3112
J. Phys. Chern. 1081, 85, 3112-3116
Magnetic, Crystallographic, and Hydrogen-Storage Characteristics of Zr,,TI,Mn, Hydrldes H. FuJII,~ F. Pourarlan, V. K. Slnha, and W. E. Wallace" Lbpaflmnt of Chemistry, Unlversby of Phtsburgh, Pittsburgh, Pennsylvanle 15260 (Recelvd: April 15, 198 1)
The (Zr,Ti)Mn2ternary alloys hydride to H/Zrl,Ti,Mnz compositions ranging from 3.6 for x = 0 to 1.6 for x = 0.5. Pressure-composition isotherms are presented for several temperatures. The shape of the isotherms suggests that a second phase, a hydride phase, exists at higher hydrogen concentrations. Hydrogen absorption and desorption occurs very rapidly. The processes are 90% complete in 1 and 5 min for absorption and desorption, respectively. The rapidity of these processes appears to be a consequence of only a thin layer of oxidation. The nonhydrides of all of the Zrl,Ti,Mnz systems show weak Pauli paramagnetism. On the other hand, the hydrides of ZrMnz and Zrl,Ti,Mnz with x = 0.3 and 0.4 exhibit ferromagnetismwith Curie temperature ranging from 143 to 185 K and with moments ranging from 0.04 to 0.42 pB/(formulaunit). The hydride of Zro.sTi,,lMn2 exhibits only enhanced paramagnetism. The Zro.sTi,-,zMnz hydride shows spin-glass behavior as a result of the occurrence of ferromagnetic clusters in a paramagnetic matrix. These complex and varied magnetic behaviors are attributed to the effects of (1) variations in the Mn-Mn spacing (and/or the electron concentrations in the 3d bands) and (2) varying local hydrogen concentrations occurring as a result of statistical fluctuations.
Introduction Recent studies in this laboratory have shown that many intermetallic compounds containing Mn as one constituent interact with hydrogen at normal conditions of temperature and pressure to produce very hydrogen-rich intermetallic hydrides. Mn-containing systems are in a sense more interesting than the corresponding Fe, Co, or Ni systems, which have been more extensively studied, because of the rapidity with which they form hydrides. ZrMn2 and TiMn2 are hexagonal Laves phase (C14 structure) systems. The former material absorbs substantial quantities of hydrogen1 but it is held too tightly for this material to be of practical significance as a hydrogen-storage material. However, systems based on ZrMn2-specially ZrMn2+,-have features which make them of potential interest for hydrogen storage?$ TiMn2 does not accept measurable quantitities of hydrogen at pressures up to 70 atm, presumably because the lattice, which is contracted compared to ZrMn2, contains interstitial sites so small that their occupancy by hydrogen is energetically unfavored. These features suggested that Zr1-,TirMn2 ternaries, which are known to occur for all values of x , might have characteristics making them of interest as a practical means of hydrogen storage. The initial intent in the present work was to examine the influence of hydrogenation on the magnetic behavior of the Zrl,Ti,Mn2. Profound changes in electronic behavior had earlier been noted for other systemsconversion of a Pauli paramagnet into a ferrimagnet (Th6Mnz3): transformation of a superconductor into a ferromagnet (ThlFe3),6etc.-and it was of interest to ascertain whether similar striking effects would occur during hydrogenation of the (Zr, Ti)Mnzternaries. If so, inferences regarding electronic band structure might be possible which would be useful in understanding how the band structure affects the ability of a system to accommodate hydrogen in the lattice. During the course of the magnetic studies, it was necessary to acquire some pressure-composition data. The pressure-composition characteristics of these systems had been studied earlier by Oesterreicher and BittnerS6 However, the pressure-composition data 'On leave from the Faculty of Integrated Arts and Sciences, Hiroshima University, Japan. 0022-3654/81/2085-3112$01.25/0
obtained in the present study differed significantly from those obtained earlier, and so these new thermodynamic data are presented along with the magnetic data acquired for the host metals and their hydrides.
