Mn2MnReO6: Synthesis and Magnetic Structure Determination of aNew Transition-Metal-Only Double Perovskite CantedAntiferromagnetMan-Rong Li,† Jason P. Hodges,§ Maria Retuerto,† Zheng Deng,† Peter W. Stephens,∥ Mark C. Croft,‡

Xiaoyu Deng,‡ Gabriel Kotliar,‡ Javier Sanchez-Benítez,⊥ David Walker,# and Martha Greenblatt*,†

†Department of Chemistry and Chemical Biology, and ‡Department of Physics and Astronomy, Rutgers, the State University of NewJersey, 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. Mn2MnReO6crystallizes in a monoclinic P21/n structure, as established by synchrotron X-ray and powder neutron diffraction (PND) methods, with eight-coordinatedA sites and rock-salt arrangement of the B and B′-site MnO6 and ReO6. Boththe structural analysis and the X-ray absorption near edge spectroscopyresults indicate mixed valence states of the B/B ′ -s i te inMn2+2Mn2+/3+Re5+/6+O6. The magnetic and PND studies evidence anantiferromagnetic (AFM) transition at ∼110 K and a transition from asimple AFM to canted AFM with net ferromagnetic component at ∼50 K.The observed Efros−Shklovskii variable-range-hopping semiconductingbehavior 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 Fermilevel by static on-site interactions and open a small gap, which is responsible for the insulating state in such a d-electron-richsystem. These results provide insight of the electronic origin of the physical properties of Mn2MnReO6 with local electronicstructure similar to that of Mn2FeReO6.

■ INTRODUCTION

Transition metal (TM) perovskite oxides (ABO3 and A2BB′O6,B and B′ are TM ions) have attracted much attention becauseof their promising technological applications.1−6 Theirmagnetic and electronic properties can be manipulated bycontrolling the valences of B/B′ ions and the geometry of theB−O−B/B′ bonds and thus the B−O−B/B′ interactions. Inconventional perovskites, the A-sites are usually occupied bymagnetically “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′ interactionsbut control the chemical valence, structure, and tolerance factor(t) variation.7 Exotic perovskites with unusually small TM ionsat the A-sites are a burgeoning area and have opened newprospects for novel multifunctional materials in competitionwith other structure types such as the corundum family.8,9 Ascompared to conventional perovskites, these exotic perovskites

can only be stabilized under high pressure; nevertheless, the Asites are not only restricted to the fine-tuning of electronconcentration and the B−O−B/B′ interactions, but alsobecome an integral part of the electronic system, and thusthe 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 beenextensively studied more recently.10−14 In this unique family,one-quarter of the A sites is generally filled with relatively largecations, such as alkali metal, alkaline-earth metals, andlanthanide ions, and the remaining three-quarters (denoted asA′) is occupied by TM ions (A′ = Cu2+, Mn3+, Co2+, or Pd2+).

Received: February 23, 2016Revised: April 8, 2016Published: April 13, 2016

Article

pubs.acs.org/cm

© 2016 American Chemical Society 3148 DOI: 10.1021/acs.chemmater.6b00755Chem. Mater. 2016, 28, 3148−3158

The large size A ions force the BO6 octahedra to tilt to stabilizethe pseudosquare-planar A′O4 configuration (Figure 1a).AA′3B4O12 quadruple perovskites typically form with cubicstructure (space group (SG) Im3). Intriguing magnetic andelectrical properties induced through the A′/B−A′/B inter-actions 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 magneticordering temperature (TC = 355 K) and large low-fieldmagnetoresistance.11 (In1−yMny)MnO3 (1/9 ≤ y ≤ 1/3,synthesized at 1773 K and 6 GPa) is a solid solution seriesof A2BB′O6-type double perovskites with the A-site partiallyoccupied by Mn2+,15 which crystallize in distorted monoclinicSG P21/n (Figure 1b) with B and B′ ordering originating fromthe charge ordering of Mn, (In3+, Mn2+)A2(Mn4+,Mn3+)B(Mn3+)B′O6 with large Jahn−Teller distortion of theB′-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 transitiontemperature (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 beenalso obtained at high pressure. Presently, the perovskitepolymorphs Mn2BSbO6 with B = Fe and Cr and Mn2FeReO6

are the only compounds in this category.16−21 Mn2BSbO6 areprepared at 5 (B = Fe) and 8 GPa (B = Cr) in competition withthe ilmenite phases. Both Mn2+2B

3+Sb5+O6 phases adopt thedistorted monoclinic P21/n (Figure 1b) with B3+ and Sb5+

ordered over the B- and B′-sites. Although high-spin (HS) d5-Mn2+ and Fe3+ and d3-Cr3+ ions occupy the A- and B-sites, theproperties of Mn2BSbO6 are not so remarkable (antiferromag-netic (AFM) insulators with TN ≈ 60 and 55 K for B = Fe andCr, 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 wasprepared, Mn2FeReO6 at 5 GPa and 1623 K with P21/ndistorted monoclinic structure. Unlike the isostructuralMn2CrSbO6 and Mn2FeSbO6 an t i f e r romagne t s ,Mn2+2Fe

