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Cu2+ in NaF: EPR study of a dynamic center
Solid State C o m m u n i c a t i o n s , V o l . 5 3 , N o . 8 pp.661-664, P r i n t e d in Great Britain.
C u 2+
IN N a F : E P R G.Magne*
1985.
0038-1098/85 $3.00 + .00 P e r g a m o n P r e s s Ltd.
STUDY OF A DYNAMIC CENTER
, H.BilI*'**
, D.Lovy*
* G r o u p e de P h y s i c o - C h i m i e d u Solide, U n i v e r s i t ~ de Gen~ve, 30 q u a i E r n e s t Ansermet, 1211 Geneva, S w i t z e r l a n d * * a n d D e p a r t m e n t of S o l i d State Physics, ANU, Canberra, A u s t r a l i a
(Received
13 A u g u s t
1984, accepted 20 N o v e m b e r
1984 b y C. W. McCombie)
A n E P R s t u d y has b e e n c a r r i e d out on a C u 2+ c e n t e r in N a F s i n g l e c r y s t a l s ( w i t h 2 % m o l e of c o p p e r ) f r o m 1.6 k - 150 K. The s y m m e t r y of the E P R s p e c t r a is tric l i n i c at 1.6 K, m o n o c l i n i c at 4.2 K, t e t r a g o n a l at T > 40 K. N o h y p e r f i n e s t r u c t u r e h a s b e e n observed. M o d e l s w h i c h i n c l u d e b o t h the J a h n - T e l l e r e f f e c t a n d a l o w s y m m e t r y c r y s t a l f i e l d are proposed.
c r y s t a l w a s t h e n grown, in a h e l i u m a t m o s p h e r e , w i t h a c h a r - c o a l t r a p ( c o o l e d t o 78 K ) c o n n e c t e d to the vessel. C r y s t a l s c o n t a i n i n g initial d o p a n t c o n c e n t r a t i o n s r a n g i n g from 0.04 to 2 m o l e % w e r e grown. A s the d i s t r i b u t i o n of the c o p p e r ions in the c r y s t a l s w a s a l w a y s found to b e i n h o m o g e neous, we use this n o m i n a l d o p a n t c o n c e n t r a t i o n t o c h a r a c t e r i z e the samples. T h e g r o w t h v e l o c i t y and the e f f e c t i v e copp e r c o n c e n t r a t i o n in the crystal t u r n e d o u t to b e c l o s e l y related.
1. I n t r o d u c t i o n C u 2+ is a w e l l - k n o w n model J a h n - T e l l e r s y s t e m w h i c h has b e e n i n v e s t i g a t e d in the p a s t in a n u m b e r of c u b i c or t r i g o n a l hosts, as for i n s t a n c e in several alkali halides. In p a r t i c u lar a c o p p e r c e n t r e o f o r t h o r h o m b i c s y m m e t r y w a s o b s e r v e d b y K u w a b a r a I in NaF. His e x p e r i m e n tal r e s u l t s seem far from being c o m p l e t e as he p u b l i s h e d d a t a o n l y around 78 K. O n the c o n t r a ry, d e t a i l e d r e s u l t s on C u 2+ in N a C I h a v e b e e n p u b l i s h e d 2 . T h i s system e x h i b i t s a s t r o n g J a h n T e l l e r e f f e c t w i t h a tetragonal E P R s p e c t r u m b e l o w a p p r o x i m a t e l y 95 K w h i c h b e c o m e s m o t i o n a l ly a v e r a g e d a b o v e this temperature. It s e e m e d to us w o r t h w h i l e to study in some d e t a i l the c o p p e r (2+) ion in N a F in o r d e r to o b t a i n a m o r e c o m p l e t e p i c t u r e r e g a r d i n g the p o s s i b l e a c t i o n o f the J a h n - T e l l e r e f f e c t in t h i s system. It t u r n e d o u t t h a t it is q u i t e difficult to i n t r o d u c e c o p p e r into NaF and, d e p e n d i n g o n the m e t h o d of p r e p a r a t i o n , one o b t a i n s r a t h e r d i f f e r e n t results. In particular, w e h a v e n o t s u c c e e d e d in r e p r o d u c i n g the centre p u b l i s h e d b y Kuwabaral. But, w e have b e e n able to c r e a t e t w o o t h e r o n e s in a r e p r o d u c i b l e manner. This letter presents experimental EPR results about o n e o f them. 2.