Experimental Section The samples were prepared by techniques which are now standard in this laboratory. The component metals were melted together several times under a stream of purified argon in a copper cold boat. The melting was accomplished by inductive heating. The samples were homogenized for a 2-h period in the cold boat at temperatures near the melting point. X-ray diffraction analysis using copper radiation with a single-crystal monochrometer revealed single-phasematerials for all of the samples studied. The method of preparation, arranged to give well-homogenized materials, is crucial for determination of the pressure-composition isotherms. Poorly homogenized materials gave poorly shaped isotherms with no defined plateau, resembling those obtained by Oesterreicher and Bittnera6 The kinetics measurements were determined by exposing the samples to hydrogen in a closed container and observing the rapidity with which pressure fell. 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. Pressure-composition isotherms were determined by normal gasometric techniques. Pressure readings were noted when equilibrium was reached and the pressure reading was constant for at least 0.5-1.0 h. In some experiments it was ascertained that there was constancy of pressure for as long as 15 h. Before measurements of magnetization and X-ray diffraction on the hydrides, the sample was hydrogenated to (1)D. Shaltiel, I. Jacob, and D. Davidov, J.Less-Common Met., 53, 117 (1977). ( 2 ) R. M. Van &sen and K. H. J. Buschow, Mater. Res. Bull., 15,1149 (1980). (3)F. Pourarian, H.Fujii, W. E. Wallace, V. K. Sinha, and H. Kevin Smith, J. Phys. Chem., preceding paper in this issue. (4)S. K. Malik, T. Takeshita,and W. E. Wallace, Solid State Commun., 23, 599 (1977). (6) S. K. Malik, W. E. Wallace, and T. Takeehita, Solid State Commun., 28, 359 (1978). (6) H. Oesterreicher and H. Bittner, Mater. Res. Bull., 13,83(1978).
0 1981 American Chemical Society
The Journal of Physlcal Chemlstty, Vol. 85, No. 2 1, 198 1 3 113
Characteristics of Zrl-xTixMnz Hydrides
1
16‘0 0
g
3
2
I
g atom H / m o l e Zro,Ti,,Mn, - .--
3
2
otom H /mole 21, ,TiO ,Mn2
I
TABLE I: Crystal Structure of Zr,, and Their Hydrides compd IO
E
c
0 1
I
0
I
1
I
2 3 Zro,Tio,Mn2
g atom H / m o l e
4
Figure 2. Pressure-composition isotherms for hydride of Zr,,Ti0~,Mn, that was not well-homogenized.
_. ._ I
I
I
I
Figure 1. (a) Pressure-composition isotherms for hydride of wellhomogenized Zro.7Tio,,Mnz. (b) Pressure-composltion isotherms for hydride of well-homogenized Zro,6Tio,,Mn,.
a known composition, hydrogen uptake being measured gasometrically, and quenched in liquid nitrogen, and the remanant gaseous hydrogen was rapidly pumped away. Pressure of 35 psi of SO2was admitted, according to the technique of Gualtieri et a1.: to poison the surface and contain the hydrogen. The sample was then allowed to warm to room temperature. Magnetization measurements were made by using a Faraday method in applied fields up to 21 kOe.
Results Pressure-Composition Isotherms and Kinetic Behavior. Pressure-composition data are shown in Figure 1,A and B, for well-homogenized Zr,,Ti,Mn2 systems with x = 0.3 and 0.4 and for a poorly homogenized host material in Figure 2. These plots show that the capacity of these systems is around 3 g-atom of hydrogen per formula unit of alloy. Capacities measured for systems with x = 0, 0.1, (7) D. M. Gualtieri, K. S. V. L. Narasimhan, and T. Takeshita, J . Appl. Phys., 47,3432 (1976).
a, A
5.026 5.480 5.0259 5.420 5.012 5.410 4.984 5.390 4.970 5.382 4.940 5.330 A V/V is the change of
c, A
8.258 8.931 8.252 8.834 8.233 8.855 8.192 8.810 8.168 8.755 8.086 8.744
TixMn2 Ternaries
V,A3 %AV/P 180.65 28.6 232.3 180.5 24.2 224.7 179.1 25.3 226.44 176.2 25.5 223.86 174.7 25.7 219.6 170.88 25.9 215.1
r
1.47 1.43 1.34 1.27 1.21 0.71
volume per unit cell.
0.2, and 0.5, together with crystallographic data obtained for ZrMnz and its hydride and the several ternaries and their hydrides, are listed in Table I. It is to be noted that there is an expansion of 25-30% when the host metals are hydrogenated. The r factors, the hydrogen density relative to that of liquid hydrogen, range from 0.7 to 1.47, showing these materials to have excellent capacity for hydrogen. The rate at which hydrogen absorbs or desorbs is shown in Figure 3. The processes are essentially complete in a time interval of 100 s. For absorption, the process is 2/3 complete in the very short interval of 5 s. It seems likely that the oxide coating, which normally inhibits hydrogen uptake, is very thin in these systems as it is in other Mncontaining intermetallics which have been studied in this lab~ratory.~,~ Magnetic Behavior. Magnetization vs. field strength measurements were made at 4.2 K, and the dependence of magnetization on temperature was established over the temperature range extending from 4 to 300 K. Results of the measurements are essentially summarized in Figures 4-8 and in Table 11. All of the host metals exhibit paramagnetism. x , the susceptibility,shows only a weak temperature dependence except at temperatures below 50 K. In that region x increases significantly with decreasing temperature. How(8) W. E. Wallace, A. Elattar, H. Imamura, R. 5. Craig, and A. G. Moldovan in “The Science and Technologyof Rare Earth Materials”,W. E. Wallace and E. C. Subbarao,Eds., Academic Press, New York, 1980, p 329.
