Journal of Crystal Growth 313 (2010) 47–50
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CaB6 single crystals grown under high pressure and high temperature Shengwei Xin, Xianyue Han, Shaocun Liu, Zhongyuan Liu, Bo Xu, Yongjun Tian, Dongli Yu n State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, Hebei 066004, People’s Republic of China
a r t i c l e in fo
Article history: Received 14 July 2010 Accepted 27 September 2010 Communicated by A.G. Ostrogorsky Available online 1 October 2010
Millimeter-sized single crystals of CaB6 are obtained by the reaction of B and Ca under 1 GPa and 950–1150 1C. The structure, chemical composition, and morphologies of the crystals are characterized by X-ray diffraction and ﬁeld emission scanning electron microscopy with energy dispersion spectrometry. Investigation of the growth mechanism of CaB6 single crystals indicates that the growth temperature affects the shape of the crystals. Electric resistivity dependence tests of temperatures from 2 to 300 K show that CaB6 single crystals are conductive materials with semi-metallic behaviors. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. High pressure A1. High temperature A1. Resistivity A1. X-ray diffraction A2. Single crystals growth B1. CaB6
1. Introduction Calcium hexaboride is extensively utilized as a neutronabsorbing material in nuclear reactors, a deoxidation agent in the production of oxygen-free copper, and as additive for refractory materials because of its excellent properties, which include a high melting point, high strength, and high chemical stability. Recently, an unexpected ferromagnetism in Ca1 xLaxB6 (x¼0.005), found by Young et al. , created a signiﬁcant interest on the origin of magnetism in CaB6 doped with La. Even though no magnetic elements were present in the CaB6 crystals, weak ferromagnetism was observed at remarkably high temperatures (Tc ¼600 K). Experiments and theoretical calculations show that the observed ferromagnetism is a result of impurities on the surface of the crystals, rather than an intrinsic property [2,3]. However, the electronic conductive nature of CaB6 is still debated. This intrinsic semi-conducting character was suggested by the analysis results of NMR , the so-called GW approximation , angle-resolved photoemission spectroscopy (ARPES) , and thermopower measurements . In band structure calculation studies , resistivity measurements [9,10], de Haas-van Alphen signals (dHvA) [9,11], and optical spectra, CaB6 crystals present semi-metal properties [2,12,13]. The preparation of singlecrystalline CaB6 has also attracted a signiﬁcant amount of attention. Otani  and Otani and Mori  synthesized large single crystals of CaB6 using the aluminum ﬂux and ﬂoating zone methods. In the aluminum ﬂux method, the synthesized CaB6 and
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Ca1 xLaxB6 crystals were found to contain iron impurities ( r0.001 wt% Fe), which come from the aluminum ﬂux. Considering the ﬂoating zone method, large CaB6 crystals have to be prepared at high growth rates because of the violent evaporation of Ca. The CaB6 crystal rods obtained were composed of about ten crystal grains and contained an impurity of 0.004 wt% 1C. Currently, there exist very few studies that discuss the use of pure elements to synthesize CaB6 single crystals. In the process of synthesizing CaB6 single crystals using pure Ca and B, an unavoidable difﬁculty is the violent evaporation of Ca. The high-pressure technique can solve the problem of Ca evaporation because it allows the extension of the temperature range for CaB6 crystal growth. In this study, we prepare single crystals of CaB6 by the direct reaction of metal Ca with B powders under high pressure and high temperature (HP–HT). The conductive behavior and the growth mechanism of the CaB6 single crystals are discussed.
2. Experimental procedure In a glove box ﬁlled with argon, Ca pellets of 99.98% purity and B powders of 99.99% purity were stacked together and pressed into rods with diameters of 8 mm and lengths of 10 mm. The rods were wrapped with Ta foil and inserted into a pure h-BN crucible. The crucible was mounted in a graphite furnace and compressed in a large volume cubic anvil press under 1 GPa. The temperature was raised from room temperature to 950, 1050, and 1150 1C for 0.5 h. The samples were kept for 3 h and then gradually cooled down to room temperature for 0.5 h. The recovered synthetic products were immersed in dilute hydrochloric acid to remove
S. Xin et al. / Journal of Crystal Growth 313 (2010) 47–50
residual Ca and oxides. The remaining B in the products was separated from the CaB6 crystals using a heavy liquid with a density of 2.40 g/cm3, which was between the densities of B (2.34 g/cm3) and CaB6 (2.45 g/cm3). The phase identiﬁcation of the product was carried out using a D8 X-ray powder diffractometer (D8 DISCOVER, Bruker AXS) with Cu Ka radiation and a microprobe to obtain X-ray information. A continuous mode was adopted with increments of 0.021, scan speed of 0.2 1/min, and scan type of locked–coupled. The morphology and composition of the CaB6 crystals were characterized by a HITACHI S-4800 ﬁeld emission scanning electron microscope (FESEM) equipped with an energy dispersive spectrometer (EDS). The electrical resistivity of the CaB6 single crystals was measured in the range of 2–300 K by the physical property measurement system (PPMS).
3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns along the reﬂection of the (1 0 0) face of the CaB6 single crystal placed on a sample stage of a single crystal silicon (inset). The strongest diffraction peak at 43.6371 corresponds to the (2 0 0) face. Other equivalent periodic peaks are the reﬂections of the (1 0 0), (3 0 0), and (4 0 0) crystal faces. The lattice constant obtained from the diffraction ˚ which is consistent with the result reported data is a¼4.159 A, previously (JCPDS card no. 31-0254, space group: Pm-3m).
Fig. 1. XRD pattern of a single CaB6 crystal. Inset shows single crystal placed on Si sample stage.
