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The praseodymium-lead system
Journal of the Less-Common Metals, 45 (1976) 275 - 281 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
THE PRASEODYMIUM-LEAD
SYSTEM*
0. D. McMASTERS and K. A. GSCHNEIDNER, Ames Laboratory-ERDA Iowa 50010 (U.S.A.)
275
and Department
Jr.
of Metallurgy, Iowa State University, Ames,
(Received August 27, 1975)
Summary X-ray diffraction, differential thermal, and metallographic methods, were used to establish the praseodymium-lead phase diagram. Eutectic reactions occur at 9.3 at.% Pb and 824 ‘C, at 55.5 at.% Pb and 1145 ‘C, at 62.5 at.% Pb and 1085 “C and at greater than 99.5 at.% Pb and 325 “C. The intermetallic compounds PraPb, Pr5Pb4, PrllPblo and PrPba decompose peritectically at 860,1455, 1365 and 1090 “C, respectively. PqPb,, Pr3Pb4 and PrPba melt congruently at 1495,118O and 1120 “C, respectively. The crystal structure data are given for these compounds. The solid solubility of lead in praseodymium is 3.5 at.% Pb at the 824 “C! eutectic temperature. The alloying characteristics of some of the Pr-Pb alloys are compared with those in the La-Pb and Pu-Pb systems.
Introduction The systematic study of the rare earth-lead alloys was continued with the determination of the praseodymium-lead phase diagram. The R-Pb systems already completed in this program are for R = ytterbium [l],yttrium [ 21, europium [ 31, dysprosium [ 41, gadolinium [ 51, lanthanum [ 61 and lutetium [7]. The properties of these alloys have been subjected to a comparative analysis [ 81 which includes predictions concerning the alloying behavior of praseodymium with lead. The results of previous Pr-Pb phase studies, summarized by Hansen [9] and Gschneidner [lo], are incomplete and inaccurate, primarily because high-purity metals were not available at that time. The structure data for PraPb [ll], Pr,Pb, [12,13], Pr,Pb, [14] and PrPba [15,16] were confirmed in this investigation. *Work performed for the U.S. Energy Research and Development Administration under Contract No. W-7405eng-82.
276
Experimental
procedure
Materials The lead used in this investigation was obtained from Cominco Products, Inc., and was specified to be 99.99% pure. The praseodymium was prepared in this laboratory by the calcium reduction of the fluoride followed by a vacuum casting of the praseodymium in tantalum crucibles. The major impurities (atomic ppm) in the praseodymium were: Ca (<50), Fe (4), Si (15), other rare-earths (
Results The praseodymium-lead phase diagram which resulted from this investigation is shown in Fig. 1. The Pr-rich portion of the diagram determined by Griffin and Gschneidner [ll] is presented in Fig. 2. Most of the phase boundaries of the Pr-Pb system were confirmed by X-ray and metallographic methods. Eutectic reactions and terminal solid solubilities The Pr-rich portion of the Pr-Pb diagram was determined by X-ray, metallographic and DTA methods [ll] . A eutectic occurs at 9.3 + 0.1 at.% Pb and 824 f 1 OC, and the solid solubility of Pb in p-Pr is 3.5 + 0.2 at.% at this temperature. There is a eutectoid reaction at 3.1 + 0.2 at.% Pb and 778 f 1 ‘C, and,the solubility of Pb in cu-Pr at this temperature is 2.2 10.2 at.% Pb. Metallographic examination of samples quenched from 760 “C showed about 1.3 at.% Pb solubility in a-Pr. Additions of praseodymium lower the 327 “C melting point of lead to about 325 ‘C, which suggests that a eutectic reaction occurs at 325 “C and with less than 0.5 at.% Pr. The solid solubility of Pr in Pb is practically negligible. The eutectic reaction at 55.5 at.% Pb and 1145 “C was established on the basis of DTA results. The microstructure of a 63.1 at.% Pb alloy was essentially that of a eutectic in which a partial separation of the Pr,Pb4 and PrPb, components had occurred. The eutectic reaction was found to be at 62.5 at.% Pb and 1085 “C. Interpretation of both the metallographic and DTA data is difficult due to the number of reactions occurring in a relatively small area of the diagram, i.e., 55 - 75 at.% Pb and 1080 - 1180 “C.
i
t
500; 400
t
300-
I
lOOiI
I.,
0;;-ATOMIC
1
50 60 PERCENT
l/_LJ
70 LEAD
80
90
Ph
Pr
2
4 ATOMIC
6 8 PERCENT
IO LEAi,
i2
14
Fig. 1. Praseodymium-lead phase diagram. Differential thermal data: A, heating arrest; v, cooling arrest; x , liquidus data. Fig. 2. Pr-rich portion of the Pr-Pb alloy system after Griffin and Gschneidner [ 111.
