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Investigations of the magnetic properties of CeRh2Si2 using lattice parameter, LIII edge and extended X-ray absorption fine structure measurements
Journal
of the Less-Common
Metals,
187
94 (1983) 187-193
INVESTIGATIONS OF THE MAGNETIC PROPERTIES OF CeRh,Si, USING LATTICE PARAMETER, L,,, EDGE AND EXTENDED X-RAY ABSORPTION FINE STRUCTURE MEASUREMENTS*
C. GODARTt
and L. C. GUPTAS
Department of Physics, Polytechnic Institute of New York, 333 Jay Street, Brooklyn, NY 11201 (U.S.A.) M. F. RAVET-KRILL Laboratoirede Magne’tisme et Structure Electroniquedes F67070 Strasbourg (France)
Solides, Universitd L. Pasteur, rue B. Pascal,
(Received February 28.1983)
Summary The lattice parameters of tetragonal CeRh,Si, are anomalous compared with those of the other members of the series CeM,Si, (M = Ru, Rh, Pd and Ag). Measurements of the susceptibility of CeRh,Si, as a function of temperature in the range 15300K reveal interesting features. At high temperatures the susceptibility obeys a Curie-Weiss law with an effective moment per cerium ion that is 20% higher than that of the free Ce3 + ion. There is a cusp-like peak at about 37 K and a relatively smaller peak at 5.2 K. The lattice parameters a and c show a slight decrease between 300 and 40 K and a rather small increase between 40 and 12 K. The L,,, absorption edge and the extended X-ray absorption fine structure spectra are unchanged between 20 and 300 K. This clearly indicates that the valence of the cerium ion does not change and that the first-nearestneighbour environment (eight rhodium atoms and eight silicon atoms) of the cerium ion is not affected by the transition at 37 K.
1. Introduction CeCu,Si, [l] appears to undergo a superconducting though the magnetic moment of the cerium atom at high close to that of the free Ce3+ ion. CePd,Si, [2] exhibits instability at intermediate temperatures; however, at
phase transition even temperatures is very an incipient moment low temperatures it
* Paper presented at the Sixteenth Rare Earth Research Conference, The Florida State University, Tallahassee, FL, U.S.A., April Z&21,1983. t Permanent address: Laboratoire de Chimie Metallurgique des Terres Rares, Equipe de Recherche du CNRS 209,l place A. Briand, F92190 Meudon, France. f Permanent address: Tata Institute of Fundamental Research, Bombay 400005, India. 0022.5088/83/$3.00
0 Elsevier Sequoia/Printed
in The Netherlands
188
undergoes a magnetic transition. Some of the europium-based systems of this structure exhibit a variety of phenomena, i.e. valence transition (EuPd,Si, [3]), magnetic ordering, and possibly mixed valence (EuAu,Si, [4]), trivalent (EuRu,Si, [S] and EuFe,Si, [5]) and divalent (EuAg,Si, [5]) configurations. Some of the ytterbium-based systems such as YbCu,Si, (mixed valence [6]) and YbPd,Ge, [7] (superconductor with T, = 1.1 K) also possess interesting properties. Therefore we have investigated the magnetic properties of various transition metal (M) isomorphs of the CeM,Si, structure [2]. In this paper we present the results of our measurements on CeRh,Si,. 2. Experimental
details
The samples were prepared by arc melting the constituents cerium (purity, 99.99% (Rare Earth Products)), rhodium (purity, 99.9% (United Minerals Corporation)) and silicon (purity, 99.999% (Mackay)) on a water-cooled copper hearth in an argon atmosphere. Almost no loss of weight occurred during melting. X-ray diffraction studies confirmed that the samples were single phase and each of the lines (including those at low angles) could be indexed in the tetragonal ThCr,Si,-type structure. Susceptibility measurements between 1.5 and 300K were performed using a Faraday-type magnetometer. The lattice parameter measurements between 12 and 300 K were performed on powdered samples using a Siemens goniometer. X-ray L,,, absorption edge and extended Xray absorption fine structure (EXAFS) measurements between 20 and 300K were performed at the Laboratoire pour l’utilisation du Rayonnement Electronique synchrotron at Orsay. 