U s u a l l y the a s - g r o w n c r y s t a l s c o n s i s t e d of a pure N a F b o d y and a m e t a l l i c c o p p e r d e p o s i t either o n its surface or as a n i n c l u s i o n in the interior. O u r e x p e r i m e n t s s h o w e d t h a t the q u a n t i t y of c o p p e r w h i c h is m i c r o s c o p i c a l l y d i s s o l v e d d e p e n d s v e r y c r i t i c a l l y on the p r e s e n c e ( o r a b s e n c e ) of o x y g e n and (OH-) g r o u p s dur i n g the c r y s t a l growth. T h e f o l l o w i n g c h e m i c a l r e a c t i o n o f the copper in the p r e s e n c e o f some oxygen traces s e e m to o c c u r :
Sample P r e p a r a t i o n
i)
n
( 2 CuF 2 ~ 2 CuF + F 2 ) ( p r o b a b l y slow ) metallic copper clusters + n F 2
2)
( NaF + C u F 2 )melt + o x y g e n N a F + m e t a l l i c copper
W h e n o x y g e n or (OH)- g r o u p s are present, e q u a t i o n (2) seems predominant. T h e d e t a i l s w e r e n o t i n v e s t i g a t e d b u t the final p r o d u c t s are : p u r e N a F crystal and m e t a l l i c copper. Indeed, o n l y the o n e of o u r g r o w t h p r o c e dures which we described above gave satisfactory and rather r e p r o d u c i b l e d o p e d crystals. Some e x p e r i m e n t s w e r e p e r f o r m e d t o introd u c e c o p p e r b y d i f f u s i o n in p u r e N a F crystals, b u t w i t h o u t success. Typically, C u F 2 , CuI, CuO, C u 2 0 or C u w e r e i n t r o d u c e d as p o w d e r s into a n i c k e l o r a q u a r t z tube in the p r e s e n c e o f a
A s this p a r t of s t u d y was n o t t o t a l l y trivial, we feel j u s t i f i e d to r e p o r t a f e w facts a b o u t s a m p l e preparation. M o s t of the c r y s t a l s w e r e g r o w n b y B r i d g m a n method. W a t e r - f r e e p o w d e r s o f N a F a n d C u F 2 were introduced into a carbon crucible and gent ly h e a t e d ( T ~ 150°C ) u n d e r h i g h v a c u u m in the c r y s t a l g r o w t h furnace. A l i q u i d n i t r o g e n trap c o n n e c t e d to the f u r n a c e e n s u r e d a low p a r t i a l p r e s s u r e of H20 , 02 . A b o u t 24 h o u r s later, the
661
662
Vol. 53, No. 8
Cu 2+ IN NaF : EPR STUDY OF A DYNAMIC CENTER
pure NaF crystal under an inert atmosphere or under NH4F.HF or C12 atmosphere. T h e sealed tube was then heated to 600oC. N o copper centers were observed in those crystals after this treatment. Suitable crystals grown by the Bridgman method contain a useable concentration of C u 2+ without any further treatment ( such as X-irradiation or annealing ). A few doped samples were X-rayed at room temperature and also at 78 K. N o changes were observed in the E P R experiments with regard to the copper signals. Electron Micro-Probe measurements reveal that aggregation of copper occurs in part of the as-grown crystals. It may be remarked that the crystal growth process mentioned above could be a suitable method for the production of metallic micro-clusters. 3.
