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Phase equilibria in the Ca-Pd system
Journal of the Less-Common
PHASE EQUIL~RIA
A. PALENZONA
307
Metals, 85 (1982) 307 - 312
IN THE Ca-Pd SYSTEM
and P. MANFRINETTI
istituto di Chimica Fisica, Uniuersitci di Genova, Corso Europa, 16132 Genoa (italy~
Palazzo deEle Scienze,
(Received January 15,1982)
Summary The equilib~um phase diagram of the Ca-Pd system was investigated throughout almost the complete composition range using differential thermal, X-ray and metallographic analyses. Four new intermetallic phases were found and the crystal structures and lattice parameters were determined for three of them: CasPd (FesC type), Ca,Pds (Mn,Ca type) and CaaPdz (ErsN& type). The compounds CaPd (CsCl type) and CaPd, (MgCu, type) have already been reported, and we confirmed all previous findings and showed additionally that CaPd, had a wide homogeneity range. The structure of the phase richest in calcium, which should have a composition close to CaePd, was not determined and work is now in progress to solve this problem.
1. Introduction The work on the Ca-Pd system reported here was undertaken as part of a programme of systematic ex~ination of the behaviour of alkaline earth metals with group VIII elements. The aim of this programme is to compare the alloying properties of divalent (alkaline earth metals), trivalent (rare earths) and tetravalent (thorium) elements, first with respect to their crystal chemistry and subsequently in terms of the behaviour of special elements like europium and ytterbium which can show anomalous valence states with interconfigurational fluctuations (ICF) in their intermetallic compounds. Such comparisons are useful, particularly in the latter case, because the existence of certain phases, their crystal structures, the likelihood of isomorphism and the values of the lattice constant dimensions can give interesting information relevant to ICF compounds. Such an investigation has recently been completed for all the platinumcontaining systems of this type [l],and as part of a similar survey of palladium-containing systems we have undertaken the study of the Ca-Pd and Sr-Pd systems for which few data are available. In this work we report the results obtained for the Ca-Pd system. 0 Elsevier ~quo~/~inted
in The Netherlands
308
2. Experimental
details
The calcium metal used in this investigation was obtained commercially (Fluka AG., Switzerland) and was refined to 99.5% purity by vacuum distillation in this laboratory. The palladium metal was obtained in powder form (Johnson-Matthey, Gt. Britain) and was outgassed under vacuum at 600 “C (purity, 99.9%). The alloys (1.0 g of each) were prepared from mixtures of the two metals in the form of fine powders pressed into pellets which were sealed in molybdenum containers for differential thermal analysis (DTA) by arc welding under pure argon. The pellets were melted in a high frequency induction furnace several times and were shaken to ensure homogeneity. The samples were subjected to thermal cycles at heating or cooling rates of 10 or 20 “C!mind1 to determine the characteristic temperatures of the diagram and were subsequently used for crystallographic and micrographic examinations. Alloys with a palladium content higher than that in CaPd, showed appreciable contamination by the container material. The DTA of these alloys was therefore carried out by heating samples prepared by arc melting under pure argon to temperatures just above the melting point using a copper hearth furnace. Alloys containing more than 85 at.% Pd were not examined because of the experimental difficulties encountered. Most of the X-ray studies were carried out on powders (Debye or Guinier methods) using Cu Ka radiation. In one case a single crystal was examined using MO Ka radiation. The intensities were calculated using the Lazy Pulverix program [2] . Special care is required in handling alloys from calcium to about CaPd as these samples are highly reactive in moisture; very good protection is achieved by using paraffin oil dried over sodium metal. After CaPd the resistance to oxidation increases with increasing palladium content and the alloys beyond CaPd, are also unreactive towards hot strong acids. The samples for microscope examination were prepared by diamond polishing under dried paraffin oil; no specific etching agent was necessary to reveal the phases present.
