Outstanding Atomic Order in Ruddlesden–Popper Oxide Microcrystals

Article pubs.acs.org/cm

Outstanding Atomic Order in Ruddlesden−Popper Oxide Microcrystals Luisa Ruiz-González,† Daniel González-Merchante,† Raquel Cortés-Gil,† José M. Alonso,‡,§ José L. Martínez,§ Antonio Hernando,‡,∥ and José M. González-Calbet*,†,‡ †

Departamento de Química Inorgánica, Facultad de Químicas, Universidad Complutense (UCM), CEI Moncloa, 28040-Madrid, Spain ‡ Instituto de Magnetismo Aplicado, UCM-CSIC-ADIF, P.O. Box 155, 28230-Las Rozas, Madrid, Spain § Instituto de Ciencia de Materiales, CSIC, Sor Juana Inés de la Cruz s/n, 28049 Madrid, Spain ∥ Departamento de Física de los Materiales, Facultad de Físicas, UCM, CEI Moncloa, 28040-Madrid, Spain S Supporting Information *

ABSTRACT: Ruddlesden−Popper’s manganates, An+1MnnO3n+1, built from the ordered intergrowth between one rock-salt and n perovskite blocks, display a wide variety of functionalities related to their physical-chemistry properties which can be in principle tuned by chemical modifications. Nevertheless, the poor thermodynamic stability of the high members constitutes an inherent impediment, limiting the development of new functionalities in this family. Actually, for n ≥ 2, defects involving disordered intergrowths between perovskite and rock-salt blocks are always present avoiding the correct characterization of their properties. For that purpose, the use of sophisticated and expensive physical methods is required. In this article, the stabilization, following a chemical strategy, of micrometric La0.5Ca2.5Mn2O7 crystalline particles exhibiting a well ordered distribution of two perovskite and one rock-salt block, according to an ideal n = 2 unit cell, is reported. This apparently long-range structural ordering is linked to an unconventional short-range order−disorder phenomenon of La and Ca cations, characterized at the atomic level, which allows a rational explanation of the crystallochemical and magnetic properties of this Ruddlesden−Popper compound.

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et al.15 in La2−2xSr1+2xMn2O7 (x = 0.4). The n = 2 RP manganates display a high magnetoresistant (MR) effect15,16 compared to other members of the series, including Mn perovskites. This fact must be related to the CMR behavior inside P layers and also the tunnelling MR mechanism between adjacent P bilayers through insulating RS ones. The main problem to this approach is the difficulty in stabilizing pure high ordered terms, which are frequently obtained as disordered intergrowths unless a controlled layer by layer method is used.17 Most studies have been devoted to La2−2xAe1+2xMn2O7 with particular attention to LaSr2Mn2O7 as a consequence of the observed CMR effect.18−20 The Sr RP phases are less distorted than Ca ones since they involve Mn octahedral angles close to 180°, a condition required for double exchange FM interactions. Less attention has been paid to the Ca system due to the difficulty in stabilizing single phases around x = 0.5. Actually, LaCa2Mn2O7 has not been prepared up to now as a single phase. However, La2−2xCa1+2xMn2O7 n = 2 members (0.6 < x < 1.0) have been reported, although microstructural studies reveal disordered intergrowths between

INTRODUCTION Strongly correlated electronic systems are cutting edge materials for the development of magnetic storage and sensing devices.1−3 Chemistry plays an important role in this behavior since the functionality of these materials is highly sensitive to both compositional and structural modifications. Among them, manganese related perovskites Ln1−xAexMnO3 (Ln = lanthanide; Ae = alkaline-earth) occupy a prominent place due to their fascinating physical-chemistry properties such as catalytic, thermoelectric, magnetocaloric, colossal magnetoresistant (CMR), charge ordering (CO), and high spin polarity effects4−7 as a consequence of the coexistence of different amounts of Mn4+ and Mn3+ as well as lattice distortions depending on the A site occupancy in ABO3 perovskite.8,9 More recently, a wide variety of functionalities have been also observed in the closely related Ruddlesden−Popper (RP) series,10−14 (AO)(ABO3)n, where n perovskite (P) blocks intergrow in an ordered way with a rock salt (RS) layer (AO) giving rise to new superstructures with quasi-2D properties. Studies of the RP phases have shown that physical-chemistry properties are sensitively dependent on the number of P layers but that, unfortunately, only few members with low n values are thermodynamically stable. CMR and ferromagnetic (FM) ordering in layered manganates were reported by Moritomo © XXXX American Chemical Society

