- Email: [email protected]
Crystal structure and magnetism of UOsAl
Journal of Magnetism and Magnetic Materials 428 (2017) 144–147
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Crystal structure and magnetism of UOsAl a,⁎
MARK
A.V. Andreev , S. Daniš , J. Šebek , M.S. Henriques , J. Vejpravová , D.I. Gorbunov , L. Havelab a b c
b
a
a
a
a,c
Institute of Physics, Academy of Sciences, Na Slovance 2, 182 21 Prague, Czech Republic Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic Dresden High Magnetic Field Laboratory (HLD-EMFL), Helmholtz-Zentrum, Dresden-Rossendorf, D-01314 Dresden, Germany
A R T I C L E I N F O
A BS T RAC T
Keywords: Uranium intermetallics Laves phases Pauli paramagnetism
Crystal structure, magnetization, and specific heat were studied on single crystal of uranium intermetallic compound UOsAl. It is a hexagonal Laves phase of MgZn2 type, space group P63/mmc, with lattice parameters a=536.4 pm, c=845.3 pm. Shortest inter-uranium distance 313 pm (along the c-axis) is considerably smaller than the Hill limit (340 pm). The compound is a weakly temperature-dependent paramagnet with magnetic susceptibility of ≈1.5*10−8 m3 mol−1 (at T=2 K), which is slightly higher with magnetic field along the a-axis compared to the c-axis. The Sommerfeld coefficient of electronic specific heat has moderate value of γ=36 mJ mol−1 K−2.
1. Introduction
magnetic susceptibility [9], whereas information about UOsAl is practically absent. Our aim was to specify if a compound with the 1:1:1 stoichiometry exists and, if so, in what structure it crystallizes and what its magnetic properties are. The existence of UOsAl was reported in [10] as a hexagonal Laves phase of the MgNi2 type. However, the calculated density, dcalc=2.31 g/cm3, was reported to be very small. It should be much higher because all UTX with T lighter than Os have density exceeding 10 g/cm3. In the present work we confirmed that UOsAl indeed exists. We determined its crystal structure, basic magnetic properties and the temperature dependence of specific heat and compared UOsAl with other isostructural and isoelectronic uranium intermetallics.
Ternary equiatomic uranium intermetallic compounds UTX (T is a late transition metal of 3d, 4d or 5d series, X is a 3- or 4-valent pmetal) form a large family of actinide materials actively studied over last decades (see for review [1]). Compounds of this family exhibit diverse magnetic properties varying from temperature-independent paramagnetism to strong ferromagnetism or antiferromagnetism. There are several isostructural groups among UTX, with the most prominent types being of the hexagonal ZrNiAl and orthorhombic TiNiSi crystal structures. Systematic study of magnetic and other electronic properties within such isostructural groups made it possible to understand influence of ligands on state of 5f electrons, which dominate the magnetic properties. In particular, rather exotic electronic systems were discovered in URhGe and UCoGe (coexistence of ferromagnetism and superconductivity [2–4]) or UCoAl (itinerant metamagnetism with very low critical field [5–7]). Very interesting change of magnetism, non-monotonously developing with composition, were found in many solid solutions between isostructural compounds [1]. Almost all compounds in the UTX series with X=Al (for T=Fe, Co, Ni, Ru, Rh, Ir and Pt) crystallize in the hexagonal ZrNiAl-type structure (ternary variant of the Fe2P type), space group P-62m [8]. All of them were studied in detail. Only UPdAl and UOsAl do not form such structure. It is known that UPdAl has the orthorhombic TiNiSi structure and is a paramagnet with weakly temperature-dependent
⁎
2. Experimental The UOsAl alloy was prepared from metals with 99.9% (U), 99.99% (Os) and 99.999% (Al) purity. Since the melting temperature of Os, 3033 °C, is higher than the boiling point of Al, 2470 °C, it is difficult to prepare a homogenous alloy with a proper concentration of both metals directly from pure elements. For this reason we made a binary precursor UOs which has the melting point 1800 °C. The use of precursor allowed us to avoid uncontrolled evaporation of Al during preparation of the UOsAl alloy. An ingot was pulled from the melt in a tri-arc furnace using the Czochralski method. Phase purity and lattice parameters were checked by standard X-ray diffractometry on powders prepared from the pulled ingot. The powder X-ray diffraction patterns
Corresponding author. E-mail address: [email protected] (A.V. Andreev).
