K Мiliyanchuk, L Havela, J C Waerenborgh - Crystal structure, magnetic and thermal properties of ucosnd - страница 1

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Chemistry of Metals and Alloys

Chem. Met. Alloys 1 (2008) 174-179 Ivan Franko National University of Lviv www. chemetal-j ournal. org

Crystal structure, magnetic and thermal properties of UCoSnD0.6

Khrystyna MILIYANCHUK1'2*, Ladislav HAVELA2, Joao C. WAERENBORGH3, Piotr M. GACZYNSKI3, Oleksandr PROKHNENKO4

1 Department of Inorganic Chemistry, Ivan Franko National University of Lviv,

Kyryla i Mefodiya St. 6, 79005 Lviv, Ukraine

2 Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University,

Ke Karlovu 5, 121 16 Prague 2, Czech Republic

3 Departamento de Quimica, Instituto Tecnologico e Nuclear/CFMC-UL, P-2686-953, Sacavem, Portugal

4 Hahn-Meitner Institute, SF-2, Glienicker Str. 100, 141 09 Berlin, Germany

* Corresponding author. Tel.: +380-32-2394506; e-mail: k_miliyanchuk@franko.lviv.ua

Received November 12, 2007; accepted May 20, 2008; available on-line September 10, 2008

The new deuteride UCoSnD06 was investigated by neutron powder diffraction, magnetic measurements, specific heat studies, and Mossbauer spectroscopy. It was shown that the insertion of deuterium atom inside the U3Co tetrahedra with concomitant lattice expansion by 3.2% leads to a pronounced enhancement of magnetism. The reason for such a behavior is a partial localization of the electronic states, which is reflected in the increase of the density of states at the Fermi level and increase of the ^-electron density on the Sn nuclei.

Deuteride / Uranium / Neutron diffraction / Specific heat / Mossbauer spectra

Introduction

UCoSn (ZrNiAl structure type, a = 7.148 A, c = 3.99 A [1], Tc = 80-88 K [2-5]) was already reported to absorb hydrogen [6]. The formed hydride UCoSnH14 is characterized by a pronounced enhancement of the magnetism. The Curie temperature increases from 82 K to 102 K and the spontaneous magnetic moment in randomly oriented powder sample increases from 0.64 jmB/f.u. to 0.75 jmB/f.u. However, studies of the isostructural URuSn compound surprisingly revealed an opposite effect. URuSnH14 exhibits a decrease of both the Curie temperature, from 54 K to 51 K, and the spontaneous magnetic moment, from 0.53 jmB/f.u. to 0.48 mB/f.u [6]. To explain the different behavior of these compounds upon hydrogenation we suggested that hydrogen atoms occupy different positions. In the present work we summarize our results from the study of the UCoSnD0 6 deuteride. The prime task was to determine the deuterium positions by a neutron powder diffraction experiment. Besides, the study of such properties as specific heat and Mossbauer spectra provided additional details on the modification of the electronic structure caused by deuteration.

Experimental details

The synthesis of UCoSnD0.6 was preceded by the preparation of corresponding intermetallic compound. The sample was prepared by arc-melting the stoichiometric amount of metals of purity at least 99.9% under argon atmosphere. The deuteride was synthesized by exposing an activated UCoSn sample to deuterium gas at p = 120 bar and subsequent thermal treatment up to T = 923 K. To quantify the amount of absorbed deuterium, a small amount of deuteride was decomposed in vacuum in a closed volume by heating up to 1023 K. The phase composition of the initial and the deuteride samples was checked by X-ray powder diffraction (XRD-3003 Seifert diffractometer, Cu-Koc radiation, step 0.02° of 26).

The crystal structure of UCoSnD0.6 was studied by neutron powder diffraction. The data were collected at Berlin Neutron Scattering Center, Hahn-Meitner-Institute (fine resolution E9 diffractometer, focusing composite germanium monochromator with reflecting plane (511), 1 = 1.797 A, step 0.08° of 26). The measurements were performed at room temperature and the task was to determine the deuterium positions.

