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D. T. PETERSON AND J. A. STRAATMANN
the significant factors affecting the concentration dependence of conductance in dilute solutions are not known a t the present time.
Acknowledgment. This work was sponsored by the Office of Saline Water, U. s. Department of the Interior, under Contract No. 14-01-0001-359.
Lanthanum-Lanthanum Hydride Phase System'
by D. T. Peterson and J. A. Straatmann Contribution No. 1874 from the Institute for Atomic Research and Department of Metallurgy, Iowa State University, Ames, Iowa (Received April 6 , 1966)
The phase relationships between lanthanum and lanthanum hydride have been determined. The addition of hydrogen increased the melting point and decreased the fcc-tobcc transformation temperature of lanthanum to a eutectoid a t 773" and 23.5 at. % hydrogen. The solubility of lanthanum hydride in fcc lanthanum ranged from 2.83 at. % at 375" to 21.3 at. % hydrogen a t 773". The enthalpy of solution was calculated to be 5.76 f 0.17 kcal. Above 773" the solubility of lanthanum hydride in bcc lanthanum increases rapidly, and complete solubility occurs at temperatures above 960". Hydrogen depressed the low-temperature transformation in lanthanum by 27 ".
Introduction The lanthanum-hydrogen system has been investigated by Mulford and Holley2 by determining pressurecomposition isotherms in the temperature range between 600 and 800". They found a significant solubility of hydrogen in the metal which increased with increasing temperature while the lower hydrogen concentration limit of the hydride decreases with increasing temperature. Pressure-composition isotherms are limited to a small temperature range because the hydrogen dissociation pressure becomes either too small or too large to measure with reasonable accuracy. General relationships between the properties of metallic hydrides and their structure have been reviewed by lib ow it^.^ The heat of formation of lanthanum hydride is -49.7 kcal/mole of Hz. The face-centeredcubic (fcc) fluorite structure of lanthanum hydride exhibits a composition range from below lanthanum dihydride to nearly lanthanum trihydride. Neutron diffraction studies indicate that the tetrahedral holes are filled by hydrogen first, and further absorption of hydrogen results in the filling of the octahedral holes The Journal of Physical Chemistry
in the structure. The effect of hydrogen on the melt ing point and structural transformations of lanthanum metal has not been investigated. In order to determine these effects and more accurately establish the solubility limits over a wider range of temperatures, this system was investigated by differential thermal analysis, isothermal equilibration, X-ray diffraction, electrical resistivity, and dilatometer techniques. The melting point and the allotropic transformation temperatures of pure lanthanum have been measured by a number of investigators. Love4 reports a lowtemperature transformation from a hexagonal-closepacked (hcp) structure to an fcc structure at 310" and a transformation from the fcc structure to a body(1) This work was performed in the Ames Laboratory of the Atomic Energy Commission. (2) R. N. R. Mulford and C . E. Holley, Jr., J. Phys. Chem., 59, 1222 (1955). (3) G. G. Libowitz, "The Solid-state Chemistry of Binary Metal Hydrides," W. A. Benjamin, Inc., New York, N. Y., 1965. (4) B. Love in "Metals Handbook," Vol. 1, American Society for Metals, Novelty, Ohio, 1961, pp 1230, 1231.
LANTHANUM-LANTHANUM HYDRIDE PHASE SYSTEM
centered-cubic (bcc) structure at 862". The bcc remains stable until the melting point of 917" is reached. These values for the high-temperature transformation and the melting temperature are in agreement with the results of other investigators. However, there is considerable disagreement about the low-temperature transition. There is disagreement as to the exact temperature or range of temperatures a t which the transition occurs and as to the extent of the transform% tion. The prior history of the metal and its shape and size seem to influence the structure of the metal below this transformation temperature. The transformation has been studied by d i l a t ~ m e t e r ,electrical ~ r e s i s t i ~ i t y , ~and . ~ X-ray diffraction technique^.^? The results of many of the investigators have been reviewed by Herrmann,lO and he reports the lowtemperature phase to be an hcp structure with a double c axis.