Fig. 2(a) shows the FESEM images of CaB6 single crystals obtained under high temperature and pressure. The clear growth steps were observed at the top of crystal (A position), indicating that the crystals were formed by means of step growth along the crystal axis direction. Analysis of the EDS spectrum reveals that the single crystal is composed of B and Ca with a ratio of B/ Ca¼6.21(70.05). The morphologies of the product can be seen from Fig. 2(b). The crystals grew on a layer of sintered B. Most of the CaB6 crystals have smooth and ﬂat surfaces. FESEM photographs of CaB6 single crystals obtained under 1 GPa and temperatures of 950, 1050, and 1150 1C are presented in Fig. 3. These CaB6 crystals have three typical morphologies: rods, cubes, and plate-like rectangular blocks. The average sizes are about 1.0 0.2 0.2, 0.3 0.3 0.3, and 0.6 0.4 0.1 mm3 for the rod-like, cubic, and plate-like crystals, respectively. Generally, cubic-structured CaB6, the shape of single crystals should be cubic. However, the results of the current study indicate that the shape of CaB6 single crystals obtained under high pressure depends on its growth temperature. The CaB6 crystals were formed through the liquid (Ca)–solid (B) state reaction  and can be expressed as Ca(l)+B(s)¼CaB6(s). When the synthesis temperature reaches the melting point of Ca, Ca melts and forms a molten pool. B is dissolved into the molten Ca and reacts with it to produce CaB6 crystals. The CaB6 crystals grow along the directions of three crystal axes. Since abundant B atoms are required to form a B framework structure in the hexaboride, the ability of B dissolution and diffusion in the molten Ca is a crucially important affective factor for the formation of CaB6. Moreover, the growth of crystals is also affected by the temperature and pressure gradients, which lead to unequal growth rates in three-dimensional directions. In the case of a growth temperature of 950 1C, the concentration of B in the molten Ca was relatively low. The limited B concentration can only maintain crystal growth in one dimension. As a result, we only obtained CaB6 crystal rods. When the temperature was increased to 1050 1C, the solubility and concentration of B dissolved in the molten Ca were enhanced. The nucleation ratio of the crystals gradually increased. When the quantity of CaB6 crystal nuclei increased, the quantity of the dissolved B became insufﬁcient for rapid crystal growth. Under this experimental condition, we obtained smaller-sized CaB6 crystal cubes. When the temperature was increased to 1150 1C, the concentration and diffusion ability of B in the molten Ca were high enough for crystal growth. In the beginning of the growth procedure, most of the CaB6 nuclei grew rapidly into two dimensions and formed small plate-like crystals because of the existence of temperature and pressure gradients in the synthesis cavity. For these small plate-like crystals, growth speed at the edges was bigger than that on the plate. Hence, under higher growth temperatures (1150 1C) and in an abundant B source, these small plate-like crystals grew into plate-like rectangular blocks.
Fig. 2. (a) FESEM photograph of single CaB6 crystals obtained, where A position is growing step of the crystal and (b) FESEM photograph of CaB6 minicrystals grown on sintered B.
S. Xin et al. / Journal of Crystal Growth 313 (2010) 47–50
Fig. 4. Resistivity of CaB6 and FZ CaB6 (reported by T. Terashima et al.) crystals as a function of temperature. The inset shows a single CaB6 crystal measured by the conventional four-probe method along the length-wise direction.
25 K, our results are similar to those of FZ CaB6 crystals reported by Terashima. In the range of 2–25 K, the resistivity of the CaB6 single crystals obtained shows a declining trend with the decrease in temperature. This is different from the slight increase in resistivity found for FZ CaB6. This discrepancy may have been caused by the Kondo effect or weak localization carriers.
Fig. 3. FESEM photographs of CaB6 single crystals obtained under 1 GPa and temperatures of (a) 950 1C, (b) 1050 1C, and (c) 1150 1C.
Millimeter-sized high quality CaB6 single crystals were successfully synthesized under high pressure and temperature using pure B powders and Ca particles. The crystal parameter of ˚ The the CaB6 crystals with a space group of Pm 3m is a¼4.159 A. morphology of single crystals was observed by FESEM. Three kinds of shapes were observed: rods, plate-like rectangular blocks, and cubes. EDS analysis revealed that the single crystal is composed of B and Ca with a ratio of B/Ca¼6.21(70.05). Resistivity measurements showed that the CaB6 single crystals obtained have a typical semi-metal electron conductive behavior. From the growth mechanism analysis of the CaB6 single crystals, we believe that B dissolution and diffusion in the molten Ca is a crucial factor that affects the shape of CaB6 single crystals formed under high pressure and temperature. Compared to the ﬂux and ﬂoating zone methods, the HP–HT method can accomplish the synthesis of CaB6 crystals through only the direct reaction of pure B and Ca. The advantages of this method include the ability to obtain cleaner crystals. This method may also be used for synthesizing other crystals of alkaline-earth metal borides.
Acknowledgements Fig. 4 presents the relationship between the temperature and the resistivities of CaB6 single crystals and FZ CaB6 crystals as reported by Terashima et al. . The measurement was accomplished using the conventional four-probe method along one axis of a CaB6 single crystal measuring 0.15 mm in length, as shown in the inset of Fig. 4. In the ﬁgure, resistivity of CaB6 single crystals drops monotonically from 300 to 2 K. The residual resistivity and room temperature resistivity are r(2 K)¼ 1.36 mO cm and r(300 K)¼2.27 mO cm, respectively. Compared to the metallic conductor, the residual resistance ratio of r(300 K)/ r(2 K)¼1.71 is very low, indicating that the CaB6 single crystals obtained have a semi-metallic conducting behavior. From 300 to
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