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METAL-NONMETAL TRANSITION AND THE MISCIBILITY GAP IN LITHIUM-METHYLAMINE
J. Buntaine, M. Sienko
To cite this version:
J. Buntaine, M. Sienko. METAL-NONMETAL TRANSITION AND THE MISCIBILITY GAP IN LITHIUM-METHYLAMINE. Journal de Physique Colloques, 1980, 41 (C8), pp.C8-36-C8-39.
�10.1051/jphyscol:1980809�. �jpa-00220193�
JOURNAL DE PHYSIQUE CoZZoque C8, suppldment au n o 8, Tome 41, a o g t 2980, page C8-36
METAL-NONMETAL TRANSITION AND THE MISCIBILITY GAP IN LITHIUM-METHYLAMINE
J . R . Buntaine and M . J . Sienko
Baker Laboratory o f C h e m i s t r y , CorneZZ U n i v e r s i t y , I t h a c a , N.Y. 24853, USA
Abstract. The electron spin resonance of lithium-methylamine solutions has been investigated a s a func- tion o f temperature and concentration. In the nonmetallic regime, the relaxation time i n c r e a s e s with temp- erature; i n t h e metallic regime, it d e c r e a s e s . The former is attributed t o motionally narrowed hyperfine in- teraction; t h e l a t t e r , t o lifetime broadened spin-orbit coupling. The change-over appears t o o c c u r i n the range 13.4-15.6 mole % Li. Kinks i n some of t h e T vs . T curves indicate t h a t liquid-liquid p h a s e
l e
separation occurs with a consolute point a t approximately 200K and 1 3 mole % Li. Calculation of a Mott- type model using a n effective dielectric c o n s t a n t s u g g e s t s t h a t t h e metal-nonmetal transition should occur a t approximately 1 2 mole % Li. Thus, a s i n metal-ammonia s y s t e m s , t h e metal-nonmetal transition appears t o be intimately connected with a miscibility gap.
Introduction. Solutions of a l k a l i metals i n liquid
ammonia have been of special i n t e r e s t b e c a u s e they 280CL~
represent, with increasing ratio of metal-to-ammo- nia, a continuous evolution from s a l t - l i k e , electro-
Solid Li + l y t i c character i n the dilute regime t o practically T(K)
i d e a l metallic behavior i n the concentrated [ l ] . 160
Dilute solutions (i . e . , l e s s t h a n 1 mole % metal) Solid NH3 + Liquid 120
have a s t h e i r principal s o l u t e s p e c i e s solvated cat- + t l l ~ l l l l l l r , , , , ~
ions M and solvated anions e-. Concentrated 4 8 12 16 20 24 28
Mole % of Li in NH3
solutions (i. e . , more t h a n 1 0 mole % metal) a r e
Figure 1. Phase diagram for lithium-ammonia.
quasi-ordered arrays of ammoniated c a t i o n s im- mersed i n a n electron g a s . A nonmetal-to-metal transition, signalled by increase i n e l e c t r i c con- ductivity, d e c r e a s e i n Hall voltage, and d e c r e a s e in thermoelectric power, occurs i n t h e range 2-8 MPM [ 2 ] . Free-energy arguments s u c h a s t h o s e given by Krumhansl [ 31 i n h i s study of the elec- tronic and thermodynamic nature of d e n s e vapors i n d i c a t e t h a t the metal-nonmetal transition could well be a s s o c i a t e d with a p h a s e separation. This indeed appears t o be t h e c a s e for a l k a l i m e t a l s i n liquid ammonia, where a liquid-liquid p h a s e sep- aration occurs a t about 4 MPM [4] .
Figure 1 shows t h e p h a s e diagram for t h e
Li-NH system. I t has two extraordinary features:
3
t h e liquid-liquid coexistence region and a d e e p e u t e c t i c where t h e saturation l i n e for equilibrium with solid NH3 i n t e r s e c t s t h a t for equilibrium with solid metal. In t h e c a s e of lithium, t h i s e u t e c t i c is a s s o c i a t e d with formation of t h e com- pound L ~ ( N H ~ ) ~ . Recently, i n a n attempt t o un- derstand t h e electronic nature of the metal-non- metal transition, we had o c c a s i o n t o investigate b y electron spin resonance the anaIogous system of lithium i n methylamine. A s reported elsewhere [S]
t h e compositional dependence of t h e electron-spin- l a t t i c e relaxation time (T ) a t 185K w a s found t o
l e
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980809
show a pronounced d e c r e a s e i n t h e range 14-18 MPM, t h e same place where conductivity a t 208K [6]
and nuclear-spin-lattice relaxation time (T ) a t 1 n 209K [ 71 showed s t e e p i n c r e a s e s . Clearly there i s a metal-nonmetal transition i n lithium-methylamine .
