoxidised to the trivalent state.17
By reducing the title compounds (see TGA analyses in theExperimental Section) the resulting materials retain the perovskitestructure and no significant decomposition is observed; however,the magnetic measurements of the reduced samples (see below)revealed the presence of small amounts of metallic cobalt. As anexample, Fig. 4 shows the XRD pattern of the residue of reductionof La1.30Sr0.70CoTiO6. Fig. 3 also shows the unit cell volume forthe residues of the thermogravimetric analyses; a remarkableexpansion of the unit cell is produced upon reduction. This isexpected on the basis of the sizes of the divalent and trivalentcobalt ions; even more, the bigger the degree of substitution ofLa3+ by Sr2+, the larger the unit cell expansion. As for the startingmaterials, the size of the cell decreases as the degree of aliovalentsubstitution increases, though in the reduced oxides this trendis less pronounced than in the starting series. The fact that the
Fig. 4 Graphic result of the fitting of XRD data for reducedLa1.3Sr0.7CoTiO6-d to an orthorhombic cell (see Table 2). Experimentaldata (circles), calculated pattern (continuous line) and their difference(continuous line at the bottom) are plotted; the positions of the Braggreflections are indicated by vertical bars.
cell volume diminishes with increasing x along the reduced seriesLa2-xSrxCoTiO6-d (where only Co2+ is present and d increases withx, see below) supports the idea that the simple ionic model doesnot always operate in real materials.16,18
3.2. Magnetic characterisation
As stated above, XRD is not suitable for determining the existenceof order of Co and Ti ions in the B¢ and B¢¢ perovskite-sites.However, it is well established that ionic ordering in the B-sitesstrongly affects the magnetic properties of double perovskites.19
For instance, in the system La2Co(MgxTi1-x)O6 magnetic suscep-tibility measurements reveal antiferromagnetic order below ~15K for x = 0 and 0.1 and below ~8 K for x = 0.2, whereasfor x = 0.3 magnetic order disappears.19 Neutron and selectedarea electron diffraction data reveal a symmetry change frommonoclinic (x £ 0.25) to orthorhombic (x ≥ 0.3) associated withthe lack of magnetic order. Therefore, we assume that there exists asimilar correlation between the onset of low-temperature magneticordering and an ordered arrangement of the B-cations in the seriesof compounds La2-xSrxCoTiO6.
Fig. 5a shows the thermal behaviour of the magnetic suscep-tibility (c) for the as-prepared La2-xSrxCoTiO6 members of theseries with x £ 0.30. For the parent oxide La2CoTiO6, x = 0, amaximum appears around 16 K, revealing an antiferromagneticorder below this temperature. For increasing values of x, theordering temperature lowers; thus for x = 0.10, 0.15, 0.20 and0.25 the materials order at around 15, 10, 8 and 4 K, respectively.For La1.7Sr0.3CoTiO6, no maximum is observed suggesting thelack of magnetic order. All the samples of the series with highdegrees of lanthanum substitution, i.e. x ≥ 0.30, display the sameparamagnetic behaviour from RT down to 2 K. As an example,Fig. 5b shows the temperature dependence of meff and c-1 (inset†)for La1.5Sr0.5CoTiO6, x = 0.5, which agrees with what is expectedconsidering the presence of paramagnetic high-spin Co2+ and Co3+
cations, both of them six-coordinated.20,21 These results suggestthat substitution of La3+ by Sr2+ induces disorder in the perovskiteB-sites; thus in the undoped compound Co and Ti would becompletely ordered (some degree of anti-site defects is possible).The order is progressively lost or limited to short-range as La3+ issubstituted by Sr2+, finally for a degree of substitution as lowas 15%, cobalt and titanium are randomly distributed amongthe B-sites and magnetic order disappears. On the other hand,since the symmetry of these double perovskites is related to theorder in the perovskite B-sites, one can assume that materialsof the La2-xSrxCoTiO6 series present monoclinic symmetry (S.G.P21/n, #14) for x < 0.30, whereas in the compositional range 0.30£ x £ 1 the symmetry is orthorhombic (S.G. Pnma, #62). Thisassumption is used to fit the corresponding XRD patterns, givingthe unit cell parameters in Table 1. Of course, neutron diffractionwould provide direct evidence to support this hypothesis; theseexperiments are planned and their results will be published in duecourse.
