Canted Antiferromagnetism on Rectangular Layers of Fe2+ in

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Canted Antiferromagnetism on Rectangular Layers of Fe2+ in Polymorphic CaFeSeO Kwing To Lai,† Alexander Christoph Komarek,† Maria Teresa Fernández-Díaz,‡ Pi-Shan Chang,†,§ Sungjoon Huh,†,∥ Helge Rosner,† Chang-Yang Kuo,† Zhiwei Hu,† Tun-Wen Pi,⊥ Peter Adler,† Vadim Ksenofontov,# Liu Hao Tjeng,† and Martin Valldor*,† †

Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Straße 40, 01187 Dresden, Germany Institute Laue Langevin, 71 Avenue des Martyrs, 39042 Grenoble Cedex 9, France § Department of Electrophysics, National Chiao Tung University, HsinChu 30100, Taiwan ∥ University of British Colombia, 2329 West Mall, Vancouver, BC V6T 1Z4, Canada ⊥ National Synchrotron Radiation Research Centre, Hsinchu 30076, Taiwan # Institut für Anorganische und Analytische Chemie, Johannes Gutenberg-Universität, 55128 Mainz, Germany ‡

S Supporting Information *

ABSTRACT: From stoichiometric amounts of CaO, Fe, and Se, pure powders and single crystals of quaternary Ca ∞2[FeSe2/2O2/2 ] can be obtained by solid-state reaction and self-flux growth, respectively. The as-synthesized compound exhibits a polymorphic crystal structure, where the two modifications have different stacking sequences of 2 2− layers. The two polymorphs have similar ∞[FeSe 2/2O2/2 ] unit cells but different crystal symmetries (Cmc21 and Pnma), of which the former is non-centrosymmetric. Fe is divalent (d6) and high-spin, as proven by X-ray spectroscopy, Mössbauer spectroscopy, and powder neutron diffraction data. The latter two, in combination with magnetic susceptibility and specific heat data, reveal a long-range antiferromagnetic spin order (TN = 160 K) with a minor spin canting. CaFeSeO is an electronic insulator, as confirmed by resistivity measurements and density functional theory calculations. The latter also suggest a relatively small energy difference between the two polymorphs, explaining their intimate intergrowth.

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reported,12 which is isostructural to CaZnSO,13 having a geometrically frustrated layered TM lattice within a polar noncentrosymmetric crystal structure. In search of related materials containing iron and two chalcogenides, we came across the title compound, CaFeSeO. A proposed crystal structure of CaFeSeO and its insulating nature were reported last year,14 and its intriguing structural complexity was first discussed in a very recent report.15 CaFeSeO has a remarkably simple composition and similar chemistry to the known CaFeSO,12 but the properties are very different. This emphasizes the influence from the anionic lattice on the physical properties of bichalcogenides. Here, we present the synthesis of CaFeSeO powder together with its magnetic properties, ordered spin structure, and data from specific heat, electric conductivity, and high-energy spectroscopic studies. The empiric observations are compared with density functional theory calculations.

INTRODUCTION Compounds containing transition metals (TMs) in a lattice of two different anions is a growing topic, mainly due to the recently found 1111-type iron-based superconductors REFeAsO (RE = La1 and other rare-earth metals), where arsenic and oxygen ions form an anionic superstructure. Similar compounds with ordering of chalcogenides (Chs) are still rare but contain novelties for chemistry and physics. A few of these compounds have low-dimensional TM lattices: LaCrS2O is reported as an Ising ferromagnetic chain compound,2−4 and La4TMCh6O (TM = Fe, Mn, Ch = S, Se) has quasi-1D columns of facesharing TMCh6 trigonal antiprisms, but their properties are still not reported.5 Ba3V2S4O3 is also a 1D magnetic system with columns of face-sharing VS6 trigonal antiprisms,6 and the first measurements of its physical properties reveal magnetic complexity.7 Also, 2D structures are reported in, for example, BaZnSO8 with layers of [ZnS2/2O2/2] tetrahedra, CeMn0.5SeO9 has [MnSe4/4] tetrahedra layers and is a magnetic insulator, and La2TM2Ch2O3 (Ch = S, Se) contain [TMCh4/4O2/4] (TM = Fe,10 Co,11 Ch = S, Se) octahedra, where strong antiferromagnetic coupling is observed in the quasi-2D TM lattice. Recently, the antiferromagnetic compound CaFeSO was © 2017 American Chemical Society

Received: August 30, 2016 Published: March 27, 2017 4271

DOI: 10.1021/acs.inorgchem.6b02098 Inorg. Chem. 2017, 56, 4271−4279

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Inorganic Chemistry

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Density Functional Theory Calculations. Scalar-relativistic density functional theory (DFT) electronic structure calculations were performed using the full-potential FPLO code,19 version fplo14.00-47. For the exchange−correlation potential, within the local density (LDA) and the general gradient correction (GGA), the parametrizations of Perdew−Wang20 and Perdew−Burke−Ernzerhof21 were chosen. The strong Coulomb repulsion on the Fe-3d shell was mimicked in the LDA+U22 and GGA+U approximation (for a broad range of U = 4−7 eV) applying the atomic limit double counting scheme. To obtain precise band structure and total energy information, the calculations were carried out on a well-converged mesh of 8000 k-points in the Brillouin zone. For all calculations, the respective experimental crystal structure has been used.

