- Email: [email protected]
Peculiarities of U2T2X hydrides
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 945–947 www.elsevier.com/locate/jmmm
Peculiarities of U2T2 X hydrides K. Miliyanchuka,, L. Havelaa, L.C.J. Pereirab, A.P. Gonc- alvesb, K. Prokesˇ c a
Charles University, Faculty of Mathematics and Physics, Department of Electronic Structures, Ke Karlovu 5, 121 16 Prague 2, Czech Republic b Instituto Tecno´logico e Nuclear/CFMC-UL, P-2686-953 Sacave´m, Portugal c Hahn-Meitner Institute, SF-2, Glieniker Str. 100, 141 09 Berlin, Germany Available online 30 October 2006
Abstract U2 Ni2 Sn belongs to a large group of U2 T2 X compounds which are very sensitive to hydrogen absorption. U2 Ni2 SnH1:8 and U2 Ni2 SnD1:8 exhibit similar magnetic behaviour—they order antiferromagnetically at T ¼ 87 K, which is much higher than T N ¼ 26 K in U2 Ni2 Sn. Deuterium lattice sites were determined by neutron diffraction. Uranium magnetic moments 0:8ð3Þ mB lie in the basal plane of the tetragonal structure. r 2006 Elsevier B.V. All rights reserved. PACS: 61.12.q; 71.27.þa; 75.50.y Keywords: Metal hydrides; Deuterides; Uranium intermetallics; Magnetic structures
1. Introduction U2 T2 X compounds (T—transition metal, X—p-element) give rise to a large unique group of uranium intermetallic hydrides. The properties of U2 T2 X compounds are very sensitive to hydrogen absorption. The pilot studies on U2 Co2 Sn hydrides showed that hydrogenation turned the non-magnetic initial compound, characterized by spinfluctuation behaviour and non-Fermi liquid features, into weak ferromagnet with T C ¼ 33:5 K, or antiferromagnet with T N ¼ 27 K, depending on the amount of hydrogen absorbed [1]. Similarly, the hydrogenation leads to stronger magnetic interactions in U2 Ni2 SnH1:8 , U2 Ni2 InH1:8 , and U2 Co2 InH1:9 [2]. The Ne´el temperature of e.g. U2 Ni2 Sn increases from 26 to 87 K in U2 Ni2 SnH1:8 . One reason for this quite general (at U intermetallics) effect can be the concomitant lattice expansion. The present work is an attempt to find out what lays behind such variations on the example of U2 Ni2 Sn, mainly
Corresponding author. Tel./fax: +420 2 2191 1351.
E-mail address: [email protected] (K. Miliyanchuk). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.157
by the crystal and magnetic structure determination by means of neutron diffraction. 2. Experimental details Both hydride and deuteride of U2 Ni2 Sn were prepared by method described in Ref. [2], which consists in the exposure of initial intermetallic to high pressure ð 100 barÞ of hydrogen or deuterium gas and subsequent thermal cycling up to 500 C. H(D) concentration in both samples was 1:8 0:1 atom=f:u: Crystal structure of U2 Ni2 SnH1:8 was studies by X-ray powder diffraction (CuKa radiation). Crystal and magnetic structure of U2 Ni2 SnD1:8 was studied by neutron diffraction (diffractometer E2 at HMI in Berlin) at T ¼ 1:8 and 120 K (below and above the ordering temperature). Both X-ray and neutron diffraction data were evaluated using full-profile Rietveld refinement. A SQUID magnetometer and extraction magnetometer (both Quantum Design) were used for magnetic studies in the temperature range of 2–300 K and in magnetic fields up to 9 T. Specific heat measurements were performed on the PPMS measuring system, using samples in the form of
ARTICLE IN PRESS K. Miliyanchuk et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 945–947
946
pellets prepared from the deuteride powder in a WC anvil applying pressure of several hundred Mbar.