Experimental Procedures The lanthanum metal used in this study was prepared by the method developed by Spedding and Daane.l1>l2 The main impurities were: Ta, 1300 ppm; Pr, 300 ppm; Ce, 300 ppm; Nd, 200 ppm; 0, 480 ppm; 104 ppm; and H, 16 ppm. The metal was stored under an argon atmosphere, and most of the handling was done in a glove box evacuated to less than 10 p and filled with argon. The exposure of the metal to atmospheric moisture was kept a t a minimum to prevent contamination. Pure hydrogen was obtained from the thermal decomposition of UH3, which had been formed by reacting hydrogen with uranium turnings. The transformation and melting temperatures were investigated by differential thermal analysis. The thermal analysis samples were prepared by placing approximately 20 g of lanthanum metal in a tantalum capsule 6 cm long and 1.9 cm in diameter. The bottom of the tantalum capsule contained a thermocouple well which was 1 cm long and 0.3 cm in diameter. The capsule containing the sample was placed in a Vycor furnace tube, the system was evacuated, and the sample was heated to a temperature between 600 and 700". The required amount of hydrogen was measured by filling the calibrated volume to the appropriate pressure. The hydrogen was then allowed to react with the lanthanum sample until it had been absorbed. By closing the stopcock in front of the furnace tube, the whole assembly could be transferred to a glove box filled with argon without exposing the sample to atmospheric moisture. A tantalum cap was welded on the thermal analysis capsule in the glove box. The samples were held under a static vacuum during
2981
the thermal analysis so that any hydrogen evolved during the heating and cooling of the sample could be detected with a manometer. The pressure in the 250ml volume of the apparatus was always less than 50 mm during the analysis of all the alloys with less than 30 at. % hydrogen. This amounted to a composition change of less than 0.1 at. % hydrogen for the thermal analysis specimens. At the high temperatures required to melt the high hydrogen alloys, higher hydrogen pressures were observed but the hydrogen gas evolved always represented a composition change of less than 1 at. % hydrogen in the specimen. A manual potentiometer which could be read to 0.1 mv was used to determine the temperature of the transformations. To determine the solubility of lanthanum hydride in lanthanum metal, lanthanum specimens which had been equilibrated with lanthanum hydride a t various temperatures were analyzed by hot vacuum extraction. The lanthanum specimens were 3 X 3 mm and 3 cm long. The specimens to be equilibrated at temperatures below 600" were first heated to a temperature between 600 and 700" for 15 min to break up any protective surface layer which could hinder the reaction of hydrogen with the sample to form a uniform hydride layer. Sufficient hydrogen was measured into the calibrated volume to constitute a 25% excess above that necessary to saturate the specimen at the temperature in question. The hydrogen was charged in increments because this procedure gave a hydride layer which was more uniform in thickness than if the hydrogen were all admitted at one time. The addition of hydrogen in increments also reduced the temperature fluctuation of the sample due to either the addition of the cool gas or the heat of reaction. The time that the samples were held at the equilibration temperature in order to ensure equilibrium varied from 2 hr at 960' to 40 hr at 259". These times were estimated on the basis of the dimensions of the specimen and the dif-
( 5 ) (a) M. Foex, Compt. Rend., 217, 501 (1943); (b) F. Barson, S. Legvold, and F. H. Spedding, Phuls. Rev., 105,418 (1957). (6) F. M. Jaeger, J. A. Bottema, and E. Rosenbohn, Rec. Trar. C h i m . , 57, 1137 (1938). (7) N. R. James, S. Legvold, and F. H. Spedding, Phys. Rea., 88, 1092 (1953). (8) W. T. Ziegler, R. A. Young, and A. L. Floyd, .'0 Am. Chem. Soc., 75, 1215 (1953). (9) G.S. Anderson, Ph.D. Thesis, Iowa State University of Science and Technology, Ames, Iowa, 1957. (10) K. R. Herrmann, Ph.D. Thesis, Iowa State University of Science and Technology, Ames, Iowa, 1955. (71) F. H.Spedding and A. H. Daane, J . Metals, 6, 1131 (1954). (12) F. H.Spedding and A. H. Daane, ibid., 6,504 (1954).
