CRYSTAL GROWTH & DESIGN
Morphologies of Ni3V2O8 Single Crystals Zhangzhen He,* Jun-Ichi Yamaura, and Yutaka Ueda Institute for Solid State Physics, UniVersity of Tokyo, Kashiwa 277-8581, Japan ReceiVed December 3, 2007; ReVised Manuscript ReceiVed January 29, 2008
2008 VOL. 8, NO. 3 799–801
ABSTRACT: In this paper, we report the morphologies of Ni3V2O8 single crystals grown by the flux method in closed and opened crucibles using SrCO3 and V2O5 as flux. Under a high vertical temperature gradient, the morphology of the grown crystals in a closed crucible is found to be pillar-like, which is different from the plate-like feature for crystals grown in an opened crucible. On the basis of the theoretical morphology of Ni3V2O8, the possible reasons for different morphologies in the crucibles are suggested. Ni3V2O8, which is composed of magnetic Ni2+ and nonmagnetic V5+ ions, has now attracted much attention in condensed matter physics. Ni3V2O8 was found to crystallize in the orthorhombic system of space group Cmca (No. 64) with lattice constants a ) 5.936(4) Å, b ) 11.420(6) Å, and c ) 8.240(5) Å.1 As shown in Figure 1, all Ni2+ ions are the arrays of edge-shared NiO6 octahedra forming Kagome-like layers, and the layers are separated by VO4 tetrahedra, resulting in a peculiar Kagome-staircase geometry.2 Ni3V2O8 has been studied using various experimental measurements, showing interesting magnetic and dielectric behaviors related to its unique structural feature.2–9 On the other hand, Ni3V2O8 single crystals have been obtained by the flux method using KVO3 or BaO/V2O5 as the flux1,4 and floating zone technique.3 In our previous study, we found that Ni3V2O8 likely displays an incongruent melt feature, and large-sized single crystals with high quality can be grown by the flux method using a mixture of SrCO3 and V2O5 as flux.10 Recently we have established a closed system for flux growth of many vanadium oxides.11,12 To gain more useful information for growth behaviors of Ni3V2O8 single crystals, here we report the morphologies of Ni3V2O8 single crystals grown in closed and opened crucibles using the same flux conditions and growth procedures. Under a high vertical temperature gradient, the morphology of the grown crystals in a closed crucible exhibits a pillar-like feature, being different from a plate-like morphology in an opened crucible. The most possible reasons for such changes in morphologies are suggested. Raw material of Ni3V2O8 was prepared by a standard solid-state reaction method. High purity reagents of NiC2O4 · 2H2O (3 N) and V2O5 (4 N) in a molar ratio of 3:1 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, and regrinding and reheating procedures several times. The product was checked using X-ray powder diffraction (XRD) and confirmed to be single phase. Crystal growth of Ni3V2O8 was carried out in a homemade electric furnace with a vertical temperature gradient of 100 °C/cm. The mixture of raw material Ni3V2O8 and 50 mol% V2O5/SrCO3 with a ratio of 2:3 was filled into two same alumina crucibles (Φ42 × 50 mm3) and then one of crucibles was capped with a cover using Al2O3 cement (C-989, Cotronics Corp.). The crucibles were put into the furnace, and then the furnace was heated up to 1000 °C and kept at 1000 °C for 10 h to ensure that the solution melts completely and homogeneously. The furnace was cooled slowly from 1000 to 800 °C at a rate of 0.5 °C h-1, which was kept at a constant temperature of 950 °C, 900 °C, 850 °C, and 800 °C for 1 h and then cooled down to room temperature at a rate of about 100 °C * Author to whom correspondence should be addressed. E-mail:
[email protected]. Tel: +81-4-7136-3436. Fax: +81-4-7136-3436.
