Effect on the Structural and Magnetic Properties - ACS Publications

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Chlorine Insertion Promoting Iron Reduction in Ba−Fe Hexagonal Perovskites: Effect on the Structural and Magnetic Properties Laura Serrador,† María Hernando,† José L. Martínez,‡ José M. González-Calbet,†,§ Aurea Varela,† F. Javier García-García,*,§ and Marina Parras*,† †

Departamento de Química Inorgánica, Facultad de CC. Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain Instituto de Ciencia de Materiales, CSIC, Cantoblanco 28049 Madrid, Spain § Centro Nacional de Microscopía Electrónica CNME, 28040 Madrid, Spain ‡

S Supporting Information *

ABSTRACT: BaFeCl0.13(2)O2.48(2) has been synthesized and studied. A proper tuning of the synthetic route has been designed to stabilize this compound as a single phase. The thermal stability and evolution, along with the magnetic and structural properties are reported here. The crystal structure has been refined from neutron powder diffraction data, and it is of the type (hhchc)2-10H. It is stable up to a temperature of 900 °C, where the composition reads BaFeCl0.13(2)O2.34(2). The study by electron microscopy shows that the crystal structure suffers no changes in the whole BaFeCl0.13(1)O3−y (2.34 ≤ 3 − y ≤ 2.48) compositional range. Refinement of the magnetic structure shows that the Fe is antiferromagneticaly ordered, with the magnetic moment parallel to the ab plane of the hexagonal structure. At higher temperature, a nonreversible phase transition into a (hchc)-4H structure type takes place with overall composition BaFeCl0.13(1)O2.26(1). Microstructural characterization shows that, in some crystals, this phase intergrows with a seemingly cubic related phase. Differences between these two crystalline phases reside in the chlorine content, which keeps constant through the phase transition for the former and disappears for the latter.

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in the anion substructure have also been tackled3 with some remarkable results in terms of the magnetic properties4 and the oxygen mobility.5 In respect to these expected modified properties, over the past decade a series of barium cobalt oxy-chlorides of general formula Ban+2ConCo2ClO3n+4 has been reported. Their crystal structures can be systematized attending to the size of the blocks of n-face-sharing octahedra [CoO6], which are linked by dimers of corner-sharing tetrahedra. The [CoO4] tetrahedra are generated after removing some oxygen atoms in the [BaO3] layer. At the same time, some chlorine comes in the layer so that the composition reads [BaX γ O δ ] with seemingly uncorrelated values for δ and γ. In all cases, the stacking sequence is kept fixed while the composition is changed according to values for x and y. To date, only two members of this series have been isolated: the n = 3, 10H− Ba5Co5XxO13,3b,c,6 and n = 4, 6H-Ba6Co6XxO16,3a,6,7 where X = Cl, F and 0 < x ≤ 1. The crystal structures for these two compounds are presented in Figure 1a and b, respectively. Two structural features of these oxy-chlorides are eye catching: (i) the halides are located in layers of the type [BaXγOδ], which are sandwiched between cobalt-containing slabs, and (ii) the halides ions are not substituting oxygen but are located at the

INTRODUCTION Solid state compounds that adopt the perovskite structure type, either in the hexagonal or cubic versions, attract considerable interest due to the broad range of physical properties found among them, for instance, superconductivity, colossal magnetoresistance, oxide ion conductivity, or ferroelectricity. Perovskite oxides of general formula ABO3 can be described as a close-packed arrangement of [AO3] layers where the B cations are sitting in all the octahedral holes. Two quite different crystal structures are formed depending on the manner in which the [AO3] layers are piled up. When the [AO3] layers form a cubic close packing (ccp) arrangement, a three-dimensional distribution of corner sharing [BO6] octahedra appears. In contrast, a hexagonal close packing (hcp) produces infinite chains of face sharing [BO6] octahedra. They give rise to the so-called 3C-polytype in the former case and 2H-polytype in the latter. In between these two end members, there exist plenty of different arrangements that the [AO3] layers can adopt. Hence, the crystal chemistry is very rich, and a vast number of different polytypes are described in the literature. This is especially true for ABO3, compounds of general stoichiometry BaBO3,1 where a number of different layer sequences are known, all of them giving rise to different compounds. 2 In what tuning of properties concerns, modification of the cation composition has been traditionally the strategy of choice. However, and to a lesser extent, changes © 2016 American Chemical Society

Received: April 12, 2016 Published: June 8, 2016 6261

DOI: 10.1021/acs.inorgchem.6b00893 Inorg. Chem. 2016, 55, 6261−6270

Article

Inorganic Chemistry

The thermal stability has been carefully analyzed, and we have found a high temperature phase with a (hchc)-4H-type crystal structure.

