Mn2MnReO6 - Rutgers Physics

Article pubs.acs.org/cm

Mn2MnReO6: Synthesis and Magnetic Structure Determination of a New Transition-Metal-Only Double Perovskite Canted Antiferromagnet Man-Rong Li,† Jason P. Hodges,§ Maria Retuerto,† Zheng Deng,† Peter W. Stephens,∥ Mark C. Croft,‡ Xiaoyu Deng,‡ Gabriel Kotliar,‡ Javier Sánchez-Benítez,⊥ David Walker,# and Martha Greenblatt*,† †

Department of Chemistry and Chemical Biology, and ‡Department of Physics and Astronomy, Rutgers, the State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, United States § Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Department of Physics & Astronomy, State University of New York, Stony Brook, New York 11794, United States ⊥ Departamento de Química Física I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Madrid E-28040, Spain # Lamont Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, New York 10964, United States S Supporting Information *

ABSTRACT: Transition-metal-only double perovskite oxides (A2BB′O6) are of great interest due to their strong and unusual magnetic interactions; only one compound, Mn2FeReO6, was reported in this category to date. Herein, we report the second transition-metal-only double perovskite, Mn2MnReO6, prepared at high pressure and temperature. Mn2MnReO6 crystallizes in a monoclinic P21/n structure, as established by synchrotron Xray and powder neutron diffraction (PND) methods, with eight-coordinated A sites and rock-salt arrangement of the B and B′-site MnO6 and ReO6. Both the structural analysis and the X-ray absorption near edge spectroscopy results indicate mixed valence states of the B/B′-site in Mn2+2Mn2+/3+Re5+/6+O6. The magnetic and PND studies evidence an antiferromagnetic (AFM) transition at ∼110 K and a transition from a simple AFM to canted AFM with net ferromagnetic component at ∼50 K. The observed Efros−Shklovskii variable-range-hopping semiconducting behavior is attributed to the three (A-site Mn2+, B-site Mn2+/3+, and B′-site Re5+/6+) interpenetrating canted AFM lattices. Theoretical calculations demonstrate that the almost fully polarized Mn states in Mn2MnReO6 are driven away from the Fermi level by static on-site interactions and open a small gap, which is responsible for the insulating state in such a d-electron-rich system. These results provide insight of the electronic origin of the physical properties of Mn2MnReO6 with local electronic structure similar to that of Mn2FeReO6.

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INTRODUCTION Transition metal (TM) perovskite oxides (ABO3 and A2BB′O6, B and B′ are TM ions) have attracted much attention because of their promising technological applications. 1−6 Their magnetic and electronic properties can be manipulated by controlling the valences of B/B′ ions and the geometry of the B−O−B/B′ bonds and thus the B−O−B/B′ interactions. In conventional perovskites, the A-sites are usually occupied by magnetically “inactive” cations (such as alkaline earth, Pb2+, Bi3+, and La3+ ions, or a mixture of them in solid solutions), which do not directly participate in the B−O−B/B′ interactions but control the chemical valence, structure, and tolerance factor (t) variation.7 Exotic perovskites with unusually small TM ions at the A-sites are a burgeoning area and have opened new prospects for novel multifunctional materials in competition with other structure types such as the corundum family.8,9 As compared to conventional perovskites, these exotic perovskites © 2016 American Chemical Society

can only be stabilized under high pressure; nevertheless, the A sites are not only restricted to the fine-tuning of electron concentration and the B−O−B/B′ interactions, but also become an integral part of the electronic system, and thus the magnetic and transport properties are controlled by the A− O−A, B−O−B/B′, and A−O−B/B′ interactions. Perovskites with the A-site partially occupied by TM, such as the A-site ordered AA′3B4O12 quadruple perovskites, have been extensively studied more recently.10−14 In this unique family, one-quarter of the A sites is generally filled with relatively large cations, such as alkali metal, alkaline-earth metals, and lanthanide ions, and the remaining three-quarters (denoted as A′) is occupied by TM ions (A′ = Cu2+, Mn3+, Co2+, or Pd2+). Received: February 23, 2016 Revised: April 8, 2016 Published: April 13, 2016 3148

DOI: 10.1021/acs.chemmater.6b00755 Chem. Mater. 2016, 28, 3148−3158

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Chemistry of Materials

Figure 1. Crystal structures of perovskite oxides with transition metals at both the A- and the B-sites. (a) AA′3B4O12-type quadruple perovskite (cubic, Im3̅, taken from ref 14) with 1:3 ordering A-site arrangement, giving icosahedral AO12, square planar A′O4, and octahedral BO6 coordination, respectively. A, green spheres; A′O4 planes, blue; BO6 octahedra, tan; O, red spheres. (b) Distorted A2BB′O6 double perovskite (monoclinic, P21/n) with rock-salt ordering B and B′ and eight-coordination AO8. A, blue spheres; BO6, lilac; B′O6, orange; O, light blue spheres. (c) GdFeO3-type ABO3 simple perovskite (orthorhombic, Pnma) with AO8 and BO6 coordination environment. A, blue spheres; BO6, orange; O, light purple.

depend upon the electron configurations and interactions of the cationic sites. Up to now, only five perovskites with only TM at A and B sites have been reported, including the ABO3-type MnVO3 (Figure 1c), 22,23 AA′ 3 B 4 O 12 -type MnCu 3 V 4 O 12 , 24 CuCu 3 V 4 O 12 , 25 and MnMn 3 Mn 4 O 12 (also known as ζMn2O3),26,27 and A2BB′O6-type Mn2FeReO6.20,21 In the present study, we succeeded in preparing Mn2MnReO6 at high pressure and temperature, the second transition-metalonly A2BB′O6 double perovskite. The crystal and magnetic structures, oxidation state of cations, and magnetic and magnetotransport properties were extensively studied both experimentally and theoretically to understand the difference between Mn2MnReO6 and Mn2FeReO6 and guide further design of novel multifunctional materials.

