Magnetic Characteristics of Gadolinium, Terbium, and Ytterbium

11 Magnetic Characteristics of Gadolinium, Terbium, and Ytterbium Hydrides in Relation to the Electronic Nature of the Lanthanide Hydrides Downloaded by CALIFORNIA INST OF TECHNOLOGY on September 22, 2017 | http://pubs.acs.org Publication Date: January 1, 1963 | doi: 10.1021/ba-1964-0039.ch011

W. E. WALLACE, YOSHIO KUBOTA, and R. L. ZANOWICK Department of Chemistry,

University

of Pittsburgh,

Pittsburgh

13, Pa.

Hydrogenation of the heavy lanthanides leads to formation of cubic dihydrides and hexagonal trihydrides, which exist over a considerable composition range. Metallic conduction is exhibited by the dihydrides but not trihydrides, suggesting that the latter lack conduction electrons, a conclusion also indicated by absence of magnetic ordering at low temperatures. Since the atomic moment is essentially unchanged by hydrogenation, the number of core electrons is unchanged in the process. One concludes that the conduction electrons are absorbed by the dissolving hydrogen to form the H- ion and hence the trihydrides are essentially ionic. The dihydrides exhibit metallic conduction and order at low temperatures considerably below the point at which ordering begins in the corresponding metal.

T h e lanthanide metals react readily w i t h hydrogen to form a series of hydrides i n w h i c h the hydrogen-metal atom ratio varies over rather wide limits. These hydrides (of a l l the lanthanides, including L a and Y , except radioactive promethium) have been examined b y conventional x-ray diffraction techniques w i t h the following conclusions (4, 9): A l l of the lanthanides form dihydrides; i n the majority of cases the dihydride w i l l readily absorb extra hydrogen, approaching or 122 Ward; Nonstoichiometric Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1963.

Downloaded by CALIFORNIA INST OF TECHNOLOGY on September 22, 2017 | http://pubs.acs.org Publication Date: January 1, 1963 | doi: 10.1021/ba-1964-0039.ch011

I I . WALLACE E T AL.

Magnetism

in Lanthanide Hydrides

123

reaching a composition corresponding to a trihydride; a n d the metal i o n cores i n the d i - and trihydrides of the light lanthanides are i n a face-centered cubic arrangement, whereas for the heavier lanthanides beginning w i t h S m the metal ion cores in the trihydrides are i n a close-packed hexagonal arrangement. T h e dihydrides of E u and Y b are exceptional i n that they absorb extra hydrogen w i t h reluctance, if at a l l (14), and the metal ion cores are i n an orthorhombic structure ( 5 ) , w h i c h can, however, be regarded as a rather simple distortion of hexagonal close packing. E u and Y b behave unlike the other lanthanide metals i n many ways, because they are divalent whereas the others are trivalent. Experiment shows nonstoichiometry to be the rule, rather than the exception, for the lanthanide hydrides. T h e metalhydrogen atom ratio, M e / H , is found to extend from about 1.8 to 2.5 i n the dihydride phases and from roughly 2.5 to 3.0 for the trihydrides. In recent years a considerable body of information has been accumulating, bearing upon the presumed electronic nature of these hydrides. T h e point of view has been held i n many quarters for a number of years that the bonding i n the lanthanide hydrides is essentially coulombic i n character—that is, these hydrides are saline i n nature, w i t h the hydrogen, of course, present as an anion. Suggestive evidence i n support of this point of view has been provided b y the magnitude of the heats of hydrogénation (2) and the ideal stoichiometry of the hydrides—trihydrides easily form w i t h trivalent metals, whereas normally only dihydrides form with the divalent metals E u and Y b . A d d i t i o n a l and more powerful support for this point of v i e w has been and is currently being provided b y the observed electrical a n d magnetic properties of the lanthanide hydrides. I n this laboratory it has been found that when D y , H o , and Y b are hydrogenated to saturation, their conductivities diminish b y five or more orders of magnitude (10). Similar observations have been made b y W a r f a n d Hardcastle (13) for the P r - H system and by Stalinski for the L a - and C e - H systems. Moreover, Stalinski noted that the resistivities of L a H and C e H decreased w i t h increasing temperature, indicating that the conductivity i n these materials was nonmetallic i n nature (11). The observed conductivity behavior clearly indicates that the fully hydrogenated lanthanide metal is lacking i n conduction band electrons. This leads to the conclusion that the hydride ion is formed b y absorbing electrons from the conduction band and the hydrogen-saturated metal represents the situation i n w h i c h this process has l e d to the complete depopulation of the conduction band. 3

