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The binary phase diagrams of thallium with gadolinium, terbium and dysprosium
Journal of the Less-Common
Metals, 136 (1988)
249 - 259
249
THE BINARY PHASE DIAGRAMS OF THALLIUM WITH GADOLINIUM, TERBIUM AND DYSPROSIUM A. SACCONE, S. DELFINO, G. CACCIAMANI and R. FERRO zstituto di Chimica Generale dell’Universita’, Genova (Italy)
Universitci di Genova,
viale Benedetto
XV,3,
(Received April 30, 1987)
Summary The Tb-Tl and Dy-Tl systems have been studied by differential thermal analysis, X-ray examination, metallography and microprobe analysis. A reinvestigation of the Gd-TI system has been also carried out in the 35 - 47 at.% thallium composition range. In the Tb-Tl system the following intermetallic compounds exist: TbzTl (decomposes at 1040 “C), TbsTl, (m.p. 1290 “C), TbsTls+, , TbTl (m.p. 1300 “C!), TbsTl, (decomposes at 1000 “C) and TbTls (m.p. 940 “C). The Dy-Tl system shows the following phases: DysTl (decomposes at 1190 “C), Dy,Tl, (m.p. 1340 “C), Dy,Tl,+, , DyTl (m.p. 1300 “C), DysTl, (decomposes at 1000 “C) and DyTl, (m.p. 940 “C). For both systems four eutectics and two peritectics occur. From the reinvestigation of the Gd-Tl system a congruent melting for the Gd,Tl, phase and the existence of a new phase (Gd,Tl, +,) have been proposed. The strict analogies between the three systems are discussed and compared with the volume contraction trends. 1. Introduction Several rare earth alloys with elements of the IIIA group have recently been investigated; among these we have previously studied the following R-In systems: Ce-In [ 11, Pr-In [2], Sm-In [3] and Cd-In [ 41; and the following R-T1 systems: La-T1 [5], Ce-Tl [l], Pr-Tl [6], Nd-Tl [7], Sm-Tl [8], Gd-Tl [4], Ho-T1 [9] and Er-Tl [9]. The general alloying behaviour of the rare earths with indium has also been discussed [lo]. Following these studies, this paper reports the results that were obtained with the Tb-Tl and Dy-Tl diagrams and with the revision of some parts of the Gd-Tl system. Previous knowledge about these systems is expressed in the literature by a series of data concerning the intermediate phases and their crystal structure as summarized, for the Tb-Tl and Dy-Tl systems, by Tables 1 and 2, where the literature data are compared with our present observations. No 0022-5088/88/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
250 TABLE
1
Crystal
structure
Phase
data of terbium-thallium
Structural
Tb,TI
typea
hexagonal, hP6-NizIn
TbST13
hexagonal, hP16-Mn$i,
phases
Lattice constantsb
AV/vC
6)
(W)
a * 5.362 c = 6.663 c/a f 1.243
-10.4
10.45
this work
a = 5.365 c = 6.694 c/a = 1.248
-9.9
10.39
11
a = 8.95 - 8.98 c = 6.60 - 6.62 c/a = 0.14
-6.8
a = 8.978 c = 6.596 c/a = 0.74
-6.3
10.15
12
Ref. cmw3)
- -5.9
10.21
_10.11
this work
TbST13
tetragonal, tI32-WsSi3
a = 12.126 c = 6.135 c/a = 0.51
-8.2
10.36
this work
TbsTl3 +x
tetragonal, related to the tI32-BaSPb3 type
a = 8.073 c = 14.281 c/a = 1.77
(-5.3)
(10.05)
this work
TbTl
cubic, cP2-CsC1 or cI2-W
a = 3.760
-12.3
11.35
13
tetragonal, tP4-AuCu(1)
a = 3.53 c = 4.24 c/a = 1.20
-12.8
11.42
14
Tb3TIS
orthorhombic, related to the oC32-Pu$‘dS type
a = 9.99 b = 8.06 c = 10.35 b/a = 0.807 c/a = 1.036
-12.8
11.94
15
TbTIB
cubic, cP4-AuCus
a = 4.682
-12.9
12.49
this work
LI= 4.679
-13.1
12.51
16
aThe
Pearson
symbol
of the unit
cell has been reported
(for
instance
hp16
gonal, P = primitive, 16 = atom number in the unit cell). bData have been summarized considering samples having either single composition. For the homogeneity ranges of the various phases, see text. cAV/V = [lo0 ( Vcell - ZV,,)/ZV,,]%; Vat = mean atomic volume.
or
: h = hexatwo-phase
information was previously available about the phase diagram of Tb-Tl, while for the Dy-Tl diagram, only a tentative version had been suggested in a preliminary report [ 181. The work of Palenzona et al. [ 191 concerning the heats of formation, and the enthalpies and entropies of melting of
251
TABLE 2 Crystal Phase
DY
zT’
DYsT’,
DYS’&+,
DyTl
DYE’&
DY%
structure
data of dysprosium-thallium
Structuml
typea
Lattice
phases
constantsb
AV/Vc
Ref.
