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A new cubic phase in the calcium-palladium system
Jo~rnu~ of the Less-Corn Metals, 32 (1973) 314-316 ‘0 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
314
SHORT COMMUNICATION
A new cubic phase in the calcium-palladium system
MARSHALL H. MENDELSOHN Department of 06268 (U.S.A.)
Chemistry
and JOHN TANAKA
and Materials
Science
Institute,
University
of
Conneeficut,
Storrs,
Conn.
(Received February 26, 1973)
Earlier work in this laboratory on the calcium hydride-palladium system’ has shown that when a 2: 1 CaH,:Pd mixture is reacted above 85O”C, a new ternary hydride, Ca,Pd,H,, is formed. When the reaction is performed from 760 to 79o”C, the known Laves-phase, CaPd,, is formed’. When the reaction was performed at 800-83o”C, a new, unknown phase was observed. The product in the latter case is always found to be contaminated with calcium hydride, and therefore it could not be determined whether this new phase was a new ternary hydride or simply an unknown alloy phase. We report here the preparation of the identical new phase, starting from pure calcium and palladium metals in an argon atmosphere. A complete powder diffraction pattern was obtained showing the material to be cubic with the cesium chloride type structure. The pure cubic phase was prepared by pressing a 5:3 mixture of pure calcium and palladium into a pellet. The pellet was then placed in a vitreous carbon or tantalum foil boat and heated at -900°C for 12 h in an argon atmosphere. The resulting material was powdered in an agate vial and reheated for 4 h at 590°C. The X-ray powder diffraction pattern of the reheated material was obtained with a Philips Debye-Scherrer camera using nickel-filtered copper radiation. Table I lists the calculated and observed d-spacings for the compound. The intensities were visually estimated with the strongest line arbitrarily set at I/I,= 100. The powder pattern for the new calcium-palladium phase was found to be similar to that of the compound CuY. A comparison of these two powder patterns is shown in Table II. The metallic radii for Cu, Y, Pd and Ca are, respectively (in A,>: 1.28, 1.82, 1.37 and 1.973. The radius ratio for YCu is 1.42, while for CaPd, the radius ratio is 1.44. On the basis of this close structural analogy, the stoichiometry of the new calcium-palladium cubic phase is assigned as 1: 1. The question may be raised as to why a 1 :l compound can only be prepared in the presence of excess calcium hydride or calcium metal. When we reheated samples of the initially prepared alloy at a temperature of 9OO”C,we found that the aIloy had decomposed. to CaPd,. The approximate amounts of CaPd, found at other temperatures were as follows (products were estimated by relative diffraction
SHORT
315
COMMUNICATIONS
pattern intensities): 95% (9Oo”C), SO><(SWC), 10% (74X), ~5% (590°C). These results suggest the following equilibrium reaction: CaPd, + Ca G 2CaPd . TABLE
I
(a=3.515
A)
~.._
doils.
d CdC.
hkl
3.52 2.49 2.03 1.758 1.572 1.435 1.243 1.172 I.112 1.060 1.015 0.975 0.940 0.879 0.853 0.829 0.806 0.786
100 110 I 11 200 210 211 220 300,22 3 10 311 222 320 321 400 410,322 4 11,330 33 1 420
---
60 100 30 70 40 80 50 20 60 IO 20 5 70 5 IO 70
3.48 2.48 2.02 I.754 I.569 I .434 1.241 1.172 I.111 I.059 1.015 0.974 0.940 0.880 0.853 0.829 (0.807) 0.786
(2)* 60 * Obtained TABLE
!;I,
1
exposure
II
(CUY)”
20 100 20 60 20 80 60 20 60 20 40 20 80 40 40 80 40 80 l
from a film with longer
-.~
Obtained
I/I,
(CaPd)
60 100 30 70 40 80 50 20 60 IO 20 5 70 5 10 70 (2)* 60 from a film with longer
d(cuYy 3.45 2.44 2.00 1.I3 1.55 1.42
1.23 1.16 1.10 1.05 1.00 0.963 0.928 0.868 0.842 0.819 0.797 0.777 exposure
d( CaPd)
3.48 2.48 2.02 1.75 1.57 1.43 1.24 1.17 I.11 1.06 1.02 0.974 0.940 0.880 0.853 0.829 (0.807)* 0.786
316
SHORT
COMMUNICATIONS
This equilibrium also explains why the compound must initially be formed in an excess of calcium metal or calcium hydride. Thermodynamically, at -8OO-900°C (the temperature needed initially to produce CaPd at a reasonable rate), CaPd, is more stable. However, in the presence of excess calcium, the equilibrium is forced to the right. The new calcium-palladium alloy was found to react with hydrogen at 560°C. The only products observed in the powder pattern were CaPd, and CaH,. In one experiment we determined the amount of calcium hydride in the mixture by pyrolysis of the product and were able to calculate the initial composition of the starting mateThis supports our hypothesis that the 1: 1 alloy (CaPd) is rial as Ca,,,,Pd,,,. stabilized in the presence of a small excess of calcium. It also indicates that most of the excess calcium in the starting materials is volatilized out of the new cubic phase. It would be interesting to determine whether or not the other known alkaline earth-noble metal Laves phases may form these CsCl-type cubic phases. We briefly looked at the Ca-Pt system and were unable to prepare a new cubic phase similar to that found in the Ca-Pd system.
REFERENCES 1 2 3 4
C. E. A. R.
Stanitski and J. Tanaka, J. Solid State Chem., 4 (1972) 331. A. Wood and V. R. Compton, Acta Cryst., 11 (1958) 429. F. Wells, Structural Inorganic Chemistry, Oxford Univ. Press., London, 1962, p. 984. F. Domagala, J. J. Rausch and D. W. Levinson, Trans. Am. Sot. Metals, 53 ($961) 137.
