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Crystal structure and the magnetic properties of the compound CeCoAl4
Jouarnll of
AU.C~ A N D COMPOUNDS ELSEVIER
Journal of Alloys and Compounds 256 (1997) 45-47
Crystal structure and the magnetic properties of the compound CeCoA14 a*
O . M o z e ' , L . D . T u n g b, J . L M . F r a n s e b, K . H . J . B u s c h o w b "Dipartimento di Fisica. Universita di Parma and Istituto Nazionale per la Fisica della Materia. Unit~z di Parma. Viale delle Scienze, 43100 Parma, haly ~Van der Waals-Zeeman Laboratory. University of Amsterdam Valckenierstraat 65. 1018 XE Amsterdam. The Netherlands
Received 16 October 1996: accepted 20 November 1996
Abstract The crystal structure of the compound CeCoAI 4 has been studied by time-of flight neutron diffraction at room temperature. This compound has the orthorhombic LaCoAl~ type of structure charactedsed by a single rare-earth site and four non-rare earth sites. The neutron diffraction data show that the compound is a tree ternary compound in whlch the Co atom occupies exclusively only one of these sites. From magnetic measurements it is derived that Ce is trivalent. The compound orders antiferromagnetically at TN= 13 K. The field dependence of the magnetisation at 4.2 K shows a spin-flop transition at about 8 T. © 1997 Elsevier Science S.A. Keywords: Cerium cobalt aluminide; Crystal structure; Magnetic properties: Neutron diffraction
1. Introduction In many intermetallic compounds of cerium, the latter atom adopts an unstable 4f shell which can lead to anomalies in the temperature dependence of the magnetic susceptibility and the electrical transport properties. Intuitively, one expects that the stability of the 4f-shell of Ce depends strongly on the extent to wh;,:h the Ce electron states hybridise with the electron states of the atoms surrounding it. This means that the physical properties mentioned depend not only on the nature of the various elements with which Ce is combined in a given crystal structure but also on the crystal structure itself and on the extent to which the Ce atoms are surrounded by atoms of the composing elements. In the present investigation we have: extended previous structural investigations of ternary Ce compounds [ 1,2] to the crystal structure and magnetic properties of the compound CeCoAI 4 in which the 3d transition element is present as minority component and where Ce is primarily surrounded by AI atoms.
2. Experimental The CeCoAI 4 compound was prepared by arc melting starting materials of at least 99% purity. After arc melting, the sample was wrapped into Ta foil and vacuum annealed *Corresponding author. 0925-8388/97/$17.00 © 1997 Elsevier Science S.A. All fights reserved PII S0925-8388(96)03103-9
inside an evacuated quartz tube at 600°(2 for several weeks. The annealed sample was investigated by X-ray diffraction. The X-ray pattern was mostly single phase, and w,,s iiidexcd according to the orthorhombic LaCoAi 4 type of structure [3]. The neutron powder diffraction experiments were made on the POLARIS high-intensity powder diffractometer, a facility present at the ISIS spallation neutron source [4]. Data was collected in the backscattering mode ( 2 0 = 1 4 5 deg.) over the entire 20 msec time frame between ISIS pulses, providing a range of accessed d-spacings from 0,4 to 3 ,A. The instrumental resolution, Adld=0.005, is constant over this range. The diffraction data was collected at 295 K, at which temperature virtually only nuclear scattering is present, because it follows from magnetic measurements made on these compounds that magnetic ordering is absent above 13 K (see below). The magne,,c measurements were made on a SQUID magnetometer in the temperature range 5-300 K in magnetic fields up to 5 T. The magnetic isotherm at 4.2 K was measured at the high-field facility of the University of Nijmegen on powder particles free to orient themselves into their equilibrium position during application of the field.
