SOTES
13.70
value of log K,, = -34.50 and for amorphous FePO4.2HzOa value of log K,, = - 30.02. These values may be compared with the value of -33.5 reported by Chang and Jacksoi~'~ as a median of (19) S. C. Chxng a n d 265 (1957).
M. L.
Jackson, Sod Sca. S o c . Am., Proc., 21,
T-ol. 65
values (-32 to - 3 5 ) obtained in direct measurements on ferric phosphate dihydrate. Acknowledgment.-W. E. Cate and M. E. Deming prepared the ferric phosphates. Inez Jenkins >Iurphy and J . TT. made the chemical analyses.
NOTES X-RAAYDIFFR-ACTION OBSERP*ATIOKS OF TH:E Pd-H? SYSTEM THROUGH THE CRITICAL REGION1 BY
k3NCLF
J. MAELlSD
AND
THOMAS R. P. GIBB, JR.
Department of Chemistru, Tufts Cniuersity, M e d f o r d , Mass. Receired S o v e m b e r 4 , 1960
The occ1u;sionof hydrogen by palladium has been studied extensively (cf. ref. 2). Two solid phases, both with a, f.c.c. structure, have been identified by X-ray d i f f r a ~ t i o n . ~ -The ~ a-phase is formed by the absorpt'ion of small amounts of hydrogen, the maximum composition is about PdHo.03 a t room temperature, and it has a lat'tice parameter only slightly larger than that of pure palladium (3.889 A.). As hydrogen is further absorbed, the a-phase becomes unstable nand the lattice suddenly expands to about' 4.018 A. (PdHo.,J to form the 0-phase. Additional hydrogen uptake is accompanied by a gradual expansion of the a-phase lattice. X-:Ray diffract'ion studies of the Pd-H2 system have, for the most part', been performed at room templerature on samples prepared either electrolytically or a t elevated t'emperatures. Either procedure involves some uncertainty as to what may occur in the specimen betmeen the time it' is charged and the time t8heX-ray exposure is made. Recognizing this problem, Owen and Jones7 and Owen and Williams8 charged the specimen from the gaseous phase and made the X-ray exposures while the specimen was maint'ained a t a definite temperature and constant hydrogen pressure. The present investigation is an extension of t'he investigation of Omen and Williams to higher temperatures and pressures. Six evolution isotherms, from 206 to 3 4 6 O , have been t,raced from 33 atni. to zero pressure. Experimental Apparatus.--The high temperature assembly used has been described previoiisly.9 The specimen temperature was - ~ _ _ (1) This research was supported b y the U. S. Atomic Energy Commission. (2) D. P. Smith, "Hydrogen in Metals," Univ. of Chicago Press, Chicago, 1948. (3) L. R '. McKeehan. Phys. Rev., 21, 334 (1923). (4) J. 0. Linde and G. Borelius, Ann. P h y s i k , 84, 747 (1927). ( 5 ) F. Kruger and G. Gehm, ibid., 16, 174 (1933). (6) G. Rosenhall, ibid., 18,150 (1933). (7) E. A. Owen and J. I. Jones, Proc. Phus. SOC.( L o n d o n ) , 49, 587 (1937); 49, 603 (1937). (8) E. A. O a e n and E. St. J. Williams, ibid., 66, 52 (1944). (9) E. J . Goon, 3 . T. Mason and T. R, P. Gibb, Jr., Reu, Sci. Inatr., 28, 342 (1957).