X-ray single-crystal and powder diffraction methods were used to investigate the crystalline nature of the compounds in this system. The computer program used to generate the powder pattern data for the compound structures was written by Yvon et al. [17]. The Nelson-Riley function was used in conjunction with the VogelKempter [ 181 extrapolation program to obtain the lattice parameters of some of the phases. Most of the structure data for these compounds are summarized in Table 1. PrsPb (25.0 at. % Pb) This phase decomposes peritectically at 860 f 5 “C and crystallizes in the AuCus-type structure with a, = 4.948 -10.005 A. As noticed in the DTA, the formation of PrsPb is quite sluggish. The magnitude of its decomposition thermal arrest could be increased by holding the samples (compositions between 15 and 30 at.% Pb) at 840 “C for one day during the DTA runs. The microstructure of a 25.1 at.% Pb sample which had been heat treated at 800 “C for one week was nearly single phase.
MngSi~ 0%
Sm sGeg
HollGelo
hexagonal Pfia/mcm
orthorhombic Pnma
tetragonal 14/mmm
Pr,Pb,
P%Pb4
P%,Pblo
-.
a c
AuCu, LIZ
cubic Pm3m
PrPbs
9.6 9.4
694.9 114.8
a = 4.66 c = 32.0 a, = 4.8596 2 3 a = 4.867
*4.948 2 5 means the error is + 5 in the last digit, e.g., 4.948 + 0.005.
HfGaz tentative
tetragonal 14l/amd
PrPb,
12.1
2.6
1146.4
2437.5
2.4
10.5
Volume contraction
515.0
121.1
Cell volume
single crystal, parameters based on front reflection powder pattern data
r141
verified in this work
P31
powder and single crystal patterns
powder patterns ill1
Remarks
powder patterns [15,161
powder patterns
based on DTA and metallographic evidence.
= 11.87 = 17.30
a = 8.377 b = 16.04 c = 8.532
a0 = 9.343 Lt1 ce = 6.813 f 1 a = 9.337 C = 6.814
a, = 4.948 t 5 a, = 4.949
Lattice* parameters (A)
Existence of compound
AuCu, Lb!
cubic PmSm
Pr,Pb
P%Pb4
structure type
Crystal class space group
Compound
Structure data for Pr-Pb intermetallic compounds
TABLE 1 2
279
The powder patterns of PrsPb were not always well resolved but were of sufficient quality to confirm the Au&a structure-type to be in agreement with the reported data [II] and isotypic CeaPb [ 191. (37.5 at.% Pb) Pr5Pb3 melts congruently at 1495 k 5 “C and crystallizes in the hexagonal Mn,Sia (I&)-type structure with the lattice parameters listed in Table 1. Both single-crystal and powder patterns were used to obtain these data, which are in agreement with the literature values [12, 131. PrsPb,
(44.4 at,% Pb) Similarly, the SmSGe,type structure reported [ 14 3 for Pr5Pb4 and lattice parameters were confirmed in this study. This compound melts peritectically at I.455 t 10 “C, and the microstructure of a 44.4 at.% Pb sample, heat-treated at 1000 “C for three days, exhibits only traces of the Pr,Pba second phase. Pr,Pb,
(47.6 at.% Pb) Although inconsistent, the micrascopic and DTA data suggested the existence of this compound. The incons~s~ncies arose because of the, presumably, greater stability of the neighboring phases, which led to segregation problems. Single-crystal Weissenberg patterns confirmed the PrllPblo phase with the HollGe Io-type structure. The complex powder patterns were partially indexed, and the lattice parameters listed in Table 1 are based on front-reflection data from both the powder and Weissenberg patterns. PrllPblo decomposes peritectically at 1365 “C. On cooling during the DTA runs, this compound did not form until about 1300 “C and under-cooling was observed in nearly all cases, as shown in Fig. 1. PrltPblo