EXPERIMENTAL SECTION

Sample Preparation. The sample for structure refinement and physical properties investigation with the composition CaFeSeO was prepared by a solid-state reaction method from commercially available starting materials: Se powder (99.999% Alfa), Fe powder (99.9% Alfa), and CaO powder. The latter was prepared by preheating CaCO3 powder (99.9% Alfa) at 950 °C for 60 h in air and directly transferring it into a glovebox, where the starting materials were mixed in equimolar amounts. The mixture was then compressed into pellets inside a stainless steel die (⦶ 13 mm) at pressure of ∼3 tonnes. Each pellet of ∼1 g was broken into pieces and filled into an alumina crucible. The crucible was sealed in the evacuated silica ampule and subsequently annealed at 840 °C for 10 h with a heating rate of 100 deg h−1. All handling of the sample was performed under an argon atmosphere in a glovebox. Small single crystals could be mechanically separated from a melt, as obtained at 1100 °C during 10 h using a stoichiometric sample in closed vessels. X-ray Diffraction. The powder X-ray diffraction measurement was carried out using a focusing camera with a Co (λ = 1.788 92 Å) radiation source (Figure S1). Single-crystal X-ray diffraction was performed at room temperature by a Bruker Venture instrument with Mo radiation (λ = 0.710 73 Å) and a CMOS Photon area detector. Neutron Diffraction. Powder neutron diffraction measurements at room temperature were performed at the D2B diffractometer at the ILL in Grenoble, France (λ = 1.59 Å). Diffraction data between 14 and 178 K from D1B (ILL, Grenoble, λ = 2.52 Å) were used to determine the magnetic structure. A restriction was added in the crystal structure refinement on the Fe−O bonds of Phases 1 and 2 to keep them similar, as Mössbauer data indicated only one Fe environment in the paramagnetic state. Both refinements were completed with Jana2006.16 Elemental Analysis. Elemental analyses and grain topology were investigated in a JEOL JSM 7100F scanning electron microscope (SEM) equipped with a field-emission gun, using an acceleration voltage of 8 kV, and a Bruker EDX-system Quantax 400 with an XFlash 6|30 silicon drift detector. Physical Property Measurements. Magnetic properties were studied using a Quantum Design magnetic property measurement system in an external field of 1 T at 2−750 K, where the sample holder was a polycarbonate capsule for T < 350 K and a quartz tube above. As the furnace has a significant magnetic signal itself in the temperature range 300−750 K, the obtained magnetic signal was shifted to overlap with the low-temperature data. Magnetization measurements up to 5 T were performed at a few chosen temperatures. Specific heat (Cp) and resistivity (ρ) measurements were performed on sintered pieces of the title compound in a Quantum Design physical property measurement system. The Cp(T) investigation was done by the nonadiabatic thermal relaxation technique, and the ρ(T) by a standard four-point method, by fastening gold wires with silver glue. As the sample was relatively insulating, an external Keithley sourcemeter 2400 was used to extract data up to 200 V at a constant current of 0.1000 μA. X-ray Absorption Spectroscopy. The soft X-ray absorption spectroscopy (XAS) at the Fe-L2,3 edges was measured at the BL08B beamline of the National Synchrotron Radiation Research Center in Hsinchu, Taiwan, with an energy resolution of about 0.2 eV. Clean sample areas were obtained by cleaving the crystals in situ at pressures in the 10−10 mbar range. The Fe-L2,3 XAS spectrum of CaFeSeO was recorded using the total electron yield method. Fe2O3 single crystals were measured simultaneously to serve as an energy reference. Mö ssbauer Spectroscopy. Mössbauer spectra of CaFeSeO were collected between 5 K and room temperature with a standard WissEl spectrometer, which was operated in the constant-acceleration mode and which was equipped with a 57Co/Rh source. The powdered sample (containing about 9 mg of natural Fe cm−2) was diluted with boron nitride to ensure homogeneous distribution and filled into a Plexiglas sample container. Spectra at different temperatures were obtained using a Janis-SHI-850-5 closed-cycle refrigerator. The isomer shifts are given relative to α-iron. The data were evaluated with the programs MossWinn17 and Recoil.18

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RESULTS Composition and Crystal Morphology. After the main sintering reaction, the sample powder was brown, giving a first hint of its electronically insulating nature. The energy dispersive X-ray analysis from eight spots on different crystallites gave the average metal composition Ca0.96(5)Fe0.96(4)Se1.07(5), which is close to the nominal composition. Due to the charging up of the sample in the electron beam and the irregular crystallite shapes (Figure 1), the oxygen content was not quantified, although the oxygen presence was confirmed in all analyses.