H II [ 0 0 1 1
Experimental Results
H II [1 1 1"1 The EPR spectra were recorded on a Varian 9 GHz spectrometer and also a home-built 36 GHz apparatus. Generally the present center is found in heavily doped-crystals ( 1-2 % mole ). The asgrown crystal axes at 78 K the spectra shown in figure i. The multiplicity of the spectrum ( re corded in (Ii0) and (001) type planes ) indicates that the center has tetragonal symmetry. The experimental g-values at 78 K are :
gll = 2.458 g l = 2.105
± ~
0.005 0.005
The EPR lines have a width which depends on the angle and ranges from i00 to 150 Gauss. No resolved hyperfine structure is observed. This spectrum is a motionally averaged one. Indeed, when the crystal is cooled to 1.6 K one observes at Ka-band frequencies the EPR spectra shown in figure ~. As can be seen from this figure, this spectrum is of lower than tetragonal - indeed triclinic - symmetry. A detailed study of the angular dependence of the resonance spectrum was realized with H rotating in (001) and (ii0) type planes. The analysis based on the angular plots obtained in the three mutually orthogonal (001) planes was performed in the usual way and the results are depicted in figure 3. The principal g-values and the Euler's angles specifying the axes have the following values : gll = 2.0124 g22 = 2.1328 g33 = 2.5139
~ ± +
H II E1 101
_)
f
U 2000G
4000G
FIGURE i. EPR spectra of Cu 2+ in NaF single crystal at 78 K. Microwave frequency : V = 9.2285 GHz.
H II [0013
H IIEl11"I
~
"
~
~
I
0.005 0.005 0.005
with the following Euler's angles :
8
= = =
9.4 ° 16.8 ° 6.7 °
+ ± ~
0.5 ° 0.5 ° 0.5 °
( The convention of Whittaker applies, definition of the angles according to Tinkham 3 ) The EPR lines are large and have line
10000 G
12600 G
FIGURE 2. E P R spectra of Cu 2+ in N a F recorded at 1.6 K, v = 35.649 GHz
Cu 2+ IN NaF : EPR STUDY OF A DYNAMIC CENTER
Vol. 53, No. 8
4.
g O
O
O
O
O
•
o
in-
o
2.40
o
o
2.20
2.OLD
b
'lo
~,0
~0
~0
Discussion
The following facts suggest that this Cu 2+ center involves modifications of its neighborhood. Firstly, we have identified a second Cu 2+ center 4 of tetragonal symmetry at low temperature which seems to be surrounded by an unaltered octahedral environment and it seems to volve the Jahn-Teller effect. The presently reported center has triclinic symmetry at very low temperature. The presence of a vacancy in the first neighborhood is unlikely since this geometry would produce a stable tetragonal center due to a strong ligand field. On the other hand, a vacancy in the second neighborhood gives rise to a C2v or a C s site symmetry of the Cu 2+ ion. This comparatively weak ligand field in conjunction with an Eg ~ ( Cg + Tlu ) Jahn-Teller effect 5 might create an off-center deformation leading to the observed very low symmetry of the copper resonance. Such a model has the further advantage to fulfil local neutrality. A second possible model, depicted in figure 4, can not be excluded at present. It contains a sodium vacancy in a [011] direction and a Cu + impurity substituting a Na + ion located in a
o O
663
e"
FIf2jRE 3. Cu2+ Angular variation of in NaF at i. 6 K when H is rotated in a (i00) plane. Circles are experimental data points while the continuous curves are c~plfEer fitted. The closed circles represetlt the r0ore accurate lines ( g-error less than 0.002 ) and the open ones are approximate positions of broad and mutually superposing lines. widths that are strongly angular dependent. For this reason the experiments at Ka-band frequency were essential. No resolved hyperfine or superhyperfine structure could be found. The Xband results at 1.6 K were virtually useless because of a insufficient resolution. The temperature dependence of the spectrum was studied at Ka-band with H being oriented along a C 4 crystal axis. The triclinic spectrum ( figure 2 ) consists of three lines ( with gvalues : gx = 2.057 ~ 0.002, gy = 2.143 ~ 0.005, gz = 2.489 ± 0.001 ) along this direction. When the temperature is increased from the initial 1.6 K the splitting between the lines corres ponding to gx and gy decreases steadily. Bet ween 1.6 K and 40 K, an exponentional law describes the temperature variation of this splitting. The parameters of this empirical fit are :
~ 0 1 ~ direction, both occupying a second neighbor position. Again, the low symmetry crystal field in conjunction with an Eg ~ ( Eg + Tlu ) Jahn-Teller effect is responsible for the observe d structure of the adiabatic ground state potential.