(57.14 at.% Pb) According to the DTA results, PrBPbcimelts congruently at 1180 + 5 “C. Microscopically, the as-cast 57.6 at.% Pb sample is nearly single phase. Evidence of the segregated-melt problem, or non-equilibrium, appeared in the microscopic examination of a 57.1 at.% Pb sample which had been heattreated at 1000 “C for three days. Some areas of this sample were two-phase, and occasionally small amounts of a third phase were present in the form of a precipitate. Reliable X-ray diffraction data could not be obtained for this phase because of the high reactivity of the samples with atmospheric gases. Pr,Pb,
PrPbB (66.67
ut.% Pb)
Equally as complex was the establishment of PrPba. The microstructure of a 66.6 at.% Pb sample is definitely single-phase and the DTA results showed that PrPba melts perit~cti~~ly at 1090 k 5 “G, Single crystals of this phase could not be obtained. Powder patterns were indexed on the basis of a large tetragonal cell, probably of the HfGaa-type structure, but without the single-crystal evidence to support this structure data, the tentative aspects of the PrPbz unit cell given in Table 1 will remain.
280
PrPb, (75.0 at.% Pb) This compound melts congruently at 1120 f 5 “C and crystallizes in the AuCua-type structure with a,, = 4.8596 + 0.0003 A. These powder pattern data are in agreement with the literature values.
Discussion The segregated-melt conditions described in the plutonium-lead alloy study [20] were also encountered in our investigation. The DTA results between 40 and 70 at.% Pb are markedly influenced by the effects of this problem. Master alloys for the DTA experiments were prepared by arcmelting, and after each run the composition was changed by adding either Pr or Pb to the tantalum crucibles containing the samples. Attempts to homogenize the samples by inductively heating them to 1200 ‘C, first in the upright position, then in the inverted position and again in the upright position, did not completely eliminate the segregation, which apparently stems from the high stability of Pr5Pb3. The microstructures of the eutectic phases in this region are invariably of a degenerate nature, which suggests that the formation of the peritectic compounds will probably be difficult. The existence of segregation was suggested above when it was stated that single crystals of PraPb4 were obtained from the top of a 48 at.% Pb DTA sample. It is believed that segregation in these alloys could be minimized by heat treating the samples at higher temperatures (1400 “C) with more thorough mixing. In restrospect, segregated-melt conditions existed in the La-Pb phase diagram determination [6] . The compound reaction temperatures in the La-Pb system are similar to those in the PrPb system, but the interpretation of the data led to different results. The LasPb4 phase reported by Merlo and Fornasini [ 141 was overlooked in the La-Pb phase study [6] . The stoichiometry of LallPblo was not well-documented and could possibly be La,Pb,, with the melting characteristics of Pr5Pb4 being applicable. This would result in LaBPbd melting incongruently at 1350 ‘C, and elimination of the 1160 “C eutectoid horizontal on the basis of segregated-melt effects. The existence of LadPbs was well-documented but an isotypic phase in the Pr-Pb system was not observed. Since PrsPb forms sluggishly, and CesPb [19] and PuaPb [20] have been reported, the possible existence of La,Pb was investigated. X-ray analysis of as-cast and heat-treated La-25 at.% Pb samples showed no evidence for the existence of the compound LasPb. Acknowledgements The authors express their appreciation to B. J. Beaudry and P. E. Palmer for preparing the praseodymium, and to H. Baker for the metallography. We wish to thank J. Ho11for his assistance in some of the experimental work.