3+Re5+O6 is a half-metallic ferrimagnet with magneticordering up to 520 K and giant positive magnetoresistancearound 220% at 5 K and 8 T,20,21 which indicates that theelectronic and magnetic properties of these materials subtly

depend upon the electron configurations and interactions of thecationic 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′3B4O12-type MnCu3V4O12,

24 Cu-Cu3V4O12,

25 and MnMn3Mn4O12 (also known as ζ-Mn2O3),

26,27 and A2BB′O6-type Mn2FeReO6.20,21 In the

present study, we succeeded in preparing Mn2MnReO6 athigh pressure and temperature, the second transition-metal-only A2BB′O6 double perovskite. The crystal and magneticstructures, oxidation state of cations, and magnetic andmagnetotransport properties were extensively studied bothexperimentally and theoretically to understand the differencebetween Mn2MnReO6 and Mn2FeReO6 and guide furtherdesign of novel multifunctional materials.

■ EXPERIMENTAL SECTIONSynthesis and Powder Synchrotron X-ray and Neutron

Diffraction. Polycrystalline Mn2MnReO6 was prepared from astoichiometric mixture of MnO (99.99%, Alfa Aesar) and ReO3(99.99%, Alfa Aesar), which was placed in a Pt capsule, pressurizedtypically over 8−12 h, and reacted at 1673 K for 1 h under 5 GPainside a MgO crucible in a multianvil press, and then quenched toroom temperature by turning off the voltage supply of the resistancefurnace as reported in previous work.28−32 The pressure wasmaintained during the temperature quenching and then decompressedslowly over 8−12 h.

Room-temperature synchrotron powder X-ray diffraction (SPXD)data of Mn2MnReO6 were recorded on beamline X-16C (λ = 0.70027Å) at the National Synchrotron Light Source (NSLS), BrookhavenNational Laboratory (U.S.). Diamond powder was used as an internalstandard. Powder neutron diffraction (PND) data were collected on a0.483 g polycrystalline sample at the POWGEN instrument, SpallationNeutron Source, Oak Ridge National Laboratory (U.S.). The PNDdata 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 inparentheses. The magnetic structure symmetry analysis was performedwith ISODISTORT software.33 The EXPGUI interface of GSASprogram34 was used for Rietveld refinement35 of the atomic andmagnetic spin structures. The magnetic form factor for Re6+ ion wastaken from published calculation.36 Representations of the crystal andspin structures were made with VESTA-3.37

X-ray Absorption Near-Edge Spectroscopy. Mn−K and Re-L3X-ray absorption near edge spectroscopy (XANES) data werecollected in both the transmission and the fluorescence modes withsimultaneous standards. All of the spectra were fit to linear pre- and

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, Im3 , 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 ABO3simple perovskite (orthorhombic, Pnma) with AO8 and BO6 coordination environment. A, blue spheres; BO6, orange; O, light purple.

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postedge backgrounds and normalized to unity absorption edge stepacross the edge.20,30,38−45 All of the XANES was performed onbeamline X-19A at the Brookhaven NSLS with a Si-111 double crystalmonochromator.Magnetism and Magnetotransport. Magnetization measure-

ments were carried out with a Quantum Design superconductingquantum interference device (SQUID) magnetometer. The magneticsusceptibility (χ) was measured in zero field cooled (ZFC) and fieldcooled (FC) conditions under 0.1 T applied magnetic field (H) fortemperatures ranging from 5 to 400 K. Isothermal magnetizationcurves were obtained at 5, 70, and 300 K under an applied magneticfield varying from −5 to 5 T. The magnetotransport properties weremeasured on a pellet sample with the standard four-probe technique ina physical property measurement system (PPMS) from QuantumDesign 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 performedwith the full-potential linearized augmented plane wave method, asimplemented in the WIEN2k package.46 The Perdew−Burke−Ernzerhof generalized gradient approximation of exchange-correlationfunctions was adopted.47 The muffin tin radii were chosen to be the2.12, 1.96, and 1.68 Bohr radii for Mn, Re, and O, respectively, and thecutoff parameter RmtKmax was 7.0. When studying the magnetic phase,we ignore the noncollinearity of magnetic moments on different sitesfor the sake of a simple yet instructive picture of the electronicstructure.

■ RESULTS AND DISCUSSION

Crystal Structure. A combined refinement of room-temperature SPXD and PND data of Mn2MnReO6 indicatesa monoclinic P21/n distorted double perovskite isostructuralwith related perovskites Mn2FeReO6

20,21 and A2MnReO6 (A =Ca, Sr).48,49 A small amount of unreacted starting MnOmaterial (around 2.9(2) wt %) was observed. A non-stoichiometric model of Mn2(Mn1−xRex)