Figure 1. Scanning electron microscopy image of CaFeSeO crystallites obtained in secondary electron contrast.

X-ray Absorption Spectroscopy. To indirectly get a hint on the oxygen composition, the oxidation state of Fe can be investigated. It is well known that the XAS spectra at the 3d TM L2,3 edges are very sensitive to the valence state, spin state, and local symmetry of 3d TMs in the solid state.23−26 Figure 2 shows that the Fe-L2,3 XAS spectrum of CaFeSeO (top) has its main weight at the same energy position as that of an Fe2+ reference, Mg0.94Fe0.04O (middle), but is shifted by more than 1 eV to lower energy with respect to the Fe3+ reference α-Fe2O3 (bottom). X-ray Diffraction. By using a Co X-ray source for the powder diffraction, it is possible to prove the relatively high phase purity of CaFeSeO (Figure S1). Almost all strong peaks can be indexed with a centered orthorhombic unit cell, from here on called Phase1 (Cmc21); a few peaks of relatively weak intensity belong to a secondary phase CaSe,27 as refined to 0.5% volume fraction in a Rietveld refinement. All the remaining minor intensities agree with the second primitive polymorph called Phase2, as found by neutron diffraction (see below). Phase2 agrees well with previously reported CaFeSeO 4272


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Figure 2. Room-temperature Fe-L2,3 XAS spectra of CaFeSeO, Mg0.96Fe0.04O, and α-Fe2O3. The latter two are references for Fe2+ and Fe3+, respectively.

(Pnma).14,15 By carefully examining single-crystal X-ray diffraction data it is obvious that the peaks that violate the Ccentering are broadened along one crystallographic direction (Figure S2). This is indicative of structural disorder: The onedimensional peak broadening can be explained by stacking faults of layers (see below). From the full-width-half-maximum of the broadened reflections the correlation length of the different periodicities can be estimated to be about the size of the unit cell, which is also confirmed by electron microscopy high-resolution imaging by Cassidy et al.15 Neutron Diffraction. High-resolution neutron diffraction data (Figure 3) on CaFeSeO powder could be modeled satisfactorily only by two different phases with crystal structures having one layer motif in two different stacking sequences (Figure 4). Although the unit cell dimensions are hardly distinguishable, the crystal symmetries of the two phases, or

Figure 4. (a) Selection of the crystal structure of CaFeSeO (Phase1) with the closest coordination of Ca and of Fe displayed with polyhedra. The buckled square layer of Fe is outlined with thicker lines. (b) Atomic and magnetic structures of the two polymorphs of CaFeSeO: Phase1 (left) and Phase2 (right). Their different stacking orders of the sublattices A, B, and C are indicated.

polymorphs (Phase1 and Phase2), are different due to a different stacking of layers. To keep the standard crystallographic settings here, the novel Phase1 is presented with switched unit cell axes relative to the already reported Phase2.14,15 However, a permutation of Phase2 into Pmcn is used to simplify the structural comparisons below (b-axis is perpendicular to layer stacking in Figure 4). The known Phase2 was added to perform a joint Rietveld refinement (Figure 3a), and the agreement parameters together with the refined parameters of Phase1 can be found in Table 1. The difference between the two phases is the orientation of every second CaFeSeO layer; see Figure 4. Whereas every second layer in the b-direction is only shifted for Phase1, these planes are additionally “flipped” with respect to the b-axis in Phase2. Hence, the two polymorphs of CaFeSeO can be described with space groups Cmc21 (Phase1) and Pnma (Phase2). The stacking of layers is ABABAB for Phase1 and ACACAC for Phase2; see Figure 4b. Interestingly, the intergrowth of the two phases seems to be intimate as the coherence length of Phase2 in Phase1 is relatively short, as estimated from the single-crystal X-ray experiment, and indicates disorder in the stacking (e.g., ABACABABACABACAC...). The steady periodicity of the A layer assures the comparably narrow peak shape of the intensities that are shared by the two phases, compared to the relatively broad intensities that break the C-centering (Phase2). The investigated powder sample contains 55 and 45 vol % of Phase1 and Phase2, respectively. Crystal Structure Description. The crystal structure of CaFeSeO can be described as a combination of a purely ionic part and a covalent sublattice. The former consists of Ca2+ ions and the latter of extended layers with the formal description 2 2− ∞[FeSe 2/2O2/2 ] . Hence, Fe is tetrahedrally coordinated by two Se and two O atoms (Figure 4a), and the paths −Se−Fe− Se−Fe− and −O−Fe−O−Fe− extend along two different crystallographic directions perpendicular to the b-axis. The pure Fe lattice can be described as a buckled square layer, but with

