Article pubs.acs.org/IC
Structural and Magnetic Properties of the Trirutile-type 1DHeisenberg Anti-Ferromagnet CuTa2O6 Aleksandr Golubev, Robert E. Dinnebier, Armin Schulz, and Reinhard K. Kremer* Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany
Hubert Langbein Institut für Anorganische Chemie der Technischen, Universität Dresden, Mommsenstr. 13, D-01062 Dresden, Germany
Anatoliy Senyshyn Forschungsneutronenquelle Heinz Maier-Leibnitz, Technische Universität München, Lichtenbergstrasse 1, D-85747 Garching, Germany
Joseph M. Law Dresden High Magnetic Field Laboratory, Helmholtz-Zentrum Dresden-Rossendorf, D-01314 Dresden, Germany
Thomas C. Hansen Institut Laue-Langevin, B.P. 156, 38043 Grenoble, France
Hyun-Joo Koo Department of Chemistry and Research Institute for Basic Sciences, Kyung Hee University, Seoul 130-701, Republic of Korea
Myung-Hwan Whangbo Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States S Supporting Information *
ABSTRACT: We prepared trirutile-type polycrystalline samples of CuTa2O6 by lowtemperature decomposition of a Cu−Ta−oxalate precursor. Diffraction studies at room temperature identified a slight monoclinic distortion of the hitherto surmised tetragonal trirutile crystal structure. Detailed high-temperature X-ray and neutron powder diffraction investigations as well as Raman scattering spectroscopy revealed a structural phase transition at 503(3) K from the monoclinic structure to the tetragonal trirutile structure. GGA+U density functional calculations of the spin-exchange parameters as well as magnetic susceptibility and isothermal magnetization measurements reveal that CuTa2O6 is a new 1D Heisenberg magnet with predominant anti-ferromagnetic nearest-neighbor intrachain spin-exchange interaction of ∼50 K. Interchain exchange is a factor of ∼5 smaller. Heat capacity and low-temperature high-intensity neutron powder diffraction studies could not detect long-range order down to 0.45 K.
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INTRODUCTION
cations offer a rich playground to identify such systems. When placed at the center of an octahedron of anions, the unevenly filled degenerate eg state leads to a Jahn−Teller (JT) instability,
The magnetism of low-dimensional quantum anti-ferromagnets is of special interest. Because of complex entanglement of spin and orbital degrees of freedom, sometimes combined with charge ordering effects, unconventional ground states may be realized. Especially compounds containing Cu2+ (3d9, S = 1/2) © 2017 American Chemical Society
Received: February 15, 2017 Published: May 8, 2017 6318
DOI: 10.1021/acs.inorgchem.7b00421 Inorg. Chem. 2017, 56, 6318−6329
Article
Inorganic Chemistry
Generally the TMTa2O6 compounds (TM = Fe, Co, Ni) crystallize in the primitive tetragonal system with space group P42/mnm (No. 136) allowing a bond variation of the TM cations to the oxygen atoms of 4:2, that is, either a compression or elongation of the oxygen octahedra along a fourfold axis. Whereas, for example, for CoSb2O6 the same space group and an analogous Co−O configuration are found, the crystal structure of CuSb2O6 exhibits a slight monoclinic distortion (space group P21/n)10,11 in with a minute bond alternation (∼0.4%) in the four equatorial Cu−O bonds of the CuO6 octahedra is observed. The monoclinic distortion nevertheless does not modify the gross magnetic features. The magnetism of CuSb2O6 is well-described by a spin S = 1/2 Heisenberg chain with nearest-neighbor spin-exchange interaction of ∼100 K.12−14 A study based on electronic structure calculations reported that a half-filled 3d3z2−r2 orbital is the magnetically active state.15 Other trirutile-type compounds with Jahn−Teller active cations like Cr2+ in CrTa2O6 exhibit analogous monoclinic distortions.16 The crystal structure and the physical properties of trirutiletype tantalates of most of the 3d transition metals have been intensively investigated.3−5 To the best of our knowledge the physical properties of trirutile-type CuTa2O6 have not been studied so far. Early attempts to synthesize CuTa2O6 from a CuO−Cu2O flux, by solid-state reaction or high-pressure synthesis resulted in perovskite-like phases with either a cubic or a pseudocubic orthorhombic crystal structure.17−20 A detailed account of the early endeavors into the Cu−Ta−O systems is given by Longo and Sleight.21 Krabbes and Langbein by employing thermal decomposition of a freeze-dried Cu−Taoxalate precursor at 700 °C succeeded in the preparation of phase-pure samples of trirutile-type CuTa2O6.22,23 By choosing such relatively mild synthesis conditions the formation of stable Cu(I) species could be prevented. In these early reports the crystal structure was described in the tetragonal trirutile type. However, a characteristic splitting of the (123) Bragg reflection was observed and ascribed to the onset of a transformation into the perovskite phase.22 Here, we report about the structural, magnetic, and thermal properties of such trirutile-type CuTa2O6 samples. By X-ray and neutron powder diffraction we find that the splitting is, in fact, due to a slight monoclinic distortion similar to that observed in CuSb2O6.10,11 At ∼500 K CuTa2O6 undergoes a structural phase transition above which the tetragonal trirutile structure is resumed. Characterization of the magnetic properties of trirutile-type CuTa2O6 by magnetic susceptibility and isothermal high field magnetization measurements indicate that CuTa2O6 constitutes a 1D Heisenberg chain with anti-ferromagnetic nearest-neighbor spin-exchange interaction. This finding is supported by electronic structure calculations. Long-range magnetic order could not be detected down to 0.45 K.
