n - American Chemical Society

Article pubs.acs.org/cm

[BaCoO3]n[BaCo8O11] Modular Intergrowths: Singularity of the n = 2 Term Rénald David,† Alain Pautrat,‡ Houria Kabbour,† Mihai Sturza,† Sergiu Curelea,‡ Gilles André,§ Denis Pelloquin,‡ and Olivier Mentré*,† †

Université Lille Nord de France, UCCS, CNRS UMR 8181, ENSCL-USTL, Villeneuve d’Ascq, France Laboratoire CRISMAT, UMR 6508-CNRS, ENSICAEN, Caen, France § Laboratoire Léon-Brillouin (LLB), CEA-Saclay, bât. 563-91191 Gif-sur-Yvette Cedex, France ‡

S Supporting Information *

ABSTRACT: The structure−property relationships for the n = 2 member (Ba3Co10O17) of the [BaCoO3]n[BaCo8O11] modular family have been studied by comparison to the n = 1 term (Ba2Co9O14). It orders antiferromagnetically at TN = 62 K with spin ordering in the common [BaCo8O11] units similar to that for n = 1. In the sandwiching perovskite [BaCoO3]n blocks, the magnetic interactions are modified from n = 1 to 2 because of the large moment on the extra (Co3) cobalt atoms, while no local moment was detected on the perovskite modules for n = 1. For n = 2, it highlights the determinant role of Co3 in the magnetic paths and magnetocrystalline anisotropy. The transport and magnetic measurements on single crystals show a strong anomaly at ∼160−170 K. Below this temperature, the conductivity regime changes from a semiconducting behavior to a variable-range hopping conduction type. Possible symmetry losses have been investigated at 100 K. A trigonal-to-monoclinic distortion is proposed but remains highly hypothetical. For both n = 1 and 2, magnetic/crystallographic considerations together with density functional theory calculations allow one to assume an almost satisfied charge Co2+/Co3+ ordered distribution between all of the cobalt positions (ideal Co 2.7+ mean valence). However, hole doping appears the most probable feature to satisfy the Co 2.8+ deduced from the oxygen stoichiometry assigned from neutron diffraction. KEYWORDS: Ba3Co10O17, Ba2Co9O14, intergrowth, magnetic structure, charge distribution, VRH

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suggest metastable states arising from the magnetic frustration between columns.8 In layered compounds, it was shown that the decoupling between CoO2 slabs in the layered Ising BaCo2(AsO4)2 also leads to an M(H) plateau, while the CoII magnetic moments reorient under an external field.9,10 Similar results have been obtained for Ban+1ConO3n−1Br (n = 5 and 6) in which magnetic blocks are spatially and magnetically isolated by central [Ba2O2Br] double layers and show a spin-flop transition into a ferromagnetic state under an external field. 11 Taking into account phenomena similar to those of other different structural types such as CoV2O612 and SrCo6O11,13 it appears that pertinent single-phased cobalt oxides would likely emulate spintronic heterosystems such as a spin valve with efficient spin polarization across tunneling barriers.14 One major interest relies on the replacement of epitaxial heterogeneous systems by single-phase nanoscaled films, as suggested in prior works.15 Within the search for new Co-based structural forms, several compounds that obey the general formula [BaCoO3]n[BaCo8O11] have recently been isolated.16−18 Here, the perovskite blocks of thickness n [BaO3]

INTRODUCTION A diversity of cobalt oxides have retained considerable attention in the last decades because of various specific physical properties assorted with versatile structural types. Thus, in the field of solid-state electrochemistry, layered LiCoO 2 show remarkable Li+ intercalation properties, 1 while Co-based perovskites possess important mixed ionic/electronic conductivity attractive for solid oxide fuel cell cathodes. 2 This general interest has been renewed by the discovery of both superconductivity3 and attractive thermoelectric power (TEP)4 in layered NaxCoO2 bronzes, in their hydrate derivatives, and in the related misfit Ca3Co4O9−δ,5 which highlight the excellent transport properties of mixed CoII/III and CoIII/IV systems. It is striking that particular magnetic properties can be accessed by taking into account the important magnetocrystalline anisotropy (MCA) of Co cations as well as their ability to exhibit various electronic configurations, namely, high spin (HS), low spin (LS), and intermediate spin (IS) depending on the temperature, pressure, or local chemical environment. 6 In fact, it is such that several Co-based compounds show particular magnetization steps arising from MCA effects. For instance, such phenomena in the columnar Ca3Co2O6 have been explained by the quantum tunneling of magnetization due to 1D macrospins,7 while more recent Monte Carlo simulations © 2011 American Chemical Society