0%
(%I
D (f3 cmp3)
hexagonal, hP6-NiaIn
(I = 5.301 c = 6.652 c/a = 1.255
-11.1
10.86
this work
hexagonal, hP16-MnsSis
a = 8.91 - 8.93 c = 6.56 - 6.69 c/a = 0.14
-1.4 --6.5
10.50-10.40
this work
a = 8.921 c = 6.584 c/p = 0.14
-6.8
10.43
12
tetragonal, related to the tI32-BasPbs type
a= 8.010
(-5.9)
(10.33)
thiswork
c= 14.290 c/a= 1.18
cubic, cP2-CsCl or cI2-W
Cl=3.743
-12.8
11.62
11
tetragonal, tP4-AuCu(1)
a= 3.52 c = 4.21 c/a= 1.20
-13.3
11.68
14
-13.2
12.15
15
a= 4.613-4.611
-13.0 m-12.3
12.6
thiswork
a= 4.612
-13.1
12.6
11
orthorhombic, related to the oC32-PusPdS type
cubic, cP4-AuCus
- a = 9.95 b= 8.03
c = 10.33 b/a = 0.801 c/a= 1.038
“The Perason symbol of the unit cell has been reported (for instance hP16 gonal, P = primitive, 16 = atom number in the unit cell). bData have been summarized considering samples having either single or composition. For the homogeneity ranges of the various phases, see text. cAV/~= [loo(vcdl - lZV,,)/XV,,]%; Vat= mean atomic volume.
: h = hexatwo-phase
TbTl, and DyTl, should also be mentioned. The Gd-Tl system had been thoroughly studied [ 41. A comparison of that diagram with those of thallium alloys with other rare earths has shown that for the great majority of compositions there are a series of close similarities. These similarities were considered to support the reliability of both the measurements and their interpretation. Nevertheless, near the 5:3 composition of the Gd-Tl system, a different interpretation of the available data seemed to be suggested. This section of the system was therefore reexamined.
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z9z
253
-
Tb
20
wt%Tl-
60
40 -at%TI
60
TI
-
Fig. 1. Tb-Tl phase diagram (V thermal effects observed on cooling).
prepared and identified: DyzT1 is isostructural with TbzTl and its parameters are reported in Table 1. The thermal effect corresponding to the peritectic reaction of the Dy,Tl is rather sluggish. The metallographic appearance suggests the existence of a homogeneity range for both phases which is fairly broad at high temperature. phase was
3.3. Phases in the proximity of TbST13 and Dy,Tl, In both systems this region corresponds to very complex phase equilibria which are characterized by the presence of two phases with very similar stoichiometries (5 :3 and 5 :3 + X). In both systems the phase with 5:3 stoichiometry is presumably the congruently melting phase, (1290 “C for TbsTls and 1340 “C for DysTl,, the latter showing the highest thermal stability in the Dy-Tl system).
254 -
wt%Tl_
40
20
60
80
I
1200
, 305 235
20
DY
40
-at
60
x)
%TI-
TI
Fig. 2. Dy-Tl phase diagram (V thermal effects observed on cooling).
For these compositions the existence of the hexagonal hP16-Mn& structure was confirmed. This structure was already reported in the literature [12] for compounds of the heavy earths (starting with gadolinium). This structure has been observed for alloys of composition either close to the theoretical composition (37.5 at.% thallium) or having a small excess of rare earth. Certain data (cell parameter variations, micrographic appearance) suggest that this phase is characterized by a homogeneity range. In the case of samples of TbST13that were cooled at a slow rate (2 “C min-I) the tetragonal t132 structure of the W&G, type (see Table 1) was also observed; this structure is typical of the R5T13 phases with the light rare earths up to samarium. Therefore Tb,Tl, is dimorphic even though no thermal effect, that could be interpreted as a solid state transformation, was observed on this composition. Considering, however, that the W&-type form was obtained only by slow cooling (while annealing at 900 - 1000 “C
255
followed by quenching resulted in the MnsSis form) we believe that the Mn,$is form is the modification stable at high temperature. This behaviour appears analogous, for instance, with that of the similar Ho&s compound, which is also dimorphic and shows both the MnsSis-type structure at high temperature [ 203 and the W&-type structure at low temperature [ 211. In both Tb-Tl and Dy-Tl systems, for compositions close to the 5:3 stoichiometric ratio, a’further thallium-richer phase was observed, whose powder photograph shows clear similarities with that of the BasPbs-type structure; we have provisionally assigned the stoichiometries R,Tls+, to these phases. Alloy samples having a composition within the range from 37.5 to 39 at.% thallium show a two-phase aspect under the optical or the electronic microscope. Their powder photographs show the two diffraction patterns characteristic of the MnsSis and BasPbs structures. As regards the mechanism of formation of these BasPbs-type phases we presume, on the basis of microscopic aspects, that a peritectic reaction occurs. Also because of the high temperature involved, it was not possible to assessthis point clearly. All the phases belonging to this region show broad solid solubility ranges which are, however, characterized by a considerable temperature dependence. The phases TbsTls+, and DysTls., form with the subsequent 1 :l phase eutectics according to the following reactions: L (43.5 + 0.5 at.% thallium) I
TbsTls+, + TbTl (T= 1210 “C)
L (45.5 + 0.5 at.% thallium) I
Dy,Tls+,