Received: December 19, 2014 Revised: January 28, 2015

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Chemistry of Materials P and RS.21−26 Since physical-chemistry properties are very sensitive to elemental composition and subtle structural changes, a comprehensive understanding of these features requires precise chemical and crystallographic knowledge. To do that, an atomic level characterization is mandatory. Transmission electron microscopy (TEM) and associated techniques such as X-ray energy dispersive (EDS) and electron energy loss spectroscopy (EELS) have shown to be powerful tools for direct visualization and detection at the local level.27−30 Nowadays, aberration corrected scanning transmission electron microscopy (STEM) offers invaluable analytical capabilities since imaging and spectroscopic techniques can be combined to get chemical information at atomic resolution,31,32 giving valuable insight into the local structure that ultimately determines their properties. Exploiting these capabilities has been mainly related to the characterization of well-ordered artificial heterostructures33,34 but is an emerging and attractive tool for solving intricate structural and analytical problems in functional complex oxides even in powdered form. In recent years, there has been a burst of activity to manipulate strongly correlated phenomena based on oxide heterostructures.35,36 Among these, particular efforts have been devoted to grow RP thin films.13,14 The synthesis of such layered oxides has been a major challenge owing to the occurrence of disordered intergrowths that result in poor materials behavior in the higher members. Molecular beam epitaxy has been the most frequently used technique to deposit layered oxides that cannot be stabilized in bulk form. In this context, the aim of this work is to develop a synthesis pathway to get an ordered high member of the Mn RP series and to shed light on the La2−2xCa1+2xMn2O7 characterization to be able to tackle the properties study on the basis of an exhaustive characterization at atomic level. For that purpose, we have chosen the La 0.5 Ca 2.5Mn2 O7 composition close to the previously reported,21 following an alternative synthesis pathway. Under these new conditions, crystalline particles of micrometric size, built from the ordered intergrowth of two P and one RS unities, have been obtained. These particles have been chemically and structurally characterized for the first time at the atomic level using aberration corrected electron microscopy. In this sense, a precise knowledge of the cation, anion, and oxidation states distribution has been obtained, providing a rational understanding of the crystallochemical, magnetic, and electric behaviors. Furthermore, the possibility of obtaining single crystal particles of micrometric size should be of great interest for fabricating microdevices37 as well as for producing larger crystals by using one of these particles as a seed for the different crystal growing processes.

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to a TEM JEOL JEM 2010. The oxygen content was confirmed by thermogravimetric analysis in a Cahn D-200 electrobalance. Basic microstructural characterization was performed on a JEOL JEM 300FEG microscope, whereas atomic scale characterization was done in a JEOL JSM-ARM200cF microscope (Cold Emission Gun) operated at 200 kV provided with a spherical aberration corrector in the probe (current emission density ∼1.4 × 10−9 A and probe size ∼0.8 A), a GIF-QuantumER spectrometer, and an Oxford INCA-350 detector. Solid semiangles between 90 and 370 mrad and 11−22 mrad were used for acquisition of high angle annular dark field (HAADF) and annular bright field (ABF) images, respectively (38 s per frame). EELS maps were acquired with a spatial resolution ∼0.4 A, over a total acquisition time of ∼2 min (current emission density of ∼1.2 × 10−8 A and a probe size of ∼1.2 A). Principal component analysis (PCA) taking 6 components was performed on the EELS data set to de-noise the spectra using the Hyperspy data analysis toolbox. Magnetic measurements were performed using a Quantum Design SQUID magnetometer. The magnetization was measured in ZFC and FC conditions as a function of temperature over the range 2 ≤ T ≤ 350 K. A hysteresis loop measurement was performed at 350 K under 7 T applied magnetic field. AC susceptibility was studied using the ACMS option of a Quantum Design PPMS (Physical Property Measurement System). The frequency range and intensity of the AC magnetic field employed were from 500 to 10000 Hz and 5 Oe, respectively. The temperature dependent resistance measurements were performed using the standard four-probe method in the temperature range 5−350 K under 0 and 14 T applied magnetic fields.