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EXPERIMENTAL METHODS

Polycrystalline 10H-BaFeCl0.13O3−y was prepared by solid state reaction from a well grinded mixture of BaCO3 (Aldrich, 99.98%), α-Fe2O3 (Aldrich, 99.98%), and BaCl2·2H2O (Merck, 99%) in a ratio of 1.87:1:0.13. The resulting mixture was heated up in an alumina crucible at 850 °C for 48 h in the air. Then, the product was grinded and reheated at 800 °C for 72 h. Finally, two successive treatments for 24 h at 820 and 900 °C, respectively, rendered a black polycrystalline powder. In all the steps, the sample was heated and cooled at a rate of 5 °C/min. The average cationic composition was determined by inductively coupled plasma (ICP), whereas the local composition and the chlorine content were analyzed by X-ray energy dispersive spectroscopy (XEDS). The average oxidation state of iron was determined by the redox titration method. Samples were dissolved in HCl (6N) with an excess of Mohr salt (NH4)2Fe(SO4)2·6H2O. The Fe2+ ions react with the possible Fe4+ present in the sample leading to Fe3+ ions. The amount of remaining Fe2+ ions is determined by titration with a 0.1N K2Cr2O7 solution. Thermogravimetric analysis (TGA), thermal stability, of the 10Hstarting material was performed in the air with a heating ratio of 5 °C· min−1 by using a SII TG/DTA6300 thermogravimetric analyzer at different temperatures. Powder X-ray diffraction, XRD, patterns were collected using Cu Kα1 monochromatic radiation (Kα1 = 0.15405 nm) at room temperature on a Panalytical X’PERT PRO MPD diffractometer equipped with a germanium 111 primary beam monochromator and X’Celerator fast detector. Neutron powder diffraction, NPD, analysis was performed at the Institut Laue Langevin Grenoble, France on the D2B diffractometer (λ = 0.1594 nm). The data were analyzed with the Rietveld method using the software package FullProf.11 Transmission electron microscopy, TEM; electron energy loss spectroscopy, and EELS; selected area electron diffraction, SAED, studies were carried out in a JEOL 2100HT. For conventional high resolution transmission electron microscopy (HRTEM), a JEOL 3000F was used. These two microscopes are attached with OXFORD INCA detectors for chemical composition analysis by X-ray energydispersive spectroscopy (XEDS). The latter is additionally equipped with an ENFINA spectrometer for electron energy loss spectroscopy, EELS. Scanning transmission electron microscopy, STEM, at atomic resolution was performed on a JEOL JEM-ARM 200cF, which is Cs probe corrected and operated at 200 kV. The scope is attached with a GIF-Quantum ERTM spectrometer and an Oxford INCA-350 detector for EELS and XEDS, respectively. Image simulations were carried out by using the EMS software package.12 In all cases, the multislice algorithm has been used for the images calculations. The magnetic susceptibility was measured in the low temperature range (2−400 K) with a simple sample holder system, inside a SQUID MPMS XL-5 manufactured by Quantum Design under an applied magnetic field of 500 Oe. For the high temperature range (300−800 K), a resistive furnace was inserted, and the sample holder was changed to a quartz type holder (optical quality with extremely low impurity levels). The field dependence of the magnetization at 5 K was carried out in the range 0−50 kOe.

Figure 1. Crystal structure models for the halogen-cobaltites 10H− Ba5Co5XxO13 (a) and 6H-Ba6Co6XxO16 (X = Cl, F; 0 < x ≤ 1) (b) along with the crystal structure model of the related cobalt oxides 5HBaCoO2.80 (c) and 12H-BaCoO2.6 (d). Coordination polyhedra for Fe are shown. Green, red, blue, and yellow spheres represent the positions of Ba, Fe, O, and Cl, respectively. The stacking sequence is indicated for all models.

center of trigonal bipyramids [Ba5X]. Hence, they are not directly coordinated to Co. For these cations, sitting in the neighboring layers, the resulting coordination is an oxygen tetrahedron. A major structural difference between the oxides and the halide-containing counterparts is worth analyzing carefully. This is the connectivity between the blocks of faces sharing octahedra at the tetrahedral level, which depends on the atomic distribution within the [BaXγOδ] planes. In the related oxides 5H-BaCoO2.80,8 Figure 1c, and 12H-BaCoO2.60,9 Figure 1d, the blocks are disconnected on both sides of the intermediate [BaO2] layer, whereas in the oxy-chlorides, the blocks are connected through dimeric units [Co2O7], which serve as bridges between the blocks. A main consequence of this connectivity pattern, apart from the overall composition modification, is that the effect of the halide doping might be described as a reducing process. Thus, the whole negative charge in the crystal structure decreases, forcing the cation sublattice to reduce the average oxidation state accordingly to neutrality. The transition metal is then forced to a lower oxidation state, and as a main consequence the possible magnetic interactions are necessarily changed. All that, and according to the published data, it happens with no changes in the cations’ substructure. The barium cobalt oxy-chlorides are the most studied hexagonal systems, and even though some other cations sitting in the B position have been studied (i.e., Fe and Mn9,10), no systematic studies performed by exclusively using other cations have been published. Looking at the results of the barium cobalt oxy-chlorides, the B ions should be stable both in tetrahedral (either isolated and connected forming dimers) and in octahedral environments. The total negative charge from the anions sublattice is modified, and the cations should be able to easily change their formal valence accordingly. Like this, besides from Co there exist several transition metals which they could perfectly fix into these needed characteristics. This is the case for Fe, and in this paper we address the synthesis and characterization of the structural and magnetic properties of the compound BaFeClxO3−y, which adopts the (hhchc)2-10H structure type. The synthetic method has been refined as to render a single phase sample, which has been confirmed by neutron powder diffraction along with transmission electron microscopy. Hence, a full characterization of the magnetic properties and of the magnetic structure have been carried out.