Table 1 Lattice parameters a and c, and unit cell volume V of UCoSn, UCoSnH14 (both X-ray data), and UCoSnD0.6 (neutron diffraction data) at room temperature, atom coordinates, displacement parameters B and coefficients of the site occupancy n of UCoSnD06 (RBragg = 6.22%, Rp = 4.50%).

UCoSn UCoSnHL4

UCoSnD06

a = 7.144(1) A a = 7.2386(6) A Da/a = 1.3% a = 7.2332(2) A Da/a = 1.2%

c = 3.993(1) A

c = 4.0189(5) A Dc/c = 0.7% c = 4.0208(2) A Dc/c = 0.7%

V = 176.5 A3

V = 182.4 A3 DV/V = 3.3%

V = 182.2 A3 DV/V = 3.2%

Atom

Site

x

y

z

в (A2)

n

U

Co1

Co2 Sn

D

3f

1a 2d

3g 4h

0.5993(8)

0

0.3333

0.2581(9) 0.3333

0 0

0.6667 0

0.6667

0

0

0.5

0

0.0634(39)

0.34 0.56 0.56 0.40 1.13

1

1 1 1

0.456(6)

The evaluation of both X-ray and neutron diffraction data was based on the Rietveld algorithm using FullProf software [7].

Measurements of DC magnetization were performed on randomly oriented fixed powder in the temperature range 5-300 K and applied external fields up to 4 T by means of a Quantum Design SQUID magnetometer.

The heat capacity measurements were performed using the Quantum Design PPMS in the temperature range 1.8-300 K. The measurements were performed on pellets prepared from deuteride powder by pressing in a hydraulic press. A special anvil cell with WC faces made it possible to reach several hundreds MPa, which turned out to be sufficient to produce mechanically stable thin pellets.

Mossbauer spectroscopy was performed at the Instituto Tecnologico e Nuclear, Sacavem, Portugal. Mossbauer spectra were collected in transmission mode using a conventional constant-acceleration spectrometer and a 10 mCi 119mSn source in CaSnO3; the velocity scale was calibrated using a 57Co source and an a-Fe foil. The absorbers were obtained by pressing the powdered samples into perspex holders. The absorber thickness was calculated on the basis of the corresponding electronic mass-absorption coefficients for 23.9 keV radiation. For the data presented in this work, the isomer shifts (IS) are given relative to the CaSnO3 source. Low-temperature spectra were collected using a liquid-helium flow cryostat with a temperature stability of ± 0.5 K. The spectra were fitted to Lorentzian profiles using a non­linear least-squares method [8]. The positions and relative intensities of the absorption lines of the Sn atoms with non-zero transferred magnetic hyperfine field were calculated by solving the complete Hamiltonian for the hyperfine interactions in the excited and ground nuclear states of the 119Sn nuclei, following the procedure described by Ruebenbauer and Birchall [9].

Results and discussion

X-ray and neutron diffraction The X-ray powder diffraction patterns of the initial alloy showed the presence of UCoSn as the major phase and a certain amount («11 %) of a spurious phase, UCo~14Sn~185 [10] (CaBe2Ge2 structure type, space group P4/nmm, a = 4.387 A, c = 9.040 A). The spurious phase remains unaffected by the deuteration, as was seen from the comparison of the diffraction patterns of the samples before and after the deuteration.

An interesting fact was revealed by the decomposition study of the UCoSn deuteride. The decomposition in vacuum by heating up to 1023 K yielded a stoichiometry of 0.7(1) D atoms per formula unit (versus 1.4 H atoms/f.u. for the corresponding hydride), while the X-ray diffraction revealed that the lattice parameters of the deuteride and the hydride are very close to each other (Table 1). In order to explain this peculiar effect we assume tentatively that a less stable hydride with twice as much hydrogen is formed initially, but decomposes rapidly into a lower hydride. This process may be assisted by the mechanical treatment (crushing) before the X-ray diffraction pattern is collected. The X-ray analysis of the decomposed sample proved that the full desorption state is indeed achieved for the deuteride. Furthermore, the refinement of the neutron powder diffraction data mentioned below showed that the UCoSnD0.6 stoichiometry is correct.