1.8
χ (10-7 m3/mol)
1.6
3. Results and discussion
1.2 U2Ni2SnH1.8
1.0
Cp /T (J/mol K2)
U2Ni2SnD1.8
1.5
1.0 U2Ni2Sn 0.5 U2Ni2SnD1.8 0.0 0
20
40
60
80
100
120
140
T (K)
(002, 400)
(321, 410)
(310)
(211)
(320)
(210)
(311)
(a)
(111)
(001, 200)
20000
(110)
30000
(201, 220)
Fig. 1. Temperature dependence of magnetic susceptibility for U2 Ni2 SnH1:8 ðm0 H ¼ 3 TÞ and U2 Ni2 SnD1:8 (m0 H ¼ 4 T, upper panel), and specific heat C p of U2 Ni2 Sn [5] and U2 Ni2 SnD1:8 (lower panel).
I (arb. units)
Both U2 Ni2 SnH1:8 and U2 Ni2 SnD1:8 crystallize in the tetragonal Mo2 FeB2 structure type (space group P4/mbm) similar to the initial compound. The lattice and crystal structure parameters are listed in Table 1. The positions of deuterium atoms were determined as (8 k) site inside the U3 Ni tetrahedra, which are coupled by sharing a face. Just one of two neighbouring tetrahedra is randomly occupied, due to the proximity of the interstitials. Magnetic measurements show that U2 Ni2 SnD1:8 orders antiferromagnetically at T N ¼ 87 K (determined as maximum in wðTÞ) similar to U2 Ni2 SnH1:8 (Fig. 1). The upturn at low temperatures, which was attributed to noncompensated spins at grain boundaries or to structure defects, is less pronounced for the deuteride. It may be a consequence of larger grain size. The susceptibility of U2 Ni2 SnD1:8 is shifted up compared to U2 Ni2 SnH1:8 due to the presence of a small amount of UH3 ðT 180 KÞ as a spurious phase. We can conclude that magnetic properties of U2 Ni2 Sn hydride and deuteride are practically identical, which means that all structure information obtained for the deuteride represents also the hydride. Unlike pure U2 Ni2 Sn, which exhibited additional magnetic reflection ð12; 12; 12Þ in neutron diffraction pattern collected in the ordered state [3], no additional reflections were observed for U2 Ni2 SnD1:8 at T ¼ 1:8 K. Instead more intensive (2 1 0) and (1 1 1) reflections were registered (Fig. 2). Thus the unit cell doubling along c, indicated at U2 Ni2 Sn is lost in the deuteride. The magnetic and crystallographic unit cells have to be identical, and we found the best fit with the model having non-collinear uranium moments of 0:8ð3Þ mB within the basal plane, similar to the situation found in U2 Pd2 In [4]. The size of the moments can be thus somewhat smaller than in U2 Ni2 Snð1:05 mB Þ. Specific heat measurements of U2 Ni2 SnD1:8 showed a pronounced magnetic anomaly at T ¼ 85 K. Although the
1.4
10000
(b) 0 0
(c) 20
40
60
80
2 (deg)
Fig. 2. Neutron powder diffraction patterns of U2 Ni2 SnD1:8 at T ¼ 120 K (a) and 1.8 K (b) and difference pattern (c).