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RESULTS AND DISCUSSION Compositional and Structural Characterization. The cationic composition as well as the chlorine content was determined on several dozens of small crystallites by XEDS analysis. In agreement with the nominal cationic composition, a Ba/Fe ratio close to 1/1 is obtained, whereas the analyzed chlorine content corresponds to x = 0.13 per unit formula, resulting in BaFeCl0.13O3−y. The average oxidation state of iron, 6262


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Inorganic Chemistry as determined by redox titration, results in a value of 3.08(2), so that y is 0.52. According to that, the composition of the obtained phase reads BaFeCl0.13(2)O2.48(2). The XRD pattern corresponding to the obtained sample is depicted in Figure S1 of the Supporting Information. All reflections can be indexed based on the hexagonal lattice for the (hhchc)2-10H polytype. The refined unit cell parameters are a = 0.57765(3) nm and c = 2.45942(15) nm. Accordingly with this structure type, the symmetry extinction conditions for the space group P63/mmc are obeyed, and no extra reflections coming from different phases are detected. The atomic scale order has been inferred from the transmission electron microscopy results, both in real and in reciprocal space. Tilting experiments in the electron microscope revealed the associated reciprocal lattice corresponding to the hexagonal (hhchc)2-10H polytype. In Figure 2, the results of one such

Figure 3. Rietveld refinement of neutron diffraction data at room temperature of 10H-BaFeCl0.13O2.48. The solid lines and overlying dots indicate the calculated and observed intensities. The second phase corresponds to the antiferromagnetic structure which exists at room temperature.

hexagonal layers building the (hhchc) 2 -10H structure. Compared to an ideal 10H-A5B5O15, the highest concentration of vacancies is located in the chlorine-containing h′-layers, in which only a percentage close to 30% of the oxygen (O3) sites are occupied. This yields [Fe2O7] pairs of corner-sharing tetrahedra (see Figure 4) as previously found in Ba5CoFe4ClO12.43.13 If this compound is taken as a reference, the maximum occupancy factor is forced to be 33%, resulting of the occupation of 1/3 of the delocalized O3 on three positions. The refined occupancy is 30% so that there exist 10% vacancies in this atomic position. This surprising feature has been previously addressed in Ba5CoFe4ClO12.43.13 Refinements of the anions’ sublattice render partial compositions of [BaO2.93(3)] for the cubic layers and [BaO2.87(3)] and [BaCl0.65O0.90(3)] for the hexagonal ones, see Table 1. The final composition is then given as BaFeCl0.130(2)O2.48(2), which corresponds to a +3.08 average oxidation state for Fe, in good agreement with the value obtained by redox titration analysis. The iron atoms are occupying three independent positions. Fe1 and Fe2 are located at the center of the face sharing octahedra that define the trimer units, sitting in the outer and central octahedra, respectively. The central octahedron consists of six identical Fe2−O distances (1.887(3)Å), whereas the terminal ones are distorted with three short Fe1−O1 (1.914(4) Å) and three long Fe1−O2 distances (2.110(4)Å; see Table 2). Fe3 is occupying the tetrahedron resulting from the insertion of the halide in the hexagonal layer. There are three long Fe3−O1 distances of 1.852(3) and another three shorter Fe3−O3 distances of 1.805(5). Note that these latter three distances show up as a result of the delocalization of 1/3 of the O3 into three positions so that the coordination of Fe3 is four (as corresponds to a tetrahedron) due to the fact that locally nowhere would all three atoms exist at the same time but only just one of them. All obtained Fe−O positions are therefore in good agreement with the previously reported values either in barium iron oxides, i.e. (hhchc)2-10H-BaFeO3−y,15 or in barium cobalt and iron oxychlorides.13 If the situation of Fe in the structure is described as rather well ordered, it is not the same case for Ba. The hexagonal h′-[BaClγOδ] planes present disorder reflected in the thermal parameters for both O3 and Ba1.

Figure 2. SAED patterns corresponding to a crystal of 10HBaFeCl0.13O2.48 oriented down (a) , (b) , and (c) . All three patterns were recorded in the same crystal. Indexation is done according to the corresponding reciprocal lattice to this structure type. Note the complete absence of extra diffracted intensity.

experiment are shown. All SAED patterns belong to the same crystal, and they were obtained by tilting the crystal while keeping the [0001]* excited. As stated here above, indexation of all observed reflections is done according to the (hhchc)210H structure type, and no indications of distortions away from this polytype were observed. For the refinements of the NPD data, the model reported by Iorgulescu et al.13 was used as a starting point. At first, the scale factor, unit cell parameters, and profile factors were allowed to refine, and just after a few refinement cycles, the convergence of the refinement was achieved. In the final refinements, the antiferromagnetic structure was also included, and the occupancy factors along the anisotropic temperature parameters were allowed to refine. The final fitting of the data along with the difference profiles between observed and calculated data are shown in Figure 3, including the refinement of the antiferromagnetic structure. The refined atomic positions are listed in Table 1, and in Table 2 the selected interatomic distances are given. The structure consists of trimers of facesharing octahedra connected through their terminal corners to two tetrahedra (Figure 4). The halogen atoms are accommodated in the hexagonal layers in the very same manner as reported previously, i.e., refs 13 and 14. These layers are then transformed into anionic-deficient layers, h′-[BaClγOδ], where Cl and O3 occupy different crystallographic positions. As a result, there are units of [Fe2O7] pairs of corner-sharing tetrahedra, which along with [ClBa5] trigonal bipyramids complete the polyhedral description of the structure. Special care has been taken when refining the anion sublattice, as this is one of the major targets of this investigation. The refined occupation parameters tell that the oxygen vacancies are located both in the cubic and in the 6263


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Inorganic Chemistry Table 1. Final Structural Parameters of the 10H-BaFeCl0.13O2.48 at Room Temperaturea Ba1 Ba2 Ba3 Fe1 Fe2 Fe3 O1 O2 O3 Cl

Wyck

x/ab

y/bb

z/cb

Biso (Å2)

occ.