Figure 1. Projection of the structure of Ni3V2O8 onto (a) the b-c plane and (b) the a-c plane. A theoretical morphology of Ni3V2O8 established on the basis of its structure is seen.
h-1. With this procedure, yellow crystals were obtained by mechanical separation from the crucible. Figure 2 shows Ni3V2O8 single crystals grown in opened and closed crucibles. Almost all of grown crystals in the opened crucible
10.1021/cg701186k CCC: $40.75 2008 American Chemical Society Published on Web 02/15/2008
800 Crystal Growth & Design, Vol. 8, No. 3, 2008
Communications
Figure 4. An enlarged view of a piece of crystal grown in a closed crucible.
Figure 2. Single crystals of Ni3V2O8 grown under a high vertical temperature gradient in (a) the opened and (b) closed crucibles.
Figure 5. An image of growth manner from solution under a high vertical temperature gradient in a closed crucible.
Figure 3. The cleaved facets of the grown crystals in an open crucible using X-ray scattering analysis.
(Figure 2a) are found to exhibit a plate-like morphology, which is similar to those obtained previously.10 The cleaved crystal facets are found to be a natural (020) plane using the X-ray diffraction technique as shown in Figure 3. On the other hand, the grown crystals in the closed crucible are found to exhibit a pillar-like morphology (Figure 2b). The qualities of the grown crystals were all checked by XRD. We note that XRD patterns obtained from crushed crystals in the closed and opened crucibles are similar, and all peaks can be indexed with the orthorhombic structure of Ni3V2O8 (ref: ICSD Code 2646). The results show that the grown crystals in the closed or opened crucibles are Ni3V2O8. Figure 4 shows an enlarged view of a piece of crystal grown in the closed crucible. A pillar-like morphology with natural facets is clearly seen. These facets are confirmed to be (020), (002), and (021) using a Bruker SMART three-circle diffractometer equipped with a CCD area detector, showing that the long direction of crystal is the a-axis.
Figure 5 shows an image of growth behavior from solution in the closed crucible. A vertical growth manner along the a-axis is clearly seen. Apart from whether the crucibles are capped with a cover using Al2O3 cement or not, we note that there are no differences in their growth conditions such as electric furnace, starting materials, flux, and heating/cooling process. However, a remarkable difference in the morphology of the grown crystals is observed: a pillar-like morphology in the closed crucible and a plate-like one in the opened crucible. To understand such changes in the morphology, the growth was similarly carried out in a furnace with a reduced vertical temperature gradient of ∼10 °C/cm. As shown in Figure 6, an irregular column-like feature is seen for almost all of crystals grown in the opened crucible, while a pillar-like morphology is seen in the closed one. However, it is noted that large-sized crystals cannot be grown due to the formation of excess spontaneous nucleation in a low vertical temperature gradient. In addition, a theoretical morphology of Ni3V2O8 as shown in Figure 1b is established using the Materials Studio Modeling program13 with its structural parameters, according to the Bravais-Friedel and Donnay-Harker (BFDH) methods.14,15 We found that the column-like morphology in the opened crucible (Figure 6) is quite similar to the theoretical one, and the pillar-like morphology as seen in Figure 4 is imaged to be the theoretical one elongated along the a-axis. Therefore, a plate-like morphology of Ni3V2O8 single crystals in the opened crucible may be considered to arise from the horizontal growth behavior on the melt surface due to a high vertical temperature gradient of 100 °C/cm. Such feature is also seen in
Communications
Crystal Growth & Design, Vol. 8, No. 3, 2008 801 In summary, Ni3V2O8 single crystals have been grown in opened and closed crucibles using a mixture of SrCO3 and V2O5 as flux under same growth conditions. However, a plate-like morphology of the grown crystals was observed under a high vertical temperature gradient in an opened crucible, while a pillar-like morphology was observed in a closed one. We found that the morphology of Ni3V2O8 grown in a closed crucible is quite similar to the theoretical one elongated along the a-axis. We suggest that such a change in morphology may originate from a decrease in vertical temperature gradient and the addition of a spontaneous pressure inside the closed crucible.