~ L..---+"--%.2",.,,,_
OF ONa"
Ag = ( gy - gx ) = a. exp ( -0.7 . T ) where a = 0.ii The whole process is reversible. Several stress experiments were performed on these systems at 1.6 K and 4.2 K. Samples ( ~--2 % mole doped ) were selected which gave a good signal to noise ratio. Yet, no repopulation effects were observed for P ~< 500 kg/cm 2. Moreover, it was impossible to saturate this center. Our experimental setup did not yield a sufficient microwave magnetic field at the sample ( P ~< 200 roW, Q -~ 5000 ) at T>~ 1.6 K.
OCu0 Cu" i--i Na + vacancy FIGURE 4. Proposed model where two defects, a sodi~n vacancy and a mcnovalent copper ion, surroumd the paracnagmetic copper ion.
664
Cu 2+ IN NaF : E P R STUDY OF A D Y N A M I C C E N T E R
A s no m o t i o n a l a v e r a g i n g of the tetragonal " h i g h t e m p e r a t u r e " s p e c t r u m is observed, one c a n c o n c l u d e that the t e t r a g o n a l d e f o r m a t i o n s o f t h i s p o t e n t i a l are deep. T h e low symmetry c r y s t a l f i e l d e f f e c t s g e n e r a t e " secondary mi n i m a " w h i c h are r a t h e r s h a l l o w w i t h respect to t h o s e of t e t r a g o n a l deformation. When the temp e r a t u r e is raised, e f f e c t i v e m o t i o n a l a v e r a g i n g o c c u r s b e t w e e n the d i f f e r e n t " s e c o n d a r y mini m a ". T h i s is i n d i c a t e d b y the large linewidths w h i c h h i d e any h y p e r f i n e structure. It is ob vious that any possible model must comply with the fact t h a t the J a h n - T e l l e r effect and crystal
f i e l d e f f e c t s c o m p e t e in this system. M o r e w o r k n e e d s to b e d o n e on this system. A c k n o w l e d g e m e n t s - T h e a u t h o r s w o u l d like to t h a n k Dr. J. B e r t r a n d of the M i n e r a l o g y Insti tute o f the U n i v e r s i t y of G e n e v a for the Elec tron Micro-Probe measurements. O n e o f the a u t h o r s (H.B) thanks the A N U for the hospitality. O n e o f us (G.M) t h a n k s Dr. Y . R a v i Sekhar for u s e f u l discussions. T h i s w o r k w a s s u p p o r t e d b y the Swiss N a t i o n a l S c i e n c e s Foundation.
REFERENCES i. 2. 3. 4. 5.
Vol. 53, No. 8
G. KUWABARA, J o u r n a l of the P h y s i c a l S o c i e t y of J a p a n 31, 1074 ( 1971 ) R. B. BORCHERTS, H. KANZAKI, H.ABE, P h y s i c a l R e v i e w B 2, 23 ( 1970 ) M. TINKHAM, " G r o u p T h e o r y A n d Q u a n t u m M e c h a n i c s " ( McGraw-Hill, N.Y, 1964 ) G. MAGNE, H. BILL, to be p u b l i s h e d G. I. BERSUKER, V. Z. POLINGER, V. P. KHLOPIN, Soviet P h y s i c s S o l i d State 24, 1402 ( 1982 )
C u 2+
IN N a F : E P R G.Magne*
1985.
