Article pubs.acs.org/IC
Geometrical Spin Frustration of Unusually High Valence Fe5+ in the Double Perovskite La2LiFeO6 Peng Xiong,† Hayato Seki,† Haichuan Guo,† Yoshiteru Hosaka,† Takashi Saito,† Masaichiro Mizumaki,‡ and Yuichi Shimakawa*,†,§ †
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Japan Synchrotron Radiation Research Institute, SPring-8, Hyogo 679-5198, Japan § Japan Science and Technology Agency, CREST, Uji, Kyoto 611-0011, Japan ‡
S Supporting Information *
ABSTRACT: A double perovskite-structure oxide La2LiFeO6 with unusually high-valence Fe5+ was synthesized using a highpressure technique. The Li+ and Fe5+ ions at the B site in the rhombohedral R3̅ perovskite structure are ordered in a rock salt manner, and the resultant tetrahedral network of Fe5+ gives geometrical spin frustration, which is consistent with a large frustration index f (|θ|/TN) ≈ 10. Mg2+ substitution for Li+ produces Fe4+ from some Fe5+ and changes the magnetic properties. The Weiss temperature is increased from −119 to 21 K by the substitution of only 1%, significantly decreasing the frustration index. The geometrical frustration of the Fe5+ spin sublattice cannot be tolerant for even a very small amount of Fe4+ disturbance.
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INTRODUCTION
CaCu3Fe4O12, and Ca2FeMnO6 is coordinated by oxygen ions octahedrally. So far, the only reported oxide that contains Fe5+ without Fe3+ at the octahedrally oxygen coordinated sites is La2LiFeO6.15 The compound was synthesized under high oxygen pressure, and it crystallized with the double perovskite structure. With the resolution of X-ray diffraction (XRD) at that time when the compound was discovered, however, detailed structure analysis was difficult, and the space group of the crystal structure and the Li+ and Fe5+ ordering were unclear. In the work reported here, a single-phase sample of La2LiFeO6 was obtained using a high-pressure synthesis technique, and the detailed crystal structure was revealed by Rietveld analysis of synchrotron X-ray diffraction (SXRD) data. The effects of Mg2+ substitution for Li+ on the structural and magnetic properties are also discussed.
Iron ions typically have ferrous (2+) and ferric (3+) oxidation states, and there are many of those ferrous and ferric oxides. Higher oxidation states of Fe, such as 4+, 5+, and 6+, have been reported. But oxides containing such unusually high valence state iron ions are rare. Compounds with the chemical formula A3FeO4 (A = Na, K, and Rb) have been reported to contain Fe5+, but the only evidence of the unusual oxidation state was provided by analyses of their chemical compositions.1−3 Although recent Mössbauer experiments for K3FeO4 revealed the Fe5+ spectrum, it changed rapidly, suggesting that this compound is unstable and easily decomposes.4,5 Another example of Fe5+ in an oxide was seen in the chargedisproportionated perovskite CaFeO3.6,7 With decreasing temperature, the instability of Fe4+ in CaFeO3 is relieved by charge disproportionation, giving Fe3+ and Fe5+ ions (2Fe4+ → Fe3+ + Fe5+). Crystal structure analysis revealed that the large Fe3+O6 octahedra and small Fe5+O6 octahedra are ordered in a rock salt manner.8,9 Mössbauer spectroscopy provided clear evidence of Fe5+ in the charge-disproportionated state: the spectrum consists of two components with different isomer shifts (ISs), and the component with the smaller IS (0.01 mm/sec) corresponds to Fe5+. Similar charge disproportionation was also found in the A-site ordered perovskite CaCu3Fe4O1210−13 and the layered double perovskite Ca2FeMnO6.10−14 Unlike the tetrahedrally oxygen-coordinated Fe5+ in A3FeO4 (A = Na, K, and Rb), Fe5+ in the charge-disproportionated CaFeO3, © XXXX American Chemical Society
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EXPERIMENTAL SECTION
Polycrystalline samples of La2Li1−xMgxFeO6 (0.0 ≤ x ≤ 0.5) were synthesized by solid-state reaction under high-pressure and hightemperature conditions. Appropriate amounts of raw oxide materials (La2O3, Li2O2, Fe2O3, and MgO) and the oxidizing agent KClO4 were well-mixed, sealed in a platinum capsule, and loaded into a cubic-anvil type high-pressure apparatus. After the samples were subjected to 8 GPa and 900 °C for 30 min, they were quenched to room temperature before the pressure was released. Received: April 6, 2016
A