314
SHORT COMMUNICATION
A new cubic phase in the calcium-palladium system
MARSHALL H. MENDELSOHN Department of 06268 (U.S.A.)
Chemistry
and JOHN TANAKA
and Materials
Science
Institute,
University
of
Conneeficut,
Storrs,
Conn.
(Received February 26, 1973)
Earlier work in this laboratory on the calcium hydride-palladium system’ has shown that when a 2: 1 CaH,:Pd mixture is reacted above 85O”C, a new ternary hydride, Ca,Pd,H,, is formed. When the reaction is performed from 760 to 79o”C, the known Laves-phase, CaPd,, is formed’. When the reaction was performed at 800-83o”C, a new, unknown phase was observed. The product in the latter case is always found to be contaminated with calcium hydride, and therefore it could not be determined whether this new phase was a new ternary hydride or simply an unknown alloy phase. We report here the preparation of the identical new phase, starting from pure calcium and palladium metals in an argon atmosphere. A complete powder diffraction pattern was obtained showing the material to be cubic with the cesium chloride type structure. The pure cubic phase was prepared by pressing a 5:3 mixture of pure calcium and palladium into a pellet. The pellet was then placed in a vitreous carbon or tantalum foil boat and heated at -900°C for 12 h in an argon atmosphere. The resulting material was powdered in an agate vial and reheated for 4 h at 590°C. The X-ray powder diffraction pattern of the reheated material was obtained with a Philips Debye-Scherrer camera using nickel-filtered copper radiation. Table I lists the calculated and observed d-spacings for the compound. The intensities were visually estimated with the strongest line arbitrarily set at I/I,= 100. The powder pattern for the new calcium-palladium phase was found to be similar to that of the compound CuY. A comparison of these two powder patterns is shown in Table II. The metallic radii for Cu, Y, Pd and Ca are, respectively (in A,>: 1.28, 1.82, 1.37 and 1.973. The radius ratio for YCu is 1.42, while for CaPd, the radius ratio is 1.44. On the basis of this close structural analogy, the stoichiometry of the new calcium-palladium cubic phase is assigned as 1: 1. The question may be raised as to why a 1 :l compound can only be prepared in the presence of excess calcium hydride or calcium metal. When we reheated samples of the initially prepared alloy at a temperature of 9OO”C,we found that the aIloy had decomposed. to CaPd,. The approximate amounts of CaPd, found at other temperatures were as follows (products were estimated by relative diffraction
SHORT
315
COMMUNICATIONS
pattern intensities): 95% (9Oo”C), SO><(SWC), 10% (74X), ~5% (590°C). These results suggest the following equilibrium reaction: CaPd, + Ca G 2CaPd . TABLE
I
(a=3.515
A)
~.._
doils.
d CdC.
hkl
3.52 2.49 2.03 1.758 1.572 1.435 1.243 1.172 I.112 1.060 1.015 0.975 0.940 0.879 0.853 0.829 0.806 0.786
100 110 I 11 200 210 211 220 300,22 3 10 311 222 320 321 400 410,322 4 11,330 33 1 420
---
60 100 30 70 40 80 50 20 60 IO 20 5 70 5 IO 70
3.48 2.48 2.02 I.754 I.569 I .434 1.241 1.172 I.111 I.059 1.015 0.974 0.940 0.880 0.853 0.829 (0.807) 0.786
(2)* 60 * Obtained TABLE
!;I,
1
exposure
II
(CUY)”
20 100 20 60 20 80 60 20 60 20 40 20 80 40 40 80 40 80 l
from a film with longer
-.~
Obtained
I/I,
(CaPd)
60 100 30 70 40 80 50 20 60 IO 20 5 70 5 10 70 (2)* 60 from a film with longer
d(cuYy 3.45 2.44 2.00 1.I3 1.55 1.42
1.23 1.16 1.10 1.05 1.00 0.963 0.928 0.868 0.842 0.819 0.797 0.777 exposure
d( CaPd)
3.48 2.48 2.02 1.75 1.57 1.43 1.24 1.17 I.11 1.06 1.02 0.974 0.940 0.880 0.853 0.829 (0.807)* 0.786
316
SHORT
COMMUNICATIONS
This equilibrium also explains why the compound must initially be formed in an excess of calcium metal or calcium hydride. Thermodynamically, at -8OO-900°C (the temperature needed initially to produce CaPd at a reasonable rate), CaPd, is more stable. However, in the presence of excess calcium, the equilibrium is forced to the right. The new calcium-palladium alloy was found to react with hydrogen at 560°C. The only products observed in the powder pattern were CaPd, and CaH,. In one experiment we determined the amount of calcium hydride in the mixture by pyrolysis of the product and were able to calculate the initial composition of the starting mateThis supports our hypothesis that the 1: 1 alloy (CaPd) is rial as Ca,,,,Pd,,,. stabilized in the presence of a small excess of calcium. It also indicates that most of the excess calcium in the starting materials is volatilized out of the new cubic phase. It would be interesting to determine whether or not the other known alkaline earth-noble metal Laves phases may form these CsCl-type cubic phases. We briefly looked at the Ca-Pt system and were unable to prepare a new cubic phase similar to that found in the Ca-Pd system.
REFERENCES 1 2 3 4
C. E. A. R.
Stanitski and J. Tanaka, J. Solid State Chem., 4 (1972) 331. A. Wood and V. R. Compton, Acta Cryst., 11 (1958) 429. F. Wells, Structural Inorganic Chemistry, Oxford Univ. Press., London, 1962, p. 984. F. Domagala, J. J. Rausch and D. W. Levinson, Trans. Am. Sot. Metals, 53 ($961) 137.