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Resistance minima in zirconium-plutonium alloys
Journal of the Less-Common Metals Elsevier Sequoia S..4., Lausanne - Printed in The Netherlands
123
Short Communication Resistance minima Ii. 0. ELLIOTT
AND
in zirconium-plutonium
alloys*
H. H. HILL
University of Califonzia, Los Alamos Scientific Laboratory, Los Alamos, N. M. 87544 (U.S.‘4 .) (Received
April 7th, 1970)
It is well-known that a minimum exists in the electrical resistivity vs. temperature of certain dilute magnetic alloys containing transition metals as solute+,“. This resistance minimum phenomenon is known as the Kondo Effect3>4. It is caused by an incremental magnetic resistivity which becomes significant in the vicinity of a characteristic temperature (the Kondo temperature) and which increases with decreasing temperature approximately in proportion to -In T. This incremental resistivity arises from a resonant exchange scattering interaction between the conduction electrons of the host material and the magnetic moments of the d electrons of the solute metal. The resonant aspect of the interaction results from the proximity of the relatively broad d levels of the impurity to the Fermi level of the host metal. Resistance minima are less common in alloys containing rare-earth impurities, probably because the 4f levels of the rare-earth impurities are usually fairly narrow and lie relatively far below the Fermi level of the host materials. (LaGd alloys exemplify this behavior.**) The most notable exception to this rule is the rare-earth metal Ce, when used as a solute. Resistance minima are observed in dilute Ce alloys formed with La6 and Pr7, in dilutes and nondilute alloys with Y, and also when Ce dilutely replaces La in LaAlz or Y in YB610. It is characteristic of pure Ce that the 4flevels in the metal lie close to its Fermi surface, and this condition apparently persists in at least several of its alloys. In those alloys, the f levels of the Ce impurity are also broadened due to their resonance with the conduction band of the host (to use the terminology of FRIEDEL~~), and the f electrons are said to occupy virtual bound states”,ll. In an attempt to elucidate the nature of the 5f levels in the transactinium metals, we have undertaken to study resistance minimum phenomena in alloys containing Pu. So far, the results indicate that the sfelectrons of Pu, at least in certain alloys, behave somewhat like the 4f electrons of Ce. Resistance minima have been observed in LaPu and PrPu alloysiz, and the purpose of this communication is to report the observation of such minima in ZrPu alloys. Also, we report several negative results in other alloy systems containing Pu. Five ZrPu alloys, containing 5, 7.5, IO, 12.5, and 16 at.% Pu, were studied. The alloys were arc cast, rolled into rod form at room temperature, and then machined into cylindrical samples $ in. diam. by I in. long. Each sample was wrapped * Work performed under the auspices of the U. S. Atomic Energy Commission. ** Throughout this communication bold face type denotes the element which constitutes major proportion in a solid solution binary alloy. J. Less-Common
the
Metals, 2.z (1970) 123--125
SHORT COMMUNICATIONS
I24
in Ta foil, sealed within a quartz capsule containing a He atmosphere, heat treated for at least I week at Goo”C, and quenched, in the unbroken capsule, to room temperature in a salt-water bath. According to published Zr-Pu phase diagramsraJ*, the alloys containing 5 12.5 at.% Pu should unquestionably have been solid solutions of Pu in h.c.p. a-phase Zr. There is disagreement as to whether ol-Zr will dissolve as much as 16 at.% Pu (at 618”CraJ*), so the 16 at.% Pu sample may not have been single phase. A pure Zr sample was made from the same stock of material as the alloys and in the same manner, except that it was annealed for 2 h at 8oo”C and furnace cooled. The Zr crystal bar used in preparing the alloys contained the following major impurities (p.p.m. by weight): 105 02, 8 Nz, 33 C, and 27 Fe. Two Pu stocks were used: the first contained 71 W, 66 Am, 35 U, 20 C, and 12.5 Ga; the second contained 62 W, 121 Am, Ig U, and 47 Fe. The resistivities of the alloys and of the pure Zr sample have been plotted as a function of temperature on a logarithmic scale in Figs. I and 2. A logarithmic temperature scale was chosen to illustrate the approximate proportionality of the incremental magnetic resistivity to -1nT below the temperature, Tmin, of the mini-
TEMPERATURE
(‘K 1
Fig. 1. Resistivity vs. temperature (log scale) for pure h.c.p. Zr and three h.c.p. ZrPu alloys. Alloy compositions are given in at.%. Data points taken on cooling are indicated by triangles; on warming, by circles. J. Less-Common Metals, 2.z (1970) r23-~26
SHORT COMMUNICATIONS
12.5
mum (where minima occurred). The incremental resistivity (e.g., for the 7.5 at.% alloy) is given, approximately, by the difference between the total resistivity of the alloy (below Tmin) and the resistivity at Tmln, t at least to the extent that the latter would have represented the residual resistivity of the alloy in the absence of magnetic interactions.