0038-1098/85 $3.00 + .00 P e r g a m o n P r e s s Ltd.
STUDY OF A DYNAMIC CENTER
, H.BilI*'**
, D.Lovy*
* G r o u p e de P h y s i c o - C h i m i e d u Solide, U n i v e r s i t ~ de Gen~ve, 30 q u a i E r n e s t Ansermet, 1211 Geneva, S w i t z e r l a n d * * a n d D e p a r t m e n t of S o l i d State Physics, ANU, Canberra, A u s t r a l i a
(Received
13 A u g u s t
1984, accepted 20 N o v e m b e r
1984 b y C. W. McCombie)
A n E P R s t u d y has b e e n c a r r i e d out on a C u 2+ c e n t e r in N a F s i n g l e c r y s t a l s ( w i t h 2 % m o l e of c o p p e r ) f r o m 1.6 k - 150 K. The s y m m e t r y of the E P R s p e c t r a is tric l i n i c at 1.6 K, m o n o c l i n i c at 4.2 K, t e t r a g o n a l at T > 40 K. N o h y p e r f i n e s t r u c t u r e h a s b e e n observed. M o d e l s w h i c h i n c l u d e b o t h the J a h n - T e l l e r e f f e c t a n d a l o w s y m m e t r y c r y s t a l f i e l d are proposed.
c r y s t a l w a s t h e n grown, in a h e l i u m a t m o s p h e r e , w i t h a c h a r - c o a l t r a p ( c o o l e d t o 78 K ) c o n n e c t e d to the vessel. C r y s t a l s c o n t a i n i n g initial d o p a n t c o n c e n t r a t i o n s r a n g i n g from 0.04 to 2 m o l e % w e r e grown. A s the d i s t r i b u t i o n of the c o p p e r ions in the c r y s t a l s w a s a l w a y s found to b e i n h o m o g e neous, we use this n o m i n a l d o p a n t c o n c e n t r a t i o n t o c h a r a c t e r i z e the samples. T h e g r o w t h v e l o c i t y and the e f f e c t i v e copp e r c o n c e n t r a t i o n in the crystal t u r n e d o u t to b e c l o s e l y related.
1. I n t r o d u c t i o n C u 2+ is a w e l l - k n o w n model J a h n - T e l l e r s y s t e m w h i c h has b e e n i n v e s t i g a t e d in the p a s t in a n u m b e r of c u b i c or t r i g o n a l hosts, as for i n s t a n c e in several alkali halides. In p a r t i c u lar a c o p p e r c e n t r e o f o r t h o r h o m b i c s y m m e t r y w a s o b s e r v e d b y K u w a b a r a I in NaF. His e x p e r i m e n tal r e s u l t s seem far from being c o m p l e t e as he p u b l i s h e d d a t a o n l y around 78 K. O n the c o n t r a ry, d e t a i l e d r e s u l t s on C u 2+ in N a C I h a v e b e e n p u b l i s h e d 2 . T h i s system e x h i b i t s a s t r o n g J a h n T e l l e r e f f e c t w i t h a tetragonal E P R s p e c t r u m b e l o w a p p r o x i m a t e l y 95 K w h i c h b e c o m e s m o t i o n a l ly a v e r a g e d a b o v e this temperature. It s e e m e d to us w o r t h w h i l e to study in some d e t a i l the c o p p e r (2+) ion in N a F in o r d e r to o b t a i n a m o r e c o m p l e t e p i c t u r e r e g a r d i n g the p o s s i b l e a c t i o n o f the J a h n - T e l l e r e f f e c t in t h i s system. It t u r n e d o u t t h a t it is q u i t e difficult to i n t r o d u c e c o p p e r into NaF and, d e p e n d i n g o n the m e t h o d of p r e p a r a t i o n , one o b t a i n s r a t h e r d i f f e r e n t results. In particular, w e h a v e n o t s u c c e e d e d in r e p r o d u c i n g the centre p u b l i s h e d b y Kuwabaral. But, w e have b e e n able to c r e a t e t w o o t h e r o n e s in a r e p r o d u c i b l e manner. This letter presents experimental EPR results about o n e o f them. 2.