BReB′O6−δ was deter-mined 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 O1site. This nonstoichiometry of the main phase is qualitativelyconsistent with the small amount of minor phase MnOobserved in the diffraction data. The refined diffraction profilesand 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 = ∼5.364Å, c = ∼7.589 Å, β = 89.95(1)°, V = ∼211.72 Å3).20 InMn2FeReO6, the B/B′-site ions are Fe3+ and Re6+, while inMn2MnReO6 they are mixed valent Mn2+/3+ and Re5+/6+ (seebelow); and Mn2+ (ri(high spin) = 0.83 Å) is much larger thanMn3+ (ri(high spin) = 0.645 Å) or Fe3+ (ri(high spin) = 0.645 Å) ascompared 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 androck-salt ordering of Mn2O6 and ReO6 octahedra (Figure 1b).The average ⟨Mn1−O⟩ distance (2.406(6) Å) of Mn1O8 iscomparable with those in isostructural Mn2

2+Fe3+Re5+O6(2.379(10) Å),20 Mn2+2Fe

3+Sb5+O6 (2.397(8) Å),17 andMn2+2Cr

3+Sb5+O6 (2.387(9) Å).19 In A2MnReO6 (A = Ca,Sr) perovskites, distortion of the B-site MnO6 octahedra can beattributed to the presence Mn2+/3+ mixed valence, since theMn3+(d4) ion is Jahn−Teller active whereas the Mn2+(d5) ion isnot. Distortion of the octahedra can be estimated from thedistortion index Δ = (1/n) × ∑[(di − dav)/dav]

2, where di anddav are individual and average M−O bonds lengths in thepolyhedron, respectively.51 Sr2MnReO5.8 with Δ = 2.1 × 10−6

possesses essentially undistorted MnO6 octahedra indicative ofMn2+ ions only, whereas Δ = 4.3 × 10−4 MnO6 distortion indexin Ca2MnReO6 was attributed to Mn2+/3+ mixed valence.49 Thiswas further verified by XANES measurements where an averageMn 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. Furtherinference of the valence states of the various sites can beobtained 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 tentativeBVS assignment for Mn2+2Mn2+/3+Re5+/6+O6 is furthercorroborated 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 anglesbetween Mn2O6 and ReO6 octahedra, which are necessary tosatisfy 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 ascompared to other Mn2+ A-site high-pressure synthesizedperovskites Mn2FeReO6 (19.7°),

20 Mn2FeSbO6 (19.3°),17 and

Mn2CrSbO6 (19.6°)19 and significantly larger than average tilt

Figure 2. Rietveld refinements of the (a) SPXD and (b) PND data inthe monoclinic P21/n structure at 300 K. In (a), asterisks (*) andcrosses (+) indicate peaks from diamond diluent (internal standard)and MnO impurity, respectively. Tick marks show the position ofallowed perovskite-phase. Right inset shows the enlarged area between45.6° and 47.6°. In (b), the tick marks show the position of allowedperovskite-phase and 2.9(2)% MnO impurity.

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angles observed in related A2MnReO6 perovskites Ca2MnReO6

(15.9°) and Sr2MnReO6 (9.0°).49

The crystal structures refined from the low-temperaturePND data (Figures S1−6, Tables 1 and 2) retain themonoclinic P21/n structure from room temperature down tothe first magnetic transition at ∼110 K. At temperatures 75 Kand lower, the magnetic ordering has lowered the crystal

symmetry to triclinic SG P1 . At 100 K, which is only ∼10 Kbelow the AFM ordering transition, the magnetic ordering andstructural response are not yet discernible by PND, and thecrystal structure is refined as nonmagnetic monoclinic P21/nstructure, although we believe the compound is actually belowthe P21/n → P1 phase transition. Details on the low-temperature magnetic crystal structures determined from

Table 1. Refined Structural Parameters and Agreement Factors Determined from SPXD and PND Data for Mn2MnReO6 at 300K, and PND Data Only at Other Temperaturesa

temperature/K

300b 200 100 75 50 25 15 5

a/Å 5.275(1) 5.272(1) 5.269(1) 5.268(1) 5.268(1) 5.267(1) 5.267(1) 5.268(1)b/Å 5.400(1) 5.394(1) 5.389(1) 5.388(1) 5.386(1) 5.385(1) 5.385(1) 5.386(1)c/Å 7.710(1) 7.704(1) 7.701(1) 7.701(1) 7.701(1) 7.700(1) 7.700(1) 7.700(1)a/deg 89.99(1) 89.97(1) 89.96(1) 89.97(1) 89.97(1)β/deg 90.02(1) 90.06(1) 90.05(1) 90.05(1) 90.05(1) 90.05(1) 90.05(1) 90.05(1)γ/deg 89.95(1) 89.95(1) 89.94(1) 89.93(1) 89.92(1)V/Å3 219.65(1) 219.06(1) 218.66(1) 218.45(1) 218.49(1) 218.43(1) 218.44(1) 218.45(1)Mn1 4e (x,y,z)x 0.0071(5) 0.0043(17) 0.0063(12) 0.0050(8) 0.0026(8) −0.0008(12) −0.0012(12) 0.0059(7)y 0.0483(3) 0.0480(15) 0.0484(11) 0.0497(6) 0.0462(6) 0.0464(10) 0.0472(10) 0.0488(6)z 0.2446(2) 0.2452(10) 0.2401(8) 0.2411(5) 0.2412(5) 0.2395(7) 0.2413(7) 0.2415(4)Biso/Å

