Inducing Ferrimagnetism in Insulating Hollandite Ba1.2Mn8o16 † † † ‡ § ∥ Amber M
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Article pubs.acs.org/cm Inducing Ferrimagnetism in Insulating Hollandite Ba1.2Mn8O16 † † † ‡ § ∥ Amber M. Larson, Pouya Moetakef, Karen Gaskell, Craig M. Brown, , Graham King, † and Efrain E. Rodriguez*, † Department of Chemistry & Biochemistry, University of Maryland, College Park, Maryland 20742, United States ‡ NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States § Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States ∥ Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States *S Supporting Information ABSTRACT: Magnetic insulators are functional materials with potential applications in spintronics and multiferroics. The hollandites AxM8O16, which contain mixed-valent transition metals, have demonstrated ferromagnetism combined with insulating behavior and provide a new platform for exploring the effects of magnetic frustration due to their “folded” triangular lattice. We have tuned the ́ hollandite BaxMn8O16 from a complex antiferromagnet with Neel temperature (TN) = 25 K to a ferrimagnet with Curie temperature (TC) = 180 K via partial cobalt substitution for manganese. Both fl BaxMn8O16 and BaxCoyMn8‑yO16 were prepared by salt ux methods, and combined neutron and X-ray diffraction confirm a distorted hollandite-type structure for both oxides. X-ray photoelectron spectroscopy reveals that the Co2+ substitution drives the average Mn oxidation state from 3.7+ to nearly 4.0+, thereby changing its d-electron count. Magnetization and resistivity measurements ff show that the cobalt-doped hollandite is a ferrimagnetic insulator, with a high TC of 180 K. On the basis of neutron di raction fi measurements, we provide the rst solution of the magnetic structure of BaxMn8O16, which consists of a complex antiferromagnet with a large magnetic unit cell. Upon substituting cobalt for manganese, the magnetic structure changes dramatically, destroying the previously large magnetic unit cell and promoting ferromagnetic alignment along the hollandite tunnel direction. The observed hysteresis at base temperature for BaxCoyMn8−yO16 is explained as arising from uncompensated spins aligned along the (200) crystallographic planes. ■ INTRODUCTION consists of edge-sharing MnO6 octahedra, so that a triangular ladder connects the Mn centers (Figure 1b). In comparison, the Transition metal oxides with triangular magnetic lattices can α fi lead to new and interesting phenomena on account of their -NaMnO2 structure contains a 2D in nite triangular lattice inherently frustrated geometry. In two-dimensional (2D) (Figure 1c). Since each wall of the hollandite BaxMn8O16 connects to the others, the overall topology of the Mn centers systems such as AxMO2 (A = alkali or alkaline earth, M = transition metal), the triangular lattice can exhibit a rich can be thought of as the triangular lattice folded into a 1D tube magnetic-structural phase diagram or exotic states such as spin (Figure 1d). − liquids.1 8 In the highly frustrated 2D Kagomélattice of The tunability of the cations in the hollandite structure type makes these oxides highly functional for various applications AxCu4−x(OH)6Cl2, the frustration can be lifted by chemical 12,13 14,15 9,10 such as molecular sieves, catalysts, and cathode disorder leading to a ferrimagnetic ground state instead. Less 16−18 explored are materials in which the triangular lattice is folded in materials in batteries. Indeed, studies of hollandites have three-dimensional space, leading to new topologies for studying already led to promising physical properties such as insulating ferromagnetism in K2Cr8O16 and metal-to-insulator transitions magnetism. Such a folded triangular lattice can be realized in a 1,19−23 group of metal oxides with the so-called hollandite structure (TMIT) with antiferromagnetism in K2V8O16. The transition metal sites in hollandite structures can accommodate type (Figure 1a). Here, we study a manganese-based folded 20,24−26 triangular lattice and the dramatic changes that occur in its valencies from 2+ to 5+, and mixed valency is a magnetic properties upon cobalt(II) substitution. common phenomenon in OMS materials. Just as mixed-valence Suib et al. coined the term octahedral molecular sieves manganates with the perovskite structure led to interesting (OMS) for frameworks featuring MnO6 octahedra arranged into one-dimensional (1D) tunnels.11 The hollandite OMS Received: October 15, 2014 features MnO6 octahedra that share edges and corners to form Revised: December 3, 2014 a2× 2 square tunnel (Figure 1a). Each wall of the 2 × 2 tunnel Published: December 19, 2014 © 2014 American Chemical Society 515 DOI: 10.1021/cm503801j Chem. Mater. 