U s u a l l y the a s - g r o w n c r y s t a l s c o n s i s t e d of a pure N a F b o d y and a m e t a l l i c c o p p e r d e p o s i t either o n its surface or as a n i n c l u s i o n in the interior. O u r e x p e r i m e n t s s h o w e d t h a t the q u a n t i t y of c o p p e r w h i c h is m i c r o s c o p i c a l l y d i s s o l v e d d e p e n d s v e r y c r i t i c a l l y on the p r e s e n c e ( o r a b s e n c e ) of o x y g e n and (OH-) g r o u p s dur i n g the c r y s t a l growth. T h e f o l l o w i n g c h e m i c a l r e a c t i o n o f the copper in the p r e s e n c e o f some oxygen traces s e e m to o c c u r :
Sample P r e p a r a t i o n
i)
n
( 2 CuF 2 ~ 2 CuF + F 2 ) ( p r o b a b l y slow ) metallic copper clusters + n F 2
2)
( NaF + C u F 2 )melt + o x y g e n N a F + m e t a l l i c copper
W h e n o x y g e n or (OH)- g r o u p s are present, e q u a t i o n (2) seems predominant. T h e d e t a i l s w e r e n o t i n v e s t i g a t e d b u t the final p r o d u c t s are : p u r e N a F crystal and m e t a l l i c copper. Indeed, o n l y the o n e of o u r g r o w t h p r o c e dures which we described above gave satisfactory and rather r e p r o d u c i b l e d o p e d crystals. Some e x p e r i m e n t s w e r e p e r f o r m e d t o introd u c e c o p p e r b y d i f f u s i o n in p u r e N a F crystals, b u t w i t h o u t success. Typically, C u F 2 , CuI, CuO, C u 2 0 or C u w e r e i n t r o d u c e d as p o w d e r s into a n i c k e l o r a q u a r t z tube in the p r e s e n c e o f a
A s this p a r t of s t u d y was n o t t o t a l l y trivial, we feel j u s t i f i e d to r e p o r t a f e w facts a b o u t s a m p l e preparation. M o s t of the c r y s t a l s w e r e g r o w n b y B r i d g m a n method. W a t e r - f r e e p o w d e r s o f N a F a n d C u F 2 were introduced into a carbon crucible and gent ly h e a t e d ( T ~ 150°C ) u n d e r h i g h v a c u u m in the c r y s t a l g r o w t h furnace. A l i q u i d n i t r o g e n trap c o n n e c t e d to the f u r n a c e e n s u r e d a low p a r t i a l p r e s s u r e of H20 , 02 . A b o u t 24 h o u r s later, the
661
662
Vol. 53, No. 8
Cu 2+ IN NaF : EPR STUDY OF A DYNAMIC CENTER
pure NaF crystal under an inert atmosphere or under NH4F.HF or C12 atmosphere. T h e sealed tube was then heated to 600oC. N o copper centers were observed in those crystals after this treatment. Suitable crystals grown by the Bridgman method contain a useable concentration of C u 2+ without any further treatment ( such as X-irradiation or annealing ). A few doped samples were X-rayed at room temperature and also at 78 K. N o changes were observed in the E P R experiments with regard to the copper signals. Electron Micro-Probe measurements reveal that aggregation of copper occurs in part of the as-grown crystals. It may be remarked that the crystal growth process mentioned above could be a suitable method for the production of metallic micro-clusters. 3.