V o l u m e 70, N u m b e r 9
September 1966
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fusion coefficients of hydrogen in thorium13 to be a t least 10 times as long as necessary to achieve 95% of the saturation value. After equilibration and cooling to room temperature, the furnace tube was transferred to a glove box. A portion of the specimen was polished and examined metallographically. In every sample, a layer of hydride phase was observed surrounding the saturated core, and the boundary between the two phases was distinct. The dark gray hydride layer was removed from the remainder of the specimen by filing and the saturated core was cut into samples for analysis. The lanthanum hydride layer surrounding the core was also analyzed from several specimens which had been especially prepared with a thicker hydride layer. The hydride layer was removed by pounding in a mortar and pestle, and the resulting chips of hydride were wrapped in tantalum foil so that they could be easily handled during the analysis. The samples were analyzed by hot vacuum extraction of the hydrogen a t 950". 'The samples were dropped into a quartz boat in the furnace tube to protect the furnace tube from the molten lanthanum. Six samples were analyzed from each equilibration specimen. The rei% tive standard deviation of all the hydrogen analyses was 0.055. This figure is based on 87 separate analyses of hydrogen concentrations from 1 at. yo hydrogen to 40 at. % hydrogen. The relative error in the hydrogen analysis was determined from the analyses of six samples from a specimen of known hydrogen concentration rind was found to be +0.0068. An X-ray diffraction powder pattern of lanthanum hydride was obtained with a Debye-Scherrer camera using nickel-filtered copper radiation. The X-ray sample was prepared by crushing lanthanum hydride in a mortar and pestle and passing the powder through a ZOO-mesh screen. The fine powder was transferred to a thin-walled capillary. The above operation was carried out in an argon-filled glove box, and the open end of the capillary was sealed with Apiezon wax before exposing it to the atmosphere. The sealed capillary was then heated to 250" to remove any distortion in the sample caused by the filing and crushing. Bulk specimens of pure lanthanum and a 10 at. % hydrogen-lanthanum alloy were examined with a diffractometer to determine the effect of hydrogen on the crystal structure of lanthanum at room tempem ture. The 10 at. % hydrogen-lanthanum alloy was prepared a t 700°, and the alloy was held a t this temperature for 3 hr to ensure equilibrium. The pure lanthanum sample was also annealed a t 700" for 3 hr, and both samples were furnace cooled from this temperature. A portion of the 10 at. % hydrogenThe Journal of Physical Chemistry
D. T. PETERSON AND J. A. STRAATMANN
lanthanum alloy was sealed in a quartz capsule under vacuum, reheated to 900" for 2 hr, and quenched in water. The three samples were then mounted in Bakelite, ground through 600-grit abrasive, and examined on the diffractometer. The samples were also examined after electropolishing in a solution of 6% perchloric a~id-menthanol'~a t a temperature of -76". The dzractometer traces obtained with these different surface preparations were the same. The effect of hydrogen on the low-temperature transformation from the hcp to the fcc structure was investigated by electrical resistivity and dilatometer measurements. The electrical resistivity samples were 2.5-mm diameter swaged lanthanum rods 12.5 mm long. A thermocouple to measure the temperature of the sample was attached by spotwelding as were tantalum leads to measure the voltage drop along the length of the rod. The dilatometer specimens were obtained from a 6.3-mm diameter machined lanthanum rod and were 25.4 mm long. An X-Y recording potentiometer was used to measure the temperature and the voltage drop or change in length of the samples.