I
2
3
45 TEMPERATURE
15
20
30
40
/‘I,
Fig. 2. Resistivity OS. temperature for Zr--12.5 and 16 at.% Pu h.c.p. alloys. The low-temperature portion of the curve for the 12.5 at.% Pu alloy is dashed to indicate uncertainty as to whether the curve tails off below 5”K, due to weak magnetic ordering of Pu spins. Data points taken on cooling are indicated by triangles; on warming, by circles.
It has been notedls-17 that the (hybridized) f bands of the early 5f metals are probably considerably broader and closer to their own respective Fermi levels, i.e., more d-like, than the 4fbands typical of the rare-earth metals. The presence of resistance minima in alloys containing Pu supports this hypothesis, since such minima readily occur with d electron solutes (specifically, 3d solutes) and with the anomalous (and most d-like) rare-earth metal, Ce, as solute. We have also measured the low-temperature resistivities of SCPU alloys containing 5, 15, 25 and 40 at.% Pu, and of YPu, PdPu, LuPu, and ThPu alloys containing 5 and IO at.% Pu in each system. No minima were observed in these alloys, with the possible exceptions of the SC alloys containing 15 and 25 at.% Pu, in which weak minima (A~~o.02 &I-cm between 4’ and 1°K) may have occurred in the vicinity of 4°K. If these are true minima, they are too weak to be unequivocally resolved in our measurements. The absence of minima in the YPu alloys is of particular interest, because such minima do occur in YCe alloys. Clearly the 5felectrons of Pu are not exactly analogous to the 4felectrons of Ce, which is to be expected. The absence of minima in the Y, Pd, Lu, Th, and possibly in the SC alloys, may be due to the absence of magnetic moments on the Pu ions in these alloys, or, J.
Less-Common
M&A-,
22 (1970)
123-126
SHORT COMMUNICATIONS
126
more likely, to the relatively greater depth of the 5flevels of the Pu below the Fermi levels of the host metals. Magnetic susceptibility measurements are needed to help resolve this question. The disappearance of the minimum in ZrPu alloys at Pu concentrations RIO at.% is in contrast to the behavior observed in LaPu and PrPu alloysr2. The curve for the 12.5 at.% Pu alloy may evidence a weak magnetic ordering of the Pu spins below - 5°K but there is no obvious evidence for such behavior in the 16 at. yO Pu alloy. The relatively early disappearance of the minimum in the ZrPu alloys may result simply from stronger interactions between the Pu ions due to their closer proximity to one another in the Zr lattice (for a given Pu concentration) than in the La and Pr lattices, but a complete understanding of the observed behavior, including an explanation of why some alloys yield no minima at all, will clearly require more detailed study and will likely involve consideration of the metallic valencies and crystal structures of the host elements, as well as possible effects due to the presence of their d bands. REFERENCES I G. J. VAN DEN BERG, in C. J. GORTER (ed.), Progress in Low Temperature Physics, Vol. IV, North-Holland, Amsterdam, 1964, p. 194. M. D. DAYBELL AND W. A. STEYERT, Rev. Mod. Phys., 40 (1968) 380. J. KONDO, Prog. Theoret. Phys. (Kyoto), 32 (1964) 37. J. R. SCHRIEFFER,J. Appl. Phys., 38 (1967) 1143. B. COQBLIN AND C. F. RATTO, Phys. Rev. Letters, 21 (1968) 1065; B. COQBLIN AND J. R. SCHRIEFFER,Phys. Rev., 185 (1969) 847. 6 T. SUGAWARA AND H. EGUCHI, J. Phys. Sot. Japan, 21 (1966) 725. H. NAGASAWA AND T. SUGAWARA,J. Phys. Sot. Japan, 23 (1967) 701. s7 T. SUGAWARA, J. Phys. Sot. Japan, 20 (1965) 2252. 9 H. H. HILL, W. N. MINER AND R. 0. ELLIOTT, Phys. Letters, 28A (1969) 588. IO M. B. MAPLE AND 2. FISK, Proc. Eleventh Internat. Conf. Low Temp. Physics, St. Andrews, 1968, p. 1288. II J. FRIEDEL, Can. J. Phys., 34 (1956) 1190; J. Phys. Radium, 23 (1962) 692. 