Acknowledgment. The authors thank Dr. D. Fu for establishing the theoretical morphology of Ni3V2O8. One of the authors (Z.H.) acknowledges the Japan Society for the Promotion of Science (JSPS) for awarding the Foreigner Postdoctoral Fellowship (P06047).
References
Figure 6. Ni3V2O8 crystals grown under a low vertical temperature gradient in (a) opened and (b) closed crucibles.
Co3V2O8 single crystals.16 The most possible reasons for such different morphology observed under a high vertical temperature gradient in the closed crucible are suggested as follows: the prevention of evaporation such as V2O5 or CO2 and the decrease in thermal diffusion from the surface of the solution in the closed crucible may decrease the vertical temperature gradient in the solution, resulting in a more profitable vertical growth manner as seen in Figure 5. Further, the addition of a spontaneous pressure inside the closed crucible at high temperature may disturb growth speeds of crystallographic axes, resulting in a change from plateto pillar-like morphology.
(1) Sauerbrei, E. E.; Faggiani, R.; Calvo, C Acta Crystallogr. 1973, B 29, 2304. (2) Rogado, N.; Lawes, G.; Huse, D. A.; Ramirez, A. P.; Cava, R. J. Solid State Commun. 2002, 124, 229. (3) Balakrishnan, G.; Petrenko, O. A.; Lees, M. R.; Paul, D. M. K. J. Phys.: Condens. Matter 2004, 16, L347. (4) Lawes, G.; Kenzelmann, M.; Rogado, N.; Kim, K. H.; Jorge, G. A.; Cava, R. J.; Aharony, A.; Entin-Wohlman, O.; Harris, A. B.; Yildirim, T.; Huang, Q. Z.; Park, S.; Broholm, C.; Ramirez, A. P. Phys. ReV. Lett. 2004, 93, 247201. (5) Lawes, G.; Harris, A. B.; Kimura, T.; Rogado, N.; Cava, R. J.; Aharony, A.; Entin-Wohlman, O.; Yildirim, T.; Kenzelmann, M.; Broholm, C.; Ramirez, A. P. Phys. ReV. Lett. 2005, 95, 087205. (6) Kenzelmann, M.; Harris, A. B.; Aharony, A.; Entin-Wohlman, O.; Yildirim, T.; Huang, Q.; Park, S.; Lawes, G.; Broholm, C.; Rogado, N.; Cava, R. J.; Kim, K. H.; Jorge, G.; Ramirez, A. P. Phys. ReV. B. 2006, 74, 014429. (7) Chaudhury, R. P.; Yen, F.; dela Cruz, C. R.; Lorenz, B.; Wang, Y. Q.; Sun, Y. Y.; Chu, C. W. Phys. ReV. B. 2007, 75, 012407. (8) Lancaster, T.; Blundll, S. J.; Baker, P. J.; Prabhakaran, D.; Hayes, W.; Pratt, F. L. Phys. ReV. B. 2007, 75, 064427. (9) Wilson, N. R.; Petrenko, O. A.; Balakrishnan, G. J. Phys.: Condens. Matter 2007, 19, 145257. (10) He, Z.; Ueda, Y.; Itoh, M. J. Cryst. Growth 2006, 297, 1. (11) He, Z.; Ueda, Y. J. Cryst. Growth 2008, 310, 171. (12) He, Z.; Ueda, Y. Unpublished. (13) Accelrys, MS Modeling Getting Started; Accelrys Software Inc.: San Diego, 2004. (14) Bravais, A. Etudes Crystallographiques; Academie des Sciences: Paris, 1913. (15) Donnay, J. D. H.; Harker, D. Am. Mineral. 1937, 22, 463. (16) He, Z.; Taniyama, T.; Itoh, M.; Ueda, Y. Cryst. Growth Des. 2007, 7, 1055.
CG701186K
