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RADIATION DEFECT AND 3d IMPURITY ABSORPTION IN MgF2 AND KMgF3 CRYSTALS
W. Sibley, S. Yun, L. Feuerhelm
To cite this version:
W. Sibley, S. Yun, L. Feuerhelm. RADIATION DEFECT AND 3d IMPURITY ABSORPTION IN MgF2 AND KMgF3 CRYSTALS. Journal de Physique Colloques, 1973, 34 (C9), pp.C9-503-C9-506.
�10.1051/jphyscol:1973984�. �jpa-00215460�
JOURNAL DE PHYSIQUE CoNoque 0, suppl6inent ali no 1 1 - 12, Tome 34, Nooembre-Dgcembre 1973, page C9-503
RADIATION DEFECT AND 3d IMPURITY ABSORPTION IN MgF, AND KMgF, CRYSTALS (*)
W. A. S I B L E Y , S. I. Y U N a n d L. N. F E U E R H E L M Physics D e p a r t m e n t , O k l a h o m a State University
Stillwater, O k l a h o m a 74074, U S A
RCumC. - Dans MgF, et KMgF,, M g ? . a approximativement la symetrie de site octaedrique.
Lorsque M g ? + est remplace par des ions 3d leurs transitions optiques sont rkgies par leurs etats de spin et leur symetrie. Lorsque des echantillons sont irradiks par des electrons, des centres F forrnes migrent vers les sites d'impurete o h ils sont pieges. Ce processus Cleve l'interdiction de symetrie des transitions et en meme temps couple les fonctions d'onde du centre F aux fonctions d'onde de I'inipurete pour Clever I'interdiction de spin. Des variations de la force de I'oscillateur de 103-105 peuvent Etre observees dans certaines transitions. Nos etudes niontrent que I'interdiction de symetrie est une regle de selection tres faible. L'accroissement d'absorption dfi a l'irradiation permet certaines suppositions sur les differentes transitions optiques des complexes ion Ctranger 3d centre F. En outre, nous mentionnons comment ces inipuretes affectent la production de defauts stables d'irradiation et comment les transitions optiques peuvent etre utilisees pour rechercher I'emplacement des interstitiels induits par irradiation.
Abstract. - In both MgF2 and KM,FI Mg?;- has approximately octahedral site symmetry.
When 3d ions are substituted for M g l t ions their optical transitions are governed by their spin states a n d symmetry. Since many transitions are both spin and symmetry forbidden, the oscillator strengths are extremely low (10-7). This is a severe limitation for most investigations and requires that long crystals be used. When saniples are irradiated with electrons F centers are produced which migrate to the impurity sites and are trapped. Such a process lifts the symmetry forbiddeness of the transitions and a t the same time couples the F center wave functions with the impurity wave functions t o lift the spin forbiddeness. Changes in oscillator strength of 103-105 can be observed for certain transitions. Our studies indicate that symmetry forbiddeness is a very weak selection rule. The increased absorption due to irradiation allows tentative assignments to be made for the various optical transitions of the 3d impurity ion F center coniplexes. I n addition, mention will be made of how these impurities affect the production of stable radiation damage defects and how the optical transitions can be used to probe for tlie location of radiation-induced interstitials.
1. Introduction. - Recently it was f o u n d t h a t the optical a b s o r p t i o n d u e t o 3d transition-ion impurities c a n b e e n h a n c e d several o r d e r s o f magnitude by t h e presence o f radiation induced F centers (Vehse a n d Sibley, 1972 a n d K a p p e r s et al., 1972). T h i s e n h a n - c e m e n t occurs because d u r i n g irradiation tlie F centers migrate t o i m p u r i t y sites a n d a r e t r a p p e d o r they a r e p r o d u c e d n e x t t o impurities initially. I n either case t h e s y m m e t r y a n d spin forbiddeness o f t h e impurity transitions is lifted. T h e p u r p o s e o f this p a p e r is t o c o m p a r e t h e radiation induced spectra f r o m K M g F , : N i a n d M g F , : N i a n d f r o m K M g F , : M n a n d M g F , : M n . I n addition, t h e radiation d a m a g e processes in these t w o types o f c r y s t a l s will be discussed.
2. Experimental procedure. - T h e crystals used f o r these experiments were purchased f r o m tlie H a r s h a w Chemical C o m p a n y a n d tlie O p t o v a c C o m p a n y o r were grown by tlie Stockbarger method.
T h e i m p u r i t y concentrations were determined by
(*) Work supported hy NSF grant G P 29545
mass spectrometry o r f r o m the optical a b s o r p t i o n d u e t o the spin-allowed transitions of N i l f a n d the known oscillator strengths o f these transitions ( F e r - guson, 1968 ; Ferguson et al., 1964 ; Balkanski a n d M o c k , 1964 a n d Ferguson a n d Guggenheini, 1966).