+ DyTl (T = 1230 “C)
3.4. TbTl and DyTl phases The compounds TbTl and DyTl both melt congruently at 1300 “C. TbTl shows the highest thermal stability in the Tb-TI system, in analogy with the behaviour of the RTl phases in the systems of the preceding rare earths with thallium. These phases have wide solid solubility ranges and (like the other RTl phases and the similar RIn compounds) have structures that have been described as belonging to the C&l type [13, 171. According to Sekizawa et aZ. [14] this is the stable structure at room and at high temperature, and the different RTI compounds undergo, at lower temperatures, a cubic to tetragonal transformation. At a certain temperature the intensities of the cubic reflection lines start to decrease and those of the tetragonal reflections begin to grow with decreasing temperature. The temperature range over which both cubic and tetragonal phases coexist, extends over about 200 K. As the temperature increases, the diffraction pattern shows extensive hysteresis. Heating to a temperature about 100 K above the starting temperature of the transformation was necessary to restore the cubic CsCl structure. The starting temperature of the transition is well defined for each compound, though the temperature range of the transition is wide. They take the
256
starting temperature T, at which the transformation starts in the cooling run, as the characteristic temperature of the transformation. The influence of the change of several conditions of sample preparations and experiments on the transformation was investigated. Intentional deviation of the composition from the ratio 1 :l always decreased the transformation temperature Z’,. The transition temperature T, has the highest value, 300 K, in GdTl. (T, = 250 K for TbTl and T, = 240 K for DyTl). Sekizawa et al. also indicated that the tetragonal-type structure may form at higher temperature by preparing polycrystalline samples of compounds by arc melting, and then heat-treating each ingot at 850 “C for 120 h. The powders obtained by crushing in an agate mortar was annealed at 600 “C for 20 h. Before annealing, all the powdered samples showed the diffraction patterns of mixtures of the cubic and tetragonal phases at room temperature. After annealing, all the compounds showed the cubic CsCl-type diffraction patterns at room temperature. This behaviour is in agreement with the data of Iandelh and Palenzona [13] who observed only the CsCl-type structure. They prepared the different RTl compounds by melting weighed quantities of the two metals in the form of turnings (after preliminary heating for 12 h at 400 “C), at about 100 “C above the melting point of the alloys. No other heat treatment was carried out; the powders obtained by melting the samples under inert gases were annealed at 300 “C. In the course of this and previous work on R-T1 systems by our group, the presence of the tetragonal structure was confirmed in powdered, nonannealed samples. Because of the good agreement between calculated and observed intensities, this structure is here described as belonging to the tP4-AuCu(1) type (that is, with a double volume cell compared to the one proposed by Sekizawa et al. [14] a = 42 aseb). All the available data seem therefore to confirm that, for the RTl phases, the CsCl-type structure is the stable structure at room temperature (and at higher temperatures). These phases, however, easily undergo a transformation (whose mechanism and kinetics deserve deeper investigation) into tetragonal structures which seem to be stable at low temperature and metastable at room temperature. 3.5. Tb3T1, and Dy,Tl, phases TbaTls and DyaTls both form peritectically at 1000 + 10 “C. The crystal structure of these phases can be considered as related to the orthorhombic oC32-PusPd, type. The lattice constants obtained from the powder photographs are in good agreement with the previously published data [ 151. The TbsTl, and DysTls phases exist over a range of compositions. For several RsTls phases the existence of a solid state transformation has been suggested [l, 5 - 71. No indication of such a transformation was obtained in this work.
257
In both systems the RsTl, phases form eutectics with the 1:3 compounds. The characteristic data of the eutectics are, in both systems, very near to 930 f 5 “C and 73.5 f 0.5 at.% thallium. 3.6. TbT13 and DyTl,phases and thallium-rich alloys TbTl, and DyTls both melt congruently at a temperature of 940 f 5 “C!. Like the other RTl, phases they are characterized by a very narrow solubility range. The structure of TbTl, and DyTls is cubic, presumably of the cP4AuCu, type, even though no superstructure lines could be observed because of the similar values of the atomic scattering factors. From the melting point of TbTls and DyTl,, the melting temperatures of the thallium-rich alloys decrease to the temperature of an invariant reaction that is very close to the melting point of pure thallium. This reaction may be considered as corresponding to a eutectic at 3 - 5 “C! below the melting point of thallium [ 51. The temperature of (Y2 p transformation of thallium does not seem to be significantly influenced by the addition of terbium or dysprosium.