281
References 1 0. D. MeMasters and K. A. Gschneidner, Jr., Trans. Metall. Sot. AIME, 239 (1967) 781. 2 0. N. Carlson, F. A. Schmidt and D. E. Diesburg, ASM Trans. Q., 60 (1967) 119. 3 0. D. McMasters and K. A. Gschneidner, Jr., J. Less-Common Met., 13 (1967) 193. 4 0. D. McMasters, T. J. O’Keefe and K. A. Gschneidner, Jr., Trans. Metall. Sac, AIME, 242 (1968) 936. 5 J. T. Dame1 and K. A. Gschneidner, Jr., J. Nucl. Mater., 29 (1969) 111. 6 0. D. MeMasters, S. D. Soderquist and K. A. Gschneidner, Jr., ASM Trans. Q., 61 (1968) 435. 7 0. D. McMasters and K, A. Gs~neidner, Jr., J. Less-Common Met., 19 (1969) 33’7. 8 K. A. Gschneidner, Jr. and 0. D. I&Masters, Mona&h. Chem., 101(197X) 1499. 9 M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 2nd edn., 1958. 10 K. A. Gschneidner, Jr., Rare Earth Alloys, Van Nostrand, Princeton, N. J., 1961. 11 R. B. Griffin and K. A. Gschneidner, Jr., Metall. Trans., 2 (1971) 2517. 12 W. Jeitschko and E. Parthe, Acta Crystallogr., 22 (1967) 551. 13 A. Palenzona and M. L. Fornasini, Atti Accad. Naz. Lincei, Cl. Sci. Fis. Mat. Nat., Rend., 40 (1966) 1040. 14 F. Merlo and M. L. Fornasini, Atti Accad. Naz. Lincei, Cl. Sci. Fis. Mat. Nat., Rend., 46 (1969) 265. 15 Y. B. Kuzma, R. V. Skolozdra and V. Ya. Markiv, Dopov. Akad. Nauk Ukr. RSR, (1964) 1070. 16 A. Palenzona, J. Less-Common Met,, 10 (1966) 290. 17 K. Yvon, W. Jeitschko and E, Parthe, A Fortran IV program for the intensity calculations of powder patterns, Univ. Penn, Lab. Res. Structure of Matter, 1969. 18 R. E. Vogel and C. P. Kempter, Acta Crystallogr., 14 (1961) 1130. 19 W. Jeitsehko, H. Nowotny and F. Benesovsky, Monatsh. Chem:, 95 (1964) 1040. 20 D. H. Wood, E. M. Cramer, P. L. Wallace and W. J. Ramsey, J. Nucl. Mater., 32 (1969) 193.
THE PRASEODYMIUM-LEAD
SYSTEM*
0. D. McMASTERS and K. A. GSCHNEIDNER, Ames Laboratory-ERDA Iowa 50010 (U.S.A.)
275
and Department
Jr.
of Metallurgy, Iowa State University, Ames,
(Received August 27, 1975)
Summary X-ray diffraction, differential thermal, and metallographic methods, were used to establish the praseodymium-lead phase diagram. Eutectic reactions occur at 9.3 at.% Pb and 824 ‘C, at 55.5 at.% Pb and 1145 ‘C, at 62.5 at.% Pb and 1085 “C and at greater than 99.5 at.% Pb and 325 “C. The intermetallic compounds PraPb, Pr5Pb4, PrllPblo and PrPba decompose peritectically at 860,1455, 1365 and 1090 “C, respectively. PqPb,, Pr3Pb4 and PrPba melt congruently at 1495,118O and 1120 “C, respectively. The crystal structure data are given for these compounds. The solid solubility of lead in praseodymium is 3.5 at.% Pb at the 824 “C! eutectic temperature. The alloying characteristics of some of the Pr-Pb alloys are compared with those in the La-Pb and Pu-Pb systems.
Introduction The systematic study of the rare earth-lead alloys was continued with the determination of the praseodymium-lead phase diagram. The R-Pb systems already completed in this program are for R = ytterbium [l],yttrium [ 21, europium [ 31, dysprosium [ 41, gadolinium [ 51, lanthanum [ 61 and lutetium [7]. The properties of these alloys have been subjected to a comparative analysis [ 81 which includes predictions concerning the alloying behavior of praseodymium with lead. The results of previous Pr-Pb phase studies, summarized by Hansen [9] and Gschneidner [lo], are incomplete and inaccurate, primarily because high-purity metals were not available at that time. The structure data for PraPb [ll], Pr,Pb, [12,13], Pr,Pb, [14] and PrPba [15,16] were confirmed in this investigation. *Work performed for the U.S. Energy Research and Development Administration under Contract No. W-7405eng-82.