Trivalent cobalt ions are d6-cations that can adopt different spin-states in an octahedral environment: first, a nonmagnetic (S =0) low-spin (LS) state (t2g
6eg0) with 1A1g ground term; second, a
† The continuous line in the inset of Fig. 5b corresponds to a Curie–Weissfit (c = C/(T - q)) in the temperature range from 150 to 300 K.
Fig. 5 (a) Temperature dependence of the magnetic susceptibility (c) forsome representative members of the as-prepared La2-xSrxCoTiO6 series(x £ 0.3); (b) Temperature dependence of meff for La1.5Sr0.5CoTiO6, theexpected values for non-interacting octahedral high-spin divalent cobaltcations are indicated; in the inset a Curie–Weiss behaviour is depicted.
magnetic (S = 2) high-spin (HS) state (t2g4e2
g) with 5T2g groundterm, which expected room-temperature meff value ranges from 5.1to 5.7 mB/Co3+; and, third, a magnetic (S = 1) intermediate-spin(IS) state (t2g
5eg1) with 3T1g ground term, which expected room-
temperature meff value can be between 2.90 and 3.62 mB/Co3+
(spin–orbit constant l = -290 cm-1).22 The existence of thesedifferent spin states arises from the competition between crystal-field effects and on-site Coulomb interaction (coupling energy).At this point it must be remarked that for Co3+, which in a givencrystal-field environment (octahedral in this case) can occur indifferent spin-states, transitions between them are possible bythermal excitation in some compounds. A prominent exampleshowing this phenomenon is LaCoO3, which has been widelyexamined and discussed.23,24 With these ideas in mind it is worthtrying to determine the spin-state of cobalt ions in the title samples.Thus, coming back to Fig. 5b, the thermal evolution of meff helpsus to assess not only the oxidation state of cobalt but also givessome indications on the actual spin states of the Co3+ ions presentin this material (it is worth noting that the behaviour of the otherparamagnetic samples is similar to that presented in Fig. 5b). AsTG measurements and chemical analyses demonstrate half of thecobalt present in La1.50Sr0.50CoTiO6 is in its trivalent state; the
presence of Co3+ in this compound is confirmed by the RT valueof meff since the experimental value (3.75 mB/f.u.) is far below thatexpected for Co2+ (in the undoped material a meff/Co of 4.81 mB wasobtained). Concerning the spin state of these trivalent cobalt ionsthe low-temperature meff as well as its evolution with temperatureshed light on this point. Indeed, the actual value of meff obtainedfor La1.50Sr0.50CoTiO6, at 2 K, (1.63 mB/f.u.), is noticeably low; thatvalue is not compatible with the presence of six-coordinated LS(diamagnetic) Co3+ and high-spin Co2+ cations in a 1 : 1 ratio. Thelowest possible value at 2 K for such model is 2.28 mB/f.u. since fordivalent cobalt the lowest expected value of the low-temperaturemoment is 3.22 mB/Co2+ or cT = 1.3 emu K/(Oe mol Co2+).20 Onthe other hand, the existence of HS-Co3+ ions can be ruled out bothby the value of the RT-meff (this should be about 5.0 mB/f.u., seeFig. 6) and by the thermal behaviour of meff plotted in Fig. 5b. Inconnection to this last point it must be stressed that although themeff of HS-Co2+ and HS Co3+ decay on cooling the experimentallow-temperature, meff is not compatible with the expected value,about 4.1 mB/f.u. for such ions (for HS-Co3+ meff at 2 K is about4.90 mB/Co3+ or cT about 3.00 emu K/(Oe mol Co3+).21,22 Thefinal hypothesis is based on considering IS-Co3+; assuming thatthe abrupt decay of the meff at low temperature is straightforwardlyexplained since the meff of these IS-Co3+ cations is expected to fallto zero at low temperatures.21,22 To account for the exact valueof the meff at any given temperature it must be considered thata reduction of the meff of HS-Co2+ is observed along the wholeLa2-xSrxCoTiO6 series, (as will be explained below) as well as thata thermal equilibrium between LS-Co3+ (the lowest energetic state)and IS-Co3+ is also possible; this makes these compounds morecomplex and interesting. In this sense magnetic measurementsof the reduced samples, in which only Co2+ is present, would givevaluable information. Unfortunately, on reduction, metallic cobaltis formed which precludes any magnetic characterisation due toits ferromagnetic nature.