DOI: 10.1021/acs.inorgchem.6b00827 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Phase identification of the samples and crystal structure analysis were performed with conventional XRD and SXRD data. The SXRD measurements at room temperature were performed at the beamline BL02B2 in SPring-8 with a wavelength of 0.774 Å. Each sample was packed into a glass capillary that was kept rotating during the measurement. The obtained data were analyzed with the Rietveld method using the program RIETAN-VENUS.16 The oxidation state of Fe was evaluated by the 57Fe Mössbauer spectroscopy measurement performed in transmission geometry with a constant-acceleration spectrometer using a 57Co/Rh radiation source. The velocity scale and the IS were determined with the relative values of α-Fe at room temperature. The spectrum was fitted to Lorentzian functions by using the standard least-squares method. X-ray absorption spectroscopy (XAS) experiments were also conducted at the beamline BL39XU in SPring-8 to estimate the oxidation state of Fe. The spectrum was obtained by a conventional transmission mode using two ionization chambers, and the powder sample was pasted uniformly on the scotch tape. The magnetic properties of the samples were measured with a commercial Quantum Design Magnetic Property Measurement System SQUID magnetometer. Temperature dependence of magnetic susceptibility was measured at 5−300 K under zero-field-cooled (ZFC) and field-cooled (FC) conditions.
Figure 2. Crystal structure of the rhombohedral double perovskite La2LiFeO6.
in CaFeO3 and SrFeO3, and it is also smaller than that for the Fe5+ component in the charge-disproportionated CaFeO3 at a low temperature.6,7 Although weak intensity (area ratio less than 4%) is also observed at ∼0.3 mm/s, this probably originates from reduced Fe ions in the surface of the sample. The Fe−K absorption edge of XAS for La2LiFeO6 is shifted to a higher energy compared to those for CaFe4+O3 and SrFe4+O3 (Figure 4), which indicates that the oxidation state of Fe in La2LiFeO6 is higher than 4+. The results of the crystal structure analysis, Mössbauer spectroscopy, and XAS all suggest that the oxidation state of Fe in La2LiFeO6 is very close to 5+. The Fe5+ oxidation state is also consistent with a simple ionic model of La2LiFeO6 with typical La3+, Li+, and O2−. It is thus concluded that La2LiFeO6 crystallizes with the R3̅ double perovskite structure, where the Li+ and Fe5+ ions are ordered in the rock salt manner. The magnetic properties of La2LiFeO6 are of particular interest because the Fe5+ arrangement in the double perovskite structure forms a tetrahedral network, which gives geometrical spin frustration if the magnetic interaction between Fe5+ is antiferromagnetic. Although geometrically frustrated anti-ferromagnets attract significant interests,20,21 the B-site ordered double perovskite structure oxides A2BB′O6 with magnetic B and nonmagnetic B′ cations are relatively less studied. The present La2LiFeO6 is such a compound and contains magnetic Fe5+ at the B site. The temperature dependences of magnetic susceptibility and inverse susceptibility of La2LiFeO6 are shown in Figure 5. The magnetic susceptibility shows a peak at 12 K, suggesting the anti-ferromagnetic-like transition takes place at that temperature (TN). Because the TN peak temperature in susceptibility measurement often differs from the actual magnetic transition temperature as seen in the frustrated double perovskites La2LiRuO6 and La2LiOsO6,22,23 d(χT)/dT − T plot analysis is also employed.24 As shown in Figure S1 in the Supporting Information, the peak temperature in the d(χT)/dT − T plot is 11 K, which is quite close to TN observed in the χ−T plot. In La2LiFeO6 the anti-ferromagnetic-like transition indeed takes place at 11−12 K. The observed susceptibility above that temperature obeys the Curie−Weiss law, χ = C/(T − θ), and the fit gives C = 1.67(4) emu-K/mol and θ = −119 K. This Curie constant is comparable with that of the theoretical value that expected for Fe5+ with d3 electron configuration (1.88 emu-K/ mol), confirming that only Fe5+ spins contribute to the magnetic properties of La2LiFeO6. The observed Weiss temperature θ is quite low compared to the TN, and the frustration index f = |θ|/ TN, which is an empirical measure of frustration by quantity,25 is ∼10, indicating that the present system is highly frustrated.