H II [ 0 0 1 1
Experimental Results
H II [1 1 1"1 The EPR spectra were recorded on a Varian 9 GHz spectrometer and also a home-built 36 GHz apparatus. Generally the present center is found in heavily doped-crystals ( 1-2 % mole ). The asgrown crystal axes at 78 K the spectra shown in figure i. The multiplicity of the spectrum ( re corded in (Ii0) and (001) type planes ) indicates that the center has tetragonal symmetry. The experimental g-values at 78 K are :
gll = 2.458 g l = 2.105
± ~
0.005 0.005
The EPR lines have a width which depends on the angle and ranges from i00 to 150 Gauss. No resolved hyperfine structure is observed. This spectrum is a motionally averaged one. Indeed, when the crystal is cooled to 1.6 K one observes at Ka-band frequencies the EPR spectra shown in figure ~. As can be seen from this figure, this spectrum is of lower than tetragonal - indeed triclinic - symmetry. A detailed study of the angular dependence of the resonance spectrum was realized with H rotating in (001) and (ii0) type planes. The analysis based on the angular plots obtained in the three mutually orthogonal (001) planes was performed in the usual way and the results are depicted in figure 3. The principal g-values and the Euler's angles specifying the axes have the following values : gll = 2.0124 g22 = 2.1328 g33 = 2.5139
~ ± +
H II E1 101
_)
f
U 2000G
4000G
FIGURE i. EPR spectra of Cu 2+ in NaF single crystal at 78 K. Microwave frequency : V = 9.2285 GHz.
H II [0013
H IIEl11"I
~
"
~
~
I
0.005 0.005 0.005
with the following Euler's angles :
8
= = =
9.4 ° 16.8 ° 6.7 °
+ ± ~
0.5 ° 0.5 ° 0.5 °
( The convention of Whittaker applies, definition of the angles according to Tinkham 3 ) The EPR lines are large and have line
10000 G
12600 G
FIGURE 2. E P R spectra of Cu 2+ in N a F recorded at 1.6 K, v = 35.649 GHz
Cu 2+ IN NaF : EPR STUDY OF A DYNAMIC CENTER
Vol. 53, No. 8
4.
g O
O
O
O
O
•
o
in-
o
2.40
o
o
2.20
2.OLD
b
'lo
~,0
~0
~0
Discussion
The following facts suggest that this Cu 2+ center involves modifications of its neighborhood. Firstly, we have identified a second Cu 2+ center 4 of tetragonal symmetry at low temperature which seems to be surrounded by an unaltered octahedral environment and it seems to volve the Jahn-Teller effect. The presently reported center has triclinic symmetry at very low temperature. The presence of a vacancy in the first neighborhood is unlikely since this geometry would produce a stable tetragonal center due to a strong ligand field. On the other hand, a vacancy in the second neighborhood gives rise to a C2v or a C s site symmetry of the Cu 2+ ion. This comparatively weak ligand field in conjunction with an Eg ~ ( Cg + Tlu ) Jahn-Teller effect 5 might create an off-center deformation leading to the observed very low symmetry of the copper resonance. Such a model has the further advantage to fulfil local neutrality. A second possible model, depicted in figure 4, can not be excluded at present. It contains a sodium vacancy in a [011] direction and a Cu + impurity substituting a Na + ion located in a
o O
663
e"
FIf2jRE 3. Cu2+ Angular variation of in NaF at i. 6 K when H is rotated in a (i00) plane. Circles are experimental data points while the continuous curves are c~plfEer fitted. The closed circles represetlt the r0ore accurate lines ( g-error less than 0.002 ) and the open ones are approximate positions of broad and mutually superposing lines. widths that are strongly angular dependent. For this reason the experiments at Ka-band frequency were essential. No resolved hyperfine or superhyperfine structure could be found. The Xband results at 1.6 K were virtually useless because of a insufficient resolution. The temperature dependence of the spectrum was studied at Ka-band with H being oriented along a C 4 crystal axis. The triclinic spectrum ( figure 2 ) consists of three lines ( with gvalues : gx = 2.057 ~ 0.002, gy = 2.143 ~ 0.005, gz = 2.489 ± 0.001 ) along this direction. When the temperature is increased from the initial 1.6 K the splitting between the lines corres ponding to gx and gy decreases steadily. Bet ween 1.6 K and 40 K, an exponentional law describes the temperature variation of this splitting. The parameters of this empirical fit are :
~ 0 1 ~ direction, both occupying a second neighbor position. Again, the low symmetry crystal field in conjunction with an Eg ~ ( Eg + Tlu ) Jahn-Teller effect is responsible for the observe d structure of the adiabatic ground state potential.