The neutron diffraction pattern of UCoSnD0.6 taken at room temperature is shown in Fig. 1. The crystal structure refinement showed that the metal atoms form a ZrNiAl-type structure (space group P62m ), similar to that of the initial compound but expanded by 3.2%. The crystal structure parameters are given in Table 1. The lattice expansion is anisotropic, with prevailing basal-plane expansion. The deuterium atoms occupy the 4h position inside the

K. Miliyanchuk et al., Crystal structure, magnetic and thermal properties of UCoSnD0 6

Fig. 1 Neutron diffraction pattern of UCoSnD06 (1= 1.797 A). The observed values (circles), the calculated values (solid line), difference pattern and the positions of the Bragg reflections of UCoSnD0.6 and the impurity phase are shown.

U3Co tetrahedra. The crystallographic stoichiometry was refined as UCoSnD0.608(8), leading to a smaller deuterium content as compared with the stoichiometry determined by the volumetric method (0.7(1) D atoms/f.u.), but both values are comparable within the experimental uncertainty. This structure was already found for UNiAlD07 [11] - a non-saturated deuteride of the isostructural compound UNiAl. The crystal structure of UCoSnD0.6 is presented in Fig. 2. The U3Co tetrahedra, which accommodate the deuterium atom, share a face, and according to "Shoemaker's exclusion rule" [12] both of them cannot be occupied at the same time. Besides, the distance between two deuterium positions (0.51 A) does not allow simultaneous occupancy of neighboring sites [13]. No additional reflections corresponding to possible ordering of the deuterium atoms were observed, therefore we suggest that the deuterium atoms occupy one out of two neighboring interstitial sites randomly. However, this model seems to fail for the mentioned hydride with the stoichiometry 1.4 H/f.u., leading to extremely short distances between the hydrogen atoms, what might be the reason of its instability. In order to satisfy the rule of minimum hydrogen-hydrogen distances 2.1 A, the hydrogen atoms should be shifted apart (zH > 0.2).

Deuterium atoms located inside the tetrahedra consisting of uranium and cobalt occupy a very typical position for metal hydrides. It is known that the crucial parameter that predetermines the magnetic properties of uranium intermetallides is the degree of f-d hybridization. The intrusion of hydrogen atoms, leading to an expansion of the unit cell and to modifications of the electronic structure via the formation of new bonds with its nearest neighbors, must have straightforward impact on magnetic properties of UCoSn.

Fig. 2 Crystal structure of UCoSnD0.6.

Magnetic properties

The measurements of the magnetic susceptibility showed that the magnetic properties of UCoSnD0.6 are very close to those of the corresponding hydride [6]. UCoSnD0.6 orders ferromagnetically, similarly to the hydride and the initial compound. In the paramagnetic range, the susceptibility curve is described by a modified Curie-Weiss law (Fig. 3). The fit parameters are presented in Table 2. The paramagnetic Curie temperature is slightly higher than that of the hydride (6P = 97 K for the hydride) and the same is true for the Curie temperature determined from the detailed low-field magnetic susceptibility measurements. The increase of the Curie temperature by approximately 2 K may be attributed to a small, but non-zero homogeneity region due to the non-stoichiometric composition of both the hydride and the deuteride.

Taking into account the reproducibility of the magnetic properties we may conclude that all the structure data obtained for the deuteride should also be valid for the hydride. However, it does not solve the puzzle with the apparent difference between the stoichiometry of the hydride and the deuteride.

Table 2 Curie temperature TC and parameters of the modified Curie-Weiss fit of the high-temperature susceptibility (effective moments meff, paramagnetic Curie temperature 6P, T-independent term с) for UCoSn and the corresponding deuteride.

 

UCoSn

UCoSnD06

Tc (K)

82

104

/4ff (iWf.u.)

