2062
D. T. P E T E R S O N AND
ture range where liquid and crystal coexist in equilibrium is rather small. This tends to make any compositional changes, which occur as we cool through this region, likewise rather small. The equilibrium relationships are essentially the same as those where we are only dealing with solid-solid equilibria-that is, as the nickel con-
v. G. F A T T O R E
Vol. 65
centration increases it is necessary to have a corresponding increase in oxygen pressure to prevent the decomposition of the spinel phase. Acknowledgments.-The author gratefully acknowledges the assistance of H. G. Schaefer in the preparation of the starting materials and G. L. Evans and J . Kuptsis in the chemical analysis.
CALCIUM-CALCIUM HYDRIDE PHASE SYSTEM1 BY D. T. PETERSON AND V. G. FATTORE Institute for Atomic Research, Iowa State Uniaersity, Ames, Iowa Sicedison S.p.A., Centro Studi e Ricerche Bollate, Milan, Italy Received May 31, 1981
The CaH2 phase diagram was studied by thermal analysis and chemical analysis 0f:quilibrated phases. The maximum Calcium metal undergoes an a h solubility of CaHs in calcium metal is 24 mole % a t the peritectic temperature of 890 tropic toransitiona t 448 f 2' and melts a t 839 f 2". An intermediatzphase, stable between 320 to 600", is produced by hydrogen. Calcium hydride shows an allotropic transformation a t 780
.
.
Introduction The calcium-calcium hydride system has been studied by several authors by determining pressurecomposition isotherms. By this method it was possible to investigate only the portion of the system above 550°, because below this temperature the pressure is too small to be measured with reasonable accuracy. Hurd and Walker2 have summarized the work up to 1931. Since that time the structure of CaH2 has been determined and Treadby Zintl and Harder.3 Johnson, et 121.)~ well and Stecherj have studied the system by pressure composition isotherms and obtained concordant results. In the temperature range of these measurements, hydrogen dissolves in the calcium phase up to about 20 mole % CaH2. From this composition to about 90 mole % CaH2, the calcium and CaH? phases coexist and, at constant temperature, the pressure is fixed. The CaH2 phase varies in composition with the hydrogen pressure and approaches very near CaH2.aas an upper limit . Pure calcium ~ m reported s by Smith, et a1..6 to be f.c.c. up to 46.2' and b.c.r. above this temperature, but contamination caused the appearance of other allotropic forms. Hydrogen was shown t o play an important role in the allotropic behavior of calcium. Other investigators have reported different allotropic forms and transformation temperatures for calcium. These results have been summarized and discussed by Schottmiller, et al.' The study of this system was expected to give some explanation of the observed phenomena. Dif(1) Contribution No. 1024. Work was performed in the Ames Laboratory of the U. S. Atomic Energy Commission. (2) C. B. Hurd and K. E. Walker, J . A m Chem. Soc., 63, 1681 (1931). (3) E. Zintl and H. Harder, 2. Elektrochem., 4 1 , 33 (1935). (4) W. C. Johnson, M. F. Stubbs, A. E. Sidwell and A. Pechukas. J . A m . (:hem. Soc., 6 1 , 318 (1939). ( 5 ) W. n. Treadwell and J. Steoher, Helu. Chzm. Acta, 36, 1820 (1953). (6) J. F. Smith, 0. N. Carlson and R. W. Vest, J . Electrochem Soc , 103,409 (1956). (7) J. C. Schottntiller, A. J. King and F. A. Kanda, J . I'hys. Chem., 62, 1140 (19.58).
