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Mössbauer isomer shifts of dilute Eu2+ ions
Volume 31A, number 2
I~HYSICS L E T T E R S
MOSSBAUER
ISOMER
SHIFTS
26 January 1970
OF DILUTE
E u 2+ I O N S
H. W. WICKMAN, M. ROBBINS, E. BUEHLER Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey, USA and E. CATALANO Lawrence Radiation Laboratory, L ivermore, California, USA Received 17 April 1969
The isomer shifts of Eu 2+ ions in CaS(CaF2) were found to increase (decrease) with increasing percent atomic concentration of Eu 2+ ions in the range 0 --/80% Sm - 151) was used. Samples with the g e n e r a l f o r m u l a CaS • xEuS and C a F 2 • x E u F 2 were p r e p a r e d by heating a p p r o p r i a t e m i x t u r e s of the components in s e a l e d , evacuated, m o l y b d e n um c o n t a i n e r s at t e m p e r a t u r e s g r e a t e r than 1300°C. X - r a y p a t t e r n s for unit cell p a r a m e t e r s were obtained with CuK~ r a d i a t i o n . In both Eu2+:CaF2 ( 1 ) a n d Eu2+:CaS, the s a m p l e s obeyed V e g a r d ' s r u l e with the l a t t i c e p a r a m e t e r s given above. In the c o n c e n t r a t i o n r e g i o n of i n t e r e s t for
-1"44I~\ -1.40
,~ ,
i - E
Eul+: CIFZ D UALETTA, lIT &L • [ 0 PRESENT WOAK
~.Z':c~s* ~[SENTw0m~ -1.3o
i
-;
-1.15
I
~o
ATOMIC % Eu2÷
Fig. 1, Isomer shifts of Eu2+:CaF2 and Eu2+:CaS versus percent atomic concentration of Et~~'+. Left-(right-) hand ordinate corresponds to Eu2+:CaF2, 77OK (Eu2+:CaS, 4.2°K). The right- and left-hand abscissae are broken at approximately the points of phase stability for CaS. xEUSo the M 6 s s b a u e r e x p e r i m e n t , the d o m i n a n t s t a t i s t ical d i s t r i b u t i o n of dopant is the isolated ion. At Eu 2+ c o n c e n t r a t i o n below ~ 4 at.%, and t e m p e r a t u r e s below 4°K, r e l a x a t i o n s p e c t r a and r e s o l v e d p a r a m a g n e t i c hfs w e r e observed. T h e s e s p e c t r a will be d i s c u s s e d e l s e w h e r e . The r e s u l t s of the e x p e r i m e n t s a r e s u m m a r ized in the figure. Note that an i n c r e a s e in i s o m e r shift c o r r e s p o n d s to an i n c r e a s e in net s e l e c t r o n d e n s i t y at the Eu2+-151 nucleus. As noted e a r l i e r , the r e l a t i o n ao(EuS)/ao(CaS ) > 59
Volume 31A, number 2
PHYSICS LETTERS
> a(EuF2)/a(CaF 2) would naively indicate a g r e a t e r lattice p r e s s u r e at low c o n c e n t r a t i o n in CaS and hence a s i m i l a r , or p o s s i b l y enhanced, effect to that found in C a F 2. The o b s e r v a t i o n of an opposite effect s u g g e s t s additional m e c h a n i s m s m u s t be considered. Although CaS and C a F 2 a r e both fcc, they differ in thai the divalent coordination in the f o r m e r is s i x - f o l d o c t a h e d r a l , and in the l a t t e r is eightfold cubic. This d i f f e r e n c e in c o o r d i n a t i o n may be significant if a d e c r e a s i n g lattice size l e a d s to i n c r e a s i n g c r y s t a l or ligand field s t r e n g t h s at the Eu 2+ locus. The differing c o o r d i n a t i o n l e a d s to a r e v e r s a l in sign of the f o u r t h - o r d e r and l e a d ing s i x t h - o r d e r t e r m s in the cubic potential. It i s r e a s o n a b l e to expect a qualitative change of this n a t u r e to lead to different i s o m e r shift t r e n d s , provided that s h i e l d i n g effects r e s u l t i n g f r o m ligand bonding to d or f o r b i t a l s a r e p r e s e n t . That such effects may produce shifts of the size r e ported here may be s e e n f r o m the known l a r g e changes of up to 1.5 c m / s e c that r e s u l t f r o m l o s s of an f - e l e c t r o n in the oxidation of Eu 2+ to Eu 3+
[2]. Because the Eu 2+ ion is an S-state, with negligible zero-field splittings, it is likely that the 5d orbitals dominate the inner 4f orbitals with respect to bonding with the ligands [3,4]. With this assumption the present m e a s u r e m e n t s give information on the relative importance of ~ and 7r bonding invloving normally unoccupied 5d levels. It is easy to show that both PTr "-"5dt2 and P~r ~ 5de~, ligand donation will be influenced by changing~ligand field strengths, and that in both cases opposite trends are expected in 6-fold versus 8-fold coordination. However, one finds that only the p~ ~ S d t 2 type donation is consistent with the experimental results. It appears therefore that the net ligand donation, in the present approximation is dominated by ~ effects. The influence of bonding is, of course, not restricted to dilute E u 2+ ions and similar effects