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RESULTS AND DISCUSSION Following the chemical strategy described in the Experimental Section, a single phase of La0.5Ca2.5Mn2O7 composition has been obtained according to the XRD study and chemical analysis. The average cation composition was checked by means of EDS and electron probe microanalysis (EPMA). The oxygen content, precisely determined by thermogravimetric analysis, indicates that the anionic sublattice is complete, according to the nominal composition. This situation is different from that previously reported for La2−2xCa1+2xMn2O7 (0.6 ≤ x ≤ 1) since iodometric titrations indicate a slight oxygen deficiency.23 The corresponding XRD pattern can be indexed on the basis of a RP n = 2 term with space group (SG) Cmc21 (No 36). Several space groups were tested in the fitting of the powder XRD data of La0.5Ca2.5Mn2O7 at room temperature. The best approach can be indexed on the basis of a RP n = 2 term with SG Cmc21 and lattice parameters: a = 1.931577(2), b = 0.536824(1), and c = 0.5340315(1) nm. This SG was previously considered for the crystallographic structure of Ca3Mn2O739,40 and La2−2xCa1+2xMn2O7 along the 0.80 < x < 1 composition range.22 However, this situation is different from that found for a similar composition, La0.4Ca2.6Mn2O7, previously reported by L. A. Bendersky et al.,23 since they proposed a tetragonal I4/mmm symmetry. In our case, the XRD pattern shows that several reflections split into the orthorhombic peaks, and thus, a tetragonal space group can be rejected. In spite of that, they suggested that a TEM study was needed to identify diffraction effects related to structural distortions.22 However, M. A. Green et al.26 initially proposed the Fmmm space group for LaCa2Mn2O7 from the Rietveld Xray refinement data, but neutron diffraction experiments were refined in the Cmcm space group with an additional perovskite impurity phase. Actually, the LaCa2Mn2O7 composition has not been prepared, up to now, as a single phase. The symmetry elucidation in these systems is a challenging question that has been the subject of different approaches usually requiring additional characterization techniques in order to provide a

EXPERIMENTAL SECTION

La0.5Ca2.5Mn2O7 was synthesized by heating stoichiometric amounts of La2O3, CaCO3, and MnO2 up to 1400 °C for 100 h, with intermediate grindings. During this process, the sample was always slowly cooled inside the furnace, while the temperature was decreased at 0.1 °C/min. This procedure has led to the stabilization of a highly homogeneous sample. X-ray diffraction (XRD) was performed using a PANalytical X’pertPRO diffractometer operating with CuKα1 radiation in Bragg− Brentano geometry at room temperature. Neutron diffraction (ND) measurement at room temperature was performed using the diffractometer D2B (Institut Laue-Langevin, Grenoble) with λ = 1.5943 Å. Crystal structure was refined by the Rietveld method using the GSAS software package with the EXPGUI interface.38 Average cationic composition was checked by means of EPMA, attached to a JEOL JXA-89000 Microprobe, and EDS (OXFORD INCA), attached B

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Chemistry of Materials more conclusive description.22,23,26 In this sense, a careful ND and TEM/STEM study has been performed. The ND pattern at room temperature can be indexed, in agreement with the XRD study, on the basis of a RP n = 2 term (SG Cmc21 and lattice parameters a = 1.93154(4), b = 0.53683(1), c = 0.53403(1) nm). Figure 1 shows the final

