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Specific heat of (U1−xThxNiSn
496
Journal of Magnetism and Magnetic Materials 90 & 91 (1990) 496-498 North- Holland
Specific heat of (U 1_:,ThJNiSn Y. Aoki, T. Suzuki, T. Fujita, H. Kawanaka
a,
T. Takabatake
and H. Fujii
a
a
Department of Physics. Faculty of Science. Hiroshima University. Hiroshima 730. Japan Faculty of Integrated Arts and Sciences, Hiroshima University, Hiroshima 730. Japan
a
The specific heat C(T) of the pseudo-binary alloys (UI_xThx)NiSn has been measured for x = 0, 0.2, 0.4. 0.6 and 1.0. An abrupt increase in magnetic entropy Sm at the unusual anti ferromagnetic transition temperature appears to indicate a first order phase transition. The value of Sm tends to approach R In 4 at liqu id N 2 temperature independently of x.
1. Introduction
Special attention has been paid to the anomalous phase transition in ternary uranium compound UNiSn, which crystallizes in the cubic MgAgAs-type structure [1-6]. As temperature is reduced, this compound exhibits a semiconductor to metal (SM) transition concurrently with a type-I antiferromagnetic (AF) ordering at TN = 43 K [6). Since an isostructural compound ThNiSn exhibits no sign of such transition, an indirect interaction between Sf-electrons of U ions is expected to play an essential role in the transition [7]. Evidently, the transition temperature TN decreases with Th substitution for U in UNiSn, as is confirmed by the temperature dependences of resistivity and magnetic susceptibility [8]. However, the interplay between the SM and AF transitions, which occur simultaneously in pure UNiSn, remains to be elucidated. In order to study a thermodynamical aspect of the anomalous transition, we carried out measurements of specific heat, C, for the pseudo-binary alloys (U1_.,Thx)NiSn with x = 0,0.2,0.4,0.6 and 1.0.
pounds, which are measured after the additional annealing. The behavior of p(T) for x = 0, 0.2 and 0.6 qualitatively agrees with the data reported in ref. [8} for the samples without the additional annealing. For x = 0.4, peT) of the present sample reduces to below ~ of the original value for all temperatures. For ThNiSn (x = 1.0), a metal- like behavior, which was seen at low temperatures before additional annealing, disappears after the annealing. The change is probably due to the ordering of the Ni and Sn atoms by annealing. However, the pronounced drop in p(T) itself and its shift to lower temperatures with Tit substitution are also observed in the present measurement. Figure.2 shows CmlT for (Ut_xThx)NiSn with x;;:::; 0.6 as a function of temperature T, where Cm is a magnetic part of the specific heat defined by Cm = (C Co)/(l - x), Co is a non-magnetic contribution estimated from the specific heat of non-magnetic ThNiSn (x = 1.0). For pure UNiSn (x = 0), we find a sharp peak at 43 K, which corresponds to the phase transition from paramagnetic semiconductor to AF metal. As Th
2. Experimental The samples used in this study were polycrystals prepared by arc-melting in a flowing argon atmosphere (see ref. [8) for details). Both resistivity and susceptibility have been reported on the same samples [8], although additional annealing was made at 800 0 C in vacuum for 30 days before the present work. The specific heat measurements were carried out from 1.2 to 80 K by using an adiabatic heat-pulse calorimeter mounted in a liquid helium cryostat.
120 100
E
80
b. ;
u
c: E
0..
60 40
Ut-xThxNi Sn
f\a
.:~,. '\:~
a xe 0
\
b x=0.2 h
c x=O.l. d x =0.6 e x = 1.0
\\
'\
\
20
3. Results and discussion T(K)
Shown in fig. 1 is the temperature dependence of the electrical resistivities for the (U1_xThx)NiSn com-
Fig . 1. Temperature dependence of electrical resistivity for (UI_xThx)NiSn.
0304-8853/90/$03.50 €> 1990 - Elsevier Science Publishers B.V. (North-Holland) and Yamada Science Foundation
Y. Aoki et 01.
I
Specific heat of (UJ _ xTh xJNiSII
1.0
08
~
::: 0.1
:J I
~ '";::
0.6
-
0.4
:.!: Y' -:
°0
.... U
f l v j0
1 ~
x:0 .2 xxx xxxxXX X
't'"ft'f:.ooeo::o8° 40 8l Tl I Kl I
e
02
u., Th.NiSn
••••
. :0 .4••• ••• .,...,.....
' -06 ')"'
.,'.• i
., X=o
••
:J;!\.
~ ~ 20
.~.
