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Crystal structure analysis of the dense Kondo system CeSix
Journal of Magnetism and Magnetic Materials 90 & 91 (1990) 433-434 North-Holl and
433
Crystal structure analysis of the dense Kondo system CefSi, M. Kohgi, M. Ito, T. Satoh, H. Asano
a, T.
Ishigaki
a
and F. Izumi
b
Department of Physics, Faculty of Sci ence, Tohoku Unicersity, Sendai 980, Japan a Institute of Materials Science, Unicersity of Tsukuba, Tsukuba 305, Japan b Nat ional lnstitute for Research in In organic Mat erials, Tsukuba 305, Japan Crystal structure analysis of CeSi x by means of neutron powder. diffraction shows that the nearest Ce-Si di stance at low temperatures decreases v..ith increasing x in spite of a corresponding increase in the unit cell volume. The result is consistent with the fact that the degree of ferromagnetic order decreases under high pressure.
The intermetallic compo und CeSi x is a typical system which shows ja cross over from a nonmagnetic heavy electron to a ferromagnetically ordered state [1,2]. At room temperature, it crystallizes in the tetragonal a-ThSi 2 type structure (l4 t/amd) for 1.8::; x ::; 2, and in the orthorhombic a-GdSi 2 type structure (Irnma) for 1.6 < x < 1.8 [1,3,4]. For 1.85 < x s; 2.0, the compound shows heavy Fermion anomalies with no magnetic ordering down to 0.1 K. It becomes ferromagnetic for x < 1.85 with a Curie temperature of aroundTtl K. However, the ferromagnetism is also strongly affected by the Kondo effect. We have performed neutron diffraction experiments on this system in order to investigate the role of crystal structure in the competition between the Kondo effect and RKKY exchange interactions. The experiments were carried out on the high resolution powder diffractometer HRP at the pulsed neutron source KENS in KEK. Diffraction patterns of compounds with x = 1.68, 1.80, 1.82 and 1.90 were recorded at several temperatures below room temperature. The first three compounds were ferromagnetic with Curie temperatures of 13, 12 and 8 K, respectively. Crystallographic data obtained at room temperature and at the low temperatures are listed in table 1.
The structure of the compound was confirmed to be a-GdSi 2type for x = 1.68 and a-ThSi 2 type for x = 1.80, 1.82 and 1.90. It should be noted that CeSil.8o was found to be a-ThSi 2 type even at 7 K, which is below the Curie tempe rature. This fact indicates that the ferromagnetism in the CeSi, system is not strongly correlated with the small orthorhombic distortion which occurs at 1.7 < x <: 1.8. For CeSit.9' we found the coexistence of two tetragonal pha ses of a-ThSi 2 type which have the same a-axis lattice constant but with slightly diffe rent (about 1.5%) c-axis lattice constant at low temperatures. X-ray diffraction [5] revealed the same phenomena for CeSi l.8~ and CeSit.9' This work also showed .that the phase with the smaller c-axis lattice constant appears below about 200 K and the relative intensity of this phase to the other one increases with decreas ing temperature. We call the phases with shorter and longer c values the LT and HT phases, respectively, in the discussion below. The intensity ratio of the LT to HT phases was found to be 64: 36 for CeSit.9 at 20 K in our measurement. The similar phenomenon has also been observed in PrSi 2 [6]. Fig, l(a) shows the composition dependence of the
Table 1 Crystal data for CeSi x ' ZCe. Z5 i(l) and ZS i(2) are the atomic pos ition parameters of the elements along the c-axis. The numbers in parentheses ar e standard deviations of the last sign ificant digit
x
1.68±0.02
1.80±0.02
T
RT
50K
RT
a (A)
4.1250(13) 4.1984(13) 13.922(5) 0.1237(6) 0.5407(6) 0.7071( 5)
4.0995(4) 4.1777(4) 13.896(2) 0.1237(3) 0.5449(3) 0.7117(4)
b (A) c (A) zCe ZSi(l ) z5i(2)
0304 -8853/90/503.50
~
4.1740(8) 13.847(3) 0.415 7(1)
1.82±0.02 20 K 4.1497(4) 13.815(1) 0.4162(3)
RT 4.17 87(3) 13.839(1) 0.4165(2)
1.90±0.01 50 K 4.1551(3) 13.817(1) 0.4161(2)
RT 4.1918(6) 13.891(2) 0.4159(1)
20 K(L1) 4.1578(7) 13.843(2) 0.4148(1)
1990 - Elsevier Science Publi shers B.Y. (North-Holland] and Yamada Science Foundat ion
20 K(H1) 4.1578(7) 14.034(2) 0.416 3(2)
M. Kohgi et al. / Crystal structure analy sis of CcSi x
434 2~
RT 50K 20 K (x=1.8 & 1.9Li) 10 20 K (x=1.9Irn x RT(ref.3) + RT(ref.4)
0
•
2~
~ <>
E ::l "0
2~2
-.;
240 1-
x -
•
)(
+ X
'r: ::l
+ x+
+
-Ir-
~
-
--<:>--'
--e----
> u
cesi,
+
x
+ -+-
--
-+-
-+-
238 t-
(a)
236
3.25
1.70
1.65
1.75
1.80
1.85
1.90
1.95
CeSi,
•
o 50K 20K(x=1.8 & 1.9Li) 20 K (x=1.911T)
. "0 10 6 ~
'$
3.20
o
~
u
a c
---_.-.~
----- - --- ---- -~
3.15
:a ;;;
8
3.10
-----..
