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On the luminescence of RbI : Pb2+ at liquid helium temperature
S. Sastry, S. Sapru
To cite this version:
S. Sastry, S. Sapru. On the luminescence of RbI : Pb2+ at liquid helium temperature. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-520-C6-521. �10.1051/jphyscol:19806136�. �jpa-00220045�
JOURNAL DE PHYSIQUE Colloque C6, suppliment au no 7, Tome 41, Juillef 1980, page C6-520
On the luminescence of RBI : Pb?+ at liquid helium temperature
S. B. S. Sastry and S. Sapru
Department of Physics, Indian Institute of Technology, Madras-600 036, India
Abstract. - Results on polarized luminescence measurements made on RbI :Pb2+ at 4.7 K and optical absorp- tion between LHeT and LNT are reported. From the luminescence data it is found that charge compensating vacancy (ccv) occupies near neighbour (28 %) and next near neighbour (72 %) sides indicating that as temperature goes down the ccv moves nearer to the impurity ion. This fact is also reflected in the optical absorption spectra of the crystal taken at these temperatures. The symmetric C-band has become asymmetric indicating that near octahedral symmetry of the impurity site has changed to lower symmetries.
1. Introduction. - Impurity ions like TIf, I n f , helium temperature (LHe) and some of the results P b 2 + , Sn2+, etc. with ns2 configuration, when doped are discussed here.
in alkali halides show absorption bands due to inira- ionic transitions, ns2-(ns) (np), in the otherwise transparent near UV region of the alkali halides.
(Prominent among these bands are denoted as A, B and C bands.) Among these the divalent impurity ions are of greater interest as these ions introduce into the crystal a charge compensating cationic vacancy (ccv) for maintaining the electrical neutrality of the crystal. One can distinguish two types of alkali halide-impurity systems : one having impurity ion smaller than the host ion it replaces and the other having an impurity ion larger in size than the host ion. NaC1:Pb2+ is a system belonging to the former group and is studied by several groups of workers. In a system where the host ion has a smaller size than the aliovalent impurity ion the ccv according to Dreyfus [2] will be predominantly in the near neighbour (nn) position and in a system where the impurity ion is smaller than the host ion the vacancy is a little farther, in next near neighbour (nnn) or third neighbour position. In our recent studies on impurity doped rubidium halides [3-51 we studied a system belonging to the latter group, namely RbI :Pb2' (Ionic radius of Rb+ is 1.47
A
and that of Pb2+ is 1.22 A). Optical absorption studies and dielectric loss measurements of this system showed interesting results [6]. It was indicated in these mea- surements that the ccv is predominantly in the third neighbour site though a small concentration of nn and nnn sites may also be occupied at temperatures around 450 K. In order to find out the site symmetry in such cases attempts have been made to study the polarized luminescence under excitation in the impu- rity band regions, in the case of NaC1 :Pb2+ [I] and KC1:Sn2+ [7].Polarized luminescence of RbI :Pb2+ has been studied under A and B band excitation at liquid
2. Experimental. - The experimental set up is the conventional one with a LHe cryostat described earlier by Scharmann et al. [8]. The crystals were studied in two configurations with crystal cut parallel to (100) and (110) planes. Optical absorption spectra were recorded by Cary 14 and Cary 17 spectrophoto- meters.
3. Result and discussion. - Under the B-band excitation (3 17 nm) the emission was observed around 411 nm. An interesting feature of this emission is that its peak shifts from about 408 nm to 412 nm as temperature increases from LHeT to 16 K [9]. Unfor- tunately the intensity of emission is. not enough to make a detailed study of its polarization characte- ristics. The emission under A-band excitation (349 nm) is at 420 nm and sharper than that observed for B- band excitation.
d (Degees I
Fig. I . - Azimuthal dependencc of polarization of Rbl :Pb2+ at 4.7 K under A-band excitation with excitation and observation in ( 100 ) direction. In set shows crystal orientation for parallel confi-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19806136
ON THE LUMINESCENCE OF RbI : Pb2+ AT LIQUID HELIUM TEMPERATURE C6-521
Figure 1 shows the azimuthal variation of pola- rization with crystal cut parallel to (100) and figure 2 shows its variation for crystal cut parallel t? (110) planes. The measurement configurations are as shown in the figures. The variation of polarization can be
Energy ieV)
Fig. 3. - Optical absorption spectra of RbI:Pb2'. a) 20 K and b) 8.7 K. Inset shows A-band absorption.