Table 1 Lattice parameters a and c, unit cell volume V of U2 Ni2 SnH1:8 and U2 Ni2 SnD1:8 measured at room temperature, and atomic parameters, thermal parameters B and the coefficients of the site occupancy n, obtained from neutron diffraction of U2 Ni2 SnD1:8 at 120 K ðRB ¼ 4:22%Þ U2 Ni2 Sn U2 Ni2 SnH1:8 U2 Ni2 SnD1:8
a ¼ 7:263ð1Þ A˚ a ¼ 7:445ð1Þ A˚ a ¼ 7:435ð1Þ A˚
c ¼ 3:695ð1Þ A˚ c ¼ 3:764ð1Þ A˚ c ¼ 3:762ð1Þ A˚
V ¼ 194:9ð1Þ A˚ 3 V ¼ 208:6ð1Þ A˚ 3 V ¼ 207:9ð1Þ A˚ 3
Atom
Site
x
y
z
B ðA˚ 3 Þ
n
U Ni Sn D
(4h) (4g) (2a) (8k)
0.1788(6) 0.3747(5) 0 0.3859(10)
0.6788(6) 0.8747(5) 0 0.8859(10)
0.5 0 0 0.5338(63)
0.33 0.58 0.24 0.92
1 1 1 0.448(6)
ARTICLE IN PRESS K. Miliyanchuk et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 945–947
magnetic entropy could not be evaluated exactly due to a non-Debye-like phonon specific heat, the comparison of the anomaly with U2 Ni2 Sn reveals that it remains a small fraction of R ln 2, meaning that the itinerant character of magnetism is preserved. The g coefficient obtained from the linear extrapolation of C=T to T ¼ 0 K is 105 mJ=mol K2 , i.e. lower than the value for U2 Ni2 Sn—172 mJ=mol K2 [5]. The position of H(D) atoms within the U3 Ni tetrahedra suggests that besides the moderate lattice expansion, also the variations of the 5f hybridization with nickel 3d states can be responsible for the dramatic increase of T N . Acknowledgements This work is a part of the research plan MSM 0021620834 that is financed by the Ministry of Education of Czech Republic. It was also supported by the Grant Agency of Charles University (Grant #224/2005) and by the action COST P16 under the project OC 146, financed
947
by the Czech Ministry of Education. The neutron experiment has been supported by the European Commission under the 6th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract no RII3-CT-2003505925 (NMI3).
References [1] K. Miliyanchuk, L. Havela, A.V. Kolomiets, A.V. Andreev, Physica B 359–361 (2005) 1042. [2] K. Miliyanchuk, L. Havela, A.V. Kolomiets, S. Danisˇ , L.C.J. Pereira, A.P. Gonc- alves, Physica B 378–380 (2006) 983. [3] F. Boure´e, B. Chevalier, L. Fourne`s, et al., J. Magn. Magn. Mater. 138 (1994) 307. [4] A. Purwanto, R.A. Robinson, L. Havela, et al., Phys. Rev. B 50 (1994) 6792. [5] K. Kindo, T. Fukushima, T. Kumada, et al., J. Magn. Magn. Mater. 140–144 (1995) 1369.
Journal of Magnetism and Magnetic Materials 310 (2007) 945–947 www.elsevier.com/locate/jmmm
Peculiarities of U2T2 X hydrides K. Miliyanchuka,, L. Havelaa, L.C.J. Pereirab, A.P. Gonc- alvesb, K. Prokesˇ c a
Charles University, Faculty of Mathematics and Physics, Department of Electronic Structures, Ke Karlovu 5, 121 16 Prague 2, Czech Republic b Instituto Tecno´logico e Nuclear/CFMC-UL, P-2686-953 Sacave´m, Portugal c Hahn-Meitner Institute, SF-2, Glieniker Str. 100, 141 09 Berlin, Germany Available online 30 October 2006
Abstract U2 Ni2 Sn belongs to a large group of U2 T2 X compounds which are very sensitive to hydrogen absorption. U2 Ni2 SnH1:8 and U2 Ni2 SnD1:8 exhibit similar magnetic behaviour—they order antiferromagnetically at T ¼ 87 K, which is much higher than T N ¼ 26 K in U2 Ni2 Sn. Deuterium lattice sites were determined by neutron diffraction. Uranium magnetic moments 0:8ð3Þ mB lie in the basal plane of the tetragonal structure. r 2006 Elsevier B.V. All rights reserved. PACS: 61.12.q; 71.27.þa; 75.50.y Keywords: Metal hydrides; Deuterides; Uranium intermetallics; Magnetic structures
1. Introduction U2 T2 X compounds (T—transition metal, X—p-element) give rise to a large unique group of uranium intermetallic hydrides. The properties of U2 T2 X compounds are very sensitive to hydrogen absorption. The pilot studies on U2 Co2 Sn hydrides showed that hydrogenation turned the non-magnetic initial compound, characterized by spinfluctuation behaviour and non-Fermi liquid features, into weak ferromagnet with T C ¼ 33:5 K, or antiferromagnet with T N ¼ 27 K, depending on the amount of hydrogen absorbed [1]. Similarly, the hydrogenation leads to stronger magnetic interactions in U2 Ni2 SnH1:8 , U2 Ni2 InH1:8 , and U2 Co2 InH1:9 [2]. The Ne´el temperature of e.g. U2 Ni2 Sn increases from 26 to 87 K in U2 Ni2 SnH1:8 . One reason for this quite general (at U intermetallics) effect can be the concomitant lattice expansion. The present work is an attempt to find out what lays behind such variations on the example of U2 Ni2 Sn, mainly
Corresponding author. Tel./fax: +420 2 2191 1351.