Fig. 6 RT meff = (8cT)12 as a function of x for the as-prepared
La2-xSrxCoTiO6 series. The upper solid line give the expected values for amixture of HS-Co2+ (4.81 mB/Co2+) and HS-Co3+ (5.10 mB/Co3+); the lowersolid line corresponds to HS-Co2+ and non-magnetic Co3+. For the valuesin the shaded area Co3+ is in IS state.
Fig. 6 shows the room temperature meff as a function of the Srcontent for the as-prepared materials. For the parent (undoped)
material the meff takes a value of 4.81 mB/f.u., which is into thenormal range observed for octahedral Co2+ (4.65–5.22 mB/Co2+)in a HS ground state (S = 3/2).20 As La3+ is substituted by Sr2+ theroom-temperature meff decreases and, although thermogravimetricand chemical analyses (see below) demonstrate that Co2+ ions areoxidised to a trivalent state upon aliovalent substitution (i.e. thecharge-compensating mechanism given in eqn (1) operates), theexplanation for this result is not straightforward. If we consider4.81 mB as the magnetic contribution per Co2+ along the seriesLa2-xSrxCoTiO6, the RT meff should increase with the presence ofHS-Co3+ as x increases. Such a variation is indicated in Fig. 6 by theupper solid line, which has been calculated considering 5.10 mB perHS-Co3+. In the case of the presence of LS-Co3+ (nonmagnetic),the room-temperature meff should decrease as x increases followingthe lower solid line. Finally, the presence of IS-Co3+ should resultin decreasing meff values, which will be located in the shaded areain Fig. 6, defined by considering the meff interval for IS-Co3+ ionsmentioned above. For the composition x = 0.1 the obtained room-temperature meff, 4.61 mB, is between the LS solid line and thelower limit of the IS range; this may indicate a mixture of LSand IS-Co3+ or LS and HS-Co3+. For the compositions 0.15 £ x£ 0.30 the room-temperature meff values are located below the LSline, which points to a decrease of the meff of HS-Co2+ below thevalue determined for the parent oxide, 4.81 mB/Co2+, and used toplot the Fig. 6. Therefore, the substitution of La3+ by Sr2+ seemsto produce a reduction of the room-temperature meff per Co2+,which can be due to a greater covalency of the cobalt-oxygenbonds, a stronger ligand field and/or a more important distortionof the [CoO6] octahedra.21 Such a reduction effect hinders theinambiguous determination of the spin-state of the Co3+ cations.In the case of compositions x = 0.4, 0.5 and 0.7; the experimentalRT-meff values are between the LS solid line and the lower limitof the IS range, thus mixtures of LS and IS-Co3+ or LS andHS-Co3+ may be possible. However, the magnetic behaviour atlow temperature of these paramagnetic samples discussed above,suggests that the LS and IS-Co3+ model is the most likely situation.This extends to the paramagnetic oxide with x = 0.3 if the decreaseof the meff of HS-Co2+ is assumed. Finally, for LaSrCoTiO6, theRT meff reaches a value of 2.97 mB/f.u., which is in the expectedrange for octahedral Co3+ in an IS ground state (S = 1, 2.90–3.62 mB/Co3+). To some extent this end-member of the series isquite different, since the thermal evolution of meff (not shown)revealed a ferromagnetic-like behaviour at high temperature(200 K) which deserves the detailed study we are currentlyperforming.