~ L..---+"--%.2",.,,,_
OF ONa"
Ag = ( gy - gx ) = a. exp ( -0.7 . T ) where a = 0.ii The whole process is reversible. Several stress experiments were performed on these systems at 1.6 K and 4.2 K. Samples ( ~--2 % mole doped ) were selected which gave a good signal to noise ratio. Yet, no repopulation effects were observed for P ~< 500 kg/cm 2. Moreover, it was impossible to saturate this center. Our experimental setup did not yield a sufficient microwave magnetic field at the sample ( P ~< 200 roW, Q -~ 5000 ) at T>~ 1.6 K.
OCu0 Cu" i--i Na + vacancy FIGURE 4. Proposed model where two defects, a sodi~n vacancy and a mcnovalent copper ion, surroumd the paracnagmetic copper ion.
664
Cu 2+ IN NaF : E P R STUDY OF A D Y N A M I C C E N T E R
A s no m o t i o n a l a v e r a g i n g of the tetragonal " h i g h t e m p e r a t u r e " s p e c t r u m is observed, one c a n c o n c l u d e that the t e t r a g o n a l d e f o r m a t i o n s o f t h i s p o t e n t i a l are deep. T h e low symmetry c r y s t a l f i e l d e f f e c t s g e n e r a t e " secondary mi n i m a " w h i c h are r a t h e r s h a l l o w w i t h respect to t h o s e of t e t r a g o n a l deformation. When the temp e r a t u r e is raised, e f f e c t i v e m o t i o n a l a v e r a g i n g o c c u r s b e t w e e n the d i f f e r e n t " s e c o n d a r y mini m a ". T h i s is i n d i c a t e d b y the large linewidths w h i c h h i d e any h y p e r f i n e structure. It is ob vious that any possible model must comply with the fact t h a t the J a h n - T e l l e r effect and crystal
f i e l d e f f e c t s c o m p e t e in this system. M o r e w o r k n e e d s to b e d o n e on this system. A c k n o w l e d g e m e n t s - T h e a u t h o r s w o u l d like to t h a n k Dr. J. B e r t r a n d of the M i n e r a l o g y Insti tute o f the U n i v e r s i t y of G e n e v a for the Elec tron Micro-Probe measurements. O n e o f the a u t h o r s (H.B) thanks the A N U for the hospitality. O n e o f us (G.M) t h a n k s Dr. Y . R a v i Sekhar for u s e f u l discussions. T h i s w o r k w a s s u p p o r t e d b y the Swiss N a t i o n a l S c i e n c e s Foundation.
REFERENCES i. 2. 3. 4. 5.
Vol. 53, No. 8
G. KUWABARA, J o u r n a l of the P h y s i c a l S o c i e t y of J a p a n 31, 1074 ( 1971 ) R. B. BORCHERTS, H. KANZAKI, H.ABE, P h y s i c a l R e v i e w B 2, 23 ( 1970 ) M. TINKHAM, " G r o u p T h e o r y A n d Q u a n t u m M e c h a n i c s " ( McGraw-Hill, N.Y, 1964 ) G. MAGNE, H. BILL, to be p u b l i s h e d G. I. BERSUKER, V. Z. POLINGER, V. P. KHLOPIN, Soviet P h y s i c s S o l i d State 24, 1402 ( 1982 )