3220
J . Phys. Chem. 1984, 88, 3220-3224
redistribution can occur in molecules with internal energy contents where the state density is ca. 5-50 ~tates/cm-l.’~Since Nethyltrifluoroacetamide has several low-frequency torsional vibrations and low-frequency vibrations involving the amine group, we expect it will achieve high state densities at low internal energies. Using available spectral data employing a direct-count procedure, and including all vibrational and rotational states, we calculated the energy dependence of the state density. A state density of 50 states/cm-l is achieved at ca. 0.1 kcal/mol for this molecule, and a state density of 100 states/cm-’ is achieved at an internal energy of 0.2 kcal/mol. The very low critical energy is primarily the result of the low torsional frequency, 25 cm-’, used for the trifluoromethyl top and the small rotational constants ( B C 1.8 GHz) of this molecule. These calculations assume a twofold potential with a 5 kcal/mol barrier for r2. The same set of assumed vibrational frequencies was used to calculate the total vibrational partition function of N-ethyltrifluoroacetamide at 300 K which is ca. 6000. Of special interest is the population of molecules with energy in excess of the threshold for rapid IVR. We estimate this population as between 80% and 96% of the total. These rapidly vibrationally redistributing molecules will have an average rotational constant which depends on the form of the internal rotation potential function and the rotational constant dependence on torsional angles. Since torsional potential functions about r z for N-ethyltrifluoroacetamide are completely unknown, an effective B C for rapidly vibrationally redistributing molecules cannot be estimated. Bands of these molecules are expected to be lifetime broadened and widths limited by the Stark field used in these experiments. N-Ethyltrifluoroacetamide is a structural analogue of ethyl trifluoroacetate. We would expect it to have a slightly lower internal energy threshold for rapid IVR since low-frequency deformation of the N-H group would contribute to its state density.
+
-
+
It is informative to compared the present results with those obtained previously for ethyl trifluoroacetate. Ethyl trifluoroacetate displays a very intense series of broad, structureless bands and in addition two weak-band series. The weak series increase in intensity at low temperature, demonstrating that the corresponding species are more stable than those producing the intense series. Ethyl trifluoroacetate has a vibrational state density of 100 states/cm-’ at an internal energy of 0.2 kcal/mol, and the origin of the intense-band series has recently been attributed to molecules whose internal energies (- 1 kcal/mol) are above the threshold for rapid vibrational redi~tribution.’~ In the rapid IVR formalism, since ground-state band spectra were not observed, the ground-state conformation cannot be established except in symmetric cases such as N-methyl- and Ntert-butyltrifluoroacetamide. For the more asymmetric trifluoroacetamides, the ground state may have a different effective torsional angle with a population too small to detect at room temperature and/or its geometry may be close to the effective torsional angle of the rapidly redistributing molecules and produce unresolvable spectra. Experimental results cannot differentiate between these two possibilities. Despite this ambiguity, it is possible to distinguish the conformation about bonds with high rotational barriers, and an unambiguous result of this study is the preferential syn conformation about the peptide bond of N-alkyltrifluoroacetamides.
Acknowledgment. This research was supported by the National Institutes of Health through Grant No. PHS-GM-29985-01. We thank Gerald Lindley for help with preliminary calculations. Supplementary Material Available: Tables containing structural parameters used in rotational constant calculations and assigned microwave bands of N-alkyltrifluoroacetamides(4 pages). Ordering information is given on any current masthead page.
Thermodynamic, Kinetlc, Structural, and Surface Studies of Y6Fe23-xMn, Alloys and Their Hydrides George Bayer and W. E. Wallace* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: November 21, 1983)
Pressure-composition isotherms at 0 and 22 OC were obtained for the systems Y6FeZ3-H2,Y&hz,-Hz, and Y6Fez3-xMnx-H2 with x = 1, 3,5, 14, and 18. These isotherms were used to derive enthalpies and entropies of desorption. The ternary systems for the regime 11.50 5 x 5 17.25 exhibit very anomalous magnetic properties and their hydrides display anomalous thermodynamics,e.g., negative entropies of desorption of hydrogen. Hydride stability was greatest for the end-membersystems. Crystallographic work shows that all ternaries and all hydrides occur in the Th6Mnz3structure, fcc and space group Fm3m. Hydrogen absorption is rapid in all cases; little activation is required. In the case of Y6Fez3a short induction period was involved for initial H2 absorption whereas there was none for Y6Mn23. By Auger spectroscopy it has been found that Mn segregatesto the surface in Y6Mnz3,whereas in Y6Fez3the surface is Fe deficient. The surface composition of Y6Mnzsresembles that observed earlier for rare earth-Mn intermetallic compounds.