3. Neutron diffraction data analysis We analysed the neutron diffraction data collected on the POLARIS diffractometer by the Rietveld technique [5],
46
O. Mo.Te et al. I Journal of AIIo.vs and Compounds 256 (1997) 4 5 - 4 7
iI
4. Experimental results and discussion
5~I/IHRII|UI[[III .10~ oi~ , ;~ o~
,
IIIIIlli[llllll I l l II ]il t~ 7g
TOF,microseconds
[
~-~ xI04
FiE. I. Ob~rved, calculated and difference neutron time-of flight diffraction pattern eft the compound CeCoAI4 at 293 K. The verticalbars indicate calculated peak positions.
using the program TFISLS based on the Cambridge Crystallography Subroutine Library, CCSL [6]. This program is able to, least-squares refinement of time-of flight neutron powder diffraction data. It is based on a peakshape function that is a convolution of a modified lkedaCarpenter iineshape and a Voigt function [7]. The appropriam nuclear scattering lengths employed in the refinement (bc~=0.4g4Xl0 -12 cm, bco=0.2490×10 -12 cm, bat=0.3449× !0 -12 cm) were taken from the most up-todate tabulation available [8]. The oriborhombic LaCoAI 4 structure (space group Pmma) was used as trial structure, with the assumption of a statistical distribution of the AI and C o atoms over the 2e, 2a 2f and 4j sites, and the additional constraint that the overall composition be fixed by the nominal composition. The AI position at 2a was chosen as origin of the unit cell. Consequently, only the following parameters were refined: unit cell constants, individual isotmpic temperature factors for all sites, relative site populations of the AI and C o atoms over the 2e, 2a 2f and 4j sites, a scale factor, and peak profile and background parameters. A total of I 118 overlapping and independent reflections were analysed.
The observed and calculated diffraction patterns for the compound investigated is displayed in Fig. I. The access to relatively small d-spacings implies a particularly accurate assessment of the thermal parameters. The refined structural, thermal and R factors are listed in Table i. It follows from the data listed in the table that there is a very strong preference for the Co atoms to occupy only one of the four available sites These results confirm conclusions reached earlier by means of X-ray diffraction [3] and show that the compound CeCoAI 4 can actually be classified as a true ternary compound. The occupation numbers of the refinement suggest that there is virtually no site interchange between Co and Al atoms. Results of magnetic measurements are displayed in Figs. 2 and 3. From the temperature dependence of the reciprocal susceptibility, shown in Fig. 2, it can be derived that CeCoAI 4 order:; antiferromagnetically below TN=I3 K. Curie-Weiss behaviour is followed above the magnetic ordering temperature, the corresponding asymptotic Curie temperature being very small ( ~ p = - 3 . 1 K). The effective moment derived from the slope of the reciprocal susceptibility equals 2.19 / ~ per formula unit, which is somewhat smaller than the value 2.54 /za/Ce expected for trivalent cerium atoms. The field dependence of the magnefisation at 4.2 K is shown in Fig. 3. The linear behaviour of the field dependence in the low-field part of the isotherm confinns the antiferromagnetic nature of the magnetic ordering. For fields higher than about 8 T the amiferromagnetism is broken via a spin-flop transition. At the highest field strength the magnetic moment is still far below the free-ion value 2.14 / h , expected for Ce s÷ moments aligned parallel to the field direction. It is well known that the presence of atomic site disorder in intermetallic compounds can seriously affect their magnetic properties because it leads to a distribution of different coordinations of the magnetic atoms. This, in turn, can lead to a distribution in local exchange fields able to prevent long-range magnetic order in the worst case. In the second place, the distribution of different coordinations can result in a distribution of local crystal fields. As a result, local moments and the crystal field induced mag-
Table 1 Atomic position parameters and site occupation of the compound CeCoAI4
(A :)
Site
Atom
xla
ylb
zlc
B
2e 2e
Ce Cr,
0.25000 0.25000
0.00000 0.00000
0.38241(17) 0.81588(33)
0.399(24) 0.228(38)
1.00( I ) 1.00(2)
2a 2f 4j
Al AI AI
0.00000 0.25000 0.06955(14)
0.00000 0.50000 0.50000
0.00000 0.04315(22) 0.70183(17)
0.724(35) 0.543(37) 0.519(23)
1.00(I) 1.00(I) 1.00(I)
Pmma; a=0.766024 nm. b=0.405616 nm: :=0691355 nm: R,r=2.1%; R~,r=1.23%; X:=2.91.