T h e samples were irradiated with I .5 M e V electrons a t various dose rates. Optical a b s o r p t i o n measurements were m a d e with a C a r y 14 spectropliotometer a n d eniission was monitored using a one-meter Czerny- T u r n e r m o n o c h r o m a t o r a n d a RCA C 31034 multi- plier phototube. Excitation spectra were obtained by monitoring t h e emission with the one-meter m o n o - c h r o m a t o r a n d exciting witli light f r o m a Spex 22 c11i no no chroma tor. Sample measurements a t low tem- peratures were m a d e with either a Displex helium refrigerator o r a Sulfrian cryostat.
3. Results. - Figure I shows a plot of tlie maximum increase in optical a b s o r p t i o n in pure a n d impurity doped MgF, crystals for tlie 230-270 n m spectral region r.ernis radiation dose. T h e impurity concen- tration in t h e doped crystals varied between 1-2 a t o m i c
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973984
C 9-504 W. A. SIBLEY, S. I. Y U N A N D L. N. FEUERHELM
/ IRRADIATED AT 3 0 0 K /
co2',
PURE N , ~ + , PURE
PURE
FIG. 1. - A plot of the maximum absorption coefficient in the range 230-270 nm for pure and doped MgF2 versus radiation dose. Three different radiation intensities were used and these
are shown o n the figure.
"/;; and the types of impurities are indicated on the
figure as are the various radiation intensities. Notice that initially in each case the absorption coefficient of the doped specimens increases with radiation faster than that of the pure specimens. This is also observed when the samples are irradiated at 77 K ; however, in this case, the MgF, : Mn crystals show a saturation of the absorption coefficient with radia- tion dose at about 20 cm-l.
The optical absorption spectra for unirradiated (dashed curves) and irradiated (solid curves) Ni2+
doped KMgF, and MgF, are shown in figure 2.
FIG. 2. - The absorption of unirradiated (dashed curves) and irradiated (solid curves) KMgF3 : Ni and MgF2 : Ni.
Note the difference i n the measurement temperatures.
The a spectrum for MgF, : Ni was taken using unpolarized light propagating along the c axis of the crystal. The sharp line structure evident on the 1 eV absorption band appears greater for MgF, : Ni simply because this spectrum was taken at 12 K and that for KMgF, : Ni was made at 77 K.
In the case of ~ n ' + doped crystals excitation spectra yield more direct information about the M ~ ' + - F center absorption than do direct absorption measu- rements. Excitation spectra for the Mn-doped mate-
rials are shown in figure 3. These data were taken by monitoring the intensity of the emission due to the Mn2+-F center complex (these bands are shown on the right side of the figure) as the sample is excited
W A V E L E N G T H l n m 1
2 5 0 3 0 0 4 0 0 6 0 0 6 5 0 7 0 0 8 0 0
3 - - r - - T - I 1 1 i 1 1
KMgF, Mn IRRADIATEO AT 3 0 0 K
- ,- 1 EXCITATION SPECTRUM
EMISSION -
m a
77K
1 M q F M n CI , IRRADIATED AT 3 0 0 K 1
5 0 9 0 3 0 2 0 19 17 15
PnOTON ENERGY(eV1
FIG. 3. - Emission and excitation spectra for irradiated KMgF, : Mn and MgF2 : Mn.
with light of different wavelengths. Even though the crystal structures of MgF, and KMgF, are quite different the spectra are very similar.
The MgF, crystal structure is such that close packed rows of like ions do not exist. This precludes the possibility of long range focussing collisions (Pooley, 1966 and Sonder and Sibley, 1972) and suggests that at low temperatures where interstitials are not mobile the interstitial may be trapped next to an Mn2'-F center complex (Buckton and Pooley, 1972). This type of event should cause the spectrum shown in figure 36 to be altered. Such an alteration is illustrated in figure 4. This figure shows excitation
W A V E L E N G T H ( n r n l
- 250 300 4 0 0 5 0 0 6 0 0
"l - 7 -
1 -
5 6 - M q s Mn CI
3
$ 4 - I R R A D I A T E D AT 300K
(L t
E 2 -
- >
t 0 /h
"l
6 -
MqF2 Mn CI
1
( b ) -
z
3 4 - I R R A D I A T E D AT 7 7 K z
W
3 0 2 0 PHOTON E N E R G Y ( e V 1
FIG. 4. - Excitation spectra measured a t 77 K for irradiated MgF2 : Mn crystals cut with the c axis perpendicular to the optical face (c I). The sample irradiated at 77 K was not warmed
prior to measurement.
spectra for two MgF, : Mn samples, cut as next neighbours from the crystal ingot, which were irra- diated at 300 K and 77 K respectively and then measured at 77 K.