4. Gd-Tl system The Gd-Tl diagram is shown in Fig. 3 according to our new investigations. A comparison with the previously published version [4] shows that the regions between 0 and approximately 36 at.% and between about 47 and 100 at.% thallium have not been altered. The central portion has been modified, particularly, for the phases in the proximity of the 5:3 stoichiometry. A peritectic formation of GdsTl, (from the liquid and GdTl) was initially proposed for the invariant equilibrium at 1200 “C. The new data as well as the re-examination of the previous measurements suggest that the 1200 “C invariant effect is due to a eutectic reaction between GdsTls, congruently melting at 1220 “C and GdTl (or perhaps more precisely between Gd,Tl,+, and GdTl). In the course of the revision of this system it was observed that, in fact, beside the MnsSis-type GdsTl, phase, a BasPbs-type phase was formed. In analogy with the previously described systems, a GdsTls.. composition is suggested for this new phase. It was not possible, however, to determine the formation mechanism of this phase. 5. General remarks In conclusion, the close similarities between the diagrams formed by these three successive rare earths have been made clear. As far as the temperatures of the different invariant equilibria are concerned, one may notice the very similar values for the thallium-rich alloys, while for the R-rich alloys
258
-
wt%TI60
60
40
20
I
r 305
235’
Gd
20
__
60
40
-
at9bTI -D
60
TI
Fig. 3. Gd-Tl phase diagram as reported in ref. 4; the region between 36 and 47 at.% thallium has been modified on the basis of this work (V thermal effects observed on cooling).
we have, for instance, gradual variations of the R5T1, and RTl melting temperatures with an inversion of the relative stability. The strict analogies among the three systems are also evidenced by the volume contraction trends with composition. These are shown in Fig. 4.
Fig. 4. Trends in volume contractions for the Gd-Tl, Tb-Tl and Dy-TI systems.
259
The rather high values reached by the volume contraction should be noted (around 14% for thallium-rich alloys), as well as the decrease in contraction corresponding to 37 - 38 at.% thallium. This behaviour seems rather characteristic of the thallium alloys; it has been already observed for the alloys with light rare earths (cerium, praseodymium and neodymium) [l, 6,7].
Acknowledgment The financial help obtained by the Italian Minister0 della Pubblica Istruzione is acknowledged with thanks. The authors’ thanks are due also to the Consiglio Nazionale delle Ricerche for their general support.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
S. Delfino, A. Saccone and R. Ferro, 2. Metallkde., 71 (1980) 165. S. Delfino, A. Saccone and R. Ferro, J. Less-Common Met., 65 (1979) 181. A. Saccone, S. Delfino and R. Ferro, J. Less-Common Met., 84 (1982) 281. S. Delfino, A. Saccone and R. Ferro, Z. Metallkde., 74 (1983) 674. S. Delfino, A. Saccone, G. Cacciamani and R. Ferro, 2. Metallkde., 76 (1985) 7. S. Delfino, A. Saccone and R. Ferro, J. Less-Common Met., 79 (1981) 47. S. Delfino, A, Saccone,,G. Borzone and R. Ferro, J. Less-Common Met., 59 (1978) 69. S. Delfino, A. Saccone, G. Borzone and R. Fen-o, Z. Anorg. Allg. Chem., 503 (1983) 184. S. Delfino, A. Saccone, G. Cacciamani and R. Ferro, Z. Metallkde., 78 (1987) 344. S. Delfino, A. Saccone and R. Ferro, J. Less-Common Met., IO2 (1984) 289. A. Palenzona and E. Franceschi, Colloq. Int. CNRS, 1 (1970) 135. E. Franceschi and A. Palenzona, J. Less-Common Met., 18 (1969) 93. A. Iandelli and A. Palenzona, J. Less-Common Met., 9 (1965) 1. K. Sekizawa, H. Chihara and K. Yasukochi, J. Phys. Sot. Jpn., 50 (1981) 3467. S. Delfino, A. Saccone, D. Mazzone and R. Ferro, J. Less-Common Met., 81 (1981) 45. J. L. Moriarty, J. E. Humphreys, R. 0. Gordon and N. C. Baenziger, Acta Crystallogr., 21 (1966) 840. N. C. Baenziger and J. L. Moriarty, Acta Crystallogr., 14 (1961) 948. S. Delfino, A. Saccone and R. Capelli, Atti XIII Cow. Naz. Chim. Inorg. Camerino (Italy), 23 - 26 September, 1980, D12. A. Palenzona and S. Cirafici, Thermochim. Acta, 9 (1974) 419. A. Palenzona, J. Less-Common Met., 16 (1968) 379. E. Franceschi, J. Less-Common Met., 37 (1974) 157.