Introduction In recent years the rare earth-Mn and rare earth-Fe compounds of 6:23 stoichiometry and their hydrides have been intensively in this laboratory and elsewhere. The R6MnZ3systems (1) V. K. Hardman, Ph.D. Dissertation, University of Missouri-Rolla, Rolla, MO, 1979. (2) F. Pourarian, E. B. Boltich, W. E. Wallace, R. S. Craig, and S.K. Malik, J . Mugn. Mugn. Mater., 21, 128 (1980). (3) F. Pourarian, E. B. Boltich, W. E. Wallace, and S.K. Malik, J. Less-Common Met., 74, 153 (1980). (4) A. T. Pedziwiatr, E. B. Boltich, W. E. Wallace, and R. S. Craig in “Electronic Structure and Properties of Hydrogen in Metals”, P. Jena and C. B. Satterthwaite, Eds., Plenum Press, New York, 1983, p 367. (5) K. Hardman, J. J. Rhyne, and W. J. James, J . Appl. Phys., 52,2049 (1981).
0022-3654/84/2088-3220$01.50/0
and their hydrides have been particularly well studied. The heavy rare earths form 6:23 compounds with both Mn and Fe: but the Fe compounds are much more difficult to prepare, and they have been the subject of much less study than their Mn analogues. Both the Mn and Fe compounds absorb significant amounts of hydrogen under ambient conditions, the Mn absorbing approximately 23 (6) J. J. Rhyne, K. Hardman-Rhyne, H. K. Smith, and W. E. Wallace, J . Less-Common Met., 94, 95 (1983). (7) Kay Hardman-Rhyne, H. Kevin Smith, and W. E. Wallace, J . LessCommon Met., 96, 201 (1984). (8) H. Kevin Smith, W. E. Wallace, and R. S.Craig, J . Less-Common Met., 94, 89 (1983). (9) W. E. Wallace, “Rare Earth Intermetallics”, Academic Press, New York, 1980, p 180.
0 1984 American Chemical Society
Y6Fea3,Mn, Alloys and Their Hydrides
H atoms per formula unit and the Fe compounds approximately 16 H atoms per formula unit. The 6:23 compounds crystallize in the ThsMnZ3structure (space group Fm3m) with 116 atoms per unit cell.Io The transition-metal atoms occupy four distinct crystallographic sites (4b, 24d, 32fl, 32f2), and the rare-earth atoms occupy the 24e site. Neutron diffraction studies across the compositional range of the Y6Fe23-xMn,system have been carried out.' These studies indicate the presence of preferential atomic ordering of Fe and Mn atoms on the four transition metal crystallographic sites. Throughout the entire compositional range, Mn atoms prefer to occupy the f2 site and Fe atoms the f, site. The Y6Fez3-,Mn, system exhibits extraordinary magnetic characteristics. The end members are magnetically ordered with Curie temperatures -500 K. The ternaries have reduced Curie temperatures compared to Y6Fe23 and Y6Mnz3,and in the intermediate composition regime 11.50 Ix I 17.25 there exists no long-range magnetic ordering." It was shown by Oesterreicher and Bittner12 that the hydrides mimic the magnetic behavior of the parent compounds, with, however, their properties shifted in a characteristic way. For example, the nonordered region occurs at higher Mn concentration in the hydrides than in the host metals. Although the structural and magnetic characeristics of the Y6Fe23-,hfnx-hydride system have been studied to some extent, there are no data in the literature regarding those properties of the Y6Fez3-,Mnx ternary system that are pertinent to hydrogen storage. Specifically, there is a lack of information