Table 2. Some Selected Bond Lengths (nm) for La0.5Ca2.5Mn2O7 from ND Data at Room Temperature Mn-01 Mn-02 Mn-02 Mn-03 Mn-03 Mn-04 La/Ca(1)-01 La/Ca(1)-01 La/Ca(1)-01 La/Ca(1)-01 La/Ca(1)-02 La/Ca(1)-02 La/Ca(1)-03 La/Ca(1)-03 Ca(2)-02 Ca(2)-02 Ca(2)-03 Ca(2)-03 Ca(2)-04 Ca(2)-04 Ca(2)-04 Ca(2)-04 Ca(2)-04

Figure 1. ND pattern (red points) and Rietveld fit (green line) of La0.5Ca2.5Mn2O7 at room temperature. The difference between experimental and calculated data is shown as a solid line above. Vertical bars show all allowed reflections of the space group Cmc21 (Rwp = 0.0480, Rp = 0.0352, and χ2 = 2.852, 20 variables).

refinement of the ND pattern. The final structural parameters resulting from that refinement are collected in Table 1, and the corresponding interatomic selected distances and bond angles are gathered in Tables 2 and 3, respectively.

site

x

y

z

104 × U1∞ (θ2)

La Cal Ca2 Mn O1 O2 O3 O4

4a 4a 8b 8b 4a 8b 8b 8b

0.5 0.5 0.31444(17) 0.10060(20) 0 0.08917(15) 0.10503(14) 0.19643(14)

0.7560(13) 0.7560(13) 0.7460(11) 0.7572(15) 0.7054(10) 0.0278(9) 0.5214(11) 0.7884(8)

0.8061(25) 0.8061(25) 0.8299(18) 0.8088(21) 0.8108(24) 1.0396830(1) 0.0795(10) 0.8083(17)

73(1) 73(1) 69 (11) 75(8) 74(13) 56(1) 157(10) 104(1)

2 2 2 2

Table 3. Some Selected Bond Angles for La0.5Ca2.5Mn2O7 from ND Data at Room Temperature O1−Mn−O2 O1−Mn−O2 O1−Mn−O3 O1−Mn−O3 O1−Mn−O4 O2−Mn−O2 O2−Mn−O3 O2−Mn−O3 O2−Mn−O4 O2−Mn−O3 O2−Mn−O3 O2−Mn−O4 O3−Mn−O3 O3−Mn−O4 O3−Mn−O4 Mn−O1−Mn Mn−O2−Mn Mn−O3−Mn

Table 1. Refined Atomic Parameters of La0.5Ca2.5Mn2O7 from ND Data at Room Temperature atom

× × × ×

0.1963(4) 0.1918(10) 0.1857(11) 0.1923(11) 0.1935(11) 0.1859(5) 0.2956(8) 0.2412(8) 0.2653(16) 0.2703(16) 0.2454(8) 0.2704(9) 0.2878(10) 0.2648(8) 0.2469(6) 0.2834(7) 0.2525(9) 0.2401(8) 0.2294(4) 0.2468(7) 0.2922(7) 0.2800(12) 0.2570(12)

In the structural refinement, the smaller Ca 2+ ion preferentially occupies the nine-coordination site, Ca(2), in the RS layer, and the remaining Ca, Ca(1), is distributed with La on the 12-coordinated site in the P blocks. In this sense, the isotropic thermal parameters for La and Ca in the P block were constrained to refine together. At this point, it is worth recalling that refinement of the La/Ca ratio at the P blocks leads to the occupancy factor values of 0.49/0.51, respectively, in a satisfactory agreement with the nominal composition for these blocks. The resulting structure from Rietveld refinement of the ND data at room temperature is depicted in Figure 2, in which the octahedral distortion is evident. As mentioned above, P blocks are occupied by La and Ca, and the different distortions associated with each cation could be responsible for the local differences on the octahedral tilt and the local changes of symmetry, thereby leading to controversial results on the XRD studies for similar compositions. Small differences between calculated and experimental intensities for some reflections can be due to preferential orientation.

