14s
A. PKBLCR AXD W.E. WALLACE
where C’ = a constant = effective area of contact between an NH3 molecule and the surface upon collision sn = sites per unit area a t time zero
a
We already have assumed b constant and found that this correlates well with the observed slopes. Moreover, u ought to be very nearly the crosssectional area of an NHa molecule and independent of the UH, surface. One thus would conclude that to should be proportional to l,/[il;H3]0so. From the data in Table 11, it can be seen that whereas the slopes and intercepts vary dircctly with toid surface area (as meas-
Vol. 66
ured by Vm), to varies in an inverse fashion with specific surface area. This suggests that so is directly related to the specific surface area, (r. I n fact, if one arbitrarily assumes so CY ~ ‘ 1 2 , then the product to[XH3]0a l l 2 should be constant for all experiments. The bottom line in Table I1 shows that the constancy is indeed quite good. This suggests that the number of sites per unit area is inversely proportional to the square root of the average particle diameter. Acknowledgment.-The author wishes to express his appreciation to Dr. J. F. Lemons of this Laboratory for his active interest and helpful direction in this work. He also wishes to thank Dr. Walton P. Ellis of this Laboratory for helpful discussions concerning the interpretation of some of the results of this work.
CR17STAL STRUCTURES OF SOME LANTHANIDE IIYDRIDES’ BY A. PEBLER A I ~ DW.E. WALLACE Dcpartment of Chemistry, University of Pittsburgh, Pittsburgh i3, Pa. Received Auguat 26, 1961
Hydrides of Pr, Nd, Sin, Tb, Dp, Ho, Er, Tm, Lu and Y have been formed and examined using X-ray diffraction techniques. The dihydrides of the several metals always were observed to be cubic. Appreciable contraction of the lattice occurs when PrH2, NdHl or SmHzabsorbs extra hydrogen. The PrHz and NdHz phases remain cubic to the highest. h.ydrogen concentration attainable, whereas the Sm-H system undergoes a transformation such that the Sm ion cores are in a cph arrangement for hydrogen concentrations exceeding that corresponding to the formula SmHl.se. Similar transformations are observed for the several heavy lanthanides studied and for yttrium, the range of stability of the dihydride being, however, considerably less than that for the alloys based on SmHs. The dependence of the lattice spacing on composition is discussed in terms of the presumed electronic nature of the lanthanide hydrides. The observed contraction of the dihydride lattice is consistent with the notion (suggested by their electrical and magnetic behavior) that the lanthanide hydrides are essentially saline in nature and the hydride ion is formed by absorption of electrons from the conduction band of the metal.
Introduction For many years it has been known that when the lanthanide metals are exposed to an atmosphere of hydrogen at elevated temperatures, reaction occurs and hydrogen is incorporated in the solid, forming what usually are termed “hydrides.” Several investigations have been carried out for the purpose of elucidating the structural features of these hydrides. Using conventional X-ray diffraction techniques the arrangement of the metal atoms or ions has been established in several cases and in one instance (the Ce-H system) the hydrogen atoms or ions were located using neutron diffraction data. To date attention has been focussed largely on the light lanthaiiides La, Ce, Pr, Nd and Sm. I n the elemental state Ce is fcc, Pr and Nd are cph, La is cph with a doubled c spacing and Sm is rhombohedral. However, each of these is observed2 to form a dihydride MeHz in which the arrangeinent of the metal ion cores is fee.+' These dihydrides have been found to possess the ability to (1) This work w3s assisted by a oontrart with the U. 6. Atomic Energy Commis.ion. (2) A. Rossi, iVature, 133, 174 (1934). (3) B. Dreyfuss-Alain, Compt. r e n d , 236, 540, 1295 (1952); 236, 1265 (1953); 237, 806 (1953). (4) R. N. R. Mulford and C. E. Holley, J . P h y s . Chem., 69, 1222 (195 5 ) . (5) C.E. Holley, 61’ al. *hid 59, 1226 (1955). (6) K. Dialer and W. Rothe, 2. Elektrochem., 69, 970 (1955). (7) JJ Stalinski. B i d acad polon. SCI. 3, 613 (1955)
absorb considerable additional hydrogen without alteration of the structure of the metallic matrix. Also, it was ascertained that to a limited extent hydrogen can be removed from the dihydride. It thus is clear that the phase based on the dihydride stoichiometry embraces a considerable range of composition, which in La-H and Ce-H extends6$’ to the trihydride composition MeH,. Neutron diffraction work on Ce& showed5 the H’s to reside in the tetrahedral interstices; thus this material possesses the fluorite structure. A similar study of a sample of composition represented by the formula CeH2.7 showed6 that all the tetrahedral interstices were filled and the additional H’s were randomly distributed in the octahedral interstices. Although direct supporting experimental evidence is lacking, i t generally is believed that (1) the situation is similar for the La, Pr and Nd dihydrides, with and without additional hvdrogen, and (2) in those hydrides in which H/Ce < 2.0, the H’s are distributed randomly over the tetrahedral sites. In 1956 Sturdy and Mulford conducted what appears to have been the first and only Investigation of a heavy lanthanide-hydrogen system--the Gd-H system.* They found by X-ray diffraction, which of course detects only the metal ion (8) G. E. Sturdy and R. N. R. Mulford J . Am Chem SOC.7 8 , 1083 (19.56I .
