STRANGE MAGNETIC BEHAVIOR IN THE LITHIUM-METHYLAMINE SYSTEM

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STRANGE MAGNETIC BEHAVIOR IN THE LITHIUM-METHYLAMINE SYSTEM

A. Stacy, D. Johnson, M. Sienko

To cite this version:

A. Stacy, D. Johnson, M. Sienko. STRANGE MAGNETIC BEHAVIOR IN THE LITHIUM- METHYLAMINE SYSTEM. Journal de Physique Colloques, 1980, 41 (C8), pp.C8-45-C8-48.

�10.1051/jphyscol:1980812�. �jpa-00220216�

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JOURNAL DE PHYSIQUE CoZZoque C8, suppZdment au n08, Tome 41, aoct 1980, page C8-44

J.R. Franz and M. Jonsont

Indiana University, Indiana

,

U.S.A.

Abstract.

-

Using t h e numerical Ilonte-Carlo technique o f G i r v i n and

on son,'

we have c a l c u l a t e d the d e n s i t i e s o f s t a t e s o f model l i q u i d b i n a r y a l l o y s . The s t r e n g t h o f the method i s i t s a b i l i t y t o accu- r a t e l y account f o r l o c a l atomic c o n f i g u r a t i o n s w h i l e a v o i d i n g e x p l i c i t rimy-atom c l u s t e r c a l c u l a t i o n s . Results w i l l be presented f o r low c o n n e c t i v i t y model a l l o y s f o r which e f f e c t s of210cal c o n f i g u r a t i o n s a r e p a r t i c u l a r l y e v i d e n t . Comparison w i t h t h e v ~ o r k o f Franz, Brouers, and Holzhey f o r t h e Cs-PU system w i l l be given. A p p l i c a t i o n of the technique t o the c a l c u l a t i o n o f t h e c o n d u c t i v i t y w i l l be d i s - cussed. I n p a r t i c u l a r , i t w i l l be shown t h a t p a r t i a l l y l o c a l i z e d s t a t e s e x i s t w i t h i n t h e bands t h a t c o n t r i b u t e t o t h e d e n s i t y o f s t a t e s b u t o n l y m i n i m a l l y t o t h e c o n d u c t i v i t y . /\nother i m p o r t a n t f e a t u r e o f t h e method i s i t s a b i l i t y t o describe Anderson l o c a l i z a t i o n . P new r e s u l t o f t h e t h e o r y i s the3 OCCUt-r-enCeof mu1 t i p l e m o b i l i t y edges s e p a r a t i n g regions o f finderson l o c a l i z a t i o n i n s i d e t h e band.

References

/1/ r1. Jonson and S.M. G i r v i n , Phys. Rev. L e t t .

43

1447 (1979)

/2/ J.R. Franz, F. Brouers and Ch. Holzhey, J. Phys. F:

Metal Phys.

10,

235 (1980).

/3/ M. Jonson and J.R. Franz, Submitted t o J. Phys. C:

S o l i d S t a t e Phys.

%This worK p a r t i a l l y supported by DMR 77-11305

t

On l e a v e from t h e I n s t i t u t e o f T h e o r e t i c a l Physics, Gtteborg, Sweden.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980812

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JOURNAL DE PHYSIQUE CoZZoque C8, s u p p l h e n t a z n o 8 , T m e 41, a o g t 1980, page C8-45

STRANGE MAGNETIC BEHAVIOR I N THE L I T H I U M - M E T H Y L A M I N E SYSTEM

A. S t a c y , D.C. Johnson and M . J . Sienko

Baker L a b o r a t o r y of C h e m i s t r y , Corne Z Z U n i v e r s i t y , I t h a c a , N. Y . 14853, USA.

Abstract. Static magnetic susceptibilities have been measured by the Faraday method over t h e range 1.8- 200 K on various solutions of lithium in methylamine. The solutions are diamagnetic; there i s a n anomaly a t the freezing point, with the solid approximately twice a s diamagnetic a s the liquid. In the concentrated regime, near t h e "expanded- metal" compound Li(CH3NH2)4, t h e susceptibility in the liquid is increasingly negative with decreasing temperature; in the solid, i t is independent of temperature. The molar susceptibility of Li(CH3NH ) i s -140 x 1 0 - ~ . Considerable electron-pairing, favored a t lower temperature apparently

2 4 occurs i n t h i s system.

Introduction. Solutions of the alkali metals i n liquid ammonia [ l ] have provided numerous unantici- pated findings on the problem of metallic systems a t low electron density. Lithium In ammonia, for ex- ample, shows liquid-liquid phase separation a s s o c - iated with a metal-nonmetal transition [2] and formation of t h e "expanded-metal" compound Li(NH3)4.