276
Experimental
procedure
Materials The lead used in this investigation was obtained from Cominco Products, Inc., and was specified to be 99.99% pure. The praseodymium was prepared in this laboratory by the calcium reduction of the fluoride followed by a vacuum casting of the praseodymium in tantalum crucibles. The major impurities (atomic ppm) in the praseodymium were: Ca (<50), Fe (4), Si (15), other rare-earths (
Results The praseodymium-lead phase diagram which resulted from this investigation is shown in Fig. 1. The Pr-rich portion of the diagram determined by Griffin and Gschneidner [ll] is presented in Fig. 2. Most of the phase boundaries of the Pr-Pb system were confirmed by X-ray and metallographic methods. Eutectic reactions and terminal solid solubilities The Pr-rich portion of the Pr-Pb diagram was determined by X-ray, metallographic and DTA methods [ll] . A eutectic occurs at 9.3 + 0.1 at.% Pb and 824 f 1 OC, and the solid solubility of Pb in p-Pr is 3.5 + 0.2 at.% at this temperature. There is a eutectoid reaction at 3.1 + 0.2 at.% Pb and 778 f 1 ‘C, and,the solubility of Pb in cu-Pr at this temperature is 2.2 10.2 at.% Pb. Metallographic examination of samples quenched from 760 “C showed about 1.3 at.% Pb solubility in a-Pr. Additions of praseodymium lower the 327 “C melting point of lead to about 325 ‘C, which suggests that a eutectic reaction occurs at 325 “C and with less than 0.5 at.% Pr. The solid solubility of Pr in Pb is practically negligible. The eutectic reaction at 55.5 at.% Pb and 1145 “C was established on the basis of DTA results. The microstructure of a 63.1 at.% Pb alloy was essentially that of a eutectic in which a partial separation of the Pr,Pb4 and PrPb, components had occurred. The eutectic reaction was found to be at 62.5 at.% Pb and 1085 “C. Interpretation of both the metallographic and DTA data is difficult due to the number of reactions occurring in a relatively small area of the diagram, i.e., 55 - 75 at.% Pb and 1080 - 1180 “C.
i
t
500; 400
t
300-
I
lOOiI
I.,
0;;-ATOMIC
1
50 60 PERCENT
l/_LJ
70 LEAD
80
90
Ph
Pr
2
4 ATOMIC
6 8 PERCENT
IO LEAi,
i2
14
Fig. 1. Praseodymium-lead phase diagram. Differential thermal data: A, heating arrest; v, cooling arrest; x , liquidus data. Fig. 2. Pr-rich portion of the Pr-Pb alloy system after Griffin and Gschneidner [ 111.
X-ray single-crystal and powder diffraction methods were used to investigate the crystalline nature of the compounds in this system. The computer program used to generate the powder pattern data for the compound structures was written by Yvon et al. [17]. The Nelson-Riley function was used in conjunction with the VogelKempter [ 181 extrapolation program to obtain the lattice parameters of some of the phases. Most of the structure data for these compounds are summarized in Table 1. PrsPb (25.0 at. % Pb) This phase decomposes peritectically at 860 f 5 “C and crystallizes in the AuCus-type structure with a, = 4.948 -10.005 A. As noticed in the DTA, the formation of PrsPb is quite sluggish. The magnitude of its decomposition thermal arrest could be increased by holding the samples (compositions between 15 and 30 at.% Pb) at 840 “C for one day during the DTA runs. The microstructure of a 25.1 at.% Pb sample which had been heat treated at 800 “C for one week was nearly single phase.
MngSi~ 0%
Sm sGeg
HollGelo
hexagonal Pfia/mcm
orthorhombic Pnma
tetragonal 14/mmm
Pr,Pb,
P%Pb4
P%,Pblo
-.
a c
AuCu, LIZ
cubic Pm3m
PrPbs
9.6 9.4
694.9 114.8
a = 4.66 c = 32.0 a, = 4.8596 2 3 a = 4.867
*4.948 2 5 means the error is + 5 in the last digit, e.g., 4.948 + 0.005.