The large size A ions force the BO6 octahedra to tilt to stabilize the pseudosquare-planar A′O4 configuration (Figure 1a). AA′3B4O12 quadruple perovskites typically form with cubic structure (space group (SG) Im3̅). Intriguing magnetic and electrical properties induced through the A′/B−A′/B interactions have been documented in these materials. For instance, the ferromagnetic (FM) semiconductor CaCu2+3Mn4+4O12 prepared at 5.8 GPa and 1273 K shows a high magnetic ordering temperature (TC = 355 K) and large low-field magnetoresistance.11 (In1−yMny)MnO3 (1/9 ≤ y ≤ 1/3, synthesized at 1773 K and 6 GPa) is a solid solution series of A2BB′O6-type double perovskites with the A-site partially occupied by Mn2+,15 which crystallize in distorted monoclinic SG P21/n (Figure 1b) with B and B′ ordering originating from the charge ordering of Mn, (In 3+ , Mn 2+ ) A 2 (Mn 4+ , Mn3+)B(Mn3+)B′O6 with large Jahn−Teller distortion of the B′-site Mn3+O6. Spin-glass-like behaviors were observed for (In1−yMny)MnO3 except for the y = 1/3 case (In2/3Mn1/3)MnO3), which is a canted antiferromagnet with transition temperature (TN) around 70 K. Double perovskite oxides with one single TM at A-site, but partial occupancies of the B-sites by different ions, have been also obtained at high pressure. Presently, the perovskite polymorphs Mn2BSbO6 with B = Fe and Cr and Mn2FeReO6 are the only compounds in this category.16−21 Mn2BSbO6 are prepared at 5 (B = Fe) and 8 GPa (B = Cr) in competition with the ilmenite phases. Both Mn2+2B3+Sb5+O6 phases adopt the distorted monoclinic P21/n (Figure 1b) with B3+ and Sb5+ ordered over the B- and B′-sites. Although high-spin (HS) d5Mn2+ and Fe3+ and d3-Cr3+ ions occupy the A- and B-sites, the properties of Mn2BSbO6 are not so remarkable (antiferromagnetic (AFM) insulators with TN ≈ 60 and 55 K for B = Fe and Cr, respectively), likely due to the nonmagnetic B′-site Sb5+ ion. Thus, further composition modulations are desired. Recently, the first transition-metal-only A2BB′O6 family perovskite was prepared, Mn2FeReO6 at 5 GPa and 1623 K with P21/n distorted monoclinic structure. Unlike the isostructural Mn2CrSbO6 and Mn2FeSbO6 antiferromagnets, Mn2+2Fe3+Re5+O6 is a half-metallic ferrimagnet with magnetic ordering up to 520 K and giant positive magnetoresistance around 220% at 5 K and 8 T,20,21 which indicates that the electronic and magnetic properties of these materials subtly

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EXPERIMENTAL SECTION

Synthesis and Powder Synchrotron X-ray and Neutron Diffraction. Polycrystalline Mn2MnReO6 was prepared from a stoichiometric mixture of MnO (99.99%, Alfa Aesar) and ReO3 (99.99%, Alfa Aesar), which was placed in a Pt capsule, pressurized typically over 8−12 h, and reacted at 1673 K for 1 h under 5 GPa inside a MgO crucible in a multianvil press, and then quenched to room temperature by turning off the voltage supply of the resistance furnace as reported in previous work.28−32 The pressure was maintained during the temperature quenching and then decompressed slowly over 8−12 h. Room-temperature synchrotron powder X-ray diffraction (SPXD) data of Mn2MnReO6 were recorded on beamline X-16C (λ = 0.70027 Å) at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (U.S.). Diamond powder was used as an internal standard. Powder neutron diffraction (PND) data were collected on a 0.483 g polycrystalline sample at the POWGEN instrument, Spallation Neutron Source, Oak Ridge National Laboratory (U.S.). The PND data were collected at 300(7.2), 200(1.0), 100(1.8), 75(7.2), 50(7.2), 25(1.8), 15(1.8), and 5(7.2) K(h) with measurement times listed in parentheses. The magnetic structure symmetry analysis was performed with ISODISTORT software.33 The EXPGUI interface of GSAS program34 was used for Rietveld refinement35 of the atomic and magnetic spin structures. The magnetic form factor for Re6+ ion was taken from published calculation.36 Representations of the crystal and spin structures were made with VESTA-3.37 X-ray Absorption Near-Edge Spectroscopy. Mn−K and Re-L3 X-ray absorption near edge spectroscopy (XANES) data were collected in both the transmission and the fluorescence modes with simultaneous standards. All of the spectra were fit to linear pre- and 3149


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Chemistry of Materials postedge backgrounds and normalized to unity absorption edge step across the edge.20,30,38−45 All of the XANES was performed on beamline X-19A at the Brookhaven NSLS with a Si-111 double crystal monochromator. Magnetism and Magnetotransport. Magnetization measurements were carried out with a Quantum Design superconducting quantum interference device (SQUID) magnetometer. The magnetic susceptibility (χ) was measured in zero field cooled (ZFC) and field cooled (FC) conditions under 0.1 T applied magnetic field (H) for temperatures ranging from 5 to 400 K. Isothermal magnetization curves were obtained at 5, 70, and 300 K under an applied magnetic field varying from −5 to 5 T. The magnetotransport properties were measured on a pellet sample with the standard four-probe technique in a physical property measurement system (PPMS) from Quantum Design at 0 and 9 T, respectively. To avoid the Joule heating effect, measurements were carried out with less than 0.5 μA current. Theoretical Calculations. First-principles calculations of Mn2MnReO6 based on density functional theory were performed with the full-potential linearized augmented plane wave method, as implemented in the WIEN2k package.46 The Perdew−Burke− Ernzerhof generalized gradient approximation of exchange-correlation functions was adopted.47 The muffin tin radii were chosen to be the 2.12, 1.96, and 1.68 Bohr radii for Mn, Re, and O, respectively, and the cutoff parameter RmtKmax was 7.0. When studying the magnetic phase, we ignore the noncollinearity of magnetic moments on different sites for the sake of a simple yet instructive picture of the electronic structure.