Subsequent study o n the temperature dependence of the electron-spin relaxation time [8] showed t h a t t h e s i g n of d ~ ~ is a s e n s i t i v e means for char- ~ / d ~ acterizing the behavior of t h e e x c e s s electron. In t h i s report w e p r e s e n t d a t a for dT / d ~ on both
l e
s i d e s o f t h e metal-nonmetal transition and show t h a t anomalies i n t h e temperature dependence a r e b e s t interpreted i n terms of liquid-liquid p h a s e separa- tion.
Preparation of Materials. Starting materials were 99.99% lithium from Lithium Corporation of America and 98% methylamine from Matheson Company. Im- purities i n t h e methylamine were 0.0% ammonia, 0.8% (max) dimethylamine, 0.6% (max) trimethyl- amine , and 0.8% (max) water. The methylamine w a s stored over freshly c u t lithium metal a t 188K for a m i n i m m of 48 hours before u s e . Subsequent a n a l y s i s by m a s s spectrometry indicated n o NH3 or H 2 0 . The lithium metal w a s c u t and weighed i n s n evacuable Dri-Lab under helium containing l e s s t h a n 1 ppm of oxygen or nitrogen.
Sample t u b e s for the ESR experiments were made from Spectrosil tubing obtained from t h e Thermal American Fused Quartz Company. D e t a i l s of tube filling and sample homogenization a r e given else- where [9] .
Measurements. ESR spectra were recorded i n t h e f i r s t derivative mode on a Varian E-12 ESR spectro- meter a t X-band frequencies with either 100- o r 10-kHz modulation frequency. Temperature control w a s facilitated with a n Oxford Instruments liquid helium flow system. The temperature s t a b i l i t y w a s
+ 0.1K over t h e entire range 4-260K.
Results. Lithium-methylamine solutions have some of t h e l o n g e s t electron-spin-lattice relaxation times known. This i s manifested in t h e ESR spectra by very narrow linewidths ranging from 0 . 5 t o 3G. In the nonmetallic regime ( a t 6.5 MPM) , a t y p i c a l
linewidth was 0.23G a t 260K and 1.07G a t 160K; in t h e metallic regime (at 21.1 MPM), 1.15G a t 260K and 0.61G a t 160K. A s c a n be s e e n , t h e linewidth expands with decreasing temperature for t h e non- metal and contracts for the metal. Also, there-was a difference i n lineshape. For t h e nonmetallic.
s a m p l e s , 6 . 5 , 8 . 2 , 9 . 3 and 1 3 . 6 MPM, the line- s h a p e w a s Lorentzian with asymmetry r a t i o A/B c l o s e t o 1. For the metallic s a m p l e s , 15.6, 18.0, and 21.1 MPM, the lineshape w a s Dysonian with asymmetry ratio A/B i n t h e range 2-3.
From the linewidth and t h e asymmetry ratio, it i s p o s s i b l e t o extract the spin-lattice relaxation time, following the procedure of Glaunsinger [ 101 .
The r e s u l t s for lithium-methylamine a r e shown i n Figures 2 and 3. Figure 2 represents t h e nonmetal- lic regime and shows t h a t T i n c r e a s e s with
l e
temperature; Figure 3 represents t h e metallic re-
9.3 MPM
10-71. !. , , u - ~ 4 ;
5
M P M
I I I I
160 210 260
T(KI
Figure 2. Spin-lattice relaxation time v s T for d i l u t e nonmetallic solutions of Li-CH3NH2.
16
MPM
Figure 3. Spin-lattice relaxation time vs T for
concentrated, metallic solutions of
Li-CH3NH2.
C8-38 JOURNAL DE PHYSIQUE
gime , where T d e c r e a s e s with temperature. Of 1 e
s p e c i a l interest a r e t h e kinks in t h e T v s T 1 e
curve s u c h a s t h a t observed i n Figure 3 for t h e 1 6 and 18 MPM solutions.
D i s c u s s i o n . The difference in behavior between me- t a l l i c and nonmetallic regimes, s o far a s relaxation time versus temperature is concerned, stems from a difference i n relaxation mechanisms. A s pointed out elsewhere [ 81 , relaxation in t h e insulating re- gime proceeds via modulation of t h e 1 4 ~ hyperfine contact interaction, whereas i n the metallic regime relaxation i s d u e t o modulation of t h e spin-orbit coupling of the electron t o the Li -I- core. For the hyperfine mechanism [ 11,121 , we e x p e c t T pT/q
l e .
where p is the d e n s i t y and q is t h e v i s c o s i t y . A t low temperatures, we d o indeed s e e t h i s behavior.