3. Results Figure 1 shows the phase diagram of the Ca-Pd system as derived from DTA, X-ray and metallographic examinations. Six compounds are formed in this system and the corresponding crystallographic data for most of them are reported in Table 1. Three phases have congruent melting points: CaaPdz (755 “C), CaPd (905 “C) and CaPdz (1300 “C). The remaining phases decompose before melting: CaePd (665 “C), Ca3Pd (570 “C) and CasPds (595 “C). Eutectic points were found at 19.0 at.% Pd (530 “C), 41.0 at.% Pd (740 “C),
309
40
atomic Fig. 1. The constitutional
60
50
‘IO
phase diagram
Pd of the Ca-Pd
system.
54.0 at.% Pd (845 “C) and 84.0 at.% Pd (1090 “C). As in the platinum systems with calcium, strontium and europium, some undercooling effects were observed in the composition range 20 - 30 at.% Pd. 3.1. Terminal solubilities and solid solutions The solubilities of palladium in calcium and of calcium in palladium are negligible as the lattice constants of the pure metals do not change significantly when they are alloyed with each other. For CaPd, an extended homogeneity range was observed from CaPd,., (a = 7.664 A) to CaPd 3.1, (a = 7.567 A). These limits were determined by X-ray powder crystallography examination of alloys with near-stoichiometric compositions that had been quenched from a high temperature. The
310
TABLE 1 Crystallographic data for the intermediate phases of the Ca-Pd system Compound
Structurea
Lattice
constants b
a
CaaPd
Fe& (oP16, Pnma)
CasPdz
Mn5C2 (mC28, C2/c) EraNi, (hR45, Rg) CSCI
CasPd2 CaPd
(cP2,Pm3m)
CaPd2
MgCuz (cF24,
Fd3m)
References
(A) C
7.699
9.937
6.691
This work
16.694
6.708
7.704
This work
p = 97.30” 8.939
-
16.900
This work
3.518
-
-
3.516 7.652 7.665
-
-
This work 141 This work
[51
aThe space group symbols are preceded by a description of the structure in the Pearson notation [ 31.
lattice constant varies linearly with the composition (Fig. 2) and the value for CaPd, should be 7.652 a. Moreover, it was possible to isolate a single crystal, nucleated from the bulk, from an ahoy of nominal composition CaPd, which appeared non-homogeneous on micrography examination. This crystal was investigated using an Enraf-Nonius CAD 4 automatic fourcircle diffractometer. Preliminary results suggested that the crystal had a composition close to the palladium-rich end of the homogeneity range and that the solid solution was formed not only by the simple replacement of calcium atoms by palladium atoms but also by a more complex mechanism. However, all the other compounds formed in the system can be regarded as “line compounds” (compounds with no or very small homogeneity ranges).
I
I 60
I
I
I
I
70
00 at.
X
I
J
Pd
Fig. 2. Lattice constant values vs. composition for the solid solutions of CaPd2: 0, one phase; @, two phase.
311
3.2. Inter-metallic compounds 3.2.1. Ca,Pd
The first compound formed in the system should correspond to CaaPd. This composition was derived from the DTA results and the micrography examination of several samples of approximately this composition. Attempts to determine the crystal structure in the absence of a single crystal which would enable a complete solution to be obtained were inconclusive. Powder photographs provided evidence for an f.c.c. structure with a lattice constant close to that of pure calcium, but the micrography examination revealed primary calcium metal, a second phase in the form of thin prismatic crystals and a small quantity of the residual eutectic. A similar situation has been found in other systems involving a divalent element and a noble metal, e.g. EusPt [l] , Sr,Pt [ 61 and more recently EugRh [7] . Work is now in progress to solve this problem. 3.2.2. Ca,Pd, Ca,Pd, and Ca3Pd, These three phases, which have not previously been detected in the system, were identified by X-ray investigations. Subsequent intensity calculations carried out using the positional parameters of HoaCo [8], SmsCo, [9] and ErsNi, [lo] confirmed these structure types. Ca,Pds and CasPdZ were brittle and well-crystallized alloys which were obtained easily, but CasPd needed prolonged annealing (1 month at 500 “C) to reach equilibrium. 3.2.3. CaPd and CaPd, The crystal structures and the lattice constants of these two phases were confirmed. Moreover a wide homogeneity range was found for CaPd, as described above. 4. Remarks The behaviour of calcium with palladium is essentially that expected between a “true metal” and a transition element, both with respect to the shape of the diagram with melting points rising from calcium towards palladium, similar to that of the Ca-Pt system and to the number of intermediate phases and their crystal structures. Most of the structure types and geometric structural correlations shown by the compounds are well known [ 111 ; they are found in many systems containing group VIII metals and appear to be independent of the nature of the partner element even when it is the major component. A more complete discussion of this subject and a comparison with the Yb-Pd and Eu-Pd systems will be possible after the determination of the Sr-Pd phase diagram which is now in progress. References A. Iandelli and A. Palenzona,J. Less-Common 2 K. Yvon, W. Jeitschko and E. Parthe, J. Appl.