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RESULTS AND DISCUSSION Crystal Structure. A combined refinement of roomtemperature SPXD and PND data of Mn2MnReO6 indicates a monoclinic P21/n distorted double perovskite isostructural with related perovskites Mn2FeReO620,21 and A2MnReO6 (A = Ca, Sr).48,49 A small amount of unreacted starting MnO material (around 2.9(2) wt %) was observed. A nonstoichiometric model of Mn2(Mn1−xRex)BReB′O6−δ was determined to give the best fit to the 300 K data where x = 0.066(2) and δ = 0.16(2) with all of the O vacancies located on the O1 site. This nonstoichiometry of the main phase is qualitatively consistent with the small amount of minor phase MnO observed in the diffraction data. The refined diffraction profiles and crystallographic results are shown in Figure 2 and Table 1. The unit cell parameters of Mn2MnReO6 (a = 5.275(1) Å, b = 5.400(1) Å, c = 7.710(1) Å, β = 90.02(1) Å, V = 219.65(1) Å3) are larger than those of Mn2FeReO6 (a = ∼5.201 Å, b = ∼5.364 Å, c = ∼7.589 Å, β = 89.95(1)°, V = ∼211.72 Å3).20 In Mn2FeReO6, the B/B′-site ions are Fe3+ and Re6+, while in Mn2MnReO6 they are mixed valent Mn2+/3+ and Re5+/6+ (see below); and Mn2+ (ri(high spin) = 0.83 Å) is much larger than Mn3+ (ri(high spin) = 0.645 Å) or Fe3+ (ri(high spin) = 0.645 Å) as compared to the differences in size between Re6+ (ri = 0.55 Å) and Re5+ (ri = 0.58 Å).50 (Mn12)A(Mn2)B(Re)B′O6 exhibits Mn1O8 coordination and rock-salt ordering of Mn2O6 and ReO6 octahedra (Figure 1b). The average ⟨Mn1−O⟩ distance (2.406(6) Å) of Mn1O8 is comparable with those in isostructural Mn22+Fe3+Re5+O6 (2.379(10) Å),20 Mn2+2Fe3+Sb5+O6 (2.397(8) Å),17 and Mn2+2Cr3+Sb5+O6 (2.387(9) Å).19 In A2MnReO6 (A = Ca, Sr) perovskites, distortion of the B-site MnO6 octahedra can be attributed to the presence Mn2+/3+ mixed valence, since the Mn3+(d4) ion is Jahn−Teller active whereas the Mn2+(d5) ion is not. Distortion of the octahedra can be estimated from the distortion index Δ = (1/n) × ∑[(di − dav)/dav]2, where di and dav are individual and average M−O bonds lengths in the polyhedron, respectively.51 Sr2MnReO5.8 with Δ = 2.1 × 10−6

Figure 2. Rietveld refinements of the (a) SPXD and (b) PND data in the monoclinic P21/n structure at 300 K. In (a), asterisks (*) and crosses (+) indicate peaks from diamond diluent (internal standard) and MnO impurity, respectively. Tick marks show the position of allowed perovskite-phase. Right inset shows the enlarged area between 45.6° and 47.6°. In (b), the tick marks show the position of allowed perovskite-phase and 2.9(2)% MnO impurity.

possesses essentially undistorted MnO6 octahedra indicative of Mn2+ ions only, whereas Δ = 4.3 × 10−4 MnO6 distortion index in Ca2MnReO6 was attributed to Mn2+/3+ mixed valence.49 This was further verified by XANES measurements where an average Mn valence of +2.3 was estimated. Similarly, in Mn2MnReO6, the Mn2O6 octahedra are highly distorted, Δ = 1.3 × 10−3, suggestive of Mn2+/3+ mixed valence at the B-site. Further inference of the valence states of the various sites can be obtained from bond valence sum (BVS, Table 2) calculations,52 which imply Mn2+ for the A-site Mn1, mixed valence Mn2+/3+ for the B-site Mn2, and Re5+/6+ for the B′-site Re. The tentative BVS assignment for Mn 2+2 Mn2+/3+Re 5+/6+O 6 is further corroborated by XANES studies shown below. The small size of the A-site Mn2+ is near the stability boundary for perovskite and leads to extreme tilt angles between Mn2O6 and ReO6 octahedra, which are necessary to satisfy A-site bonding requirements. The average tilting angle, Φ = (180° − θ)/2 where θ is the average B−O−B′ angle, determined for Mn2MnReO6 of Φ = 20.8° is the highest as compared to other Mn2+ A-site high-pressure synthesized perovskites Mn2FeReO6 (19.7°),20 Mn2FeSbO6 (19.3°),17 and Mn2CrSbO6 (19.6°)19 and significantly larger than average tilt 3150


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Table 1. Refined Structural Parameters and Agreement Factors Determined from SPXD and PND Data for Mn2MnReO6 at 300 K, and PND Data Only at Other Temperaturesa temperature/K 300b a/Å 5.275(1) b/Å 5.400(1) c/Å 7.710(1) a/deg β/deg 90.02(1) γ/deg V/Å3 219.65(1) Mn1 4e (x,y,z) x 0.0071(5) y 0.0483(3) z 0.2446(2) Biso/Å2 1.06(4) μx μy μz |μ| Mn2 2c (0,1/2,0)c Biso/Å2 0.49(4) μx μy |μ| Re 2d (1/2,0,0) Biso/Å2 0.47(2) μx μy |μ| O1 4e (x,y,z)c x 0.3420(4) y 0.2992(4) z 0.0697(3) Biso/Å2 0.85(4) O2 4e (x,y,z) x 0.1930(4) y 0.8264(3) z 0.0593(3) Biso/Å2 1.25(3) O3 4e (x,y,z) x −0.1187(3) y 0.4313(3) z 0.2631(3) Biso/Å2 0.81(3) χ2 0.62 Rp (%) 6.13 Rwp (%) 2.62