Received: July 22, 2011 Revised: October 21, 2011 Published: November 11, 2011 5191

dx.doi.org/10.1021/cm202106t | Chem. Mater. 2011, 23, 5191−5199

Chemistry of Materials

Article

Figure 1. Panorama of [BaCoO3]n[BaCo8O11] intergrowths [n = 1 (a), 2 (b), and 5 (c)] and related compounds (d). X = Cl and Br in the Ba2Co4XO7 case. resistivity measurements, a large excess of K2CO3 was added and the mixture was heated to 850 °C for 72 h and cooled down to room temperature (5 °C/h down to 600 °C and then rapidly). The product was finally washed with boiling water to remove K2CO3. Energydispersive spectrometry (EDS) analysis of single crystal Ba3Co10O17 has been performed (see Figure S1 in the Supporting Information), and no trace of potassium has been detected. Single-Crystal XRD and Powder ND. Room temperature and 100 K single-crystal XRD experiments were carried out on a X8 Bruker SMART APEX diffractometer using Mo Kα radiation. Intensities have been extracted and corrected from Lorentz polarization. An empirical absorption correction was then applied using the SADABS program,20 and the refinement was processed using Jana 2000.21 High-resolution powder ND analysis was carried out at the LLB. The magnetic structure has been determined from data collected with G41 (λ = 2.426 Å) and 3T2 (λ = 1.2254 Å) diffractometers. Refinements were carried out using the Fullprof program.22 Transport and Magnetic Measurements. In-layer and perpendicular resistivity measurements were performed using a Quantum Design PPMS with two different four-probe configurations for ρ ab and ρ c (see Figure S2 in the Supporting Information). Magnetic measurements were carried out with a MPMS Squid magnetometer (Quantum Design). Typical measurements were performed using zero-field-cooling (ZFC) and field-cooling (FC) procedures under a 0.1 T field on two different samples (single crystals and polycrystalline). Computational Methods. DFT calculations were performed using the Vienna ab initio simulation package (VASP).23 Calculations were carried out within the generalized gradient approximation (GGA) for the electron exchange and correlation corrections using the Perdew−Wang24 functional and the frozen-core projected wavevector method.25 A planewave energy cutoff of 400 eV, a total energy convergence threshold of 10−6, and 50 k points in the irreducible Brillouin zone were used.

layers sandwich central [BaCo 8O11] blocks. The oxides Ba2Co9O14 and Ba3Co10O17 correspond to the n = 1 and 2 terms,16,17 while the partial incorporation of Ga in the samples yield the n = 5 member, that is, Ba6(Ga,Co)13O2618 (see Figure 1). These series deserve attention at several levels if one considers the targeted specificities mentioned above. First, [BaCo 8O11] contains CdI2-like layers at the origin of most of the fascinating physical particularities mentioned for layered cobaltites (transport, thermoelectricity, MCA, and frustrated magnetism). Second, it was shown by neutron diffraction (ND) that, in the n = 1 term, [BaCo8O11] units are ferrimagnetic and order antiferromagnetically by supersuperexchanges (SSEs) across the diamagnetic perovskite subunits (LS CoIII, S = 0).17 We also note that magnetization steps have been observed in the preliminary characterization of Ba2Co4XO7 (X = Cl, Br), which contains the [BaCo8O11] units separated by [Ba3X2O3] double layers.19 This suggests the possibility for even weakest interblock exchanges through thicker perovskite [BaCoO3]n spacers, e.g., by increasing the n value from n = 1 to 2. Finally, the n = 2 term remains mainly uncharacterized considering the preliminary reports of its transport and magnetic measurements performed on multiphased samples.16 In fact, the chemical difficulties encountered to produce the pure n = 2 compound (Ba3Co10O17) are limiting factors for routine characterizations. Here we used single crystals and multiphased polycrystalline materials. Our results stem from X-ray diffraction (XRD) and ND, transport and magnetic measurements, and density functional theory (DFT) calculations.