1
Met., 80 (1981) P71. Crystallogr., 10 (1977)
73.
312 3 W. B. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys, Vol. 2, Pergamon, Oxford, 1967. 4 A. Iandelli, G. L. Olcese and A. Palenzona, J. Less-Common Met., 76 (1980) 317. 5 E. A. Wood and V. B. Compton, Acta Crystallogr., I1 (1958) 429. 6 A. Palenzona, J. Less-Common Met., 78 (1981) P49. 7 A. Iandelli and A. Palenzona, unpublished work. 8 K. H. J. Buschow and A. S. van der Goot, J. Less-Common Met., 18 (1969) 309. 9 J. M. Moreau and D. Paccard,Acta Crystallogr., Sect. B, 32 (1976) 1654. 10 J. M. Moreau, D. Paccard and D. Gignoux, Acta Crystallogr., Sect. B, 30 (1974) 2122. 11 E. Parthe and J. M. Moreau, J. Less-Common Met., 53 (1977) 1.
PHASE EQUIL~RIA
A. PALENZONA
307
Metals, 85 (1982) 307 - 312
IN THE Ca-Pd SYSTEM
and P. MANFRINETTI
istituto di Chimica Fisica, Uniuersitci di Genova, Corso Europa, 16132 Genoa (italy~
Palazzo deEle Scienze,
(Received January 15,1982)
Summary The equilib~um phase diagram of the Ca-Pd system was investigated throughout almost the complete composition range using differential thermal, X-ray and metallographic analyses. Four new intermetallic phases were found and the crystal structures and lattice parameters were determined for three of them: CasPd (FesC type), Ca,Pds (Mn,Ca type) and CaaPdz (ErsN& type). The compounds CaPd (CsCl type) and CaPd, (MgCu, type) have already been reported, and we confirmed all previous findings and showed additionally that CaPd, had a wide homogeneity range. The structure of the phase richest in calcium, which should have a composition close to CaePd, was not determined and work is now in progress to solve this problem.