200

100

75

50

5.272(1) 5.394(1) 7.704(1)

5.269(1) 5.389(1) 7.701(1)

5.268(1) 5.388(1) 7.701(1) 89.99(1) 90.05(1) 89.95(1) 218.45(1)

5.268(1) 5.386(1) 7.701(1) 89.97(1) 90.05(1) 89.95(1) 218.49(1)

90.06(1)

90.05(1)

219.06(1)

218.66(1)

0.0043(17) 0.0480(15) 0.2452(10) 1.00(11)

1.41(17)

0.40(6)

0.0063(12) 0.0484(11) 0.2401(8) 0.60(8)

1.23(13)

0.24(4)

0.0050(8) 0.0497(6) 0.2411(5) 0.61(4)

0.0026(8) 0.0462(6) 0.2412(5) 0.66(4)

1.9(1)

2.7(1)

1.9(1)

2.7(1)

1.28(7)

1.60(8)

3.4(1) 3.4(1)

4.2(1) 4.2(1)

0.24(1)

0.13(1)

0.1(1) 0.1(1)

0.1(1) 0.1(1)

25

15

5.267(1) 5.385(1) 7.700(1) 89.96(1) 90.05(1) 89.94(1) 218.43(1)

5.267(1) 5.385(1) 7.700(1) 89.97(1) 90.05(1) 89.93(1) 218.44(1)

−0.0008(12) 0.0464(10) 0.2395(7) 0.54(6) 1.4(2) 2.8(1) 1.0(2) 3.3(1)

−0.0012(12) 0.0472(10) 0.2413(7) 0.59(7) 1.7(1) 3.2(1) 1.4(2) 3.9(1)

1.43(11) −0.2(3) 4.7(1) 4.7(1) 0.14(2) −0.6(4) 0.3(2) 0.6(4)

1.23(11) 0.6(2) 4.3(2) 4.3(2) 0.17(2) −1.5(3) 0.2(2) 1.5(3)

5 5.268(1) 5.386(1) 7.700(1) 89.97(1) 90.05(1) 89.92(1) 218.45(1) 0.0059(7) 0.0488(6) 0.2415(4) 0.41(4) 1.3(1) 2.9(1) 2.1(1) 3.7(1) 1.41(8) −0.6(2) 4.8(1) 4.8(1) 0.15(2) −1.0(3) −0.8(1) 1.2(2)

0.3440(10) 0.2985(10) 0.0695(8) 0.81(8)

0.3444(8) 0.2982(8) 0.0690(6) 0.74(6)

0.3451(5) 0.2976(5) 0.0693(4) 0.76(3)

0.3433(5) 0.2962(5) 0.0691(4) 0.69(3)

0.3446(7) 0.2967(7) 0.0689(5) 0.60(3)

0.3451(7) 0.2973(7) 0.0703(6) 0.68(4)

0.3448(4) 0.2978(4) 0.0694(3) 0.61(4)

0.1942(11) 0.8267(11) 0.0573(8) 1.11(9)

0.1930(8) 0.8268(8) 0.0582(6) 0.73(6)

0.1931(5) 0.8272(5) 0.0584(4) 0.76(3)

0.1906(5) 0.8277(4) 0.0585(4) 0.66(2)

0.1904(8) 0.8272(7) 0.0576(5) 0.63(3)

0.1928(8) 0.8274(7) 0.0583(6) 0.70(4)

0.1919(5) 0.8276(4) 0.0583(7) 0.80(3)

−0.1196(7) 0.4303(7) 0.2635(6) 0.53(6) 1.30 8.70 3.67

−0.1187(4) 0.4312(4) 0.2629(4) 0.58(2) 2.13 6.54 2.72

−0.1204(4) 0.4316(4) 0.2633(4) 0.58(2) 2.32 6.49 2.81

−0.1191(6) 0.4331(6) 0.2624(6) 0.59(3) 1.42 8.97 3.80

−0.1169(6) 0.4332(6) 0.2627(5) 0.48(3) 1.48 9.41 3.88

−0.1185(4) 0.4315(4) 0.2626(7) 0.51(2) 2.03 6.54 2.65

−0.1196(9) 0.4343(9) 0.2632(8) 0.70(7) 1.31 4.50 3.73

The structural refinements were performed in monoclinic space group P21/n; equivalent positions are (x,y,z) and (1/2 − x,1/2 + y,1/2 − z). At temperatures 75−5 K, the magnetic structural refinements were performed in a C-centered 2 × 2 × 1 supercell in triclinic SG C1̅; additional symmetry elements were added as constraints to generate all of the required equivalent positions. All of the parameters listed in this table are transformed to the parent P21/n unit cell to aid comparison across the temperature range. bCombined refinement of SPXD and PND data. c Nonstoichiometry is refined in the 300 K refinement: Mn2 site is occupied by 93.4(2)% Mn and 6.6(2)% Re; O1 site is occupied by 92(1)% O and 8(1)% vacancy. a

symmetry to triclinic SG P1̅. At 100 K, which is only ∼10 K below the AFM ordering transition, the magnetic ordering and structural response are not yet discernible by PND, and the crystal structure is refined as nonmagnetic monoclinic P21/n structure, although we believe the compound is actually below the P21/n → P1̅ phase transition. Details on the lowtemperature magnetic crystal structures determined from

angles observed in related A2MnReO6 perovskites Ca2MnReO6 (15.9°) and Sr2MnReO6 (9.0°).49 The crystal structures refined from the low-temperature PND data (Figures S1−6, Tables 1 and 2) retain the monoclinic P21/n structure from room temperature down to the first magnetic transition at ∼110 K. At temperatures 75 K and lower, the magnetic ordering has lowered the crystal 3151