2.0

2.1

6P (K)

77

102

С (10-8 m3/mol)

1.3

1.1

I

о о

3.0 2.5 2.0 1.5 1.0 0.5

0.0

0      50     100     150     200     250 300

T (K)

Fig. 3 Temperature dependence of the inverse magnetic susceptibility of UCoSn (measured in magnetic field 6 T) and of UCoSnD0.6 (measured in magnetic field 4 T). The solid lines represent the modified Curie-Weiss fit (see Table 2 for the corresponding parameters).

Fig. 4 Temperature dependence of the specific heat of UCoSnD0.6. The solid line represents total fitted electron and phonon contributions.

Specific heat measurements

Specific heat data were collected for UCoSnD0.6 down to T = 2.2 K (Fig. 4). The temperature dependence is characterized by a pronounced anomaly at the temperature of the magnetic phase transition.

The /-coefficient of electronic specific heat was determined by linear extrapolation of CP/T vs. T 2 to T = 0 K. The value of /increases upon deuteration and reaches 98(1) mJ/mol K2 (/ = 61 mJ/mol K2 for UCoSn [14]), which reflects the increase of the density of states at the Fermi level N(£F). The reason can be further localization of the electronic levels (but one has to keep in mind that the experimental /-value reflects N(£F) in the magnetically split state), the additional contribution of deuterium atoms to the total density of states, or the shift of the peaks of the electronic density.

In order to estimate the phonon contribution we have chosen the Debye model, which worked well in this case. The best fit was obtained for the model described by 9 branches (apparently the contribution of the metal atoms), with the Debye temperature of 217.5 K. The suggested model described the specific heat curve in the whole temperature range, while the extra specific heat in the proximity of the magnetic phase transition was evaluated to estimate the magnetic entropy as

Smag(T ) =

•Cmag(T')

T'

dT \

Such procedure yields a small magnetic entropy of 1.5 J mol/K (0.26*Rln2), showing that the system is still far from the localization limit.

Mossbauer spectroscopy

Mossbauer spectra for UCoSnD0.6 were collected at room temperature and at T = 4.2 K, i.e. in the paramagnetic region and in the magnetically ordered state.

The room temperature Mossbauer spectrum (Fig. 5) exhibits a broad peak, which could not be described by a single line. Therefore, a quadrupole splitting had to be considered in agreement with the

T

0

Table 3 Estimated hyperfine parameters (isomer shift IS, quadrupole splitting QS, hyperfine magnetic field Bhf, asymmetry parameter of electric field gradient (EFG) h, 6 and j polar and azimuthal angles specifying the direction of Bhf with respect to the main axes of the EFG tensor, and line-width Г) from the 119Sn Mossbauer spectra of UCoSn and UCoSnD0.6 obtained at different temperatures T.

 

T (K)

IS (mm/s)

QS (mm/s)

Bhf (T)

h

6(deg)

j(deg)

G(mm/s)

UCoSna

80

1.75

0.56

-

-

-

-

0.93

UCoSnD0 6b

300

1.78(1)

0.64(1)

-

-

-

-

0.96(2)

UCoSna

4.2

1.765

-0.55

8.40

0.22

90

90

0.76

UCoSnD06b

4.2

1.89(4)

-0.93(3)

8.56(1)

0.36

90

0

1.08(1)

QS was recalculated considering that for the ET = 23.875 keV transition of    mSn, 1 mm/s corresponds to

19.253(6) MHz

a Data from [15]. The isomer shift IS is relative to the Ba119mSnO3 source. b Present work. The isomer shift IS is relative to the Ca119mSnO3 source.

low local symmetry of the Sn atoms. The hyperfine parameters extracted from the fit, describing two Lorentzians with equal line widths and relative areas, are given in Table 3. The lines are unresolved since the corresponding widths are larger than the quadrupole splitting. The hyperfine parameters of UCoSnD0.6 are close to those obtained for UCoSn.