Results and Discussion The lanthanum-lanthanum hydride phase diagram is presented in Figure 1. The dotted lines indicate phase boundaries that were not definitely established in this study but were drawn from consideration of the thermodynamic rules that govern the construction of phase diagrams. The fcc to bcc transformation a t 860" and the melting point at 918" found for pure lanthanum are in agreement with t,he values reported by Love.* Differential thermal analysis revealed that the addition of hydrogen increased the melting point of lanthanum and decreased the fcc to bcc transformation temperature. The decrease in the transformation temperature continued down to 773" where the bcc lanthanum undergoes a eutectoid decomposition to fcc lanthanum plus lanthanum hydride. The eutectoid composition was 23.5 at. Yo hydrogen. The thermal arrest due to this eutectoid decomposition was found in alloys that contained up to 60 at. % ' hydrogen. The melting points were measured on samples up to the 40.3 at. % hydrogen alloy which melted a t 1079". The melting temperatures of specimens above this hydrogen composition could not be measured owing to the high dissociation pressure. The thermal analysis results shown on Figure 1 are from heating curves and were averaged from a number of analyses of each (13) D. T. Peterson and D. G . Westlake, J. Phys. Chem., 64, 649 (1960). (14) E. N. Hopkins, D. T. Peterson, and H. H. Baker, submitted to the 19th AEC Metallography Group Meeting, April 1965.
LANTHANUM-LANTHANUM HYDRIDE PHASE
2983
SYSTEM
Table I: Isothermal Equilibration Data 1100
----At.
Temp,
. V
% hydrogen-Hydride Metal
OC
phase
259 340 345 375 450 595 652 692
2.9 1.1 2.8 4.6 9.3 12.0
700
14.5
% hydrogen---
-At.
Temp,
phase
OC
63.6
740 795 826 840 859 912 922 960
1.1
Hydride phase
Metal phase 17.5 24.2 25.2 25.5
58.7 29.8
31.8
49.5 46.3
39.6
a THERMAL ANALYSIS a ISOTHERMAL EOUILIMIATIOW
400
-
t
a La+
L a LO 200
L
2 W 0 K
w
LoHx
a
I
I
I
I
I
I
I
0
10
20
30
40
50
60
I I m
ATOMIC PERCENT HYDROGEN
Figure 1. Lanthanum-lanthanum hydride phase diagram.
alloy. The usual heating and cooling rates were 5 " / min, and changes in this rate had little effect on the transformation temperatures. The arrests on heating and on cooling always agreed within 4". The lowtemperature hcp to fcc transformation was not observed by thermal analysis in either the pure lanthanum or the lanthanum-hydrogen alloys. This was expected since the enthalpy of this transformation has been reported to be only 67 cal by Berg.I6 The solubility limits of lanthanum hydride in lanthanum metal were determined by isothermal equilibration and the results are tabulated in Table I. These solubility limits could not be detected by thermal analysis and could be checked only partially by metallography. The solubility of lanthanum hydride in fcc lanthanum ranged from 2.8 at. % hydrogen at 375" to 21.3 at. % hydrogen a t 733". The solubility of lanthanum hydride in lanthanum metal at 259" was found to be 1.1 at. yo hydrogen. This value falls on the extension of the solubility curve for the fcc region but it is not certain whether the metal was in the cubic or hexagonal modification at this temperature. If it was in the hexagonal form, the lanthanum hydride solubility was not appreciably
9
10
II
12
13
14
15
16
17
18
19
20
RECIPROCAL DEGREES KELVIN x 104
Figure 2. The solubility of lanthanum hydride in lanthanum as a function of temperature.
changed by the transformation. A plot of the logarithm of the solubility of lanthanum hydride in fcc lanthanum as a function of reciprocal temperature is shown in Figure 2 . An analytical expression was fitted to the data by a least-squares method. This equation was that log C = -1260 i 30/T 2.402 f 0.507, where C is the at. % hydrogen. The enthalpy of solution calculated from this equation is +5.76 f 0.17 kcal. Above 773" the solubility of lanthanum hydride in bcc lanthanum increased rapidly, and complete solubility occurs a t temperatures above 960". This rapid increase in the solubility of hydrogen in lanthanum metal along with a rapid decrease in the hydrogen concentration of the coexisting lanthanum hydride is
+
(15) J. R. Berg, Ph.D. Thesis, Iowa State University of Science and Technology, Ames, Iowa., 1961.