12 H. H. HILL. R. 0. ELLIOTT AND W. N. MINER, Proc. Colloque Internat. C.N.R.S. sur Les Elements de; Terres Rares, Paris-Grenoble, May, Ig6g (to be piblished). I3 A. A. BOCHVAR, S. T. KONOBEEVSKY, V. I. KUTAITSEV, T. S. MENSHIKOVA AND N. T. CHEBOTAREV,Proc. Second U. N. Internat. Conf. on the Peaceful Uses of At. Energy, Geneva, 1958, Vol. 6, p. 184. I4 J. A. C. MARPLES,J. Less-Common Metals, 2 (1960) 331. I5 J. FRIEDEL, J. Phys. Chem. Solids, I (1956) 175. 16 Y. A. ROCHER, Advan. in Phys., II (1962) 233. I7 E. A. KMETKO, Proc. Third Mater. Res. Symp. on Electronic Density of States, Gaithersburg, Md., Nov., 1969, Nat. Bur. Std. (to be published). J. Less-Common
Metals, 22 (1970) 123-126
123
Short Communication Resistance minima Ii. 0. ELLIOTT
AND
in zirconium-plutonium
alloys*
H. H. HILL
University of Califonzia, Los Alamos Scientific Laboratory, Los Alamos, N. M. 87544 (U.S.‘4 .) (Received
April 7th, 1970)
It is well-known that a minimum exists in the electrical resistivity vs. temperature of certain dilute magnetic alloys containing transition metals as solute+,“. This resistance minimum phenomenon is known as the Kondo Effect3>4. It is caused by an incremental magnetic resistivity which becomes significant in the vicinity of a characteristic temperature (the Kondo temperature) and which increases with decreasing temperature approximately in proportion to -In T. This incremental resistivity arises from a resonant exchange scattering interaction between the conduction electrons of the host material and the magnetic moments of the d electrons of the solute metal. The resonant aspect of the interaction results from the proximity of the relatively broad d levels of the impurity to the Fermi level of the host metal. Resistance minima are less common in alloys containing rare-earth impurities, probably because the 4f levels of the rare-earth impurities are usually fairly narrow and lie relatively far below the Fermi level of the host materials. (LaGd alloys exemplify this behavior.**) The most notable exception to this rule is the rare-earth metal Ce, when used as a solute. Resistance minima are observed in dilute Ce alloys formed with La6 and Pr7, in dilutes and nondilute alloys with Y, and also when Ce dilutely replaces La in LaAlz or Y in YB610. It is characteristic of pure Ce that the 4flevels in the metal lie close to its Fermi surface, and this condition apparently persists in at least several of its alloys. In those alloys, the f levels of the Ce impurity are also broadened due to their resonance with the conduction band of the host (to use the terminology of FRIEDEL~~), and the f electrons are said to occupy virtual bound states”,ll. In an attempt to elucidate the nature of the 5f levels in the transactinium metals, we have undertaken to study resistance minimum phenomena in alloys containing Pu. So far, the results indicate that the sfelectrons of Pu, at least in certain alloys, behave somewhat like the 4f electrons of Ce. Resistance minima have been observed in LaPu and PrPu alloysiz, and the purpose of this communication is to report the observation of such minima in ZrPu alloys. Also, we report several negative results in other alloy systems containing Pu. Five ZrPu alloys, containing 5, 7.5, IO, 12.5, and 16 at.% Pu, were studied. The alloys were arc cast, rolled into rod form at room temperature, and then machined into cylindrical samples $ in. diam. by I in. long. Each sample was wrapped * Work performed under the auspices of the U. S. Atomic Energy Commission. ** Throughout this communication bold face type denotes the element which constitutes major proportion in a solid solution binary alloy. J. Less-Common