Fujii et al.
The Journal of Physical Chemistry, Vol. 85, No. 21, 1981
3114
c
IO0
0 .-c
~ ~ 0 . 4
Q
$
60
a,
V
-
0.3
20
N
5
-2
+k
0
N
-00
0
E
\
I
5
c
0.5
0
o,
0;
2b
4b
610
sb
Id0
0
Time ( s e c ) Flgure 3. Desorption of hydrogen from the hydride of Zro,,Tio,3Mn2 at 100 OC (upper diagram) and absorption of hydrogen by Zro,,Tio,3Mn2 at 50 OC (lower diagram).
0
IO
5
H
15
20
(kOe)
Flgure 4. Magnetization vs. field at 4.2 K for ZrMn2 and the hydrides of the ternaries Zrl-,TI,Mnz, with x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5.
TABLE 11: Magnetic Data of Zr,-,Ti,Mn2 Ternaries and Their Hydrides
I
I
I
I
magnetization
at 21 koe og,
emu/ g
0.86
0.04
1.03
0.04
3.21
0.12
10.6 12.3 3.00 a
Ms,C~B/ (formula unit)
0.35 0.42 0.10
x at 298 K, emu/ (formula unit)
1.60 x 10-3 4.2 x 10-3 1.44 x 10-3 4.6 x 10-3 1.51 x 10-3 4.4 x 10-3 1.54x 10-3 5.1 x 10-3 1.64x 10-3 5.6 x 10-3 2.05 x 10-3 5.5 x 10-3
T,., K PPa
143
PPa -0 PPa'
136
PP4
162
PPa
Flgure 5. TMA's for ZrMn2 and Zr,,TiO,,Mnz and their hydrides. The arrow below T, indicates the Curie temperature for Z T M ~ ~ H ~ . ~ .
185 I
I
PPa
212
Pauli paramagnets.
ever, the susceptibility does not show a Curie-Weiss dependence, and hence the low-temperature behavior is thought to be an impurity effect. The hydrides exhibit five types of magnetization-temperature behavior. These are summarized in Table 111.
Discussion of Results Magnetic Data. Before we attempt to discuss the results summarized in Table I11 and Figures 4-8, it is well to review what has been learned about Mn-Mn coupling in earlier studies of pertinent related Mn systems. In extensive studies of alloys of Mn with other 3d transition metals, it has been observeds that there is a critical MnMn distance of -2.8 A. When the Mn-Mn distance, d, is less than 2.8, the Mn d electrons are delocalized and
-
0
(ZrogTiO2 ) Mnz 21 I hoe I
Od
I
I
-
I
200
IO0
T (K) Flgure 6, TMA's for Zro,8Tio,zMn2 and Zr0,8Ti0,zMnzH3,2The behavior of the hydride at low temperatures was dependent upon its historydependlng upon whether it was cooled in a field (dashed Ilne) or not (full lines). Measurements were made in applied fields of 4.4 and 21.1 kOe. The fieid was 21.1 kOe in the field-cooled sample.
~~
(9) See, for example, R. S. Tebble and D. 3. Craik, "Magnetic Material", Wiley-Interscience,New York, 1969, p 61.