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EXPERIMENTAL SECTION

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Synthesis. In order to prepare a large amount of Ba3Co10O17 polycrystalline sample for ND experiments, a stoichiometric mixture of BaCO3 and Co3O4 was ground in an agate mortar, then placed in an alumina boat, and heated to 950 °C (100 °C/h) for 72 h. The mixture could not be formed pure, and a consequent amount of Ba 2Co9O14 was systematically detected. To elaborate large single crystals for

RESULTS AND DISCUSSION Transport Properties. The electric conductivity of Ba3Co10O17 measured on a polycrystalline sample has been briefly reported in ref 15 and indicates a semiconducting behavior. 5192



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However, the anomaly mentioned at ∼160 K was not discussed, maybe because of the traces of Co3O4 and BaCoO3 present in the sample. As mentioned in the Experimental Section, our synthesis conditions never lead to a single-phase polycrystalline sample of this compound except of the n = 1 term such that transport properties have been performed on single crystals. The in-layer resistivity ρ ab and perpendicular resistivity ρ c are shown on Figure 2. As a general statement, it

Figure 3. In-plane (ab plane) MR at 14 T with B parallel to the c axis. Inset: in-plane resistance under 14 T.

negative orbital MR in the B//ab configuration. It is consistent with our experimental results here showing, within our resolution, no negative MR for B//ab (see Figure 4a). Note,

Figure 2. Single-crystal perpendicular (black) and in-plane (red) resistivity versus the temperature (K). The inset shows the ρ c/ρ ab ratio as a function of T.

turns out that Ba3Co10O17 displays a higher resistivity along the c axis than in the ab plane. As shown in the inset of Figure 2, the ratio ρ c/ρ ab is around 3.2 from room temperature down to T* ∼ 160 K and then increases notably to reach more than 102 at 70 K. This gives 1 order of magnitude of the electronic anisotropy γ of the sample. At T*, a resistive transition discernible on both ρ c and ρ ab occurs but is more pronounced in plane, where the increase of ρ ab is 1 order of magnitude. ρ ab(T) shows two distinct regimes at both sides of T*. From room temperature to 150 K, the resistivity is characteristic of a thermally activated semiconducting system [ρ = A exp(−Δ/kT)] with a low gap, Δ = 0.0592 eV, according to the good linearity of the Arrhenius plot. Below T*, the temperature dependence of ρ ab becomes more pronounced, which suggests a T −1/n (n > 1) dependence for log(ρ). To distinguish between the Efros− Shklovskii model26 [log(ρ) ∝ T −1/2; coulomb gap with strong interelectronic interaction independent of the system dimensionality] and a Mott variable-range hopping (VRH) model27 [log(ρ) ∝ T −1/(D+1); hopping between Anderson localized states], we plot the Zabrodskii parameter28 W = −T d ln(ρ)/dT with a slope corresponding to the exponential −1/n coefficient. Here we find −1/n = −0.31(4) as for a D = 2 bidimensional Mott VRH system well compatible with the above-mentioned anisotropy of ρ. This localized origin is at the basis of early explanations for 1D BaCoO3 also associated with VRH of the conductivity and TEP29 but questions the origin of the transition observed here. In fact, the change from the semiconducting to VRH regime involves a reinforced electronic localization at the transition, likely due to disorder and/or lowering of the crystal symmetry. To shed more light on this transition and of the relevance of the electronic anisotropy, we have performed magnetoresistance (MR) measurements for ρ ab with different directions of the magnetic field (in-plane B//ab and out-of-plane B//c configurations). MR is small and negative around the transition for B//c (typically 1 or 2% under a 14 T magnetic field), with a clear discontinuity at T* (Figure 3). A negative component of MR is expected in the localized 2D VRH regime, when the MR is of orbital origin.30 In addition, an ideal 2D case would not show a

Figure 4. Field dependence of the in-plane MR with B parallel to the plane and to the c axis at (a) T = 100 K and (b) T = 180 K.

however, that a small positive component emerges at the highest fields. At high fields, the out-of-plane MR also presents a small upward curvature, which reflects this positive component. Such behavior of the VRH MR corresponds to a spin Zeeman effect31 because of the alignment of electron spins, which can be strong due to spin−orbit coupling. This component is expected in both geometries, i.e., B//c and B//ab, as observed. When T > T*, the MR is purely negative for B//c and is almost zero for B//ab, underlying a pure orbital origin (Figure 4b). Then, the appearance of the positive component of MR at high fields for T < T* is also consistent with the observed change in the 14 T MR magnitude at T* (Figure 3). Following this analysis, we conclude that the spin Zeeman contribution is reinforced below the transition temperature T*, likely via a change in the spin−orbit coupling. In order to probe the origin of the transition, the crystal structure of Ba 3Co10O17 at 100 K has also been investigated. Structural Data. The Rietveld refinement using powder ND data of a 70(1)% wt Ba3Co10O17/30(5)% wt Ba2Co9O14 mixture shows no significant deviation of the O content in both 5193