3. Results 3.1. Susceptibility measurements The susceptibility is plotted against the temperature between 1.5 and 300 K in Fig. 1. The inverse of the susceptibility is also plotted against temperature in this figure. The behaviour in the temperature range 130-300 K obeys the CurieWeiss law x = C/( T- 0) with a paramagnetic Curie temperature f3of 72 K and an effective moment pee of about 2.9 ps per cerium ion which is about 20% higher than the moment of the free Ce3 + ion. This suggests that rhodium-derived d bands contribute significantly to the magnetization of the sample. The susceptibility exhibits a cusp-like peak at 37 K which could be due either to antiferromagnetic ordering or to a valence transition. We used X-ray diffraction measurements of the lattice constant and L,,, absorption edge measurements (see below) at about 37 K to investigate the possibility of a change in valence. The results of these measurements do not agree with a valence transition near 37 K and hence suggest a magnetic transition. A second smaller susceptibility peak is observed at 5.2 K which may be due to the magnetism of the Rh 4d bands in this compound. This is consistent with the large effective magnetic moment observed at high temperatures.
189
I 0
100 T(K)
Fig. 1. Susceptibility
’ 200
lo
and inverse susceptibility
of CeRh,Si,
us. temperature.
It would be of interest to study the magnetic properties of LaRh,Si,. These measurements should be helpful in understanding the magnetic behaviour of the rhodium d bands. In this respect the measurements of Felner and Nowik [S] on RRh,B, systems (R E Y, La, Lu) are noteworthy. There is evidence of itinerant magnetic ordering (due essentially to the rhodium bands) in these systems. 3.2. Latticeparameter measurements The method of least squares was used to deduce the lattice parameters a and c from diffraction lines obtained at diffraction angles 28 lying between 90 and 150”. The values a = 4.0g3 A and c = lo.& A are in good agreement with previous results (a = 4.086 A and c = 10.17 A [9]). A very interesting feature becomes apparent when the lattice parameters of the series CeM,Si, are considered (Table 1). The value of the parameter a is smallest for CeRh,Si, and the value of c does not lie between those for CeRu,Si, and CePd,Si,. In the RRh,Si, series [9] the lanthanide contraction takes place in the c constant, in contrast with other series such as RPd,Si, where it takes place in the a constant. This implies that some subtle differences in the nature of the bonding in this compound may exist. The results of the lattice parameter measurements are shown as a function of temperature from 12 to 300 K in Fig. 2. Both a and c decrease slowly as the temperature decreases from 300 to 50 K. The overall decrease in the parameters in this temperature interval is Aa x 0.01 A and AC zz 0.005 A. There is a slight increase in the lattice constants near 37 K and they remain almost constant between 37 and 20 K. If a valence transition had taken place at or near 37 K much larger changes in the lattice parameters would have been observed.
TABLE
1
Lattice parameters of CeM,Si, ion Compound
and the Ce-Ce and Ce-M distances for the neighbours
a
of the cerium
Ce- Ce
Ce- Ce
Ce- Ce
Ce-M
Ce-M
4.19 4.09 4.21 4.25
5.93 5.78 5.95 6.01
5.72 5.85 5.81 6.11
3.22 3.26 3.26 3.40
5.28 5.23 5.33 5.45
(4
CeRu,Si, CeRh,Si, CePd,Si, CeAg,Si,
4.19 4.09 4.21 4.25
lNN, first-nearest neighbour; “Four neighbours. “Eight neighbours. ’ 16 neighbours.
9.78 10.18 9.98 10.64
2NN, second-nearest
neighbour;
3NN, third-nearest neighbour.