2 1.06(4) 1.00(11) 0.60(8) 0.61(4) 0.66(4) 0.54(6) 0.59(7) 0.41(4)μx 1.4(2) 1.7(1) 1.3(1)μy 1.9(1) 2.7(1) 2.8(1) 3.2(1) 2.9(1)μz 1.0(2) 1.4(2) 2.1(1)|μ| 1.9(1) 2.7(1) 3.3(1) 3.9(1) 3.7(1)Mn2 2c (0,1/2,0)c

Biso/Å2 0.49(4) 1.41(17) 1.23(13) 1.28(7) 1.60(8) 1.43(11) 1.23(11) 1.41(8)

μx −0.2(3) 0.6(2) −0.6(2)μy 3.4(1) 4.2(1) 4.7(1) 4.3(2) 4.8(1)|μ| 3.4(1) 4.2(1) 4.7(1) 4.3(2) 4.8(1)Re 2d (1/2,0,0)Biso/Å

2 0.47(2) 0.40(6) 0.24(4) 0.24(1) 0.13(1) 0.14(2) 0.17(2) 0.15(2)μx −0.6(4) −1.5(3) −1.0(3)μy 0.1(1) 0.1(1) 0.3(2) 0.2(2) −0.8(1)|μ| 0.1(1) 0.1(1) 0.6(4) 1.5(3) 1.2(2)O1 4e (x,y,z)c

x 0.3420(4) 0.3440(10) 0.3444(8) 0.3451(5) 0.3433(5) 0.3446(7) 0.3451(7) 0.3448(4)y 0.2992(4) 0.2985(10) 0.2982(8) 0.2976(5) 0.2962(5) 0.2967(7) 0.2973(7) 0.2978(4)z 0.0697(3) 0.0695(8) 0.0690(6) 0.0693(4) 0.0691(4) 0.0689(5) 0.0703(6) 0.0694(3)Biso/Å

2 0.85(4) 0.81(8) 0.74(6) 0.76(3) 0.69(3) 0.60(3) 0.68(4) 0.61(4)O2 4e (x,y,z)x 0.1930(4) 0.1942(11) 0.1930(8) 0.1931(5) 0.1906(5) 0.1904(8) 0.1928(8) 0.1919(5)y 0.8264(3) 0.8267(11) 0.8268(8) 0.8272(5) 0.8277(4) 0.8272(7) 0.8274(7) 0.8276(4)z 0.0593(3) 0.0573(8) 0.0582(6) 0.0584(4) 0.0585(4) 0.0576(5) 0.0583(6) 0.0583(7)Biso/Å

2 1.25(3) 1.11(9) 0.73(6) 0.76(3) 0.66(2) 0.63(3) 0.70(4) 0.80(3)O3 4e (x,y,z)x −0.1187(3) −0.1196(9) −0.1196(7) −0.1187(4) −0.1204(4) −0.1191(6) −0.1169(6) −0.1185(4)y 0.4313(3) 0.4343(9) 0.4303(7) 0.4312(4) 0.4316(4) 0.4331(6) 0.4332(6) 0.4315(4)z 0.2631(3) 0.2632(8) 0.2635(6) 0.2629(4) 0.2633(4) 0.2624(6) 0.2627(5) 0.2626(7)Biso/Å

2 0.81(3) 0.70(7) 0.53(6) 0.58(2) 0.58(2) 0.59(3) 0.48(3) 0.51(2)χ2 0.62 1.31 1.30 2.13 2.32 1.42 1.48 2.03Rp (%) 6.13 4.50 8.70 6.54 6.49 8.97 9.41 6.54Rwp (%) 2.62 3.73 3.67 2.72 2.81 3.80 3.88 2.65

aThe 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). Attemperatures 75−5 K, the magnetic structural refinements were performed in a C-centered 2 × 2 × 1 supercell in triclinic SG C1; additionalsymmetry elements were added as constraints to generate all of the required equivalent positions. All of the parameters listed in this table aretransformed to the parent P21/n unit cell to aid comparison across the temperature range. bCombined refinement of SPXD and PND data.cNonstoichiometry 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 and8(1)% vacancy.

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PND are given in the later section on magnetic properties andstructure.XANES. The main edge features at 3-d TM K edges are

dominated by 1s to 4p transition peak-features, along with astep-continuum-onset-feature. The 4p features can be compli-cated by splitting into multiple features by the local atomiccoordination/bonding and by admixed 3d configurations.Nevertheless, these features manifest a chemical shift to higherenergy with increasing valence, allowing the use of the K edgeto chronicle the evolution of the transition metal valence statein compounds.20,30,38−45

In Figure 3a, the Mn K main edges of Mn2MnReO6 andMn2FeReO6 are plotted along with those of standard Mncompounds with varying valence.20,38,39 The Mn2FeReO6 andMn2MnReO6 compounds both have the same distortedmonoclinic P21/n perovskite structure.28 Therefore, the Mn