ferential thermal analysis, equilibration experiments and X-ray examination were the experimental methods employed in this work. Experimental Methods Materials .-The
calcium metal used in this investigation was purified by distillation a t 950" under a pressure not exceeding 1 X 10-5 mm. Chemical and spectrographic analysis of this calcium showed that its purity was about 99.94%, if the hydrogen and oxygen contents were not considered. The main impurities are: Mg, 300 p.p.m.; C, 100 p.p.m.; Si, 100 p.p.n!.; N,50 p.p.m.; Fe, 20 p.p.m. Oxygen was not determined. The hydrogen content varied from 120-220 p.p.m., corresponding to 0.24-0.44 mole 7 0 of CaH2. To avoid contamination, the calcium always was kept out of air and handled in a glove box previoudy evacuated to 5-10 p and filled with argon whose purity was over 99.9%. Very pure hydrogen for charging the specimens was obtained by heating uranium hydride above 300". Thermal Analysis.-The calcium (7-8 g. of distilled crystals) was placed into the thermal analysis capsule in the glove boy. The capsule was of type 304 stainless steel and was 6.5 cm. long, 1.9 em. inside diameter and 1.5 mm. wall thickness. The suitability of type 304 stainless steel as,an inert crucible for molten calcium was verified by ana1yzin.g the calcium after heating to 900" in the thermal anallpis capsule. The total increase in metallic impurities was only 0.03 atomic c;. In addition, the melting point was determ i n d in a tantalum thermal analysis capsule and the same melting trmpcmture was found as in stainless steel capsules. There was a thermocouple well 1.3 cm. deep and 3 mm. in diameter in the bottom of the capsule. The top end ivas only partially closed by welding on a cover in order to allow rapid Rems of h j diogen to the calcium during charging. The capsule was placed in a 45 cm. long quartz tube whirh was clobed at on(>end and at the other end a stopper with a stopcock was sealed ujth Bpiezon W wax. ,4fter this operation the tube was taken from the glove box and joined to the charging apparatus. This consisted of a resistance furnace for heating the specimen, a calibrated volume and a nianometer for measuring the hydrogen, a uranium hydride hydrogen generator and a mechanical vacuum pump. The apparatus was evacuated and filled with a known amount of pure hydrogen which was allowed to react with the specimen a t about 510-530O. After charging to the desired concentration of CaH2, the quartz tube was transferred again to the glove box. The capsule was tahen out and the cover completely sealed by welding. The capsule then could he handled out of the glove boy without danger of contamination and it was ready for the thermal analysis. The loss of hT drogen from the capsule bl diffusion through the walls of the capsule during the thermal analysis was reduced by placing thr. capsule in a close-fitting quartz tube which \vas cwaciiatcd :md closed The :\mount of hydrogen
THECALCIUM-CALCIUM HYDRIDE PHASESYSTEM
Nov., 1981
which escaped was determined by measuring the pressure with a manometer. In all cases, the change in composition due to the evolution of hydrogen was negligible. A differential thermal analysis was used to give greater sensitivity in detecting small heat effects. The sample thermocouple (Chromel-.4lumel) was calibrated a t the melting point of a K.B.S. standard aluminum sample and s sample of electrolytic silver. The differential thermocouple was placed in an empty capsule similar to the sample capsule but shorter and located just below the quartz tube. The thermal analyses were repeated several times for each sample a t various rates of heating and cooling. Except for the transformation of 320-360", the heating and cooling arrests came a t identical temperatures and were not significantly changed by changes in the heating or cooling rate. Equilibration Experiments.-The calcium rods used for these experiments were 1.9 cm. long and 1.3 cm. in diameter. These rods were obtained by melting distilled calcium crystals in a pure iron crucible in the glove box filled with argon, and casting into a steel mold. After casting, the calcium rods were machined in a lathe in the glove box to a smooth surface and the desired size. The calcium cylinder was placed in an open stainless steel capsule and loaded into the Vycor charging tube in the glove box. The charging was done a t the temperature chosen for the equilibration. Hydrogen was added to the sample to transform 2 5 3 0 % of the calcium to CaH2. After adding the hydrogen, the temperature of the sample was maintained for 5 hours to be sure that equilibrium was reached. The charged specimen consisted of a very brittle outside layer of the hydride phase and a soft core of the metal phase. In the glove box, the two layers were separated and samples were taken to be analyzed for hydrogen. These analyses were done by a hot vacuum extraction method, to be published. The accuracy of the method was i 2 % of the amount of hydrogen present. X-Ray Examination.-A number of samples of various hydrogen contents were examined a t room temperature by powder X-ray diffraction. The powder obtained by filing the metal phase or by crushing the CaHz phase was.passed through a 250 mesh screen and placed in a 0.3 mm. diameter glass capillary. This operation was performed in the glove box and the open end of the capillary was sealed with Apieeon W wax before bringing the capillary into the air. Some specimens were heated to 300-500° in the capillary to remove the distortion caused by filing or crushing. The diffraction patterns were obtained with copper K CY radiation.