which typically effects an axial elongation of the octahedron. In compounds made of JT-unstable ions, co-operative JT distortion takes place, to lower the local symmetry and to minimize the total energy.1 Lowering the orbital degeneracy by distortions generally implies a distinct directionality of the spinexchange interaction to neighboring cations via intermediate anions often resulting in low-dimensional quantum magnetism with dominant spin exchange along chains or within layers. The pronounced orbital directionality makes it per se difficult to conclude on spin-exchange anisotropy from sheer structural motifs or cation−cation distances. Recently, modern electronic structure calculations have proved to be very useful to identify the dominant spin-exchange interactions in such compounds.2 Compounds of composition TMX2O6 (TM = divalent 3d transition metal cation, X = pentavalent transition metal cation) often crystallize with the tritutile structure type (see Figure 1),
Figure 1. Perspective view of the room-temperature crystal structure of CuTa2O6, where the blue, pink, yellow, green, and cyan circles represent Cu, Ta, O(1), O(2), and O(3) atoms, respectively.
in which the divalent transition metal cations TM are arranged in square planar layers separated by neighboring double layers of edge-connected XO6 octahedra.3,4 The magnetic properties of trirutile-type TM2+X2O6 (TM = Fe, Co, Ni; X = Ta, Sb) compounds are often characterized by typical features of lowdimensional magnetism, that is, broad anti-ferromagnetic shortrange ordering maxima in the magnetic susceptibilities at temperatures comparable to the dominant intrachain or intraplanar exchange interaction.5−8 Eventually, weaker interchain or interplanar spin exchange leads to long-range magnetic ordering at significantly lower temperatures involving magnetic entropies being markedly reduced from the theoretically expected values.5,6 Recently, we reinvestigated the magnetic properties of the trirutile compound NiTa2O6 in more detail9 and found that NiTa2O6 must be described as a quasi-one-dimensional (1D) Heisenberg S = 1 spin chain with an anti-ferromagnetic spinexchange interaction of ∼19 K. Even though interchain spinexchange coupling is by ∼2 orders of magnitude smaller, it causes long-range anti-ferromagnetic ordering at ∼10 K.
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EXPERIMENTAL SECTION
Polycrystalline samples were prepared by decomposing a freeze-dried Cu−Ta-oxalate precursor at 700 °C following the procedure described in detail previously.22,23 The composition of the products was determined by microprobe analysis employing a TESCAN TS5130MM scanning electron microscope (SEM) equipped with an INCA crystal spectrometer. Phase purity of the products was checked by X-ray powder diffraction using Cu Kα1 and Mo Kα1 radiation in Debye−Scherrer geometry (capillary diameter 0.2 mm). Neutron powder diffraction was performed on the high-resolution powder diffractometer SPODI (λ = 1.55 Å) at the MLZ (Munich) and on the medium-resolution high-intensity two-axis diffractometer D20 (ILL, 6319
DOI: 10.1021/acs.inorgchem.7b00421 Inorg. Chem. 2017, 56, 6318−6329
Article
Inorganic Chemistry
Figure 2. (left) Typical SEM scan across a polycrystalline CuTa2O6 sample and (right) result of the EDX analysis. Grenoble) at a wavelength of λ = 1.89 Å.24,25 Profile refinements of the powder diffraction patterns were performed with the FullProf software package.26 Field cooled (fc) and zero-field cooled magnetic susceptibilities of a polycrystalline sample of CuTa2O6 (∼38 mg) were measured with an MPMS SQUID magnetometer (Quantum Design, San Diego) in the temperature range of 0.45−300 K. Pulse field isothermal magnetization up to ∼60 T was measured at the Hochfeld-Magnetlabor at the Helmholtz-Zentrum (Dresden-Rossendorf) using a compensated coil setup.27 The same sample was also measured in an MPMS SQUID (Quantum Design, San Diego magnetometer up to 7 T to put the pulse field measurements on an absolute scale. Electron paramagnetic resonance (EPR) spectra were collected at ∼9.48 GHz with a Bruker ER 040XK X-band spectrometer in an ER73 electromagnet controlled by a BH-015 field controller that was calibrated against the resonance of 2,2-diphenyl-1-picrylhydrazyl (DPPH). Heat capacities were determined employing a PPMS system (Quantum Design, San Diego) in the temperature range of 0.4−20 K. The powder sample (∼3 mg) was intimately mixed with Apiezon N vacuum grease to improve the thermal contact. The heat capacities of the sample holder and the vacuum grease were determined in a preceding measurement cycle and subtracted from the total heat capacities. Raman spectra were collected on a Jobin Yvon Typ V 010 LabRAM single grating spectrometer with ∼1 cm−1 spectral resolution. The spectrometer setup included a double super razor edge filter, a Peltier-cooled CCD camera, and a Mikrocryo cryostat with a copper coldfinger. Measurements were performed with linearly polarized He/ Ne gas laser light of 632.817 nm wavelength with a power of less than 1 mW. The light beam was focused on a 10 μm spot on the top surface of the sample using a microscope. Measurements were taken at temperatures ranging between 25 and 500 °C.