RADIATION DEFECT A N D 3d IMPURITY ABSORPTION IN MgF2 AND KMgF3 CRYSTALS C9-505
Peak positions and oscillator strengths of ~ n transitions ~ '
Assignment KMnF3 77 K KMgF3:Mn 77 K MnF2 300 K MgF2:Mn 300 K
I r r a d i a t e d a t 77 K I r r a d i a t e d a t 300 K (oh) Peak O s c i l l a t o r Peak O s c i l l a t o r Peak
O s c i l l a t o r Peak O s c i l l a t o r 6 6 p o s i t i o n s t r e n g t h p o s i t i o n s t r e n g t h p o s i t i o n s t r e n g t h p o s i t i o n s t r e n g t h
')-- nm(cm-l) ~ ( x I o - ~ ) nm(cm-l) f ( ~ 1 0 - ~ ) nm(cm-l) ~ ( x I o - ~ ) nm(cm-l) ~ ( x I o - ~ )
Peak positions and oscillator strengtlis of Ni2+ transitions
Assignment KNiF3 300 K KMgF3:Ni 77 K NiF2 300 K MgF2:Ni 12 K
I r r a d i a t e d a t 77 K I r r a d i a t e d a t 300 K
(Oh) Peak O s c i l l a t o r Peak O s c i l l a t o r Peak O s c i l l a t o r Peak O s c i l l a t o r 3 3 p o s i t i o n s t r e n g t h p o s i t i o n s t r e n g t h p o s i t i o n s t r e n g t h p o s i t i o n s t r e n g t h
*zg( F)- m ( a - l ) f ( x ~ o - ~ ) m ( c m -1 ) f ( ~ l o - ~ ) n m c m - f ( x 1 0 - ~ ) nm(cm-l) f ( x 1 0 - ~ )
1 1
TZg( D) 463.0(21600) l . g C 450.0(22222) 12.ab 469.5 - 480.8 1.1 470.0(21277) 1 1 . 3
0.8 (21300 - 20800) v e r y weak
a no n o t i c e a b l e change a f t e r i r r a d i a t i o n .
b e f o r e i r r a d i a t i o n . o s c i l l a t o r s t r e n g t h a t 20 K.
C9-506 W. A . SIBLEY. S. I . Y U N A N D L. N. FEUERHELM 4. Discussion. - It is clear from figure 1 that
impurity doping enhances the radiation induced absorption in the spectral range around 260 nm which is the position of the F band in pure MgF,.
The observation that the 77 K and 300 K irradiations of MgF, : Co produce almost identical growth curves suggests that impurities can stabilize intersti- tials to prevent correlated interstitial-vacancy recombi- nation and thus enhance the growth curve.
From the data shown in figures 2 and 3 tentative assignments can be made for the 3d impurity-F center transitions. Some perturbation on the impurity due to the F center is expected and so we compare the peak positions for the M n 2 + transitions in KMnF, and MnF, (Stout, 1959 and Sibley et a / . , 1973) with those of the irradiated doped KMgF, and MgF,.
Table I gives the tentative assignments for M n 2 + type centers and table I1 for N i 2 + type centers (Fer- guson, 1968 and Ferguson et al., 1964). The oscillator strengths shown for the impurity-F center complexes are lower limits since in the estimate we assumed
that each impurity had an F center next to it. Even so it should be noted that the oscillator strengths for the con~plexes are orders of magnitude greater, in the case of M n 2 + , than those for the pure M n Z + transitions and some greater than those for the pure Ni2+ transitions.
The data illustrated by figure 4 indicate that when an interstitial is present next t o the impurity-vacancy complex certain of the impurity transitions are enhanced and in some cases split in energy. This is not unex- pected ; however, the magnitude of the change and the possibility of splitting energy levels indicates that these types of studies could prove t o be very powerful in our attempts to specify interstitial confi- gurations and to understand the effect of impu- rities on the damage process (Sonder and Sibley,
1972).
Acknowledgment. - The authors wish to express their appreciation to W. E. Vehse who supplied the KMgF, : Ni samples for this investigation.
References
BALKANSKI, M. and MOCK, P., J. Chem. Phys. 40 (1964) 1897. POOLEY, D., Proc. Phys. Soc. 87 (1966) 245 and 257.
BUCKTON, M. R. and POOLEY, D., J. Phys. C 5 (1972) 1553. SIBLEY, W. A., YUN, S. I. and VEHSE, W. E., J. Phys. C 6 (1973) FERGUSON, J., GUGGENHEIM, H. J. and WOOD, D. L., J. Chem. 1 1 0 5
Phys. 40 (1964) 822.
FERGUSON, J. and GUGGENHEIM, H. J., J. Clzern. Phys. 44 (1966) SONDER, E. and SIBLEY, W. A., In Point Defects in Solids, edited
1095. by J. H. Crawford and L. Slifkin (Plenum Press,
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