Occ.
47
O. Moze et al. I Journal of Alloys and Compounds 256 (1997) 4.5-47 16 '
L
oo
i
[
/
C~OAI-
i
.-"
1,00 F-
t
J
0.8o,
C6COAI4 4.2K
.~
c ~
i
o o ° °
o.~--
$ g"
0.40.
'I~
i
[ _ _.
L
100
200
•
i
i o,-'a
•
1
." 000
300
J 0
_. 4
Temperature(K)
!
i 8
12
16
2O
Field 0")
Fig. 2, temperature dependenceof the reciprocal magnetic sumeptibility for the compound CeCoAI.. measured in I T.
Fig. 3. Field dependence of the magnetic moment in the compound CeCoAI~ at 4.2 K.
netocrystalline anisotropy can become locally different. This. in turn, affects the local exchange interactions and reinforces the first effect in preventing long-range magnetic order. Even when local differences in exchange fields and crystal fields are not large enough to suppress longrange magnetic order, they can affect the sharpness of the
close above it. The lack of saturation can conveniently be explained in such cases by field induced level crossing. Further experiments including measurements of the electrical resistivity, the specific heat and neutron diffraction in the magnetically ordered regime will currently be undertaken.
magnetic ordering transition. Inspection of the data listed in Table I shows that the atomic site disorder is virtually absent in the compound CeCoAI 4. This is in concord with the absence of smearing out effects in the magnetic ordering transition observed in the temperature dependence of the magnetic susceptibility (Fig. 2) and also with the relative sharpness of the spinflop transition observed in the field dependence of the magnetisation (Fig. 3). Finally, we wish to discuss the comparatively low value of the magnetisation reached at 4.2 K after the spin-flop transition and the lack of saturation in the highfield region. We mentioned already that a value of 2 . 1 4 / ~ would be expected for Ce ~÷ moments aligned parallel to the field direction when crystal field splitting of the 2 J + I ground manifold is either absent altogether or when crystal field splitting leads to a [-+5/2> ground state. Apparently this situation does not apply to CeCoAi 4. More likely is a situation in which crystal field splitting has led to a crystal field split ground state, consisting primarily of the [+_3/2> doublet state. The corresponding moment would then be equal to 1.3 P-n per Ce atom, which is close to the value found in the highest field in Fig. 3. Another possibility is a [-+ l / 2 > ground state with the [ + 3 1 2 > doublet state very
Acknowledgments The authors wish to thank R.M. lbberson for assistance with the neutron diffraction experiments. Provision of neutron facilities by DRAL (Daresbury and Rutherford Laboratories) is gratefully acknowledged. This research has been supported by the Dutch Technology Foundation
(STW). References [I] O. Moze and K.H.J. Bu.~how, J. Alloys Cutup,, 235 (1996) 62. [2] O. Moz¢, S.A.M. Mentink.GJ. Nieuwenhuysand K.HJ. Bu~how, J. Magn. Magn. Mater.. 1.5G(1995) 345-348. [3] R.M. Rykhal. O.S. Zarechnyukand Ya.P.Yarmolyuk. Dopov. Akad. Nauk. Ukr. RSR. Ser. A. (19'~7) 265, [41 S. Hull and J. Mayers, Rep,wt 891118, 1989, (Rutherford Appleton Laboratory). [5] H.M. Rietveld, J. Appl. Cryst.. 2 (1969) 65. [6] P.J. Brownand J.C. Matthewman.Report 851112, 1985o(Rutherford Appleton Laboratory). [7] W.I.F. David, S. Hull and R.M. lbberson, Report 901000, 19gO, (Rutherford Appleton Laboratory). [8] V.F. Sears, Neutron scatteringlengths and cross sections, in Neutron Ne~w. Vol. 3, Gordon and Breach Science Publishers, 1992, p. 26.