1. Introduction The work on the Ca-Pd system reported here was undertaken as part of a programme of systematic ex~ination of the behaviour of alkaline earth metals with group VIII elements. The aim of this programme is to compare the alloying properties of divalent (alkaline earth metals), trivalent (rare earths) and tetravalent (thorium) elements, first with respect to their crystal chemistry and subsequently in terms of the behaviour of special elements like europium and ytterbium which can show anomalous valence states with interconfigurational fluctuations (ICF) in their intermetallic compounds. Such comparisons are useful, particularly in the latter case, because the existence of certain phases, their crystal structures, the likelihood of isomorphism and the values of the lattice constant dimensions can give interesting information relevant to ICF compounds. Such an investigation has recently been completed for all the platinumcontaining systems of this type [l],and as part of a similar survey of palladium-containing systems we have undertaken the study of the Ca-Pd and Sr-Pd systems for which few data are available. In this work we report the results obtained for the Ca-Pd system. 0 Elsevier ~quo~/~inted
in The Netherlands
308
2. Experimental
details
The calcium metal used in this investigation was obtained commercially (Fluka AG., Switzerland) and was refined to 99.5% purity by vacuum distillation in this laboratory. The palladium metal was obtained in powder form (Johnson-Matthey, Gt. Britain) and was outgassed under vacuum at 600 “C (purity, 99.9%). The alloys (1.0 g of each) were prepared from mixtures of the two metals in the form of fine powders pressed into pellets which were sealed in molybdenum containers for differential thermal analysis (DTA) by arc welding under pure argon. The pellets were melted in a high frequency induction furnace several times and were shaken to ensure homogeneity. The samples were subjected to thermal cycles at heating or cooling rates of 10 or 20 “C!mind1 to determine the characteristic temperatures of the diagram and were subsequently used for crystallographic and micrographic examinations. Alloys with a palladium content higher than that in CaPd, showed appreciable contamination by the container material. The DTA of these alloys was therefore carried out by heating samples prepared by arc melting under pure argon to temperatures just above the melting point using a copper hearth furnace. Alloys containing more than 85 at.% Pd were not examined because of the experimental difficulties encountered. Most of the X-ray studies were carried out on powders (Debye or Guinier methods) using Cu Ka radiation. In one case a single crystal was examined using MO Ka radiation. The intensities were calculated using the Lazy Pulverix program [2] . Special care is required in handling alloys from calcium to about CaPd as these samples are highly reactive in moisture; very good protection is achieved by using paraffin oil dried over sodium metal. After CaPd the resistance to oxidation increases with increasing palladium content and the alloys beyond CaPd, are also unreactive towards hot strong acids. The samples for microscope examination were prepared by diamond polishing under dried paraffin oil; no specific etching agent was necessary to reveal the phases present.
3. Results Figure 1 shows the phase diagram of the Ca-Pd system as derived from DTA, X-ray and metallographic examinations. Six compounds are formed in this system and the corresponding crystallographic data for most of them are reported in Table 1. Three phases have congruent melting points: CaaPdz (755 “C), CaPd (905 “C) and CaPdz (1300 “C). The remaining phases decompose before melting: CaePd (665 “C), Ca3Pd (570 “C) and CasPds (595 “C). Eutectic points were found at 19.0 at.% Pd (530 “C), 41.0 at.% Pd (740 “C),
309
40
atomic Fig. 1. The constitutional
60
50
‘IO
phase diagram
Pd of the Ca-Pd
system.
54.0 at.% Pd (845 “C) and 84.0 at.% Pd (1090 “C). As in the platinum systems with calcium, strontium and europium, some undercooling effects were observed in the composition range 20 - 30 at.% Pd. 3.1. Terminal solubilities and solid solutions The solubilities of palladium in calcium and of calcium in palladium are negligible as the lattice constants of the pure metals do not change significantly when they are alloyed with each other. For CaPd, an extended homogeneity range was observed from CaPd,., (a = 7.664 A) to CaPd 3.1, (a = 7.567 A). These limits were determined by X-ray powder crystallography examination of alloys with near-stoichiometric compositions that had been quenched from a high temperature. The
310
TABLE 1 Crystallographic data for the intermediate phases of the Ca-Pd system Compound
Structurea
Lattice
constants b
a
CaaPd
Fe& (oP16, Pnma)
CasPdz
Mn5C2 (mC28, C2/c) EraNi, (hR45, Rg) CSCI
CasPd2 CaPd
(cP2,Pm3m)
CaPd2
MgCuz (cF24,
Fd3m)
References
(A) C
7.699
9.937
6.691
This work
16.694
6.708
7.704
This work
p = 97.30” 8.939
-
16.900
This work
3.518
-
-
3.516 7.652 7.665
-
-
This work 141 This work
[51
aThe space group symbols are preceded by a description of the structure in the Pearson notation [ 31.