2b 4f 4f 4e 2a 4f 12k 12k 6h 2c

0 1/3 2/3 0 0 2/3 0.8371(8) 0.1494(5) 0.634(3) 1/3

0 2/3 1/3 0 0 1/3 0.6744(17) 0.2984(11) 0.269(6) 2/3

0.25 0.1273(3) 0.0391(3) 0.10761(13) 0 0.17775(14) 0.14849(14) 0.04696(13) 0.25 0.25

c 0.5(1) 0.32(9) 0.26(5) 0.24(9) 0.27(5) 0.90(7) 0.27(6) 0.9(4) 0.63(1)

1 1 1 1 1 1 0.977(9) 0.958(9) 0.301(11) 0.651(12)

M (μB)

3.05(10) 2.03(15) 3.14(9)

Fit parameters: Rp = 3.92, Rwp = 5.12, Rexp = 2.39, χ2 = 4.57, RB = 5.15, Rmag = 8.33 ba = 5.7756(2) Å, c = 24.5973(1) Å, P63/mmc (RT). Anisotropic thermal factor for Ba1: B11 = 0.023856, B22 = 0.023856, B33 = 0.000933, B12 = −0.011928.

a c

position. Note that such behavior has also been observed for BaMn0.4Fe0.6O2.7.16 To perform a further structural study of this h′-[BaCl0.65O0.9] layer, difference Fourier maps were calculated (Figure 4). From this, an important oxygen disorder is evidenced inside the layer. As can be seen, O3 is displaced by 0.37 Å away from its central symmetric position around the 3-fold axis, giving rise to a broad nucleon density around the ideal atomic position. This situation has been found in different oxyhalides such as Ba6Co6ClO15.57 or Ba6Co6FxO16−δ6 and 15R-BaFeF0.2O2.44.5 In oxyfluoride phases, a fraction of fluorine ions is displaced inside the hexagonal layers, from the center of the bipyramidal site toward the coordination sphere of the iron sites and, therefore, [Fe2O7] tetrahedra, more or less anionic deficient [Fe2(FO)9−x] octahedra are generated. On the contrary, in 10H-BaFeCl0.13O2.48 the calculated difference Fourier map shows an isotropic shape for the Cl atoms, suggesting that they remain in the central position of the barium pyramids, and they are not present in the Fe coordination sphere. The same detail is also found in other oxychloride, such as Ba5(CoFe)5ClxO15−y.13 The atomic structure of this compound was further studied by high resolution electron microscopy. We have used both the conventional (CTEM) and the scanning transmission electron microscopy modes (STEM). Obviously, the most informative direction is down [110], and then this direction is the one we have used to image all crystals. The results of the STEM and the CTEM experiments are presented in Figures 5 and S2, Supporting Information, respectively. The images presented in Figure 5a, b, and c, in HAADF, BF, and ABF modes, respectively, have been recorded in a 10H-BaFeCl0.13O2.48

Table 2. Selected Interatomic Distances for 10HBaFeCl0.13O2.48 atoms

d (Å)

Ba1−O1 Ba1−O3 Ba1−Cl Ba2−O1 Ba2−O2 Ba2−Cl Ba3−O1 Ba3−O2 Ba3−O2 Cl−Ba1 Cl−Ba2

2.981(4) × 6 3.188(15) × 6 3.334(4) × 3 2.935(6) × 6 2.701(6) × 3 3.018(7) 3.186(7) × 3 2.901(3) × 6 2.806(6) × 3 3.334(2) × 3 3.018(7) × 2

atoms

d (Å)

Fe1−O1 Fe1−O2

1.914(4) × 3 2.110(4) × 3

Fe2−O2

1.887(3) × 6

Fe3−O1 Fe3−O3

1.852(5) × 3 1.805(5) × 3

Fe1 − Fe2 Fe1−Fe3 Fe3−Fe3

2.647(3) 3.756(3) 3.549(2)

Figure 4. (a) Crystal structure model of the 10H-BaFeCl0.13O2.48 oriented down as refined from neutron powder diffraction data. The Ba, Fe, O, and Cl atoms are represented by green, red, blue, and yelow spheres, respectively. The antiferromagnetic spin ordering is included. The moments lie parallel to the ab plane of the hexagonal structure (arbitrary alignment along a is chosen). Ba1 is represented with the refined anisotropic temperature factor. In b, the calculated Fourier map within the h′-BaCl0.65O0.9 layer (z = 0.25) is shown.