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RESULTS AND DISCUSSION Figure 1 shows the SXRD pattern, which indicates that the obtained La2LiFeO6 is a single phase and crystallizes in the
Figure 1. SXRD pattern of La2LiFeO6 and result of Rietveld refinement.
perovskite structure. Superstructure reflections such as 111 (2θ = 10.01°) and 11−1 (10.15°) in the rhombohedral lattice indicate that the B-site Li and Fe ions are ordered. Although the previous XRD analysis did not give the crystal structure with either the R3̅m or R3̅ space group,15 the present Rietveld refinement revealed that the R3̅ structure model gives a better goodness of fit. The result implies that the a−a−a− octahedral tilt is stabilized in the double perovskite structure.17 No apparent vacancies are observed for the oxygen sites. The refinement result for the B-site occupancies confirms the fully ordered rock salt type of arrangement of Li and Fe as shown in Figure 2. The obtained structure parameters are listed in Table I. The bond distances obtained from the crystal structure refinement enabled us to estimate the oxidation states of the cations by a bond valence sum (BVS) method,18,19 and those states are also listed in Table I. Because no reliable bond valence parameter for Fe5+ has been reported, that for Fe4+ was used for the calculation.9 The BVS result of 4.8 for Fe strongly suggests the Fe in La2LiFeO6 is in an unusually high oxidation state. The oxidation state of Fe in La2LiFeO6 was also examined by Mössbauer spectroscopy and XAS measurements. The Mössbauer spectrum at room temperature (Figure 3) mainly consists of a single component with the IS of −0.42 mm/s, which is in good agreement with the result of a previous study.15 This negative IS value is much smaller than the ISs for the Fe4+ typically observed B
DOI: 10.1021/acs.inorgchem.6b00827 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table I. Refined Structural Parameters, Selected Bond Lengths, and Bond Valence Sum Values of La2LiFeO6 at Room Temperaturea z
B (Å2)
M−O (Å)
BVS
0
0.250 04(9)
0.203(5)
3.0
0 0 0.3476(9)
1/2 0 0.5846(3)
0.3(2) 0.12(2) 0.34(4)
2.495(2) × 3 2.685(2) × 3 2.954(2) × 3 2.728(2) × 3 2.003(2) × 6 1.857(2) × 6
x
atom
site
La
6c
0
Li Fe O
3b 3a 18f
0 0 0.2188(9)
y
1.4 4.8
a
In the structure analysis the rhombohedral crystal structure was refined with the hexagonal cell. The cation (M)−O bond lengths and BVSs calculated from the refinement result are also listed. Space group; R3̅. a = 5.447 11(3) Å, c = 13.135 99(5) Å, and Rwp = 5.62%.
ature,22,23 because each compound contains S = 3/2 ions with d3 electron configuration, 3d3 (Fe5+), 4d3 (Ru5+), or 5d3 (Os5+), at the B site in La2LiBO6. The magnetic transition temperatures of La2LiFeO6, La2LiRuO6, and La2LiOsO6 are, respectively, 12, 24, and 30 K, while the Weiss temperatures are, respectively, −119, −184, and −154 K. The frustration indices are thus ∼10, 8, and 5, indicating that all the S = 3/2 spins in the tetrahedral networks are somewhat geometrically frustrated. Considering that La2LiFeO6 crystallizes in the rhombohedral R3̅ space group, while both La2LiRuO6 and La2LiOsO6 crystallize in the P21/n symmetries,22,23 the highly frustration index ∼10 in the present La2LiFeO6 might be related to this high symmetry crystal structure. Mg2+ is then substituted for Li+ in La2LiFeO6 to see how the crystal structure and magnetic properties would change. Up to 50% substitution there was no change in the structure symmetry, and thus the rhombohedral double perovskite structure was retained. As shown in Figure 6, the lattice parameters increases
Figure 3. Mössbauer spectrum of La2LiFeO6 at room temperature. The dots are experimental data, and the red line shows the fit.