E-mail address: [email protected] (K. Miliyanchuk). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.157
by the crystal and magnetic structure determination by means of neutron diffraction. 2. Experimental details Both hydride and deuteride of U2 Ni2 Sn were prepared by method described in Ref. [2], which consists in the exposure of initial intermetallic to high pressure ð 100 barÞ of hydrogen or deuterium gas and subsequent thermal cycling up to 500 C. H(D) concentration in both samples was 1:8 0:1 atom=f:u: Crystal structure of U2 Ni2 SnH1:8 was studies by X-ray powder diffraction (CuKa radiation). Crystal and magnetic structure of U2 Ni2 SnD1:8 was studied by neutron diffraction (diffractometer E2 at HMI in Berlin) at T ¼ 1:8 and 120 K (below and above the ordering temperature). Both X-ray and neutron diffraction data were evaluated using full-profile Rietveld refinement. A SQUID magnetometer and extraction magnetometer (both Quantum Design) were used for magnetic studies in the temperature range of 2–300 K and in magnetic fields up to 9 T. Specific heat measurements were performed on the PPMS measuring system, using samples in the form of
ARTICLE IN PRESS K. Miliyanchuk et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 945–947
946
pellets prepared from the deuteride powder in a WC anvil applying pressure of several hundred Mbar.
1.8
χ (10-7 m3/mol)
1.6
3. Results and discussion
1.2 U2Ni2SnH1.8
1.0
Cp /T (J/mol K2)
U2Ni2SnD1.8
1.5
1.0 U2Ni2Sn 0.5 U2Ni2SnD1.8 0.0 0
20
40
60
80
100
120
140
T (K)
(002, 400)
(321, 410)
(310)
(211)
(320)
(210)
(311)
(a)
(111)
(001, 200)
20000
(110)
30000
(201, 220)
Fig. 1. Temperature dependence of magnetic susceptibility for U2 Ni2 SnH1:8 ðm0 H ¼ 3 TÞ and U2 Ni2 SnD1:8 (m0 H ¼ 4 T, upper panel), and specific heat C p of U2 Ni2 Sn [5] and U2 Ni2 SnD1:8 (lower panel).
I (arb. units)
Both U2 Ni2 SnH1:8 and U2 Ni2 SnD1:8 crystallize in the tetragonal Mo2 FeB2 structure type (space group P4/mbm) similar to the initial compound. The lattice and crystal structure parameters are listed in Table 1. The positions of deuterium atoms were determined as (8 k) site inside the U3 Ni tetrahedra, which are coupled by sharing a face. Just one of two neighbouring tetrahedra is randomly occupied, due to the proximity of the interstitials. Magnetic measurements show that U2 Ni2 SnD1:8 orders antiferromagnetically at T N ¼ 87 K (determined as maximum in wðTÞ) similar to U2 Ni2 SnH1:8 (Fig. 1). The upturn at low temperatures, which was attributed to noncompensated spins at grain boundaries or to structure defects, is less pronounced for the deuteride. It may be a consequence of larger grain size. The susceptibility of U2 Ni2 SnD1:8 is shifted up compared to U2 Ni2 SnH1:8 due to the presence of a small amount of UH3 ðT 180 KÞ as a spurious phase. We can conclude that magnetic properties of U2 Ni2 Sn hydride and deuteride are practically identical, which means that all structure information obtained for the deuteride represents also the hydride. Unlike pure U2 Ni2 Sn, which exhibited additional magnetic reflection ð12; 12; 12Þ in neutron diffraction pattern collected in the ordered state [3], no additional reflections were observed for U2 Ni2 SnD1:8 at T ¼ 1:8 K. Instead more intensive (2 1 0) and (1 1 1) reflections were registered (Fig. 2). Thus the unit cell doubling along c, indicated at U2 Ni2 Sn is lost in the deuteride. The magnetic and crystallographic unit cells have to be identical, and we found the best fit with the model having non-collinear uranium moments of 0:8ð3Þ mB within the basal plane, similar to the situation found in U2 Pd2 In [4]. The size of the moments can be thus somewhat smaller than in U2 Ni2 Snð1:05 mB Þ. Specific heat measurements of U2 Ni2 SnD1:8 showed a pronounced magnetic anomaly at T ¼ 85 K. Although the
1.4
10000
(b) 0 0
(c) 20
40
60
80
2 (deg)
Fig. 2. Neutron powder diffraction patterns of U2 Ni2 SnD1:8 at T ¼ 120 K (a) and 1.8 K (b) and difference pattern (c).