the
Metals, 2.z (1970) 123--125
SHORT COMMUNICATIONS
I24
in Ta foil, sealed within a quartz capsule containing a He atmosphere, heat treated for at least I week at Goo”C, and quenched, in the unbroken capsule, to room temperature in a salt-water bath. According to published Zr-Pu phase diagramsraJ*, the alloys containing 5 12.5 at.% Pu should unquestionably have been solid solutions of Pu in h.c.p. a-phase Zr. There is disagreement as to whether ol-Zr will dissolve as much as 16 at.% Pu (at 618”CraJ*), so the 16 at.% Pu sample may not have been single phase. A pure Zr sample was made from the same stock of material as the alloys and in the same manner, except that it was annealed for 2 h at 8oo”C and furnace cooled. The Zr crystal bar used in preparing the alloys contained the following major impurities (p.p.m. by weight): 105 02, 8 Nz, 33 C, and 27 Fe. Two Pu stocks were used: the first contained 71 W, 66 Am, 35 U, 20 C, and 12.5 Ga; the second contained 62 W, 121 Am, Ig U, and 47 Fe. The resistivities of the alloys and of the pure Zr sample have been plotted as a function of temperature on a logarithmic scale in Figs. I and 2. A logarithmic temperature scale was chosen to illustrate the approximate proportionality of the incremental magnetic resistivity to -1nT below the temperature, Tmin, of the mini-
TEMPERATURE
(‘K 1
Fig. 1. Resistivity vs. temperature (log scale) for pure h.c.p. Zr and three h.c.p. ZrPu alloys. Alloy compositions are given in at.%. Data points taken on cooling are indicated by triangles; on warming, by circles. J. Less-Common Metals, 2.z (1970) r23-~26
SHORT COMMUNICATIONS
12.5
mum (where minima occurred). The incremental resistivity (e.g., for the 7.5 at.% alloy) is given, approximately, by the difference between the total resistivity of the alloy (below Tmin) and the resistivity at Tmln, t at least to the extent that the latter would have represented the residual resistivity of the alloy in the absence of magnetic interactions.
I
2
3
45 TEMPERATURE
15
20
30
40
/‘I,
Fig. 2. Resistivity OS. temperature for Zr--12.5 and 16 at.% Pu h.c.p. alloys. The low-temperature portion of the curve for the 12.5 at.% Pu alloy is dashed to indicate uncertainty as to whether the curve tails off below 5”K, due to weak magnetic ordering of Pu spins. Data points taken on cooling are indicated by triangles; on warming, by circles.
It has been notedls-17 that the (hybridized) f bands of the early 5f metals are probably considerably broader and closer to their own respective Fermi levels, i.e., more d-like, than the 4fbands typical of the rare-earth metals. The presence of resistance minima in alloys containing Pu supports this hypothesis, since such minima readily occur with d electron solutes (specifically, 3d solutes) and with the anomalous (and most d-like) rare-earth metal, Ce, as solute. We have also measured the low-temperature resistivities of SCPU alloys containing 5, 15, 25 and 40 at.% Pu, and of YPu, PdPu, LuPu, and ThPu alloys containing 5 and IO at.% Pu in each system. No minima were observed in these alloys, with the possible exceptions of the SC alloys containing 15 and 25 at.% Pu, in which weak minima (A~~o.02 &I-cm between 4’ and 1°K) may have occurred in the vicinity of 4°K. If these are true minima, they are too weak to be unequivocally resolved in our measurements. The absence of minima in the YPu alloys is of particular interest, because such minima do occur in YCe alloys. Clearly the 5felectrons of Pu are not exactly analogous to the 4felectrons of Ce, which is to be expected. The absence of minima in the Y, Pd, Lu, Th, and possibly in the SC alloys, may be due to the absence of magnetic moments on the Pu ions in these alloys, or, J.