AU.C~ A N D COMPOUNDS ELSEVIER
Journal of Alloys and Compounds 256 (1997) 45-47
Crystal structure and the magnetic properties of the compound CeCoA14 a*
O . M o z e ' , L . D . T u n g b, J . L M . F r a n s e b, K . H . J . B u s c h o w b "Dipartimento di Fisica. Universita di Parma and Istituto Nazionale per la Fisica della Materia. Unit~z di Parma. Viale delle Scienze, 43100 Parma, haly ~Van der Waals-Zeeman Laboratory. University of Amsterdam Valckenierstraat 65. 1018 XE Amsterdam. The Netherlands
Received 16 October 1996: accepted 20 November 1996
Abstract The crystal structure of the compound CeCoAI 4 has been studied by time-of flight neutron diffraction at room temperature. This compound has the orthorhombic LaCoAl~ type of structure charactedsed by a single rare-earth site and four non-rare earth sites. The neutron diffraction data show that the compound is a tree ternary compound in whlch the Co atom occupies exclusively only one of these sites. From magnetic measurements it is derived that Ce is trivalent. The compound orders antiferromagnetically at TN= 13 K. The field dependence of the magnetisation at 4.2 K shows a spin-flop transition at about 8 T. © 1997 Elsevier Science S.A. Keywords: Cerium cobalt aluminide; Crystal structure; Magnetic properties: Neutron diffraction
1. Introduction In many intermetallic compounds of cerium, the latter atom adopts an unstable 4f shell which can lead to anomalies in the temperature dependence of the magnetic susceptibility and the electrical transport properties. Intuitively, one expects that the stability of the 4f-shell of Ce depends strongly on the extent to wh;,:h the Ce electron states hybridise with the electron states of the atoms surrounding it. This means that the physical properties mentioned depend not only on the nature of the various elements with which Ce is combined in a given crystal structure but also on the crystal structure itself and on the extent to which the Ce atoms are surrounded by atoms of the composing elements. In the present investigation we have: extended previous structural investigations of ternary Ce compounds [ 1,2] to the crystal structure and magnetic properties of the compound CeCoAI 4 in which the 3d transition element is present as minority component and where Ce is primarily surrounded by AI atoms.
2. Experimental The CeCoAI 4 compound was prepared by arc melting starting materials of at least 99% purity. After arc melting, the sample was wrapped into Ta foil and vacuum annealed *Corresponding author. 0925-8388/97/$17.00 © 1997 Elsevier Science S.A. All fights reserved PII S0925-8388(96)03103-9
inside an evacuated quartz tube at 600°(2 for several weeks. The annealed sample was investigated by X-ray diffraction. The X-ray pattern was mostly single phase, and w,,s iiidexcd according to the orthorhombic LaCoAi 4 type of structure [3]. The neutron powder diffraction experiments were made on the POLARIS high-intensity powder diffractometer, a facility present at the ISIS spallation neutron source [4]. Data was collected in the backscattering mode ( 2 0 = 1 4 5 deg.) over the entire 20 msec time frame between ISIS pulses, providing a range of accessed d-spacings from 0,4 to 3 ,A. The instrumental resolution, Adld=0.005, is constant over this range. The diffraction data was collected at 295 K, at which temperature virtually only nuclear scattering is present, because it follows from magnetic measurements made on these compounds that magnetic ordering is absent above 13 K (see below). The magne,,c measurements were made on a SQUID magnetometer in the temperature range 5-300 K in magnetic fields up to 5 T. The magnetic isotherm at 4.2 K was measured at the high-field facility of the University of Nijmegen on powder particles free to orient themselves into their equilibrium position during application of the field.