K edge of the Mn2FeReO6 compound provides a goodapproximation of the A-site Mn2+ contribution to theMn2MnReO spectrum. Indeed, the dotted vertical lines indicatespectral features that are similar, albeit with reduced intensity,to those in the Mn2FeReO6 spectrum. The low energy onset of

Table 2. Selected Interatomic Distances (Å) and Angles(deg) Determined from SPXD and PND Data forMn2MnReO6 at 300 K, and PND Data Only at 200 and 100K

temperature/K

300 200 100

Mn1O8

−O1 2.119(3) 2.118(10) 2.144(7)−O1 2.604(3) 2.621(10) 2.593(7)−O1 2.778(3) 2.665(10) 2.799(7)−O2 2.107(3) 2.127(10) 2.088(7)−O2 2.654(3) 2.641(11) 2.613(8)−O2 2.657(3) 2.664(10) 2.678(7)−O3 2.145(3) 2.120(10) 2.135(7)−O3 2.177(3) 2.188(10) 2.169(7)⟨Mn1−O⟩ 2.405(3) 2.393(10) 2.402(7)BVS(Mn1) 2.07Mn2O6

−O1 (×2) 2.173(2) 2.181(5) 2.181(4)−O2 (×2) 2.086(2) 2.085(6) 2.082(4)−O3 (×2) 2.156(2) 2.154(6) 2.158(5)⟨Mn2−O⟩ 2.138(2) 2.140(6) 2.140(4)BVS(Mn2) 2.36ReO6

−O1 (×2) 1.896(2) 1.886(5) 1.881(4)−O2 (×2) 1.927(2) 1.916(6) 1.921(4)−O3 (×2) 1.966(2) 1.962(6) 1.963(5)⟨Re−O⟩ 1.930(2) 1.921(6) 1.922(4)BVS(Re) 5.0O1−Mn2−O2 87.9(1) 87.8(2) 87.9(2)

92.2(1) 92.2(2) 92.1(2)O1−Mn2−O3 85.6(1) 86.0(2) 85.8(2)

94.4(1) 94.0(2) 94.2(2)O2−Mn2−O3 85.4(1) 85.2(2) 85.0(2)

94.7(1) 94.8(2) 95.0(2)O1−Re−O2 88.7(1) 89.1(2) 88.9(2)

91.3(1) 90.9(2) 91.1(2)O1−Re−O3 87.9(1) 88.3(2) 87.7(2)

92.1(1) 91.7(2) 92.3(2)O2−Re−O3 87.5(1) 88.1(2) 87.7(2)

92.6(1) 91.9(2) 92.3(2)Mn2−O1−Re 136.1(1) 135.9(3) 136.0(2)Mn2−O2−Re 140.3(1) 141.0(3) 140.5(2)Mn2−O3−Re 138.5(1) 138.7(3) 138.2(2)⟨Mn2−O−Re⟩ 138.4(1) 138.5(3) 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 aweighted difference spectrum (with normalization) to estimate the B-site Mn spectrum in Mn2MnReO6. (b) The Mn−K pre-edge spectralregion for the same compounds as shown in (a). Note the spectra havebeen displaced vertically for clarity. The energy region indicated by thearrow and “i” label indicates the region of excess intensity in theMn2MnReO6 spectrum relative to the Mn2FeReO6 spectrum. (c) TheRe-L3 edges of Mn2MnReO6, Mn2FeReO6, along with the edges for aseries of Re standard compounds in various d-configurations/valencestates: the ∼d0-Re7+ SrFe3/4Re1/4O6; the ∼d1-Re6+ Ba2MnReO6; andthe ∼d2-Re5+ Ca2CrReO6. The spectra have been displaced verticallyfor clarity. The bimodal A and B WL-feature components are,respectively, due to transitions into the t2g and eg ligand-field-split final5d states. With increasing Re valence (waning 5d-t2g hole count), A-feature intensity should decrease and the chemical shift of the WLfeature should be toward lower energy.

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Mn2MnReO6 spectrum also clearly supports such a Mn2+

component. However, the reductions in spectral intensity inregions 1 and 2 (see label arrows in the figure) as well as theexcess intensity position 3 suggest a higher valent contributionfrom 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 weightingfactor, and the resulting difference spectrum was renormalizedto unity absorption step across the edge. The differencespectrum (see “Diff.” curve, green line in Figure 3a) manifests aprominent peak, displaced in energy, well above the Mn2+

