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ENDOR investigations on CdF2: Eu2+ and SrF2 : Eu2+ systems
Volume 30A, number 6
PHYSICS
LE]?TERS
References
3. 4. 5. 6.
1. F. Jona and G. Shirane, F e r r o e l e c t r i c c r y s t a l s (The McMillan Company, New York, 1962). 2. A . E . Lundin, G. M. Mikhailov and S. P. Habuda, Soviet P h y s i c s - J E T P 13 (1961) 903.
ENDOR
INVESTIGATIONS
ON
CdF2:
1 7 N o v e m b e r 1969
G. Burns, Phys. Rev. 123 (1961) 1634. J. L. Bjorkstam, Bull. Am. Phys. Soe. 14 (1969) 635. E.Schempp, Ph.D. Thesis, Brown U n i v e r s i t y , 1968 T. Oja, Bull. Am. Phys. Soc. 14 (1969) 31.
Eu 2 + and
SrF 2
: Eu 2+
SYSTEMS
R. V A L E N T I N
I I. Physikalisches Institut der Technisvhen Hochschule , Darmstadt~Germany Received 9 October 1969
ENDOR m e a s u r e m e n t s of the 19F nuclei in the f i r s t four shells of CdF 2 and SrF 2 containing Eu 2+ are reported. The r e s u l t s are compared with CaF 2 • Eu 2+
ENDOR i n v e s t i g a t i o n s at X - b a n d w e r e p e r f o r m e d on C d F 2 : Eu 2+ a n d S r F 2 : Eu 2+ ( c i r c a 0.05 m o l % Eu2~). A q u a r t z r e s o n a t o r a s d e s c r i b e d in r e f . 1 w a s u s e d . T h e r f f i e l d w a s p r o d u c e d in two d i f f e r e n t w a y s . In t h e f i r s t c a s e , t h e E N D O R c o i l w a s p a r t of a p a r a l l e l r e s o n a n t c i r c u i t , t u n e d by a s e r v o - s y s t e m . In t h e s e c o n d c a s e , t h e E N D O R c o i l f o r m e d p a r t of a 50 ~ l i n e , t e r m i n a t e d w i t h 50 ~2 o u t s i d e t h e d e w a r . T h e b e s t s i g n a l n o i s e ratio was achieved with a simultaneous modulat i o n of t h e n u c l e a r r e s o n a n c e f i e l d a n d t h e s t a t i c Ho - f i e l d . W i t h H o in [100] d i r e c t i o n , m e a s u r e m e n t s w e r e d o n e on a l l s e v e n E P R t r a n s i t i o n s . F i g . 1 s h o w s a s p e c t r u m of C d F 2 : E u 2+. T h e s p e c t r a a r e d e s c r i b e d by t h e s p i n H a m i l t o n i a n a s u s e d by B a k e r [7],
M.-3/2
01~
M--5/2
O~2X,
~Ho~"~2
H = -gn~nHoel + eA sSCl + + ( ( A p + CAd) ( 3 S z e l z
-S el),
w h e r e ~ i n d i c a t e s t h e p o s i t i o n of t h e f l u o r i n e n u c l e i . On t a b l e 1, t h e c o n s t a n t s m e a s u r e d a r e c o m p a r e d w i t h t h o s e of C a F 2 : Eu 2+ [3]. T h e i s o t r o p i c p a r t of t h e i n t e r a c t i o n w i t h the f i r s t s h e l l i s m a i n l y c a u s e d by s p i n p o l a r i z a t i o n o f t h e o u t e r 5s a n d 5p e l e c t r o n s of t h e Eu z + - ion, w h i c h i s due to e x c h a n g e i n t e r a c t i o n w i t h t h e 4 f e l e c t r o n s [4]. A c c o r d i n g to t h e l a t t i c e c o n s t a n t b e c o m i n g s m a l l e r , 1A s b e c o m e s m o r e n e g a t i v e in C a F 2 t h a n i n S r F 2. S u r p r i s i n g , h o w e v e r , i s t h a t 1A s b e c o m e s l e s s n e g a t i v e f r o m C a F 2 to C d F 2. T h e s a m e t e n d e n c y w a s o b s e r v e d in C d F 2 Gd 3+ a n d C a F 2 : G d 3+ [5,6].