Figure 3. Room-temperature X-ray powder diffraction pattern (Cu Kα1 radiation) of a polycrystalline sample of CuTa2O6. (inset) The angular region near the (123)tet and the (220)tet Bragg reflection with fits of Lorentzian lines (blue) and the deconvolution of the split doublet at 2Θ = 52.7° into four separate lines of equal intensity (black solid lines).
to an additional, though smaller splitting. The inset in Figure 3 displays fits of the (220)tet reflection (2Θ = 55°) assuming a Lorentzian profile resulting in a full width at half-maximum (fwhm) value of ∼0.25°. Using an fwhm similar to that of the (220)tet Bragg reflection and surmising identical intensities the two split reflections around d = 1.74 Å (2Θ = 52.7°) can be well-fitted indicating that their broadening is due to an additional splitting of ∼0.2°. Crystal Structure Refinement. In the following we describe the room-temperature and the high-temperature crystal structures as determined by X-ray and neutron highresolution powder diffraction (see Figure 4). By raising the temperature to greater than 500 K, we detected a structural phase transition from the monoclinic room-temperature structure to the tetragonal trirutile-type high-temperature structure. By a series of X-ray powder diffraction experiments at variable temperatures up to 600 K we followed the splitting of the (123)tet Bragg reflection. Figure 5 shows the splitting versus temperature together with a critical power law calculated assuming a mean field critical exponent 1/2 and TC = 503(3) K according to
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RESULTS Sample Characterization. Figure 2 displays a typical SEM picture and an energy-dispersive X-ray spectroscopy (EDX) analysis of the composition. The sample particles exhibit a fluffy morphology with typical particle sizes of 20−30 μm. According to the EDX analysis the composition of the heavy atoms amounted to 11.2(1) atom % for the Cu content and 20.8(4) atom % for the Ta content corresponding to a composition ratio of 1:1.86(5). X-ray powder diffraction patterns collected at room temperature with Cu Kα1 radiation (Figure 3) exhibit the same splitting of characteristic Bragg reflections as noted before.22,23 For example, the (123)tet Bragg reflection at d = 1.74 Å (2Θ = 52.7°) splits into two reflections of equal intensity with a distance between the two reflections of ∼0.5°. Considering the width of the nearby (220)tet Bragg reflection we noticed also a broadening of these the two reflections, which is possibly due
(Δ2Θ)123 = I0t 1/2 + a0 + a1T (Δ2Θ)123 = a0 + a1T
for T < TC
for T > TC
(1)
The crystal structure of CuTa2O6 at room temperature was analyzed in the space group P21/n by a profile refinement of X6320
DOI: 10.1021/acs.inorgchem.7b00421 Inorg. Chem. 2017, 56, 6318−6329
Article
Inorganic Chemistry
structure. In the profile refinement we assumed equal displacement factors for all atoms and a Thompson−Cox− Hastings profile of the reflections (FullProf, NPR = 7). The resolution function of the diffractometer was determined from a diffraction pattern of a LaB6 standard sample measured with the same instrumental and capillary parameters. Apparent broadening of the Bragg reflections in the CuTa2O6 diffraction pattern was subsequently modeled by expanding the particle size and shape contributions into spherical harmonics, cmnYmn, with m = 0, 2, 4, and even ± n; |n| ≤ m appropriate for Laue class 2/m (FullProf, size model No. 15) and fitting the expansion coefficients cmn. The average particle size amounted to 32(8) nm consistent with the SEM results. The background was constructed by superposing Chebychev polynomials of higher degree. A first attempt to refine the pattern collected at 500 K was made by refining the Ta atom parameters but fixing the oxygen positional parameters and assuming the monoclinic structure. The refinement converged (Bragg R-factor 2.5%) to lattice parameters a ≈ b and a monoclinic angle β ≈ 90° indicating tetragonal symmetry of the crystal structure. The refinement improved slightly (Bragg R-factor 2.34% (Rf-factor 2.32%) if the tetragonal trirutile structure and the space group P42/mnm (No. 136) was assumed. The refined structural parameters are summarized in Tables 1 and 2. In Table S1 of the Supporting Information the atom coordinates are listed and compared with those of CuSb 2 O 6 , CoTa 2 O 6 , and NiTa2O6.14,30,31 A small additional Bragg reflection was detected at d ≈ 3.75 Å in both patterns, which we ascribe to a trace (