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Chemistry of Materials Table 2. Selected Interatomic Distances (Å) and Angles (deg) Determined from SPXD and PND Data for Mn2MnReO6 at 300 K, and PND Data Only at 200 and 100 K temperature/K 300 Mn1O8 −O1 −O1 −O1 −O2 −O2 −O2 −O3 −O3 ⟨Mn1−O⟩ BVS(Mn1) Mn2O6 −O1 (×2) −O2 (×2) −O3 (×2) ⟨Mn2−O⟩ BVS(Mn2) ReO6 −O1 (×2) −O2 (×2) −O3 (×2) ⟨Re−O⟩ BVS(Re) O1−Mn2−O2 O1−Mn2−O3 O2−Mn2−O3 O1−Re−O2 O1−Re−O3 O2−Re−O3 Mn2−O1−Re Mn2−O2−Re Mn2−O3−Re ⟨Mn2−O−Re⟩

200

100

2.119(3) 2.604(3) 2.778(3) 2.107(3) 2.654(3) 2.657(3) 2.145(3) 2.177(3) 2.405(3) 2.07

2.118(10) 2.621(10) 2.665(10) 2.127(10) 2.641(11) 2.664(10) 2.120(10) 2.188(10) 2.393(10)

2.144(7) 2.593(7) 2.799(7) 2.088(7) 2.613(8) 2.678(7) 2.135(7) 2.169(7) 2.402(7)

2.173(2) 2.086(2) 2.156(2) 2.138(2) 2.36

2.181(5) 2.085(6) 2.154(6) 2.140(6)

2.181(4) 2.082(4) 2.158(5) 2.140(4)

1.896(2) 1.927(2) 1.966(2) 1.930(2) 5.0 87.9(1) 92.2(1) 85.6(1) 94.4(1) 85.4(1) 94.7(1) 88.7(1) 91.3(1) 87.9(1) 92.1(1) 87.5(1) 92.6(1) 136.1(1) 140.3(1) 138.5(1) 138.4(1)

1.886(5) 1.916(6) 1.962(6) 1.921(6)

1.881(4) 1.921(4) 1.963(5) 1.922(4)

87.8(2) 92.2(2) 86.0(2) 94.0(2) 85.2(2) 94.8(2) 89.1(2) 90.9(2) 88.3(2) 91.7(2) 88.1(2) 91.9(2) 135.9(3) 141.0(3) 138.7(3) 138.5(3)

87.9(2) 92.1(2) 85.8(2) 94.2(2) 85.0(2) 95.0(2) 88.9(2) 91.1(2) 87.7(2) 92.3(2) 87.7(2) 92.3(2) 136.0(2) 140.5(2) 138.2(2) 138.2(2)

Figure 3. (a) Mn−K edge spectra for Mn2MnReO6, and Mn2FeReO6, along with those of a series of standard compound spectra: Mn2+O, LaMn3+O3, and CaMn4+O3. The spectrum labeled as “Diff.” is a weighted difference spectrum (with normalization) to estimate the Bsite Mn spectrum in Mn2MnReO6. (b) The Mn−K pre-edge spectral region for the same compounds as shown in (a). Note the spectra have been displaced vertically for clarity. The energy region indicated by the arrow and “i” label indicates the region of excess intensity in the Mn2MnReO6 spectrum relative to the Mn2FeReO6 spectrum. (c) The Re-L3 edges of Mn2MnReO6, Mn2FeReO6, along with the edges for a series of Re standard compounds in various d-configurations/valence states: the ∼d0-Re7+ SrFe3/4Re1/4O6; the ∼d1-Re6+ Ba2MnReO6; and the ∼d2-Re5+ Ca2CrReO6. The spectra have been displaced vertically for clarity. The bimodal A and B WL-feature components are, respectively, due to transitions into the t2g and eg ligand-field-split final 5d states. With increasing Re valence (waning 5d-t2g hole count), Afeature intensity should decrease and the chemical shift of the WL feature should be toward lower energy.

PND are given in the later section on magnetic properties and structure. XANES. The main edge features at 3-d TM K edges are dominated by 1s to 4p transition peak-features, along with a step-continuum-onset-feature. The 4p features can be complicated by splitting into multiple features by the local atomic coordination/bonding and by admixed 3d configurations. Nevertheless, these features manifest a chemical shift to higher energy with increasing valence, allowing the use of the K edge to chronicle the evolution of the transition metal valence state in compounds.20,30,38−45 In Figure 3a, the Mn K main edges of Mn2MnReO6 and Mn2FeReO6 are plotted along with those of standard Mn compounds with varying valence.20,38,39 The Mn2FeReO6 and Mn2MnReO6 compounds both have the same distorted monoclinic P21/n perovskite structure.28 Therefore, the Mn

K edge of the Mn2FeReO6 compound provides a good approximation of the A-site Mn2+ contribution to the Mn2MnReO spectrum. Indeed, the dotted vertical lines indicate spectral features that are similar, albeit with reduced intensity, to those in the Mn2FeReO6 spectrum. The low energy onset of 3152