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Table 1. Crystallographic Data and Refinement Parameters for Ba3Co10O17 at 293 and 100 K temperature

293 K

formula, Mw symmetry, space group unit cell (Å)

Ba3Co10O17, Mw, 439 trigonal, R3̅m (No. 166) a = 5.69480(10) a = 5.6820(4) c = 35.9597(9) c = 35.8762(7) V = 1009.96(4) V = 1003.0(2) 3 3 Bruker X8 Bruker X8 0.7107 0.7107 6.28 6.32 hexagonal platelet, black hexagonal platelet, black ω, φ ω, φ 4.17−50.44 4.18−34.99 2.91 3.9 −12 ≤ h ≤ 12, −12 ≤ k ≤ 12, −78 ≤ h ≤ 45 −8 ≤ h ≤ 9, −9 ≤ k ≤ 8, −54 ≤ h ≤ 50 11 544 4064 1153/1400 435/563 0.5 × 0.4 × 0.1 0.5 × 0.4 × 0.1 41 40 L.S. on F, Jana2000 L.S. on F, Jana2000 2.1/2.85 4.47/7.16 1.92/2.03 3.95/4.14 1.32 2.22 4.21/−2.15 7.03/−5.80 0.00141(7) 0.00210(31)

Z equipment λ(Mo Kα) graphite monochromator (Å) density calcd (g/cm3) shape, color scan mode θ(min−max) (deg) R(int) (%) recording reciprocal space no. of measured reflns no. of indep reflns [I > 2σ(I)], total crystal dimensions (mm) no. of refined param refinement method, program R1(F) [I > 2σ(I)]/R1(F 2) [all data] (%) wR2(F 2) [I > 2σ(I)]/wR2(F 2) [all data] (%), where w = 1/σ 2(Fo2) GOF max/min residual electronic density (e/Å3) ref extinction coefficient

100 K

thermal parameters. In fact, only the cell change into the C-centered monoclinic lattice (am = −2ah − bh, bm = −bh, cm = 2/3ah + 1/3bh + 1/3ch) allows a slight improvement of the global refinement process, as listed in Table 2. However, the greatest number of refined parameters and strong residual electron density (∼11 e/Å3) close to Ba do not yield a clear conclusion. Furthermore, the splitting of M−O distances compared to the R3̅m model often lies similarly within the estimated standard deviation values. Only we can attest that, according to this hypothesis, the octahedral Co2 atoms would be split into Co2a and Co2b, which would be associated with different bond valence sums (BVSs), +3.36 and +3.39. It could picture a minor charge disproportion at the transition. This model proposed in Figure 5 remains highly hypothetical. Pertinent distances at both room temperature (R3̅m) and 100 K (C2/m model) are compared in Table S4 in the Supporting Information. For comparison, pertinent interatomic distances in Ba2Co9O14 are listed in Table S5 in the Supporting Information. Charge Ordering. Prior to examination of the magnetic properties, it is interesting to compare the n = 2 and 1 terms on the basis of their density of states (DOS). Indeed, one should recall that in Ba2Co9O14 (n = 1) the refined magnetic structure from powder ND data showed a distribution of HS CoII and LS CoIII that order below TN = 39 K. The [BaCo8O11] blocks formed by the central CoO2 layer and the terminal tetrahedra are ferrimagnetic, while the magnetic exchanges between them are expected to be weak (because they are achieved solely by SSE paths).17 This gives rise to a 2D magnetic picture. The DOS projected on the d states of the Co atoms calculated for the ferromagnetic states of Ba2Co9O14 are presented in Figure 6a. This is consistent with the spin distribution refined from ND analysis. Then, a comparison with calculations on Ba3Co10O17 is quite interesting (Figure 6b). The spatial expansion of the intermediate perovskite block in the n = 2