Figure 3. Powder neutron diffraction data of CaFeSeO at room temperature (a) and at 14 K (b), where the observed data are presented with open circles and the calculated pattern with a solid line. The Bragg positions are marked with vertical lines for both polymorphs of the title phase, below which the difference plot (obsd −calcd) is displayed. 4273


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are relatively large, superexchange interactions have to be responsible for the spin−spin interactions. Consistently, the Fe−O−Fe (angle: 112°) superexchange results in an antiferromagnetic interaction, while the Fe−Se−Fe (angle: 98°) interaction is ferromagnetic. Both interactions are consistent with the Kanamori−Goodenough rules.28,29 As the paths involve two different mediating ions, O2− or Se2−, it is difficult to directly compare the magnetic interaction strengths. Temperature Dependence of Magnetic Moment. The refined ordered magnetic moments of Fe for different temperatures are displayed in Figure 5. The magnetic moment

Table 1. Data of Phase1 from the Rietveld Refinement of Atomic Structure from Neutron Powder Diffraction Data at 293 K (Paramagnetic); The Shortest Interatomic Distances and Relevant Angles Are Listed Phase1 Cmc21 (No.36) 293(2) 3.8818(2) 13.1599(6) 5.9106(3) 301.94(3) Se 4a 0 0.6549(2) 0.2793(6) 0.0106(4) Fe 4a 0 0.0435(2) 0.4192(5) 0.0130(4) Ca 4a 0 0.6696(3) 0.7875(8) 0.0062(6) O 4a 0 0.9259(3) 0.2428(6) 0.0133(6) relative volume [%] 55.24(6) common profile fit parameters: GOF = 2.11, Rp = 0.045, wRp = 0.059 selected distances [Å] Fe−O 1 × 1.955(5), 1 × 1.866(5) Fe−Se 2 × 2.569(3) Ca−O 2 × 2.327(3) Ca−Se 1 × 2.913(6), 1 × 3.010(6), 2 × 3.017(4) Fe−Fe 2 × 3.169(4), 2 × 3.8818(2) Selected angles [degrees] Fe−O−Fe 112.1(2) Fe−Se−Fe 98.1(1)

space group temp [K] a [Å] b [Å] c [Å] V [Å3] atom Wyckoff x y z Ueq

Figure 5. Temperature dependence of the magnetic moment of Fe, as estimated by Rietveld calculations on neutron diffraction data. The inset displays the magnetic scattering intensities as a function of temperature and diffraction angles.

two different Fe−Fe distances that both are too large (>3 Å) for direct metal−metal interactions. In the layers, Fe−Se and Fe−O distances are within expectations (Tables 1 and S3). The bond angle Fe−O−Fe is 112° for Phase1, which is relatively close to the tetrahedral angle (109.5°) that is often found in covalent systems with hybridized orbitals. The corresponding Fe−Se−Fe angles are notably smaller, 98° in Phase1. Calcium is situated within a strongly distorted octahedron consisting of four Se and two O, where the two O atoms are direct neighbors (cis-like, Figure 4a). Magnetic Spin Structure. For the two phases, the magnetic structure could be refined with magnetic space groups Cm′c21′ (Phase1) and Pnm′a (Phase2), respectively (Table 2). All other possible irreducible representations for

was set equal for Phase1 and Phase2 in the neutron data refinement, giving an ordered magnetic moment of 3.40(6) μB Fe−1 at the lowest temperature. It is possible to see the typical moment increase as can be expected for a second-order phase transition. Thermal fluctuations are important even far below the Néel temperature of about 160 K. The magnetic moment is somewhat lower than the expected moment for a high-spin Fe2+ (d6, S = 2, 4 μB) ion. This might be attributed to either disorder in the stacking of the CaFeSeO layers or the obvious covalence between Fe and Se that lowers the amount of ordered moment on Fe. Magnetic Susceptibility. The magnetic susceptibility (χ) of CaFeSeO powder (Figure 6) reveals that no Curie−Weisslike paramagnetism is observed even up to 750 K. However, in the temperature range 165−150 K, there is a strong increase in

Table 2. R-Values from the Rietveld Refinement of the Magnetic Structure at 14 K Measured at the D1B Diffractometer with a Long Neutron Wavelength of 2.52 Åa Phase1 magnetic space Cm′c21′ group temp (K) 14 magnetic R-values R = 0.0188, wR2 = 0.0267 common profile fit parameters: GOF = 3.09, Rp =

Phase2 Pnm′a 14 R = 0.0280, wR2 = 0.0414 0.0541, wRp = 0.0755

a

The crystal structure was taken from the structural refinement of the D2B data (λ = 1.59 Å, Table 1), and only the magnetic structure was refined.