CO
/
3.3. L, and extended X-ray absorption fine structure measurements The L,,, absorption edge is the result of a transition of one electron from the deep 2p core level to the 5d band. The energy of this transition is shifted if there is a change in the valence state of the absorbing ion. When two different stable valence states are present in the material or when there are configuration fluctuations, the L,,, spectrum has a bimodal shape. The value of the average valence (for configuration fluctuations) can in principle be determined by fractional superposition of the edges corresponding to the two valence states. However, in the cerium compounds some difficulties arise in the determination of the average valence using this procedure [lo]. However, any change in the valence state must produce changes in the qualitative features of the L,,, absorption spectrum. Figure 3 shows the L,,, absorption spectrum in the range 5700-5800 eV at various temperatures above and below 37 K. The shape of the L,,, edge is unaffected by the change in temperature which implies that the valence is unchanged. EXAFS measurements were performed at 300 and 25 K in the range 56506100 eV. The two spectra are qualitatively identical and there is no difference in the frequency of the Kronig oscillations. The contribution of the first-nearest neighbours of the cerium atom can be reproduced (Fig. 4) by assuming that there
191
I
c 1
lo23 +++: ti
t
t
+ +
tii,
4
4.
:
A a
t
+t+
t
t +
44t
?
.
4A8-
I
5700
(K)
Fig. 2. Lattice parameters a and c of CeRh,Si, Fig. 3. L,,, absorption
edge of CeRh,Si,
E(oV)
5780
us. temperature.
US.energy at various temperatures.
Fig. 4. Partial EXAFS spectrum of the newest-neighbour
shell.
are eight rhodium atoms at a distance of 3.27 A ({(a/2)’ + (c/4)‘} 1/2) and eight silicon atoms at a distance of 3.16 A ({ (~2~‘~/2)~ +c2(1/2 -2)“~‘~‘) where z = 0.375 for the ThCr,Si,-type structure. Additional measurement are necessary to obtain better statistics and to] be able to study the’contribution of other higher
192
order neighbours. However, it is clear from the present measurements that the transition at 37 K has little or no effect on the first-nearest-neighbour environment (eight rhodium and eight silicon atoms) of the cerium ion. In Table 1 we show the Ce-Ce and Ce-M distances for the cerium and M neighbours of a cerium ion in various CeM,Si, compounds calculated from experimentally determined lattice parameters [9-11,121. The first-, second- and third-nearest neighbours are assigned on the basis of the ThCr,Si,-type structure. Two important points emerge from this table. First the Ce-Ce distance between second-nearest neighbours in CeRh,Si, is less than that between third-nearest neighbours as expected,, whereas this situation is reversed in CeRu,Si, and CePd,Si,. This may cause an enhancement of the CeCe interaction in the rare earth plane. Secondly the distance between the 16 Ce-M second-nearest neighbours in CeRhzSi, is anomalously small compared with the results for the other compounds. In addition the ionic radius of rhodium is larger than those of ruthenium or palladium and hence there may be stronger d-f hybridization between cerium and rhodium. These effects may be responsible for the rather large TN (about 37 K) found for CeRh,Si, compared with the TN of about 10 K found for CePdzSi,, CeAuzSi, and CeAg,Si,, for the absence of a moment instability in contrast with the results for CePdzSi, and CeRu,Si, [11] and for the anomalously large cerium moment observed at high temperatures. Measurements using other techniques such as neutron diffraction examination are necessary to determine the topology of the magnetic ordering which appears at 37 K and to define the contribution of the rhodium d band magnetism. Further EXAFS measurements are in progress to study the change in the contribution of the second- and higher-order neighbours of the cerium ion at the transition.
Acknowledgments The work at the Polytechnic Institute of New York was supported in part by the National Science foundation under Grant DMR-8202726. Two of the authors (C.G. and L.C.G.) would like to thank Professor R. D. Parks for his stimulating role in initiating this project and for numerous helpful discussions.