The disorder situation in the hexagonal h′-[BaCl0.65O0.9] layer is reflected in the thermal factors for barium constituting that nonclose-packed layer. The refinement of the anisotropic thermal factors for Ba1 atoms gives rise to the xy-plane ellipsoids shown in Figure 4. These anisotropic thermal factors reveal the local disorder due to the presence of a large number of oxygen vacancies in the [BaCl0.65O0.9] layer indicating significant delocalization of this atom around its average

Figure 5. (a, b, and c) High resolution images in HAADF, BF, and ABF modes as recorded in a 10H-BaFeCl0.13O2.48 crystal oriented along the [110] direction. By using the ABF mode, the anion sublattice is made visible, and the chlorine and oxygen atoms are then seen. By making a common analysis of all three types of images, all the atoms can be located in the projected structure, the inset on the left side of the panel. Green, red, yellow, and blue spheres indicate Ba, Fe, Cl, and O positions. 6264


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Inorganic Chemistry crystal oriented along the [110] direction. By making a common analysis of all three types of images, all the atoms can be located in the projected structure. In a, the intensity of the spots reproduces the Ba atomic structure in projection. The Fe positions are also clearly observed, even though much more faint, as a consequence of the difference in Z between these two elements. In b, the BF image is inserted, and the image after removing the elastically scattered electrons, the so-called ABF, is presented in c. In the latter, the positions of the anions, chlorine and oxygen, are observed and, furthermore, distinguished from each other. Like this, we can assign the Ba, Fe, Cl, and O atomic columns as green, red, yellow, and blue spheres. The agreement with the model refined from NPD data is evident. Besides, no further differences between the atomic positions as refined are observed. Magnetic Properties. Even at room temperature, the NPD data show heavy magnetic contributions, which correspond to a 3D-AFM ordering. We refined from data collected at room temperature using the propagation vector k = [0,0,0] for the magnetic structure. Two models have been reported in 15R and 6H Ba/Fe oxyfluorides leading to different magneto-crystalline anisotropy. 6H-Ba0.8Sr0.2FeF0.2O2.48 and 15R-BaFeF0.2O2.424 and BaFeO2F17 show antiferromagnetic order, but whereas in the 6H-phase moments are laying in the ab plane, in both 15Rforms they are aligned to the c axis. In the studied 10HBaFeCl0.13O2.48, the best fit was obtained from a magnetic model which assumes antiferromagnetic interactions between nearest neighbors (NN) where the moments lie in the (ab) plane. This model results in a magnetic structure consisting of ferromagnetic sheets with the magnetic moments stacked antiferromagnetically perpendicular to the c axis (Figure 4). The magnetic moment values obtained at RT are M(Fe1) = 3.05 μB, M(Fe2) = 2.03 μB, and M(Fe3) = 3.14 μB. By comparing these values with those corresponding to Fe3+ (μeff = 5.91 μB in the spin only model), it seems that the antiferromagnetic order involves strong AFM exchanges. In fact, the magnetization curve versus temperature (Figure 6a) shows an increase of the susceptibility at a temperature close to 720 K that could be associated with the Néel temperature (TN), with a significant ferromagnetic component (probably on the stacking AF moment along the c axis). A similar magnetic behavior is found in 6H-Ba0.8SrFeF0.2O2.45 or in 15RBaFeF0.2O2.44 compounds with TN values about 700−730 K, but up to now, it has not been described for any oxychloride. This robust AFM is given by AFM exchange interactions Fe3+− O−Fe3+ at 180° consistent with those predicted by the Goodenough rules.18 The incorporation of chlorine in the hexagonal layers gives rise to a chemical reduction increasing the Fe 3+ content. The oxygen vacancies created are accommodated in such a way that face-sharing octahedra are replaced by corner-sharing dimers. This leads to a change in the Fe−O−Fe connectivity from 90° to nearly 180° (Fe3−O−Fe3 = 159.8°), reinforcing the magnetic exchanges between Fe3+ cations. For comparison, in the nonchlorinated 10H-BaFeO2.80, the cooperative AF interactions persist above room temperature although the TN has not been determined.15a The magnetic moment for the Fe2 located in central face sharing octahedra is 1 μB lower than the corresponding value to Fe1 and Fe3. This feature could be due to a higher covalence bond Fe2−O in this central octahedral site, as it had been already observed in 15R-BaFeO2F17 and 10H-BaFeO2.8.15a This difference in the magnetic moment gives rise to a ferrimagnetic structure, with no compensated magnetic component, that

Figure 6. (a) Magnetization versus temperature under an applied magnetic field of 500 Oe in the temperature range between 2 and 800 K for BaFeCl0.13O2.48. (b) Magnetization versus magnetic field at 5 K. The connection of the different set of data (T ≈ 400 K) is very good, after a small correction due to the difference in the background signal coming from the different type and the mass of the sample holder.

could be the origin of the divergence in the susceptibility curve versus temperature between ZFC−FC below 400 K (Figure 6a). This feature could also explain the weak ferromagnetic moment (0.45 emu/g) at 5 K evidenced from the hysteresis loop (Figure 6b). Thermal Stability. The results of the thermogravimetric analysis, TGA, of the sample are presented in Figure 7. The TG curve under air and up to 1100 °C is shown in Figure 7a, and the evolution of this compound can be pictured as follows. The sample displays a slight oxidation in between 360 and 520 °C; above this temperature, the weight loss runs up to 1100 °C. The process splits up into two. In the first step, a weight loss of Δw ≈ 0.9% takes places up to a temperature close to 900 °C, where a sharp change in the slope of the curve occurs. On further heating and up to 1100 °C, a progressive weight loss happens, which is not smooth but with an inflection point at around 1050 °C. The resulting total weight loss is Δw ≈ 2.12%. During the cooling process, the weight increases by Δw ≈ 1.04% down to room temperature. This quite different value points toward the idea of an irreversible phase transition or chemical reactions happening. This is confirmed by the XRD pattern of the final product shown in Figure 7b, which does clearly indicate the disappearance of the 10H-polytype phase (compare with the pattern presented in Figure S1 where all diffraction peaks from this phase are observed). A careful indexing of this new set of reflections could be done based on a (hchc)-4H structure type. The unit cell parameters were refined to values close to a = 0.576 nm and c = 0.987 nm, by using the P6 3/mmc space group. Additionally, several unindexed 6265


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in the structure. Note that this strong thermal stability connects well with previous studies in some related phases. It seems to be a general feature of fluorinated and chlorinated Co/Fehexagonal polytypes, which also present a good structural stability up to 900 or 1000 °C.5,17,19 The TGA plot from RT up to 1050 °C is presented in Figure 8a, whereas the XRD of the final product is shown in Figure 8b.