3.05
•
c
__
(b) 1.65
1.70
1.75
1.80
1.85
1.90
1.95
x
Fig. 1. (a) Un it cell volume at room temerature and at low temperatures; (b) near neighbor Ce-si distances at low temperatures vs. composition for alloys in the system CeSix-
unit cell volume at room temperature (open circles) and at the low temperatures (closed marks) . The volume at room temperature shows a gradual increase with increasing x , in good agreement with the previous work (crosses) [3,4] except for a slight difference at x = 1.68. At low temperatures, nearly the same temperature dependence was observed although the volume of the HT phase for CeSil. 9 (closed triangle) is rather large. The increase of the unit cell volume with increasing x has been a mystery. Shaheen et al. [3] showed that applying pressure to the ferromagnetic compounds causes a decrease in saturation momen ts and Curie temperatures. This is phenomenologically the same as the case where x increases. Since volume contraction is naturally expected under hydrostatic pressure, the fact that the volume increases with increasing x would contradict to
the pressure effect if the ferromagnetic instability in this system were strongly correlated with the unit cell volume. In Fig. l(b), we show the composition dependence of the atomic distances between a Ce atom and its 12 nearest neighbor Si atoms at low temperatures (20 and 50 K). For x ~ 1.8, they are grouped into two values which come from the fourfold and eightfold tetragonal coordination of Si atoms around a Ce atom. For CeSi1.68 , they split into two values for each coordination because of the lower point symmetry in the a-GdSi 2 structure, consequently we take the average value as the distance to the Si atoms for each coordination in the discussion below. For the CeSil. 9 case, the values for both LT and HT phases arc shown by circles and triangles , respectively. One can see that the distance from the Ce atom to the Si atoms with fourfold coordination (closed marks) is shorter than that to the Si atoms with eightfold coordination (open marks) and that, if the HT phase of CeSil. 9 is disregarded, it decreases with increasing x, whereas the latter increases with increasing x. This fact may reconcile the contradiction stated above becau se a decrease of the nearest Ce-Si atomic distance is also expected und er hydrostatic pres sure. Therefore, the present experiments suggest that the strength of the c-f hybridization in this system is correlated with the distance betwe en nearest Ce and Si atoms rather than with the - unit cell volume. It is not clear, however, whether we can safely disregard the role of the HT phase for compounds with x near 1.9 in the above discussion. We speculate that the LT ph ase is the one which is extended from the compounds with lower x values, since the LT phase becomes dominant at low temperatures and its lattice parameters are a natural extension of the values of the lower x compounds. References
Il] H. Yashima and T. Satoh, Solid State Commun. 41 (1982) 723. [2] H. Yashima, H. Mori, T. Satoh and K. Kohn, Solid State Commun. 43 (1982) 193. H . Yashima, N. Sato, H. Mori and T. Satoh, ibid, 595. [3] SA Shaheen and J.S. Schilling, Phys. Rev. D 35 (1987) 6880. (4) \V.H . Lee, R.N . Shelton, S.K. Dhar and K.A. Gschne idner, Jr. , Phys. Rev. D 35 (1987) 8523. [5) Y. Mur ashit a. J. Sakurai and T. Satoh, J. Phys. Soc. Jpn. (submitted). [6] \V.H. Dijkman, Thesis, Unive rsity of Amsterdam (1982); we thank J. Pierre for informing us about this paper.