2015, 27, 515−525 Chemistry of Materials Article 41 hollandite, Ba1.2Mn8O16. Starting reagents included KCl (99.2%, J.T. Baker), Mn2O3 (99%, Sigma-Aldrich), Co3O4 (74%-gravimetric Co, 42 Sigma-Aldrich), and Ba(NO3)2 (99.999%, Sigma-Aldrich). Reagents were used without further purification. A powder mixture with a molar ratio of 1.2:4:12 of Ba(NO3)2, Mn2O3, and KCl was ground with an agate mortar and pestle to obtain a target stoichiometry of Ba1.2Mn8O16 (BMO). For the cobalt-doped sample, a stoichiometric mixture of the metals was mixed with the KCl fl ux in a 1:12 ratio to target a stoichiometry of Ba1.2CoMn7O16 (BCMO). Both materials were then heated in covered alumina crucibles under ambient atmosphere. Heating was maintained at a rate of 100 K/h up to 1123 K, soaked for 72 h, then cooled to room temperature at 100 K/h. The obtained samples were washed in DI water to dissolve KCl and dried at 373 K for 1 h. Small impurities of BaCO3 were removed by stirring samples in 1 M HCl for 30 min, followed by washing with DI H2O until pH returned to neutral. For inductively coupled plasma mass spectrometry (ICP-MS) measurements, ∼10 mg of sample was stirred in 5 mL of concentrated HCl until completely dissolved then diluted appropriately for analysis. For resistivity measurements, sample powders were pressed into pellets 1 mm thick and 13 mm in diameter. Pellets were sintered at 1173 K for 24 h. Diffraction, Magnetization, and Spectroscopy. Room temper- ature powder X-ray diffraction (XRD) data were collected on a Bruker D8 X-ray diffractometer with Cu Kα radiation, λ = 1.5418 Å (step size = 0.013°, with 2θ ranging from 8°−140°). Time-of-flight (TOF) neutron powder diffraction (NPD) data were collected on the high intensity powder diffractometer (HIPD) beamline at the Lujan Neutron Scattering Center at Los Alamos Neutron Science Center. Figure 1. (a) Hollandite structure viewed down the b axis. (b) Tunnel Low temperature constant wavelength (CW) neutron diffraction data wall of hollandite showing a triangular ladder. (c) 2D triangular lattices were collected on the BT-1 high resolution powder neutron α in -NaMnO2 and (d) the triangular lattice tube in hollandite. Oxygen diffractometer at the NIST Center for Neutron Research. Cu(311) atoms are not shown for clarity in c and d. and Ge(311) monochromators produced neutron beams of λ = 1.5403 Å and λ = 2.078 Å, respectively. Scans were taken at 5, 10, and 50 K for BMO, and at 10, 40, 80, 120, 160, 180, 250, and 300 K for BCMO. 27−29 phenomena such as charge, spin, and orbital ordering, Magnetic susceptibility measurements were carried out using a mixed Mn3+/Mn4+ cations in hollandites could also lead to magnetic property measurement system (Quantum Design MPMS). similar phenomena. Transition metal site doping in the Both field-cooled (FC) and zero-field-cooled (ZFC) magnetic hollandites has already been undertaken by various researchers susceptibility measurements were taken from 2 K−300 K in direct 30−35 current mode with an applied magnetic field of 0.01 T (100 Oe). to tune the chemical properties. Determining the fi location(s) of dopant atoms, as well as any potential charge Hysteresis measurements were carried out at 2 K in magnetic elds between ±7T. ordering, is key to understanding new physics in the hollandites A Quantum Design physical property measurement system (PPMS) as well. was used for temperature dependent resistivity measurements using a The propensity of OMS materials to develop crystallographic four-probe or Van der Pauw geometry.43,44 Sintered pellets were defects, such as twinning and/or intergrowth of more than one mounted onto a PPMS puck with electrical continuity established by OMS phase, make it difficult to grow crystals or prepare highly using silver paste to connect gold wires from the pellet to the sample − crystalline powders.36 38 Recently, Talanov et al. and Moetakef mount. et al. have demonstrated high quality single crystal growth of X-ray photoelectron spectroscopy (XPS) measurements were taken hollandite oxides through flux methods.39,40 Therefore, we have in a high sensitivity Kratos AXIS 165 X-ray photoelectron sought to prepare highly crystalline and phase-pure Mn-based spectrometer operating in hybrid mode to analyze the composition fl and average transition metal oxidation state at the surface. For the Mn hollandites by employing salt ux methods in order to fully 2p peaks, monochromatic Al Kα at 280 W was used as the X-ray explore the relationship between crystal structure and magnetic energy source, whereas both Mg Kα and monochromatic Al Kα properties. sources were utilized (at 280 W) in studying the Co 2p peaks in efforts We report the nuclear and magnetic structures of BaxMn8O16 to better resolve peak overlaps. CasaXPS software was used for fi fi (BMO) and a cobalt-doped analogue, BaxCoyMn1−yO2 quanti cation and peak tting after application of Shirley backgrounds (BCMO). A combination of X-ray and neutron powder using relative sensitivity factors from the Kratos vision library. Pressure −8 diffraction, X-ray photoelectron spectroscopy, magnetometry, was at or below 5 × 10 Torr during data collection, and charge and electronic transport studies were utilized to investigate the neutralization was required to minimize surface charging.