http://dx.doi.org/10.1016/j.jmmm.2016.12.007 Received 30 August 2016; Received in revised form 14 November 2016; Accepted 3 December 2016 Available online 05 December 2016 0304-8853/ © 2016 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 428 (2017) 144–147
A.V. Andreev et al.
were refined by means of Rietveld analysis using the Fullprof/Winplotr software. X-ray back-scattered Laue patterns were used to check the single-crystalline state of the ingot. The ingot appeared to be polycrystalline, nevertheless, we succeeded in extracting small (~20 mg) single crystals. The single crystal of UOsAl was polished and studied using a scanning electron microscope (SEM) Superprobe 733 equipped with energy dispersive X-ray spectrometer (EDS) EDAX. Quantitative chemical analysis was carried out on the crystal with the electron beam operating at 20 kV. Single crystal X-ray diffraction was undertaken in order to unambiguously solve the crystalline structure of UOsAl. A single crystal with dimensions 0.03×1.3×0.2 mm3 was glued on the top of a glass fiber and mounted onto the goniometer head. The diffracted intensities were collected at ambient temperature using a four-circle diffractometer Gemini of Agilent, equipped with a Mo lamp (λ (MoKα) =0.71073 Å), graphite monochromator and a CCD detector Atlas. The software CrysAlis [11] was used to collect and reduce the data, and to perform the absorption correction (face indexing of the crystal shape in combination with a Gaussian correction based on spherical harmonic functions). The structure solution was found by the program Superflip [12], and Jana2006 package [13] performed the structure refinements. Steady-field magnetization was measured in the temperature range 2–300 K in fields up to 14 T applied along the principal crystallographic axes using the PPMS-14 cryomagnetic installation. The magnetization in pulsed magnetic fields (pulse duration 20 ms) up to 59 T was measured at T=2 K at the Dresden High Magnetic Field Laboratory. The high-field magnetometer used for the measurements is described in detail in Ref. [14]. Absolute values of the magnetization were calibrated using data obtained in static magnetic fields. The temperature dependence of specific heat was measured by relaxation method using PPMS-14.
Fig. 1. Back-scattered Laue patterns of the UOsAl single crystal along the a (top) and c (bottom) axes. Table 1 Crystal data and details of UOsAl structural determination. Crystal system Structure type Space group Lattice constants (pm) a, b C Unit-cell volume V (nm3) Formula units/cell Formula weight Calculated density (g/cm3) Absorption coefficient (cm−1) Range in hkl Total/Unique/Observed reflections Rint No. of variables Goodness-of-fit (on F2) Final residuals Largest diff. peak/hole (e/Å3)
3. Results and discussion Powder diffraction analysis of the ingot showed that UOsAl has a hexagonal Laves phase, but not of the MgNi2 type, as reported in [10]. The diffraction pattern is well described by crystal structure of the MgZn2 type (space group P63/mmc) with traces of an unidentified impurity phase. On the other hand, analysis of single crystals (extracted from the center of ingot) showed that they are single-phase. Thus, the traces of second phase in the ingot could be attributed to intergrain boundaries or surface layer. Therefore, detailed study of the crystal structure was performed on a single crystal. Its chemical composition was determined as U 32.29 at%, Os 34.61 at% and Al 33.10 at%, i.e., in good agreement with the 1:1:1 stoichiometry. Back-scattered Laue patterns along the principal axes of the single crystal show its high quality (Fig. 1). Details on the data collection and refinement procedure, lattice parameters and final refinement factors are given in Table 1. The atomic coordinates and equivalent isotropic displacement parameters are listed in Table 2. The structure has 4 formula units per elementary cell. The calculated density, dcalc=14.36 g/cm3, is in a good agreement with the value measured directly (dobs=14.28 g/cm3). Our conclusion about the MgZn2-type structure of UOsAl is supported by the fact that the same structure was observed in the related compound with excess of Al on account of Os, UOs0.5Al1.5 [15]. The same crystal structure was also observed for a high-temperature modification of UFeAl [6,8]. The crystal structure of UOsAl is shown in Fig. 2. U atoms occupy the 4f sites, whereas Os and Al share randomly the 2a and 6h sites. The U atom is elevated from the basal plane by z=0.0647, so the interuranium distance dU-U along the c-axis is (0.5–2z)c=313 pm. The second distance (direction is almost in the basal plane) is longer, 336 pm. Therefore, unlike the UTX compounds with the ZrNiAl structure, the shortest dU-U is along the c-axis, not in the basal plane. Since shortest dU-U is substantially below the Hill limit for uranium (340 pm [16]), no magnetic order might be expected in UOsAl.
