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NMR INVESTIGATIONS OF THE SUPERIONIC CONDUCTORS MAg4I5 (M = K,Rb) AT
PRESSURES UP TO 0,7 GPa
H. Huber, M. Mali, J. Roos, D. Brinkmann
To cite this version:
H. Huber, M. Mali, J. Roos, D. Brinkmann. NMR INVESTIGATIONS OF THE SUPERIONIC
CONDUCTORS MAg4I5 (M = K,Rb) AT PRESSURES UP TO 0,7 GPa. Journal de Physique
Colloques, 1984, 45 (C8), pp.C8-75-C8-78. �10.1051/jphyscol:1984815�. �jpa-00224313�
N M R I N V E S T I G A T I O N S
OFTHE S U P E R I O N I C CONDUCTORS M A ~ (M ~
=I K , R ~ ~ AT PRESSURES UP TO
0 , 7GPa
H. Huber, M. Mali, J. Roos and D'. Brinkmann
Physik-Institut, University of Zurich, Scmnberggasse 9, 8001 Ztlrich, SAtzerZand
Resume
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Un nouvel e t a t de phasea
haute pression aete
mis en evidence dans l e RbAgqIg. La conductibilite s e r a i t plus p e t i t e que dans 1 ' & a t 6.La comportement des temps de relaxation du rubidium e t de l ' a r g e n t suggere que, pour l e RbAg415 e t l e KAg415, l a conductibilite dans l ' e t a t 6 e s t indbpendante de l a pression jusqu'a 0,57 GPa.
Abstract
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In RbAg415 a new high-pressure phase has been detected which could have a lower conductivity than the 6 phase. Rb and Ag relaxation studies suggest t h a t in the 6 phases of RbAg415 and KAg415 the conduc- t i v i t y i s independent of pressure u p t o 0.57 GPa.Superionic conductors a r e solid s t a t e systems whose ionic conductivity approaches values of 1 0 0 (am)-' t h a t a r e typical of those found in 1 iquid e l e c t r o l y t e s . This c l a s s of new materials has therefore received large a t t e n t i o n f o r i t s technological application, e.g. as solid s t a t e b a t t e r i e s and fuel c e l l s . Nuclear magnetic reso- nance (NMR) has proved t o be a powerful technique in yielding valuable information on s t a t i c and dynamic properties of superionic conductors. However, most NMR studies, see f o r instance /1-3/, a r e being performed a t standard pressure. In t h i s paper we present what we believe a r e the f i r s t NMR studies in superionic conductors a t higher pressure. Preliminary r e s u l t s together with d e t a i l s of the high-pressure equipment have been already published /4/.
I
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HIGH PRESSURE APPARATUSHere we only summarize the main features of our pressure apparatus, f o r d e t a i l s we r e f e r t o /4/. Hydrostatic pressure up t o 0.7 GPa i s produced in a two stage pressure generator manufactured by NOVA SWISS AG /5/ and transmitted by helium gas t o the NMR probe, a cross-sectional view of which i s shown in Fig. la. All hatched parts were machined from 1 / 1 hard Berylco 25. The pressure vessel B contains a sample space 8 mm in diameter by 10 mn long with the NWR c o i l . The radio-frequency ( r f ) feedthrough i s d i r e c t l y connected t o the tuning capacitor E which can be tuned by means of a movable s t a i n l e s s s t e e l cylinder F which i s insulated from the vessel.
Crucial parts of the pressure vessel a r e the sealing plug C with the sealing ring G and the rf feedthrough, d e t a i l s a r e given in Fig. I b . The construction of the seal- ing i s based on work of Trappeniers e t a l . / 6 / while t h a t of the r f feedthrough has followed the design of Jonas /7/.
The NMR probe head f i t s into a cryostat designed f o r the temperature range 77 t o 300 K and consisting of a vacuum-shielded cylinder of 28 nun inner diameter. Low tem- peratures a r e achieved by blowing cold nitrogen or helium gas a t the probe head. The cryostat together with the probe head i s employed in a 5.1 T superconducting magnet of Oxford Instruments.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984815
C8-76 JOURNAL DE PHYSIQUE
Fig.1. High-pressure NMR probe head ( f r o m Ref. / 4 / ) .
( a ) O v e r a l l c r o s s - s e c t i o n a l view: (A) gland n u t , (B) pressure vessel,
(C)
p l u g w i t h NMR c o i l and r.f.f eedthrough, (D) gland n u t , (F) t u n i n g c y l i n d e r , ( Q ) c a v i t y f o r PT 100 p l a t i n u m r e s i s t o r .
( b ) D e t a i l s o f high- pressure seal and feed- through:
(G) Berylco s e a l i n g r i n g , (H) c u r v a t u r e of 1 mn rad., ( I ) c o a x i a l transmission 1 ine, (K) Berylco s e a l i n g cone, ( L ) gland nut,
(MI
Vespel washer, ( N ) A r a l d i t sealed ends, (0) Vespel c l o s u r e cap, (P) epoxy sealed feedthrough.