60
26January1970
(perhaps including nonmonotonic v a r i a t i o n s ) should be o b s e r v a b l e in other l a t t i c e s containin~ dilute M S s s b a u e r atoms. F u r t h e r , the change in i s o m e r Shift may be r e f l e c t e d by c o r r e s p o n d i n g v a r i a t i o n s in spin d e n s i t y which c o n t r i b u t e to m a g n e t i c hyperfine i~iteractions in the dilute s p e c i e s . E P R hfs s t u d i e s would appear m o r e s e n s i t i v e to this v a r i a t i o n than the M S s s b a u e r technique. In c o n c l u s i o n we note that the i s o m e r shift suggested by an e x t r a p o l a t i o n of the low c o n c e n t r a t i o n shifts to higher c o n c e n t r a t i o n s a p p r o a c h ing x = 100 yields an i s o m e r shift close to that o b s e r v e d in E u ~ . T h i s situation is not t r u e , bowever, in Eu~'~:CaS. This d i f f e r e n c e may be du to band effects and m a g n e t i c exchange i n t e r a c tions which a r e operative at higher concentration~ of Eu 2+ ions in CaS. A t t e m p t s to p u r s u e this poim were l i m i t e d by the finding that solid solutions of CaS • x E u S with 20 ~ x ~ 8 0 are not formed with our present preparation method. However, a sample with x = 80 showed an isomer shift of - 1.250 ± 0.004 c m / s e c versus - 1.260 ± 0.004 for EuS. Here, the origin of this relative change is not obvious, since one m a y look for a direct 4f~---Sd magnetic interaction between E u 2+ ions [3] tojead to a decreased shielding as the numbez • of Eu Z + neighbors is increaed between x = 80 and x = 100. The data show, however, a small decrease of I~(0)[ 2 between these two points. It is a p l e a s u r e to thank C. F. W a g n e r for exp e r i m e n t a l a s s i s t a n c e in the c o u r s e of these measurements.
Refe~'ences 1. H. Maletta, W. Heidrich and R. L. M~ssbauer, Phys. Letters 25A (1967) 295. 2. P. Brix, S. Hflfner, P. Kienle and D. Quitman, Phys. Letters 13 (1964) 140. 3. S. Methfessel, Z. Angew. Phys. 18 (1965) 414. 4. J. Danon, J. Phys. Chem. Solids 27 (1966) 1953.
I~HYSICS L E T T E R S
MOSSBAUER
ISOMER
SHIFTS
26 January 1970
OF DILUTE
E u 2+ I O N S
H. W. WICKMAN, M. ROBBINS, E. BUEHLER Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey, USA and E. CATALANO Lawrence Radiation Laboratory, L ivermore, California, USA Received 17 April 1969
The isomer shifts of Eu 2+ ions in CaS(CaF2) were found to increase (decrease) with increasing percent atomic concentration of Eu 2+ ions in the range 0 --
-1"44I~\ -1.40
,~ ,
i - E
Eul+: CIFZ D UALETTA, lIT &L • [ 0 PRESENT WOAK
~.Z':c~s* ~[SENTw0m~ -1.3o
i
-;
-1.15
I
~o
ATOMIC % Eu2÷
Fig. 1, Isomer shifts of Eu2+:CaF2 and Eu2+:CaS versus percent atomic concentration of Et~~'+. Left-(right-) hand ordinate corresponds to Eu2+:CaF2, 77OK (Eu2+:CaS, 4.2°K). The right- and left-hand abscissae are broken at approximately the points of phase stability for CaS. xEUSo the M 6 s s b a u e r e x p e r i m e n t , the d o m i n a n t s t a t i s t ical d i s t r i b u t i o n of dopant is the isolated ion. At Eu 2+ c o n c e n t r a t i o n below ~ 4 at.%, and t e m p e r a t u r e s below 4°K, r e l a x a t i o n s p e c t r a and r e s o l v e d p a r a m a g n e t i c hfs w e r e observed. T h e s e s p e c t r a will be d i s c u s s e d e l s e w h e r e . The r e s u l t s of the e x p e r i m e n t s a r e s u m m a r ized in the figure. Note that an i n c r e a s e in i s o m e r shift c o r r e s p o n d s to an i n c r e a s e in net s e l e c t r o n d e n s i t y at the Eu2+-151 nucleus. As noted e a r l i e r , the r e l a t i o n ao(EuS)/ao(CaS ) > 59
Volume 31A, number 2
PHYSICS LETTERS