HfGaz tentative
tetragonal 14l/amd
PrPb,
12.1
2.6
1146.4
2437.5
2.4
10.5
Volume contraction
515.0
121.1
Cell volume
single crystal, parameters based on front reflection powder pattern data
r141
verified in this work
P31
powder and single crystal patterns
powder patterns ill1
Remarks
powder patterns [15,161
powder patterns
based on DTA and metallographic evidence.
= 11.87 = 17.30
a = 8.377 b = 16.04 c = 8.532
a0 = 9.343 Lt1 ce = 6.813 f 1 a = 9.337 C = 6.814
a, = 4.948 t 5 a, = 4.949
Lattice* parameters (A)
Existence of compound
AuCu, Lb!
cubic PmSm
Pr,Pb
P%Pb4
structure type
Crystal class space group
Compound
Structure data for Pr-Pb intermetallic compounds
TABLE 1 2
279
The powder patterns of PrsPb were not always well resolved but were of sufficient quality to confirm the Au&a structure-type to be in agreement with the reported data [II] and isotypic CeaPb [ 191. (37.5 at.% Pb) Pr5Pb3 melts congruently at 1495 k 5 “C and crystallizes in the hexagonal Mn,Sia (I&)-type structure with the lattice parameters listed in Table 1. Both single-crystal and powder patterns were used to obtain these data, which are in agreement with the literature values [12, 131. PrsPb,
(44.4 at,% Pb) Similarly, the SmSGe,type structure reported [ 14 3 for Pr5Pb4 and lattice parameters were confirmed in this study. This compound melts peritectically at I.455 t 10 “C, and the microstructure of a 44.4 at.% Pb sample, heat-treated at 1000 “C for three days, exhibits only traces of the Pr,Pba second phase. Pr,Pb,
(47.6 at.% Pb) Although inconsistent, the micrascopic and DTA data suggested the existence of this compound. The incons~s~ncies arose because of the, presumably, greater stability of the neighboring phases, which led to segregation problems. Single-crystal Weissenberg patterns confirmed the PrllPblo phase with the HollGe Io-type structure. The complex powder patterns were partially indexed, and the lattice parameters listed in Table 1 are based on front-reflection data from both the powder and Weissenberg patterns. PrllPblo decomposes peritectically at 1365 “C. On cooling during the DTA runs, this compound did not form until about 1300 “C and under-cooling was observed in nearly all cases, as shown in Fig. 1. PrltPblo
(57.14 at.% Pb) According to the DTA results, PrBPbcimelts congruently at 1180 + 5 “C. Microscopically, the as-cast 57.6 at.% Pb sample is nearly single phase. Evidence of the segregated-melt problem, or non-equilibrium, appeared in the microscopic examination of a 57.1 at.% Pb sample which had been heattreated at 1000 “C for three days. Some areas of this sample were two-phase, and occasionally small amounts of a third phase were present in the form of a precipitate. Reliable X-ray diffraction data could not be obtained for this phase because of the high reactivity of the samples with atmospheric gases. Pr,Pb,
PrPbB (66.67
ut.% Pb)
Equally as complex was the establishment of PrPba. The microstructure of a 66.6 at.% Pb sample is definitely single-phase and the DTA results showed that PrPba melts perit~cti~~ly at 1090 k 5 “G, Single crystals of this phase could not be obtained. Powder patterns were indexed on the basis of a large tetragonal cell, probably of the HfGaa-type structure, but without the single-crystal evidence to support this structure data, the tentative aspects of the PrPbz unit cell given in Table 1 will remain.
280
PrPb, (75.0 at.% Pb) This compound melts congruently at 1120 f 5 “C and crystallizes in the AuCua-type structure with a,, = 4.8596 + 0.0003 A. These powder pattern data are in agreement with the literature values.