Jan., 1962
CRYSTAL STRTJCTURES OF SOME LANTHANIDE HYDRIDES
cores, the usual fcc dihydride. This phase was observed to exist from GdH1.8 to GdH2.8. At higher hydrogen concentrations a second phase appeared, in which the Gd ion cores were in a cph arrangement, ‘The two-phase region extended from GdH2.3to CJdHz.as, above which only the hexagonal phase existed up to GdH2.91, the highest hydrogen concentration obtained in their study. hlulford has predictedg similar behavior for all the heavy lanthanides except europium and ytterbium.’O The present study was undertaken to increase the amount of information available concerning the constitution of heavy lanthanide hydrides, to ascertain whether or not Mulford’s prediction is correct and, if so, to locate the several phase boundaries using standard X-ray diffraction techniques. The lanthanides originally included in the study extended from Sm to Lu, with the exception of Eu and Yb. When Sm was observed to form a hexagonal phase, the coverage was extended to P r and Kd, and, in addition, the chemically similar element Y was included. Experimental The lanthanide metals were obtained from the Nuclear Corp. of America, Burbank, Cal. They were analyzed spectroscopically by the supplier and were found to be 99% pure or better. The hydrogen used was obtained from a commercial cylinder and purified by passing through a Deoxo unit, a liquid nitrogen trap and finally a heated palladium tube. The following procedure was employed in preparing the metal for hydrogen: 0.1 to 0.4-g. samples were cut from the stock supply, the cutting being done under mineral oil. The samples then were polished under mineral oil with progressively finer abrasive papers ending with type 600A. They then were washed successively with CC4, acetone and ether. (With the more reactive lighter lanthanons weighing was also under oil.) After weighing, t h e sample was introduced into the preparation train (vacuum system and appropriate gas metering equipment) and evacuated to high vacuum for a t leaat 4 hr. It then was heated gradually to 600” and hydrogen then was admitted. Hydrides with compositions approximating the dihydride usually formed quite rapidly. For higher hydrogen contents the temperature had to be lowered and higher pressures (up to 1 atm.) had to be applied. After attaining the desired composition, the temperature was reduced and the sample was annealed at 200 to 300’ for at, least 4 hr. Upon removal the hydrides were placed under oil and kept there to retard oxidation and/or decomposition. X-Ray diffraction photographs were taken of the samples using a 114.6 mm. diameter camera arranged for Straumanis loading. Filtered CrKa radiation was used. The powder specimens were prepared by crushing the sample under oil and sucking the oil suspension up into a fine capillary which then was sealed off with plastic cement. I n the case of the very sensitive Sm trihydride the capillary was filled under helium in a dry box. Except for some of the higher hydrides, the lines in the diffraction pattern for the single phase materials were quite sharp, permitting accurate determination of the lattice parameters. Extrapolation to 90’ Bragg angle was made using Taylor’s or Cohen’s method.11
Results Experimental effort was largely concentrated on the five metals Sm, Dy, Ho, Tm and Nd. The (9) R. N. R. Mulford. private oommunication. (10) Variouri evidence suggests that these elements are divalent in the metallic state in contrast with the remaining lanthanides, which are trivalent. For E u and Yb the dihydride is the highest hydride formed. These dihydrities are orthorhombic, possessing the CaH2 structure. See W. L. Kortit and J. C. Warf, Acta Cry& 9, 452 (1956). (11) See, for example, L. V. Azaroff and M J. Buerger, “The Powder Method in X-Ray Crystallography,” MoGraw-Hill Book Co.,Inc., New York N. Y 1958 p. 238 ff.