measured with an accuracy of h 3 " or better. All diffraction patterns n-ere obtained with the Straumanis type G.E. powder camera and with nickel-filtered copper radiation. Materials.-The palladium metal n-as of 99.87, (specified) purity and was obtained from A . D. Mackay, Inc. Commercially available hydrogen was purified by passing it through a Deoxo purifier, then through a column of Drierite, and finally through a hot uranium getter. Experimental Procedure.-Tvo methods of charging the specimen with hydrogen m-ere employed. I n method (1) the hydrogen was admitted a t room temperature into the evacuated system to the desired pressure. The temperature was then raised to its proper value (the temperature of the isotherm under study). Both temperature and pressure were kept constant one hour or more before making the exposure. After the exposure, the specimen was allowed to cool to room temperature. Before the next exposure was made, the specimen was heated to the temperature of the isotherm, the pressure changed and maintained a t this new value. In method (2) the hqdrogen was admitted as in (1) and the temperature adjusted to the value of the isotherm. Bfter each exposure the pressure was changed and maintained constant for one hour; this was done, however, without cooling the specimen to room tempersture. Both methods gave similar results. Lattice Parameter Calculation.-The lattice constants were obtained by the extrapolation method of Bradley and Jay,lo using. the 422, 420, 311 and 400 lines. I n some cases, the broad diffraction bands, caused by the beryllium sample holder, completely masked the 422 and 420 lines. I n these instances care was taken not t o alter the position of the specimen during a series of isothermal exposures at different pressures. It was assumed that the absorption and asymmetry errors similarly affected the reflections from a particular plane (331) a t the various pressures, and the extrapolation method could therefore be used (assuming constant slope).
Results Parameter values for the hydrogen charged specimen, held a t 206, 258 and 346O, are listed in Table I, along with the various pressures employed. Figure 1 is a plot of the lattice parameter uersus hydrogen gas pressure for each of the six isotherms traced. In the favorable cases, i.e., when the 422, 420, 311 and 400 reflections were unmasked, the broadened lines of the @-phase yiel4ed parameters with an accuracy of *0.004 A , while the parameters of the a-phase are less accurate due to the very diffuse appearance of the lines. At the higher temperatures the lines appeared to be partially re$olved and (he accuracy for the 346' isotherm is i 0.003 A. In the few instances when the a-phase and the p-pha-e coexisted, the lines of the a-phase were so weak that measurement was impossible. The presence of the 0-phase is indicated in the table, however, so that it may be known when the tu70 phases coexisted. (10) A. J. Bradley and A. A. Jay, Proc. P h y s . Soc. ( L o n d o n ) ,44, 563 (1932).
July, l%iI
1271
SOTER
TABLE I ISOTHERMAL T'AR~ATIO?~OF LATTICEPARAMETER OF Par.LADIU.M HYDIRIDE WITH HYDROGES G a s PRESSURE
--
7 - 2 5 8 ' Isotherm-Pressure, Parameter, A . atm. a P
206O [sotherni--
Prespure,
atm.
22.3 13.3 5 1 4.5 4 2 X8
2.7 1 i 5 I 5.0
1.8 4.4 14 6
P.iramete-, K. Ly
. . ...
B
4.043 4.040 4.022 4 020 4 015
Faint Faint Faint :3.!116 4 01:i 3.!)14 . 3,$)0!) . . . Faint 4 025 Faint 4.022 Failit 1 0 2 2 F:titit 4 OI!) 4 038 . . 4 034 4 029 , 4 027 Faint 4 02'7 Faint 4 025 Faint 4 023 Faint. 1 022 3.920
23.3 19.4
16.8 11 8 1 1 :i 10 5
10.3 0 $1 !).$)
9 5 9.4
9 3 9 2 7 8 '7 1 4.4 2 0
...
4.035 4.031 4.027 4.022 4.021 4.021 4.021 4.020 4.010 4.020 4.016
346' Isotherm Pres- Paramsure, $ter, atm. A.
33.7 31.7 30.8 28.6 26.9 25.5 22.0 .. 19.4 ... 17.3 ... 19.2 Faint 11.3 Faint 4 . 0 1 i 7.9 3.934 . , 4.1 .. 3,928 . . . ... 3.926 .. 3.916 .., , . ... 3.912 .. ... , . . . . , . . . . . , . .