with regard to the thermodynamic and kinetic features of hydride formalion for these materials, which is a significant deficiency. To provide needed thermodynamic information, pressurecomposition isotherms were obtained over the entire composiltion range. These data provide information pertaining to hydride stability and to phase relationships in these quaternary systems. Studies were also undertaken to ascertain in a qualitative fashion if there are significant differences in hydrogen sorption kinetic behavior between Fe-rich and Mn-rich compounds of the YsFe23,Mnx system. Additionally, surface analysis of Y6FeZ3and Y6MnZ3was performed by using Auger spectroscopy to ascerl ain if there are differences between surface and bulk compositions for these systems and, if so, to correlate these differences with their kinetic behavior. Experimental Section The Y6Fez3-,MnX ( x = 0, 1, 3, 5, 14, 18, 23) alloys were prepared by melting together 99.9% pure metals in a 540-kHz induction-heated, water-cooled, copper boat furnace under a Ti-gettered flowing Ar atmosphere. The alloys were melted 5 times to ensure homogeneity. Compositions were determined by synthesis. Mn loss was taken care of by adding an appropriate excess amount of Mn. The Mn content of the alloy was controlled to better than 0.5%. The alloys were annealed at 900 OC for approximately 6 days, except for Y6Mnzj, which was annealed for 1 h in the induction furnace. Sample quality and latlice constants were established from X-ray (Cu Ka) powder diffraction patterns obtained by using a Diano XRD-700 powder diffractometer equipped with a graphite beam monochromator. The samples were all single phase following annealing to the limil of detection by X-ray diffraction. To obtain X-ray diffraction patterns on the hydrides, the samples were hydrogenated at PHI = 55 atm and SOz was introduced to poison the surface of the sample according to the method of Gualtieri et al.I3 The pressure-composition isotherms were obtained by using 2.5-3.5 g of virgin sample at temperatures of 0 and 22 O C (f0.5 "C) and employing rather standard gasometric techniques. A (10) J. V. Florio, R. E. Rundle, and A. I. Snow, Acta Crystallogr., 5, 449 (1952). (1 1) H. R. Kirchmayr, "Habilitations Schrift", Technische Hochschule, Vienna, Austria, 1968. (12) H. Oesterreicher and H. Bittner, Phys. Status Solidi A , 41, KlOl (1977). (13) D. M. Gualtieri, K. S.V. L. Narasimhan, and T. Takeshita, J . Appl. Phys., 47, 3432 (1976).
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3221 TABLE I: Crystal-Structure Dataa for Y,Fe,,-,Mn,
Y,Fe,,-,Mn,
a, A
compd
I
Y, FeiMn,,
-_
Y,Fe,Mn,,H,, Y,Fe,Mn,, Y6Fe,Mn,,H,,
6
and
Hydrides
12.084 12.573 12.094 12.584 12.110 12.723 12.138 12.788 12.265 12.814 12.331 12.855 12.45 8 12.880
v,A 3
AV/V
rb
1764.5 1987.5 1768.9 1992.8 1776.0 2059.5 1788.3 2091.3
0.126
0.94
0.127
0.94
0.160
0.95
0.169
0.97
1845'0
0.140
1.08
2104.0
1875'0 0.133 1.10 2124.3 Y,Mn,, 1933S 0.105 1.15 ' Mn2$ H26 2136.7 a All systems occurred in the Th, Mn,, , as evidenced by X-ray diffraction measurements o n powders. b The r factor is the hydrogen concentration per unit volume relative to liquid hydrogen. The particle density of liquid hydrogen at 20 K is 4.2 x lozz atoms/cm3.