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Chemistry of Materials Mn2MnReO6 spectrum also clearly supports such a Mn2+ component. However, the reductions in spectral intensity in regions 1 and 2 (see label arrows in the figure) as well as the excess intensity position 3 suggest a higher valent contribution from the Mn at the B-site. To gain insight into the spectral contribution from the Mn at the B-site, the Mn2FeReO6 spectrum has been used as a Mn2+ A-site estimate and has been subtracted with a 2/3 weighting factor, and the resulting difference spectrum was renormalized to unity absorption step across the edge. The difference spectrum (see “Diff.” curve, green line in Figure 3a) manifests a prominent peak, displaced in energy, well above the Mn2+ chemical shift regime. On the other hand, the difference spectrum is not fully shifted into a clear Mn3+ regime. Thus, we interpret the Mn K main edge measurements on Mn2MnReO6 in terms of an A-site Mn2+ state, and a B-site Mn mixed-valence state with a Mn2+/Mn3+ admixture. The spectral structure and chemical shift of the Mn pre-edge features (see Figure 3b) can be used to qualitatively estimate cooperative valence changes.30,38−44 As has been noted previously, an enhancement of the pre-edge feature intensity is present in Mn2FeReO6 and Mn2MnReO6-type compounds due to the d/p hybridization allowed by the noncentrosymmetric highly distorted local coordination.28 The single, low energy onset feature, typical of Mn2+, can be clearly seen in both the MnO and the Mn2FeReO6 pre-edge spectra. In the standard compounds, higher Mn valence pre-edge spectral features can be seen to be broader and occur at higher energies. The pre-edge spectrum of Mn2MnReO6 has an onset quite similar to that of Mn2FeReO6; however, there is a clear excess of spectral intensity at a higher energy consistent with the proposed interpretation of a higher valence on the Mn−B-sites. Thus, the pre-edge results are qualitatively consistent with the main edge results discussed above. The L3 edges of TM are dominated by very intense “white line” (WL) features due to dipole transitions into final d-states. The octahedral oxygen coordination imposes a ligand field (LF) splitting of the d-states, into lower energy, 6X degenerate, t2g and higher energy, 4X degenerate, eg multiplets. This LF splitting can be clearly observed at the Re-L3 edges as splitting of the WL feature into A (t2g related) and B (eg related) features as illustrated by the Re-L3 edge for the d0, Re7+ compound, SrFe3/4Re1/4O6, in Figure 3c.20,40−45 In general, decreases in the 5d-electron count (increases in the 5d-hole count) lead to enhancement in the relative A-feature intensity, although matrix element and bonding/band structure effects can lead to variations in the A−B feature splittings and intensities. In Figure 3c, the general trend of increasing relative A-intensity with increasing valence can be seen. Another indicator of the Re d-configuration/valence state is the chemical shift of the WL feature. Referring to Figure 3c, one should note the systematic chemical shift upward in WL-feature centrum energy in the sequence of ∼d2-Re5+, ∼d1-Re6+, and ∼d0-Re7+ spectra. Consistent with the observation and expectation of Fe3+ in Mn2FeReO6, previous work by our group concluded a ∼d2-Re5+ state for this compound. By directly comparing the Mn2MnReO6 and Mn2FeReO6 spectra in Figure 3, one notes two points: first, the A-feature of Mn2MnReO6 is somewhat enhanced relative to that of Mn2FeReO6, consistent with a somewhat higher Re-5d hole count in Mn2MnReO6; second, there is a clear excess of intensity on the high-energy side of the Mn2MnReO6 WL feature, evidence for a higher energy

chemical shift in this compound. Both of these observations are consistent with a Re state in Mn2MnReO6 that is shifted higher in valence and lower in d-electron count relative to the ∼d2-Re5+ state in Mn2FeReO6. Moreover, this would also be consistent with the mixed Mn2+/Mn3+ B-site state proposed above. Thus, for Mn2MnReO6, overall the XANES results appear to support an A-site Mn2+ state; a mixed Mn2+/Mn3+ Bsite state; and a mixed ∼d2-Re5+/∼d1-Re6+ B′-site Re state. Magnetic Properties and Structures. The temperaturedependent ZFC and FC magnetic susceptibility of Mn2MnReO6 at 0.1 T (Figure 4a) exhibits two transitions: a

Figure 4. (a) Temperature-dependent ZFC and FC magnetic susceptibility (χ) of Mn2MnReO6 at H = 0.1 T up to 400 K. Inset shows the CW fitting of paramagnetic region of the 1/χ versus temperature plot. (b) Isothermal magnetization (M) versus magnetic field (H) at 5, 70, and 300 K between −5 and 5 T.

shallow AFM transition at around 110 K and a sharp increase around 50 K, which has a significant FM component. At higher temperatures, the compound follows the Curie−Weiss (C−W) law, and the negative Weiss temperature (θ = −92.2(2) K) is consistent with the AFM transition seen at 110 K. The effective magnetic moment (μeff) derived from the C−W fit of 1/χ(T) over the paramagnetic regime (inset of Figure 4a) is 9.27 μB/fu, which is close to the theoretical value (9.78 μB /fu) corresponding to two Mn2+, one Mn3+, and one Re5+, or 9.74 μB/fu corresponding to three Mn2+ and one Re6+. Upon cooling below 50 K, the sharp down-turn around 40 K of the ZFC curve and the notable divergence between the ZFC and FC plots at lower temperature are attributed to canted-AFM (CAFM) below 50 K. CAFM is consistent with Dzyaloshinskii−Moriya (DM)53,54 allowed in the monoclinic space group of Mn2MnReO6. The appearance of this transition in magnetic susceptibility is very similar to the CAFM transition observed at around 80 K in the closely related perovskite In1−yMnyMnO3 (y = 1/3).15 Thus, Mn2MnReO6 fully orders in two steps: in the 3153