of the phases (Table S3 in the Supporting Information). However, the relatively high values of the reliability factor cannot be improved, by tuning any of the structural parameters: final Rf (RBragg) = 6.65% (9.92%) and 7.16% (12.3%) for Ba3Co10O17 and Ba2Co9O14, respectively. At least for the major phase, such behavior is reminiscent of the high concentration of defects observed by transmission electron microscopy. As was recently reported for the layered Ba1.9Ca2.1YFe5O13,32 the high concentration of stacking faults and compositional heterogeneities was simulated by the introduction of two similar phases in the refinement process. In our case, the presence of both the n = 1 and 2 terms excludes a similar modelization. For Ba3Co10O17, the crystal structure refined from single-crystal XRD at room temperature yields low reliability factors associated with the possibility of refining all atoms with anisotropic thermal parameters [a = 5.69480(10) Å, c = 35.9597(9) Å, space group R3̅m, R1 = 2.1%, and wR2 =1.92%], which assert the good quality of the selected crystal, essential for the pertinence of our low-temperature analysis. Details of the data collection and refined parameters are given in Table 1 for room temperature and 100 K according to R3̅m symmetry. The fully collected reciprocal space has been used to calculate the precession frames corresponding to hk0, hk1, h0l, h1l, 0kl, and 1kl layers. They show no additional peak that would result from a supercell ordering. The R3̅m model using data collected at 100 K data converged to final R values, R1 = 4.47% and wR2 = 2.95%. However, it is striking that, opposite to the room temperature solution, one Co and one O positions show not positive definite anisotropic thermal parameters. We also note that, in spite of the low temperature, several thermal parameters are greater than those at room temperature, within greater final R values. Then different rhombohedral subgroups have been tried, namely, R3̅ and R3, that do not improve the quality of the refinement and also lead to a number of discrepancies of the 5194



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Table 2. Comparison of the Structures and Refinement Parameters for Ba3Co10O17 at 100 K for Different Possible Space Groups space group

R3̅m

R3̅

R3

C2/m

a (Å) b (Å) c (Å) angles (deg) indep refls/(I > 3σ) Robs/wRobs no. of param weighting scheme max/min Δρ (e/Å3) Ba

5.6820(4)

5.6820(4)

5.6820(4)

35.872(7)

35.872(7)

35.872(7)

435/563 4.47/3.95 40 1/σ 2 7.03/−5.80 1 × 6c 1 × 3a 1 × 18 h 1 × 9d 2 × 6c 1 × 3b 2 × 18 h 1 × 9e 1 × 6c

660/876 4.85/4.44 36 1/σ 2 6.54/−7.45 1 × 6c 1 × 3a 1 × 9d 3 × 6c 1 × 3b

1118/1570 4.81/4.74 59 1/σ 2 6.50/−7.67 3 × 3a

2 × 18f 1 × 9e 1 × 6c

5 × 9b 2 × 3a

0.0058(2) < UBa < 0.0067(3) 0.0036(9) < UCo < 0.0058(7) 0.0043(18) < UO < 0.012(3) some O atoms do not have defined thermal anisotropic parameters

0.0057(2) < UBa < 0.0065(3) 0.0042(17) < UCo < 0.0058(6) 0.0046(16) < UO < 0.0117(16) no O atoms have defined thermal anisotropic parameters

0.00554(18) < UBa < 0.0063(2) 0.0035(13) < UCo < 0.0052(6) negative < UO < 0.016(10) one O atom is not anisotropically defined

9.838(3) 5.6828(13) 12.396(3) β=105.263(12) 916/1272 3.23/3.93 90 1/σ 2 11.59/−15.88 1 × 2a 1 × 4i 1 × 4f 3 × 4i 1 × 2c 1 × 2d 2 × 8j 3 × 4i 1 × 4e 1 × 2b 0.0046(2) < UBa < 0.0056(3) 0.0027(4) < UCo < 0.0048(6) 0.003(2) < UO < 0.011(4) all atoms are considered with anisotropic thermal parameters

Co

O

range for Uiso or Ueq

refinement of thermal parameters and comments

7 × 3a 1 × 9b

Figure 5. Crystalline structure of Ba3Co10O17 (left). The projection on the ab plane is shown in the case of R3̅m symmetry (right, top) and of C2/m distortion (right, bottom) to highlight the Co2 site splitting into two (Co2a and Co2b) different sites from rhombohedral to monoclinic symmetry.