3

Support for the thesis that the hydrogen i n the lanthanide hydrides is anionic and the fully hydrogenated lanthanide metal is devoid of conduction electrons is also provided b y the study of the magnetic properties of the lanthanide hydrides carried on i n this laboratory i n recent years. T h e elemental lanthanides are known to order magnetically at reduced temperatures and it is generally believed the localized 4/ electrons i n adjacent metal ion cores do not overlap appreciably. The interactions w h i c h produce ferromagnetism and antiferromagnetism i n the lanthanides are hence not due to direct exchange. Instead they result from indirect exchange—that is, exchange v i a the conduction band electrons. It follows, therefore, that hydrogenating these metals w i l l have strong implications as regards their tendencies to order magnetically. Hence studies of the magnetic behavior of the lanthanide hydrides, particularly the strength of their tendency to order, provide further evidence i n support of the presumed electronic nature of these hydrides. This provided the original incentive for investigating the magnetic behavior of this group of hydrides. Earlier investigations have dealt w i t h the magnetic characteristics of holmium (6) and europium hydrides (15). T h e present paper is concerned w i t h a similar study of gadolinium, terbium, a n d ytterbium Ward; Nonstoichiometric Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1963.

ADVANCES IN

124

CHEMISTRY SERIES

hydrides and the utilization of the results obtained to document further the thesis set forth above.


Experimental

Details

T h e lanthanide metals were obtained from the Nuclear Corp. of America, Burbank, Calif. T h e y were analyzed spectroscopically by the supplier and found to be 9 9 % pure or better. The hydrogen was obtained from a commercial cylinder and purified by passing through a Deoxo unit and a l i q u i d nitrogen trap and finally by diffusion through a heated palladium tube. In preparing the metal for hydrogénation 0.1- to 0.4-gram samples were cut from the stock supply, the cutting being done under mineral oil. T h e samples were then polished under mineral oil w i t h progressively finer abrasive papers, ending with T y p e 600A. They were then washed successively w i t h C C 1 , acetone, and ether. After weighing, the sample was introduced into the preparation train (vacuum system and appropriate gas metering equipment) and evacuated to h i g h vacuum for at least 4 hours. It was then heated gradually to 500° and hydrogen was admitted. Hydrides w i t h compositions approximating the dihydride formed rapidly i n most cases. F o r higher hydrogen contents the temperature had to be lowered and higher pressures (up to 1 atm.) had to be applied. After the desired composition had been attained, the temperature was reduced and the sample was annealed at 200° to 300° for at least 4 hours. U p o n removal, the hydrides were either placed under oil and kept there to retard oxidation a n d / o r decomposition or were introduced immediately into the equipment used to measure susceptibilities. Susceptibilities were measured from 4 ° K . to room temperature using the Faraday method. A Varian 6-inch magnet provided w i t h a Sucksmith gap (12) was used and the forces were measured w i t h the automatic recording balance w h i c h has been described i n detail ( 1 ). The field gradient was calibrated using N i C l solution and elemental iron. Measurements were always made at several field strengths to test for possible field dependencies of the measured susceptibilities. T h e observed susceptibility can vary w i t h the field strength for either of two reasons: (1) the presence of ferromagnetic impurity w h i c h tends to saturate at higher fields and (2) an intrinsic property of the sample under investigation. T h e latter occurs only under conditions of very high field and very low temperatures ( < 4 ° K . ) or at temperatures close to the Curie or Néel point. 4

2

Results

It has been tacitly assumed to this point that the dissolving hydrogen acquires its supernumerary electron from the conduction band of the metal. There is, of course, another possibility—that some or all of the electrons are taken from the core. If so, the atomic moment w o u l d change. F o r terbium the alteration i n moment is rather large; for a loss of 1 electron from the core its moment would decrease by 1.7 μ (Bohr magnetons). T e r b i u m hence is a favorable case for verifying that the number of core electrons is essentially invariant during the hydrogénation process, and its hydrides were studied rather thoroughly. The gadolinium and ytterbium hydrides were studied somewhat less thoroughly. Β

T e r b i u m H y d r i d e s . T h e measured susceptibilities at 300° K . of eleven terb i u m hydrides at a variety of field strengths are given i n Table I. These values can be compared w i t h the value 8.7 Χ 1 0 e.m.u. per gram obtained for elemental T b . Thus the hydrides are seen to be very strongly paramagnetic materials but not quite so strongly paramagnetic as the parent metal. T h e temperature dependence of the susceptibilities of a number of the samples is represented i n Figures 1, 2, and 3. Over most of the temperature range the susceptibilities of the hydrides are represented b y the Curie-Weiss law, χ = C/(T — Θ ) . This is also true for the parent metal for temperatures above its Curie point. Below a certain temperature, T , however, the χ values show significant departures from the Curie-Weiss law. - 4

d

Ward; Nonstoichiometric Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1963.