A- rP
149
c, ,
;
I
/
Nd
Srn
Tb DY
Ho Er
Tm Lu
Y
I
0
H/Me.
2
3
Fig. 1.-Resume of information concerning phase relationships in several lanthanide metal-hydrogen and the Y-H systems. The single phase regions based on the dihydrides (cubic) and trihydrides (hexagonal) are indicated by C and H, respectively. Regions in which two phases coexist are crosshatched. Compositions for which diffraction patterns were obtained are indicated by the vertical line or lines: two-phase; single cubic phase; % ,single hexagonal p h a s e - r he phase boundaries whose positions are considered less certain are dashed.
-I--,
-
others were examined only to the extent necespary to establish that their behavior is substantially the same as that of their more thoroughly investigated neighbors in the Periodic Table. I n all cases powder diffraction patterns were obtained for the metal containing varying amounts of hydrogen. The patterns were analyzed to ascertain whether the system was single phase or heterogeneous and to determine the structure(s) of the phase or phases present. When the sharpness and intensities of the lines permitted, lattice parameters were evaluated. The results obtained are summarized in Fig. 1 and 2 and Table I. Jn regard to Fig. 1 it is to be noted that no range of primary solubility is indicated. Since earlier work has shown6J that at elevated temperatures several of the metals take appreciable quantities of hydrogen into solid solution, there is undoubtedly some solubility even at room temperature. However, this solubility is probably quite small; at least this seems to be the case for Bo-H, the one system which was examined to ascertajn the extent of the primary solubility. The metallic phase coexisting with HoHa was examined and its lattice parameters were found to be indistinguishable from those of pure holmium. T t also is to be noted regarding Fig. 1 that in some cases (for example, the Dy-H system) a number of compositions were examined and the phase bound-
A. PEBLEA A N D \V8 E. WALLACE
160
1
I
T’ol. 66
I
TABLE I X-RAY D E S B I T I E OF~ LANDI- AND TRI-HYDRIDES
LATTICUPARA METERS^
5.53
TKANIDE
MeEz
AND
nteI-Ig
poubt/
(A,) no (A,) c5 (A,) co/ar phex. Pr 5.518 (5.517“) Nd 5.464 ( 5 470’) Sm 5.374(5.376“) 3.782 6.779 1.792 1.082 Gd (5.303d) 3.73d 6.71d 1.80 1.084 Tb 5.246 3.700 6.658 1.799 1.093 Dy 5.201 3.671 6.615 1.802 1.097 Ho 5.165 3.642 6.560 1.801 1.094 Er 5.123 3.621 6.526 1.802 1.102 Tm 5.090 3.599 6.489 1.803 1.104 Lu 5.033 3.558 6.443 1.811 1.108 Y 5.205 3.672 6.659 1.813 1.103 a For the dihydride the parameter listed is either measured directly or interpolated to the composition RieH2. For the “trihydride” the parameters given are for MeHt or, if less hydrogen could be introduced into the metal, the ammeters are for the most hydrogen-rich sample that could%e formed. In no case v a s the hydrogen-metal ratio less than 2.90. * Densities of the cubic form (represented as poub.) are those estimated for the hypothetical cubic MeH,. Data of Holley, et ai. (ref. 5 . ) . d D n t a of Sturdy and PIlulford (ref. 8). Metal
no
I
5 39k
510
-
c I
I
,:TmHx C t H ,
I
wider and the total cont.ra,ction of the lattice spacing is increased accordingly. It is of some interest to ascertain the cause of the decrease in lattice dimensions when the dihydrides absorb extra hydrogen. This phenomenon can be rat,ionalized in terms of the concepts held by the present authors concerning the elect’ronicnaOure of these hydrides. The view is held (I) that the lanthanide hydrides are essentially salt-like in character with the hydrogen of course present as an anion and (2) that the hydrogen acquires its electron upon entry into the metal from the electrons in the conduction band. Support for the first point12 is provided by such things as the observed heat’s of hydrogenation,6 the stoichiometry of the hydrides (trihydride formed with trivalent metals whereas only dihydride can be formed with the divalent metals Eu and Yb) and their positions in the Periodic Table. More recently considerable support for both points has been provided by measurements of the electrical conductivities of certain lanthanide hydrides. la Upon complete hydrogenation the conductivity falls by six or more orders of magnit,ude with Dy, Ho and Yb. Stalinski found a similar behavior for La and Ce and moreover obtained additional evidence for the non-met,allic character of these hydrides by noting t’hat their resistivities decreased with increasing temperature.l4 The conductivity behavior is consistent with the not,ion t,hat the hydride ion is formed using electrons in the conduction band. This also is supported by magnet’icwork carried out by Trombe15 for GdI&, by S t a l i n ~ k i l for ~ , ~the ~ La-H and Ce-H systems and by Kubota and Wd(12) Actually, the presumed saline character of these hydrides should be restricted t o the ca8e of the trihydride. Alloys less rich in hydrogen, including the dihydride, exhibit metallic conduction and hence must be regarded as a t least partly metallic in nature. (13) T.Peltz and W. E. Wallace, unpublished. (14) B. S t a h s k i , Bull. m a d . polon. sci., 5, 1001 (1957); 7 , 269 (1959). (16) F. Trombe, Compt. rend., 219, 182 (1944). (16) B. S t a h s k i , Bull. mad. pdon. sci., 6, 997 11957).