these k = 0 magnetic structures can be excluded. We obtained a collinear antiferromagnetic spin structure with the magnetic moments pointing along the crystallographic c or a direction for Phase1 and Phase2, respectively (Figure 4b); that is, the spin directions are identical for each layer of type A, B, or C. This suggests that the spin interactions within the layers are stronger than between the layers. As the interatomic Fe−Fe distances

Figure 6. Temperature-dependent magnetic molar susceptibility in a constant magnetic field of 1 T. Both field-cooling (FC) and zero fieldcooling (ZFC) data are displayed. 4274


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Inorganic Chemistry χ, below which spin domains seem to form, as field-cooled and zero field-cooled data do not superimpose. This is also seen in low-field measurements (0.05 T) recorded by others.15 As the neutron diffraction data suggest an antiferromagnetic state, in agreement with the small molar susceptibility values, domains with minor residual ferromagnetic components have to occur. The most convincing spin-ordering description is, thus, a canted antiferromagnet. As a relatively large magnetic field is used here, it is possible to conclude that the domain effect is intrinsic. Isothermal Magnetization. From the size of the magnetic polarization in the hysteresis curve (Figure 7) it is possible to

Figure 8. 57Fe Mössbauer spectra of CaFeSeO at different temperatures, where the measured data are represented by filled circles. The solid lines correspond to the best fits.

and can be described by a single magnetic Fe site. The spectral shapes are typical for combined quadrupolar and magnetic hyperfine interactions. For instance, at 150 K only four lines are discernible in the spectrum, in contrast to the usual six-line pattern found in many magnetic iron-based compounds. The spectra were evaluated using a full Hamiltonian analysis within the thin absorber approximation as described previously.31,33 The results obtained from the analysis of the Mössbauer spectra are summarized in Table 3. The electric field gradient (efg) reveals only a small deviation from axial symmetry (asymmetry parameter η = 0.12). It is noteworthy that the QS is nearly independent of temperature. This suggests that the splitting of orbitals of e-parentage, due to the deviation from ideal tetrahedral symmetry, is larger than the thermal energy even at room temperature, which leads to a temperatureindependent valence contribution to the efg. The two polymorphs of CaFeSeO that are apparent in the neutron diffraction studies are indistinguishable in the Mössbauer spectra. This assures that the assumption of equal magnetic moments in the two phases is justified, as different moments would lead to different hyperfine fields Bhf and thus to a splitting in the spectra. The ratio of Bhf(150 K)/Bhf(5 K) = 0.56 compares very well with the corresponding ratio of 1.9 μB/3.4 μB = 0.56 of the magnetic moments determined from the neutron diffraction data (Figure 5). The hyperfine field Bhf is composed of three contributions, namely, the Fermi contact term Bc, the dipolar contribution Bdip, and the orbital contribution BL. Because tetrahedral coordination contradicts first-order orbital momentum, all the contributions to Bhf are proportional to the average ordered iron spin ⟨Sz⟩,34 and thus Bhf(T) reflects the temperature dependence, M(T), of the magnetization. In agreement with the crystal and magnetic structure (cf. Figure 4b) Bhf and the principal axis VZZ of the efg are noncollinear; namely, Bhf is essentially perpendicular to VZZ. While the spins and thus Bhf are oriented along the c- or adirections for Phases1 and 2, respectively, VZZ mainly reflects the strongly distorted FeSe2O2 coordination sphere with much shorter Fe−O bonds compared to the longer Fe−Se bonds.

Figure 7. Isothermal magnetizations at temperatures indicated next to the curves. The dashed line represents an extrapolation to zero fields for the 300 K data. The double arrow marks the suggested resulting moment originating from a possible spin canting at 150 K.

conclude that a canted antiferromagnetic state explains the observations: The remanence is only about 0.04 μB. As 4 μB is the theoretical moment for an S = 2 (high-spin Fe2+), the average canting angle at 150 K can be determined as sin−1(0.04/4) ≈ 0.5°. On further increasing the field, the canting increases slightly as expected. At 2 K, this canting seems to have diminished somewhat but is still significant. The reason for not observing the spin canting in the neutron data is its relatively small contributions and that its additional scattering overlaps with the diffraction intensities of the nuclear scattering. At 300 K, the magnetization curve consists of two parts: at low fields a superparamagnetic-like increase in the magnetic signal reveals a minor ferromagnetic impurity, whereas at higher fields the spin system acts as expected for a paramagnet at high temperatures, as the magnetization is proportional to the field. By extrapolating to zero fields at 300 K, the remnant magnetic signal is about 0.0034 μB, representing about 0.15% Fe-metal in the sample.30 Mössbauer Spectroscopy. The 57Fe Mössbauer spectra of CaFeSeO powder are shown in Figure 8. The spectra at 293 and 170 K consist of a single quadrupole doublet, which verifies the absence of long-range magnetic order in this temperature range and suggests that the local coordination environment of Fe in Phases1 and 2 must be very similar. The isomer shift (IS) of 0.73 mm s−1 and the quadrupole splitting (QS) of 1.31 mm s−1 at 293 K are consistent with high-spin Fe2+ in an approximate tetrahedral coordination environment (e3t23 electron configuration). The IS value is about 0.05 mm s−1 more positive than in oxyselenides featuring a FeSe3O coordination environment,31,32 which indicates a more ionic character of the average chemical bonding in CaFeSeO due to the presence of a second oxygen atom in the FeSe2O2 building block. The spectra at 5 and 150 K evidence magnetic ordering 4275