coupling is antiferromagnetic. For interatomic distances exceeding 2.8 A, Mn develops a local moment and the
Charwterlstlcs of Zr,-,Ti,Mn,
Hydrides
The Journal of Physical Chemistty, Vol. 85, No. 2 1, 198 1 3115
TAB:LE 111: Types of Magnetization-Temperature Behavior
-type 1
system
most conspicuous feature(s)
M vs. T indicates magnetic ordering,
ZrMn2H3.6
2 3
small magnetic moment
M linear with H
zr0.9v0.1~2H3.4
history-dependent M vs. T behavior
zr0.8n0.2Mn2H3.2
4
zr0.7v0.3Mn2H3.0 and Zr0.6~0.4Mn2H2.8
5
zro.8~0.5Mn2H1.6
normal M vs. T behavior for magnetically ordered state atypical M vs. T relationship
T (K)
Figure 7. TMA for Zr0.BTi0.4Mn2 and its hydrlde. The hydrlde exhibits ferromagnetism, whereas the host metal behaves as a Pauli paramagneit. I
I
‘
I
I
\\ I
I
I
200
100
T(K) Figure 8. TMA for Zro,,Tio~,Mn2and its hydride. This material seems to couple antiferromagnetlcally.I
coupling is ferromagnetic. Stearns’O has advanced the thesis that the Mn-Mn coupling in elemental Mn (where d < 2.8) is via the RKKY interaction, Le., indirect via the delocadizated electrons. The concentration of delocalized electrons is high and because of this she postulates that the coupling is antiferromagnetic. On this basis it follows that, if the Mn-Mn nearest-neighbor distance is increased beyond 2.8 and/or if the concentration of delocalized electrons is decreased, ferromagnetic coupling will ensue. For simplicity we term these two effects the “distance increase” and “ec decrease” effects. Barmowski“ has studied the hydrogenation of elemental Mn. IJnder several kilobars of pressure, Mn is found to hydrogenate to MnHo,,. This hydride is found to be ferromagnetic with T,= 345 K and psat= 0.035 pa/atom. (IO) IM.B. Stearns, Phys. Today, 34 (Apr, 1978). (11)13. Baranowski, 2.Phys. Chern. (Frankfurt urn Main), 114,59 (1979).
character of magnetism weak ferromagnetism, or ferrimagnetism at low temperatures paramagnetic at 4.2 K low temperature ordering, spin-glass state ferromagnetic antiferromagnetic (?)
Since hydrogenation invariably enlarges the lattice, it appears that the ferromagnetism for MnHo.83is at least partly a consequence of the distance effect. Studies of Y6Mna and Th,@na and their hydrides have led Wallace to conclude12that in these hydrides H acts as an electron acceptor, a point of view also held by Buschow.13 If this also occurs in MnHo.s3,as seems likely, then there is also an ec decrease, which also strengthens ferromagnetic exchange a t the expense of antiferromagnetic exchange. Buschow has studied13the magnetism of the hydrides of the cubic Laves phase compound YMn2. The host intermetallic is a Pauli paramagnet. In studies of the YMn2 hydrides it is noted that, as the hydrogen content is progressively increased, the material transform from a Pauli paramagnet into a ferromagnet (-0.4 pB/(formula unit) and then back to a Pauli paramagnet. This peculiar behavior has ita rough, and more complex, counterpart in the systems examined in the present study in that there is oscillation between antiferromagnetic and ferromagnetic behavior as the Zr content and the Zr/Ti ratio in the system are varied. Consider the YMn2hydrides studied by Buschow. As YMn2 is hydrided, the two effects described above are brought into play. Lattice expansion favors ferromagnetism. So does the ec decrease, at least for a time, and for the reasons cited above. However, in time the concentration of the itinerant d electrons has been so depleted and the distances have been so enlarged that coupling is too weak to sustain ferromagnetism. Thus, the material reverts, to paramagnetic behavior at the highest hydrogen concentrations. Somewhat analogous behavior is observed for the ZrI-,TixMn2 hydrides. For x = 0.5 (Figure 8) the interactions are such as to generate an essentially antiferromagnetic system. With increasing Zr content the material becomes a rather well-behaved ferromagnet (Figure 7). This is in part a distance effect; the lattice is enlarged as Zr replaces Ti and the Mn-Mn distances are increased. There is, however, an added complication. Because of the rising stability of the hydride at higher Zr contents, the hydrogen concentration increases. Thus, there is also an ec effect. Together these bring on ferromagnetism. At higher Zr concentrations (Figures 5 and 6) the system develops first spin-glass and then paramagnetic behavior. For zro.6Tj,,2hin2H, the following seems likely the system consists of ferromagnetic clusters embedded in a paramagnetic matrix. The system is essentially paramagnetic, and, were the hydrogen distributed uniformly throughout the lattice, it would exhibit only paramagnetism. However, because of statistical fluctuations the hydrogen concentration in some regions is low and the Mn atoms form (12)W. E. Wallace, Z . Phys. Chern. (Frankfurt am Main), 116, 219 (1979). (13)K.H. J. Buschow In “Hydrides for Energy Storage”, A. F. Andresen and A. J. Maeland, Eds., Pergamon Press, Oxford, 1978, p 273.