chemical shift regime. On the other hand, the differencespectrum is not fully shifted into a clear Mn3+ regime. Thus, weinterpret the Mn K main edge measurements on Mn2MnReO6in terms of an A-site Mn2+ state, and a B-site Mn mixed-valencestate 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 estimatecooperative valence changes.30,38−44 As has been notedpreviously, an enhancement of the pre-edge feature intensityis present in Mn2FeReO6 and Mn2MnReO6-type compoundsdue to the d/p hybridization allowed by the noncentrosym-metric highly distorted local coordination.28 The single, lowenergy onset feature, typical of Mn2+, can be clearly seen inboth the MnO and the Mn2FeReO6 pre-edge spectra. In thestandard compounds, higher Mn valence pre-edge spectralfeatures can be seen to be broader and occur at higher energies.The pre-edge spectrum of Mn2MnReO6 has an onset quitesimilar to that of Mn2FeReO6; however, there is a clear excessof spectral intensity at a higher energy consistent with theproposed interpretation of a higher valence on the Mn−B-sites.Thus, the pre-edge results are qualitatively consistent with themain 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 LFsplitting can be clearly observed at the Re-L3 edges as splittingof 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-holecount) lead to enhancement in the relative A-feature intensity,although matrix element and bonding/band structure effectscan lead to variations in the A−B feature splittings andintensities. In Figure 3c, the general trend of increasing relativeA-intensity with increasing valence can be seen. Anotherindicator of the Re d-configuration/valence state is the chemicalshift of the WL feature. Referring to Figure 3c, one should notethe systematic chemical shift upward in WL-feature centrumenergy 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 theMn2MnReO6 and Mn2FeReO6 spectra in Figure 3, one notestwo points: first, the A-feature of Mn2MnReO6 is somewhatenhanced relative to that of Mn2FeReO6, consistent with asomewhat higher Re-5d hole count in Mn2MnReO6; second,there is a clear excess of intensity on the high-energy side of theMn2MnReO6 WL feature, evidence for a higher energy

chemical shift in this compound. Both of these observationsare consistent with a Re state in Mn2MnReO6 that is shiftedhigher in valence and lower in d-electron count relative to the∼d2-Re5+ state in Mn2FeReO6. Moreover, this would also beconsistent with the mixed Mn2+/Mn3+ B-site state proposedabove. Thus, for Mn2MnReO6, overall the XANES resultsappear to support an A-site Mn2+ state; a mixed Mn2+/Mn3+ B-site state; and a mixed ∼d2-Re5+/∼d1-Re6+ B′-site Re state.

Magnetic Properties and Structures. The temperature-dependent ZFC and FC magnetic susceptibility ofMn2MnReO6 at 0.1 T (Figure 4a) exhibits two transitions: a

shallow AFM transition at around 110 K and a sharp increasearound 50 K, which has a significant FM component. At highertemperatures, the compound follows the Curie−Weiss (C−W)law, and the negative Weiss temperature (θ = −92.2(2) K) isconsistent with the AFM transition seen at 110 K. The effectivemagnetic 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 9.74μB/fu corresponding to three Mn2+ and one Re6+. Upon coolingbelow 50 K, the sharp down-turn around 40 K of the ZFCcurve and the notable divergence between the ZFC and FCplots at lower temperature are attributed to canted-AFM(CAFM) below 50 K. CAFM is consistent with Dzyaloshin-skii−Moriya (DM)53,54 allowed in the monoclinic space groupof Mn2MnReO6. The appearance of this transition in magneticsusceptibility is very similar to the CAFM transition observed ataround 80 K in the closely related perovskite In1−yMnyMnO3 (y= 1/3).15 Thus, Mn2MnReO6 fully orders in two steps: in the

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

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

expected, but there is no experimental evidence for magneticdisorder. The isothermal magnetization (M) versus temper-ature (T) loops at 5, 70, and 300 K are far from saturation(Figure 4b), which is characteristic of an AFM system. Thesmall hysteresis at 5 K is probably from the FM componentwith remnant magnetization of 0.03 μB/fu.The lowest temperature magnetic spin structure of

Mn2MnReO6 was determined from PND data collected at 5K. Comparison of the 5 K with the 300 K PND profilesrevealed many new reflections at 5 K, presumed to be magneticin origin (>10 in addition to magnetic reflections observedfrom the minor MnO impurity phase). The Mn2MnReO6magnetic reflections were indexed on a C-centered 2 × 2 × 1

supercell of the parent perovskite (P21/n), indicating amagnetic propagation vector k = (1/2,1/2,0). The magneticsymmetry analysis was performed with ISODISTORT.33

Details of the magnetic symmetry analysis are given in theSupporting Infomation. It is found that a magnetic propagationvector k1 = (1/2,1/2,0) and magnetic space group (MSG) PS1 alone can only account for the AFM transition observed at∼110 K. The second transition at ∼50 K, which is FM in natureand induces canting of the AFM spin structure, cannot bedescribed with primary k1 alone, and canting is accountedsimply by addition of a secondary magnetic propagation vectork2 = (0,0,0) of MSG P21′/n′.Having deduced that the 5 K magnetic structure of