344
"'1 [31g
I
D1
I
15
I
I
16
I
I
17
[
I
18
]
i ~.2
19 MHz
Fig. 1. ENDOR of Eu 2+ with F - in CdF 2 :Eu 2+
Ho//[lO0 ] , M = - ~ ' ~ - ~ , m 153 = - 2~ ; T= 4.2°K, 7 = 1 sec. (For E P R o f CdF 2 : Eu2 + see ref. 2)
Volume 30A, number 6
PHYSICS
LETTERS
B e c a u s e of s m a l l c o v a l e n t c o n t r i b u t i o n s , h o w e v e r , it i s d i f f i c u l t to d e t e r m i n e the l a t t i c e d i s tortions quantitatively. S r F 2 : E u 2+ s h o w s the v a l u e of the d i p o l e d i p o l e i n t e r a c t i o n f o r ~ A p + 2Ad~ w i t h i n the l i m i t s of e r r o r . S i n c e 8r "+ and Eu 2+ h a v e an a l m o s t i d e n t i c a l i o n i c r a d i u s , l a t t i c e d i s t o r t i o n s s h o u l d not o c c u r . F u r t h e r m o r e , the c o v a l e n t p a r t s should be s m a l l e r b e c a u s e of the l a r g e r l a t t i c e c o n s t a n t . T h i s r e s u l t , t h e r e f o r e , i s to be e x p e c t e d . T h e e x p e r i m e n t a l (Ap + Ad) v a l u e s of the f l u o r i n e n u c l e i in the t h i r d a n d f o u r t h s h e l l a r e in good a g r e e m e n t a l s o with the v a l u e s c a l c u l a t e d for point dipoles.
Table 1. Eu 2+ 1A s
CdF 2 - 1.94
CaF2[3] - 2.23
SrF 2 - 1.93 3.80
(lAp + l a d )
4.10
4.01
1A d (calc.)
5.825
5.604
4.675
2As
0.012
- 0.007
0.000
(2Ap + 2Ad)
0.815
0.785
0.666
2A d (cale.)
0.830
0.798
0.666
3As (3Ap + 3Ad)
0.000 0.366
-
0.000 0.293
3Ad (ealc.)
0.366
0.352
0.293
4As (4Ap + 8Ad)
0.000 0.216
-
0.000 0.173
4A d (calc.)
0.216
0.208
0.173
ao
5.39
5.46
5.80
17 November 1969
T h e a u t h o r i s i n d e b t e d to P r o f e s s o r D r . B. E l s c h n e r f o r h i s i n t e r e s t in t h i s w o r k and s t i m u l a t i n g d i s c u s s i o n s ; to M r . K. B a b e r s c h k e f o r his c o - o p e r a t i o n ; to P r o f e s s o r D r . S. H u f n e r f o r s u p p l y i n g a S r F 2 : Eu 2+ c r y s t a l ; to M r . W. - J . B e c k e r f o r the u s e of h i s c o m p u t e r p r o g r a m , and to the D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t f o r p r o v i d i n g p a r t of the e q u i p m e n t .
Units ~fA s, An andAA are expressed in MHz, those of a o in A. The e~rors o~'the constants 1A amount to * 10 kHz, of 2A, 3A and 4.4 to ~: 5 kHz. The calculated values of the dipole interaction constants A d are based upon an undistorted lattice, a o is the lattice constant.
References 1. K. Baberschke, W. Rhein, G. Klein and R. Valentin, Z. Angew. Physik 25 (1968) 265. 2. H.J. Gl~iser and D. Geist, Z. Naturforsch. 23a (1968) 1980. 3. J. M. Baker and J. P. Hurrell. Proc. Phys. Soc. 82 (1963) 242. 4. R. E. Watson and A. J. F r e e m a n , Phys. Rev. Letters 6 (1961) 277. 5. R. H. Borcherts, T. Cole and T. Horn. J. Chem. Phys. 49 (1968) 4880. 6. H.Bill, Phys. Letters 29A (1969) 593. 7. J . M . B a k e r , J . P h y s . C 1 (1968) 1670.
On a c c o u n t of the l a t t i c e c o n s t a n t b e c o m i n g s m a l l e r , ( 1 A n + l A d ) i n c r e a s e s f r o m S r F 2 t~ C d F o . A s l i g h t l y p o s i t i v e v a l u e i s found f o r "~A_ in C d F 2 : Eu 2+, in a c c o r d a n c e w i t h C d F 2 : Gd 3+~ T h e v a l u e s of (2A u + 2A d) in C d F 2 and C a F 2 i s 15 k H z r e s p . 13 k H z l o w e r than the v a l u e c a l c u l a t e d f o r a puriely d i p o l a r i n t e r a c t i o n . A c c o r d i n g to r e f . 7, s u c h d e v i a t i o n s m a y be b a s e d upon l a t t i c e d i s t o r t i o n s , c a u s e d by the r e p l a c e m e n t of the Cd 2+ r e s p . Ca 2+ - i o n by the l a r g e r Eu 2+ - i o n . ***
*
*
345
PHYSICS
LE]?TERS
References
3. 4. 5. 6.