Discussion The segregated-melt conditions described in the plutonium-lead alloy study [20] were also encountered in our investigation. The DTA results between 40 and 70 at.% Pb are markedly influenced by the effects of this problem. Master alloys for the DTA experiments were prepared by arcmelting, and after each run the composition was changed by adding either Pr or Pb to the tantalum crucibles containing the samples. Attempts to homogenize the samples by inductively heating them to 1200 ‘C, first in the upright position, then in the inverted position and again in the upright position, did not completely eliminate the segregation, which apparently stems from the high stability of Pr5Pb3. The microstructures of the eutectic phases in this region are invariably of a degenerate nature, which suggests that the formation of the peritectic compounds will probably be difficult. The existence of segregation was suggested above when it was stated that single crystals of PraPb4 were obtained from the top of a 48 at.% Pb DTA sample. It is believed that segregation in these alloys could be minimized by heat treating the samples at higher temperatures (1400 “C) with more thorough mixing. In restrospect, segregated-melt conditions existed in the La-Pb phase diagram determination [6] . The compound reaction temperatures in the La-Pb system are similar to those in the PrPb system, but the interpretation of the data led to different results. The LasPb4 phase reported by Merlo and Fornasini [ 141 was overlooked in the La-Pb phase study [6] . The stoichiometry of LallPblo was not well-documented and could possibly be La,Pb,, with the melting characteristics of Pr5Pb4 being applicable. This would result in LaBPbd melting incongruently at 1350 ‘C, and elimination of the 1160 “C eutectoid horizontal on the basis of segregated-melt effects. The existence of LadPbs was well-documented but an isotypic phase in the Pr-Pb system was not observed. Since PrsPb forms sluggishly, and CesPb [19] and PuaPb [20] have been reported, the possible existence of La,Pb was investigated. X-ray analysis of as-cast and heat-treated La-25 at.% Pb samples showed no evidence for the existence of the compound LasPb. Acknowledgements The authors express their appreciation to B. J. Beaudry and P. E. Palmer for preparing the praseodymium, and to H. Baker for the metallography. We wish to thank J. Ho11for his assistance in some of the experimental work.
281
References 1 0. D. MeMasters and K. A. Gschneidner, Jr., Trans. Metall. Sot. AIME, 239 (1967) 781. 2 0. N. Carlson, F. A. Schmidt and D. E. Diesburg, ASM Trans. Q., 60 (1967) 119. 3 0. D. McMasters and K. A. Gschneidner, Jr., J. Less-Common Met., 13 (1967) 193. 4 0. D. McMasters, T. J. O’Keefe and K. A. Gschneidner, Jr., Trans. Metall. Sac, AIME, 242 (1968) 936. 5 J. T. Dame1 and K. A. Gschneidner, Jr., J. Nucl. Mater., 29 (1969) 111. 6 0. D. MeMasters, S. D. Soderquist and K. A. Gschneidner, Jr., ASM Trans. Q., 61 (1968) 435. 7 0. D. McMasters and K, A. Gs~neidner, Jr., J. Less-Common Met., 19 (1969) 33’7. 8 K. A. Gschneidner, Jr. and 0. D. I&Masters, Mona&h. Chem., 101(197X) 1499. 9 M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 2nd edn., 1958. 10 K. A. Gschneidner, Jr., Rare Earth Alloys, Van Nostrand, Princeton, N. J., 1961. 11 R. B. Griffin and K. A. Gschneidner, Jr., Metall. Trans., 2 (1971) 2517. 12 W. Jeitschko and E. Parthe, Acta Crystallogr., 22 (1967) 551. 13 A. Palenzona and M. L. Fornasini, Atti Accad. Naz. Lincei, Cl. Sci. Fis. Mat. Nat., Rend., 40 (1966) 1040. 14 F. Merlo and M. L. Fornasini, Atti Accad. Naz. Lincei, Cl. Sci. Fis. Mat. Nat., Rend., 46 (1969) 265. 15 Y. B. Kuzma, R. V. Skolozdra and V. Ya. Markiv, Dopov. Akad. Nauk Ukr. RSR, (1964) 1070. 16 A. Palenzona, J. Less-Common Met,, 10 (1966) 290. 17 K. Yvon, W. Jeitschko and E, Parthe, A Fortran IV program for the intensity calculations of powder patterns, Univ. Penn, Lab. Res. Structure of Matter, 1969. 18 R. E. Vogel and C. P. Kempter, Acta Crystallogr., 14 (1961) 1130. 19 W. Jeitsehko, H. Nowotny and F. Benesovsky, Monatsh. Chem:, 95 (1964) 1040. 20 D. H. Wood, E. M. Cramer, P. L. Wallace and W. J. Ramsey, J. Nucl. Mater., 32 (1969) 193.