References F. Steglich, J. Aarts, C. D. Bredl, W. Lieke, D. Mescbede, W. Franz and H. Schafer, Phys. Reu. Lett., 43(1979)1982. 2 V. Murgai, S. Raaen, L. C. Gupta and R. D. Parks, in P. Wachter and H. Boppart (eds.), Valence Instabilities, North-Holland, Amsterdam, 1982,p. 537. 3 E. V. Sampathkumaran, R. Vijayaraghavan, K. V. Gopalakrishnan, R. G. Piley, H. G. Devare, L. C. Gupta, B. Post and R. D. Parks, in L. H. Falicov, W. Hanke and M. B. Maple (eds.), Valence Fluctuations in Solids, North-Holland, Amsterdam, 1981,p. 193. 1
193
4 5 6 7 8 9 10
11 12
L. C. Gupta, V. Murgai, Y. Yeshurun and R. D. Parks, in P. Wachter and H. Boppart (eds.), Valence Instabilities, North-Holland, Amsterdam, 1982, p. 225. E. Bauminger, D. Froindlich, I. Nowick, S. Ofer, I. Felner and I. Mayer, Phys. Reu. Lett., 30 (1973) 1053. B. C. Sales and D. K. Wohlleben, Phys. Reu. Lett., 35 (1975) 1240. G. W. Hull, J. H. Wernick, T. H. Geballe, J. V. Waszczak and J. E. Bernardini, Phys. Rev. B, 24 (11) (1981) 6715. I. Felner and I. Nowik, Phys. Rev. Lett., 45 (1980) 2128. R. Ballestracci, C.R. Acad. Sci., St%-.B, 282(1976) 291. R. Ballestracci and G. Astier, CR. Acad. Sci., St+. B, 286(1978) 109. K. R. Bauchspiess, W. Boksch, E. Holland Moritz, H. Launois, R. Pott and D. Wohlleben, in L. M. Falicov, W. Hanke and M. B. Maple (eds.), Valence Fluctuations in Solids, North-Holland, Amsterdam, 1981, p. 417. L. C. Gupta, C. Godart, D. MacLaughlin and R. D. Parks, unpublished data, 1983. I. Mayer and J. Cohen, J. Less-Common Met., 29 (1972) 221.
of the Less-Common
Metals,
187
94 (1983) 187-193
INVESTIGATIONS OF THE MAGNETIC PROPERTIES OF CeRh,Si, USING LATTICE PARAMETER, L,,, EDGE AND EXTENDED X-RAY ABSORPTION FINE STRUCTURE MEASUREMENTS*
C. GODARTt
and L. C. GUPTAS
Department of Physics, Polytechnic Institute of New York, 333 Jay Street, Brooklyn, NY 11201 (U.S.A.) M. F. RAVET-KRILL Laboratoirede Magne’tisme et Structure Electroniquedes F67070 Strasbourg (France)
Solides, Universitd L. Pasteur, rue B. Pascal,
(Received February 28.1983)
Summary The lattice parameters of tetragonal CeRh,Si, are anomalous compared with those of the other members of the series CeM,Si, (M = Ru, Rh, Pd and Ag). Measurements of the susceptibility of CeRh,Si, as a function of temperature in the range 15300K reveal interesting features. At high temperatures the susceptibility obeys a Curie-Weiss law with an effective moment per cerium ion that is 20% higher than that of the free Ce3 + ion. There is a cusp-like peak at about 37 K and a relatively smaller peak at 5.2 K. The lattice parameters a and c show a slight decrease between 300 and 40 K and a rather small increase between 40 and 12 K. The L,,, absorption edge and the extended X-ray absorption fine structure spectra are unchanged between 20 and 300 K. This clearly indicates that the valence of the cerium ion does not change and that the first-nearestneighbour environment (eight rhodium atoms and eight silicon atoms) of the cerium ion is not affected by the transition at 37 K.