Less-Common
M&A-,
22 (1970)
123-126
SHORT COMMUNICATIONS
126
more likely, to the relatively greater depth of the 5flevels of the Pu below the Fermi levels of the host metals. Magnetic susceptibility measurements are needed to help resolve this question. The disappearance of the minimum in ZrPu alloys at Pu concentrations RIO at.% is in contrast to the behavior observed in LaPu and PrPu alloysr2. The curve for the 12.5 at.% Pu alloy may evidence a weak magnetic ordering of the Pu spins below - 5°K but there is no obvious evidence for such behavior in the 16 at. yO Pu alloy. The relatively early disappearance of the minimum in the ZrPu alloys may result simply from stronger interactions between the Pu ions due to their closer proximity to one another in the Zr lattice (for a given Pu concentration) than in the La and Pr lattices, but a complete understanding of the observed behavior, including an explanation of why some alloys yield no minima at all, will clearly require more detailed study and will likely involve consideration of the metallic valencies and crystal structures of the host elements, as well as possible effects due to the presence of their d bands. REFERENCES I G. J. VAN DEN BERG, in C. J. GORTER (ed.), Progress in Low Temperature Physics, Vol. IV, North-Holland, Amsterdam, 1964, p. 194. M. D. DAYBELL AND W. A. STEYERT, Rev. Mod. Phys., 40 (1968) 380. J. KONDO, Prog. Theoret. Phys. (Kyoto), 32 (1964) 37. J. R. SCHRIEFFER,J. Appl. Phys., 38 (1967) 1143. B. COQBLIN AND C. F. RATTO, Phys. Rev. Letters, 21 (1968) 1065; B. COQBLIN AND J. R. SCHRIEFFER,Phys. Rev., 185 (1969) 847. 6 T. SUGAWARA AND H. EGUCHI, J. Phys. Sot. Japan, 21 (1966) 725. H. NAGASAWA AND T. SUGAWARA,J. Phys. Sot. Japan, 23 (1967) 701. s7 T. SUGAWARA, J. Phys. Sot. Japan, 20 (1965) 2252. 9 H. H. HILL, W. N. MINER AND R. 0. ELLIOTT, Phys. Letters, 28A (1969) 588. IO M. B. MAPLE AND 2. FISK, Proc. Eleventh Internat. Conf. Low Temp. Physics, St. Andrews, 1968, p. 1288. II J. FRIEDEL, Can. J. Phys., 34 (1956) 1190; J. Phys. Radium, 23 (1962) 692. 12 H. H. HILL. R. 0. ELLIOTT AND W. N. MINER, Proc. Colloque Internat. C.N.R.S. sur Les Elements de; Terres Rares, Paris-Grenoble, May, Ig6g (to be piblished). I3 A. A. BOCHVAR, S. T. KONOBEEVSKY, V. I. KUTAITSEV, T. S. MENSHIKOVA AND N. T. CHEBOTAREV,Proc. Second U. N. Internat. Conf. on the Peaceful Uses of At. Energy, Geneva, 1958, Vol. 6, p. 184. I4 J. A. C. MARPLES,J. Less-Common Metals, 2 (1960) 331. I5 J. FRIEDEL, J. Phys. Chem. Solids, I (1956) 175. 16 Y. A. ROCHER, Advan. in Phys., II (1962) 233. I7 E. A. KMETKO, Proc. Third Mater. Res. Symp. on Electronic Density of States, Gaithersburg, Md., Nov., 1969, Nat. Bur. Std. (to be published). J. Less-Common
Metals, 22 (1970) 123-126