CRYSTAL GROWTH & DESIGN
Flux Growth of β-Cu2V2O7 Single Crystals in a Closed Crucible
2008 VOL. 8, NO. 7 2223–2226
Zhangzhen He* and Yutaka Ueda Institute for Solid State Physics, UniVersity of Tokyo, Kashiwa, Chiba 277-8581, Japan ReceiVed August 8, 2007; ReVised Manuscript ReceiVed March 9, 2008
ABSTRACT: Large single crystals of quasi-one-dimensional spin compound Cu2V2O7 with β-form are successfully grown at a slow cooling rate using the flux method in a closed crucible. The grown crystals are analyzed by X-ray diffraction (XRD), electron probe microanalysis (EPMA), and thermal analysis techniques. Magnetic behaviors of the grown crystals investigated by means of magnetic and heat capacity measurements are in good agreement with those of β-Cu2V2O7. Introduction Search for one-dimensional (1D) magnetic materials has been one of the most active fields in solid state chemistry and physics, which has brought various interesting magnetic behaviors. Novel spin-Peierls transition and Haldane spin-liquid ground-state are observed in 1D spin-1/2 system CuGeO31 and spin-1 system PbNi2V2O8,2 respectively, giving a keen issue in this respect. Compounds with a chain structure have attracted much attention and current interests on 1D magnetic materials are mainly focused on copper- and vanadium-based oxides containing Cu2+ ions (3d9) or V4+ ions (3d1). Many copper- or vanadium-oxides such as PbCu2(PO4)2,3 BaCuSi2O6,4 SrCu2O3,5 BaCu2V2O8,6 (VO)2P2O7.7 and NaV2O5,8 etc., are characterized to have a spin singlet ground-state with a finite spin gap, while BaCu2Si2O7,9 BaCu2Ge2O7,10 CuSiO3,11 and Sr2V3O912 are found to undergo a three-dimensional (3D) magnetic ordering at low-temperature. Such different ground states observed in copper- or vanadiumbased oxides provided an interesting issue to investigate the competition between spin quantum fluctuation and their interchain interactions in 1D spin systems. Cu2V2O7, a cupper-vanadium oxide composed of Cu2+ (3d9, S ) 1/2) ions and tetrahedral VO4 (3d0, S ) 0), is found to have a peculiar chain structure,13 which exhibits three phases of high-temperature forms (γ- and β-forms) and low-temperature form (R-form). γ-form13 crystallizes triclinic system of space group P-1¨ with a ) 5.0873(1) Å, b ) 5.8233(1) Å, c ) 9.4020(1) Å, R ) 99.780(3)°, β ) 97.253(3)°, and γ ) 97.202(3)°, and β-form14 crystallizes in monoclinic system of space group C2/c with a ) 7.685(5) Å, b ) 8.007(3) Å, c ) 10.29(2) Å, and β ) 110.27(5)°, whereas the R-form15 crystallizes in the orthorhombic system of space group Fdd2 with a ) 20.645(2) Å, b ) 8.383(7) Å, and c ) 6.442(1) Å. As shown in Figure 2, one of the most significant differences in their structural features is seen in the different orientations of chains built by Cu2+ ions:13 chains are parallel to [011] and [011j] in the R-form and to [110] and [1j10] in the β-form, whereas all chains in the γ-form are parallel to same direction of the a-axis. Structural phase transition between R- and β-forms of Cu2V2O7 is reported to occur at ∼712 °C.16 Also, it is noted that the R-form is the only stable phase at ambient conditions, whereas both the β- and γ-forms are metastable.13 Cu2V2O7 with rich structural features has attracted much attention and its magnetic behaviors have been studied,17–19 showing different magnetic properties between the R- and
Figure 1. Peculiar chain structure of Cu2V2O7 built by Cu2+ ions: (a) R-form, (b) β-form, and (c) γ-form.
Figure 2. Single crystals of β-Cu2V2O7 grown by the flux method in a special process.
