Chem. Mater. 2008, 20, 5643–5651
5643
Unique Crystal Structure and Anomalous Magnetic Behavior of Quaternary U2ScB6C3 V. H. Tran,*,† P. Rogl,‡ T. Mori,§ H. Ripplinger,| and K. Schwarz W. Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław, Poland, Institute of Physical Chemistry, UniVersity of Vienna, Wa¨hringerstr. 42, A-1090 Wien, Austria, National Institute for Materials Science, Namiki 1-1, Tsukuba, 305-044, Japan, Institute of Physical and Theoretical Chemistry, Vienna UniVersity of Technology, Getreidemarkt 9/156, A-1060 Wien ReceiVed May 9, 2008. ReVised Manuscript ReceiVed June 15, 2008
The crystal structure of the quaternary uranium-based intermetallic U2ScB6C3 was derived from X-ray single-crystal counter data. The compound crystallizes in a unique structure type with the space group P6/mmm. The unit cell dimensions are as follows: a ) 0.65096(2) nm and c ) 0.34265(2) nm. Specific heat of the nonmagnetic phonon reference Th2ScB6C3 and measurements of the magnetic susceptibility, magnetization, specific heat, and electrical resistivity on polycrystalline sample U2ScB6C3 are reported. The experimental data revealed that Th2ScB6C3 possesses low-lying optical modes, whereas U2ScB6C3 with enhanced electronic specific-heat coefficient at low temperatures (γ ) 40 mJ/molU · K2) orders ferromagnetically below TC ) 61(0.5) K, followed by presumably a spin reorientation below 45 K. The investigated U2ScB6C3 compound is characterized by an easy-plane anisotropy, which is evidenced by the experimental data and is consistent with the two-ion anisotropic interaction theory. The temperature dependencies of magnetization, specific heat, and electrical resistivity of U2ScB6C3 were analyzed with help of the spin-wave theory and compared to the closely related β-UB2C compound. The magnetism of U2ScB6C3 is attributed to the polarized 5f3 states and a strong hybridization of the U 6d with B/C 2p states. Band structures using the FLAPW+LO approach were calculated for Th2ScB6C3 and U2ScB6C3. The theoretical data confirm the metallic properties for all studied compounds and magnetic ground state in U2ScB6C3.
Introduction Compounds with p-electron metalloids (M) and alkali and those with alkaline earth metal (A) form in a rich variety of crystal structures.1 Most of them belong to a class of socalled Zintl phases, and their electronic structures are rather well-determined based on an electron counting rule. However, when the A element is changed by a transition metal (T) or f electron metal (R, rare earth/An, actinoids), new characteristics of chemical bonding appear, and therefore the electronic structure of the compounds dramatically changes. In the ionic-like Zintl phase compounds, the physical properties are influenced by the covalent bonding of M ions, whereas in the T-M and R-M or An-M compounds, the hybridized metallic d/f-sp bonding plays a key role. As a consequence of the changing bonding characteristics, many new exotic phenomena, including Kondo effect (for instance, in CeAl22), spin fluctuation (UAl23), heavy-fermion behavior * Corresponding author. E-mail:
[email protected]. Tel.: (4871)3435021. Fax: (4871) 3441029. † Polish Academy of Sciences. ‡ University of Vienna. § National Institute for Materials Science. | Institute of Physical and Theoretical Chemistry, Vienna University of Technology.
(1) Chemistry, Structure and Bonding of Zintl Phases and Ions; Kauzlarich, S. M., Ed.; VCH Publishers: New York, 1996. (2) Buschow, K. H. J.; van Daal, H. J. Phys. ReV. Lett. 1969, 23, 408. (3) Trainor, R. J.; Brodsky, M. B.; Culbert, H. V. Phys. ReV. Lett. 1975, 34, 1019.