11
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PHASE DIAGRAM OF RbAg415 AND KAg415The f i r s t a p p l i c a t i o n o f our NMR high-pressure equipment d e a l t w i t h t h e two i s o s t r u c t u r a l s u p e r i o n i c conductors RbAgqIg and KAgqIg i n which Ag i s the mobile i o n /8/.
These compounds a r e o f special i n t e r e s t since they e x h i b i t two s u p e r i o n i c phases a and separated by a f i r s t - o r d e r phase t r a n s i t i o n a t temperature Tc, whereas a low- temperature y phase i s low-conducting. Our previous NMR work i n these compounds i s described i n /9-12/.
The quadrupolar s p l i t t i n g o f the 8 7 ~ b o r t h e 3 9 NMR l i n e can be employed t o d e t e r - ~
mine t h e pressure-vs.-temperature phase diagram o f these compounds. The s p l i t t i n g a r i s e s from the i n t e r a c t i o n o f t h e n u c l e a r e l e c t r i c quadrupole moment eQ o f e i t h e r 87Rb o r 3 9 K (both w i t h nuclear s p i n 3 / 2 ) w i t h t h e e l e c t r i c f i e l d g r a d i e n t (EFG) tensor a t t h e n u c l e a r s i t e . A t h i g h magnetic f i e l d s t h e spectrum c o n s i s t s o f a cen- t r a l l i n e s h i f t e d w i t h respect t o t h e Larmor frequency, and f l a n k e d by a s a t e l l i t e 1 in e on e i t h e r side. Since t h e s h i f t i s p r o p o r t i o n a l t o (eQVzz/h)2 where VZz i s t h e maximum p r i n c i p a l component o f t h e EFG tensor, t h e quadrupole s p l i t t i n g i s very sen- s i t i v e t o any s t r u c t u r a l changes associated w i t h a phase t r a n s i t i o n . E s p e c i a l l y , t h e appearance o f a new c e n t r a l l i n e (and i t s s a t e l l i t e s ) i n t h e spectrum i s i n d i c a t i v e o f a t r a n s i t i o n i n t o a new phase o f lower symmetry. I n t h i s way we have detected i n RbAgqIg a new phase which we c a l l t h e 6 phase. By measuring the Rb spectrum as a f u n c t i o n o f both temperature and pressure t h e 0-6 and y-6 phase boundaries were de- termined y i e l d i n g t h e phase diagram as shown i n Fia. 2. The s o l i d l i n e reoresents r e s u l t s o f A l l e n and Lazarus /13/ who s t u d i e d the phase diagram by measuring t h e i o n i c c o n d u c t i v i t y up t o 0.6 GPa.
I n KAg415 t h e search f o r the 6 phase was hampered by t h e f a c t t h a t f o r experimental reasons t h e resonance o f l o g Ag ( s p i n 1/2) r a t h e r than t h a t o f 39K had t o be used.
As w i l l be discussed i n Section 111, t h e t r a n s i t i o n i n t o t h e 6 phase can be moni- t o r e d by a sudden increase o f t h e s p i n - l a t t i c e r e l a x a t i o n time. P r e l i m i n a r y r e s u l t s p o i n t t o t h e existence o f a 6 phase i n KAgqIg appearing c l o s e t o 0.7 GPa which i s the h i g h e s t pressure o b t a i n a b l e i n our NMR probe head a t present.
work, t h e broken l i n e i s a guide t o
x
-
t h e eye. S o l i d l i n e : r e s u l t s from-
Ref. /13/.120 140 160 180
Temperature [K]
I 1 1
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RELAXATION STUDIES I N RbAgqIg AND KAgq15Due t o t h e d i f f u s i v e motion o f t h e Ag ions and o t h e r dynamic p r o p e r t i e s o f t h e crys- t a l , the Rb and Ag n u c l e i a r e acted on by time-dependent e l e c t r i c and magnetic f i e l d s which g i v e r i s e t o r e l a x a t i o n e f f e c t s . The r e l a x a t i o n time TI i s t h e t i m e constant f o r t h e nuclear s p i n system t o approach thermal e q u i l i b r i u m w i t h i t s envi- ronment. We have measured T1 o f t h e s t a t i o n a r y Rb and o f t h e mobile Ag ions.
F i g . 3 shows t h e temperature dependence o f the r e l a x a t i o n r a t e o f the 87Rb c e n t r a l l i n e a t a pressure o f 0.68 GPa together w i t h t h e corresponding data a t standard pressure
/ l o / .
Except f o r t h e r e g i o n around Tc, t h e r e l a x a t i o n r a t e i n t h e a and 8 phase a t standard pressure can be f i t t e d by two Bloembergen-Purcell -Pound (BPP) type formulas:1 = C 1 1/T;=2C2&z
Here, C1 and Cp which a r e p r o p o r t i o n a l t o eQV,,/h, a r e a measure f o r t h e amplitude o f t h e f l u c t u a t i n g EFG tensors a t t h e Rb s i t e , u i s t h e Larmor frequency, and v = voexp(-E/kT) i s t h e t h e r m a l l y a c t i v a t e d jump frequency o f t h e d i f f u s i n g Ag ions.