Hexagonal MgZn2 P63/mmc (no. 194) 536.41(4) 845.27(7) 0.21063(2) 4 455.21 14.36 136.98 −7→7, −7→7, −11→10 2829/135/127 0.14 8 1.86 R=0.052; Rw=0.104 4.34/−4.08
Table 2 Atomic coordinates and thermal parameters of UOsAl. All atomic sites are fully occupied. Atom
Site
x
y
z
Ueq (Å2)
U1 M1 M2
4f 6h 2a
1/3 0.17082 0
2/3 0.34165 0
0.56467 1/4 0
0.0137 (7) 0.0119 (4) 0.0129(5)
M1=0.54 Os+0.46 Al M2=0.49 Os+0.51 Al
However, there exist at least two uranium intermetallic compounds with the same MgNi2-type structure, which exhibit magnetic ordering despite such short dU-U, UNi2 [1,17,18] and U2Fe3Ge [19,20]. UNi2 with dU-U=306 pm is a ferromagnet with the Curie temperature TC=23 K with very low spontaneous magnetic moment Ms=0.05– 0.08 μB per formula unit (f.u.). U2Fe3Ge has even shorter dU-U, only 285 pm, but it is also ferromagnet with TC=55 K and Ms=1.0 μB/f.u. (i.e., 0.5 μB per U atom). In both compounds the 3d metal (Ni or Fe) is
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Journal of Magnetism and Magnetic Materials 428 (2017) 144–147
A.V. Andreev et al.
Fig. 2. Crystal structure of UOsAl (with coordinates of U atoms).
Fig. 4. Temperature dependence of magnetic susceptibility of a UOsAl single crystal measured along the main axes in 7 T. The inset shows similar results for URuAl [1].
Fig. 3. Magnetization curves of a UOsAl single crystal measured along the main axes at 2 K. The inset shows similar results for URuAl [1]. Dashed lines are guides for eye.
non-magnetic. Therefore, it is interesting to check whether UOsAl is another exception from the Hill rule. Fig. 3 shows the field dependence of magnetization at T=2 K in field applied along the principal axes of the UOsAl crystal. Both curves are linear up to highest field 59 T. Neither a spontaneous magnetic moment nor a field-induced anomaly (found in URuAl, compound with 4d-electron analog of Os) is observed. In URuAl, whose magnetization curves are presented in the inset of Fig. 3, a broad S-shape anomaly interpreted as suppression of spin fluctuations is seen in the magnetization curve along the direction with higher susceptibility (it is the c-axis in URuAl having the ZrNiAl-type structure). The temperature dependence of magnetic susceptibility of UOsAl, compared again with URuAl (inset), is presented in Fig. 4. UOsAl exhibiting almost temperature-independent response without any anomalies, is qualitatively different from URuAl, where a broad maximum is observed along the c-axis around 50 K. The susceptibility value in UOsAl, ≈1.5*10−8 m3 mol−1, can be associated with the Pauli susceptibility, reflecting a moderately enhanced density of states at the Fermi level, N(EF), in the case of broad 5f-band materials. It also corresponds to magnitude of a temperature-independent contribution into susceptibility in those U intermetallics, which follow the CurieWeiss behavior otherwise [1]. The magnetic susceptibility of UOsAl is almost isotropic. Nevertheless, a slightly lower value for magnetic field along the c-axis is in agreement with the shorter dU-U in this direction.
Fig. 5. Temperature dependence of the specific heat of UOsAl plotted as Cp/T vs T. The inset shows Cp/T vs T2 at low temperatures.