compound could considerably reduce the interblock exchanges if one assumes the same intrablock configuration and the presence of nonmagnetic (LS CoIII) cations in the full perovskite block. Surprisingly, our DFT calculations rule out this hypothesis and break down the 2D magnetic picture for Ba3Co10O17. In fact, from the topology of the DOS projected (Figure 6) on the Co d states calculated from a ferromagnetic configuration (for simplification purposes), the same Co atoms as those in Ba2Co9O14 carry out moments, but unexpectedly Co3 in the perovskite type layer also appears to be magnetic. In Figure 5, Co1(CoO2), Co2(CoO2), Co(perov.), Co(interm.),

and Co(Td) correspond to Co1, Co2, Co3, Co4, and Co5, respectively, for Ba3Co10O17 and to Co5, Co4, Co2, Co1, and Co3 for Ba2Co9O14 (from ref 17). It follows that (i) Co2(CoO2) and Co(interm.) have similar topologies in both compounds with a simple picture corresponding to empty eg states and filled t2g states indicating a LS configuration compatible with Co3+ (d6). (ii) Co1(CoO2) and Co(Td) have also similar topologies in both compounds; the up-spin d bands are completely filled, while the down-spin d bands are partially empty, which indicates a HS configuration. 5195



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Figure 6. Topologies of the DOS projected on the Co d states for each type of Co atom site for (a) Ba 2Co9O14 and (b) Ba3Co10O17 calculated for a simple ferromagnetic unit cell in the GGA. The Fermi level is indicated by vertical dotted lines. The plots are annotated with the corresponding Co atom as follows: Co1(CoO2) and Co2(CoO2) correspond to atomic sites within the CdI2-type layer (edge-sharing octahedral layer), Co(perov.) corresponds to the site in the perovskite layer (one layer in the case of Ba 2Co9O14 and two layers in the case of Ba3Co10O17), Co(interm.) corresponds to the octahedral site within the intermediate layer of polyhedra sandwiched between the CdI 2-type layer and the perovskite layer, and Co(Td) corresponds to the tetrahedral site.

Co2+/Co3+ charge-ordering results from crystallographic considerations. However, the Ba3Co10O17 charge distribution is not as straightforward. Compared to Ba2Co9O14, a similar charge distribution is reasonable in the [BaCo8O11] subunits, according to unmodified interatomic distances between the two compounds. The assignment of Co3 to HS Co3+ leads to an average Co2.7+ valence. Then, hole doping appears to be the most probable scheme within an almost satisfied charge ordering to reach the mean Co2.8+ valence inherent to the absence of oxygen vacancies from powder ND data. Ideally, the charge distribution is as follows: octahedral HS Co12+ (S = 3/2), tetrahedral HS Co52+ (S = 3/2), octahedral HS Co33+ (S = 2), octahedral LS Co23+ (S = 0), and LS Co43+ (S = 0). The introduction of onsite repulsion U to Co atoms (GGA + U) in supercells accounting for the experimental magnetic structure and more insight into the Co3 site will be presented elsewhere. Magnetic Properties. Taking into consideration the large concentration of extended defects in the powder samples as the

(iii) Co(perov.), which exhibits a LS state in Ba2Co9O14, shows a modified topology toward a higher spin configuration in Ba3Co10O17. Ba3Co10O17/Ba2Co9O14 are predicted to be metallic/semimetallic (calculation of the ferromagnetic spin configuration within the GGA). These results do not reproduce the insulating behavior observed experimentally because of neglect of the electron correlation of the Co 3d states and the ferromagnetic unit cell considered through the calculations. However, according to the good agreement between the calculated and observed distribution of magnetic and nonmagnetic Co sites for n = 1, the general features of the DOS are relevant for the probable oxidation states and spin configurations of both compounds. For n = 2, the unexpected magnetic moment of ∼1.95 μ B for Co3 validates a magnetic Co2+ or Co3+ state. The oxidation states assigned to each Co site are given in Table 3 from DOS and ND experiments (see below) for both compounds. As was already reported for Ba2Co9O14,17 a perfect 5196



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Table 3. Electronic Configurations of the Co Atoms in Ba2Co9O14 and Ba3Co10O17, Deduced from the Combined Analysis of the Magnetic Moments Obtained Experimentally and through DFT Calculations, DOS Topologies, and BVS Calculationsa atom

environment

Co1 Co2 Co3 Co4 Co5

Oh Oh Td Oh Oh

interm. layer perovskite layer

Co1 Co2 Co3 Co4 Co5

Oh Oh Oh Oh Td

CoO2 layer CoO2 layer perovskite layer interm. layer

CoO2 layer CoO2 layer

exptl M DFT M (μ B) (μ B) Ba2Co9O14 0b 0.14 0b 0.32 2.65b 2.24 0b 0.06 1.44b 2.34 Ba3Co10O17 1.44 2.37 0 0.1 2.87 1.93 0 0.18 2.57 2.31