89.4(4) 88.5(5) 87.0(4) 86.4(4) 177.0(5) 90.7(4) 91.2(5) 175.83(31) 92.7(4) 175.1(4) 89.8(5) 93.5(4) 88.0(33) 90.9(4) 91.4(4) 163.7(5) 162.7(4) 169.2(6)

In order to get more complete structural information and to elucidate whether short-range order−disorder phenomena, quite frequent on RP high members, are present, an exhaustive TEM characterization was performed. Figure 3a−c shows three representative SAED patterns for La0.5Ca2.5Mn2O7 along [100], [01̅1], and [001], indexed on the basis of an orthorhombic (Cmc21) n = 2 R-P cell, in agreement with ND data. However, the absence of additional reflections on the basal plane (Figure 3a) differs from the observation of satellite reflections associated with a 2D-inconmensurate modulation previously reported in La2−2xCa1+2xMn2O7,23 as a plausible consequence of CO between Mn4+ and Mn3+. The authors23 initially considered the possibility of chemical ordering between Ca and La but always discarded the option of anionic vacancy ordering in spite of the observed oxygen deficiency. At this point, it is worth C

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structural ordered situation supports the improvement of the procedure followed here. On the one hand, the use of temperatures as high as 1400 °C, which favors the diffusion process in a solid state reaction, and, on the other hand, a very slow cooling rate (0.1 °C/min), against the more traditionally used fast quenching, have led to the growth of microcrystals of around 2 μm (see Supporting Information, Figure SI2) with an unprecedented high order degree. A full understanding of the La0.5Ca2.5Mn2O7 crystallochemistry requires information about the cation distribution at the atomic level. For that purpose, a STEM study with atomic resolution is mandatory. Representative HAADF images along [011̅ ] are shown in Figure 4a. Atomically resolved

Figure 2. Structural model of La0.5Ca2.5Mn2O7.

Figure 4. (a) HAADF image along [01̅1]. A schematic model for the cationic position has been inserted; (b) HAADF image used for the EELS study (the studied area is marked with a green square). (c) HAADF image of the region of interest recorded at the same time as the EELS data set acquisition. (d) EELS spectra sum acquired over the area marked in b, showing the La-M4,5, Ca-L2,3, Mn-L2,3, and O-K signals. Chemical maps obtained from La-M4,5 (e), Ca-L2,3 (f), and Mn-L2,3 (g) signals.

columns corresponding to La (Z = 57), Ca (Z = 20), and Mn (Z = 25), arranged in an ordered intergrowth between two P and one RS blocks (with periodicities of 0.39 and 1.9 nm along perpendicular directions), are clearly observed. The brightest dots should be related to a preferential occupation of the heaviest cation, i.e., La, while the less intense to Ca. It should be noticed that the RS sites always exhibit the same brightness, while different brightness randomly alternate at the P site. At the P central position, a variable occupation of A sites can be inferred since clear contrast differences are observed, while the peripheral positions of the P blocks, which equally correspond to the RS sites, always show the same brightness. These atomic columns show lower intensity contrast, similar to the lowest of the central P blocks, suggesting that Ca is the majority occupant of these sites. This situation is in agreement with the Rietveld refinement that supports a preferential occupation of the Ca atoms at the 9-fold coordination sites, i.e., at the RS block. For LaCa2Mn2O7, the ideal model (see inset) comprises La always at the central position of the P block, whereas Ca is at the RS. In La0.5Ca2.5Mn2O7, RS sites seem to be all Ca occupied, as in LaCa2Mn2O7, while the Ca remaining is randomly distributed with La at the P central position. In order to check such a distribution, atomically resolved maps by EELS were acquired. Figure 4b shows the selected area of a HAADF image along [011̅ ] where EELS data were acquired. The HAADF image, recorded simultaneously with EELS acquisition, is depicted in Figure 4c. In order to simultaneously analyze the three signals, La, Ca, and Mn