Metals, 136 (1988)
249 - 259
249
THE BINARY PHASE DIAGRAMS OF THALLIUM WITH GADOLINIUM, TERBIUM AND DYSPROSIUM A. SACCONE, S. DELFINO, G. CACCIAMANI and R. FERRO zstituto di Chimica Generale dell’Universita’, Genova (Italy)
Universitci di Genova,
viale Benedetto
XV,3,
(Received April 30, 1987)
Summary The Tb-Tl and Dy-Tl systems have been studied by differential thermal analysis, X-ray examination, metallography and microprobe analysis. A reinvestigation of the Gd-TI system has been also carried out in the 35 - 47 at.% thallium composition range. In the Tb-Tl system the following intermetallic compounds exist: TbzTl (decomposes at 1040 “C), TbsTl, (m.p. 1290 “C), TbsTls+, , TbTl (m.p. 1300 “C!), TbsTl, (decomposes at 1000 “C) and TbTls (m.p. 940 “C). The Dy-Tl system shows the following phases: DysTl (decomposes at 1190 “C), Dy,Tl, (m.p. 1340 “C), Dy,Tl,+, , DyTl (m.p. 1300 “C), DysTl, (decomposes at 1000 “C) and DyTl, (m.p. 940 “C). For both systems four eutectics and two peritectics occur. From the reinvestigation of the Gd-Tl system a congruent melting for the Gd,Tl, phase and the existence of a new phase (Gd,Tl, +,) have been proposed. The strict analogies between the three systems are discussed and compared with the volume contraction trends. 1. Introduction Several rare earth alloys with elements of the IIIA group have recently been investigated; among these we have previously studied the following R-In systems: Ce-In [ 11, Pr-In [2], Sm-In [3] and Cd-In [ 41; and the following R-T1 systems: La-T1 [5], Ce-Tl [l], Pr-Tl [6], Nd-Tl [7], Sm-Tl [8], Gd-Tl [4], Ho-T1 [9] and Er-Tl [9]. The general alloying behaviour of the rare earths with indium has also been discussed [lo]. Following these studies, this paper reports the results that were obtained with the Tb-Tl and Dy-Tl diagrams and with the revision of some parts of the Gd-Tl system. Previous knowledge about these systems is expressed in the literature by a series of data concerning the intermediate phases and their crystal structure as summarized, for the Tb-Tl and Dy-Tl systems, by Tables 1 and 2, where the literature data are compared with our present observations. No 0022-5088/88/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
250 TABLE
1
Crystal
structure
Phase
data of terbium-thallium
Structural
Tb,TI
typea
hexagonal, hP6-NizIn
TbST13
hexagonal, hP16-Mn$i,
phases
Lattice constantsb
AV/vC
6)
(W)
a * 5.362 c = 6.663 c/a f 1.243
-10.4
10.45
this work
a = 5.365 c = 6.694 c/a = 1.248
-9.9
10.39
11
a = 8.95 - 8.98 c = 6.60 - 6.62 c/a = 0.14
-6.8
a = 8.978 c = 6.596 c/a = 0.74
-6.3
10.15
12
Ref. cmw3)
- -5.9
10.21
_10.11
this work
TbST13
tetragonal, tI32-WsSi3
a = 12.126 c = 6.135 c/a = 0.51
-8.2
10.36
this work
TbsTl3 +x
tetragonal, related to the tI32-BaSPb3 type
a = 8.073 c = 14.281 c/a = 1.77
(-5.3)
(10.05)
this work
TbTl
cubic, cP2-CsC1 or cI2-W
a = 3.760
-12.3
11.35
13
tetragonal, tP4-AuCu(1)
a = 3.53 c = 4.24 c/a = 1.20
-12.8
11.42
14
Tb3TIS
orthorhombic, related to the oC32-Pu$‘dS type
a = 9.99 b = 8.06 c = 10.35 b/a = 0.807 c/a = 1.036
-12.8
11.94
15
TbTIB
cubic, cP4-AuCus
a = 4.682
-12.9
12.49
this work
LI= 4.679
-13.1
12.51
16
aThe
Pearson
symbol
of the unit
cell has been reported
(for
instance
hp16
gonal, P = primitive, 16 = atom number in the unit cell). bData have been summarized considering samples having either single composition. For the homogeneity ranges of the various phases, see text. cAV/V = [lo0 ( Vcell - ZV,,)/ZV,,]%; Vat = mean atomic volume.
or
: h = hexatwo-phase
information was previously available about the phase diagram of Tb-Tl, while for the Dy-Tl diagram, only a tentative version had been suggested in a preliminary report [ 181. The work of Palenzona et al. [ 191 concerning the heats of formation, and the enthalpies and entropies of melting of
251
TABLE 2 Crystal Phase
DY
zT’
DYsT’,
DYS’&+,
DyTl
DYE’&
DY%
structure
data of dysprosium-thallium
Structuml
typea
Lattice
phases
constantsb
AV/Vc
Ref.