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Table 3. Mössbauer Parameters Obtained from the Evaluation of the Experimental Spectra at Various Temperatures T (K)a T

IS

QS

293 170 150 5

0.734(1) 0.814(1) 0.824(1) 0.871(1)

1.312(2) 1.325(2) +1.365(3)b +1.351(3)b

η

Bhf

10.38(3) 18.54(2)

0.146(6) 0.118(4)

Ω

Γ

86.4(2) 84.4(2)

0.268(3) 0.281(2) 0.292(4) 0.290(2)

IS (mm s−1) corresponds to the isomer shift, QS (mm s−1) to the quadrupole splitting, Bhf(T) to the magnetic hyperfine field, η to the asymmetry parameter of the electric field gradient (efg), Ω (deg) to the polar angle describing the orientation of Bhf in the principal axis system of the efg, and Γ (mm s−1) to the full line width at half-maximum. Best fits at 150 and 5 K were obtained with an azimuthal angle Φ of 90°. bIn these spectra QS was calculated according to QS = eQVZZ/2(1 + η2/3)1/2, where Q is the quadrupole moment of the excited 57Fe nucleus. a

Thermodynamics. As expected, the specific heat of sintered polycrystalline CaFeSeO saturates close to the Dulong−Petit limit (3RN = 99.77 J mol−1 K−1, where R = 8.3145 J mol−1 K−1 and N = 4). Near 160 K there appears an anomaly (Figure 9). Its broadness, asymmetry, and the fact that

Figure 10. Temperature-dependent electric resistivity of a polycrystalline CaFeSeO piece. The dashed line presents an Arrhenius-like activation behavior.

energy of 0.19 eV, as obtained from a single crystal.14 Thus, the grain boundaries in the polycrystalline sample, presented here, are hampering the electron conduction. Further, the reported transparency of single crystals14 suggests that the number of charge carriers should be relatively small, which might agree with electron hopping between imperfection sites in the crystal structure. That the data deviate from the activation-like behavior at lower temperatures is, naturally, caused by the relatively high absolute resistance (>1 GΩ). Density Functional Theory. In order to investigate the crystal structure and the underlying electronic structure of CaFeSeO on microscopic grounds, DFT calculations were carried out in different approximations: Independent of the particular density functional calculation, LDA or GGA,19 we obtain a metallic ground state for a nonmagnetic treatment (Figure 11a). The electronic density of states (DOS) shows a high-lying part of the valence band dominated by Fe-3d states (between −1 and 1 eV) and a lower lying part dominated by Se-4p (between −4.5 and −2.5 eV) and O-2p states (between −6.5 and −4.5 eV). The contribution of Ca to the occupied states is negligible. However, the metallic ground state is in strong contrast to the experimentally observed insulating behavior. Allowing for spin polarization, we can stabilize different magnetic configurations where the two with the lowest energy (almost degenerate, see Figure S4) exhibit a magnetic structure as obtained from the neutron data (Figure 4b). Fe2+ is found in the high-spin state with a magnetic moment of about 3.4 μB. The respective DOSs exhibit a small band gap, about 0.15 eV for LDA and 0.6 eV for GGA (Figure 11b), which is too small compared with the optical experiment.14 The Fe-3d states show a rather large spin split of about 3 eV. All other in-plane spin configurations (Figure S3) exhibit a metallic ground state, although the spin split for the Fe-3d states is comparable.

Figure 9. Temperature-dependent specific heat Cp(T) of CaFeSeO. The dashed line represents the Dulong−Petit limit (3RN). The inset displays the Cp/T(T) curves on heating (up) and cooling (dw), where a dashed line is an estimate of the phononic contribution.