Both isostructural ferromagnets, UNi2 and U2Fe3Ge, also exhibit the caxis as the hard magnetization direction. The paramagnetic state of UOsAl without any trace of magnetic ordering is confirmed also by the specific-heat measurements (Fig. 5). The C/T vs. T2 dependence shows a weak upturn below about 10 K (inset in Fig. 5). It yields a moderate value of the Sommerfeld coefficient, γ=36 mJ mol−1 K−2. The Debye temperature estimated from the linear slope of C/T vs. T2 can be evaluated as ΘD=200 K. This value is comparable to ΘD found for the isostructural compounds UNi2 (250 K [18]) and U2Fe3Ge (213 K [19]). The Sommerfeld coefficient of UOsAl is inferior to 65 mJ mol−1 K−2 in UNi2 and 88 mJ mol−1 K−2 (i.e., 44 mJ mol−1 K−2 per U atom) in U2Fe3Ge. This reflects a lower density of states at the Fermi level in UOsAl and indicates that this compound is a broad-band weakly correlated material. It explains also the weak magnetic behavior of UOsAl. 4. Conclusions The UTAl compound with T=Os of the 5d series, despite its different crystal structure, has paramagnetic ground state as the 146
Journal of Magnetism and Magnetic Materials 428 (2017) 144–147
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[2] D. Aoki, A. Huxley, E. Ressouche, D. Braithwaite, J. Flouquet, J.-P. Brison, E. Lhotel, C. Paulsen, Nature 413 (2001) 613. [3] N.T. Huy, A. Gasparini, D.E. de Nijs, Y. Huang, J.C.P. Klaasse, T. Gortenmulder, A. de Visser, A. Hamann, T. Görlach, H.V. Löhneysen, Phys. Rev. Lett. 99 (2007) 067006. [4] J. Vejpravová-Poltierová, J. Pospíšil, J. Prokleška, K. Prokeš, A. Stunault, V. Sechovský, Phys. Rev. B 82 (2010) 180517(R). [5] A.V. Andreev, R.Z. Levitin, Y.F. Popov, R.Y. Yumaguzhin, Sov. Phys. Solid State 27 (1985) 1145. [6] V. Sechovsky, L. Havela, F.R. de Boer, J.J.M. Franse, P.A. Veenhuizen, J. Sebek, J. Stehno, A.V. Andreev, Physica B 142 (1986) 283. [7] A.V. Andreev, M.I. Bartashevich, T. Goto, K. Kamishima, L. Havela, V. Sechovský, Phys. Rev. B 55 (1997) 5847. [8] A.V. Andreyev, M.I. Bartashevich, Phys. Met. Metallogr. 62 (2) (1986) 50. [9] V.H. Tran, R. Troć, Int. J. Mod. Phys. B 7 (1993) 850. [10] G. Petzow, S. Steeb, Proceeding of 3rd Int. Conf. on Peaceful Uses of Atomic Energy, 11, 1965, p. 169 [11] CrysAlis PRO, Agilent Technologies, Version 1.171.37.31 [12] L. Palatinus, G. Chapuis, J. Appl. Cryst. 40 (2007) 786. [13] V. Petříček, M. Dušek, L. Palatinus, Z. Krist. 229 (2014) 345. [14] Y. Skourski, M.D. Kuz’min, K.P. Skokov, A.V. Andreev, J. Wosnitza, Phys. Rev. B 83 (2011) 214420. [15] G. Petzow, S. Steeb, Z. Met. 55 (1964) 453. [16] H.H. Hill, W.N. Miner (Ed.)Plutonium and Other Actinides, AIME, New York, 1970, p. 2. [17] V. Sechovsky, L. Havela, J. Hrebik, A. Zentko, P. Diko, J. Sternberk, Physica B 130 (1985) 243. [18] C. Schnitzer, E. Gmelin, G. Hilscher, Physica B 130 (1985) 237. [19] M.S. Henriques, D.I. Gorbunov, J.C. Waerenborgh, L. Havela, A.B. Shick, M. Diviš, A.V. Andreev, A.P. Gonçalves, J. Phys.: Condens. Matter 25 (2013) 066010. [20] M.S. Henriques, D.I. Gorbunov, A.V. Andreev, Z. Arnold, S. Surblé, S. Heathman, J.-C. Griveau, E.B. Lopes, J. Prchal, L. Havela, A.P. Gonçalves, Phys. Rev. B 89 (2014) 054407. [21] A.V. Andreev, K. Shirasaki, J. Šebek, J. Vejpravová, D.I. Gorbunov, L. Havela, S. Daniš, T. Yamamura, J. Alloy. Compd. 681 (2016) 275.