I I . WALLACE ET AL.

Magnetism in Lanthanide Hydrides

T[°K]

Figure 1. Inverse susceptibility of Tb, TbH , vs. temperature 195

G./e.m.u. Data for TbHi. for clarity displaced upward by 5 units. temperature, T , for Tb 9S

and

TbH

198

Arrow indicates Curie

c

Values of T , the Weiss constant, Θ, a n d the atomic moment computed from the Curie constant, C, are given i n Table I I . F o r most of the hydrides i n the region of the dihydride the susceptibility rises to maximum 40° and 50° K . (Figures 1, 2, and 3 ) . F o r T b H . 8 there is i n a d d i tion a m i n i m u m χ at about 20° K . T h e maximum implies the onset of some type of antiferromagnetic ordering and the temperature at w h i c h this begins is the Néel point, T . T h e values of T are also given i n Table I I . G d H a n d G d H . T h e susceptibilities of G d H and G d H at 300° K . were found to be 1.56 Χ 1 0 a n d 1.36 X 1 ( H e.m.u. per gram, respectively. T h e tem perature dependence of their susceptibilities is shown i n F i g u r e 4. Here again one d

2

N

0

N

2

3

2

3

- 4


ADVANCES IN CHEMISTRY SERIES


126

5h

SO

Ο

IOO

I50

200

250

300

[τ °K.] Figure 2. Inverse susceptibilities TbH , TbH , 2

Data for TbH .

2 08

Table 1.

01

2

08

vs. temperature TbH

for

210

G./e.m.u. and TbHz.io displaced upward by 2 and 4 units, respectively Susceptibilities of Terbium Hydrides at 300° Κ Ί0*Χ , e.m.u./g. α

Field otrengin, Koe. 8.5 10.5 13.4 15.4 16.8 18.2 18.9 19.6 20.0

TbH\.v

b

2.19 2.20 2.18 2.20 2.18 2.17 2.19 2.20 2.19

TbHx. )8 2.16 2.15 2.16 2.15 2.14 2.17 2.18 2.19 2.18

TbHï.oi 2.18 2.17 2.17 2.19 2.18 2.18 2.19 2.20 2.20

TbH . 2i

TbHz.w TbU2.% g TbU2.2 2 TbH

2.05 2.06 2.07 2.08 2.06 2.05 2.08 2.09 2.09

2.17 2.16 2.15 2.18 2.15 2.17 2.18 2.19 2.19

2

2.20 2.18 2.19 2.20 2.21 2.19 2.21 2.21 2.21

2

2.18 2.16 2.17 2.19 2.15 2.16 2.18 2.19 2.19

2.17 2.11 2.16 2.10 2.14 2.15 2.18 2.19 2.19

2.20 2.18 2.19 2.17 2.18 2.20 2.21 2.22 2.22


g 5 TbHi 2 2 2 2 2 2 2 2 2

17 16 16 19 16 16 18 19 19

II.

W A L L A C E

Magnetism

ET A L .

in Lanthanide

127

Hydrides

notes a w i d e range of temperature over w h i c h Curie-Weiss behavior is obeyed a n d a Néel point for the dihydride. Values for the atomic moment, T , T , and Θ, for these materials are also listed i n Table I I . Y b and Y b H . Information on these two materials was only semiquantitative, since the equipment employed was designed for use w i t h either strongly paramag netic or ferromagnetic materials. L o c k showed that elemental ytterbium is weakly paramagnetic and its susceptibility is nearly independent of temperature (7). R e sults obtained for Y b i n this study were similar. Measurements showed that Y b H was even more weakly paramagnetic than Y b a n d m a y even be diamagnetic. χ was of the order of 10— e.m.u. per gram for both Y b and Y b H . N

d

2

2

6


2

τ [·κ] ' Figure 3.

Inverse susceptibilities vs. temperature for TbH TbHt.B*, TbHt.9 , and TbH*.v

2

ss

S

Data for TbHTbHt.Bs, 6 units respectively

G./e.m.u. and TbHz.v displaced

upward

by 2, 4, and



128 Table II.

Results of Magnetic Studies of Terbium and Gadolinium Hydrides T

Sample

N

°

7V ° 240 40 38 70 75 55

°K.