0%
(%I
D (f3 cmp3)
hexagonal, hP6-NiaIn
(I = 5.301 c = 6.652 c/a = 1.255
-11.1
10.86
this work
hexagonal, hP16-MnsSis
a = 8.91 - 8.93 c = 6.56 - 6.69 c/a = 0.14
-1.4 --6.5
10.50-10.40
this work
a = 8.921 c = 6.584 c/p = 0.14
-6.8
10.43
12
tetragonal, related to the tI32-BasPbs type
a= 8.010
(-5.9)
(10.33)
thiswork
c= 14.290 c/a= 1.18
cubic, cP2-CsCl or cI2-W
Cl=3.743
-12.8
11.62
11
tetragonal, tP4-AuCu(1)
a= 3.52 c = 4.21 c/a= 1.20
-13.3
11.68
14
-13.2
12.15
15
a= 4.613-4.611
-13.0 m-12.3
12.6
thiswork
a= 4.612
-13.1
12.6
11
orthorhombic, related to the oC32-PusPdS type
cubic, cP4-AuCus
- a = 9.95 b= 8.03
c = 10.33 b/a = 0.801 c/a= 1.038
“The Perason symbol of the unit cell has been reported (for instance hP16 gonal, P = primitive, 16 = atom number in the unit cell). bData have been summarized considering samples having either single or composition. For the homogeneity ranges of the various phases, see text. cAV/~= [loo(vcdl - lZV,,)/XV,,]%; Vat= mean atomic volume.
: h = hexatwo-phase
TbTl, and DyTl, should also be mentioned. The Gd-Tl system had been thoroughly studied [ 41. A comparison of that diagram with those of thallium alloys with other rare earths has shown that for the great majority of compositions there are a series of close similarities. These similarities were considered to support the reliability of both the measurements and their interpretation. Nevertheless, near the 5:3 composition of the Gd-Tl system, a different interpretation of the available data seemed to be suggested. This section of the system was therefore reexamined.
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z9z
253
-
Tb
20
wt%Tl-
60
40 -at%TI
60
TI
-
Fig. 1. Tb-Tl phase diagram (V thermal effects observed on cooling).
prepared and identified: DyzT1 is isostructural with TbzTl and its parameters are reported in Table 1. The thermal effect corresponding to the peritectic reaction of the Dy,Tl is rather sluggish. The metallographic appearance suggests the existence of a homogeneity range for both phases which is fairly broad at high temperature. phase was
3.3. Phases in the proximity of TbST13 and Dy,Tl, In both systems this region corresponds to very complex phase equilibria which are characterized by the presence of two phases with very similar stoichiometries (5 :3 and 5 :3 + X). In both systems the phase with 5:3 stoichiometry is presumably the congruently melting phase, (1290 “C for TbsTls and 1340 “C for DysTl,, the latter showing the highest thermal stability in the Dy-Tl system).
254 -
wt%Tl_
40
20
60
80
I
1200
, 305 235
20
DY
40
-at
60
x)
%TI-
TI
Fig. 2. Dy-Tl phase diagram (V thermal effects observed on cooling).
For these compositions the existence of the hexagonal hP16-Mn& structure was confirmed. This structure was already reported in the literature [12] for compounds of the heavy earths (starting with gadolinium). This structure has been observed for alloys of composition either close to the theoretical composition (37.5 at.% thallium) or having a small excess of rare earth. Certain data (cell parameter variations, micrographic appearance) suggest that this phase is characterized by a homogeneity range. In the case of samples of TbST13that were cooled at a slow rate (2 “C min-I) the tetragonal t132 structure of the W&G, type (see Table 1) was also observed; this structure is typical of the R5T13 phases with the light rare earths up to samarium. Therefore Tb,Tl, is dimorphic even though no thermal effect, that could be interpreted as a solid state transformation, was observed on this composition. Considering, however, that the W&-type form was obtained only by slow cooling (while annealing at 900 - 1000 “C
255
followed by quenching resulted in the MnsSis form) we believe that the Mn,$is form is the modification stable at high temperature. This behaviour appears analogous, for instance, with that of the similar Ho&s compound, which is also dimorphic and shows both the MnsSis-type structure at high temperature [ 203 and the W&-type structure at low temperature [ 211. In both Tb-Tl and Dy-Tl systems, for compositions close to the 5:3 stoichiometric ratio, a’further thallium-richer phase was observed, whose powder photograph shows clear similarities with that of the BasPbs-type structure; we have provisionally assigned the stoichiometries R,Tls+, to these phases. Alloy samples having a composition within the range from 37.5 to 39 at.% thallium show a two-phase aspect under the optical or the electronic microscope. Their powder photographs show the two diffraction patterns characteristic of the MnsSis and BasPbs structures. As regards the mechanism of formation of these BasPbs-type phases we presume, on the basis of microscopic aspects, that a peritectic reaction occurs. Also because of the high temperature involved, it was not possible to assessthis point clearly. All the phases belonging to this region show broad solid solubility ranges which are, however, characterized by a considerable temperature dependence. The phases TbsTls+, and DysTls., form with the subsequent 1 :l phase eutectics according to the following reactions: L (43.5 + 0.5 at.% thallium) I
TbsTls+, + TbTl (T= 1210 “C)
L (45.5 + 0.5 at.% thallium) I
Dy,Tls+,