Figure 3. SAED patterns corresponding to La0.5Ca2.5Mn2O7 along (a) [100], (b) [01], and (c) [001]. (d) Representative HRTEM image along [01] showing a very well ordered n = 2 RP term. A good agreement between the experimental and calculated image (inserted) taking into account the ordered intergrowth between P and RS blocks can be observed.

recalling that in La0.5Ca2.5Mn2O7, the oxygen sublattice is complete, and therefore, no satellite reflections are observed. Even more, to check this point, we have stabilized reduced phases that clearly show the presence of satellite reflections (see Supporting Information, Figure SI1). However, diffuse streaking has not been observed along the [01̅1] and [001] direction, suggesting an ordered n = 2 term without intergrowth of higher terms as described by other authors. Actually, this is confirmed in the high resolution electron microscopy (HREM) image along [01̅1] (Figure 3d) showing uninterrupted periodicities of 1.9 and 0.39 nm, characteristic of an ordered RP n = 2 term. A good agreement between the experimental and calculated image (inserted), taking into account the ordered intergrowth between P and RS blocks, can be observed. This unusual and apparently fully D

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Chemistry of Materials spectra were recorded using a dispersion of 0.25 eV. The sum spectra obtained over an area of 3.05 nm2 are depicted in Figure 4d. The chemical maps obtained from La-M4,5, Ca-M4,5, and Mn-L2,3 signals are shown in Figure 4e−g. These elemental mappings unambiguously indicate that Mn is located at the B site of the P block, La is always placed at the A site of this block, while Ca is at the RS block but also at the P block randomly alternating with La. This situation reflects that the A site of the P blocks is sometimes preferentially occupied by La and sometimes by Ca. This fact has a relevant structural effect because the different A site occupations influence the Mn−O− Mn distortion.27 It is well known that a higher distortion is associated with Ca compared to La. In this sense, this variable and unusual occupation of the A site at the P block suggests the presence of local differences on the octahedral tilt and then local changes of symmetry that could be responsible for the controversial discussion on the XRD data which provides average structural information. In order to get information about the oxygen sublattice, ABF imaging have been obtained.41−43 Using a collection angle between 11 and 22 mrad, the ABF image along [011̅ ] is obtained (Figure 5). It should be noticed that a different scale

Figure 6. Comparison of the Mn-L2,3 signals corresponding to representative punctual EELS spectra of La0.5Ca2.5Mn2O7 (red line), Mn2+ (black line), Mn3+ (blue line), and Mn4+ (green line) standards.

In order to get information about the Mn 4+ /Mn 3+ distribution at the B perovskite site, a spectrum-imaging study was performed (see Supporting Information, Figure SI3). Again, the Mn and ZL signals were simultaneously collected for the correct Mn oxidation state interpretation at the atomic level. Alternatively, the spectra extracted unit cell by unit cell, using a smaller window, have been obtained (Figure 7). As a result, the energy shift on the manganese signal depending on the selected unit cell (marked in yellow) is observed. Even more, these differences are subtly linked to the A composition in P blocks.

Figure 5. Atomically resolved ABF image of La0.5Ca2.5Mn2O7 along [01̅1]. Oxygen atomic positions are in blue. The schematic projection is depicted in the inset.

of contrasts with respect to the HAADF image since the heaviest cation, La, is here seen as the darkest contrast, whereas oxygen atoms are seen as less dark but clearly resolved as can be inferred by comparing with the schematic projection depicted in the inset. A good agreement with the characteristic distribution of the n = 2 model is obtained. In this point, it is worth recalling that oxygen positions are sufficiently clear in the proposed structural model, but the additional details in the image would require detailed simulations. Apart from this elemental chemical identification, an EELS study for the manganese oxidation state has been done. In order to increase the energy resolution, the energy dispersion was set to 0.05 eV, and punctual analysis corresponding to the Mn signal was performed in 20 crystals. Simultaneously to this signal, the zero loss peak (ZL) was also acquired by using dual EELS. Figure 6 shows the characteristic experimental EELS data for the Mn signal, perfectly calibrated as a consequence of the concurrent collection of the ZL, compared to well-known standards for Mn4+ (CaMnO3), Mn3+(LaMnO3), and Mn2+ (CaMnO2). The presence of Mn4+ and Mn3+ with a higher ratio of the first one, in agreement with the nominal composition (75% Mn4+ and 25% of Mn3+), is evident.