Mn2MnReO6 MSG P1 is a canted AFM with two magneticpropagation vectors, a single phase spin and nuclear structurefor Mn2MnReO6 was modeled and refined with GSAS in SGC1 (the C-centered supercell was chosen rather than P1 setting,because this allows easier comparison between nuclearstructures above and below the magnetic transition). Magneticconstraints were included to preserve the orthogonal spinstructures of the primary k1 and secondary k2. In considerationof the small sample size limiting diffraction data quality anddesire not to over complicate the refinement, the 21 screw axissymmetry element of the parent Mn2MnReO6 structure wasalso included by adding constraints on the nuclear positioncoordinates. This fixed the total number of free nuclear positionparameters to equal that of the parent Mn2MnReO6 of SG P21/n, allowing straightforward structural comparisons at any of themeasured diffraction temperatures. The final neutron diffractionprofile fit for Mn2MnReO6 at 5 K, with all the crystal andmagnetic constraints discussed included, is shown in Figure 5a,and refined structure and magnetic parameters are listed inTable 1. An effective magnetic moment μMn1 = 3.7(1) μB wasdetermined for Mn1A, which is moderately less than μ ≈ 5 μBexpected for Mn2+(d5) ions. The deficiency in magneticmoment suggests that some magnetic disorder may beassociated with the A-site even at 5 K. At the B- and B′-sites,however, the determined magnetic moments μMn2 = 4.8(1) μBand μRe1 = 1.2(2) μB match within error the expected values ofμ(Mn2+(d5)) ≈ 5 μB and μ(Re6+(d1)) ≈ 1 μB for fully orderedmagnetic sites. The determined spin arrangements and therefined crystal structure of Mn2MnReO6 at 5 K are shown inFigure 5b. The magnetic spin structure can be considered toconsist of three interpenetrating CAFM lattices, one CAFMlattice 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 theAFM transition around 110 K is due to the major phase,although they diminished in intensity, and structural refine-ments of Mn2MnReO6 were completed using the previouslydescribed nuclear and spin structure model in SG C1 . At 50 and75 K, the FM components from the modeled secondary k2magnetic propagation vector were determined to be insignif-icant and so were eliminated by setting μx = 0 for all TM sites;at this point, the spin structure modeled is a simple collinearAFM of MSG PS1 with all moments parallel to the my direction(b-axis direction of parent P21/n structure). This is consistentwith the magnetic susceptibility data, discussed previously, inthat the transition from a simple AFM to canted AFM with netFM component occurs at ∼50 K.

Magnetotransport Properties. The resistivity (ρ) versustemperature (T) plots of Mn2MnReO6, down to 60 K at 0 and9 T (Figure 6), show characteristic semiconducting behavior.

Figure 5. (a) Rietveld refinement plots of the PND data at 5 K. Thetick marks show the position of allowed perovskite-phase and MnOimpurity [(2.9(2)%]. (b) Magnetic crystal structure of Mn2MnReO6 at5 K; blue = Mn at A-site, lilac = Mn at B-site, orange = Re at B′-site.

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The resistivity was too high to measure accurately below 60 K.Measured ρ values of Mn2MnReO6 at 0 and 9 T showed littlefield 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 and9 T, respectively. The temperature dependence of ρ wasdetermined by trial fitting the relations (1/T)p, which followsthe relation with p = 1/2 over the widest ranges oftemperatures and indicates an Efros−Shklovskii variable rangehopping (ES-VRH) mechanism (ρ = ρ0 exp(T0/T)

1/2, where T0

is the characteristic temperature) of localized carriers in thepresence of a parabolic Coulomb gap,55 as seen in the linear fitin the plot of ln ρ versus 1/T1/2 in the inset of Figure 6. Theextracted T0

1/2 (171 K1/2) and ρ0 (3.3× 10−4 Ω cm) are in linewith those of ES-VRH in A2MnReO6 (ρ0/T0

1/2 = 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 magneto-resistance (MR) of Mn2MnReO6 with A2MnReO6 andisostructural Mn2FeReO6,

6,20 in A2MnReO6 the A cations arenot magnetic; hence the A sublattice cannot interfere with thehalf-metallic ferrimagnetic Mn−O−Re paths, and, therefore,when an external magnetic field is applied, the conductivityincreases because the Mn−O−Re electron/hole mobility isenhanced in one spin direction (negative MR). In the case ofMn2FeReO6, Fe−O−Re paths are still ferrimagnetic and half-metallic; however, the presence of magnetically ordered Mnspins can interfere with the spins of the Fe−O−Re sublattice,and when an external magnetic field is applied the newmagnetic structure between Mn and Fe/Re hinders the halfmetallicity and increases the resistivity of the material (positiveMR).21 In Mn2MnReO6, the absence of magnetoresistance isattributed basically to the absence of ferrimagnetic Mn−O−Resublattice. Because Sr2MnReO6 is ferrimagnetic and presentssimilar oxidation states, it seems that the presence of Mncations on A sites turns the magnetic structure of the Bsublattice into an AFM structure. This AFM magnetic structurehinders the possibility of a half-metallic state between Mn and

Figure 6. Temperature (T)-dependent resistivity (ρ) plots ofMn2MnReO6 at 0 and 9 T, respectively. Inset shows the linear fit tothe plot of ln ρ versus T−1/2, indicating one-dimensional ES-VRHconduction mechanism.

Figure 7. Calculated density of states of Mn2MnReO6 in the simplified collinear AFM phase. (a) The total density of states computed by LSDA+Umethod (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” denotesspin-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 mainlyin 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 aremainly occupied, can be obtained by simply inverting the “up” and “dn” labels.

