THESCANDIUM-YTTRIUM-HYDROGEN SYSTEM
3973
The Scandium-Y ttrium-Hydrogen System'
by M. L. Lieberman and P. G. Wahlbeck Department of Chemistry, Illinois Institute of Technology, Chicago, Illinoia
60616
(Received June 14, 1965)
Equilibrium hydrogen pressures up to 1 atm. have been measured for the scandiumyttrium-hydrogen system a t 700, 800, and 900". These data indicate a large primary solid solution region of hydrogen in scandium-yttrium alloys and a three condensed phase region. The latter region indicates limited miscibility of the two metal dihydrides which was confirmed by X-ray diffraction analysis. The phase diagram a t 700" has been constructed, and the relative partial molal and differential thermodynamic properties of hydrogen have been calculated. For scandium-yttrium alloys with a t least 60 atomic yoyttrium, evidence for a trihydride phase has been found a t 242". Hysteresis has been observed in the hydrogenation reaction to produce the trihydride phase.
Introduction Although numerous binary metal-hydrogen systems have been investigated, relatively few ternary metalhydrogen systems have been studied. In 1948, Smith2 listed only 15 ternary systems which had been studied. In 1962, Gibb3 listed additional ternary systems which had been investigated. The only ternary system involving two rare earth metals that has been investigated has been the cerium-lanthanum-hydrogen system studied by Sieverts and Reasons for the study of the scandium-yttriumhydrogen system were the determination of (1) pressure-temperature-composition (P-T-C) data, (2) the phase diagram, (3) thermodynamic data, (4) possible existence of marked changes in the thermodynamic data as found for the binary metal-hydrogen systems, ( 5 ) ability to predict the thermodynamic data for the ternary system from the thermodynamic data of the binary systems, and (6) ability to predict the phase diagram from known data for the binary systems such as phase diagrams, thermodynamic properties, and structural parameters for metals and compounds. Scandium and yttrium hydrides are very stable toward the evolution of gaseous hydrogen and the possibility existed that hydrides of the alloys might be more stable than those of either of the metal-hydrogen systems. Each of the two component systems had been investigated previously. Beaudry and Daanebfound that the scandium-yttrium system shows complete solid solubility a t all compositions. Yannopoulos, Edwards,
and Wahlbecks obtained P-T-C data for the yttriumhydrogen system; these data indicate formation of primary solid solution and apparently hydrogen-deficient dihydride and trihydride phases with intermediate two condensed phase regions. Lieberman and Wahlbeckl obtained P-T-C data for the scandiumhydrogen system which show formation of primary solid solution and apparently hydrogen-deficient dihydride phases with an intermediate two condensed phase region. No evidence for a scandium trihydride phase was observed.
Experimental Section Apparatus. A Sieverts' apparatus, modified so that equilibrium hydrogen pressures between lo-* mm. and 1 atm. could be measured, was used to obtain pressure-temperaturecomposition data. The apparatus has been described elsewhere.lf+' Materials. The scandium used in this research was (1) Based on a thesis by IM.L. Lieberman submitted to the Illinois Institute of Technology in partial fulfillment of the requirements for the Ph.D. degree, June 1965. Presented before the Physical Chemistry Division at the 149th National Meeting of the Bmerican Chemical Society, Detroit, Mich., April 1965. (2) D. P. Smith, "Hydrogen in Metals," University of Chicago Press, Chicago, Ill., 1948. (3) T. R. P. Gibb, Jr., Progr. Inorg. Chem., 3 , 315 (1962). (4) A. Sieverts and A. Gotta, 2.Elektrochem., 3 2 , 105 (1926). (5) B. J. Beaudry and A. H. Daane, Trans. A I M E , 2 2 7 , 865 (1963). (6) L. N. Tannopoulos. R. K. Edwards, and P. G. Wahlbeck, J .
Phys. Chem., 69, 2510 (1965). (7) 11.L. Lieberman and P. G. Wahlbeck, ibid., 69, 3514 (1965).