3. Neutron diffraction data analysis We analysed the neutron diffraction data collected on the POLARIS diffractometer by the Rietveld technique [5],
46
O. Mo.Te et al. I Journal of AIIo.vs and Compounds 256 (1997) 4 5 - 4 7
iI
4. Experimental results and discussion
5~I/IHRII|UI[[III .10~ oi~ , ;~ o~
,
IIIIIlli[llllll I l l II ]il t~ 7g
TOF,microseconds
[
~-~ xI04
FiE. I. Ob~rved, calculated and difference neutron time-of flight diffraction pattern eft the compound CeCoAI4 at 293 K. The verticalbars indicate calculated peak positions.
using the program TFISLS based on the Cambridge Crystallography Subroutine Library, CCSL [6]. This program is able to, least-squares refinement of time-of flight neutron powder diffraction data. It is based on a peakshape function that is a convolution of a modified lkedaCarpenter iineshape and a Voigt function [7]. The appropriam nuclear scattering lengths employed in the refinement (bc~=0.4g4Xl0 -12 cm, bco=0.2490×10 -12 cm, bat=0.3449× !0 -12 cm) were taken from the most up-todate tabulation available [8]. The oriborhombic LaCoAI 4 structure (space group Pmma) was used as trial structure, with the assumption of a statistical distribution of the AI and C o atoms over the 2e, 2a 2f and 4j sites, and the additional constraint that the overall composition be fixed by the nominal composition. The AI position at 2a was chosen as origin of the unit cell. Consequently, only the following parameters were refined: unit cell constants, individual isotmpic temperature factors for all sites, relative site populations of the AI and C o atoms over the 2e, 2a 2f and 4j sites, a scale factor, and peak profile and background parameters. A total of I 118 overlapping and independent reflections were analysed.
The observed and calculated diffraction patterns for the compound investigated is displayed in Fig. I. The access to relatively small d-spacings implies a particularly accurate assessment of the thermal parameters. The refined structural, thermal and R factors are listed in Table i. It follows from the data listed in the table that there is a very strong preference for the Co atoms to occupy only one of the four available sites These results confirm conclusions reached earlier by means of X-ray diffraction [3] and show that the compound CeCoAI 4 can actually be classified as a true ternary compound. The occupation numbers of the refinement suggest that there is virtually no site interchange between Co and Al atoms. Results of magnetic measurements are displayed in Figs. 2 and 3. From the temperature dependence of the reciprocal susceptibility, shown in Fig. 2, it can be derived that CeCoAI 4 order:; antiferromagnetically below TN=I3 K. Curie-Weiss behaviour is followed above the magnetic ordering temperature, the corresponding asymptotic Curie temperature being very small ( ~ p = - 3 . 1 K). The effective moment derived from the slope of the reciprocal susceptibility equals 2.19 / ~ per formula unit, which is somewhat smaller than the value 2.54 /za/Ce expected for trivalent cerium atoms. The field dependence of the magnefisation at 4.2 K is shown in Fig. 3. The linear behaviour of the field dependence in the low-field part of the isotherm confinns the antiferromagnetic nature of the magnetic ordering. For fields higher than about 8 T the amiferromagnetism is broken via a spin-flop transition. At the highest field strength the magnetic moment is still far below the free-ion value 2.14 / h , expected for Ce s÷ moments aligned parallel to the field direction. It is well known that the presence of atomic site disorder in intermetallic compounds can seriously affect their magnetic properties because it leads to a distribution of different coordinations of the magnetic atoms. This, in turn, can lead to a distribution in local exchange fields able to prevent long-range magnetic order in the worst case. In the second place, the distribution of different coordinations can result in a distribution of local crystal fields. As a result, local moments and the crystal field induced mag-
Table 1 Atomic position parameters and site occupation of the compound CeCoAI4
(A :)
Site
Atom
xla
ylb
zlc
B
2e 2e
Ce Cr,
0.25000 0.25000
0.00000 0.00000
0.38241(17) 0.81588(33)
0.399(24) 0.228(38)
1.00( I ) 1.00(2)
2a 2f 4j
Al AI AI
0.00000 0.25000 0.06955(14)
0.00000 0.50000 0.50000
0.00000 0.04315(22) 0.70183(17)
0.724(35) 0.543(37) 0.519(23)
1.00(I) 1.00(I) 1.00(I)
Pmma; a=0.766024 nm. b=0.405616 nm: :=0691355 nm: R,r=2.1%; R~,r=1.23%; X:=2.91.