40 T (K)
~~:t'FltftOO'tP 60
60
Fig. 2. Temperature dependence of CmlT of the alloys (U1_ .•Thx)NiSn for x = 0,0.2,0.4 and 0.6. The inset illustrates Cml! against T 2 at low temperatures.
concentration x increases, the peak in CmlT substantially diminishes and shifts to lower temperatures. The shift of the peak in CmlT indicates decrease of the AF ordering temperature TN and the apparent diminution of the peak might indicate a spread-out distribution of T by partial substitution of Th for U. This distribution is expected to arise from inhomogeneity of U sites or from disorder of the Ni and Sn sites introduced by Th doping . We note that the temperature where CmlT has the peak is almost the same as that for the maximum temperature-derivative of peT). In addition to the peak at TN' we find a shoulder around 30 K for x = O. With Th doping, the shoulder appears to change into a hump shifting to a lower temperature around 20 and 10 K for x = 0.4 and 0.6, respectively. The origin of the hump is not clear at present. For ThNiSn (x = 1.0), a
497
linear dependence of CIT on T 2 at low temperatures gives y = 0.9 (mJ/K 2mol) per formula unit and a Debye temperature e D = 248 K. The inset of fig. 2 shows C IT versus T 2 for x ::5 0.6 below 10 K. The curves for m ~ 2 x::5 0.2 can be reasonably fitted to CmIT= y + uT with y = 20, 7 (mJ/K 2mol U), and 0 = 0.09, 0.57 (mJ/K 4mol U) for x = 0,0.2, respectively. For these U compounds, y may be attributed to the density-of-states enhanced at Fermi level due to Sf-electrons and 0 mainly to the magnon excitation in the AF state (the large values of 0 cannot be explained by variation of phonon contribution). The magnet ic entropy Sm per U ion, which is estimated from Cm, is shown in fig. 3. Sm tends to approach R In 4 at liquid nitrogen temperature independently of x. This suggests four nearly degenerate levels lying within a liquid-nitrogen temperature scale, which is relevant to the AF ordering. For pure UNiSn (x = 0), Sm tends to jump discontinuously at TN' suggesting a first order phase transition. This possibility has been pointed out also by Bykovetz et al. as an interpretation of their Mossbauer experiment [2]. In order to estimate the crystal electric field (CEF) contribution, we assume the valence of U to be 4 + rather than 3 + taking into consideration the ionic radii, because lattice parameters increase with Th doping [8]. The ground state of (5f)2 configuration of free U 4 + is 3 H 4 multiplet with 9 degeneracy. In a cubic CEF, however, the state splits into two triplets, one doublet and one singlet. There exists a region of CEF parametes where a triplet state and a singlet state are involved in the ground state (OS) and the first exited state (1ES) corre sponding to the value R In 4 of Sm' Moreover, the existence of the parameter region in which the level scheme has a singlet as and a triplet 1ES gives a possible explanation of the first order phase transition. However, what implication involved in AF and SM transitions is still unclear and further investigations are needed.
12 -------------------Rln4---------U,••Th.NiSn
::::>
References
I
-0
E
-. "" .....
6
e
VI
4
20
40
60
60
T (KI Fig . 3. Temperature dependence of magnetic entropy Sm for (UI_ xThx)NiSn.
(II T .T.M . Palstra, GJ. Nieuwenhuys, R.F.M . Vlastuin, J.A . Mydosh and K.H.J. Buschow, J. App\. Phys. 63 (1988) 4279 . [2) N. Bykovetz, W.N . Herman, T. Yuen, Chan-SOD Jee, C.L. Lin and J .E. Crow, J. App\. Phys. 63 (1989) 4127 . [3] R.C. Albers. A .M. Boring. G.H.O. Daalderop and F.M. Mueller, Phys. Rev. B 36 (1987) 3661. [4) H. Fujii, H . Kawanaka, T. Takabatake, M. Kurisu, Y. Yam aguch i, J. Sakurai, H. Fuj iwara, T. Fujita and I. Oguro, J. phy s. Soc. Jpn. 58 (1989) 2495.
498
Y. Aoki et al. / Specific heat
{5J M. Yethiraj, R.A. Robinson, J.J. Rhyne, J.A. Gotaas and K.H.J. Buschow, J. Magn, Magn. Mat. 79 (1989) 355. {6J H. Kawanaka, H. Fujii, M. Nishi, T. Takabatake, K. Motoya, Y. Uwatoko and Y. Ito, J. Phys, Soc. Jpn. 58 (1989) 3481.
of(~
_xThxJNiSn
[7] K. Takegahara and T. Kasuya, Solid State Commun., to be published. [8J H. Fujii, H. Kawanaka, T. Takabatake, E. Sugiura, K. Sugiyama and M. Date, J. Magn, Magri. Mat. 87 (1990) 235.