K.

θ,°

Meff, Bohr Magnetons

°K.

9.85 9.8 TbHi.ge 40 9.7 46 9.8 TbH2.oi 50 9.4 TbH2-o8 40 9.4 TbH2-io b b N.O. 9.3 TbH2.2i b b N.O. 9.4 TbH2.4o N.O. 8 -2 9.5 TbH -88 N.O. 8 -2 9.5 TbH2.92 N.O. 8 -5 9.7 TbH .95 N.O. TbH «97 8 -7 9.7 GdH 21 55 +3 7.7 N.O. GdH 9 -3 7.3 TN. Néel temperature ( N.O. means not observed ). T . Temperature below which departures from Curie-Weiss law become noticeable. Θ. Weiss constant. μ ττ. Effec tive moment. Below about 160° K. linearity of l/μ vs. Τ is only approximate for these two samples. The data between 75° and 160° K . show small but significant systematic negative deviations from the straight line which represents the results well above 160° and between 40° and 75° K . The cause of these deviations is unknown but TbH .« is known to be a two-phase mixture of the d i - and trihydrides. TbH .2i is close to the phase boundary and it, too, may be two-phase ( see 9 ). The anomalous magnetic behavior may in these cases be due to the fact that they are mixtures. 235

Tb

?

TbHi.95

230 -11 -13 -6 +7 -7


2

2

2

2

3

A

D

β

b

2

2

Discussion. T h e exchange interactions between the h i g h l y localized 4 / elec trons i n the lanthanide metals take place v i a the delocalized electrons i n the con duction b a n d (3). Hence, i n the fully hydrogenated metal, i f the conduction elec trons are absorbed the tendency for magnetic alignment is expected to b e greatly suppressed as compared to the parent metal or perhaps removed entirely. Earlier work gave no indication of ordering i n fully hydrogenated holmium (6)—i.e., H0H3—and a similar situation is indicated i n the present work for G d H and T b H . Stalinski (11) studied the magnetic properties of C e H , and found that it remains paramagnetic to the lowest temperatures studied. However, his measure ments extended to only 80° K . If magnetic ordering occurs at all, i t is expected to develop at temperatures considerably lower than this. Hence, Stalinski's results are not conclusive. It seems likely that i f ordering at lower temperatures were sought, results w o u l d be negative. Since the packing of the metal ion cores i n the element and i n the trihydride is essentially the same ( 9 ) , the absence of magnetic ordering i n the latter strongly suggests that they lack conduction electrons. Furthermore, the lanthanide moment is nearly the same i n the hydride a n d i n the corresponding element [(10) a n d Table I I ] , indicating that the conduction elec trons have not been absorbed into the lanthanide core. It thus appears that they have been consumed b y the dissolving hydrogen to form the H ~ ion. 3

3

3

T h e results for Y b a n d Y b H , even though only semiquantitative, are con sistent w i t h the ideas i n the preceding paragraph. T h e susceptibility of a para magnetic metal can originate i n at least two ways—from the electrons i n the conduc tion b a n d (Pauli paramagnetism) and from the core electrons, i f the core contains an unfilled shell or subshell ( L a n g e v i n paramagnetism). If the latter predomi nates, χ varies w i t h temperature according to either Curie's l a w , χ = C / Γ , or the Curie-Weiss l a w , χ = C/(T—Θ), If, however, the susceptibility originates largely w i t h the conduction electrons, χ is essentially independent of temperature. This, as pointed out above, is the situation for Y b , suggesting that χ for Y b derives i n the m a i n from the conduction electrons. Hence, hydrogénation, w h i c h absorbs these electrons, should produce a decrease i n χ. This is observed to happen. 2


II.

W A L L A C E

ET A L .

129

Magnetism in Lanthanide Hydrides


70

θ"

50

ÏOÔ

Î5Ô

20Ô

25Ô

30Ô

'

Τ [°K.]

Figure 4. Inverse susceptibilities GdHs.oi

vs. temperature

for

and GdHs.oo

G./e.m.u.