(CeAl34), and superconductivity (UGe25,6), occur. The appearance of superconductivity in the latter compound, and in two other uranium-based URhGe7 and UIr,8 is unexpected owing to the coexistence of ferromagnetism and unconventional superconductivity. A particularly interesting feature is the fact that in all cases the Curie temperature TC is always higher than the superconducting critical temperature TSC. However, there are only three known compounds with such behavior, so we are still far from understanding the nature of coexisting superconductivity with ferromagnetism. Naturally, any comparative study on itinerant electron ferromagnets is highly desired. In this context we have recently reported on fundamental physical properties of itinerant ferromagnetism in β-UB2C close to a superconducting transition.9,10 β-UB2C is a high-temperature modification adopting the rhombohedral ThB2C-type structure containing (4) Andres, K.; Graebner, J. E.; Ott, H. R. Phys. ReV. Lett. 1975, 35, 1779. (5) Saxena, S. S.; Agarwal, P.; Ahilan, K.; Grosche, F. M.; Haselwimmer, R. K. W.; Steiner, M. J.; Pugh, E.; Walker, I. R.; Julian, S. R.; Monthoux, P.; Lonzarich, G. G.; Huxley, A.; Sheikin, I.; Braithwaite, D.; Flouquet, J. Nature (London ) 2000, 406, 587. (6) Huxley, A.; Sheikin, I.; Ressouche, E.; Kernavanoi, N.; Braithwaite, D.; Calemczuk, R.; Flouquet, J. Phys. ReV. B 2001, 63, 144519. (7) Aoki, D.; Huxley, A.; Ressouche, E.; Braithwaite, D.; Flouquet, J.; Brison, J.-P.; Lhotel, E.; Paulsen, C. Nature (London) 2001, 413, 613. (8) Akazawa, T.; Hidaka, H.; Fujiwara, T.; Kobayashi, T. C.; Yamamoto, E.; Haga, Y.; Settai, R.; Onuki, Y. J. Phys.: Condens. Matter 2004, 16, L29. (9) Tran, V. H.; Rogl, P.; Andre´, G.; Boure´e, F. J. Phys.: Condens. Matter 2006, 18, 703.
10.1021/cm801267a CCC: $40.75 2008 American Chemical Society Published on Web 08/15/2008
5644
Chem. Mater., Vol. 20, No. 17, 2008
three puckered metal layers per unit cell sandwiched between planar layers of BC nonmetal layers (space group R3jm).11 Some time ago, the authors12,13 reported on hexagonal quaternary boron carbides, (Th,U)2ScB6C3, as an ordered variant of the rhombohedral ThB2C-type structure. The crystal structure of hexagonal Th2ScB6C3, which was derived from room-temperature X-ray powder diffractometry, was solved by combined Rietveld and Patterson methods,12 and was found to adopt a unique structure type with space group P6/mmm (unit cell dimensions: a ) 0.660296(7) nm, c ) 0.358421(4) nm). U2ScB6C3 is isotypic and preliminary investigations showed it to be ferromagnetic below TC ) 56 K.13 However, until now, no detailed structural investigations on single crystals were made and no detailed study on physical properties was hitherto pursued. In view of the interesting physical behavior of β-UB2C and with respect to its close structural relationship with U2ScB6C3, it deemed fruitful to investigate the fundamental properties of U2ScB6C3. Therefore, we present in this work details on an X-ray single-crystal study and on low-temperature physical properties obtained from measurements of the magnetization and specific heat of polycrystalline U2ScB6C3 samples. As we will show below, the substitution of one U atom by one Sc atom in UB2C results in a new, truly uranium quaternary compound, U2ScB6C3, with a unique crystal structure type. Regarding magnetic properties, we conclude that U2ScB6C3 exhibits anomalous behavior, with respect to two successive phase transitions. To better understand the close relationship between crystal structure, bonding, and magnetic properties, we have performed band structure calculations using a full potential linear augmented plane wave plus local orbital method for Th2ScB6C3 and U2ScB6C3. Analyzing the results of theoretical calculations, we give a consistent interpretation of the experimental data. Experimental Details Preparation. Polycrystalline samples of U2ScB6C3 and Th2ScB6C3 were prepared by argon arc-melting of the elements (99.9 mass %) on a water-cooled copper hearth. Prior to melting, the B,C-powder mixture was compacted in a steel die without the use of lubricants. Annealing was performed in a Knudsen-type graphite crucible for 150 h at 1200 °C in a tungsten sheet metal high-vacuum furnace at 10-4 Pa. The boron-carbide compound is slightly sensitive to moisturesthus, all handling was performed under argon or under protective water-free liquids (cutting of specimen under glycerine, etc.). Small single-crystal specimens of U2ScB6C3, suitable for single-crystal X-ray diffraction experiments, were isolated after mechanical fragmentation of the middle part of the polycrystalline melted ingots. X-ray and Electron Diffraction. X-ray examination of polycrystalline materials was performed at room temperature in a (10) Tran, V. H.; Khan, R. T.; Bauer, E.; Rogl, P. Physica B 2008, 403, 1375. (11) Rogl, P.; Fischer, P. J. Solid State Chem. 1989, 78, 294. (12) Rogl, P.; Mori, T.; Tanaka, T. Structure and Properties of Quaternary Th2ScB6C3. Presented at the 16th IUPAC Conference on Thermodynamics, concurrent with the 10th Symposion on Thermodynamics of Nuclear Materials, Dalhousie University, Halifax, Canada, August 611, 2000. (13) Mori, T.; Tanaka, T.; Rogl, P. Crystal Structure and Properties of Quaternary Actinoid Boron Carbides, Th2ScB6C3 and U2ScB6C3. Proceedings of the International Conference on Actinoids, Japan, 2002. Published in J. Nucl. Sci. Technol. 2002, Suppl. 3, 122.