1. Introduction CeCu,Si, [l] appears to undergo a superconducting though the magnetic moment of the cerium atom at high close to that of the free Ce3+ ion. CePd,Si, [2] exhibits instability at intermediate temperatures; however, at
phase transition even temperatures is very an incipient moment low temperatures it
* Paper presented at the Sixteenth Rare Earth Research Conference, The Florida State University, Tallahassee, FL, U.S.A., April Z&21,1983. t Permanent address: Laboratoire de Chimie Metallurgique des Terres Rares, Equipe de Recherche du CNRS 209,l place A. Briand, F92190 Meudon, France. f Permanent address: Tata Institute of Fundamental Research, Bombay 400005, India. 0022.5088/83/$3.00
0 Elsevier Sequoia/Printed
in The Netherlands
188
undergoes a magnetic transition. Some of the europium-based systems of this structure exhibit a variety of phenomena, i.e. valence transition (EuPd,Si, [3]), magnetic ordering, and possibly mixed valence (EuAu,Si, [4]), trivalent (EuRu,Si, [S] and EuFe,Si, [5]) and divalent (EuAg,Si, [5]) configurations. Some of the ytterbium-based systems such as YbCu,Si, (mixed valence [6]) and YbPd,Ge, [7] (superconductor with T, = 1.1 K) also possess interesting properties. Therefore we have investigated the magnetic properties of various transition metal (M) isomorphs of the CeM,Si, structure [2]. In this paper we present the results of our measurements on CeRh,Si,. 2. Experimental
details
The samples were prepared by arc melting the constituents cerium (purity, 99.99% (Rare Earth Products)), rhodium (purity, 99.9% (United Minerals Corporation)) and silicon (purity, 99.999% (Mackay)) on a water-cooled copper hearth in an argon atmosphere. Almost no loss of weight occurred during melting. X-ray diffraction studies confirmed that the samples were single phase and each of the lines (including those at low angles) could be indexed in the tetragonal ThCr,Si,-type structure. Susceptibility measurements between 1.5 and 300K were performed using a Faraday-type magnetometer. The lattice parameter measurements between 12 and 300 K were performed on powdered samples using a Siemens goniometer. X-ray L,,, absorption edge and extended Xray absorption fine structure (EXAFS) measurements between 20 and 300K were performed at the Laboratoire pour l’utilisation du Rayonnement Electronique synchrotron at Orsay. 3. Results 3.1. Susceptibility measurements The susceptibility is plotted against the temperature between 1.5 and 300 K in Fig. 1. The inverse of the susceptibility is also plotted against temperature in this figure. The behaviour in the temperature range 130-300 K obeys the CurieWeiss law x = C/( T- 0) with a paramagnetic Curie temperature f3of 72 K and an effective moment pee of about 2.9 ps per cerium ion which is about 20% higher than the moment of the free Ce3 + ion. This suggests that rhodium-derived d bands contribute significantly to the magnetization of the sample. The susceptibility exhibits a cusp-like peak at 37 K which could be due either to antiferromagnetic ordering or to a valence transition. We used X-ray diffraction measurements of the lattice constant and L,,, absorption edge measurements (see below) at about 37 K to investigate the possibility of a change in valence. The results of these measurements do not agree with a valence transition near 37 K and hence suggest a magnetic transition. A second smaller susceptibility peak is observed at 5.2 K which may be due to the magnetism of the Rh 4d bands in this compound. This is consistent with the large effective magnetic moment observed at high temperatures.
189
I 0
100 T(K)
Fig. 1. Susceptibility
’ 200
lo
and inverse susceptibility
of CeRh,Si,
us. temperature.