β-forms of Cu2V2O7. Generally, R-Cu2V2O7 displays a canted antiferromagnetic ordering below the Neel temperature (TN) of ∼34 K,17–19 whereas β-Cu2V2O7 displays a Heisenberg antiferromagnetic ordering below TN ≈ 26 K18 or a dimer-like system with a singlet ground state.19 To elucidate the magnetic nature of β-Cu2V2O7, it is necessary to obtain a large-sized single crystal for magnetic measurements or even further studies on spin dynamics. In this study, we report that large sized β-Cu2V2O7 single crystals are successfully grown by the flux method in a special process. Also, magnetic behaviors of the grown crystals are investigated by means of magnetic and heat capacity measurements. Experimental Section
* To whom correspondence should be addressed. Present address: Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China. E-mail:
[email protected].
A mixture of high-purity reagents of CuO (4N, 11 g), V2O5 (4N, 24.7g), and SrCO3 (4N, 10 g) was ground carefully and homogenized
10.1021/cg7007478 CCC: $40.75 2008 American Chemical Society Published on Web 05/30/2008
2224 Crystal Growth & Design, Vol. 8, No. 7, 2008
He and Ueda
Figure 4. Typical EDS spectrum acquired from β-Cu2V2O7 single crystals coated with carbon.
Figure 3. X-ray diffraction patterns calculated on the basis of structural parameters and obtained from the crashed β-Cu2V2O7 single crystals.
thoroughly with ethanol (99%) in an agate mortar. The mixture was packed into an alumina crucible (Φ42 × 50 mm3) and the crucible was then capped with a cover using Al2O3 cement (C-989, Cotronics Corp.). Such a closed crucible was put into a homemade electric furnace with an adjustable temperature gradient and then the furnace was heated up to 950 °C and kept at 950 °C for 10 h to ensure that the solution melts completely and homogeneously. The furnace was cooled slowly to 750 °C at a rate of 0.5 °C/h while being kept at a constant temperature several times, and then cooled to room temperature at a rate of about 100 °C/h. With this procedure, some crystals with a size of 2 × 2 × 7 mm3 were obtained by mechanical separation from the crucible. The X-ray powder diffraction (XRD) data were collected at room temperature in the range 2θ ) 10-80° with a scan step width of 0.02° and a fixed counting time of 4 s using an MXP21AHF (Mac Science) powder diffractometer with graphite monochromatized CuKR radiation. Chemical analysis was performed using an electron probe microanalysis (EPMA) system (JEOL JSM-5600, Oxford Link ISIS). Thermal analysis was performed using a TG-DTA 2000s (Mac Science) apparatus in air at a heating/cooling rate of 10 °C/min. Magnetic susceptibility was measured using a superconducting quantum interference device (MPMS5S, Quantum Design) magnetometer and heat capacity was measured by a relaxation method using a commercial physical property measurement system (PPMS, Quantum Design).
Results and Discussion Figure 2 shows β-Cu2V2O7 single crystals grown by flux method in a special process. Figure 3 shows the XRD patterns calculated from structural parameters and obtained from the crushed grown crystals. It is found that all peaks in the observed XRD pattern can be well indexed in a monoclinic system and identified to diffraction peaks from β-Cu2V2O7. Also, the lattice constants of a ) 7.679(2) Å, b ) 8.031(2) Å, c ) 10.117(3) Å, and β ) 110.21(1)° determined by using a high-purity Si powder as an internal standard are close to those reported in ref 14. Figure 4 shows a typical EDS spectrum of the grown crystals. No other metal elements except for Cu and V were detected and the Cu:V:O molar ratio was calculated to be close to 2:2:7, agreeing with the formula of Cu2V2O7. The above results show that the grown crystals are β-Cu2V2O7 and have high quality. In addition, the orientations of the crystal surfaces were confirmed by X-ray scattering analysis. As seen in Figure 5,
Figure 5. X-ray scattering peaks for the cleaved facets of the grown crystals.
Figure 6. Thermal behaviors of β-Cu2V2O7 on heating and cooling regimes.