Volume 69, Sumber 11 Xozember 1965
M. L. LIEBERMAN AND P. G. WAHLBECK
3974 _L_
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Figure 1. Isothermal data a t 700 i 0.7”. Symbols indicate respective compositions: 0, 0%; 0, 10.44%; V, 20.20%; 39.69%; 0 ,59.72%; 8,79.99%; El, 90.07%; and A, 1007,. Open symbols designate absorption points and solid symbols designate desorption points.
from the same supply as that used in the study of the Sc-H ~ y s t e m . ~It was prepared from ScZ03, obtained from the American Scandium Corp., by reduction at the Ames Laboratory of Iowa State University. An analysis of the metal by the Ames Laboratory showed the following impurities: 0, 1805 p.p.m.; N, 80 p.p.m.; H, 15 p.p.m.; Y, 0.03%; Er, -. :: lhi
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Figure 6. Plots of a" us. r: 0, 0 atomic % Y ; 0, 10.44 atomic % Y; V, 20.20 atomic % Y ; 0,39.69 atomic % Y ; A, 59.72 atomic yo Y. For the three condensed phase region of each curve (except 0 atomic 5 Y), one would expect a constant value of ~ E of ca. I -23.8 kcal./g.-atom of H,
The Journal of Physical Chemistry
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THESCAXDIUM-YTTRIUM-HYDROGEN SYSTEM
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Atomic ratio, r.
Figure 7 . Plots of 3, us. r for the scandium-hydrogen system, 0, and the yttrium-hydrogen system, V.
yttrium-hydrogen6 system. I n the case of the primary solid solution region of hydrogen in scandium-yttrium alloys, one observes a trend from the actual minimum in the A& z’s. composition plot for hydrogen in scandium-rich alloys (similar to the case of the scandiumhydrogen system) to the less noticeable change of curvature for hydrogen in yttrium-rich alloys (similar to the case of the yttrium-hydrogen system). The ASH vs. composition plots for hydrogen in solution with scandium, yttrium, and scandium-yttrium alloys are all very similar since the hydrogen atoms enter similar sites in the metal lattice in all cases. An explanation for the marked changes in relative partial niolal properties was made on the basis of Rees’IO statistical mechanical model by Lieberman and Wahlbeck.7 It was assumed that hydrogen atoms enter sites of the metal lattice which have the lowest energy or relative partial molal enthalpy associated with them. The successive filling of sites of differing energies produces the marked changes observed in relative partial molal enthalpies and entropies. One is able to predict qualitatively the observed changes in relative partial molal thermodynaniic properties. For the scandium-hydrogen7 and yttrium-hydrogen6 systems, one finds nearly identical values for ASH and rather small differences in ARH. The similarity of ASH values is a result of the hexagonal crystal structures of scandium and yttrium in the temperature range studied and indicates that the hydrogen atoms occupy similar positions in each metal. The small differences in AP;, values indicate that the hydrogen atoms are bonded more strongly for the case of yttrium than for scandium. Only slight changes are seen in ASH for the case of hydrogen in scandium-yttrium alloys from the case for scandium and yttrium. The differences between the ASH values are in general less than the estimated
uncertainty of hO.5 e.u. This similarity in AS, values is probably due to the structural similarities of the metals and alloys. Observation of Figures 5 and 6 indicates that ABH us. composition curves for the primary solid solution region for scandium-yttrium alloys other than the 10 atomic % yttrium alloys are nearly identical with the yttrium curve within the estimated experimental error of *500 cal. On the basis of Reedlo statistical mechanical model, it is reasonable to assume that the hydrogen atoms prefer to occupy low energy sites near yttrium atoms rather than higher energy sites near scandium atoms. The values of A€& in the multiphase regions are understood if one assumes that the yttrium dihydride phase is produced by addition of hydrogen to the primary solid solution since the hydrogen atoms preferentially occupy sites near yttrium atoms. After entering the multiphase regions, the values of ABHinitially correspond well with AH for the two condensed phase region of the yttriuni-hydrogen system. For scandium-yttrium alloy samples with 10, 20, 40, and 60 atomic % yttrium, when three condensed phases are present, A& corresponds closely with AH for the two condensed phase region of the scandium-hydrogen system. One may explain these observations by assuming that the yttrium atoms are removed from the primary solid solution by forniation of the yttrium dihydride phase; a solid solution of hydrogen in primarily scandium remains which gives results similar to those of the scandium-hydrogen system. The phase relationships for the scandiuni-yttriumhydrogen system are summarized with the phase diagram, Figure 2. A solid solution of hydrogen dissolved in the metals and alloys at all alloy compositions is shown on the phase diagram. This is an expected consequence of the solid solution of hydrogen in both scandium and yttrium and the complete miscibility of the metals in each other. The phase diagram leads one to conclude that there is limited miscibility of the metal dihydrides in each other. The limited miscibility was confirmed by X-ray diffraction analysis. One would expect the miscibility of the dihydrides to be dependent on their crystal structures. The structure of YH2has been determined by Lundin and Klodtgas being face-centered cubic with a lattice parameter of 5.201 *I. The structure of ScH2 has been determined by XcGuire and I