Table 1 Lattice parameters a and c, unit cell volume V of U2 Ni2 SnH1:8 and U2 Ni2 SnD1:8 measured at room temperature, and atomic parameters, thermal parameters B and the coefficients of the site occupancy n, obtained from neutron diffraction of U2 Ni2 SnD1:8 at 120 K ðRB ¼ 4:22%Þ U2 Ni2 Sn U2 Ni2 SnH1:8 U2 Ni2 SnD1:8
a ¼ 7:263ð1Þ A˚ a ¼ 7:445ð1Þ A˚ a ¼ 7:435ð1Þ A˚
c ¼ 3:695ð1Þ A˚ c ¼ 3:764ð1Þ A˚ c ¼ 3:762ð1Þ A˚
V ¼ 194:9ð1Þ A˚ 3 V ¼ 208:6ð1Þ A˚ 3 V ¼ 207:9ð1Þ A˚ 3
Atom
Site
x
y
z
B ðA˚ 3 Þ
n
U Ni Sn D
(4h) (4g) (2a) (8k)
0.1788(6) 0.3747(5) 0 0.3859(10)
0.6788(6) 0.8747(5) 0 0.8859(10)
0.5 0 0 0.5338(63)
0.33 0.58 0.24 0.92
1 1 1 0.448(6)
ARTICLE IN PRESS K. Miliyanchuk et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 945–947
magnetic entropy could not be evaluated exactly due to a non-Debye-like phonon specific heat, the comparison of the anomaly with U2 Ni2 Sn reveals that it remains a small fraction of R ln 2, meaning that the itinerant character of magnetism is preserved. The g coefficient obtained from the linear extrapolation of C=T to T ¼ 0 K is 105 mJ=mol K2 , i.e. lower than the value for U2 Ni2 Sn—172 mJ=mol K2 [5]. The position of H(D) atoms within the U3 Ni tetrahedra suggests that besides the moderate lattice expansion, also the variations of the 5f hybridization with nickel 3d states can be responsible for the dramatic increase of T N . Acknowledgements This work is a part of the research plan MSM 0021620834 that is financed by the Ministry of Education of Czech Republic. It was also supported by the Grant Agency of Charles University (Grant #224/2005) and by the action COST P16 under the project OC 146, financed
947
by the Czech Ministry of Education. The neutron experiment has been supported by the European Commission under the 6th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract no RII3-CT-2003505925 (NMI3).
References [1] K. Miliyanchuk, L. Havela, A.V. Kolomiets, A.V. Andreev, Physica B 359–361 (2005) 1042. [2] K. Miliyanchuk, L. Havela, A.V. Kolomiets, S. Danisˇ , L.C.J. Pereira, A.P. Gonc- alves, Physica B 378–380 (2006) 983. [3] F. Boure´e, B. Chevalier, L. Fourne`s, et al., J. Magn. Magn. Mater. 138 (1994) 307. [4] A. Purwanto, R.A. Robinson, L. Havela, et al., Phys. Rev. B 50 (1994) 6792. [5] K. Kindo, T. Fukushima, T. Kumada, et al., J. Magn. Magn. Mater. 140–144 (1995) 1369.