Article pubs.acs.org/crystal
Floating Zone Single Crystal Growth of γ‑CoV2O6 with Substantially Enhanced Crystal Size and Quality Yvo Drees,† Stefano Agrestini,† Oksana Zaharko,‡ and Alexander Christoph Komarek*,† †
Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Strasse 49, 01187 Dresden, Germany Laboratory for Neutron Scattering, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
‡
S Supporting Information *
ABSTRACT: Here, we report the growth of centimeter-sized γ-CoV2O6 single crystals by the optical floating zone method which are suitable for neutron scattering experiments. Our floating zone grown single crystals have higher magnetic ordering temperatures of 7.8 K, much more clearly visible magnetization plateaus, and substantially enhanced saturation magnetizations compared to fluxgrown single crystals, which is also important with regard to the presence of orbital moment contributions within these S = 3/2 systems. Our centimeter-sized single crystals grown by the optical floating zone technique open the way toward the measurement of the peculiar magnetic properties of this intriguing system by means of elastic and inelastic neutron scattering and have been characterized by Xray and neutron diffraction measurements.
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INTRODUCTION
Quasi one-dimensional systems have attracted enormous attention in contemporary solid state physics due to their rich physical properties and magnetic phase diagrams. Within such systems one-dimensional chains or ladders are formed by the magnetic ions. CuGeO3 and NaV2O5 are two prominent examples of low-dimensional S = 1/2 compounds.1,2 Whereas S = 1/2 systems with linear chain structure are governed by quantum spin fluctuations and often exhibit a spin singlet ground state, the S = 1 systems also often exhibit nonmagnetic ground states and behave according to Haldane’s conjecture.3 However, quasi-one-dimensional S = 3/2 cobaltate systems tend to exhibit a three-dimensional Néel ordering at the lowest temperature like for example that observed in CoNb2O6.4 Also CoV2O6 is such a low-dimensional system which crystallizes in two polymorphs, α-CoV2O6 and γ-CoV2O6.5 The crystal structure of γ-CoV2O6 consists of one-dimensional chains of edge-sharing CoO6 octahedra that are separated by nonmagnetic vanadium (V5+) oxide blocks forming quasi-1D magnetic chains along the crystallographic b-axis; see Figure 1. γ-CoV2O6 is antiferromagnetic with TN ≈ 7 K. The CoV2O6 system has attracted considerable attention recently, due to the observation of 1/3-magnetization plateaus within the isothermal magnetization curves of γ-CoV2O66−13 and due to an unusual high orbital contribution to the magnetic moment of αCoV2O66,14 as well as the proposed existence of soliton excitations in γ-CoV2O6.8 γ-CoV2O6 single crystals grown by flux methods had relatively low antiferromagnetic ordering temperatures and saturated magnetic moments of only 2.4 μB,15 which is even lower than the expected spin-only value for this cobalt vanadate with Co2+ in the high-spin state. However, one peculiarity of © XXXX American Chemical Society
Figure 1. (a) Crystal structure of γ-CoV2O6 consisting of Co−O octahedral chains running in the b-direction, green/blue/orange balls: Co-/O-/V-ions. (b) One CoO6 chain running in the b-direction together with the Co−Co bonds (red).
this intriguing system is the presence of additional orbital moment contributions which are most pronounced for αCoV2O69,14 But already pure powder samples of γ-CoV2O6 exhibit saturation magnetizations larger than 3 μB.8 Additionally, there is the need for obtaining large single crystals that are suitable for typical (inelastic) neutron scattering experiments, which is usually more difficult to realize with flux growth15 or Received: October 15, 2014 Revised: January 23, 2015
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DOI: 10.1021/cg5015303 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Crystal Growth & Design chemical vapor transport14 techniques. Hence, we tried to increase both the single crystal quality and the obtainable crystal size by choosing the crucible-free floating zone technique and report the growth of the triclinic brannerite polymorph (γ-CoV2O6) as centimeter-sized single crystals which are suitable for studying the intriguing properties of CoV2O6 by means of single crystal neutron diffraction and spectroscopy.