It would be of interest to study the magnetic properties of LaRh,Si,. These measurements should be helpful in understanding the magnetic behaviour of the rhodium d bands. In this respect the measurements of Felner and Nowik [S] on RRh,B, systems (R E Y, La, Lu) are noteworthy. There is evidence of itinerant magnetic ordering (due essentially to the rhodium bands) in these systems. 3.2. Latticeparameter measurements The method of least squares was used to deduce the lattice parameters a and c from diffraction lines obtained at diffraction angles 28 lying between 90 and 150”. The values a = 4.0g3 A and c = lo.& A are in good agreement with previous results (a = 4.086 A and c = 10.17 A [9]). A very interesting feature becomes apparent when the lattice parameters of the series CeM,Si, are considered (Table 1). The value of the parameter a is smallest for CeRh,Si, and the value of c does not lie between those for CeRu,Si, and CePd,Si,. In the RRh,Si, series [9] the lanthanide contraction takes place in the c constant, in contrast with other series such as RPd,Si, where it takes place in the a constant. This implies that some subtle differences in the nature of the bonding in this compound may exist. The results of the lattice parameter measurements are shown as a function of temperature from 12 to 300 K in Fig. 2. Both a and c decrease slowly as the temperature decreases from 300 to 50 K. The overall decrease in the parameters in this temperature interval is Aa x 0.01 A and AC zz 0.005 A. There is a slight increase in the lattice constants near 37 K and they remain almost constant between 37 and 20 K. If a valence transition had taken place at or near 37 K much larger changes in the lattice parameters would have been observed.
TABLE
1
Lattice parameters of CeM,Si, ion Compound
and the Ce-Ce and Ce-M distances for the neighbours
a
of the cerium
Ce- Ce
Ce- Ce
Ce- Ce
Ce-M
Ce-M
4.19 4.09 4.21 4.25
5.93 5.78 5.95 6.01
5.72 5.85 5.81 6.11
3.22 3.26 3.26 3.40
5.28 5.23 5.33 5.45
(4
CeRu,Si, CeRh,Si, CePd,Si, CeAg,Si,
4.19 4.09 4.21 4.25
lNN, first-nearest neighbour; “Four neighbours. “Eight neighbours. ’ 16 neighbours.
9.78 10.18 9.98 10.64
2NN, second-nearest
neighbour;
3NN, third-nearest neighbour.
CO
/
3.3. L, and extended X-ray absorption fine structure measurements The L,,, absorption edge is the result of a transition of one electron from the deep 2p core level to the 5d band. The energy of this transition is shifted if there is a change in the valence state of the absorbing ion. When two different stable valence states are present in the material or when there are configuration fluctuations, the L,,, spectrum has a bimodal shape. The value of the average valence (for configuration fluctuations) can in principle be determined by fractional superposition of the edges corresponding to the two valence states. However, in the cerium compounds some difficulties arise in the determination of the average valence using this procedure [lo]. However, any change in the valence state must produce changes in the qualitative features of the L,,, absorption spectrum. Figure 3 shows the L,,, absorption spectrum in the range 5700-5800 eV at various temperatures above and below 37 K. The shape of the L,,, edge is unaffected by the change in temperature which implies that the valence is unchanged. EXAFS measurements were performed at 300 and 25 K in the range 56506100 eV. The two spectra are qualitatively identical and there is no difference in the frequency of the Kronig oscillations. The contribution of the first-nearest neighbours of the cerium atom can be reproduced (Fig. 4) by assuming that there
191
I
c 1
lo23 +++: ti
t
t
+ +
tii,
4
4.
:
A a
t
+t+
t
t +
44t
?
.
4A8-
I
5700
(K)
Fig. 2. Lattice parameters a and c of CeRh,Si, Fig. 3. L,,, absorption
edge of CeRh,Si,
E(oV)
5780
us. temperature.
US.energy at various temperatures.
Fig. 4. Partial EXAFS spectrum of the newest-neighbour
shell.