3.3. Cobalt oxidation state
Fig. 7 shows the evolution of the amount of Co3+ as a function ofthe quantity of La3+ substituted by Sr2+ (x in La2-xSrxCoTiO6)as obtained from both redox titration and thermogravimetricanalysis. The solid line corresponds to the expected valuesassuming charge neutrality for stoichiometric compounds (i.e. nooxygen vacancies or d = 0). The amounts of trivalent cobalt asobtained by both independent techniques fully agree; giving bothtechniques conclusive evidence to support the presence of trivalentcobalt in doped compounds; as an example, Fig. 8 shows the TGanalyses for some materials (x = 0.00, 0.10, 0.30, 0.70 and 1.00).Fig. 8 and the d values in Table 2 clearly suggest that the amount
Fig. 7 Expected (solid line) and experimental amounts of Co3+ asdetermined by TGA (triangles) and chemical analyses (circles) as afunction of the strontium content for the as-prepared La2-xSrxCoTiO6
series.
Fig. 8 Experimental results of the thermogravimetric analyses for as-pre-pared materials La2-xSrxCoTiO6 (x = 0.00, 0.10, 0.30. 0.70 and 1.00).
of removable oxygen, associated to trivalent cobalt, increases asthe degree of lanthanum substitution by strontium does.
Two main conclusions can be obtained from the data in Fig. 7:(a) since the average oxidation state of cobalt (i.e. the amountof trivalent ions) increases as x does, the charge-compensatingmechanism of aliovalent substitution (La3+ by Sr2+) proposed ineqn (1) operates in an increasing way along the whole series; (b)since the experimental values and those expected (solid line) onthe basis that no oxygen vacancies are created fully agree (withinthe experimental errors) it can be concluded that oxidation of Co2+
is by far the main effect of lanthanum substitution. This result iscompletely different to what is found in a similar series of doubleperovskites La2-xSrxNiTiO6 in which the amount of trivalent nickelremains very low (ca. 0.3% to 3%) along the whole series.17 This isundoubtedly related to the stability of trivalent state of 3d metals:high oxidation states of cobalt are very common whereas for nickelthey are less usual.
4. Conclusions
New perovskite materials have been prepared by substitution ofLa3+ by Sr2+ in the double perovskite La2CoTiO6. The existencerange of the series La2-xSrxCoTiO6-d was found to extend up to x =1. For substitution degrees of La3+ up to 15% cobalt and titaniumare ordered, though the order is progressively lost as x increases;for x ≥ 0.30 no ordering is observed as evidenced by magneticmeasurements.
An interesting finding is that the aliovalent substitution resultsin the oxidation of the equivalent amount of Co2+ to Co3+. Thespin state of such Co3+ cations is actually a mixture of low andintermediate-spin, although for the last member of the series,LaSrCoTiO6, the intermediate-spin is likely the ground state. Onthe other hand, under reducing conditions the hole defects, Co3+,are annihilated while oxygen vacancies are produced. This mech-anism makes possible the structural stability of the perovskitesLa2-xSrxCoTiO6-d even under severe reducing conditions (pO2 ª10-28 atm). The ability of these materials to present either cobaltions in a mixed oxidation state or large amounts of anion vacanciesdepending upon the atmosphere makes them interesting to befurther investigated regarding their electrical and electrochemicalproperties, and hence, their usefulness in some electrochemicaldevices.
Acknowledgements
We thank the Ministerio de Ciencia e Innovacion and Comu-nidad de Madrid for funding the projects MAT2007-64486-C07-01, MAT2010-19837-C06-01, PIB2010JP-00181 and S2009/PPQ-1626 respectively. Financial support from Universidad San Pablois also acknowledged.
We thank Mr Paco Gomez from CTR Repsol for helping uswith the SEM studies.
Notes and references
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