BVS

ideal spin state (mean Co2.7+)

3.18b 3.09b 2.10b 3.36b 2.09b

Co3+ Co3+ Co2+ Co3+ Co2+

(LS) (LS) (HS) (LS) (HS)

2.08 3.36 2.61 3.12 2.36

Co3+ Co3+ Co3+ Co3+ Co2+

(HS) (LS) (HS) (LS) (HS)

a

The geometry around each Co atom is indicated. The DFTcalculated magnetic moments were obtained from a ferromagnetic spin configuration in a simple unit cell for simplification purposes and to reduce our computational task because the antiferromagnetic experimental spin configuration would require doubling of the unit cell. bValues taken from ref 17.

Figure 7. (a) M/H versus temperature measured for single crystals of Ba3Co10O17. We note that the transition at T* was not observed for multiphased polycrystalline samples. (b) Magnetization.

basis of further work, the magnetic susceptibility has been measured on two samples, namely, the 70 wt % Ba3Co10O17/30 wt % Ba2Co9O14 mixture (as determined from our powder ND analysis) and a sample of handily isolated single crystals (m ∼ 2.6 mg). For this latter sample, χ(T) at H = 0.1 T is shown on Figure 4. FC and ZFC procedures do not provide evidence of thermal hysteresis, disregarding ferromagnetic and/or glassylike behavior. The anomaly at T* ∼ 160 K coincides well with those observed upon electric measurement. As shown in the inset of Figure 7a, ZFC and FC cycles confirm a reversible transition. The sharp peak at TN ∼ 62 K pictures the 3D antiferromagnetic ordering. Both T* and TN are not discernible in χ(T) for the polycrystalline compound (see figure S6 in the Supporting Information). Long-range magnetic interactions are likely broken by the extended defects that we have evidenced in the polycrystals by high-resolution electron microscopy. Electron microscopy fine analyses of these defects will be presented in further work. M(H) curves measured on single crystals at low temperature show the occurrence of a very small hysteresis for B < 0.05 T and of a weak net magnetic moment under an applied field (Figure 7b). The remanent moment is very weak (∼5 × 10−3 μ B/Co) and could result from uncompensated ferromagnetic alignment, possibly related to the small amount of hole doping mentioned above. The Curie−Weiss fit from 200 to 350 K leads to μ eff = 7.2 μ B/f.u. and θ CW = −112.15 K, smaller than the calculated μ eff = 9.64 μ B/f.u. in the spin-only approximation for the ideal charge distribution. The significant difference between the experimental and expected values could be due to the relatively high conductivity at room temperature, which denotes a degree of electronic delocalization. On both sides of T*, we observe a separation into two Curie−Weiss regimes with rather similar effective moments but different antiferromagnetic fluctuations (Figure 7a). Indeed, μ eff= 7.2 μ B/f.u. and θ CW = −112 K for T > T*, whereas μ eff = 7 μ B/f.u. and θ CW = −150 K for TN < T < T*. Those characteristics are not those of a genuine chargeordering transition and are more indicative of a structural

distortion with a change of the spin−orbit coupling, as suggested in the Transport Properties section. Finally, contrary to our expectations, TN is greater for n = 2 than for n = 1, which comforts the determining role of the magnetically active Co3 of the perovskite blocks in the exchange paths for n = 2. The magnetic structure detailed below comforts its crucial role. The powder ND data from 50 to 1.8 K show evidence below 40 K of magnetic satellites of Ba2Co9O14 (n = 1; Figure 8a). They can be indexed using the propagation vector k(n=1) = 3/2c*, which indicates a doubling of the magnetic periodicity compared to the crystallographic unit cell. The refined magnetic structure17 is shown in Figure 8b. The magnetic lines of Ba3Co10O17 (n = 2) are rather different because the main strong line is already almost saturated at 50 K and corresponds to the (101) + k with k = 3/2c* (Figure 8b). Contrary to the n = 1 case (moment lying in the ab plane), no magnetic satellites of the 00l reflections exist that picture the magnetic moment parallel to c. Several magnetic models have been tested. In fact, only the introduction of magnetic moments on Co3 leads to a convergence of the refinement process; Rmagn = 8.11% and 14.6% for Ba3Co10O17 and Ba2Co9O14, respectively. The magnetic moments of the n = 1 and 2 compounds are gathered in Table 3. Parts a and b of Figure 9 show that, apart from the different MCA, the magnetic ordering of the [BaCo8O11] units remains similar in the two compounds. However, for n = 2, the magnetic interplay is largely modified by magnetic Co3 via two extra superexchange paths: Co5---Co3 (d = 3.95 Å; Co−O−Co = 157°) and Co3---Co3 (d = 3.829 Å; Co−O−Co = 180°), which are predicted to be antiferromagnetic from Kanamori−Goodenough rules in the approximation of 180° angles. As suggested in Figure 9c,d, according to the modular aspects of both compounds with ferrimagnetic [BaCo8O11] units separated by spacers, the change of MCA from n = 1 to 2 could reflect the dominant effect of dipolar 5197