no significant hysteresis is seen on cooling and heating (Figure 9 inset) suggests a second-order phase transition. This agrees with the magnetic transition observed in the susceptibility data (Figure 6) and neutron diffraction data (Figure 5). However, by integrating the released entropy (S), only 0.47 J mol−1 K−1 is found, in contrast to the theoretical Smag = R ln(2S + 1)) = 13.4 J mol−1 K−1 (S = 2). The missing magnetic entropy at 160 K indicates that spin fluctuations slow down already far above the spin ordering temperature and the complete magnetic entropy is released within a much larger temperature range. Hence, to estimate the true magnetic entropy, a phononic background has to be obtained by measuring a nonmagnetic reference, such as the presumably isostructural, hypothetical CaZnSeO, but so far this compound is not reported. No further features are observed, meaning that the room-temperature crystallographic symmetries probably are valid down to 2 K, confirming the observations from the neutron diffraction data. Electrical Conductivity. The insulating nature of the title compound was already expected due to its brown color. The temperature dependence of the resistivity (ρ, Figure 10) on a sintered polycrystalline pellet reveals that a normal thermal activation behavior of the conductivity describes the data above 250 K. Thereby, the calculated activation energy is about 0.31 eV, which is larger than the previously reported activation 4276


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DISCUSSION CaFeSeO possesses a rare crystal structure. CaFeO2 is known but structurally completely different,35 and other CaFeCh2 (Ch = S, Se, Te) are unknown. The crystal structure of CaFeSO12 is hexagonal and very different from that of the title compound, although the chemical relationship is obvious. Instead, the title compound can be described as a distorted version of BaTMSO (TM = Zn,8 Co36), which, in turn, stems from SrZnO2.37 The orthorhombic unit cells of Phase1 CaFeSeO (Table 1) and BaTMSO (a = 4.0, b = 12.8, c = 6.1 Å, Cmcm) are similar, but the centrosymmetry is broken in the title compound: the mirror plane perpendicular to the c-axis in BaTMSO is replaced by a 21 screw axis in Phase1 CaFeSeO (Figure 12). Thereby the

Figure 11. Density of states as obtained from DFT calculations. The color codes are used to separate the contributions from the different elements, and the Fermi energy is at zero. (a) GGA non-spinpolarized; (b) GGA spin-polarized ground state (AFM_udud, Figure S3); (c) GGA + U (U = 6 eV, AFM_udud).

The calculation results are reminiscent of the situation in NiO, where the gap in the spin-polarized state is much too small without an explicit treatment of the strong correlations of the transition metal 3d electrons. Therefore, we approximate the strong Coulomb repulsion (U) in the Fe-3d shell in a mean field way by applying the so-called GGA + U. For a broad range of U values, the ground state is gapped (U3d = 4−7 eV; see Figure S3) with gaps having the appropriate size. Due to the localization of the Fe-3d orbitals, modeling the strong Coulomb repulsion, the resulting magnetic moment of Fe2+ becomes larger with about 3.8 μB (U = 6 eV). For the two magnetic configurations with the lowest total energy we obtain a gap size between about 2.2 and 2.6 eV (Figures 11c and S4). Considering the energy of magnetically ordered states, we find a strong preference for the antiferromagnetic in-plane order observed in the neutron experiment (Figure S4, compare with Figure 4). Within a simple isotropic Heisenberg model, a rough estimate for the in-chain magnetic exchange coupling along the Fe−O−Fe chains yields an antiferromagnetic J∥ = 120−70 K (for U = 4−7 eV). According to the calculations, the interlayer couplings, J⊥, are ferromagnetic and about 50 times smaller than J∥, as the lowest lying curves in the inset of Figure S4 are essentially degenerate. Hence, an accurate estimate of the interlayer couplings and the respective magnetic order from the DFT + U calculations would be, at best, on the border of accuracy. To understand the experimentally observed intergrowth for layers of two different structural polymorphs, we calculated the respective total energies of Phase1 (Cmc21) and Phase2, as taken from previous reports (Pnma),14,15 including the strong Coulomb repulsion for Fe-3d electrons within the GGA + U (U = 6 eV). We find that the two energies are similar, with only a slight preference (about 18 meV per atom) of Phase1. This would explain the intimate mixture of the two polymorphs. A relatively rapid cooling after the main synthesis, as in the here presented case, should result in domain formation of both phases. A slower cooling might produce a more homogeneous phase with Cmc21 symmetry.

Figure 12. Comparison of the Phase1 CaFeSeO (up) and BaTMSO (down, TM = Zn,8 Co36) crystal structures, where the 21 screw axis (up) and the mirror plane (down) are marked.

distortion is probably induced by the relatively smaller alkaline earth metal ion Ca2+ compared to Ba2+. The non-centrosymmetric Phase1 possesses electric polarity due to the crystal structure symmetry, which has to be addressed when discussing the magnetic properties. In the first report on CaFeSeO,14 a centrosymmetric crystal structure symmetry was used, Pnma (No. 62). We tried this centrosymmetric structure in our neutron diffraction refinement, but the agreement was very poor. The indicated structural disorder from our single-crystal data was the key to try an additional centered phase (Phase1) in a joint refinement, which resulted in good agreement with observations. The existence of Phase1 is further supported by the magnetic data: Spin canting resulting in a residual global ferro-component, as observed in the magnetic data, is allowed only if the inversion symmetry is broken. Moreover, by estimating the total energies through a DFT calculation, Phase1 and Phase2 are energetically rather similar. This would explain why the first report on CaFeSeO states that sample phase purity was not reached,14 perhaps due to the presence of our Phase1. The possibility of having two polymorphs of CaFeSeO is given by the crystal structure: The layers are compatible in either ABAB or ACAC stacking due to fitting charge distributions at the layer interfaces. Both stacking sequences result in more Columbic attraction possibilities than repulsions; that is, none of the equally charged ions end up on the first coordination spheres. Perhaps the two polymorphs should not be seen as low- (Phase1) and high- (Phase2) temperature phases, but instead in an order−disorder relation, as layers B 4277