433
Crystal structure analysis of the dense Kondo system CefSi, M. Kohgi, M. Ito, T. Satoh, H. Asano
a, T.
Ishigaki
a
and F. Izumi
b
Department of Physics, Faculty of Sci ence, Tohoku Unicersity, Sendai 980, Japan a Institute of Materials Science, Unicersity of Tsukuba, Tsukuba 305, Japan b Nat ional lnstitute for Research in In organic Mat erials, Tsukuba 305, Japan Crystal structure analysis of CeSi x by means of neutron powder. diffraction shows that the nearest Ce-Si di stance at low temperatures decreases v..ith increasing x in spite of a corresponding increase in the unit cell volume. The result is consistent with the fact that the degree of ferromagnetic order decreases under high pressure.
The intermetallic compo und CeSi x is a typical system which shows ja cross over from a nonmagnetic heavy electron to a ferromagnetically ordered state [1,2]. At room temperature, it crystallizes in the tetragonal a-ThSi 2 type structure (l4 t/amd) for 1.8::; x ::; 2, and in the orthorhombic a-GdSi 2 type structure (Irnma) for 1.6 < x < 1.8 [1,3,4]. For 1.85 < x s; 2.0, the compound shows heavy Fermion anomalies with no magnetic ordering down to 0.1 K. It becomes ferromagnetic for x < 1.85 with a Curie temperature of aroundTtl K. However, the ferromagnetism is also strongly affected by the Kondo effect. We have performed neutron diffraction experiments on this system in order to investigate the role of crystal structure in the competition between the Kondo effect and RKKY exchange interactions. The experiments were carried out on the high resolution powder diffractometer HRP at the pulsed neutron source KENS in KEK. Diffraction patterns of compounds with x = 1.68, 1.80, 1.82 and 1.90 were recorded at several temperatures below room temperature. The first three compounds were ferromagnetic with Curie temperatures of 13, 12 and 8 K, respectively. Crystallographic data obtained at room temperature and at the low temperatures are listed in table 1.
The structure of the compound was confirmed to be a-GdSi 2type for x = 1.68 and a-ThSi 2 type for x = 1.80, 1.82 and 1.90. It should be noted that CeSil.8o was found to be a-ThSi 2 type even at 7 K, which is below the Curie tempe rature. This fact indicates that the ferromagnetism in the CeSi, system is not strongly correlated with the small orthorhombic distortion which occurs at 1.7 < x <: 1.8. For CeSit.9' we found the coexistence of two tetragonal pha ses of a-ThSi 2 type which have the same a-axis lattice constant but with slightly diffe rent (about 1.5%) c-axis lattice constant at low temperatures. X-ray diffraction [5] revealed the same phenomena for CeSi l.8~ and CeSit.9' This work also showed .that the phase with the smaller c-axis lattice constant appears below about 200 K and the relative intensity of this phase to the other one increases with decreas ing temperature. We call the phases with shorter and longer c values the LT and HT phases, respectively, in the discussion below. The intensity ratio of the LT to HT phases was found to be 64: 36 for CeSit.9 at 20 K in our measurement. The similar phenomenon has also been observed in PrSi 2 [6]. Fig, l(a) shows the composition dependence of the
Table 1 Crystal data for CeSi x ' ZCe. Z5 i(l) and ZS i(2) are the atomic pos ition parameters of the elements along the c-axis. The numbers in parentheses ar e standard deviations of the last sign ificant digit
x
1.68±0.02
1.80±0.02
T
RT
50K
RT
a (A)
4.1250(13) 4.1984(13) 13.922(5) 0.1237(6) 0.5407(6) 0.7071( 5)
4.0995(4) 4.1777(4) 13.896(2) 0.1237(3) 0.5449(3) 0.7117(4)
b (A) c (A) zCe ZSi(l ) z5i(2)
0304 -8853/90/503.50
~
4.1740(8) 13.847(3) 0.415 7(1)
1.82±0.02 20 K 4.1497(4) 13.815(1) 0.4162(3)
RT 4.17 87(3) 13.839(1) 0.4165(2)
1.90±0.01 50 K 4.1551(3) 13.817(1) 0.4161(2)
RT 4.1918(6) 13.891(2) 0.4159(1)
20 K(L1) 4.1578(7) 13.843(2) 0.4148(1)
1990 - Elsevier Science Publi shers B.Y. (North-Holland] and Yamada Science Foundat ion
20 K(H1) 4.1578(7) 14.034(2) 0.416 3(2)
M. Kohgi et al. / Crystal structure analy sis of CcSi x
434 2~
RT 50K 20 K (x=1.8 & 1.9Li) 10 20 K (x=1.9Irn x RT(ref.3) + RT(ref.4)
0
•
2~
~ <>
E ::l "0
2~2
-.;
240 1-
x -
•
)(
+ X
'r: ::l
+ x+
+
-Ir-
~
-
--<:>--'
--e----
> u
cesi,
+
x
+ -+-
--
-+-
-+-
238 t-
(a)
236
3.25
1.70
1.65
1.75
1.80
1.85
1.90
1.95
CeSi,
•
o 50K 20K(x=1.8 & 1.9Li) 20 K (x=1.911T)
. "0 10 6 ~
'$
3.20
o
~
u
a c
---_.-.~
----- - --- ---- -~
3.15
:a ;;;
8
3.10
-----..