are eight rhodium atoms at a distance of 3.27 A ({(a/2)’ + (c/4)‘} 1/2) and eight silicon atoms at a distance of 3.16 A ({ (~2~‘~/2)~ +c2(1/2 -2)“~‘~‘) where z = 0.375 for the ThCr,Si,-type structure. Additional measurement are necessary to obtain better statistics and to] be able to study the’contribution of other higher
192
order neighbours. However, it is clear from the present measurements that the transition at 37 K has little or no effect on the first-nearest-neighbour environment (eight rhodium and eight silicon atoms) of the cerium ion. In Table 1 we show the Ce-Ce and Ce-M distances for the cerium and M neighbours of a cerium ion in various CeM,Si, compounds calculated from experimentally determined lattice parameters [9-11,121. The first-, second- and third-nearest neighbours are assigned on the basis of the ThCr,Si,-type structure. Two important points emerge from this table. First the Ce-Ce distance between second-nearest neighbours in CeRh,Si, is less than that between third-nearest neighbours as expected,, whereas this situation is reversed in CeRu,Si, and CePd,Si,. This may cause an enhancement of the CeCe interaction in the rare earth plane. Secondly the distance between the 16 Ce-M second-nearest neighbours in CeRhzSi, is anomalously small compared with the results for the other compounds. In addition the ionic radius of rhodium is larger than those of ruthenium or palladium and hence there may be stronger d-f hybridization between cerium and rhodium. These effects may be responsible for the rather large TN (about 37 K) found for CeRh,Si, compared with the TN of about 10 K found for CePdzSi,, CeAuzSi, and CeAg,Si,, for the absence of a moment instability in contrast with the results for CePdzSi, and CeRu,Si, [11] and for the anomalously large cerium moment observed at high temperatures. Measurements using other techniques such as neutron diffraction examination are necessary to determine the topology of the magnetic ordering which appears at 37 K and to define the contribution of the rhodium d band magnetism. Further EXAFS measurements are in progress to study the change in the contribution of the second- and higher-order neighbours of the cerium ion at the transition.
Acknowledgments The work at the Polytechnic Institute of New York was supported in part by the National Science foundation under Grant DMR-8202726. Two of the authors (C.G. and L.C.G.) would like to thank Professor R. D. Parks for his stimulating role in initiating this project and for numerous helpful discussions.
References F. Steglich, J. Aarts, C. D. Bredl, W. Lieke, D. Mescbede, W. Franz and H. Schafer, Phys. Reu. Lett., 43(1979)1982. 2 V. Murgai, S. Raaen, L. C. Gupta and R. D. Parks, in P. Wachter and H. Boppart (eds.), Valence Instabilities, North-Holland, Amsterdam, 1982,p. 537. 3 E. V. Sampathkumaran, R. Vijayaraghavan, K. V. Gopalakrishnan, R. G. Piley, H. G. Devare, L. C. Gupta, B. Post and R. D. Parks, in L. H. Falicov, W. Hanke and M. B. Maple (eds.), Valence Fluctuations in Solids, North-Holland, Amsterdam, 1981,p. 193. 1
193
4 5 6 7 8 9 10
11 12
L. C. Gupta, V. Murgai, Y. Yeshurun and R. D. Parks, in P. Wachter and H. Boppart (eds.), Valence Instabilities, North-Holland, Amsterdam, 1982, p. 225. E. Bauminger, D. Froindlich, I. Nowick, S. Ofer, I. Felner and I. Mayer, Phys. Reu. Lett., 30 (1973) 1053. B. C. Sales and D. K. Wohlleben, Phys. Reu. Lett., 35 (1975) 1240. G. W. Hull, J. H. Wernick, T. H. Geballe, J. V. Waszczak and J. E. Bernardini, Phys. Rev. B, 24 (11) (1981) 6715. I. Felner and I. Nowik, Phys. Rev. Lett., 45 (1980) 2128. R. Ballestracci, C.R. Acad. Sci., St%-.B, 282(1976) 291. R. Ballestracci and G. Astier, CR. Acad. Sci., St+. B, 286(1978) 109. K. R. Bauchspiess, W. Boksch, E. Holland Moritz, H. Launois, R. Pott and D. Wohlleben, in L. M. Falicov, W. Hanke and M. B. Maple (eds.), Valence Fluctuations in Solids, North-Holland, Amsterdam, 1981, p. 417. L. C. Gupta, C. Godart, D. MacLaughlin and R. D. Parks, unpublished data, 1983. I. Mayer and J. Cohen, J. Less-Common Met., 29 (1972) 221.