+ DyTl (T = 1230 “C)
3.4. TbTl and DyTl phases The compounds TbTl and DyTl both melt congruently at 1300 “C. TbTl shows the highest thermal stability in the Tb-TI system, in analogy with the behaviour of the RTl phases in the systems of the preceding rare earths with thallium. These phases have wide solid solubility ranges and (like the other RTl phases and the similar RIn compounds) have structures that have been described as belonging to the C&l type [13, 171. According to Sekizawa et aZ. [14] this is the stable structure at room and at high temperature, and the different RTI compounds undergo, at lower temperatures, a cubic to tetragonal transformation. At a certain temperature the intensities of the cubic reflection lines start to decrease and those of the tetragonal reflections begin to grow with decreasing temperature. The temperature range over which both cubic and tetragonal phases coexist, extends over about 200 K. As the temperature increases, the diffraction pattern shows extensive hysteresis. Heating to a temperature about 100 K above the starting temperature of the transformation was necessary to restore the cubic CsCl structure. The starting temperature of the transition is well defined for each compound, though the temperature range of the transition is wide. They take the
256
starting temperature T, at which the transformation starts in the cooling run, as the characteristic temperature of the transformation. The influence of the change of several conditions of sample preparations and experiments on the transformation was investigated. Intentional deviation of the composition from the ratio 1 :l always decreased the transformation temperature Z’,. The transition temperature T, has the highest value, 300 K, in GdTl. (T, = 250 K for TbTl and T, = 240 K for DyTl). Sekizawa et al. also indicated that the tetragonal-type structure may form at higher temperature by preparing polycrystalline samples of compounds by arc melting, and then heat-treating each ingot at 850 “C for 120 h. The powders obtained by crushing in an agate mortar was annealed at 600 “C for 20 h. Before annealing, all the powdered samples showed the diffraction patterns of mixtures of the cubic and tetragonal phases at room temperature. After annealing, all the compounds showed the cubic CsCl-type diffraction patterns at room temperature. This behaviour is in agreement with the data of Iandelh and Palenzona [13] who observed only the CsCl-type structure. They prepared the different RTl compounds by melting weighed quantities of the two metals in the form of turnings (after preliminary heating for 12 h at 400 “C), at about 100 “C above the melting point of the alloys. No other heat treatment was carried out; the powders obtained by melting the samples under inert gases were annealed at 300 “C. In the course of this and previous work on R-T1 systems by our group, the presence of the tetragonal structure was confirmed in powdered, nonannealed samples. Because of the good agreement between calculated and observed intensities, this structure is here described as belonging to the tP4-AuCu(1) type (that is, with a double volume cell compared to the one proposed by Sekizawa et al. [14] a = 42 aseb). All the available data seem therefore to confirm that, for the RTl phases, the CsCl-type structure is the stable structure at room temperature (and at higher temperatures). These phases, however, easily undergo a transformation (whose mechanism and kinetics deserve deeper investigation) into tetragonal structures which seem to be stable at low temperature and metastable at room temperature. 3.5. Tb3T1, and Dy,Tl, phases TbaTls and DyaTls both form peritectically at 1000 + 10 “C. The crystal structure of these phases can be considered as related to the orthorhombic oC32-PusPd, type. The lattice constants obtained from the powder photographs are in good agreement with the previously published data [ 151. The TbsTl, and DysTls phases exist over a range of compositions. For several RsTls phases the existence of a solid state transformation has been suggested [l, 5 - 71. No indication of such a transformation was obtained in this work.
257
In both systems the RsTl, phases form eutectics with the 1:3 compounds. The characteristic data of the eutectics are, in both systems, very near to 930 f 5 “C and 73.5 f 0.5 at.% thallium. 3.6. TbT13 and DyTl,phases and thallium-rich alloys TbTl, and DyTls both melt congruently at a temperature of 940 f 5 “C!. Like the other RTl, phases they are characterized by a very narrow solubility range. The structure of TbTl, and DyTls is cubic, presumably of the cP4AuCu, type, even though no superstructure lines could be observed because of the similar values of the atomic scattering factors. From the melting point of TbTls and DyTl,, the melting temperatures of the thallium-rich alloys decrease to the temperature of an invariant reaction that is very close to the melting point of pure thallium. This reaction may be considered as corresponding to a eutectic at 3 - 5 “C! below the melting point of thallium [ 51. The temperature of (Y2 p transformation of thallium does not seem to be significantly influenced by the addition of terbium or dysprosium.