compounds with T=Fe and Ru, i.e. 3d and 4d analogues of Os. Hightemperature modification of UFeAl, isostructural with UOsAl, is paramagnetic as well. This general feature can be associated with the 5f-d hybridization, which is stronger, due to a larger overlap of the d and 5f states in the energy scale, than for the T metals from the very end of a given transition metal series. The overlap may be enhanced for the 5d states by stronger spin-orbit interaction. The splitting of the 5d3/ 5 and 5d5/2 states contributes to the total 5d band width. It is, however, interesting to mention that a few-percent substitution of Os for Co induces a ferromagnetic state in the band metamagnet UCoAl [21]. So, Os in such situation behaves again very similar to Fe or Ru. Band-structure calculations based on these experimental data on crystal structure and magnetic properties are planned to be performed in collaboration with theoreticians. Acknowledgement This work was supported by the Czech Science Foundation (project 16-03593S). The work was partly performed in MLTL (http://mltl.eu/ ), which is supported within the program of Czech Research Infrastructures (Project No. LM2011025). We acknowledge the support of HLD at HZDR, member of the European Magnetic Field Laboratory (EMFL). References [1] V. Sechovsky, L. Havela, K.H.J. Buschow (Ed.)Handbook of Magnetic Materials 11, Elsevier, Amsterdam, 1998, p. 1.
147
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Crystal structure and magnetism of UOsAl a,⁎
MARK
A.V. Andreev , S. Daniš , J. Šebek , M.S. Henriques , J. Vejpravová , D.I. Gorbunov , L. Havelab a b c
b
a
a
a
a,c
Institute of Physics, Academy of Sciences, Na Slovance 2, 182 21 Prague, Czech Republic Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic Dresden High Magnetic Field Laboratory (HLD-EMFL), Helmholtz-Zentrum, Dresden-Rossendorf, D-01314 Dresden, Germany
A R T I C L E I N F O
A BS T RAC T
Keywords: Uranium intermetallics Laves phases Pauli paramagnetism
Crystal structure, magnetization, and specific heat were studied on single crystal of uranium intermetallic compound UOsAl. It is a hexagonal Laves phase of MgZn2 type, space group P63/mmc, with lattice parameters a=536.4 pm, c=845.3 pm. Shortest inter-uranium distance 313 pm (along the c-axis) is considerably smaller than the Hill limit (340 pm). The compound is a weakly temperature-dependent paramagnet with magnetic susceptibility of ≈1.5*10−8 m3 mol−1 (at T=2 K), which is slightly higher with magnetic field along the a-axis compared to the c-axis. The Sommerfeld coefficient of electronic specific heat has moderate value of γ=36 mJ mol−1 K−2.
1. Introduction
magnetic susceptibility [9], whereas information about UOsAl is practically absent. Our aim was to specify if a compound with the 1:1:1 stoichiometry exists and, if so, in what structure it crystallizes and what its magnetic properties are. The existence of UOsAl was reported in [10] as a hexagonal Laves phase of the MgNi2 type. However, the calculated density, dcalc=2.31 g/cm3, was reported to be very small. It should be much higher because all UTX with T lighter than Os have density exceeding 10 g/cm3. In the present work we confirmed that UOsAl indeed exists. We determined its crystal structure, basic magnetic properties and the temperature dependence of specific heat and compared UOsAl with other isostructural and isoelectronic uranium intermetallics.
