Crystal Growth and Magnetic Properties of BaCo 2 V 2 O 8 Zhangzhen He, Desheng Fu, To ˆru Kyo ˆmen, Tomoyasu Taniyama, and Mitsuru Itoh* Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan ReceiVed April 9, 2005 Single crystals of BaCo 2 V 2 O 8 were grown in a furnace with a high vertical temperature gradient using spontaneous nucleation method at a slow cooling rate. The crystals exhibit a characteristic morphology with growth lines, crystal veins, and the original nucleus at the center. The quality and good single crystallinity of the crystal are confirmed using X-ray powder diffraction and Laue technique. Magnetic and heat capacity measurements show that this crystal exhibits a transition from one-dimensional magnetic behavior into three-dimensional antiferromagnetic ordering at ∼5 K. An exotic magnetic anisotropy is also observed. Introduction One-dimensional (1D) quantum spin chain systems have been actively studied in condensed matter physics since the discovery of their fascinating magnetic properties. In par- ticular, compounds with a linear chain structure have attracted much attention in experimental and theoretical views: PbNi 2 V 2 O 8 is a typical example of spin-1 1D haldane antifer- romagnet systems 1 and BaCu 2 V 2 O 8 is a spin-1/2 alternating chain system with a large spin gap of about 230 K. 2 Major interest in 1D spin chain systems, however, is currently focusing on spin-1/2 or spin-1 systems, while a number of spin-3/2 compounds including CsVCl 3 , 3,4 BaCo 2 Si 2 O 7 , 5,6 and CoNb 2 O 6 7 also demonstrate interesting magnetic properties. In this study, we first report on the synthesis and mag- netic properties of BaCo 2 V 2 O 8 single crystal, which be- longs to 1D spin chain system with spin-3/2. The crystal structure of BaCo 2 V 2 O 8 has tetragonal symmetry of space group I41/acd with a ) 12.444(1) Å, c ) 8.415(3) Å, and Z ) 8, 8 similar to PbNi 2 V 2 O 8 and BaCu 2 V 2 O 8 . As shown in Figure 1, all magnetic Co 2+ ions are equivalent with arrays of edge-shared CoO 6 octahedra forming screw chains along the c-axis and the screw chains separated by nonmagnetic VO 4 (V 5+ ) tetrahedra and Ba 2+ ions resulting in a quasi-1D structural arrangement. Experimental Section A polycrystalline sample was synthesized by a standard solid- state reaction method using high purity reagents of BaCO 3 (4 N), CoC 2 O 4 ‚2H 2 O (3 N), and V 2 O 5 (4 N) as the starting components in the molar ratio 1:2:1. The reagents were weighed separately and mixed with ethanol, then ground carefully and thoroughly homog- enized in an agate mortar. The mixtures were packed into an alumina crucible and calcined at 930 °C in air for 60 h with several intermediate grindings. Finally, the product was pressed into pellets and sintered at 950 °C for 20 h. The polycrystalline sample was checked by X-ray powder diffraction (XRD) and confirmed to be single phase. Crystal growth of BaCo 2 V 2 O 8 was carried out in a homemade electric furnace with adjustable vertical temperature gradient. The polycrystalline sample (150 g) was melted in an alumina crucible at 1050 °C and kept at 1050 °C for 10 h. The furnace was slowly cooled to 950 °C at a rate of 1 °Ch -1 and then cooled to room temperature at a rate of 100 °Ch -1 . With this procedure, dark green crystals were obtained by mechanical separation from the crucible. The XRD data of crystals were collected at room temperature in the angular range of 2θ ) 10-60° with a scan step width of 0.02° and a fixed counting time of 4 s using a MXP18AHF (Mac Science) powder diffractometer with a graphite monochromatized Cu KR radiation. The dc magnetic susceptibility was measured with the applied field of 1 T from 300 to 2 K and the heat capacity was measured by a relaxation method. These measurements were performed in a Quantum Design physical property measurement system (PPMS). * To whom correspondence should be addressed. E-mail: Mitsuru_Itoh@ msl.titech.ac.jp. (1) Uchiyama, Y.; Sasago, Y.; Tsukada, I.; Uchinokura, K.; Zheludev, A.; Hayashi, T.; Miura, N.; Bo ¨ni, P. Phys. ReV. Lett. 1999, 83, 632. (2) He, Z.; Kyomen, T.; Itoh, M. Phys. ReV. B 2004, 69, 220407. (3) Itoh, S.; Endoh, Y.; Kakurai, K.; Tanaka, H. Phys. ReV. Lett. 1995, 74, 2375. (4) Itoh, S. et al. Phys. ReV. B 1999, 59, 14406. (5) Adams, R. D.; Layland, R.; Datta, T.; Payen, C. Polyhedron 1993, 12, 2075. (6) Adams, R. D.; Layland, R.; Payen, C.; Datta, T. Inorg. Chem. 1996, 35, 3492. (7) Kobayashi, S.; Mitsuda, S.; Ishikawa, M.; Miyatani, K.; Kohn, K. Phys. ReV. B 1999, 60, 3331. (8) Wichmann, R.; Mu ¨ller-Buschbaum, Hk. Z. Anorg. Allg. Chem. 1986, 532, 153. Figure 1. Crystal structure of BaCo2V2O8. Octaheadra, tetraheadra, large ball, and small ball represent CoO6, VO4, Ba, and O, respectively. 2924 Chem. Mater. 2005, 17, 2924-2926 10.1021/cm050760e CCC: $30.25 © 2005 American Chemical Society Published on Web 05/07/2005