1
e
0
+HEMAL A N A ~ S I S ' EQUILIBRATION EXPERIMENTS
2063
'
8
8
1
,
P C a and a C a n e
7 and a CaHp
! t I
$0
?k
do
do
I
daHZ
MOLE PER CENT CaHp
Fig. 1.-The
calcium-calcium hydride phase diagram.
I
1
I
I
I
Results The calcium-calcium hydride system is quite similar to the barium-barium hydride system* except for the additional complexity arising from the allotropy of calcium. The phase diagram is shown in Fig. 1. The melting point of calcium mas found to be 839 i 2'. The thermal arrest for the liquidus and solidus rose smoothly with increasing hydrogen coiitent to a peritectic at 890' and 24 mole % CaHz. The solid solubility limit of CaHz in calcium was observed by both thermal analysis and equilibration experiments. The solid solubility limit incrt:ased with temperature, and, in the range 780-890', it was in good agreement with the values obtained by Johnson, et aL14and by Treadwell and Stecherj through pressure-composition isotherm studies. Thermal analysis showed a phase transition in pure calcium metal a t 448 i 2' on heating and 442 h 2' on cooling which must be the f.c.c.+b.c.c. transformation found bv Smith, et d 6 A new phase, gamma, appears in this temperature range with increasing hydrogen contents. This gamma phase seems to not be an allotropic modification of calcium metal which is stabilized by hydrogen but a calcium-hydrogen intermediate phase that can exist only in the temperature range 320-600'. Postulated detail (8) D. T. Peterson and hl. Indig, J. Am. Chem. Soc., 82, 5645 (1960).
MOLE % CaH2, Fig. 3.--Postulated extent of the ?-phase in the calciumcalcium hydride system.
of the calcium-calcium hydride system in this range is shown in Fig. 2 . A peritectoid decomposition of a-calcium solid solution into ,&calcium and the a-phase is postulated a t about 449'. A specimen of the calcium which contained 220 p.p.m. hydrogen and gave an arrest a t 448' on heating and 438' on cooling was placed in a sealed tantalum thermal analysis capsule and degassed by heating for 2 days at 900' under a high vacuum. The melting point was unchanged from 839' and the transformation temperature was unchanged on heating, but on cooling mas raised to 442'. The thermal arrest, which was rather indistinct originally, was sharp, and occurred at an invariant temperature after degassing. hnalysis of the calcium after degassing showed 110 p.p.m. of hydrogen. These observations supported the conclusion that pure calcium has only two allotropic forms, but the origin of the y-phase could not
2064
D. T. PETERSON AND V. G. FATTORE
be firmly established. The y-phase a t 600 f 5' and 11 mole % CaHz undergoes a peritectoid transformation to ,&calcium and CaH2. At 320360' and near 1.5 mole yo CaH2, an eutectoid decomposition of y into a-calcium and calcium hydride takes place. This thermal arrest was the only one which was not found at essentially the same temperature during heating as during cooling. The arrest was always found a t 360' on heating and a t 320' on cooling. Decreasing the rate of heating or cooling did not significantly change the temperature at which this arrest was observed. X-Ray diffraction a t room temperature showed the presence of f.c.c. calcjum with a lattice constant of 5.592 * 0.002 A. in equilibrium with CaH2. The constant lattice of the pure calcium (110 p.p.m. Hz) was 5.59 rt 0.002 A. The composition of the CaHz phase in equilibrium with the metal phase could not be determined by analyzing the CaHz layer on the outside of equilibration specimens. The hydrogen contents of samples of the CaHz layer were erratic and generally below the lower composition limit for CaH: obtained by thermal analysis and pressure - composition isotherms. The