we note that the cleaved facets of the grown crystals are natural (100) planes. Figure 6 shows thermal behavior of the crushed β-Cu2V2O7 crystals. Four endothermic dips are clearly observed at ∼556, ∼595, ∼610, and ∼766 °C on the heating regime. We suggest that the dip at ∼766 °C is linked with the melting of β-Cu2V2O7 and other dips are associated with the complicated phase transitions between β-, γ-, and R-forms. Such phase transitions are also confirmed by four exothermic peaks on the cooling regime. These findings are in good agreement with the results reported previously.20 In our recent studies on the growth of Ni3V2O8, Co3V2O8, and β-Mn2V2O7 single crystals,21–24 we found that SrV2O6 is a considerable flux for growth of vanadate crystals because of its low melting points of ∼650 °C in the V2O5-SrO system.25 After testing different ratios of CuO, V2O5, and SrCO3, we selected the solution system with the ratio of Cu2V2O7: SrV2O6 ) 1:1 for growth of β-Cu2V2O7. As noted in ref. 21–23, many important points are suggested for this growth process: to allow slow spontaneous nucleation only at the surface of the melt and to avoid the formation of excess nucleation in the system, a
Flux Growth of β-Cu2V2O7 in a Closed Crucible
Figure 7. Magnetic susceptibility of β-Cu2V2O7 single crystals measured in an applied field of 1 T along the a-axis.
Crystal Growth & Design, Vol. 8, No. 7, 2008 2225
Figure 9. Heat capacity data of β-Cu2V2O7 single crystals measured in zero magnetic field.
previous studies on Zn-doped Cu2V2O7 polycrystalline samples.18 Recently, many interesting magnetic behaviors such as large paramagnetic anisotropy and field-induced spin-flop transition are also observed in single crystals of β-Cu2V2O7.26 Conclusions
Figure 8. Magnetization vs magnetic field curves at 5 K.
high vertical temperature gradient of 100 °C/cm and a very slow cooling rate of 0.5 °C/h are expected. Further, to prevent inclusions of the melt into the crystal because of overcooling of the melt, the furnace was kept at a constant temperature several times in the cooling process. Also, because β-Cu2V2O7 is a metastable phase at ambient conditions,13 it is necessary to keep a finite pressure inside the crucible at high temperature and to carefully avoid the evaporation of V2O5 at high temperature, resulting in an unsteady solution system during the growth. The alumina crucible was capped with a cover using Al2O3 cement to be the closed system. In addition, to prevent the occurrence of structural transition of R-β forms, the furnace was cooled rapidly from 750 to 600 °C in this process. Figure 7 shows the temperature dependence of magnetic susceptibilities from 5 to 300 K, which are measured in an applied field of 1 T along the crystallographic a-axis (to be perpendicular to the natural (100) facets of the grown crystals). The susceptibilities exhibit a broad peak at around 50 K, indicative of 1D short-range ordering. The susceptibility decreases with decreasing temperature, while a Curie-like upturn is seen below 26 K. Figure 8 shows magnetization (M) as a function of applied field (H) at 5 K. The linear behavior of magnetization is seen and no magnetization saturation is seen up to 5 T. Furthermore, hysteresis and remanent magnetization near H ) 0 are not observed. These magnetic features suggest that the ground-state of β-Cu2V2O7 is likely an antiferromagnetic (AF) ordering. Figure 9 shows heat capacity data of the grown crystals in zero magnetic field. A λ-like peak is seen at ∼26 K, indicating the onset of an antiferromagnetic ordering at ∼26 K, which is in good agreement with the magnetic data. These combined results of magnetic susceptibility, magnetization, and heat capacity provide concrete evidence that the grown crystals we obtained in a closed crucible are β-Cu2V2O7 single crystals with high purity, which also support an antiferromagnetic ordering at ∼26 K in β-Cu2V2O7 suggested by the
Large-sized β-Cu2V2O7 single crystals have been successfully grown by the flux method in a closed crucible. The grown crystals were confirmed to be β-Cu2V2O7 and have high quality based on XRD and EPMA measurements. The complicated phase transitions between β-, γ-, and R-forms were also confirmed by the thermal analysis. Magnetic susceptibility, magnetization, and heat capacity measurements showed that β-Cu2V2O7 is a 1D antiferromagnet with a Neel temperature of ∼26 K. We envisage that the first successful growth of large sized β-Cu2V2O7 single crystals will stimulate further structural and magnetic studies of other polymorphs of Cu2V2O7 as well. Acknowledgment. The authors thank Ms. Y. Kiuchi for her assistance with EPMA measurement and Dr. J. Yamaura for determination of the orientations of crystal surfaces. Z.H. acknowledges the Japan Society for the Promotion of Science (JSPS) for the awarding of the Foreigner Postdoctoral Fellowship (P06047).
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