Tran et al. Guinier-Huber X-ray camera with an Image Plate recording system (Cu KR radiation) employing an internal standard of 99.9999% pure Si (aSi ) 0.5431065 nm). Quantitative Rietveld refinements of X-ray powder data were performed employing the FULLPROF program.14 X-ray powder diffraction at room temperature indicates that the as-cast and annealed samples are single-phased. For single-crystal X-ray diffraction experiments, the single-crystal specimens of U2ScB6C3 were glued (covered by glue) freestanding to the tip of a glass rod and were inspected on a GADDS-D8 instrument, which also served for determination of the unit cell dimensions and quality control of the crystal specimens. Single-crystal X-ray intensity data for U2ScB6C3 were collected on a four-circle Nonius Kappa diffractometer equipped with a CCD area detector (graphite monochromated Mo KR radiation, λ ) 0.071073 nm). Orientation matrix and unit cell parameters for a hexagonal crystal system were derived using the program DENZO.15 Absorption correction was taken from the program SORTAV,15 and the structure was refined with aid of the SHELXS-97 program system.16 Crystal structure data are standardized using the program Structure Program Typix.17 Transition electron diffraction patterns were taken from a crushed splinter of Th2ScB6C3 in am Hitachi H-500 TEM operated at 100 kV. Physical Property Measurements. Magnetization of powdered specimens of U2ScB6C3 was measured using a SQUID magnetometer (Quantum Design) in fields up to µ0H ) 5 T and in the temperature range 1.7-400 K. Specific heat, Cp(T), measurements on polycrystalline samples of U2ScB6C3 and Th2ScB6C3 were performed in the temperature range 1.8-100 K, using a thermal relaxation method. The specific heat was measured using a Quantum Design Physical Property Measurement System. The specific heat data of Th2ScB6C3 were used to subtract the phonon background heat capacity of U2ScB6C3. Electrical resistivity was measured on a bar-shaped sample by applying a standard four-probe ac technique at temperatures down to 2 K. Electronic Band Structure Calculations. Electronic band structure calculations were performed for U2ScB6C3 and Th2ScB6C3, employing a full-potential linear augmented plane wave (FLAPW) approach based on the density functional theory using the WIEN2K program package.18 The FLAPW approach was used to solve the Kohn-Sham equations self-consistently and the generalized gradient approximation of Perdew et al.19 was used to describe the exchange-correlation energy. The atomic sphere radii of U(Th), Sc, B, and C were kept as 2.5(2.5), 2.0, 1.3, and 1.3 au.
Results and Discussion Structural Investigations and Crystal Chemistry. Although ternary compounds ThB2C and UB2C and the quaternary compounds (Th,U)2ScB6C3 melt congruently, the sections ThB2C - “ScB2C” and β-UB2C - “ScB2C” are not quasibinary due to the instability of the high-temperature (14) Roisnel, T.; Rodriguez-Carvajal, J. Mater.-Sci.-Forum 2001, 378, 118. (15) NoniusKappaCCD,ProgramPackageCOLLECT,DENZO,SCALEPACK, SORTAV; Nonius: Delft, The Netherlands. (16) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refinement; University of Go¨ttingen: Germany, 1997; Windows version by McArdle, Natl. Univ. Ireland, Galway. (17) Parthe´, E.; Gelato, L.; Chabot, B.; Penzo, M.; Cenzual, K.; Gladyshevskii, R. TYPIX - Standardized Data and Crystal Chemical Characterization of Inorganic Structure Types; Springer: Verlag, 1994. (18) Blaha, P.; Schwarz, K.; Madsen, G.; Kvasnicka, D.; Luitz, J. Program for calculating crystal properties, WIEN2k; Vienna University of Technology: Vienna, 2001; 3-9501031-1-2. (19) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865.