Ternary equiatomic uranium intermetallic compounds UTX (T is a late transition metal of 3d, 4d or 5d series, X is a 3- or 4-valent pmetal) form a large family of actinide materials actively studied over last decades (see for review [1]). Compounds of this family exhibit diverse magnetic properties varying from temperature-independent paramagnetism to strong ferromagnetism or antiferromagnetism. There are several isostructural groups among UTX, with the most prominent types being of the hexagonal ZrNiAl and orthorhombic TiNiSi crystal structures. Systematic study of magnetic and other electronic properties within such isostructural groups made it possible to understand influence of ligands on state of 5f electrons, which dominate the magnetic properties. In particular, rather exotic electronic systems were discovered in URhGe and UCoGe (coexistence of ferromagnetism and superconductivity [2–4]) or UCoAl (itinerant metamagnetism with very low critical field [5–7]). Very interesting change of magnetism, non-monotonously developing with composition, were found in many solid solutions between isostructural compounds [1]. Almost all compounds in the UTX series with X=Al (for T=Fe, Co, Ni, Ru, Rh, Ir and Pt) crystallize in the hexagonal ZrNiAl-type structure (ternary variant of the Fe2P type), space group P-62m [8]. All of them were studied in detail. Only UPdAl and UOsAl do not form such structure. It is known that UPdAl has the orthorhombic TiNiSi structure and is a paramagnet with weakly temperature-dependent
⁎
2. Experimental The UOsAl alloy was prepared from metals with 99.9% (U), 99.99% (Os) and 99.999% (Al) purity. Since the melting temperature of Os, 3033 °C, is higher than the boiling point of Al, 2470 °C, it is difficult to prepare a homogenous alloy with a proper concentration of both metals directly from pure elements. For this reason we made a binary precursor UOs which has the melting point 1800 °C. The use of precursor allowed us to avoid uncontrolled evaporation of Al during preparation of the UOsAl alloy. An ingot was pulled from the melt in a tri-arc furnace using the Czochralski method. Phase purity and lattice parameters were checked by standard X-ray diffractometry on powders prepared from the pulled ingot. The powder X-ray diffraction patterns
Corresponding author. E-mail address: [email protected] (A.V. Andreev).
http://dx.doi.org/10.1016/j.jmmm.2016.12.007 Received 30 August 2016; Received in revised form 14 November 2016; Accepted 3 December 2016 Available online 05 December 2016 0304-8853/ © 2016 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 428 (2017) 144–147
A.V. Andreev et al.
were refined by means of Rietveld analysis using the Fullprof/Winplotr software. X-ray back-scattered Laue patterns were used to check the single-crystalline state of the ingot. The ingot appeared to be polycrystalline, nevertheless, we succeeded in extracting small (~20 mg) single crystals. The single crystal of UOsAl was polished and studied using a scanning electron microscope (SEM) Superprobe 733 equipped with energy dispersive X-ray spectrometer (EDS) EDAX. Quantitative chemical analysis was carried out on the crystal with the electron beam operating at 20 kV. Single crystal X-ray diffraction was undertaken in order to unambiguously solve the crystalline structure of UOsAl. A single crystal with dimensions 0.03×1.3×0.2 mm3 was glued on the top of a glass fiber and mounted onto the goniometer head. The diffracted intensities were collected at ambient temperature using a four-circle diffractometer Gemini of Agilent, equipped with a Mo lamp (λ (MoKα) =0.71073 Å), graphite monochromator and a CCD detector Atlas. The software CrysAlis [11] was used to collect and reduce the data, and to perform the absorption correction (face indexing of the crystal shape in combination with a Gaussian correction based on spherical harmonic functions). The structure solution was found by the program Superflip [12], and Jana2006 package [13] performed the structure refinements. Steady-field magnetization was measured in the temperature range 2–300 K in fields up to 14 T applied along the principal crystallographic axes using the PPMS-14 cryomagnetic installation. The magnetization in pulsed magnetic fields (pulse duration 20 ms) up to 59 T was measured at T=2 K at the Dresden High Magnetic Field Laboratory. The high-field magnetometer used for the measurements is described in detail in Ref. [14]. Absolute values of the magnetization were calibrated using data obtained in static magnetic fields. The temperature dependence of specific heat was measured by relaxation method using PPMS-14.