Volume 70, Number B
September 1966
2984
in agreement with the solubility limits found by Mulford and HolleyS2 This type of relationship has been observed in a number of metal-hydrogen systems. Lanthanum hydride coexisting with lanthanum metal a t room temperature was found by X-ray diffraction to have the fcc fluorite structure reported by Holley, Rfulford, and Ellinger.16 The lattice constant was 5.669 A which is in satisfactory agreement with the value of 5.667 A reported by the above authors. This fcc hydride would not be expected to be able to form a continuous solid solution with the bcc lanthanum metal. Consequently a phase transition in lanthanum hydride has been postulated although no evidence for this transition was found in the thermal analyses. If the phase transition is at a high temperature, it would be very difficult to establish because of the high hydrogen dissociation pressure. The crystal structure of pure lanthanum and lanthanum-hydrogen alloys at room temperature was investigated with an X-ray diffractometer using bulk samples. The pure lanthanum metal structure was found to be a mixture of fcc lanthanum and hcp lanthanum. The lanthanum-hydrogen alloy also contained both crystaI forms of Ianthanum but the fcc peaks were slightly stronger than in the pure metal. A sample of this alloy was quenched from a region where all the hydrogen was in solution and reexamined on the X-ray diffractometer. The results showed that both cryslal forms of lanthanum mere still present but the ratio of fcc lanthanum to hcp lanthanum had greatly increased. A slight increase of the diffraction angles was also found, and this indicated an increase in the lattice parameter of lanthanum apparently due to the retention of some hydrogen in solution. The effect of hydrogen on the temperature of the hcp to fcc transformation was studied by dilatometer and electrical resistivity measurements. Figure 3 shows the expansion of pure lanthanum and a 3 at. % ' hydrogen-lanthanum alloy as a function of temperature. The hysteresis in the transformation temperature was found by both the dilatometer and electrical resistivity measurements. The results indicated that a slight excess of hydrogen over the solubility limit in this temperature range lowers the temperature of the hcp to fcc transformation. The transformation temperatures were not changed by variation in the heating and cooling rate. The transformation temperatures shown in Figure 1 are midpoint values of the transformation on heating. The pure lanthanum transformed a t 324" and the addition of hydrogen apparently results in an eutectoid reaction of fcc lanthanum to hcp plus lanthanum hydride at 297".
The Journal of Physical Chemistry
D. T. PETERSON AND J. A. STRAATMANN
+IO
c
-I5[
-PURE LANTHANUM
, 100
\\26 ,206
200
,
,
300
400
TEMPERATURE
Figure 3.
J
----3 a % HYDROGEN-LANTHANUM
,
,
500
600
,
O C
Relative changes in length us. temperature.
Hydrogen has been found to be extensively soluble in lanthanum at elevated temperatures and to stabilhe the bcc phase by raising the melting temperatures and also lowering the transition to fcc lanthanum. The increase in the melting temperature is similar to the effect of hydrogen in the alkaline earth metals. The lowering of the transition to the bcc form is analogous to the effect of hydrogen in zirconium and titanium but differs from the effect of hydrogen in calcium and strontium. The fcc to hexagonal phase transition in lanthanum is lowered slightly by hydrogen but the kinetics and completeness of the transformation are not affected. The interpretation of the influence of hydrogen on the phase transitions in the rare earth metals will probably not be possible until more of these systems have been studied.
Acknowledgments. The authors wish to express their appreciation to Rlr. Ardis Johnson for the necessary welding, RIr. John Omohundro and RIr. David Dennison for their experimental assistance during the electrical resistivity and dilatometer measurements, and Rlr. Earl Hopkins and Mr. Harlan Baker for their assistance in developing metallography techniques. (16) C. E. Holley, Jr., R. N. R. Mulford, and F. H. Ellinger, J . Phys. Chem., 59, 1226 (1955).