Jan., 1962
C R Y s T a L 8TRVCTURES O F &.)ME
LANTHANIDE HYDRIDES
151
Cubic Dihvdrtdes lacel7 for the Ho-H and Tb-H systems. In each / / / / I / I I I I I I E80 case the magneton number is imchanged by hydroI genation, indicating that the electrons used to form the hydride ion are not abstracted from the core. On the basis of the evidence presented in the preceding paragraph one concludes that as hydrogen is added to the dihydride, electrons are progressively removed from the conduction band. T t appears that this would produce a denser structure for two reasons: (a) the conduction electrons in the dihydride screen and hence weaken the interaction between the oppositely charged lanthanide and hydrogen ions; as they are consunied when hydrogen Hexagonal Trthydrides is added, their screening power is reduced, the interactions increase and hence the interionic distances arlz expected to decrease; (b) as has been discussed in detail by Nabarro and Varley,l* the quasi-free condixction electrons in a metal due to their Fermi (or kinetic) energy materially contribute t o repulsion and hence a,ct to expand the solid; thus as they are absorbed by the dissolving hydrogen this repulsive tendency is decreased, permitting contraction in the lattice. These considerations make the origin of the contraction of the hydrogen-rich dihydrides clear but do not, of course, clarify why the structure changes Y L a c e Pr Nd Pm Srn Ed Tb Dy Ho Er Tm Lu, to hexagonal for the metals beginning with samarFig. 3.-Lattice parameters uemtis atomic number for ium. The data in column 6 of Table I show that lanthanide dihydrides and samplea approaching the comupon changing t o the hexagonal form the system position of the trihydrides: 0,this study; A, ref. 5 ; x, achieves, a reduction in density of roughly 10%. ref. 8. Data for YH2 and YHn also are included. It is not clear whether the structural change and the concomitant density reduction are merely a pairs of hydride ions occur separated by a distance matter of reducing the electrostatic energy of the of 0 . 4 0 4 ~or~about 1.5 An According to Libowitz system or whether other factors are involved. and O;ibblBthe ionic radius of H- is about 1.3 A. Work is under way in this Laboratory a t present to Thus the undistended structue entails too close evaluate the Madelung number of the hexagonal an approach of the hydride ions and elongation and cubic forms of the trihydride. This should in along the hexagonal axis occurs. It is evidently tjime contribute to the elucidation of the factors this which produces the abnormally high axial responsible for the cubic to hexagonal transfornia- ratio. I n Fig. 3 are plotted lattice parameters for the tion observed for the heavy lanthanides. The data in Table I show that the axial ratio dihydrides and trihydrides (or iiearly so) of the exceeds the value for the close packing of spheres by various lanthanides studied to date. These data about 10%. It appears that the distension along give clear evidence of the well-known lanthanide the hexagonal dxi s develops primarily because of contraction. Acknowledgment.-The authors would like to strong repulsion between the hydride ions in certain of the tetrahedral sites; using a model of the express their appreciation to Professor R. S. Craig inndistended hexagonal trihydride, it is noted that for his generous assistance in connection with many of the experimental aspects of the present work. 117) Y. Kubota and W. E. Wallace, to be published.
'
3
(181 F. R N Nsbarro and J. H, 0, Varley, PTOC. Cambridge Phil. BOG,,48, 318 11952)
(IS) G , G, Libowitv and T (1966).
R P
d t b b , Jr., J . Phys, Chcm., 60, 510