1. F. Jona and G. Shirane, F e r r o e l e c t r i c c r y s t a l s (The McMillan Company, New York, 1962). 2. A . E . Lundin, G. M. Mikhailov and S. P. Habuda, Soviet P h y s i c s - J E T P 13 (1961) 903.
ENDOR
INVESTIGATIONS
ON
CdF2:
1 7 N o v e m b e r 1969
G. Burns, Phys. Rev. 123 (1961) 1634. J. L. Bjorkstam, Bull. Am. Phys. Soe. 14 (1969) 635. E.Schempp, Ph.D. Thesis, Brown U n i v e r s i t y , 1968 T. Oja, Bull. Am. Phys. Soc. 14 (1969) 31.
Eu 2 + and
SrF 2
: Eu 2+
SYSTEMS
R. V A L E N T I N
I I. Physikalisches Institut der Technisvhen Hochschule , Darmstadt~Germany Received 9 October 1969
ENDOR m e a s u r e m e n t s of the 19F nuclei in the f i r s t four shells of CdF 2 and SrF 2 containing Eu 2+ are reported. The r e s u l t s are compared with CaF 2 • Eu 2+
ENDOR i n v e s t i g a t i o n s at X - b a n d w e r e p e r f o r m e d on C d F 2 : Eu 2+ a n d S r F 2 : Eu 2+ ( c i r c a 0.05 m o l % Eu2~). A q u a r t z r e s o n a t o r a s d e s c r i b e d in r e f . 1 w a s u s e d . T h e r f f i e l d w a s p r o d u c e d in two d i f f e r e n t w a y s . In t h e f i r s t c a s e , t h e E N D O R c o i l w a s p a r t of a p a r a l l e l r e s o n a n t c i r c u i t , t u n e d by a s e r v o - s y s t e m . In t h e s e c o n d c a s e , t h e E N D O R c o i l f o r m e d p a r t of a 50 ~ l i n e , t e r m i n a t e d w i t h 50 ~2 o u t s i d e t h e d e w a r . T h e b e s t s i g n a l n o i s e ratio was achieved with a simultaneous modulat i o n of t h e n u c l e a r r e s o n a n c e f i e l d a n d t h e s t a t i c Ho - f i e l d . W i t h H o in [100] d i r e c t i o n , m e a s u r e m e n t s w e r e d o n e on a l l s e v e n E P R t r a n s i t i o n s . F i g . 1 s h o w s a s p e c t r u m of C d F 2 : E u 2+. T h e s p e c t r a a r e d e s c r i b e d by t h e s p i n H a m i l t o n i a n a s u s e d by B a k e r [7],
M.-3/2
01~
M--5/2
O~2X,
~Ho~"~2
H = -gn~nHoel + eA sSCl + + ( ( A p + CAd) ( 3 S z e l z
-S el),
w h e r e ~ i n d i c a t e s t h e p o s i t i o n of t h e f l u o r i n e n u c l e i . On t a b l e 1, t h e c o n s t a n t s m e a s u r e d a r e c o m p a r e d w i t h t h o s e of C a F 2 : Eu 2+ [3]. T h e i s o t r o p i c p a r t of t h e i n t e r a c t i o n w i t h the f i r s t s h e l l i s m a i n l y c a u s e d by s p i n p o l a r i z a t i o n o f t h e o u t e r 5s a n d 5p e l e c t r o n s of t h e Eu z + - ion, w h i c h i s due to e x c h a n g e i n t e r a c t i o n w i t h t h e 4 f e l e c t r o n s [4]. A c c o r d i n g to t h e l a t t i c e c o n s t a n t b e c o m i n g s m a l l e r , 1A s b e c o m e s m o r e n e g a t i v e in C a F 2 t h a n i n S r F 2. S u r p r i s i n g , h o w e v e r , i s t h a t 1A s b e c o m e s l e s s n e g a t i v e f r o m C a F 2 to C d F 2. T h e s a m e t e n d e n c y w a s o b s e r v e d in C d F 2 Gd 3+ a n d C a F 2 : G d 3+ [5,6].