Magnetic [3] and CESR [4] investigations of L i ( N q 4 indicated that both the liquid and the cubic form of t h e solid are b e s t characterized a s "nearly free electronic" though verging on themetal-nonmetal transition. In a n attempt t o push c l o s e r t o t h i s transition, we have undertaken a study of the lithium-methylamine system. The bulky CH3 group in place of a n ammonia H should serve t o expand the mean Li-Li spacing and bring u s closer t o the elusive Mott transition. A s indicated elsewhere [S]

,

ESR studies of fluid and frozen solutions of L i in CH3NH2 sug- g e s t t h a t the electronic properties of Li(CH3NH2)4 cannot be described in t h e context of a nearly free- electron material but signal incipient localization in the very strong scattering regime. This report describes findings on t h e magnetic susceptibility of the Li-CH3NH2 system both a s a function of composition and temperature.

was stored and handled in a n evacuable Dri-Lab under helium with N and 0 impurities a t l e s s than 1

2 2

ppm l e v e l s . The CH3NH2, 98% grade from Mathe- son Co., originally had a t maximum 0.0% NH3,O .8%

(CH3)2NH, 0.6% (CH3)3N, and 0.8% H20. It was freed of H 2 0 by storage over freshly c u t Li a n d d i s - tillation therefrom. M a s s spectrometric analysis of the CH3NH2 used indicated H 2 0 and NH3 t o be below 0.1%.

Sealed buckets for the sample containment were fashioned from thin-walled (3 mm ID, 4 . 2 mm OD) Spectrosil tubing obtained from the Thermal American Fused Quartz Co. Special precautions (e.g., non- metallic forceps) were taken t o ensure freedom from magnetic contamination. The samples were made a s follows: The Spectrosil was constricted a t t h e expected seal-off point

a

mm above the bottom of t h e tube, and connected t o a ground g l a s s joint via a silica-to-Pyrex graded s e a l . The entire assembly was cleaned with HN03-HF, rinsed with distilled deionized water, and dried in a drying oven. The tube was then outgassed a t 1000°C t o 3 x 1 0 - ~ Torr.

The evacuated tube w a s loaded in the Dri-Lab with a precisely weighed portion c u t from the center of a lithium bar. The weighing was done on a high pre- Preparation of Materials

samples

were prepared cision Cahn Electrobzilance G2. The sample tube using standard vacuum techniques [63

.

Li metal, containing t h e Li w a s then transferred t o a vacuum 99.99%, obtained from Lithium Corp. of America, l i n e , evacuated t o 5 x 1 0 - ~ Tom, and filled with

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C8-46 JOURNAL DE PHYSIQUE

CH3NH2 from a g a s volume previously calibrated s o that a measured pressure corresponded t o a known mass of condensed CH3NH2. With the end of t h e sample in liquid nitrogen, the tube was s e a l e d off.

Only sgmples which showed no trace of color in the sealing process were considered acceptable.

Samples were stored a t 77K in t h e undissolved s t a t e . They could be kept indefinitely without de- composition. Immediately before magnetic measure- ment,they were homogenized by warming t o room temperature for 5 t o 20 minutes, depending on t h e time required t o hang the sample in the Faraday apparatus.

Measurements. Magnetic susceptibilities were measured over t h e range I. 8-2 00 K by the Faraday method, using apparatus and procedure elsewhere described [7]

.

Temperature was controlled t o better than 0.1 K with a n Oxford Instruments temperature controller using a Au(O.O7%Fe)-Chrome1 P thermocouple with liq

N 2

reference. The region 1.8-80 K w a s monitored with a CryoCal calibrated germanium resistance thermometer; 80-200 K, with the Au- C hromel thermocouple. Susceptibility measurements, a l l ofwhich were found t o be field independent, were corrected for the diamagnetism of the sample holder. Possible transition t o the supercon- ducting s t a t e was examined using a n a . c . mutual- inductance apparatus which has been described elsewhere [8]

.

Results. Eight samples of Li-CH3NH2 ranging in composition from 8.0 t o 19.5 MPM (mole % metal) were examined. In addition, pure CH3NH2 was measured a t room temperature, 50 K, and 4 K. The gram-susceptibility of CH3NH2 was observed t o be -0.867 X I O - ~ independent of temperature. This cor- responds t o a molar susceptibility of -26.9 x i n excellent agreement with t h e literature value -27.0 x [9]

.