Figure 7. (a) Thermogravimetric analysis of 10H-BaFeCl0.13O2.48 under air from room temperature up to 1100 °C. (b) X-ray diffraction pattern of the TGA final product. Indexation is done based on a (hchc)-4H structure type. The set of extra maxima should be associated with a cubic related BaFeO2.5+δ.

reflections, indicated in the pattern, are present. These lines, along with the data from electron microscopy, vide infra, are interpreted because of the presence of a cubic related compound, which might correspond to a BaFeO2.5+δ like phase as a small impurity. This result indicates that the reduction process of the 10H sample is accompanied by a phase transition to a new (hchc)4H structure type. To clarify and to add some more light into the structural change associated with this reduction process, we have designed two TGA complementary experiments at two temperature ranges: (i) from room temperature up to 900 °C and (ii) up to 1050 °C. The TGA curve from RT up to 900 °C was carried out in two subsequent heating/cooling cycles (Figure S3 Supporting Information). The reduction/oxidation process is reversible in this temperature range. The final product is identified by XRD as 10H-BaFeClxOy (Figure S3) without any extra maxima which could indicate the partial decomposition or transformation of the starting material, which is confirmed by SAED and HREM. Compositional analysis by XEDS evidenced that the chlorine content remains constant through the whole process of heating and cooling. Therefore, we learned that the loss of weight (Δw = 1.15%) is associated with the decrease of the oxygen content (0.16 per unit formula). The resulting stoichiometry for this reduced high temperature 10H sample is BaFeCl0.13O2.34. The average Fe oxidation state that follows is 2.81, which corresponds to 19% Fe2+ and 81% Fe3+. Accordingly, it can be stated that hexagonal 10H-polytype is stable up to temperatures close to 900 °C without significant chlorine being lost and with the oxygen content ranging from 2.48(2) (10H-BaFe +3.1 Cl 0.13 O 2.48(2) ) to 2.34 (4H-BaFe+2.81Cl0.13O2.34(3)). We also here conclude that the oxygen vacancies are randomly distributed through all oxygen positions

Figure 8. (a) Thermogravimetric analysis of 10H-BaFeCl0.13O2.48 under air from room temperature up to 1050 °C. (b) X-ray diffraction pattern of the TGA final product. Indexation is done based on a (hchc)-4H structure type. The set of extra maxima should be associated with a cubic related BaFeO2.5+δ.

In this temperature range, the redox process is clearly nonreversible, and it is accompanied by a phase transition into a 4H polytype, as evidenced in the XRD pattern. From 900 °C up to 1050 °C, the total weight loss corresponds to Δw = 0.73%, whereas during the cooling process an increase of 0.67% in weight is observed showing a nonreversible red-ox process in this temperature range. The XRD pattern of the final product shows that the main phase corresponds to a 4H polytype, although the cubic BaFeO2.5+δ maxima of very weak intensity are also observed (marked with arrows in the pattern). This result tells us that, under these thermal conditions, a small chlorine loss invariably occurs. Therefore, a decomposition of the oxy-chloride into oxide necessarily takes place to some extent. From these data, the resulting composition is close to BaFeCl0.13O2.26(1); this is assuming that the only phase appearing in the transformation process would be a 4H type phase. Hence, and once we have exposed the existence of a minority phase, the final corresponding formula for the 4H type phase is the subject of additional analysis as follows. All attempts to obtain the 4H phase by solid state reaction from BaCO3, BaCl2, and Fe2O3 as starting materials were unsuccessful. We assume that the reason is the high temperature and long reaction times required that provoke the chlorine leaving the reaction media and giving rise to the corresponding Ba−Fe oxides. After this, the strategy decided upon was to anneal the 10H-BaFeCl0.13O2.48 in the temperature 6266


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Inorganic Chemistry range between 900 and 1050 °C by using different reaction times. This phase transition might suffer from the very same problems that the direct synthesis tried before. On one hand, the lower the temperature, the longer the reaction times that are required, and therefore increasing the chlorine loss. On the other hand, slow cooling from the temperature of the reaction down to room temperature results in a significant reoxidation into the starting 10H- phase. The conclusion is that the best result is obtained by annealing in the air, at 1050 °C, and using short reaction times (from 30 min to 1 h) followed by quenching in liquid nitrogen. Figure 9 shows an XRD powder

Figure 9. Rietveld refinement of the profile of the X-ray diffraction data of the 4H-BaFeCl0.13O2.5−δ.