344
"'1 [31g
I
D1
I
15
I
I
16
I
I
17
[
I
18
]
i ~.2
19 MHz
Fig. 1. ENDOR of Eu 2+ with F - in CdF 2 :Eu 2+
Ho//[lO0 ] , M = - ~ ' ~ - ~ , m 153 = - 2~ ; T= 4.2°K, 7 = 1 sec. (For E P R o f CdF 2 : Eu2 + see ref. 2)
Volume 30A, number 6
PHYSICS
LETTERS
B e c a u s e of s m a l l c o v a l e n t c o n t r i b u t i o n s , h o w e v e r , it i s d i f f i c u l t to d e t e r m i n e the l a t t i c e d i s tortions quantitatively. S r F 2 : E u 2+ s h o w s the v a l u e of the d i p o l e d i p o l e i n t e r a c t i o n f o r ~ A p + 2Ad~ w i t h i n the l i m i t s of e r r o r . S i n c e 8r "+ and Eu 2+ h a v e an a l m o s t i d e n t i c a l i o n i c r a d i u s , l a t t i c e d i s t o r t i o n s s h o u l d not o c c u r . F u r t h e r m o r e , the c o v a l e n t p a r t s should be s m a l l e r b e c a u s e of the l a r g e r l a t t i c e c o n s t a n t . T h i s r e s u l t , t h e r e f o r e , i s to be e x p e c t e d . T h e e x p e r i m e n t a l (Ap + Ad) v a l u e s of the f l u o r i n e n u c l e i in the t h i r d a n d f o u r t h s h e l l a r e in good a g r e e m e n t a l s o with the v a l u e s c a l c u l a t e d for point dipoles.
Table 1. Eu 2+ 1A s
CdF 2 - 1.94
CaF2[3] - 2.23
SrF 2 - 1.93 3.80
(lAp + l a d )
4.10
4.01
1A d (calc.)
5.825
5.604
4.675
2As
0.012
- 0.007
0.000
(2Ap + 2Ad)
0.815
0.785
0.666
2A d (cale.)
0.830
0.798
0.666
3As (3Ap + 3Ad)
0.000 0.366
-
0.000 0.293
3Ad (ealc.)
0.366
0.352
0.293
4As (4Ap + 8Ad)
0.000 0.216
-
0.000 0.173
4A d (calc.)
0.216
0.208
0.173
ao
5.39
5.46
5.80
17 November 1969
T h e a u t h o r i s i n d e b t e d to P r o f e s s o r D r . B. E l s c h n e r f o r h i s i n t e r e s t in t h i s w o r k and s t i m u l a t i n g d i s c u s s i o n s ; to M r . K. B a b e r s c h k e f o r his c o - o p e r a t i o n ; to P r o f e s s o r D r . S. H u f n e r f o r s u p p l y i n g a S r F 2 : Eu 2+ c r y s t a l ; to M r . W. - J . B e c k e r f o r the u s e of h i s c o m p u t e r p r o g r a m , and to the D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t f o r p r o v i d i n g p a r t of the e q u i p m e n t .
Units ~fA s, An andAA are expressed in MHz, those of a o in A. The e~rors o~'the constants 1A amount to * 10 kHz, of 2A, 3A and 4.4 to ~: 5 kHz. The calculated values of the dipole interaction constants A d are based upon an undistorted lattice, a o is the lattice constant.
References 1. K. Baberschke, W. Rhein, G. Klein and R. Valentin, Z. Angew. Physik 25 (1968) 265. 2. H.J. Gl~iser and D. Geist, Z. Naturforsch. 23a (1968) 1980. 3. J. M. Baker and J. P. Hurrell. Proc. Phys. Soc. 82 (1963) 242. 4. R. E. Watson and A. J. F r e e m a n , Phys. Rev. Letters 6 (1961) 277. 5. R. H. Borcherts, T. Cole and T. Horn. J. Chem. Phys. 49 (1968) 4880. 6. H.Bill, Phys. Letters 29A (1969) 593. 7. J . M . B a k e r , J . P h y s . C 1 (1968) 1670.
On a c c o u n t of the l a t t i c e c o n s t a n t b e c o m i n g s m a l l e r , ( 1 A n + l A d ) i n c r e a s e s f r o m S r F 2 t~ C d F o . A s l i g h t l y p o s i t i v e v a l u e i s found f o r "~A_ in C d F 2 : Eu 2+, in a c c o r d a n c e w i t h C d F 2 : Gd 3+~ T h e v a l u e s of (2A u + 2A d) in C d F 2 and C a F 2 i s 15 k H z r e s p . 13 k H z l o w e r than the v a l u e c a l c u l a t e d f o r a puriely d i p o l a r i n t e r a c t i o n . A c c o r d i n g to r e f . 7, s u c h d e v i a t i o n s m a y be b a s e d upon l a t t i c e d i s t o r t i o n s , c a u s e d by the r e p l a c e m e n t of the Cd 2+ r e s p . Ca 2+ - i o n by the l a r g e r Eu 2+ - i o n . ***
*
*
345