Figure 4. Normalized Fe−K edge XAS spectrum of La2LiFeO6. The spectra of Fe4+ in SrFeO3 and CaFeO3 are also plotted for comparison. Figure 6. Changes in the hexagonal lattice parameters with the Mg substitution for Li in La2Li1−xMgxFeO6. Dotted lines are guides.
linearly with increasing Mg substitution, confirming the successful substitution of Mg2+ for Li+ in the double perovskite structure. A typical SXRD pattern of La2Li0.5Mg0.5FeO6 and the result of the Rietveld refinement are given in the Supporting Information (Figure S2). The refinement result shows the occupancies of 0.50(1)/0.50(−) for Li/Mg, suggesting that Mg2+ substitutes for Li+ at the B site, although it was difficult to precisely refine the occupancies. The refined Fe−O bond length 1.903(4) Å increases from 1.857(2) Å in the nonsubstituted sample, indicating that the Mg2+ substitution for Li+ produces Fe4+ or Fe3+ for the Fe5+ site. The Mössbauer spectrum of La2Li0.5Mg0.5FeO6 also shows the presence of a lower oxidation component than Fe5+, as shown in the Supporting Information (Figure S3). The additional intensity in the spectrum is reproduced with a singlet with an IS of 0.13 mm/s, which can
Figure 5. Temperature dependence of the magnetic susceptibility (left axis) and the inverse susceptibility (right axis) of La2LiFeO6 measured under a 10 kOe magnetic field.
It is interesting to compere the present results of La2LiFeO6 to those of La2LiRuO6 and La2LiOsO6 reported in the literC
DOI: 10.1021/acs.inorgchem.6b00827 Inorg. Chem. XXXX, XXX, XXX−XXX
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be attributed to Fe4+. Therefore, the Mg2+ substitution for Li+ produces Fe4+ in the Fe5+ site, as described by La2(Li+1−xMg2+x)(Fe5+1−xFe4+x)O6. In Figure 7, the magnetic susceptibility and the inverse susceptibility of La2Li1−xMgxFeO6 (0.0 ≤ x ≤ 0.5) are plotted as a
CONCLUSIONS A single-phase sample of a double perovskite La2LiFeO6 was synthesized at high pressure (8 GPa) and temperature (900 °C) condition. The Rietveld structure refinement with SXRD data revealed that the space group of La2LiFeO6 is R3̅ and the Li+ and Fe5+ ions at the B site of the rhombohedral perovskite cell are ordered in a rock salt manner. The unusually high oxidation state of Fe5+ ions was confirmed by Mössbauer spectroscopy and XAS. The tetrahedral arrangement of Fe5+ gives geometrical spin frustration of anti-ferromagnetic Fe5+ spins. The Weiss temperature is −119 K, and the anti-ferromagnetic transition temperature is 12 K, giving a frustration index of ∼10. Substitution of Mg2+ for up to 50% of the Li+ produces Fe4+ for some of Fe5+, which is confirmed by the Mössbauer spectra. The substitution strongly affects the magnetic interactions in the Fe spin sublattice, and the Weiss temperature increases significantly, decreasing the frustration index. The Fe5+ spin sublattice is disturbed by even a very small amount of Fe4+, and thus the frustration cannot be tolerant.
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Figure 7. Temperature dependence of the magnetic susceptibility (left axis, lines and markers) and the inverse susceptibility (right axis, dotted lines) of La2Li1−xMgxFeO6 (0.0 ≤ x ≤ 0.5) measured under a 10 kOe magnetic field.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00827. Magnetic transition temperature analysis by d(χT)/dT − T plot for La2LiFeO6, synchrotron X-ray diffraction pattern with result of Rietveld refinement, and Mössbauer spectrum of La2Li0.5Mg0.5FeO6 (PDF)
function of temperature. Note that even very small amounts of the Mg substitution changes the magnetic behaviors drastically. For these Mg-substituted samples, the temperature dependence of the inverse susceptibility shows anomalies at ∼120 K and above 120 K shows linear behaviors. Fitting the data above 120 K to the Curie−Weiss law gives Curie constants for all the samples are almost the same as that for the nonsubstituted La2LiFeO6, but the Weiss temperatures change greatly (Figure 8). θ increases
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The synchrotron radiation experiments at SPring-8 were performed with the approval of the Japan Synchrotron Radiation Research Institute (Proposal Nos. 2014B1078, 2015A1014, and 2016A1650). This work was supported by a grant for the Joint Project of Chemical Synthesis Core Research Institutions from MEXT and by JST-CREST Program. The work was also partly supported by Grants-in-Aid for Scientific Research (Nos. 16H00888 and 16H02266).
Figure 8. Changes in the Curie constant and the Weiss temperature of La2Li1−xMgxFeO6 (0.0 ≤ x ≤ 0.05). Dotted lines are guides.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.6b00827 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b00827 Inorg. Chem. XXXX, XXX, XXX−XXX