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EXPERIMENTAL SECTION
The γ-CoV2O6 single crystals were grown by the traveling solvent floating zone method. Feed and seed rods were prepared by high temperature solid state reaction. Therefore, appropriate amounts of V2O5 (Acros, 99.6+%) and CoCO3 (Alfa Aesar, 99.5%) were ground together and sintered several times for 20 h at 720 °C in platinum crucibles in air with intermediate grindings until a single phase αCoV2O6 sample was obtained. The phase pure powder samples were then hydrostatically pressed into rods and, again, sintered at 720 °C in air in a platinum boat crucible. A Crystal Systems Corp. four-mirror optical floating zone image furnace was used for the final floating zone growth procedure. The growth atmosphere was composed of an argon−oxygen mixture with a 7:1 Ar/O2 ratio and with a total pressure of 2.5 bar. Regarding the complicated CoO-V2O5 phase diagram,16 the development of a stable molten zone turned out to be extremely difficult, which might be the reason that CoV2O6 single crystals have not been grown by the floating zone technique so far. A substantial decrease in temperature was always needed in order to increase the stability of the floating zone. Our growth procedure had to be optimized in order to cope with the difficult incongruently melting character of γ-CoV2O6. Finally, we were able to improve the stability of the melt during the floating zone growth by the initial application of an additional vanadium(V) oxide pellet of 1 cm thickness between feed and seed rods that has been molten together with parts of the feed and seed rod in order to establish an appropriate flux and to reach an appropriate point in the CoO-V2O5 phase diagram16 where γ-CoV2O6 can be synthesized. Thus, the melt could be stabilized by the application of 218 W lamp power, and there was no need to decrease the temperature strongly after the initial melting procedure. Suitable growth conditions could be reached only with very slow growth rates of ∼0.8 mm/h, where the feed and seed rods were inversely rotated at 16 and 12 rpm, respectively. During the floating zone growth process a facette became apparent. Single crystal X-ray diffraction measurements have been performed on a Bruker D8 VENTURE single crystal X-ray diffractometer. Therefore, a small single crystalline sphere that has been obtained from our large single crystal was measured using Mo−Kα radiation (λ = 0.71069 Å). Thus, 171 748 reflections have been collected up to 2Θmax = 143.1° (H = −19 → 19, K = −23 → 23, L = −12 → 12) with a redundancy of 14.6 and an internal R-value that amounts to Rint = 2.4%. The structural refinement has been performed with the Jana2006 software.17,18
Figure 2. Photo of one of the single crystals cut from the as-grown boule that are suitable for elastic and inelastic neutron scattering experiments.
the CIF (Supporting Information). Our results are in agreement with previous structural studies.19 The refinement of the Co occupancies indicates a Co:V ratio of r = 50.3(1)%, which is close to the nominal stoichiometric value of 1:2. Thus, our single crystal X-ray diffraction measurements confirm an essentially stoichiometric sample composition of our γ-CoV2O6 single crystals. Parts of the single crystal were crushed to powder and analyzed by means of powder X-ray diffraction (using Cu−Kα radiation). Only the pure triclinic phase of γ-CoV2O6 appears within the powder X-ray diffraction measurements. Neither impurity phases nor the monoclinic phase of α-CoV2O6 is visible within the powder X-ray diffraction patterns; see Figure 4. The lattice parameters obtained from the Rietveld refinement with space group P1̅ amount to a = 7.169(5) Å, b = 8.884(6) Å, c = 4.805(3) Å, α = 90.28(1), β = 93.67(1), γ = 102.19(1). We studied our large centimeter-sized single crystals also by means of neutron diffraction at the Morpheus and at the TriCS diffractometers at the PSI in Villigen. In Figure 5 various rocking scans of our centimeter-sized single crystals are shown. As can be seen, there are no other crystallites that appear in more than 180° rocking scans with neutrons that are penetrating the entire sample volume. Furthermore, the peak widths obtained by Gaussian fits of Bragg reflections are on the order of 0.8−0.9°, indicating a very homogeneous crystallization of our whole centimeter-sized single crystal. Furthermore, we were able to detect third-integer magnetic reflections at 4.6 K. In Figure 6 a quick scan with an area detector is shown, which clearly reveals the presence of these third-integer magnetic reflections within the H0L plane of reciprocal space. The measurement of the magnetization of γ-CoV2O6 reveals an antiferromagnetic ordering temperature of TN = 7.8 K (peak of magnetic susceptibility χ shown in Figure 7), which is even higher than the reported values for pure powder samples in literature (i.e., 7 K6,8). The inverse susceptibility of γ-CoV2O6 in the b-direction can be fitted with a Curie−Weiss law between about 60 and 400 K. The fitted effective moment amounts to 5.38(22) μB and the Weiss-temperature amounts to 27.2(4) K, which is very roughly in accordance with previous reports.8 As stated in ref 8, the moment is distinctly increased compared to the expected moment of a S = 3/2 system. This has been attributed to spin−orbit coupling which can be observed for