3116
The Journal of Physical Chemistry, Vol. 85, No. 21, 1981
ferromagnetic clusters. Thermal activation of these clusters leads to the positive temperature coefficient of magnetizaiton for T < 100 K. The necessity for thermal activation is a consequence of the energy barrier associated with the interaction of the clusters with the moments arising from the highly polarizable matrix. In Zro,eTio.lMn2H3.4the higher hydrogen concentration and/or the enlarged lattice inhibits the statistical fluctuations of the interstitial hydrogen and suppresses the formation of clusters coupled ferromagnetically. This material then exhibits only paramagnetism. The magnetism-temperature behavior of ZrMn2H3.6 (Figure 5 ) indicates that this material is ferromagnetic,14 which superficially is puzzling in the light of the trends described in the preceding paragraphs. It seems likely that the contrasting behavior of the hydrides of the three hosta with x = 0, 0.1, and 0.2 is a consequence of varying interstitial site occupancy by hydrogen. In ZrMn2 ( x = 0) sites in a given family, sites are all equivalent energetically and will be occupied statistically. This is not so for Zro.gTb,lMnzand Zro.sT&.2Mnz.In a given family of sites, those with a Ti near neighbor will be energetically unfavorable because of the diminished size of the interstitial site. Hydrogen will hence cluster in those sites not having Ti as a near neighbor. This clustering tendency will obviously diminish in the sequence x = 0.2,0.1,and 0. (This will be another factor in paramagnetism of Zro.gTi,-,lMn2H3.r.) As hydrogen “spreads out” and becomes uniformly distributed among the accessible interstitial sites on passing from x = 0.1 to 0, the interaction evidently again becomes favorable for the development of ferromagnetic order at low temperatures. However, in this case T,is low and the saturation moment is s m d , -0.02 pB/(Mn atom). It is of interest to note that this is of the same order as the Mn moment observedll in MnHo.@,Le., 0.035pB/(Mn atom). The complexity of the magnetic results obtained for these hydrides grows out of the variation of both the Mn-Mn separation and the hydrogen content, both of (14) See also I. Jacob, D. Davidov, and D. Shaltiel, J . Magn. Magn. Mater., 20, 226 (1980). They also observe ferromagnetism for this hydride, with a Curie temepature of 145 K.
Fujll et ai.
which significantly influence exchange. Hydrogenation Thermodynamics and Kinetics. The well-defined plateau regions seen in Figure 1,A and B, differ from the data previously obtained6 for (Zr,Ti)Mn2 hydrides. In preliminary work in the present investigation, pressure-compositionisotherms were obtained which gave no plateau region (Figure 2). The behavior exhibited in Figure 2, which contrasts with the requirements of the phase rule, is characteristic of a poorly homogenized material. AH and AS for hydrogen release were found to be 33 kJ/(mol of Hz) and 90 J/(K.mol of Hz). These quantities are larger than the values obtained for ZrMn2+,hydrides recently in this lab~ratory.~ This is expected since the particular (Zr,Ti)Mn2hydrides studied lacked the specific magnetic features which led to the small AH and AS values for hydrides of nonstoichiometric ZrMnz. The present AH and A S values are close to those obtained for the paradigm hydrogen-storage material LaNi5.1S The rate of uptake and release of hydrogen by the (Zr,Ti)Mnz alloys is phenomenally rapid. This is in keeping with the behavior of other Mn-containing intermetallics recently studied in this laborat~ry.~*~ Hydrogen entry or removal is normally inhibitedls by an ever-present oxide layer on these very surface-active materials. This layer, which is -200 A thick on LaNi5,17makes this intermetallic relatively sluggish for hydriding or dehydriding; hydrogen atoms must traverse -200 A of oxide in these processes. The oxide films growing on Mn intermetallics are consistently observed to be -50 8,thick. This is found to be true for (Zr,Ti)Mn2intermetallics, which accounts for their rapidity in absorbing and releasing hydrogen. Acknowledgment. The magnetic studies were supported by a contract with the Army Research Office. Studies of hydrogen absorptivity were carried out under a contract with the Koppers Co., Pittsburgh, PA. (15) W. E.Wallace, R. S. Craig, and V. U. S. b o , Adu. Chem. Ser., No. 106,207 (1980). (16) W. E. Wallace, R. F. Karlicek, Jr., and H.Imamura, J. Phys.
Chem., 83, 2009 (1979). (17) H.C. Siegmann, L. Schlapbach, and C. R. Brundle, Phys. Reu. Lett., 40,1972 (1978).