The Mossbauer spectrum of UCoSnD0.6 obtained at T = 4.2 K (Fig. 5) shows that, besides the main phase with a well-defined magnetic hyperfine field, there is a contribution from an impurity phase in the middle of the spectrum. Therefore it was necessary to reproduce the shape of the impurity component prior to the determination of the lines of the main phase, split by a magnetic hyperfine field. The hyperfine interaction parameters for UCoSnD0.6, obtained after this procedure, are presented in Table 3. In the magnetically ordered state, a non-zero magnetic hyperfine field is found on the Sn site for both UCoSn and the deuteride. The deuteration does not have significant impact on the value of the magnetic hyperfine field Bhf, while the values of the quadrupole splitting and isomer shift (IS) are changed. The increase of IS between room temperature and T = 4.2 K, deduced from the present data, was expected due to a second order Doppler effect. The IS value is significantly lower compared to b-Sn (2.78(5) mm/s), which corresponds to a lower s-state density at the Sn nuclei in the compound. This could be understood as due to the involvement of the Sn states in the hybridization. Then we can speculate that the increase of IS from UCoSn to UCoSnD0.6 is due to a reduced hybridization of the Sn states in the deuteride. The increase of the line-width for the deuteride compared to the initial compound is more pronounced at low temperature than for the samples measured at room temperature and might point to a higher degree of atomic disorder in the deuteride, or to a certain homogeneity region. Some changes were observed for the parameters of the electric field gradient (EFG). The direction of the main z-axis of the EFG does not change, but the x- and y-axes are interchanged. For UCoSnD0.6 a slight improvement of the goodness of fit parameter was registered for the values 6= 73° and j= 23° of the polar and azimuthal angles defining the direction of Bhf in the coordinate system of the EFG. This improvement is, however, not significant and therefore there is not enough evidence to suggest a deviation of the direction of the main axes of the EFG, which might be attributed to the presence of deuterium atoms in the crystal lattice. In fact, as they are not nearest neighbors of Sn atoms, their presence should not have a strong effect on the EFG at the Sn nuclei.

Fig. 5 Sn Mossbauer spectra of UCoSnD06 taken at room temperature (upper panel) and at T = 4.2 K (lower panel). The calculated fit is shown by a solid line.

Conclusions

The combined investigation of the crystal structure and physical properties of UCoSnD0.6 explained the strong effect of hydrogenation/deuteration on the magnetism of UCoSn. UCoSnD0.6 is a ferromagnet with an extremely high (for UTX ternary compounds, where T - transition metal, X - p-element) Curie temperature, exceeding 100 K. Based on the obtained results we may conclude that the mechanism responsible for the enhancement of magnetism in UCoSn is the decrease of the degree of f-d hybridization, leading to a certain step in the localization of 5f electronic states. The following facts support this idea: (i) the proximity of deuterium atoms to both U and Co must result in the formation of bonds, therefore withdrawing electronic density from the hybridized band; (ii) the increase of the density of states at the Fermi level can be a consequence of the band narrowing; (iii) the increase of the s-density of states on the Sn nucleus means again withdrawal of Sn electronic states from the hybridized band. This tendency was already observed for other uranium intermetallic compounds upon hydrogenation. For example, it was argued that the considerable increase of the ordering temperature of UNiAl after hydrogenation is due to a reduction of the hybridization effect [16]. Moreover, further studies of the U(Fe1-xNix)AlHy system [17,18] clearly illustrated that hydrogenation strengthens the localization of uranium 5f electrons. In the latter case hydrogenation induces magnetic order in the non-magnetic U(Fe0.15Ni0.85)Al compound, and leads to a large increase of the ferromagnetic ordering temperature and the value of the uranium magnetic moment for the U(Fe0.3Ni0.7)Al compound. However another issue still remains open - what is the reason for the different impact of hydrogenation on the magnetism of UCoSn and URuSn. It will be the subject of our future investigation.

Acknowledgements

This work was part of the research plan MSM 0021620834 financed by the Ministry of Education of the Czech Republic. It was also supported by the Grant Agency of the Czech Republic under grant No. 202/07/418 and by the action COST P16 under the project OC 146, financed by the Ministry of Education of the Czech Republic.

References

[I] A.E.   Dwight,   in:   B.C. Giessen (Ed.),

Developments in the Structural Chemistry of Alloy Phases, Plenum, New York, 1969, p. 181. [2]  A.V. Andreev, M.I. Bartashevich, Phys. Met.

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