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RESULTS AND DISCUSSION A large single crystal of several centimeters in length and about 5−6 mm in diameter was successfully grown by the traveling solvent floating zone technique; see Figure 2. Wavelength dispersive X-ray fluorescence (WDX) measurements indicate an essentially stoichiometric composition within the expected accuracy of these measurements, i.e., a Co:V-ratio r of 34.1(2)%/65.9(2)%. X-ray scattering intensities within the HK0, H0L, and 0KL planes of reciprocal space that have been obtained in single crystal X-ray diffraction measurements are shown in Figure 3. As can be seen, there are no additional twin domains or crystallites, etc. The structural refinement yields a reliably small goodness of fit (GoF) and R-values that amount to GoF = 1.80 and R = 1.85%, Rw = 4.98%. The atomic positions are listed in B
DOI: 10.1021/cg5015303 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Crystal Growth & Design
Figure 3. Precession map within the HK0, H0L, and 0KL planes of reciprocal space calculated from a single crystal X-ray diffraction measurement of our γ-CoV2O6 single crystal. No other crystallites or twin domains are visible in these single crystal X-ray diffraction measurements.
Figure 4. Rietveld refinement results of powder X-ray diffraction measurements on powders obtained from our crushed floating-zone grown single crystals.
Figure 5. Room-temperature rocking scans obtained during neutron measurements on our bulk centimeter-sized single crystals at the Morpheus and at the TriCS diffractometer (the instrumental resolution is roughly 0.4°).
high-spin Co(II) in octahedral coordination.8 Furthermore, our γ-CoV2O6 crystal exhibits very well pronounced magnetization
plateaus within isothermal magnetization curves at 2.5 K; see Figure 7. C
DOI: 10.1021/cg5015303 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Crystal Growth & Design
for flux-grown single crystals has been reported to be 6 K only.15 Hence, we were not only able to increase the obtainable crystal size enormously by the traveling solvent floating zone method such that our single crystal sizes have become suitable for neutron scattering experiments but we were also able to enhance the single crystal quality and the magnetic properties of our single crystals significantly. Interestingly, also the easyaxis observed in our crystals differs from that reported in ref 15. However, our results are in agreement with X-ray magnetic circular dichroism measurements.14
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CONCLUSION Despite the difficulties arising from the CoO-V2O5 phase diagram,16 we managed to grow centimeter-sized single crystals of γ-CoV2O6 by the traveling solvent floating zone technique. Our crystals exhibit a sharp magnetic transition at TN = 7.8 K and extremely well pronounced magnetization plateaus within magnetization curves at 2.5 K together with a saturation magnetization of 3.2 μB, which is larger than the expected spinonly value for a S = 3/2 system. These large single crystals are suitable for elastic and inelastic neutron scattering experiments which will be able to give deeper insight in the intriguing magnetic properties of this system.
Figure 6. A scan with an area detector at 4.6 K at the TriCS diffractometer is shown. Besides the nuclear Bragg reflections also the third-integer magnetic reflections are nicely visible. All these neutron measurements confirm the high single crystal quality of our sizable single crystals.
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ASSOCIATED CONTENT
* Supporting Information S
Crystallographic information file. This material is available free of charge via the Internet at http://pubs.acs.org.
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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 We thank the team of U. Burkhardt for WDX measurements and the team of H. Borrmann for powder X-ray diffraction measurements. We thank H. Borrmann and L. H. Tjeng for helpful discussions. This work is partially based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland.
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Figure 7. Magnetic susceptibility of γ-CoV2O6 as a function of temperature and magnetization measurements of γ-CoV2O6 in the bdirection and two perpendicular directions at low temperatures revealing an easy-axis direction parallel to b.
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DOI: 10.1021/cg5015303 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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