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Figure 9. Comparison of the refined magnetic structures of Ba2Co9O14 (a) from ref 17 and Ba3Co10O17 (b). The magnetic Co sites are indicated by white filled circles inside gray polyhedra. The spin directions are represented by arrows on the magnetic Co sites. The refined magnetic moments are also indicated. A proposed scheme of spin interactions is shown in Ba2Co9O14 by dipolar couplings (c) and Ba3Co10O17 by site-to-site exchanges (d).

Figure 8. Powder ND pattern for the Ba3Co10O17/Ba2Co9O14 mixture (a): variable-temperature data with evidence of two series of magnetic phases below T = 50 K; (b) refined structural and magnetic phases at 1.8 K.

couplings33 in the former against Co−O−Co exchanges in the latter because of the magnetic Co3 intermediate. The difference between both compounds in terms of the magnetic or nonmagnetic Co cations in the perovskite subunits could be related to the corner-sharing (n = 2) or face-sharing (n = 1) connectivity. Here, according to the mean octahedral Co3+−O for these central cations (1.89 Å for n = 2 vs 1.93 Å for n = 1), it appears that the HS Co23+ (ionic radius = 0.61 Å) nature (n = 1) would be favored by corner sharing while face sharing would create chemical pressure effects, responsible for the LS Co33+ (ionic radius = 0.545 Å) in the n = 2 member.

at lower temperatures. A slight monoclinic distortion is proposed as possibly being at the origin of this anomaly, but the small improvement of the refinement and the resulting distances barely divergent from the initial model do not play in favor of this hypothesis. Further investigations would be necessary to understand the origin of this transition.

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ASSOCIATED CONTENT * Supporting Information EDS spectrum of Ba3Co10O17, four-contact configurations, atomic positions, interatomic distances and calculated BVSs, and a comparison between the susceptibilities measured for single crystals and multiphased polycrystalline samples. This material is available free of charge via the Internet at http://pubs.acs.org. S

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CONCLUSION The n = 2 member of the family of compound [BaCoO3]n[BaCo8O11] was characterized from the transport and magnetic properties point of view using both powder and single-crystal samples. ND indicates 3D antiferromagnetic spin ordering below 65 K, which rules out the 2D magnetic picture expected by extrapolation from the n = 1 member in which the Co atoms of the perovskite block are nonmagnetic (LS Co3+). The Co atoms in the perovskite block of the n = 2 member exhibit a strong magnetic moment compatible with HS Co3+, as deduced from ND, crystallographic considerations, and DFT-based electronic structure calculations. In addition, an anomaly is observed around 160−170 K concomitantly in the magnetic susceptibility and resistivity curves. From the transport properties point of view, it corresponds to a change from a classical semiconducting behavior at higher temperatures to a VRH mechanism

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

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ACKNOWLEDGMENTS The Fonds Européen de Développement Régional (FEDER), CNRS, Rég ion Nord Pas-de-Calais, and Ministèr e de l’Enseignement Supérieur et de la Recherche are acknowledged for funding of the X-ray diffractometers. R.D. thanks the ENS of Lyon for financial support. This work was carried out under the framework of the MAD-BLAST project supported by the 5198



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(32) Demont, A.; Dyer, M. S.; Sayers, R.; Thomas, M. F.; Tsiamtsouri, M.; Niu, H. N.; Darling, G. R.; Daoud-Aladine, A.; Claridge, J. B.; Rosseinsky, M. J. Chem. Mater. 2010, 22, 6598. (33) Ostrovsky, S.; Haase, W.; Drillon, M.; Panissod, P. Phys. Rev. B 2001, 64, 134418.

ANR (Grant ANR-09-BLAN-0187-01). This work was supported by the FP7 European Initial Training Network SOPRANO (Grant GA-2008-214040).

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