Fig. 1. Back-scattered Laue patterns of the UOsAl single crystal along the a (top) and c (bottom) axes. Table 1 Crystal data and details of UOsAl structural determination. Crystal system Structure type Space group Lattice constants (pm) a, b C Unit-cell volume V (nm3) Formula units/cell Formula weight Calculated density (g/cm3) Absorption coefficient (cm−1) Range in hkl Total/Unique/Observed reflections Rint No. of variables Goodness-of-fit (on F2) Final residuals Largest diff. peak/hole (e/Å3)
3. Results and discussion Powder diffraction analysis of the ingot showed that UOsAl has a hexagonal Laves phase, but not of the MgNi2 type, as reported in [10]. The diffraction pattern is well described by crystal structure of the MgZn2 type (space group P63/mmc) with traces of an unidentified impurity phase. On the other hand, analysis of single crystals (extracted from the center of ingot) showed that they are single-phase. Thus, the traces of second phase in the ingot could be attributed to intergrain boundaries or surface layer. Therefore, detailed study of the crystal structure was performed on a single crystal. Its chemical composition was determined as U 32.29 at%, Os 34.61 at% and Al 33.10 at%, i.e., in good agreement with the 1:1:1 stoichiometry. Back-scattered Laue patterns along the principal axes of the single crystal show its high quality (Fig. 1). Details on the data collection and refinement procedure, lattice parameters and final refinement factors are given in Table 1. The atomic coordinates and equivalent isotropic displacement parameters are listed in Table 2. The structure has 4 formula units per elementary cell. The calculated density, dcalc=14.36 g/cm3, is in a good agreement with the value measured directly (dobs=14.28 g/cm3). Our conclusion about the MgZn2-type structure of UOsAl is supported by the fact that the same structure was observed in the related compound with excess of Al on account of Os, UOs0.5Al1.5 [15]. The same crystal structure was also observed for a high-temperature modification of UFeAl [6,8]. The crystal structure of UOsAl is shown in Fig. 2. U atoms occupy the 4f sites, whereas Os and Al share randomly the 2a and 6h sites. The U atom is elevated from the basal plane by z=0.0647, so the interuranium distance dU-U along the c-axis is (0.5–2z)c=313 pm. The second distance (direction is almost in the basal plane) is longer, 336 pm. Therefore, unlike the UTX compounds with the ZrNiAl structure, the shortest dU-U is along the c-axis, not in the basal plane. Since shortest dU-U is substantially below the Hill limit for uranium (340 pm [16]), no magnetic order might be expected in UOsAl.
Hexagonal MgZn2 P63/mmc (no. 194) 536.41(4) 845.27(7) 0.21063(2) 4 455.21 14.36 136.98 −7→7, −7→7, −11→10 2829/135/127 0.14 8 1.86 R=0.052; Rw=0.104 4.34/−4.08
Table 2 Atomic coordinates and thermal parameters of UOsAl. All atomic sites are fully occupied. Atom
Site
x
y
z
Ueq (Å2)
U1 M1 M2
4f 6h 2a
1/3 0.17082 0
2/3 0.34165 0
0.56467 1/4 0
0.0137 (7) 0.0119 (4) 0.0129(5)
M1=0.54 Os+0.46 Al M2=0.49 Os+0.51 Al
However, there exist at least two uranium intermetallic compounds with the same MgNi2-type structure, which exhibit magnetic ordering despite such short dU-U, UNi2 [1,17,18] and U2Fe3Ge [19,20]. UNi2 with dU-U=306 pm is a ferromagnet with the Curie temperature TC=23 K with very low spontaneous magnetic moment Ms=0.05– 0.08 μB per formula unit (f.u.). U2Fe3Ge has even shorter dU-U, only 285 pm, but it is also ferromagnet with TC=55 K and Ms=1.0 μB/f.u. (i.e., 0.5 μB per U atom). In both compounds the 3d metal (Ni or Fe) is
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A.V. Andreev et al.
Fig. 2. Crystal structure of UOsAl (with coordinates of U atoms).
Fig. 4. Temperature dependence of magnetic susceptibility of a UOsAl single crystal measured along the main axes in 7 T. The inset shows similar results for URuAl [1].