3.05
•
c
__
(b) 1.65
1.70
1.75
1.80
1.85
1.90
1.95
x
Fig. 1. (a) Un it cell volume at room temerature and at low temperatures; (b) near neighbor Ce-si distances at low temperatures vs. composition for alloys in the system CeSix-
unit cell volume at room temperature (open circles) and at the low temperatures (closed marks) . The volume at room temperature shows a gradual increase with increasing x , in good agreement with the previous work (crosses) [3,4] except for a slight difference at x = 1.68. At low temperatures, nearly the same temperature dependence was observed although the volume of the HT phase for CeSil. 9 (closed triangle) is rather large. The increase of the unit cell volume with increasing x has been a mystery. Shaheen et al. [3] showed that applying pressure to the ferromagnetic compounds causes a decrease in saturation momen ts and Curie temperatures. This is phenomenologically the same as the case where x increases. Since volume contraction is naturally expected under hydrostatic pressure, the fact that the volume increases with increasing x would contradict to
the pressure effect if the ferromagnetic instability in this system were strongly correlated with the unit cell volume. In Fig. l(b), we show the composition dependence of the atomic distances between a Ce atom and its 12 nearest neighbor Si atoms at low temperatures (20 and 50 K). For x ~ 1.8, they are grouped into two values which come from the fourfold and eightfold tetragonal coordination of Si atoms around a Ce atom. For CeSi1.68 , they split into two values for each coordination because of the lower point symmetry in the a-GdSi 2 structure, consequently we take the average value as the distance to the Si atoms for each coordination in the discussion below. For the CeSil. 9 case, the values for both LT and HT phases arc shown by circles and triangles , respectively. One can see that the distance from the Ce atom to the Si atoms with fourfold coordination (closed marks) is shorter than that to the Si atoms with eightfold coordination (open marks) and that, if the HT phase of CeSil. 9 is disregarded, it decreases with increasing x, whereas the latter increases with increasing x. This fact may reconcile the contradiction stated above becau se a decrease of the nearest Ce-Si atomic distance is also expected und er hydrostatic pres sure. Therefore, the present experiments suggest that the strength of the c-f hybridization in this system is correlated with the distance betwe en nearest Ce and Si atoms rather than with the - unit cell volume. It is not clear, however, whether we can safely disregard the role of the HT phase for compounds with x near 1.9 in the above discussion. We speculate that the LT ph ase is the one which is extended from the compounds with lower x values, since the LT phase becomes dominant at low temperatures and its lattice parameters are a natural extension of the values of the lower x compounds. References
Il] H. Yashima and T. Satoh, Solid State Commun. 41 (1982) 723. [2] H. Yashima, H. Mori, T. Satoh and K. Kohn, Solid State Commun. 43 (1982) 193. H . Yashima, N. Sato, H. Mori and T. Satoh, ibid, 595. [3] SA Shaheen and J.S. Schilling, Phys. Rev. D 35 (1987) 6880. (4) \V.H . Lee, R.N . Shelton, S.K. Dhar and K.A. Gschne idner, Jr. , Phys. Rev. D 35 (1987) 8523. [5) Y. Mur ashit a. J. Sakurai and T. Satoh, J. Phys. Soc. Jpn. (submitted). [6] \V.H. Dijkman, Thesis, Unive rsity of Amsterdam (1982); we thank J. Pierre for informing us about this paper.