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Chemically this new system has many possibilities for substitutions, of which one would be to exchange Ca for a divalent lanthanide, e.g., Eu2+ or Sm2+, in search of fluorescent properties. This would be especially interesting, as the local environment of the Ca site is rare. Further, iron might be substituted for other transition metals, and perhaps the electronic correlations can be weakened by electron/hole doping.

can be easily transferred into C by a general movement of all Fe atoms in the layer; see Figure 13. As a consequence, a perfect

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CONCLUSIONS Through a temperature-optimized solid-state reaction in closed ampules, it was possible to obtain samples of the quaternary compound CaFeSeO with >99% purity, enabling comprehensive investigations of the crystal structure and magnetic properties. The title compound exhibits a polymorphic crystal structure, where the two modifications differ in their stacking of layers. One polymorph is non-centrosymmetric, but the absolute relative energy difference between the polymorphs is small. Fe is divalent (d6), high-spin (S = 2), and coordinated by two O and two Se in a distorted tetrahedron. The magnetic ground state is similar for both polymorphs: collinear antiferromagnetic (TN = 160 K) as refined by neutron diffraction. However, a minor spin canting is observed by SQUID magnetometry and is allowed by the non-centrosymmetry in one polymorph. CaFeSeO is an electrical insulator due to strong electronic correlations, as confirmed by X-ray absorption spectroscopy and DFT calculations.

Figure 13. Relation between the B and C layers (Figure 4) in the two CaFeSeO polymorphs, using Phase1 as starting point: Phase1 has Fe at the red positions in the intermediate layer, and Phase2 has Fe at the yellow sites as shown by the arrows.

mixture of both phases will be found at higher temperatures, and, on slow cooling or long heat treatment at low temperatures, Phase1 should dominate in the sample. On optimizing the postannealing heat treatment, a pure sample of Phase1 might be obtained. Here, we have slightly more of Phase1 in the sample (55%), but even 80% of Phase1 was reported by others.15 The tetrahedral Fe coordinations are the same for both polymorphs, and the spatial difference between the two sites is relatively small (∼1.2 Å). Further, there is only a relatively low energy barrier between the sites, as the O−O distance is about 3.5 Å, making the shortest Fe−O distance, on moving from one Fe site to the next one, almost within acceptable limits for solid-state compounds. Naturally, due to Coulomb interactions, the site shift is necessarily a global phenomenon here, meaning that all Fe atoms in the central row in Figure 13 have to shift simultaneously. Naturally, defects on the Fe sites might exist in each structure layer. However, these are assumed to be scarce because all X-ray and neutron diffraction peaks from in-layer periodicities are resolution limited, indicating dominating long-range atomic order within each layer. In comparison to BaCoSO, which is a collinear antiferromagnet without hysteretic magnetic remanence,36,38 the title compound shows a small but significant spin canting, leaving a residual ferromagnetic component during magnetizations (Figure 7). This spin ground state contains something unexpected outside the suggested canted antiferromagnet. Usually, the canting of the antiferromagnetic state is flopped in a single transition with normally a small coercive field. Here, the release of the spin canting is quick, but it is relatively slowly reached on increasing the field, which was obvious by using the same field change rate. This dragging effect on polarizing the system and the quick release indicate that another factor has to be taken into consideration. The electronic distribution among the d orbitals of Fe is e3t23, allowing for only minor orbital contributions to the magnetic moment in a tetrahedron. The non-centrosymmetric crystal structure in Phase1 makes Dzyaloshinky−Moriya interactions possible,39,40 but also a time-dependent, glassy-like, effect might cause the observation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02098. X-ray powder diffraction data at room temperature; slices through the Ewald sphere of observed data from singlecrystal diffraction; additional density functional theory calculation results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Martin Valldor: 0000-0001-7061-3492 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Chun Fu Chang for organizing P.-S.C.’s student exchange. P.A. gratefully acknowledges useful discussions with G. Wortmann. Yu. Prots, S. Hückmann, and H. Borrmann are acknowledged for XRD measurements and U. Burkhardt for the SEM investigations. S.H. thanks the Max Planck Gesellschaft−University of British Columbia Center for Quantum Materials collaboration. 4278


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Canted Antiferromagnetism on Rectangular Layers of Fe2+ in

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