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supercell of the parent perovskite (P21/n), indicating a magnetic propagation vector k = (1/2,1/2,0). The magnetic symmetry analysis was performed with ISODISTORT.33 Details of the magnetic symmetry analysis are given in the Supporting Infomation. It is found that a magnetic propagation vector k1 = (1/2,1/2,0) and magnetic space group (MSG) PS1̅ alone can only account for the AFM transition observed at ∼110 K. The second transition at ∼50 K, which is FM in nature and induces canting of the AFM spin structure, cannot be described with primary k1 alone, and canting is accounted simply by addition of a secondary magnetic propagation vector k2 = (0,0,0) of MSG P21′ /n′. Having deduced that the 5 K magnetic structure of Mn2MnReO6 MSG P1̅ is a canted AFM with two magnetic propagation vectors, a single phase spin and nuclear structure for Mn2MnReO6 was modeled and refined with GSAS in SG C1̅ (the C-centered supercell was chosen rather than P1̅ setting, because this allows easier comparison between nuclear structures above and below the magnetic transition). Magnetic constraints were included to preserve the orthogonal spin structures of the primary k1 and secondary k2. In consideration of the small sample size limiting diffraction data quality and desire not to over complicate the refinement, the 21 screw axis symmetry element of the parent Mn2MnReO6 structure was also included by adding constraints on the nuclear position coordinates. This fixed the total number of free nuclear position parameters to equal that of the parent Mn2MnReO6 of SG P21/ n, allowing straightforward structural comparisons at any of the measured diffraction temperatures. The final neutron diffraction profile fit for Mn2MnReO6 at 5 K, with all the crystal and magnetic constraints discussed included, is shown in Figure 5a, and refined structure and magnetic parameters are listed in Table 1. An effective magnetic moment μMn1 = 3.7(1) μB was determined for Mn1A, which is moderately less than μ ≈ 5 μB expected for Mn2+(d5) ions. The deficiency in magnetic moment suggests that some magnetic disorder may be associated with the A-site even at 5 K. At the B- and B′-sites, however, the determined magnetic moments μMn2 = 4.8(1) μB and μRe1 = 1.2(2) μB match within error the expected values of μ(Mn2+(d5)) ≈ 5 μB and μ(Re6+(d1)) ≈ 1 μB for fully ordered magnetic sites. The determined spin arrangements and the refined crystal structure of Mn2MnReO6 at 5 K are shown in Figure 5b. The magnetic spin structure can be considered to consist of three interpenetrating CAFM lattices, one CAFM lattice for each TM A, B, and B′-site. At intermediate temperatures of 15, 25, 50, and 75 K, the magnetic reflections were observable and evidence that the AFM transition around 110 K is due to the major phase, although they diminished in intensity, and structural refinements of Mn2MnReO6 were completed using the previously described nuclear and spin structure model in SG C1̅. At 50 and 75 K, the FM components from the modeled secondary k2 magnetic propagation vector were determined to be insignificant and so were eliminated by setting μx = 0 for all TM sites; at this point, the spin structure modeled is a simple collinear AFM of MSG PS1̅ with all moments parallel to the my direction (b-axis direction of parent P21/n structure). This is consistent with the magnetic susceptibility data, discussed previously, in that the transition from a simple AFM to canted AFM with net FM component occurs at ∼50 K. Magnetotransport Properties. The resistivity (ρ) versus temperature (T) plots of Mn2MnReO6, down to 60 K at 0 and 9 T (Figure 6), show characteristic semiconducting behavior.

intermediate regime 110 down to 50 K, the susceptibility and NPD show that ordering is partial, but the unordered part is paramagnetic-like, but not frozen in frustrated unordered arrangement. If the unordered component were frustrated with spin-glass like arrangement, the ZFC and FC would be split between 110 and 50 K. Given the variety of magnetic ions and the relatively complex magnetic structure of Mn2MnReO6 (see Figure 5b), some disorder due to frustration might be

Figure 5. (a) Rietveld refinement plots of the PND data at 5 K. The tick marks show the position of allowed perovskite-phase and MnO impurity [(2.9(2)%]. (b) Magnetic crystal structure of Mn2MnReO6 at 5 K; blue = Mn at A-site, lilac = Mn at B-site, orange = Re at B′-site.

expected, but there is no experimental evidence for magnetic disorder. The isothermal magnetization (M) versus temperature (T) loops at 5, 70, and 300 K are far from saturation (Figure 4b), which is characteristic of an AFM system. The small hysteresis at 5 K is probably from the FM component with remnant magnetization of 0.03 μB/fu. The lowest temperature magnetic spin structure of Mn2MnReO6 was determined from PND data collected at 5 K. Comparison of the 5 K with the 300 K PND profiles revealed many new reflections at 5 K, presumed to be magnetic in origin (>10 in addition to magnetic reflections observed from the minor MnO impurity phase). The Mn2MnReO6 magnetic reflections were indexed on a C-centered 2 × 2 × 1 3154


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is the characteristic temperature) of localized carriers in the presence of a parabolic Coulomb gap,55 as seen in the linear fit in the plot of ln ρ versus 1/T1/2 in the inset of Figure 6. The extracted T01/2 (171 K1/2) and ρ0 (3.3× 10−4 Ω cm) are in line with those of ES-VRH in A2MnReO6 (ρ0/T01/2 = 2.9 × 10−4/ 248, 1.1 × 10−5/272, and 6.2 × 10−4/194 Ω·cm/K1/2 for A = Ca, Sr, and Ba, respectively).6 When comparing the half-metallic behavior and magnetoresistance (MR) of Mn2MnReO6 with A2MnReO6 and isostructural Mn2FeReO6,6,20 in A2MnReO6 the A cations are not magnetic; hence the A sublattice cannot interfere with the half-metallic ferrimagnetic Mn−O−Re paths, and, therefore, when an external magnetic field is applied, the conductivity increases because the Mn−O−Re electron/hole mobility is enhanced in one spin direction (negative MR). In the case of Mn2FeReO6, Fe−O−Re paths are still ferrimagnetic and halfmetallic; however, the presence of magnetically ordered Mn spins can interfere with the spins of the Fe−O−Re sublattice, and when an external magnetic field is applied the new magnetic structure between Mn and Fe/Re hinders the half metallicity and increases the resistivity of the material (positive MR).21 In Mn2MnReO6, the absence of magnetoresistance is attributed basically to the absence of ferrimagnetic Mn−O−Re sublattice. Because Sr2MnReO6 is ferrimagnetic and presents similar oxidation states, it seems that the presence of Mn cations on A sites turns the magnetic structure of the B sublattice into an AFM structure. This AFM magnetic structure hinders the possibility of a half-metallic state between Mn and

Figure 6. Temperature (T)-dependent resistivity (ρ) plots of Mn2MnReO6 at 0 and 9 T, respectively. Inset shows the linear fit to the plot of ln ρ versus T−1/2, indicating one-dimensional ES-VRH conduction mechanism.