pattern along with the result of the Le-Bail refinements of the resulting 4H type phase (P63/mmc space group and unit cell parameters a = 0.576292(6) and c = 0.987891(8) nm). The extra reflections, denoting the presence of a cubic related compound, as mentioned above, are inserted in a second refinement cycle as a secondary phase with the space group Pm3̅m; the resulting parameter is close to a = 0.40838(4) nm. In the electron microscope, the 4H phase is clearly dominant. However, we could find some crystals showing a peculiar feature worth further study. In Figure 10, a typical image of this type of crystal is shown. Clearly, the crystal contains two different areas which give a rather different contrast in the HAADF-STEM mode. The major component presents a 4H crystal structure type, and the minor one seems to correspond to the cubic type structure mentioned before. In Figure 10b, the contrast in both domains is better appreciated as well as the interphase. Interpretation of the contrast is easily done based on hchc-4H and ccc-3C structures, which is in full agreement with the electron diffraction experiments, vide infra. Differences between these two phases were further studied by electron diffraction and XEDS analysis. After analyzing several crystals, we concluded that the cubic phase is present as a minority component although the observed extensions of the domains change from crystal to crystal. Figure 10c shows the electron diffraction pattern of a cubic domain of roughly 0.4 nm indexed according to a cubic perovskite unit cell. In Figure 10e, the diffraction pattern after moving the incident beam of electrons into a neighbor hexagonal domain is presented and indexed correspondingly. Finally, the pattern shown in Figure 10d was obtained by selecting an area covering roughly half of a cubic and half of a hexagonal domain. A most eye-catching detail of the electron diffraction experiment is the splitting observed in the diffraction spots, in such a way that the resulting cubic and hexagonal cells are metrically slightly different. This is furthermore proved by the presence of the extra diffraction

Figure 10. (a) High resolution STEM image of a crystal oriented down [1−10]. In the area shown, there exists clearly a portion of the crystal with a completely different contrast to that expected for a 4H structure type. Besides, the recorded XEDS patterns are inset in the corresponding areas. To make the differences much more apparent, we present a portion of the image in a at higher magnification in b. In c, d, and e, three electron diffraction patterns are presented which were recorded in the same crystal and down the same direction, but different areas. Parts c and d are indexed according to a cubic and hexagonal perovskite respectively, while e was obtained by diffracting in an area covering both domains.

peaks in the powder diffraction pattern. The results of the compositional analysis, inset in the corresponding areas in Figure 10a, indicate that chlorine is completely absent in the cubic domains while it is present in the 4H structure type, and furthermore the Ba/Fe and chlorine ratios keep constant with respect to the starting 10H phase. All these data together suggest that this phase corresponds to a cubic related BaFeO2.5+δ. At this point, we turn our attention into carrying out a detailed characterization of the 4H phase. Careful tilting experiments were carried out in the 4H polytype, and the results are collected in Figure 11. In a, b, c, and d, the , , , and [001] zone axis electron diffraction patterns, respectively, are presented. Note that a−c patterns were recorded in the same crystal. Indexation is done according to the hexagonal unit cell and space group symmetry for this structure type. However, and as it is apparent in these diffraction patterns, the presence of extra diffracted intensity 6267


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Figure 11. (a, b, c, and d) The , , , and [0001] zone axis electron diffraction patterns corresponding to 4H sample are presented. All the patterns are recorded in the same crystal.

indicates that the real atomic structure is far from being perfectly well ordered. This extra diffracted intensity seems to define quite sharp satellite reflections. The set of reflections that define what in principle constitute the basic structure are indexed according to unit cell parameters and symmetry extinction conditions in agreement with a 4H polytype. A detailed description of these extra reciprocal space features is presented in the Supporting Information, section II.1. The results of the high resolution CTEM are shown in Figure 12 where an image along [110], in a, is presented. The digital diffraction pattern is inset and oriented with respect to the image. It agrees with the previously shown SAED, Figure 12a. The figure is completed with two magnified areas of the image, in b and c, where the corresponding simulated images are inserted. The contrast observed suggests a rather well ordered atomic structure, but however, some fine deviations of the contrast are clearly present, as small areas of darker contrast. Therefore, the origin of the satellites’ reflections can be attributed to such contrast deviations. The similarities between the simulation and the experimental images, which are in fact acceptable, might well be interpreted as the cation substructure being basically undistorted as long as the contrast is dominated by them in any high resolution CTEM image. Thus, we are forced to interpret the differences based on distortions in the anion sublattice. This conclusion is further supported by the high resolution STEM images. These images, as seen in Figure 13, show a contrast in agreement with an (hchc)-4H structure type. In the pictures, the sequence of cubic and hexagonal planes is indicated, and slight differences can be observed between the expected contrast, for an ideal 4H-type structure, and the real atomic structure of the compound under study. We here work out a plausible structure model for this compound based on (i) the barium atom positions sitting on the c planes move along the hexagonal axis of the structure. The short distance between these hexagonal layers, where the chlorine incorporation should happen, provokes the rearrangement of the Ba atoms in the adjacent cubic layers. This is additionally supported by the knowledge gained from the 10Hstructure type. In this model, the chlorine atoms are incorporated in the h′ layers as we assume it happens in the 4H structure type. As a result of this, the Ba atoms sitting in the neighbor c layers slightly move apart along the hexagonal axis. (ii) The anion positions are well-defined on the cubic layers,

Figure 12. (a) High resolution image of a 4H-BaFeCl0.13O2.26 crystal oriented down . The digital diffraction pattern is inset and oriented with respect to the image. Note the clear presence of welldefined satellite reflections. (b and c) Two areas of an image with the simulated one are presented. The inset in b and c fits perfectly with the experimental ones at Δt = 1.6 nm and Δf = −30 and Δf = −70, respectively. Green, red, and blue spheres indicate Ba, Fe, and anion positions.