Figure 7. (a−c) HAADF images corresponding to La0.5Ca2.5Mn2O7 along [01̅1] simultaneously recorded to EELS acquisition. The schematic projection of the different cationic distributions, blue = Ca (RS blocks), green = La/Ca (P blocks), and red = Mn, is depicted behind each HAADF image. On the right of each image, EELS spectra sums corresponding to the three different marked P unit cells (in yellow) are placed. The energy shift in the Mn-L2,3 signal suggests differences in the Mn oxidation state. The corresponding enlarged HAADF image of the selected P unit cell is included for clarity behind each EELS spectrum. E

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The isothermal data at 350 K for the magnetic field until 7 T confirm the lack of FM long-range order (see inset in Figure 9).

The most common situation is depicted in Figure 7a where the A sites surrounding Mn atoms display a mixture of intensities and the Mn signal fits better with a Mn3+/Mn4+ mixture. We have also found places in which the Mn signal fits better with Mn3+ as depicted in 7b. Notice that in this case the brightest contrast corresponding to La at the P site is more evident. The third situation is shown in Figure 7c where the Mn signal reflects a mixture of Mn3+ and Mn4+ but clearly displaced to Mn4+. It is remarkable that in such situations the intensity of the atomic columns surrounding Mn are clearly less intense, suggesting, in agreement with the previous HAADF and EELS study, a higher Ca concentration. The schematic projection of the different cationic distributions observed in each area is depicted in Figure 7a−c. The ensemble of results allows us to propose an average situation with Mn3+ and Mn4+ randomly distributed, while local deviations comprising a majority of Mn4+ associated with the chemical differences at the A sites have been found. The La0.5Ca2.5Mn2O7 magnetic properties can now be discussed on the basis of the local study concerning Mn oxidation states. This scenario indicates the absence of long-range CO at room temperature but suggests the eventual localization of Mn3+ and Mn4+ surrounded by La3+ and Ca2+ rich environment, respectively. Even more, observed localization ensures the avidity of every A cation for the adequate Mnn+ to keep electroneutrality. Following this trend, a total ordered pattern of Mn3+ and Mn4+ should involve a different La and Ca ordered pattern than the one observed. Actually, CO has not been observed at room temperature. Nevertheless, FM double exchange interactions due to the presence of hopping between Mn3+ and Mn4+ appear weakly at high temperature (Figure 8)

Figure 9. ZFC and FC magnetization measurements of La0.5Ca2.5Mn2O7 as a function of temperature at 1000 Oe. Inset a shows isothermal data at 350 K for the magnetic field until 7 T. Inset b shows temperature dependence of the imaginary part of AC susceptibility. The frequency range and intensity of the used AC magnetic field were from 500 to 10000 Hz and 5 Oe.

At 283 K, the magnetization decreases, whereas the resistivity shows an anomalous increase matching with CO temperature (see insets a and b in Figure 8). At 125 K, besides this CO state, long antiferromagnetic (AFM) order appears. At T lower than 60 K, ZFC and FC curves are different (Figure 9). This behavior could reflect disorder and frustration effects such as spin glass as a consequence of the competition of the different Mn−O−Mn interactions. However, AC susceptibility measurements (see inset b in Figure 9) do not show a maximum at that temperature which shifts upward with increasing frequency, ruling out the existence of a spin-glass-like magnetic state. Taking into account the orthorhombic distortion in this compound, the anomalous behavior detected at 60 K could be related to a spin canting effect in the magnetic structure.