4. Gd-Tl system The Gd-Tl diagram is shown in Fig. 3 according to our new investigations. A comparison with the previously published version [4] shows that the regions between 0 and approximately 36 at.% and between about 47 and 100 at.% thallium have not been altered. The central portion has been modified, particularly, for the phases in the proximity of the 5:3 stoichiometry. A peritectic formation of GdsTl, (from the liquid and GdTl) was initially proposed for the invariant equilibrium at 1200 “C. The new data as well as the re-examination of the previous measurements suggest that the 1200 “C invariant effect is due to a eutectic reaction between GdsTls, congruently melting at 1220 “C and GdTl (or perhaps more precisely between Gd,Tl,+, and GdTl). In the course of the revision of this system it was observed that, in fact, beside the MnsSis-type GdsTl, phase, a BasPbs-type phase was formed. In analogy with the previously described systems, a GdsTls.. composition is suggested for this new phase. It was not possible, however, to determine the formation mechanism of this phase. 5. General remarks In conclusion, the close similarities between the diagrams formed by these three successive rare earths have been made clear. As far as the temperatures of the different invariant equilibria are concerned, one may notice the very similar values for the thallium-rich alloys, while for the R-rich alloys
258
-
wt%TI60
60
40
20
I
r 305
235’
Gd
20
__
60
40
-
at9bTI -D
60
TI
Fig. 3. Gd-Tl phase diagram as reported in ref. 4; the region between 36 and 47 at.% thallium has been modified on the basis of this work (V thermal effects observed on cooling).
we have, for instance, gradual variations of the R5T1, and RTl melting temperatures with an inversion of the relative stability. The strict analogies among the three systems are also evidenced by the volume contraction trends with composition. These are shown in Fig. 4.
Fig. 4. Trends in volume contractions for the Gd-Tl, Tb-Tl and Dy-TI systems.
259
The rather high values reached by the volume contraction should be noted (around 14% for thallium-rich alloys), as well as the decrease in contraction corresponding to 37 - 38 at.% thallium. This behaviour seems rather characteristic of the thallium alloys; it has been already observed for the alloys with light rare earths (cerium, praseodymium and neodymium) [l, 6,7].
Acknowledgment The financial help obtained by the Italian Minister0 della Pubblica Istruzione is acknowledged with thanks. The authors’ thanks are due also to the Consiglio Nazionale delle Ricerche for their general support.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
S. Delfino, A. Saccone and R. Ferro, 2. Metallkde., 71 (1980) 165. S. Delfino, A. Saccone and R. Ferro, J. Less-Common Met., 65 (1979) 181. A. Saccone, S. Delfino and R. Ferro, J. Less-Common Met., 84 (1982) 281. S. Delfino, A. Saccone and R. Ferro, Z. Metallkde., 74 (1983) 674. S. Delfino, A. Saccone, G. Cacciamani and R. Ferro, 2. Metallkde., 76 (1985) 7. S. Delfino, A. Saccone and R. Ferro, J. Less-Common Met., 79 (1981) 47. S. Delfino, A, Saccone,,G. Borzone and R. Ferro, J. Less-Common Met., 59 (1978) 69. S. Delfino, A. Saccone, G. Borzone and R. Fen-o, Z. Anorg. Allg. Chem., 503 (1983) 184. S. Delfino, A. Saccone, G. Cacciamani and R. Ferro, Z. Metallkde., 78 (1987) 344. S. Delfino, A. Saccone and R. Ferro, J. Less-Common Met., IO2 (1984) 289. A. Palenzona and E. Franceschi, Colloq. Int. CNRS, 1 (1970) 135. E. Franceschi and A. Palenzona, J. Less-Common Met., 18 (1969) 93. A. Iandelli and A. Palenzona, J. Less-Common Met., 9 (1965) 1. K. Sekizawa, H. Chihara and K. Yasukochi, J. Phys. Sot. Jpn., 50 (1981) 3467. S. Delfino, A. Saccone, D. Mazzone and R. Ferro, J. Less-Common Met., 81 (1981) 45. J. L. Moriarty, J. E. Humphreys, R. 0. Gordon and N. C. Baenziger, Acta Crystallogr., 21 (1966) 840. N. C. Baenziger and J. L. Moriarty, Acta Crystallogr., 14 (1961) 948. S. Delfino, A. Saccone and R. Capelli, Atti XIII Cow. Naz. Chim. Inorg. Camerino (Italy), 23 - 26 September, 1980, D12. A. Palenzona and S. Cirafici, Thermochim. Acta, 9 (1974) 419. A. Palenzona, J. Less-Common Met., 16 (1968) 379. E. Franceschi, J. Less-Common Met., 37 (1974) 157.