The resistivity was too high to measure accurately below 60 K. Measured ρ values of Mn2MnReO6 at 0 and 9 T showed little field dependence having values around 6.80 and 8.10 Ω cm (300 K), and 1.49 × 106 and 1.87 × 106 Ω cm (50 K) at 0 and 9 T, respectively. The temperature dependence of ρ was determined by trial fitting the relations (1/T)p, which follows the relation with p = 1/2 over the widest ranges of temperatures and indicates an Efros−Shklovskii variable range hopping (ES-VRH) mechanism (ρ = ρ0 exp(T0/T)1/2, where T0

Figure 7. Calculated density of states of Mn2MnReO6 in the simplified collinear AFM phase. (a) The total density of states computed by LSDA+U method (solid line) in comparison with the one computed by LSDA method. Parts (b) and (c) show the projected density of states of Mn and Re, respectively, with d-character computed by LSDA+U method (U(Mn) = 5 eV and U(Re) = 2 eV); “up” denotes spin-up channel and “dn” denotes spin-down channel. For clarity, in (b) and (c) the density of states are summed over one-half of the Mn or Re sites, on which occupancies are mainly in the spin-up channel, as indicated by the plots. The density of states of the other one-half of the Mn/Re sites, on which the spin-down orbitals are mainly occupied, can be obtained by simply inverting the “up” and “dn” labels. 3155



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CONCLUSION In this work, we prepared the second transition-metal-only double perovskite, Mn2MnReO6, at high pressure and temperature, and studied its crystal and magnetic structures, oxidation states of the constituent magnetic ions, and the physical properties including magnetism and magnetotransport behavior. Unlike the ferromagnetic coupling of the B-site Fe-spins in the parent Mn2FeReO6, the B-site Mn-spins are coupled antiferromagnetically in Mn2MnReO6, resulting in three interpenetrating canted antiferromagnetic lattices and absence of magnetoresistance. First-principles calculations results are consistent with the experimental observations, and reveal that the strong correlated electronic interactions significantly change the electronic structure near the Fermi level by opening a gap, in agreement with the high resistivity observed in Mn2MnReO6. Although the crystal and local electronic structures of Mn2BReO6 (B = Mn and Fe) are very similar, their physical properties are dramatically different, antiferromagnetic insulator for B = Mn, and ferromagnetic half-metal for B = Fe. These findings imply that transition-metal-only double perovskite compounds can be synthesized under high pressure and temperature with their chemical and physical behaviors strongly dependent on the spin structures and electronic interactions in the ground state. So far, only two compounds are known in this family; thus, other new compounds are expected for interesting properties and bottom-up material design rules.

Re, and therefore no MR is expected. In addition, the magnetic structure also explains the insulating behavior of the compound.

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CALCULATIONS First-principles calculations using spin-polarized local density approximation (LSDA) confirm that the system strongly favors the magnetic phase in that the total energy of a FM state is several electronvolts per formula lower than that of nonmagnetic states. Furthermore, LSDA can stabilize an AFM state with total energy 0.21 eV per formula lower than the FM state. It is notable that the calculated collinear AFM state is very compatible with the low-temperature noncollinear AFM configurations observed (Figure 5b), as it preserves the sublattice AFM alignment of A-site and B-site Mn atoms, as well as the Re atoms. Mn2MnReO6 differs from Mn2FeReO6 in the overall AFM configuration: in the latter, the Fe-spins are coupled ferromagnetically, while in the former, the Mn-spins (B sites) are coupled antiferromagnetically. However, their local electronic structures at the TM sites are similar: A-site (B-site) Mn features a large magnetic exchange splitting and a relatively small crystal field splitting, and therefore they are almost fully polarized with a large magnetic moment; in contrast, the Re site features a relatively small magnetic moment due to a relatively small exchange splitting. In the calculated AFM state, the Mn moments are around 4.1 and 3.9 μB at A- and B-sites, respectively, while the Re moment is around 0.25 μB. These observations are in accordance with the refined spin structure determined in the previous section, although the magnetic moments are slightly underestimated especially at the Re site. Although LSDA provides a reasonable description of the magnetic properties of Mn2MnReO6, it suggests a metallic ground state as shown by the density of states of the AFM model in Figure 7a, which is not consistent with the measured large resistivity. It is likely that strong correlations play an important role in determining the ground state. To access the effect due to strong electronic interactions, the LSDA+U calculations were performed by considering on-site interactions on Mn and Re sites in the mean-field level. The interaction parameters Ueff = U − J of 5 and 2 eV are applied for Mn and Re, respectively, which are reasonable values used for investigating similar transition metal oxides.56,57 The LSDA +U calculations stabilize the same AFM state as the LSDA calculations. The magnetic moments on Mn and Re sites are ∼4.4 and 0.5 μB, respectively, which are slightly enhanced due to the enhanced exchange splitting by on-site interactions and are closer to the experimental measurements. Moreover, the interactions significantly change the electronic structure around the Fermi level by opening a small gap, as shown by the total and projected density of states of Mn and Re sites in Figure 7. The density of states near the Fermi level is mainly from Re dorbitals, with little contributions from the Mn d-orbitals. This is because the Mn sites are almost fully polarized, and one-half of the Mn d-orbitals in the majority spin channel are fully occupied, while the other one-half in the minority spin channel are empty. In the LDA+U scenario, the Mn states could be easily driven away from the Fermi level by static on-site interactions. This is very different from the case in Mn2FeReO6, where the partial occupancy in the minority spin channel of Fe sites pins the d states at the Fermi level, which could not be moved away easily by electronic interactions.20 These findings provide insights into the electronic origin of the various differences in the properties of Mn2MnReO6 and Mn2FeReO6.

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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.chemmater.6b00755. Determination of magnetic spin structure; symmetry operations for magnetic space group Ps1̅ (Table S1); the refined PND data between 5 and 300 K (Figures S1−6); and the spin structure at 5 K (Figure S7) (PDF) Crystallographic information files (ZIP)

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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 This work was supported by the NSF-DMR-1507252 grant. X.D. and G.K. are supported by the NSF-DMREF project DMR-1435918. J.S.-B. is supported by the Spanish projects MAT2013-41099-R and RyC-2010-06276. A portion of this research at ORNL’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Use of the NSLS, Brookhaven National Laboratory was supported by the DOE BES (DE-AC02-98CH10886). We would like to thank Ms. J. Hanley at LDEO in Columbia University for making the high pressure assemblies.

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REFERENCES

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