Fig. 2. - Az~muthal dependence of polarlzatlon of Rbl :Pb2' at 4.7 K under A-band excitahon with excitation in ( 100 ) or ( 110 ) direction and observation in
<
110 ) direction. In set shows crystal orientation for parallel configuration.estimated if one knows the site symmetry of the impurity [lo]. If a single site symmetry is assumed i.e., the ccv is in one site only (either C2 or C4), it is difficult to explain the observed shape. However a combina- tion of the two site symmetries (C4 and C2) are tried with appropriate phase difference. The full lines are such theoretically calculated polarization curves with about 72
%
sites having C4 symmetry (vacancy in the nnn position) and about 28%
with C2 symmetry (vacancy in the nn position). The fit is in good agree- ment in both cases with the same ratio of C4 and C2.This shows that at LHeT the nnn vacancies are more predominant than nn vacancies and the third neigh- bour vacancies, even if present, do not seem to effect the polarization characteristics of the luminescing centre.
Figure 3 shows typical optical absorption spectrum recorded at 20 K and 8.7 K. The 8.7 K spectrum has been shifted on the OD axis by 5 units for clarity.
The 20 K spectrum has been analysed by a nonlinear least square programme on IBM 370 computer. The broken lines show the analysed bands. We find that the B-band shows splitting below 70 K.
he
structure of the C-band is clearly revealed, but C1, C2 and C3 do not remain symmetric in contrast to the obser- vation at 77 K [6].The B-band corresponds to the transition 1A,,;3T2, and 3Eu and is vibration induced. Vibronic transitions being broad, this splitting is not normally observed.
In the present system the splitting is observed and should be due to the 1A,,-3T2u and 1A1,:3Eu transi- tions. In KC1 :Sn2+ and RbCI : Sn2+ simllar B-band splittings have been observed by Tsuboi et al. [ll].
In RbI : Pb2+ the splitting is observed only below 70 K, it could be due to some changes in the crystal field levels at these temperatures. Lowering of symmetry could lead to such splittings. One likely reason for this could be that the ccv moves nearer to the impu- rity ion (to nnn or even nn sites).
Acknowledgments. - The authors are grateful to Prof. C. Ramasastry for his very helpful discussion ; Prof. A. Scharmann for his interest in this work and Dr. D. Schwabe for his help in the polarization measurements. They are also grateful to Dr. Rolfe (N.R.C. Canada) for supplying some RbI :Pb2+
samples and Mr. Labbe (N.R.C.) for his help in taking optical absorption spectra.
References [I] COLLINS, W. C. and CRAWFORD Jr., J. H., Phys. Rev. B 5
(1972) 633.
[2] DREYFUS, R. W., Phys. Rev. 121 (1961) 1675.
[3] SASTRY, S. B. S. and BALASUBRAMANIAM, K., (i) Phys. Status Solidi (a) 47 (1978) 711. (ii) Phys. Status Solidi (b) 90 (1978) 375.
[4] SASTRY, S. B. S, and SAPRU, S., Phys. S t a m Solidi (a) 48 (1978) K189.
[5] SASTRY, S. B. S. and SAPRU, S., Proc. Symp. Nucli. Soli. Phys.
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[6] SASTRY, S. B. S. and BALASUBRAMANIAM, K., J. Phys. C.
Solid State Physics 11 (1978) 4213.
[7] COATSWORTH, L. L., JACOBS, P. W. M. and KAMISHINA, Y., Proc. Int. Nat. Conf. on Defects in Ion Solids, Gatlinburg (U.S.A.) (1977) 94.
[8] SCHARMANN, A., SCHWABE, D. and WEYLAND, D., J. Lum.
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191 SASTRY, S. B. S., SCHARMANN, A. and SCHWABE, D., Phys.
Status Solidi (b) 85 (1978) K15.
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