Figure 1 shows the gram-susceptibility vs temperature for a concentrated (17.04 MPM) solution of Li in CH3NH2. The observed susceptibility is everywhere negative. In the liquid (above 155 K ) , t h e susceptibility i n c r e a s e s t o l e s s negative val-

T(K)

Fig.1. 17.04 mole % Li in CH3NH2 u e s a s the temperature r i s e s . In the solid (below 155 K), the susceptibility i s about twice a s negative and is practically independent of temperature.

There is a slight Curie t a i l below 10 K; its smallness t e s t i f i e s t o the extremely high purity of the materials.

Figure 2 shows the behavior of the most dilute (8.0 MPM) solution studied. In t h e liquid, the slope is again positive and becomes considerably more s o above 180 K; in the solid,the susceptibility is pratically independent of temperature.

T(K)

Fig. 2 8.0 MPM Li in CH3NH2

Figure 3 shows how the susceptibilitytracks through the melting point 155 K. A l l the susceptibility curves were determined i n the rising-temperature mode, a s hysteresis effects were smaller in that direction.

Each data point, however, was taken a t a n equili- brated temperature. There w a s no drift with time.

Table I summarizes t h e observed and derived parameters for a l l t h e samples investigated. The nota- tion i s a s follows: Column I gives the concentration of t h e sample i n MPM. Column I1 gives t h e observed gram-susceptibflity of the solid extrapolated t o 155 K. (Values quoted are believed t o be reliable t o within 10%). Column I11 shows the gram-suscep-

tibility calculated for the compound L ~ ( C H ~ N H ~ ) ~

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Fig. 3 13.7 MPM Li in CH3NH2

a t 155 K, assuming the solid contains only CH3NH2 and Li(CH3NH2)4. Column IV gives the observed gram-susceptibility of t h e liquid extrapolated t o 155 K. Columns 11, 111, and IV are i n units of emu/g. Column V (10'8emu/g K) gives t h e slope of the observed gram-susceptibility curve above 180 K, the melting point of pure solid CH3NH2. For three of the samples (15 . I , 17.0, 19.5' MPM), bucket correction was determined directly over the whole temperature range. For the other samples, bucket correction w a s determined indirectly by using the above three samples t o fix the susceptibility of CH3NH2 and Li(CH3NH2)4 a t 155 K and then using those values t o compute a force correction for the containers. For column 111

rdwas

calculated a s follows :

value for the gram-susceptibility of the compound L ~ ( C H ~ N H ~ ) ~

is

-(I .lt0.3)emu/g. The molar s u s - ceptibility i s -140 x

Table I

X c ~ d Xliq

MPM dgiq/dT

g g

0 -0.867

Discussion. There i s a growing evidence that the Li-CH3NHZ system resembles the Li-NH3 system in showing both a miscibility gap and compound formation [2,5]. In Li-C H3NH2 the miscibility gap appears a t -13 MPM and 200 K [2]; in Li-NH3, a t 4.35 MPM and 21 0 K [lo]. The deep eutectic, which appears t o be intimately connected with the compound formation, occurs a t 88.8 K for Li(NH3)4 and 155

K

for Li(CH3NH2)4.

Assuming t h a t we have an "expanded metal" compound of t h e type Li(CH3NH2)4, we c a n calculate the expected susceptibility, using the model previously developed for L ~ ( N H ~ ) ~ [3]

.

Unfortunately, some of the key parameters a r e not available, s o they have t o be estimated. Y a m a m o t o e t l [Ill give the density of Li-CH3NH2 solutions down t o 223 K up t o 1 0 MPM. If we extrapolate their data t o 20 MPM, we get 0.61 g/cc. Lowering df t h e temperature i s not expected t o change t h e density of t h e solution very much, a s i t is mainly decided by e--e- repulsion, not thermal motion. For Li(NH3)4 there i s a 2% increase in density on solidification, SO if we apply t h e same correction here we end up with

0.62 g/cc for Li(CH3NH2)4. The corresponding mol- a r volume is 211 c c . Assuming one free e- per Li, we get conduction electron density q of 2.8 x 10 2 1 per c c

.

For nearly free electrons, t h e Pauli susceptibility per c c i s given by

C 2. 2

4" PA

(3~'h$''(I-

sa)

-T-

where m* i s he effective mass and p o is the Bohr magneton. Assuming m*=m

,

we g e t k 0 . 2 0 8 X I O ' ~ per c c . corresponding t o xg=O. 335 x10 -6 per g, o r

~ = 4 3 . 9 x 1 0 - ~ per mole. The molar diamagnetic core corrections a r e -0.6 for ~i'and:-27 x 1 0 - ~ for CH3NH2, leading t o -1 08 f o r ~ i ( C Y N ~ ) ~ . Therefore, we expect a n e t molar susceptibility of -64 x 1 0 - ~ . The experimental value -140 x 1 0 - ~ is more than twice a s diamagnetic. The discrepancy becomes even greater if we note t h a t Li(CH3NH2)4 is in t h e strong scattering regime [5], s o t h a t the itinerant electron diamagnetism of Landau (given by the -m2/3m* 2 term in the above equation) probably should be ignored. Clearly there must be some kind of electron pairing in Li(CW3NH2)4.