stainless steel system with Bourdon tube type gauges was used for the equilibrium points above atmospheric pressure. To obtain each subsequent lower equilibrium pressure point, an increment of 100 mL or less of hydrogen gas was removed by displacing water in a gas buret. For the equilibrium points below atmospheric pressure, increments of hydrogen were removed by using a glass vacuum system equipped with a Toepler pump. Time periods of 30 min or more were required in order to establish equilibrium at each pressure. The Beattie-Bridgeman equation of state for hydrogen was employed in the calculation of equilibrium points. The kinetics experiments involved a stainless steel gas-handling vacuum apparatus. Two precision pressure gauges were used to record the hydrogen equilibrium pressure covering the range of approximately 1-50 atm. The temperature of the sample zone was controlled to 23 f 1 OC. Approximately 2.0-2.5 g of sample was used in the form of coarse granules. The kinetics measurements were determined by exposing the samples to hydrogen in the gas-handling apparatus and observing the rapidity of pressure drop. This was the technique used to determine the rate of absorption of hydrogen. Desorption was determined by observing the pressure buildup in the closed system as the sample was permitted to desorb hydrogen. Surface studies of Y6Fez3and Y6Mn23were accomplished by using a Varian Auger spectrometer equipped with depth profiling capabilities. Quantitative surface analysis of the alloys was performed by the determination of relative surface quantities of the individual elements. The sensitivity factors for calculating absolute Mn/Y and Fe/Y ratios were obtained from the bulk Mn/Y and Fe/Y signals, assuming that Mn/Y = Fe/Y = 2316 = 3.83. In both samples the oxygen content was also monitored, by using the KLzLz line.
Results It is well-known that all parent compounds of the Y6Fez3,Me, system exist in the Th6Mnz3structure. The compounds prepared in this study were confirmed as having this structure. The results also confirm the negative deviation from Vegard's law in the lattice parameters, as found p r e v i ~ u s l y . ~ ~ It' ~was also observed that all of the hydrides occurred in the Th6Mnz3structure. Crystalstructure information for the host metals and their hydrides is given in Table I. There is the usual increase in unit-cell size accompanying hydrogenation. Included in the table is the volumetric capacity of these hydrides compared to liquid hydrogen, designated as r. These systems have hydrogen density comparable with that of liquid H2. Desorption pressure-composition isotherms obtained for the members of the Y3Fe23-xMnxsystem indicate the absence of flat, (14) H. Oesterreicher, H. F. Bittner, and F. J. Parker, Magn. Lett., 1, 89 ( 1978).
3222 The Journal of Physical Chemistry, Vol. 88, No. 15, 1984
Bayer and Wallace
TABLE II: Enthalpy' and Entropyb of Desorption Data for the Y6Fe2*,Mn,-H System at Various compd x = 12' x = 14c x = 16c Y6Fe23 34.2 (87.1) 34.0 (105.5) Y6Fez2Mn 33.2 (80.2) 35.8 (107.5) Y6Fe20Mn3 61.7 (169.7) 48.6 (149.1) 47.8 (168.7) Y6Fe18Mn5 13.6 (15.0) 13.5 (51.1) Y6Fe9Mn14 11.3 (-3.3) 16.8 (40.0) 24.8 (84.3) Y6Fe5Mn18 5.4 (-25.7) 8.3 (1.4) Y6Mn23 61.3 (172.9)
'2&
Hydrogen/Alloy Ratios x = 18C x = 20c 34.1 (133.4) 39.6 (140.7) 40.6 (160.5) 13.6 (67.3) 20.3 (79.1) 16.0 (75.7) 15.7 (39.4) 13.2 (41.6) 54.2 (157.4) 48.2(151.2)
- HH;(g) values in kJ/mol of H2. b2SH- &,O(g) values (in parentheses) in J/(Kmol of H2).
c~
IO
0 moles H /mole
Figure 1. Desorption isotherms for the Y6Fe23-H (X, 22 OC;0,0 OC) and Y6Mn23-H (A,22 O C ; 0,0 "c) systems.
toi
Mn
OIIOY
alloy.
r
compd Y6Fe23
Y6Fe2&fn Y6Fe20Mn3 Y6Fel8MnS
IO0:
VI
a
20 unit
TABLE I11 Initial Induction Periods for Absorption and Time Required for 50% Desorption init absn desorpn indn perd,' time,b
E
:
atoms /formulo
Figure 3. H, pressure as a function of Mn content at three hydride compositions; 14, 16,and 18 g-atom of hydrogen per formula unit of
c
E
= mol of H/mol of alloy.