Journal of Magnetism and Magnetic Materials 90 & 91 (1990) 496-498 North- Holland
Specific heat of (U 1_:,ThJNiSn Y. Aoki, T. Suzuki, T. Fujita, H. Kawanaka
a,
T. Takabatake
and H. Fujii
a
a
Department of Physics. Faculty of Science. Hiroshima University. Hiroshima 730. Japan Faculty of Integrated Arts and Sciences, Hiroshima University, Hiroshima 730. Japan
a
The specific heat C(T) of the pseudo-binary alloys (UI_xThx)NiSn has been measured for x = 0, 0.2, 0.4. 0.6 and 1.0. An abrupt increase in magnetic entropy Sm at the unusual anti ferromagnetic transition temperature appears to indicate a first order phase transition. The value of Sm tends to approach R In 4 at liqu id N 2 temperature independently of x.
1. Introduction
Special attention has been paid to the anomalous phase transition in ternary uranium compound UNiSn, which crystallizes in the cubic MgAgAs-type structure [1-6]. As temperature is reduced, this compound exhibits a semiconductor to metal (SM) transition concurrently with a type-I antiferromagnetic (AF) ordering at TN = 43 K [6). Since an isostructural compound ThNiSn exhibits no sign of such transition, an indirect interaction between Sf-electrons of U ions is expected to play an essential role in the transition [7]. Evidently, the transition temperature TN decreases with Th substitution for U in UNiSn, as is confirmed by the temperature dependences of resistivity and magnetic susceptibility [8]. However, the interplay between the SM and AF transitions, which occur simultaneously in pure UNiSn, remains to be elucidated. In order to study a thermodynamical aspect of the anomalous transition, we carried out measurements of specific heat, C, for the pseudo-binary alloys (U1_.,Thx)NiSn with x = 0,0.2,0.4,0.6 and 1.0.
pounds, which are measured after the additional annealing. The behavior of p(T) for x = 0, 0.2 and 0.6 qualitatively agrees with the data reported in ref. [8} for the samples without the additional annealing. For x = 0.4, peT) of the present sample reduces to below ~ of the original value for all temperatures. For ThNiSn (x = 1.0), a metal- like behavior, which was seen at low temperatures before additional annealing, disappears after the annealing. The change is probably due to the ordering of the Ni and Sn atoms by annealing. However, the pronounced drop in p(T) itself and its shift to lower temperatures with Tit substitution are also observed in the present measurement. Figure.2 shows CmlT for (Ut_xThx)NiSn with x;;:::; 0.6 as a function of temperature T, where Cm is a magnetic part of the specific heat defined by Cm = (C Co)/(l - x), Co is a non-magnetic contribution estimated from the specific heat of non-magnetic ThNiSn (x = 1.0). For pure UNiSn (x = 0), we find a sharp peak at 43 K, which corresponds to the phase transition from paramagnetic semiconductor to AF metal. As Th
2. Experimental The samples used in this study were polycrystals prepared by arc-melting in a flowing argon atmosphere (see ref. [8) for details). Both resistivity and susceptibility have been reported on the same samples [8], although additional annealing was made at 800 0 C in vacuum for 30 days before the present work. The specific heat measurements were carried out from 1.2 to 80 K by using an adiabatic heat-pulse calorimeter mounted in a liquid helium cryostat.
120 100
E
80
b. ;
u
c: E
0..
60 40
Ut-xThxNi Sn
f\a
.:~,. '\:~
a xe 0
\
b x=0.2 h
c x=O.l. d x =0.6 e x = 1.0
\\
'\
\
20
3. Results and discussion T(K)
Shown in fig. 1 is the temperature dependence of the electrical resistivities for the (U1_xThx)NiSn com-
Fig . 1. Temperature dependence of electrical resistivity for (UI_xThx)NiSn.
0304-8853/90/$03.50 €> 1990 - Elsevier Science Publishers B.V. (North-Holland) and Yamada Science Foundation
Y. Aoki et 01.
I
Specific heat of (UJ _ xTh xJNiSII
1.0
08
~
::: 0.1
:J I
~ '";::
0.6
-
0.4
:.!: Y' -:
°0
.... U
f l v j0
1 ~
x:0 .2 xxx xxxxXX X
't'"ft'f:.ooeo::o8° 40 8l Tl I Kl I
e
02
u., Th.NiSn
••••
. :0 .4••• ••• .,...,.....
' -06 ')"'
.,'.• i
., X=o
••
:J;!\.
~ ~ 20
.~.