A f i t t o t h e data y i e l d s
vO = 8 . 7 ~ 1 0 ' ~ Hz, E = 0.13 eV, C1 = 5.0x1012 Hz, C2 = 6 . 7 ~ 1 0 ~ ' Hz.
Fig. 3. Relaxation r a t e o f 07Rb i n RbAgqIg a t 5.17 T and 0.68 GPa. Dashed l i n e s : data from Ref.
/ l o /
a t standard pressure. V e r t i c a l dashed l i n e s : phase boundaries a t 0.68 GPa.C8-78 JOURNAL DE PHYSIQUE
The 14 % increase o f t h e r a t e on a p p l y i n g h i g h pressure i s w e l l accounted f o r by t h e corresponding increase o f C1 and C2 and hence o f , V, due t o t h e l a t t i c e com- pression. Apparently t h e a c t i v a t i o n energy and the attempt frequency vo o f t h e Ag d i f f u s i o n a r e n o t changed.
The c r i t i c a l c o n t r i b u t i o n t o t h e r e l a x a t i o n r a t e a t t h e high-temperature s i d e of Tc has been e x p l a i n e d by t h e f o r m a t i o n and m i g r a t i o n o f c l u s t e r s i n which t h e Ag ions a r e p a r t i a l l y ordered as they a r e i n t h e 5 phase
/ l o / .
By a p p l y i n g pressure t h e c r i t i c a l c o n t r i b u t i o n i s " s h i f t e d " t o h i g h e r temperatures, i.e. i n t h e same d i r e c - t i o n as Tc i s s h i f t e d . We thus conclude t h a t t h e c l u s t e r model i s a l s o a p p l i c a b l e a t h i g h e r pressure. Together w i t h our r e s u l t t h a t v0 and E do n o t depend on pres- sure, t h i s seems t o i n d i c a t e t h a t pressure ( a t l e a s t up t o 0.68 GPa) does n o t a l t e r the microscopic d e t a i l s o f t h e Ag d i f f u s i o n mechanism. I n t h i s c o n t e x t i t would be i n t e r e s t i n g t o see whether t h e Haven r a t i o i s i n f l u e n c e d by pressure. A t standard pressure t h i s r a t i o decreases when c r o s s i n g Tc from above /11,12/.A t t h e 5-6 phase t r a n s i t i o n t h e Rb r e l a x a t i o n r a t e i s d r a s t i c a l l y reduced. However, the r a t e i n the 6 phase e x h i b i t s t h e same a c t i v a t i o n energy o f 0.13 eV as found i n the a and 6 phase. T h i s r e s u l t c o n t r a s t s w i t h t h e n e a r l y temperature independent r a t e o f t h e y phase
/ l o / .
Hence, beside t h e i r s t r u c t u r a l d i f f e r e n c e s t h e y and 6 phase can a l s o be d i s t i n g u i s h e d i n terms o f t h e Ag dynamics as probed by t h e Rb r e - l a x a t i o n . I n a simple model o f independently d i f f u s i n g Ag i o n s t h e parameter C i s p r o p o r t i o n a l t o t h e number o f d i f f u s i n g Ag ions. A r e d u c t i o n o f 1/T1 c o u l d then be t r a c e d back t o a s m a l l e r value o f t h e "attempt" frequency vo which would s h i f t t h e 1/T1 maximum t o h i g h e r temperatures and p o s s i b l y a decreased number o f mobile ions and hence a lower c o n d u c t i v i t y .The l o 9 A g r e l a x a t i o n times have been measured i n 5-KAg415 w i t h no s i g n i f i c a n t change compared t o t h e values a t standard pressure. However, i n b o t h the y and 6 phase t h e Ag r a t e s a r e extremely small which could p o i n t t o a lower c o n d u c t i v i t y . T h i s reduc- t i o n o f t h e Ag r e l a x a t i o n r a t e s was u t i l i z e d t o determine the phase boundaries i n KAg415.
I n conclusion, we have shown t h a t i n the temperature range 120 t o 175 K t h e a p p l i c a - t i o n o f h y d r o s t a t i c pressure up t o about 0.57 GPa must have a n e g l i g i b l e e f f e c t on the Ag c o n d u c t i v i t y . T h i s i s p a r a l l e l e d by an o b s e r v a t i o n t h a t i n t h e a phase above 330 K t h e c o n d u c t i v i t y i s independent o f pressure up t o about 2 GPa /14/. However, a t pressures h i g h e r than 0.57 GPa t h e o r y phase transforms i n t o a new phase, where t h e Ag c o n d u c t i v i t y c o u l d be lower than t h a t i n t h e 0 phase.
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