3 997 3.986 3.971 3.961 3.951 3.941 3.937 3.933 3.929 3.927 3.923 3.919 3.916
Fig. 1.-Hydrogen
evolution isothrrms.
which is also observed in dissociation pressure measurements. The runs are listed in Table I in the order in which they were made. ... A11 diffraction patterns taken at 206' of the pphase caontained broadened lines in the back re.. , . . ... .. ... flection region. Prolonged heat treatment a t the 4 1 .. ... ... .. ... temperature of the isotherm did not improve the 3.7 . ... ... .. ... definition of the lines. When the a-phase appeared 3 4 .. ... ... .. ... on diminishing the pressure, its presence was first 0 . 0 3 .00x . ... ... .. ... indicated by the appearance of low angle lines. On further decrease of pressure high angle lines were Discussion produced, but these were more diffuse than the PFrom a consideration of Fig. 1 i t is evident that, phase lines. The definition of the lines improved in crossing the 1,wo-phase region, the 206' isotherm somewhat with rising temperature. Thus, the exhibits ,some unusual feat,ures not observed for patterns taken at 346' gave moderately sharp the other three isotherms studied below the critical lines in the back reflection region. The crossing solution temperature. St,arting with the p-phase of isotherms in the lower left-hand region of Fig. 1 at) high pressure, it is seen that' the lattice param- is, of course, due to the opposing effect of H-content eter of the p-phase decreases gradually with de- and thermal expansion. The parameters of the a- and P-phases where creasing hydrogen pressure. The rate of change in lattice parameter increases as the t,wo-phase they coexist are plotted in Fig. 2 for each temperaregion is approached. When the pressure has been ture investigated. The results from Owen's and lowered to 5.1 atm., the a-phase makes its ap- William's investigation are also included. When pearance. Both the a- and the P-phase continue the two phases coexist over a range of pressures to coexist as the pressure is further diminished, the at one particular temperature, the parameters a-phase cwntinuously increasing in amount. When recorded are those measured a t the pressure when the pressure is decreased beyond 3.7 atm., the p- the p-phase disappears with decreasing pressure. phase disappears entirely and the parameter of the From Fig. 2 it is evident that the parameter for a-phase then decreases linearly with decreasing the p-phase when it disappears is approximately pressure. It should be pointed out' t'hat the lat'tice the same a t all temperatures from 60' to about constants of boi:h t,he a- and the p-phase decrease 230'. The parameter of the a-phase, however, in crossing the ( a b') region Then the pressure is (measured when the p-phase disappears) increases lowered (cf. ref'. 8). This is contrary to what is gradually with temperature. From temperatureexpected in a two-phase region of an alloy system concentration measurements, it is known that the in a state of thermal equilibrium. In such a case concentration of hydrogen in the p-phase, measured the param.eters (concentrations) of the two phases a t the right-hand end of the plateaus, decreases remain constant' n-ithin the (a p) field. I t would with increasing temperature. Since the p-phase seem, therefore, that the system is not in a state lattice contracts when hydrogen is removed from of equilibrium. Owen and WilliamsY arrived a t it, it appears that up to about 230' this contraction the same conclu3ion from their investigation of the is exactly balanced by the thermal expansion of 60, 80, 100 and 120' isotherms. The 206' iso- the lattice. For temperatures higher than about therm is of the 5;amc form as those investigated by 230' the contraction due to decreasing hydrogen concentration in the p-phase lattice dominates. these authors. Figure 1 shows that a t 206' two different sets of The data indicate a critical solution temperature parametem were obtained for the p-phase in the of 308" in fair agreement with the value of 295.3' region where the a- and the @-phasescoexist. The estimated by Gillespie and Galstaun.ll difference is believed t,o be due to the previous (11) L. J. Gillespie and L. 8. Osls%aun,J. Am. Chem. Soc., 68, 2565 history of the sample. This is a form of hysteresis (1936). 7 8 5.8