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C8-48 JOURNAL DE PHYSIQUE

No temperature dependence is expected for pauli paramagnetism or core diamagnetism. Hence the positive value of d X / d ~ observed in Fig.1 above 155

K

must be due t o a d e c r e a s e i n the electronpair- ing. The situation in Fig. 2 appears t o be more complicated. Not only d o e s t h e lower concentration (8.0 MPM) put u s on the dilute s i d e of t h e miscibility gap but the relatively larger amount of e x c e s s

Acknowledgment. This research was sponsored by NSF under grant No. DMR 78-12238 and w a s supported i n part by t h e US AFOSR and the Materialscience Center a t Cornell University.

Refere nces.

[

11

For review of metal-ammonia systems, s e e J. C

.

Thomson, "Electrons in Liquid Ammonia", Clarendon Press, Oxford, 19 76

C H ~ N H ~ means we have t o go t o a higher tempera-

r21

See. for the paper J.R.Buntainet M . J. Sienko in t h i s same colloquium.

ture before a l l t h e CH,NH:, h a s redissolved. The

., -

[3] W.S .Glaunsinger,S .Zolotov,M. J.Sienko, data in table I show that dilution makes the solution J . C h e m . P h y s , s 4756 (1972)

[4] W.S.Glaunsinger, M.J.Sienko, J.Chem.Phys.

more diamagnetic. Hence, a s we rise in tempera-

-

62 -1883 (1975) ture in t h e 8 MPM solution, two effects cancel out.

Increasing diamagnetism d u e to dilution is compen- s a t e d by decreasing diamagnetism due t o electron depairing. Above 180 K, a l l t h e solid CH3NH2 i s gone, s o d X / d ~ reflects the effect of temperature alone.

In comparison with NH3 solutions, the CH3NH2 solutions show similar susceptibility behavior when calculated permole of Li after correction for a l l the solvent in the solution. The apparent molar

x

of Li d e c r e a s e s with increasing concentration and in- c r e a s e s with increasing temperatures. The observa- tion of this [12] originally led Ogg [13] t o postulate electron pairs trapped in solvation c a v i t i e s . The equilibrium 2&$

e;^

is shifted t o the right within- creasing concentration and decreasing temperature.

Such

a

model is too naive for concentrated solutions, but replacement of NH3 by CH3NH2 does indeed displace the o n s e t of metallic behavior t o

higher concentrations.

The molar susceptibilities of Li(NH3)4 and

-

6 L ~ ( c H ~ N H ~ ) ~ a r e + 8 0 x 1 0 - ~ and -140x10

,

resp.

The reason for t h e electron pairing that l e a d s t o t h i s dramatic difference is not clear. One intrigu- ing possibility i s that L ~ ( c H ~ N H ~ ) ~ represents t h e

+ -

intermediate M M s t a t e t h a t was calculated t o be s t a b l e when isolated M O atoms are gradually brought together t o form a metal. That would ex- plain the diamagnetism observed here and a l s o might account for t h e "strong scattering" behavior of t h e Tle relaxation time in the CESR [5]

.

[51

P. P.Edwards ,A. R . Lusis ,M

.

J.Sienko, J.Chem.

phys.

72

3103 (1980)

[6] J. R. Buntaine ,Ph.D . t h e s i s , Cornell 1980 [7] J.E.Young,jr,Ph.D.thesis, Cornell 1971 [81 W.G.Fisher,Ph.D. t h e s i s , Cornell 1978 [9] Landolt-~Grnstein " Zahlenwerte und Funktionen"

6th ed,vol I1,part 1 0 , S ~ r i n g e r ~ 1 9 6 7 ~ p . 6 8

[lo]

D.E .Loeffler,Ph.D.thesis ,Stanford, 1949 [ l l ] M .Yamamoto,Y .Nakamura,M .Shimoji, J.Chem.

Soc., Faraday Trans.

67

2292 (19 71)

[12] S.Freed,H.G.Thode,J.Chem.Phys

.7

85 (1939) [13] R.A.Ogg,jr,J.Am.Chem.Soc.

2

155 (1946)