40 T (K)
~~:t'FltftOO'tP 60
60
Fig. 2. Temperature dependence of CmlT of the alloys (U1_ .•Thx)NiSn for x = 0,0.2,0.4 and 0.6. The inset illustrates Cml! against T 2 at low temperatures.
concentration x increases, the peak in CmlT substantially diminishes and shifts to lower temperatures. The shift of the peak in CmlT indicates decrease of the AF ordering temperature TN and the apparent diminution of the peak might indicate a spread-out distribution of T by partial substitution of Th for U. This distribution is expected to arise from inhomogeneity of U sites or from disorder of the Ni and Sn sites introduced by Th doping . We note that the temperature where CmlT has the peak is almost the same as that for the maximum temperature-derivative of peT). In addition to the peak at TN' we find a shoulder around 30 K for x = O. With Th doping, the shoulder appears to change into a hump shifting to a lower temperature around 20 and 10 K for x = 0.4 and 0.6, respectively. The origin of the hump is not clear at present. For ThNiSn (x = 1.0), a
497
linear dependence of CIT on T 2 at low temperatures gives y = 0.9 (mJ/K 2mol) per formula unit and a Debye temperature e D = 248 K. The inset of fig. 2 shows C IT versus T 2 for x ::5 0.6 below 10 K. The curves for m ~ 2 x::5 0.2 can be reasonably fitted to CmIT= y + uT with y = 20, 7 (mJ/K 2mol U), and 0 = 0.09, 0.57 (mJ/K 4mol U) for x = 0,0.2, respectively. For these U compounds, y may be attributed to the density-of-states enhanced at Fermi level due to Sf-electrons and 0 mainly to the magnon excitation in the AF state (the large values of 0 cannot be explained by variation of phonon contribution). The magnet ic entropy Sm per U ion, which is estimated from Cm, is shown in fig. 3. Sm tends to approach R In 4 at liquid nitrogen temperature independently of x. This suggests four nearly degenerate levels lying within a liquid-nitrogen temperature scale, which is relevant to the AF ordering. For pure UNiSn (x = 0), Sm tends to jump discontinuously at TN' suggesting a first order phase transition. This possibility has been pointed out also by Bykovetz et al. as an interpretation of their Mossbauer experiment [2]. In order to estimate the crystal electric field (CEF) contribution, we assume the valence of U to be 4 + rather than 3 + taking into consideration the ionic radii, because lattice parameters increase with Th doping [8]. The ground state of (5f)2 configuration of free U 4 + is 3 H 4 multiplet with 9 degeneracy. In a cubic CEF, however, the state splits into two triplets, one doublet and one singlet. There exists a region of CEF parametes where a triplet state and a singlet state are involved in the ground state (OS) and the first exited state (1ES) corre sponding to the value R In 4 of Sm' Moreover, the existence of the parameter region in which the level scheme has a singlet as and a triplet 1ES gives a possible explanation of the first order phase transition. However, what implication involved in AF and SM transitions is still unclear and further investigations are needed.
12 -------------------Rln4---------U,••Th.NiSn
::::>
References
I
-0
E
-. "" .....
6
e
VI
4
20
40
60
60
T (KI Fig . 3. Temperature dependence of magnetic entropy Sm for (UI_ xThx)NiSn.
(II T .T.M . Palstra, GJ. Nieuwenhuys, R.F.M . Vlastuin, J.A . Mydosh and K.H.J. Buschow, J. App\. Phys. 63 (1988) 4279 . [2) N. Bykovetz, W.N . Herman, T. Yuen, Chan-SOD Jee, C.L. Lin and J .E. Crow, J. App\. Phys. 63 (1989) 4127 . [3] R.C. Albers. A .M. Boring. G.H.O. Daalderop and F.M. Mueller, Phys. Rev. B 36 (1987) 3661. [4) H. Fujii, H . Kawanaka, T. Takabatake, M. Kurisu, Y. Yam aguch i, J. Sakurai, H. Fuj iwara, T. Fujita and I. Oguro, J. phy s. Soc. Jpn. 58 (1989) 2495.
498
Y. Aoki et al. / Specific heat
{5J M. Yethiraj, R.A. Robinson, J.J. Rhyne, J.A. Gotaas and K.H.J. Buschow, J. Magn, Magn. Mat. 79 (1989) 355. {6J H. Kawanaka, H. Fujii, M. Nishi, T. Takabatake, K. Motoya, Y. Uwatoko and Y. Ito, J. Phys, Soc. Jpn. 58 (1989) 3481.
of(~
_xThxJNiSn
[7] K. Takegahara and T. Kasuya, Solid State Commun., to be published. [8J H. Fujii, H. Kawanaka, T. Takabatake, E. Sugiura, K. Sugiyama and M. Date, J. Magn, Magri. Mat. 87 (1990) 235.