Figure 13. HAADF and ABF images of a 4H crystal oriented along . The atomic positions in projection are indicated. Green, red, and blue spheres indicate Ba, Fe, and anion positions. Two points are to be stressed: (i) in the HAADF image, distortions of the Ba atoms along the hexagonal axis are observed, and (ii) the anion subtructure as observed in the image corresponds to the average, out of the present disorder, in projection.

while in the hexagonal ones the corresponding contrast is changing from layer to layer. In some cases, they are better described as a continuous rod parallel to the (therefore contained in the c plane), while in other cases there is blurred and more delocalized dark contrast. 6268


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well with the complexity of the analysis just by the sight of the STEM images.

All these structural features have been additionally studied by neutron powder diffraction. The synthetic route to access this phase forced us to run the neutron powder diffraction data with a much lesser amount of sample than in the case of the 10H phase. Furthermore, and as we have explained already, the crystal structure is disordered, and not only that, the type of disorder observed is completely “non-refineable.” Even more, the contribution of the magnetic structure is also there at room temperature. Even like this, we collected the powder data and refined against the 4H structure type. The results of the refinements are given in Table S1. The powder pattern, after the Rietveld matching, is presented in Figure S5. In agreement with what we have already suggested, this refinement confirms the following points: (i) the Cl anions are inserted in the hexagonal layers, with Cl and O occupying distinct crystallographic positions as can be observed in the 10H BaFeCl0.13O2.48. (ii) The Ba1 gives an anomalous high displacement parameter (≈ 5.9 Å2) that points toward the splitting of this position. By doing so, we could refine the positions of two Ba atoms slightly out of plane z = 0, one up and the other down (with z ≈ ±0.036). Note that this displacement is observed in the STEM images. (iii) The refined composition is BaFeCl0.13(1)O2.26(1). The resulting structure model is shown in Figure 14.

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CONCLUSIONS A BaFeCl0.13(2)O2.48(2) has been successfully synthesized as a single phase by solid state reaction. Tuning of the synthesis route ended up in several intermediate steps with different annealing temperatures in between 800 and 900 °C. The magnetic and structural properties are reported here. BaFeCl0.13(2)O2.48(2) adopts a (hhchc)2-10H structure type, and it presents antiferromagnetic order involving strong antiferromagnetic exchanges (TN close to 720 K). The study is completed with the thermal stability. The thermal evolution of this compound shows that it is stable up to 900 °C, where the composition becomes BaFeCl0.13(2)O2.34(2). The whole process, involving a redox reaction, is fully reversible. The average oxidation state of Fe decreases from +3.1 to +2.8, and the study by electron microscopy tells that the crystal structure is unaffected and the oxygen vacancies seem randomly distributed. If the temperature is further increased, a (hchc)-4H structure type condenses out. The average composition of the crystals is ≈ BaFeCl0.13(1)O2.26(1), as calculated from thermal analysis data, also confirmed by XEDS and neutron diffraction data. This result is a very interesting one as long as it is the first time, to our knowledge, that a barium iron oxychloride is reported to adopt such a crystal structure type. Not only that, but this structure type has never been reported within these chlorine containing systems. Besides, another surprise popped up when we could detect a cubic related phase to intergrowth with it. This cubic related phase does not contain Cl, at the same time that the Ba/Fe ratio keeps constant. Such findings allow us to argue that there exists interplay between the presence of Cl and the structure type preferred, when the oxygen content is reduced. The magnetic structure of the (hchc)-4H phase is similar to that found in 10H-BaFeCl0.13(2)O2.48(2). The antiferromagnetic exchange interactions are also rather robust as reflected in the remarkable TN close to 730 K.

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Figure 14. Crystal structure representation of the 4H-polytype as refined from neutron powder diffraction data. Atomic positions for Ba, Fe, O, and Cl are presented as green, red, blue, and yellow, respectively. The refined magnetic structure is depicted. In the picture the anion positions are presented as fully occupied although the refinements show vacancies randomly distributed in the hexagonal layers, [BaClγOδ].

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00893. Microstructural characterization (HREM) and thermogravimetric study of 10H-BaFeCl0.13O2.48(2) sample; structural characterization from NPD data, microstructural characterization by SAED, and magnetic properties of 4H-BaFeCl0.13O2.26(1) (PDF)

The composition for this phase clarifies that a disorder situation is necessarily present. As a starting point, we restrict ourselves to the observed 4H unit cell, so that the anion compositional range is from ABO3 to ABO2, which is made up of B−O6 octahedra and B−O4 tetrahedra, respectively. The fact that the anion sublattice content is somewhere in between says that a disorder mixture of both environments could be present. Like this, what we should locally find in projection is a mixture of all possible situations. This is reflected in the high displacement parameter value refined for O2, which is sitting on the hexagonal layers. As a consequence, we suggest that this might be the reason for the observed differences in the anion position within these layers. Besides, the chlorine content, which keeps constant through the phase change (0.13 per unit formula), gives a composition of 0.26 per single layer, clearly lower than for the 10H structure type of 0.65. This detail agrees

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support through research projects MAT2014-54372R and CSD2009-00013 is acknowledged. We thank Dr. M. T. ́ for assistance in collecting the neutron powder Fernández-Diaz diffraction data. 6269


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