Occ.
47
O. Moze et al. I Journal of Alloys and Compounds 256 (1997) 4.5-47 16 '
L
oo
i
[
/
C~OAI-
i
.-"
1,00 F-
t
J
0.8o,
C6COAI4 4.2K
.~
c ~
i
o o ° °
o.~--
$ g"
0.40.
'I~
i
[ _ _.
L
100
200
•
i
i o,-'a
•
1
." 000
300
J 0
_. 4
Temperature(K)
!
i 8
12
16
2O
Field 0")
Fig. 2, temperature dependenceof the reciprocal magnetic sumeptibility for the compound CeCoAI.. measured in I T.
Fig. 3. Field dependence of the magnetic moment in the compound CeCoAI~ at 4.2 K.
netocrystalline anisotropy can become locally different. This. in turn, affects the local exchange interactions and reinforces the first effect in preventing long-range magnetic order. Even when local differences in exchange fields and crystal fields are not large enough to suppress longrange magnetic order, they can affect the sharpness of the
close above it. The lack of saturation can conveniently be explained in such cases by field induced level crossing. Further experiments including measurements of the electrical resistivity, the specific heat and neutron diffraction in the magnetically ordered regime will currently be undertaken.
magnetic ordering transition. Inspection of the data listed in Table I shows that the atomic site disorder is virtually absent in the compound CeCoAI 4. This is in concord with the absence of smearing out effects in the magnetic ordering transition observed in the temperature dependence of the magnetic susceptibility (Fig. 2) and also with the relative sharpness of the spinflop transition observed in the field dependence of the magnetisation (Fig. 3). Finally, we wish to discuss the comparatively low value of the magnetisation reached at 4.2 K after the spin-flop transition and the lack of saturation in the highfield region. We mentioned already that a value of 2 . 1 4 / ~ would be expected for Ce ~÷ moments aligned parallel to the field direction when crystal field splitting of the 2 J + I ground manifold is either absent altogether or when crystal field splitting leads to a [-+5/2> ground state. Apparently this situation does not apply to CeCoAi 4. More likely is a situation in which crystal field splitting has led to a crystal field split ground state, consisting primarily of the [+_3/2> doublet state. The corresponding moment would then be equal to 1.3 P-n per Ce atom, which is close to the value found in the highest field in Fig. 3. Another possibility is a [-+ l / 2 > ground state with the [ + 3 1 2 > doublet state very
Acknowledgments The authors wish to thank R.M. lbberson for assistance with the neutron diffraction experiments. Provision of neutron facilities by DRAL (Daresbury and Rutherford Laboratories) is gratefully acknowledged. This research has been supported by the Dutch Technology Foundation
(STW). References [I] O. Moze and K.H.J. Bu.~how, J. Alloys Cutup,, 235 (1996) 62. [2] O. Moz¢, S.A.M. Mentink.GJ. Nieuwenhuysand K.HJ. Bu~how, J. Magn. Magn. Mater.. 1.5G(1995) 345-348. [3] R.M. Rykhal. O.S. Zarechnyukand Ya.P.Yarmolyuk. Dopov. Akad. Nauk. Ukr. RSR. Ser. A. (19'~7) 265, [41 S. Hull and J. Mayers, Rep,wt 891118, 1989, (Rutherford Appleton Laboratory). [5] H.M. Rietveld, J. Appl. Cryst.. 2 (1969) 65. [6] P.J. Brownand J.C. Matthewman.Report 851112, 1985o(Rutherford Appleton Laboratory). [7] W.I.F. David, S. Hull and R.M. lbberson, Report 901000, 19gO, (Rutherford Appleton Laboratory). [8] V.F. Sears, Neutron scatteringlengths and cross sections, in Neutron Ne~w. Vol. 3, Gordon and Breach Science Publishers, 1992, p. 26.