analytical method had been shown to give satisfactory results on homogeneous samples of CaHz so the low results must have been due to occlusions of calcium metal during the growth of the CaHz layer. The thermal arrest at, 780' was found to originate in the CaHz phase and was interpreted as a phase transition in CaHz similar to that found in BaH2. This could not be checked by high temperature X-ray diffraction because of the reactivity and high dissociation pressure of CaH2 in this temperature range. The X-ray diffraction pattern of CaH2 a t room temperature mas found to be identical with that reported by Zintl and Hardera except that a few of the very weak lines were not observed. Discussion The melting point of calcium metal has been reported at temperatures between 839 and 851 ' by several investigators. Variation in the hydrogen content could be responsible for these different results. On the basis of the effect of hydrogen on the melting temperature and of the purity of the calcium used in this investigation, it seems reasonable to give 839 f 2' as the melting point for pure calcium metal. The allotropy of calcium has been studied by a number of investigators using methods such as
Vol. 65
differential thermal analysis, electrical resistivity, dilatometry, thermal expansion, X-ray analysis and thermoelectric power. There was considerable disagreement in the results which can be divided into two groups: those showing only one transition and those showing more than one transition. Rinck,g Graf,'O Ebert, et al.," Schulze, et a1.,l2 and Smith, et U E . , ~ found only one transformation which was observed a t 450-460'. Bastien,Ia Grafl* Schulze,16 Sheldon,I6 Melsert, et al.," and Schottmiller, et al.,' found two transformations: the f i s t in the range 270-350' and the second in the range 460-610'. Calcium was reported to be f.c.c. up to the first transition, h.c.p. from the first to the second and b.c.c. above the second transition. In addition, some of these authors found a low symmetry or "complex" phase between 270-460' which was introduced by contaminants. None of the investigators reported the hydrogen content of their specimens although the calcium which was used might have contained significant quantities of hydrogen. Smith and Bernsteinl* studied the influence of specific contaminants on the calcium transformations. They reported that the presence of the h.c.p. phase was associated with hydrogen contamination of the calcium. This investigation shows the importance of hydrogen in the allotropy of calcium and supports the conclusion that pure calcium undergoes only one allotropic transformation which is the transition from f.c.c. to b.c.c. a t 448'. However, a small amount of hydrogen induces the appearance of a y-phase, the presence of which in the specimens of some previous investigators can explain the differing results reported. The identity of the temperature range over which gamma is stable, and the temperature range reported by Sheldon and by Schottmiller, et al., for the h.c.p. phase leave little doubt that CY is the h.c.p. phase. (9) E. Rinck, Compt. rend., 192, 421 (1931). (10) L. Graf, MetaZZwirfscho/t, l a , 649 (1933). (11) F.Ebert, H.Hartmann and H. Peisker, Z. onorg. u.dZgem. Chcm., 213, 126 (1933). (12) A. Schulze and H. Schulte-Overberg, MetaZZwirtuchaff, 12, 633 (1933). (13) P.Bastien, Compt. rend., 198, 831 (1934). (14) L. Graf. Physlk. Z.,36, 551 (1934). (15) A. Schulre, {bid., 36, 595 (1935). ( 1 6 ) E. A. Sheldon, Thesiu, Syracuse University. 1949. (17) H. Melsert, T. J. Tiedema and N. G . Burgers, Acta Cryst., 9, 525 (1956). (18) J. F. Smith and B. T. Bernstein, J . Electrochem. Soc., 106,448 (1959).