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CONCLUSIONS The above experimental results provide a precise knowledge of La0.5Ca2.5Mn2O7 in terms of structure and composition at the atomic level. A well-ordered orthorhombic n = 2 term has been stabilized through a solid state reaction at high temperature followed by very slow cooling. Isolated defects leading to some disorder, previously found systematically for similar compositions,21−23 have not been detected in spite of being the most frequent situation when chemical strategies are used in similar materials. This situation contrasts with previous results, not only for manganese oxides but for different oxides belonging to the RP series, that evidence the difficulty to stabilize pure n = 2 with such a high order degree, which can only be dealt with using physical methods that control the growth layer by layer. However, order−disorder phenomena concerning the La and Ca positions have been identified. All of the A sites in the RS blocks (AO) are occupied by Ca, whereas the A sites of the P ones (ABO3) show a mixed occupancy of La and the remaining Ca. La is at the P site, while Ca occupies the RS and some P positions. Since this situation induces differences at the local environment of Mn, local degrees of the [MnO6]o octahedral distortion are expected, which could be the origin of the controversial interpretation for similar compositions22,23,26 of the XRD in terms of symmetry. The manganese oxidation state

Figure 8. FC magnetization curve of La0.5Ca2.5Mn2O7 as a function of temperature at 1000 Oe. (a) Temperature dependence of electrical resistance under 0 and 14 T of the applied magnetic field. (c) Electrical resistance and imaginary part of AC susceptibility (500 Hz, 5 Oe) as a function of the temperature.

as previously observed for other authors in close compositions.21 Such interactions could be related, according to our EELS data, to the most observed distribution of Mn (Figure 7a) where a random distribution of Mn3+ and Mn4+ takes place. It is worth mentioning that only FM fluctuations (and no FM order) are assumed at high temperatures as a consequence of the [MnO6]o tilting in the orthorhombic structure of La0.5Ca2.5Mn2O7 that drastically decreases the strength of the FM double exchange interactions. F

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Chemistry of Materials

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has been unambiguously identified with atomic resolution, and although an average random distribution has been detected, a detailed study shows differences on the manganese oxidation state depending on the local composition of Ca and La. A preferential location of Mn4+ and Mn3+ in rich Ca2+ and La3+ environments, respectively, can be suggested using a fine probe provided by the aberration corrected electron microscope. The properties of this phase can then be rationally discussed on the basis of the exhaustive chemical and structural characterizations.

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ASSOCIATED CONTENT

S Supporting Information *

Characteristic SAED pattern along [100] corresponding to La 0.5 Ca 2.5 Mn 2 O 6.5 ; low magnification TEM images of La0.5Ca0.5Mn2O7 showing microcrystals around 2 μm; and HAADF image corresponding to La0.5Ca2.5Mn2O7 along [01̅1] in which the selected area used for EELS analysis is marked. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+) (34) 91 394 43 52. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Spanish Ministry of Innovation, Science and Technology and Spanish Ministry of Economy and Competitiveness through Research Projects CSD2009-00013, TSI-020100-2011-280, MAT2011-23068, and MAT2012-37109-C02-01. We are grateful to Dr. E. Suard for assistance at ILL. R.C-G. acknowledges a postdoctoral fellowship from Moncloa Campus of International Excellence (UCM). We thank the National Center for Electron Microscopy (UCM) for facilities.

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ABBREVIATIONS Ln, lanthanide; Ae, alkaline-earth; CMR, colossal magnetoresistance; CO, charge ordering; RP, Ruddlesden−Popper; P, perovskite; RS, rock salt; FM, ferromagnetism; TEM, transmission electron microscopy; EDS, X-ray energy dispersive; EELS, electron energy loss spectroscopy; STEM, scanning transmission electron microscopy; XRD, X-ray diffraction; ND, neutron diffraction; HAADF, high angle annular dark field; ABF, annular bright field; PCA, principal component analysis; EPMA, electron probe microanalysis; HREM, high resolution electron microscopy; ZFC-FC, zero field cooled and field cooled curves; ZL, zero loss peak

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REFERENCES

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DOI: 10.1021/cm504679r Chem. Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/cm504679r Chem. Mater. XXXX, XXX, XXX−XXX