Y6Mn23
IO-'
min
S
8 4 3 4 0
31 36 23 23 21
'Initial absorption induction period. bTime required for 50% desorption.
N
I
motes H /mole OIIOY
Figure 2. Desorption isotherms for the Y6Fe20Mn3-H(X, 22 OC;0,0 "C)and Y6Fe9Mn14-H(X, 22 OC;0,0 "c).
broad plateaus over the covered range of equilibrium hydrogen pressures (30-10-3 atm). There is some evidence for the occurrence of narrow, sloped "plateau" in this ternary system. It would be very difficult to determine a specific dissociation pressure for these compounds at the temperatures studied. Isotherm data for the Y6FeZ3-H and Y6Mn23-H systems are shown in Figure 1. Data for the Y6Fe2,Mn3-H and Y6Fe9Mn14--Hsystems are shown These data are representative of the entire in Figure 2. Y,Fe2,,Mn,-H system. The data obtained for the Y6Fe23-H and Y6Mn23-H systems are similar to those obtained by Smith.'5J6 (15) H. K.Smith, Ph.D. Dissertation, University of Pittsburgh, Pittsburgh, PA, 1983,p 40. (16)E. B. Boltich, F. Pourarian, W. E. Wallace, H. K. Smith, and S. K. Malik, Solid State Commun., 23 599 (1977).
Figure 3 indicates the hydrogen equilibrium pressure at three fixed hydride compositions (H = 14, 16, 18) for the entire range of Y6Fepa-,Mn, compounds studied. It is apparent that the equilibrium pressure at all three hydride compositions increases as x is increased from 0 (Y6Fe2,) to 5 (Y6Fe18Mn5).As x is further increased from 5 (Y6Fel,Mn5) to 23 (Y6MnZ3),a decrease in equilibrium pressure is observed. One deviation from this trend occurs at x = 1 (y6Fe22M1-1). The relative partial molar enthalpies 2HH - ",O(g) and the relative partial molar entropies 23, - SH,'(g) of desorption were computed from the pressure-composition data obtained at 0 and and 22 'C, utilizing the van't Hoff expression for 2RH - ",'(g), 2SH - S,,O(g) from AG = -RT In PH2and AS = AH/T - AG/T. These thermodynamic values as a function of hydrogen content are given in Table 11. Interesting absorption behavior was found for the Y&,3,Mn, system. The induction time for the initial hydrogen loading is dependent upon the Mn content of the sample. Increasing the Mn content shortens the induction time of the initial loading into the virgin alloy, as seen in Table 111. After initial loading and degassing, hydrogen absorption is extremely rapid for all compounds, with 50% saturation occurring within 4.5-5.0 s at 25 OC for all compounds studied. Figure 4 illustrates this rapid absorption behavior.
YsFez3,Mn,
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3223
Alloys and Their Hydrides
101
t
x/
-
--y--
X',C/Y
I
I
I
I
I
,
I
I
,
I
LL \
t
I
I
Y6Mn23
Time Sputtered ( s e c )
Figure 6. T/R ratio (T = Fe or Mn; R = Y) during depth profiling of 20
'0
40
60
Y6Mnz3and Y6Fe2,.
80
Absorption T i m e ( s e c )
Figure 4. Hydrogen absorption by Y6Fez3-,Mn, at 23 'C. 2.01
'
'
I
I
'
I
'
'
I
I
01 0
'
100
'
2 0I 0
'
300
T i m e Sputtered
0
IO0
200
300
400
500
Desorption T i m e ( s e c )
Figure 5. Hydrogen desorption by Y6Fez3-,Mn, at 23
"c.
Hydrogen desorption is also rapid for these compounds. However, it occurs to only a small extent (-5%) at 23 OC and atmospheric pressure. Figure 5 indicates this behavior. In the case of desorption, an increased Mn content in the compound favors more rapid desorption. This is illustrated in Table I11 as the time required for 50%release of removable hydrogen. It should also be noted that degassing of the samples at 200 "C under vacuum (