Crystal Structure and Magnetic Properties of U2ScB6C3
compound ScB2C,20 which decomposes at temperatures below 1700 °C into ScB2 and ScB2C2.21 Preliminary investigation of the solubility limit of Sc in β-UB2C revealed random replacement of about 15% of uranium by scandium atoms.22 Complete indexation of the single-crystal data set for U2ScB6C3 prompted a hexagonal unit cell with a lattice relation close to the structure type of ThB2C (isotypic β-UB2C): a(U2ScB6C3) ) a(β-UB2C) and c(U2ScB6C3) ) 1 /3c(β-UB2C). Systematic extinctions in powder and singlecrystal data prompt the possible space groups in high Laue symmetry: P6/mmm, P6j2m, P6jm2, P6mm, and P622, from which the highest symmetric one was chosen for further structure solution and refinement. Heavy uranium atoms were unambiguously located in the 2c sites of P6/mmm, leaving the 1a sites for occupation by scandium atoms. Knowledge of the structural chemistry of actinoid (An) boron carbides with the general formula AnB2C23 points toward a structure with alternating metal and nonmetal layers. For our model with the short c-axis only one nonmetal layer in z ) 0.5 is possible, for which the nonmetal layer of β-UB2C (ThB2C type11) seems to be most appropriate for U2ScB6C3. The electron density peaks revealed in the difference Fourier synthesis Fobs - FU,Sc are fully consistent with this structure model, which refined satisfactorily and finally converged at RF2 ) 0.033. No significant deviation from full atom occupancy was evident. Refinement, however, prompted a small random replacement of 0.948(4) Sc + 0.052 U1 atoms in the Sc site (1a), indicating a small homogeneity region (∼1 at. %) of the quaternary compound, U2+xSc1-xB6C3. Moreover, from the calculated anisotropic atomic displacement parameters for the U atoms, we see that vibrating amplitude in the ab plane is larger than that along the c axis because U11()0.0044 × 10-2 nm2) are larger than U33()0.0031 × 10-2 nm2). Concerning displacement parameters for B and C, one expects that these atoms with a large Uiso of 0.0052 × 10-2 nm2 should be vibrating more violently than U and Sc atoms do. Table 1 summarizes the X-ray single crystal data for U2ScB6C3. Rietveld refinement of the X-ray room temperature intensity pattern of Th2ScB6C3, reaching low residual values RF ) 0.032, RI ) 0.058 for a set of 45 independent reflections (xB ) 0.2637(1)), proved isotypism with the crystal structure of U2ScB6C3. TEM data for Th2ScB6C3 confirm the setting of the unit cell. The electron diffraction patterns of the a*-b* and a*-c* planes in Figure 1 reveal no indication of the existence of superstructures. The Rietveld data for Th2ScB6C3 and U2ScB6C3 are available on request. The structure type of U2ScB6C3 (see Figure 2) is one of the layer-type boron carbides YB2C, ThB2C, and β-UB2C type:23 planar 36-Kagome´ metal layers (U2Sc) sandwiched between planar nonmetal Kagome´ networks 6B(6B + 3C)2 in z ) 1/2. Each boron atom in the structure (20) Bauer, J. J. Less Common Met. 1982, 87, 45. (21) Shi, Y.; Leithe-Jasper, A.; Tanaka, T. J. Solid State Chem. 1999, 148, 250. (22) Rogl, P.; Rupp, B.; Felner, I.; Fischer, P. J. Solid State Chem. 1993, 104, 337. (23) Rogl, P. Ternary Metal Boron Carbon Systems; Effenberg, G., Ed.; ASM International: Materials Park, OH, 1998; pp 1-525.
Chem. Mater., Vol. 20, No. 17, 2008 5645 Table 1. Single-Crystal Data for U2+xSc1-xB6C3; Space Group P6/mmm; No. 191; Origin at Center; Anisotropic Atomic Displacement Parameters Uij in [102 nm2]; Bij ) 8π2Uij formula from refinement
U2.05Sc0.95B6C3
data collection; radiation
Nonius Kappa CCD; Mo KR redundancy > 7
crystal size density (Mg/m3) a [nm]; c [nm]
28 × 28 × 42 µm3 7.48 0.65096(2); 0.34265(3)
data collection, 2Θ range
2 < 2Θ < 72.19°; 90 s/frame
total number of frames reflections in refinement mosaicity number of variables RF2 ) Σ|F02 - Fc2|/ΣF02 RI GOF extinction (Zachariasen)
389 for 8 sets; scan width ) 2° 153 Fo >4σ(Fo) of 153