At the highest temperatures, however, the relaxa- tion time l e v e l s off. This o c c u r s , apparently, b e c a u s e the molecular motion modulation frequency h a s become s o high t h a t it no longer i s an effective relaxation mechanism. The spin-orbit coupling mechanism [ 12,131 i s beginning t o compete effec- tively.
The spin-orbit-relaxation mechanism is sub- stantially different from the hyperfine mechanism.
I t is expected to go a s T VAT, where A i s l e
the equivalent conductivity. Figure 3 a g r e e s with this prediction a s Tle d e c r e a s e s with rising T and T d e c r e a s e s with rising A (A i n c r e a s e s
l e
with concentration. )
The kinks in the Tle vs . T c u r v e s , s e e n for both t h e 16 MPM and 18 MPM s a m p l e s , were f i r s t thought t o be d u e t o errors i n t h e a n a l y s i s . The appropriate spectra were reanalyzed with t h e result that t h e kinks were indeed found t o be r e a l . They a r e now believed t o a r i s e from phase separation of the lithium-methylamine solutions i n t o two liquid l a y e r s , a more concentrated one t h a t is l e s s d e n s e and r i s e s out of the ESR-measuring region and a more d i l u t e one t h a t is more d e n s e and s i n k s into the ESR-measuring region.
From Figure 3 it may be noted t h a t t h e relaxa- tion time a t 160K for the 16 MPM solution is about 3.5 x s e c ; from Figure 2 it should be noted
t h a t Tie a t 160K for t h e 9 . 3 MPM is 3.2 x s e c . The coincidence of t h e s e relaxation times s u g g e s t s t h a t t h e ESR cavity is s e e i n g t h e same solution. For the 9 . 3 MPM it i s believed t o be a homogeneous phase of t h a t composition cooled from 260 t o 160K without p h a s e separation. For t h e 1 6 MPM, it i s believed t h a t cooling from 260 t o 160K s p l i t s t h e phase i n t o an upper one of con- centration 16 MPM and a lower one of concentration 9 . 3 MPM. In other words, a t 160K, it is believed t h a t 9.3 MPM and 16 MPM a r e coexistent and in equilibrium with e a c h other.
A similar coincidence occurs when we compare t h e 8 . 2 MPM and 1 8 MPM solutions. As c a n be s e e n from Figure 3, Tle for 18 MPM a t 160K is 1 . 8 x s e c ; from Figure 2 , Tle for 8.2 MPM a t 160K is 1 . 6 x s e c . Again it is believed t h a t a t 160K t h e 8.2 MPM and 1 8 MPM solutions a r e i n equilibrium with e a c h other. Mihen t h e 1 8 MPM solution is cooled from 260 t o 160K, p h a s e separation occurs t o s p l i t out the 8 . 2 MPM solu- tion, which, being more d e n s e , c o l l e c t s in the measuring zone.
Evidence for p h a s e separation was a l s o ob- served i n two samples t h a t were being studied by differential thermal a n a l y s i s (DTA) for p o s s i b l e evidence t h a t a compound Li(CH NH ) was being
3 2 4
formed. D e t a i l s of the DTA experiment will be pub- l i s h e d e l s e w h e r e , but t h e pertinent f a c t s a r e a s fol- lows: Samples of 16.4 MPM and 18.0 MPM were being homogenized by ultrasonic vibration prepara- tory t o DTA study. As soon a s e a c h sample was re- moved from t h e ultrasonic d e v i c e , two zones of col- o r were observed visually. The sample temperature w a s 188K, which i s above t h e liquidus curve on t h e p h a s e diagram. The separation appeared t o be quite similar t o t h a t observed i n M-NH solutions; ex-
3
c e p t that t h e l e s s d e n s e , bronze p h a s e w a s not s o highly reflective.
A second p i e c e of evidence w a s t h a t for t h e 16.4 and 1 8 . 0 MPM samples the DTA compound fusion peak, e a s i l y observed for Li(CH NH2)* in
3
other solutions, w a s e i t h e r small or nonexistent.
This w a s probably d u e t o t h e more concentrated,
less d e n s e p h a s e rising above the position of t h e thermocouple, which w a s located in the bottom of the DTA c e l l .
Figure 4 shows what the phase diagram for t h e lithium-methylamine system probably looks like.
The consolute concentration is placed a t about 12- 1 3 MPM, a s this is the mean of the concentrations we believe from t h e T studies are in equilibrium
1 e
with e a c h other. The consolute temperature i s placed a t about 200K, a s t h i s temperature is just below the 208K where Toma e t a l . [ 61 measured conductivity a s a function of concentration but ob- served no anomalies.
Solufian
240 Salid Li
+
Solullan 223