lattice constant varies linearly with the composition (Fig. 2) and the value for CaPd, should be 7.652 a. Moreover, it was possible to isolate a single crystal, nucleated from the bulk, from an ahoy of nominal composition CaPd, which appeared non-homogeneous on micrography examination. This crystal was investigated using an Enraf-Nonius CAD 4 automatic fourcircle diffractometer. Preliminary results suggested that the crystal had a composition close to the palladium-rich end of the homogeneity range and that the solid solution was formed not only by the simple replacement of calcium atoms by palladium atoms but also by a more complex mechanism. However, all the other compounds formed in the system can be regarded as “line compounds” (compounds with no or very small homogeneity ranges).
I
I 60
I
I
I
I
70
00 at.
X
I
J
Pd
Fig. 2. Lattice constant values vs. composition for the solid solutions of CaPd2: 0, one phase; @, two phase.
311
3.2. Inter-metallic compounds 3.2.1. Ca,Pd
The first compound formed in the system should correspond to CaaPd. This composition was derived from the DTA results and the micrography examination of several samples of approximately this composition. Attempts to determine the crystal structure in the absence of a single crystal which would enable a complete solution to be obtained were inconclusive. Powder photographs provided evidence for an f.c.c. structure with a lattice constant close to that of pure calcium, but the micrography examination revealed primary calcium metal, a second phase in the form of thin prismatic crystals and a small quantity of the residual eutectic. A similar situation has been found in other systems involving a divalent element and a noble metal, e.g. EusPt [l] , Sr,Pt [ 61 and more recently EugRh [7] . Work is now in progress to solve this problem. 3.2.2. Ca,Pd, Ca,Pd, and Ca3Pd, These three phases, which have not previously been detected in the system, were identified by X-ray investigations. Subsequent intensity calculations carried out using the positional parameters of HoaCo [8], SmsCo, [9] and ErsNi, [lo] confirmed these structure types. Ca,Pds and CasPdZ were brittle and well-crystallized alloys which were obtained easily, but CasPd needed prolonged annealing (1 month at 500 “C) to reach equilibrium. 3.2.3. CaPd and CaPd, The crystal structures and the lattice constants of these two phases were confirmed. Moreover a wide homogeneity range was found for CaPd, as described above. 4. Remarks The behaviour of calcium with palladium is essentially that expected between a “true metal” and a transition element, both with respect to the shape of the diagram with melting points rising from calcium towards palladium, similar to that of the Ca-Pt system and to the number of intermediate phases and their crystal structures. Most of the structure types and geometric structural correlations shown by the compounds are well known [ 111 ; they are found in many systems containing group VIII metals and appear to be independent of the nature of the partner element even when it is the major component. A more complete discussion of this subject and a comparison with the Yb-Pd and Eu-Pd systems will be possible after the determination of the Sr-Pd phase diagram which is now in progress. References A. Iandelli and A. Palenzona,J. Less-Common 2 K. Yvon, W. Jeitschko and E. Parthe, J. Appl.
1
Met., 80 (1981) P71. Crystallogr., 10 (1977)
73.
312 3 W. B. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys, Vol. 2, Pergamon, Oxford, 1967. 4 A. Iandelli, G. L. Olcese and A. Palenzona, J. Less-Common Met., 76 (1980) 317. 5 E. A. Wood and V. B. Compton, Acta Crystallogr., I1 (1958) 429. 6 A. Palenzona, J. Less-Common Met., 78 (1981) P49. 7 A. Iandelli and A. Palenzona, unpublished work. 8 K. H. J. Buschow and A. S. van der Goot, J. Less-Common Met., 18 (1969) 309. 9 J. M. Moreau and D. Paccard,Acta Crystallogr., Sect. B, 32 (1976) 1654. 10 J. M. Moreau, D. Paccard and D. Gignoux, Acta Crystallogr., Sect. B, 30 (1974) 2122. 11 E. Parthe and J. M. Moreau, J. Less-Common Met., 53 (1977) 1.