CRYSTAL GROWTH & DESIGN
Flux Growth and Magnetic Anomalies of Co3V2O8 Crystals
2007 VOL. 7, NO. 6 1055-1057
Zhangzhen He,*,†,‡ Tomoyasu Taniyama,† Mitsuru Itoh,† and Yutaka Ueda‡ Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan, and Institute for Solid State Physics, UniVersity of Tokyo, Kashiwa 277-8581, Japan ReceiVed August 11, 2006; ReVised Manuscript ReceiVed February 5, 2007
ABSTRACT: Large single crystals of the novel vanadate Co3V2O8 are grown at a slow cooling rate using the flux method. X-ray diffraction and chemical analyses show that the grown crystals have high quality. Magnetic susceptibility and heat capacity measurements confirm that Co3V2O8 exhibits unusual magnetic behaviors, where five magnetic phase transitions occur below 15 K along the magnetic easy a-axis. Introduction Geometrically frustrated magnetic systems have been actively studied in condensed matter physics due to the discovery of their fascinating magnetic phenomena. In general, geometrical frustration arises as the spatial arrangement of the spins in a specific lattice such as an equilateral triangle and a tetrahedron cannot simultaneously minimize all the interaction energies. Major research efforts involving geometrically frustrated magnetic systems are currently focusing on compounds with Kagome (two-dimensional) or pyrochlore (three-dimensional) structures, showing exotic ground states such as spin glasses, spin liquids, and spin ices.1 Compounds with general formula of M3V2O8 (M ) Mg, Zn, Cu, Ni, and Co) are found to have orthorhombic structures with new variants of a Kagome lattice.2-4 All M2+ ions are arranged as the arrays of edge-shared MO6 octahedra forming Kagome-like layers, and the layers are separated by VO4 tetrahedra, resulting in a peculiar Kagomestaircase geometry. A detailed structural description can be found elsewhere.3,4 Because of their unique structural features, Mg3V2O8 and Zn3V2O8 are found to be excellent photocatalysts for O2 evolution,2 while Ni3V2O8 are found to exhibit various interesting magnetic behaviors such as field-induced magnetic transitions,5 a complicated magnetic phase diagram,6 and magnetically driven ferroelectric order,7 which are different from those observed in conventional Kagome compounds and which have attracted much experimental and theoretical attention. Co3V2O8, a member of M3V2O8, is composed of magnetic Co2+ ions (S ) 3/2) and nonmagnetic V5+ ions and is isostructural to Ni3V2O8. Recently, magnetic and dielectric properties of Co3V2O8 were also studied,8-11 which indicate a significant difference between Co3V2O8 and Ni3V2O8. Ferroelectric transition seen in Ni3V2O8 does not occur in Co3V2O8, although these compounds have similar crystal structure and magnetic phases. Co3V2O8 crystals have been reported to be obtained from the melt12 and by the floating zone technique.8,13 However, the phase diagrams of the CoO-V2O5 system clearly indicate that Co3V2O8 melts incongruently at ∼1000 °C.14 Recently, Co3V2O8 crystals have also been grown using K2OV2O5 as the flux.9 In fact, however, no description for detailed growth of Co3V2O8 single crystals could be seen elsewhere. In this paper, we report flux growth of large Co3V2O8 single crystals and further investigate magnetic behaviors of the grown * To whom correspondence should be addressed. † Tokyo Institute of Technology. ‡ University of Tokyo.
crystals in terms of ac magnetic susceptibility and heat capacity measurements. Experimental Section Raw material of Co3V2O8 for crystal growth was prepared by a standard solid-state reaction method using high-purity reagents CoC2O4‚ 2H2O (3 N) and V2O5 (4 N) in a molar ratio of 3:1. The reagents were weighed separately and mixed with ethanol (99%), then ground carefully, and homogenized thoroughly in an agate mortar. The mixture was packed into an alumina crucible and calcined at 900 °C in air for 40 h, followed by cooling to room temperature; the regrinding and reheating procedures were repeated several times. The product was checked using X-ray powder diffraction (XRD) and confirmed to be single phase. Crystal growth of Co3V2O8 was carried out in a homemade electric furnace with an adjustable temperature gradient. Raw material Co3V2O8 (∼101.5 g) and a mixture of V2O5 (22.7 g), SrCO3 (27.6 g), and BaCO3 (36.9 g) were melted in an alumina crucible at 1000 °C and kept at 1000 °C for 10 h to ensure that the solution melts completely and homogeneously. The furnace was cooled slowly to 700 °C at a rate of 0.5 °C h-1 while keeping at a constant temperature several times, and then it was cooled to room temperature at a rate of about 100 °C h-1. With this procedure, dark blue crystals with a size of 3 × 3 × 1 mm3 were obtained by mechanical separation from the crucible. The XRD data were collected at room temperature in the range 2θ ) 10°- 60° with a scan step width of 0.02° and a fixed counting time of 4 s using an MXP18AHF (Mac Science) powder diffractometer with graphite monochromatized Cu KR radiation. Chemical analysis was performed using an electron probe microanalysis (EPMA) system (JEOL JSM-5600•Oxford Link ISIS). The orientations of the crystal surfaces were confirmed using X-ray Laue backscattering analysis. ac magnetic susceptibility was measured with an amplitude of 10 Oe at different frequencies, and heat capacity was measured in terms of a relaxation method using a commercial Physical Property Measurement System (PPMS, Quantum Design).
Results and Discussion Figure 1 shows Co3V2O8 crystals grown by the flux method and their platelike morphology. The cleaved crystal facets are found to be a natural (010) plane using the Laue X-ray backreflection technique. Figure 2 shows a typical XRD pattern of crushed Co3V2O8 crystals. It is found that all peaks can be indexed with the orthorhombic structure of Co3V2O8 (ref: ICSD Code 2645), and no impurity phases were detected. Figure 3 shows a typical EDS spectrum of Co3V2O8. No another metal elements except for Co and V were confirmed, and the molar ratio of Co/V/O was calculated to be 3:2:8, in agreement with the formula of Co3V2O8. These results clearly show that the grown crystals are Co3V2O8 and have high quality. Like growth of Ni3V2O8 single crystals,15 since Co3V2O8 melts incongruently at ∼1000 °C,14 the flux growth of Co3V2O8
10.1021/cg060539u CCC: $37.00 © 2007 American Chemical Society Published on Web 04/07/2007
1056 Crystal Growth & Design, Vol. 7, No. 6, 2007
He et al.
Figure 1. Single crystals of Co3V2O8 grown by the flux method.
Figure 4. (a) ac magnetic susceptibility measured in an excitation field of 10 Oe at a frequency of 1000 Hz. (b) The frequency dependence of ac magnetic susceptibilities along the a-axis.
Figure 2. Powder X-ray diffraction pattern of Co3V2O8 crystal: the bottom ticks denote the Bragg reflection positions.
Figure 3. A typical EDS spectrum acquired from Co3V2O8 crystals coated with carbon.
single crystals is considerably selected. We used V2O5, SrCO3, and BaCO3 as the flux separately; however, large Co3V2O8 single crystals with high quality could not be obtained: other phases such as Co2V2O7 were appeared. A steady phase of SrV2O6 and BaV2O6 with their melting points of