> a(EuF2)/a(CaF 2) would naively indicate a g r e a t e r lattice p r e s s u r e at low c o n c e n t r a t i o n in CaS and hence a s i m i l a r , or p o s s i b l y enhanced, effect to that found in C a F 2. The o b s e r v a t i o n of an opposite effect s u g g e s t s additional m e c h a n i s m s m u s t be considered. Although CaS and C a F 2 a r e both fcc, they differ in thai the divalent coordination in the f o r m e r is s i x - f o l d o c t a h e d r a l , and in the l a t t e r is eightfold cubic. This d i f f e r e n c e in c o o r d i n a t i o n may be significant if a d e c r e a s i n g lattice size l e a d s to i n c r e a s i n g c r y s t a l or ligand field s t r e n g t h s at the Eu 2+ locus. The differing c o o r d i n a t i o n l e a d s to a r e v e r s a l in sign of the f o u r t h - o r d e r and l e a d ing s i x t h - o r d e r t e r m s in the cubic potential. It i s r e a s o n a b l e to expect a qualitative change of this n a t u r e to lead to different i s o m e r shift t r e n d s , provided that s h i e l d i n g effects r e s u l t i n g f r o m ligand bonding to d or f o r b i t a l s a r e p r e s e n t . That such effects may produce shifts of the size r e ported here may be s e e n f r o m the known l a r g e changes of up to 1.5 c m / s e c that r e s u l t f r o m l o s s of an f - e l e c t r o n in the oxidation of Eu 2+ to Eu 3+
[2]. Because the Eu 2+ ion is an S-state, with negligible zero-field splittings, it is likely that the 5d orbitals dominate the inner 4f orbitals with respect to bonding with the ligands [3,4]. With this assumption the present m e a s u r e m e n t s give information on the relative importance of ~ and 7r bonding invloving normally unoccupied 5d levels. It is easy to show that both PTr "-"5dt2 and P~r ~ 5de~, ligand donation will be influenced by changing~ligand field strengths, and that in both cases opposite trends are expected in 6-fold versus 8-fold coordination. However, one finds that only the p~ ~ S d t 2 type donation is consistent with the experimental results. It appears therefore that the net ligand donation, in the present approximation is dominated by ~ effects. The influence of bonding is, of course, not restricted to dilute E u 2+ ions and similar effects
60
26January1970
(perhaps including nonmonotonic v a r i a t i o n s ) should be o b s e r v a b l e in other l a t t i c e s containin~ dilute M S s s b a u e r atoms. F u r t h e r , the change in i s o m e r Shift may be r e f l e c t e d by c o r r e s p o n d i n g v a r i a t i o n s in spin d e n s i t y which c o n t r i b u t e to m a g n e t i c hyperfine i~iteractions in the dilute s p e c i e s . E P R hfs s t u d i e s would appear m o r e s e n s i t i v e to this v a r i a t i o n than the M S s s b a u e r technique. In c o n c l u s i o n we note that the i s o m e r shift suggested by an e x t r a p o l a t i o n of the low c o n c e n t r a t i o n shifts to higher c o n c e n t r a t i o n s a p p r o a c h ing x = 100 yields an i s o m e r shift close to that o b s e r v e d in E u ~ . T h i s situation is not t r u e , bowever, in Eu~'~:CaS. This d i f f e r e n c e may be du to band effects and m a g n e t i c exchange i n t e r a c tions which a r e operative at higher concentration~ of Eu 2+ ions in CaS. A t t e m p t s to p u r s u e this poim were l i m i t e d by the finding that solid solutions of CaS • x E u S with 20 ~ x ~ 8 0 are not formed with our present preparation method. However, a sample with x = 80 showed an isomer shift of - 1.250 ± 0.004 c m / s e c versus - 1.260 ± 0.004 for EuS. Here, the origin of this relative change is not obvious, since one m a y look for a direct 4f~---Sd magnetic interaction between E u 2+ ions [3] tojead to a decreased shielding as the numbez • of Eu Z + neighbors is increaed between x = 80 and x = 100. The data show, however, a small decrease of I~(0)[ 2 between these two points. It is a p l e a s u r e to thank C. F. W a g n e r for exp e r i m e n t a l a s s i s t a n c e in the c o u r s e of these measurements.
Refe~'ences 1. H. Maletta, W. Heidrich and R. L. M~ssbauer, Phys. Letters 25A (1967) 295. 2. P. Brix, S. Hflfner, P. Kienle and D. Quitman, Phys. Letters 13 (1964) 140. 3. S. Methfessel, Z. Angew. Phys. 18 (1965) 414. 4. J. Danon, J. Phys. Chem. Solids 27 (1966) 1953.