Fig. 3. Magnetization curves of a UOsAl single crystal measured along the main axes at 2 K. The inset shows similar results for URuAl [1]. Dashed lines are guides for eye.
non-magnetic. Therefore, it is interesting to check whether UOsAl is another exception from the Hill rule. Fig. 3 shows the field dependence of magnetization at T=2 K in field applied along the principal axes of the UOsAl crystal. Both curves are linear up to highest field 59 T. Neither a spontaneous magnetic moment nor a field-induced anomaly (found in URuAl, compound with 4d-electron analog of Os) is observed. In URuAl, whose magnetization curves are presented in the inset of Fig. 3, a broad S-shape anomaly interpreted as suppression of spin fluctuations is seen in the magnetization curve along the direction with higher susceptibility (it is the c-axis in URuAl having the ZrNiAl-type structure). The temperature dependence of magnetic susceptibility of UOsAl, compared again with URuAl (inset), is presented in Fig. 4. UOsAl exhibiting almost temperature-independent response without any anomalies, is qualitatively different from URuAl, where a broad maximum is observed along the c-axis around 50 K. The susceptibility value in UOsAl, ≈1.5*10−8 m3 mol−1, can be associated with the Pauli susceptibility, reflecting a moderately enhanced density of states at the Fermi level, N(EF), in the case of broad 5f-band materials. It also corresponds to magnitude of a temperature-independent contribution into susceptibility in those U intermetallics, which follow the CurieWeiss behavior otherwise [1]. The magnetic susceptibility of UOsAl is almost isotropic. Nevertheless, a slightly lower value for magnetic field along the c-axis is in agreement with the shorter dU-U in this direction.
Fig. 5. Temperature dependence of the specific heat of UOsAl plotted as Cp/T vs T. The inset shows Cp/T vs T2 at low temperatures.
Both isostructural ferromagnets, UNi2 and U2Fe3Ge, also exhibit the caxis as the hard magnetization direction. The paramagnetic state of UOsAl without any trace of magnetic ordering is confirmed also by the specific-heat measurements (Fig. 5). The C/T vs. T2 dependence shows a weak upturn below about 10 K (inset in Fig. 5). It yields a moderate value of the Sommerfeld coefficient, γ=36 mJ mol−1 K−2. The Debye temperature estimated from the linear slope of C/T vs. T2 can be evaluated as ΘD=200 K. This value is comparable to ΘD found for the isostructural compounds UNi2 (250 K [18]) and U2Fe3Ge (213 K [19]). The Sommerfeld coefficient of UOsAl is inferior to 65 mJ mol−1 K−2 in UNi2 and 88 mJ mol−1 K−2 (i.e., 44 mJ mol−1 K−2 per U atom) in U2Fe3Ge. This reflects a lower density of states at the Fermi level in UOsAl and indicates that this compound is a broad-band weakly correlated material. It explains also the weak magnetic behavior of UOsAl. 4. Conclusions The UTAl compound with T=Os of the 5d series, despite its different crystal structure, has paramagnetic ground state as the 146
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compounds with T=Fe and Ru, i.e. 3d and 4d analogues of Os. Hightemperature modification of UFeAl, isostructural with UOsAl, is paramagnetic as well. This general feature can be associated with the 5f-d hybridization, which is stronger, due to a larger overlap of the d and 5f states in the energy scale, than for the T metals from the very end of a given transition metal series. The overlap may be enhanced for the 5d states by stronger spin-orbit interaction. The splitting of the 5d3/ 5 and 5d5/2 states contributes to the total 5d band width. It is, however, interesting to mention that a few-percent substitution of Os for Co induces a ferromagnetic state in the band metamagnet UCoAl [21]. So, Os in such situation behaves again very similar to Fe or Ru. Band-structure calculations based on these experimental data on crystal structure and magnetic properties are planned to be performed in collaboration with theoreticians. Acknowledgement This work was supported by the Czech Science Foundation (project 16-03593S). The work was partly performed in MLTL (http://mltl.eu/ ), which is supported within the program of Czech Research Infrastructures (Project No. LM2011025). We acknowledge the support of HLD at HZDR, member of the European Magnetic Field Laboratory (EMFL). References [1] V. Sechovsky, L. Havela, K.H.J. Buschow (Ed.)Handbook of Magnetic Materials 11, Elsevier, Amsterdam, 1998, p. 1.
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