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

■ CALCULATIONS

First-principles calculations using spin-polarized local densityapproximation (LSDA) confirm that the system strongly favorsthe magnetic phase in that the total energy of a FM state isseveral electronvolts per formula lower than that of non-magnetic states. Furthermore, LSDA can stabilize an AFM statewith total energy 0.21 eV per formula lower than the FM state.It is notable that the calculated collinear AFM state is verycompatible with the low-temperature noncollinear AFMconfigurations observed (Figure 5b), as it preserves thesublattice AFM alignment of A-site and B-site Mn atoms, aswell as the Re atoms. Mn2MnReO6 differs from Mn2FeReO6 inthe overall AFM configuration: in the latter, the Fe-spins arecoupled ferromagnetically, while in the former, the Mn-spins (Bsites) are coupled antiferromagnetically. However, their localelectronic structures at the TM sites are similar: A-site (B-site)Mn features a large magnetic exchange splitting and a relativelysmall crystal field splitting, and therefore they are almost fullypolarized with a large magnetic moment; in contrast, the Re sitefeatures a relatively small magnetic moment due to a relativelysmall exchange splitting. In the calculated AFM state, the Mnmoments are around 4.1 and 3.9 μB at A- and B-sites,respectively, while the Re moment is around 0.25 μB. Theseobservations are in accordance with the refined spin structuredetermined in the previous section, although the magneticmoments are slightly underestimated especially at the Re site.Although LSDA provides a reasonable description of the

magnetic properties of Mn2MnReO6, it suggests a metallicground state as shown by the density of states of the AFMmodel in Figure 7a, which is not consistent with the measuredlarge resistivity. It is likely that strong correlations play animportant role in determining the ground state. To access theeffect due to strong electronic interactions, the LSDA+Ucalculations were performed by considering on-site interactionson Mn and Re sites in the mean-field level. The interactionparameters Ueff = U − J of 5 and 2 eV are applied for Mn andRe, respectively, which are reasonable values used forinvestigating similar transition metal oxides.56,57 The LSDA+U calculations stabilize the same AFM state as the LSDAcalculations. The magnetic moments on Mn and Re sites are∼4.4 and 0.5 μB, respectively, which are slightly enhanced dueto the enhanced exchange splitting by on-site interactions andare closer to the experimental measurements. Moreover, theinteractions significantly change the electronic structure aroundthe Fermi level by opening a small gap, as shown by the totaland projected density of states of Mn and Re sites in Figure 7.The density of states near the Fermi level is mainly from Re d-orbitals, with little contributions from the Mn d-orbitals. This isbecause the Mn sites are almost fully polarized, and one-half ofthe Mn d-orbitals in the majority spin channel are fullyoccupied, while the other one-half in the minority spin channelare empty. In the LDA+U scenario, the Mn states could beeasily driven away from the Fermi level by static on-siteinteractions. This is very different from the case in Mn2FeReO6,where the partial occupancy in the minority spin channel of Fesites pins the d states at the Fermi level, which could not bemoved away easily by electronic interactions.20 These findingsprovide insights into the electronic origin of the variousdifferences in the properties of Mn2MnReO6 and Mn2FeReO6.

■ CONCLUSIONIn this work, we prepared the second transition-metal-onlydouble perovskite, Mn2MnReO6, at high pressure and temper-ature, and studied its crystal and magnetic structures, oxidationstates of the constituent magnetic ions, and the physicalproperties including magnetism and magnetotransport behav-ior. Unlike the ferromagnetic coupling of the B-site Fe-spins inthe parent Mn2FeReO6, the B-site Mn-spins are coupledantiferromagnetically in Mn2MnReO6, resulting in threeinterpenetrating canted antiferromagnetic lattices and absenceof magnetoresistance. First-principles calculations results areconsistent with the experimental observations, and reveal thatthe strong correlated electronic interactions significantly changethe 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 ofMn2BReO6 (B = Mn and Fe) are very similar, their physicalproperties are dramatically different, antiferromagnetic insulatorfor B = Mn, and ferromagnetic half-metal for B = Fe. Thesefindings imply that transition-metal-only double perovskitecompounds can be synthesized under high pressure andtemperature with their chemical and physical behaviors stronglydependent on the spin structures and electronic interactions inthe ground state. So far, only two compounds are known in thisfamily; thus, other new compounds are expected for interestingproperties and bottom-up material design rules.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.chemma-ter.6b00755.

Determination of magnetic spin structure; symmetryoperations for magnetic space group Ps1 (Table S1); therefined PND data between 5 and 300 K (Figures S1−6);and the spin structure at 5 K (Figure S7) (PDF)Crystallographic information files (ZIP)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: martha@rutchem.rutgers.edu.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the NSF-DMR-1507252 grant.X.D. and G.K. are supported by the NSF-DMREF projectDMR-1435918. J.S.-B. is supported by the Spanish projectsMAT2013-41099-R and RyC-2010-06276. A portion of thisresearch at ORNL’s Spallation Neutron Source was sponsoredby the Scientific User Facilities Division, Office of Basic EnergySciences, U.S. Department of Energy. Use of the NSLS,Brookhaven National Laboratory was supported by the DOEBES (DE-AC02-98CH10886). We would like to thank Ms. J.Hanley at LDEO in Columbia University for making the highpressure assemblies.

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