Data for GdH . i e 0

displaced upward by 40 units

The foregoing remarks do not hold, of course, for the dihydrides of the triva lent lanthanides. T h e y exhibit metallic conduction (10), as w o u l d be expected, since their conduction band is only somewhat depleted. One w o u l d expect them to display a tendency to order at l o w temperatures, b u t it seems not unreasonable to expect that this tendency w o u l d be weaker than the corresponding element, i n view of the decreased electron concentration, a n d the ordering w o u l d hence occur at lower temperatures. This was i n fact observed for H o H , w h i c h exhibits (6) a Néel point at 8 ° K . , as compared to 135° K . for H o . It is also observed i n the present work for the terbium dihydrides, whose Néel points are 40° to 50° K . , whereas that for the element is 241° K . These properties are compatible w i t h the notion that hydrogen i n a l l the lanthanide hydrides is anionic. O n this basis the dihydrides appear as an intermediate form between the truly metallic elements on the one hand and the truly ionic or saline trihydrides on the other. 2

Most, but not a l l , of the observed magnetic behavior of the lanthanide h y drides fits i n w i t h the notion that the hydrogen-saturated metal lacks conduction electrons. T h e hydride of europium is an exception; E u H , the highest hydride that has been formed i n this laboratory, has been shown (15) to be ferromagnetic below 25° K . This w o u l d ordinarily i m p l y the existence of electrons i n the con1 8 6



130

auction band. However, TîuO, a known insulator, also exhibits ferromagnetism at low temperatures (8). Elemental europium is atypical in many respects and perhaps in the case of europium dihydrides the interactions can occur without the intervention of the conduction electrons. Two aspects of the properties of the hydrides reported in the present paper merit attention. First, G d H exhibits a Néel point, despite the fact that it derives from a metal which alone among the strongly magnetic lanthanides never becomes antiferromagnetic. Second, T b H shows an increase of susceptibility with decrease of temperature at temperatures below 20° K. This implies a change in the nature of the ordering in this material at this point, seemingly toward ferromagnetic ordering. T b H 8, alone among the terbium hydrides, shows a positive Weiss constant, which is a further indication of a tendency toward ferromagnetic ordering. Change from antiferromagnetic to ferromagnetic ordering at the lowest temperature is the rule for the elemental lanthanides but heretofore has not been observed for the lanthanide hydrides. The point of greatest interest is the extreme sensitivity of this effect to the hydrogen content, and hence conduction electron concentration, of the sample. The rise in susceptibility is not observed for T b H i nor T b H . Evidently the electron concentrations and/or the Tb-Tb spacing in these materials is not favorable for the formation of the ferromagnetic structure at low temperatures, whereas the conditions in T b H are conducive to this type of ordering. The fact that so slight a change in composition can produce so radical a change in behavior is probably no more than another indication of the very slight difference in energy of the ferromagnetic and antiferromagnetic modes of coupling. 2


2 0 8

2 0

2 0

2 1 0

2 0 8

Literature Cited (1) Butera, R. Α., Craig, R. S., Cherry, L. V., Rev. Sci. Instr. 32, 708 (1961). (2) Dialer, K., Rothe, W., Z. Elektrochem. 59, 970 (1955). (3) Elliott, R. J., Phys. Rev. 124, 346 (1961). (4) Holley, C. E., Jr., Mulford, R. N. R., Ellinger, F. H., J. Phys. Chem. 59, 1226 (1955). (5) Korst, W. L., Warf, J. C., Acta Cryst. 9, 452 (1956). (6) Kubota, Y., Wallace, W. E., J. Appl. Phys. Suppl. 33, 1348 (1962). (7) Lock, J. M., Proc. Phys. Soc. (London, 70B, 476 (1957). (8 Matthias, B., Bozorth, R. M., Van Vleck, J. H., Phys. Rev. Letters 7, 160 (1961). (9) Pebler, Α., Wallace, W. Ε., J. Phys. Chem. 66, 148 (1962). (10) Peltz, T., Wallace, W. E., unpublished measurements. (11) Stalinski, B., Bull Acad. Polon. Sci. 5, 1001 (1957); 7, 269 (1959). (12) Sucksmith, W., Proc. Roy. Soc. (London) A170, 551 (1939). (13) Warf, J. C., Hardcastle, K., Final Report, Office of Naval Reserve, Contract Nonr 228(15), August 1961. (14) Warf, J. C., Hardcastle, K., J. Am. Chem. Soc. 83, 2207 (1961). (15) Zanowick, R. L., Wallace, W. E., Phys. Rev. 126, 536 (1962). RECEIVED September 6, 1962. Work assisted by a contract with the U . S. Atomic Energy Commission. Portions taken from a thesis presented by R. L. Zanowick to the Graduate Faculty, University of Pittsburgh, in partial fulfillment of the requirements for the Ph. D. degree, January 1962.


Magnetic Characteristics of Gadolinium, Terbium, and Ytterbium

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