Identification of Suzuki-phase in CsCl: Eu2+, Mn2+ crystals

Solid State Communications, Vol. 69, No. 11, pp. 1109-1112, 1989. Printed in Great Britain.

0038-1098/89 $3.00 + .00 Pergamon Press plc

I D E N T I F I C A T I O N O F S U Z U K I - P H A S E IN CsCI:Eu 2+ , Mn 2+ CRYSTALS K.K. Tewari* and S.D. Pandey Physics Department, Kanpur University, P.P.N. College Campus, Kanpur 208 001, India and P. Chand Physics Department, Indian Institute of Technology, Kanpur 208 016, India

(Received 7 October 1988 by C.N.R. Rao) The emission and excitation spectra of Eu 2+ and Mn 2+ ions in close proximity and forming separate aggregated precipitates have been investigated in CsC1 crystal at room temperature by photostimulated luminescence. The associated aggregates of Eu 2÷ and Mn 2÷ ions are also identified by EPR. The Suzuki-phase aggregates of both Eu 2÷ and Mn 2+ dipoles are proposed to exist in as grown samples, through observance of emission bands at 434 nm and 530 nm, respectively. A radiative energy transfer from Eu 2+ to Mn 2÷ ions is also observed. 1. I N T R O D U C T I O N N U M E R O U S studies have been made in alkali halides singly doped with divalent cations. The substitutional divalent ions create an equal number of host cation vacancies for charge neutrality and can be studied by various techniques such as optical [1, 2], EPR [3], ionic thermo current (ITC) [4] and ionic conductivity [5]. The (I-V) dipoles of impurities could aggregate to form various stable and/or metastable aggregates/ precipitates. An ordered metastable aggregate of (I-V) dipoles so called the Suzuki-phase, has been studied by various techniques including optical [1, 2] emissionexcitation and EPR [3]. Recently, optical spectroscopic studies in NaC1 crystals doubly doped with divalent cations (Eu 2+, Mn 2+ ; Pb 2+ , Mn 2+ ) have been undertaken [6, 7]. The interesting result of non-radiative energy transfer from one type of divalent impurity ion to another has been reported [6, 7]. It may be rather interesting to investigate possibility of such an energy transfer in other alkali halide hosts. In this communication accordingly we report the results of energy transfer from Eu 2+ to Mn 2+ ions in doubly doped CsCl single crystals. 2. E X P E R I M E N T A L Single crystals of CsC1 doubly doped with europium (0.0007mol %) and manganese (0.44mo1%) were grown by the Stockbarger technique. Samples of * Address for correspondence: Physics Department, DAV College, Kanpur 208 001, India.

suitable thickness for optical studies were cleaved from the as grown crystal and polished. Optical emission and excitation spectra were recorded with Spex model 1902 Fluorolog. The excitation source was a 150-W Xenon lamp. The luminescence was observed perpendicular to the direction of excitation. The excitation and emission spectra were corrected for lamp intensity and photomultiplier sensitivity, respectively. The EPR spectra were recorded on a Varian E-109 X-band spectrometer with 100 kHz field modulation. 3. RESULTS A N D DISCUSSION Figure 1 shows the RT emission spectra of a one month old CsCI:Eu 2+, Mn 2+ sample excited with 365 nm and 445 nm radiations. The emission spectrum excited with 365nm comprises mainly two bands peaking at 445 nm and 530 nm. The relative intensities of these bands are comparable. Further, the emission spectrum excited with 445 nm shows only one strong band peaking at 530 nm. The intensity of 530 nm band obtained with 445 nm excitation is nearly double as compared to the intensity of corresponding band obtained with 365 nm excitation. Figure 2 shows the RT excitation spectra for the 445nm and 530nm emissions. The spectra show a common excitation band at 365nm wavelength for both 445 nm and 530 nm emissions. Excitation spectrum for 530nm emission further has a very strong excitation at 445 rim. Figure 3 shows the EPR spectra of CsCI:Eu 2÷, Mn 2÷ sample at RT. The EPR spectrum of as grown

1109

1110

THE SUZUKI-PHASE IN CsCI:Eu 2+, Mn 2+ CRYSTALS

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Fig. 1. Emission spectra at RT of an as grown CsCI:Eu 2+, Mn 2÷ sample with 365nm excitation ( ) and 445nm excitation ( - - - ) .

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(n m) Fig. 2. Excitation spectra at RT of an as grown CsCI : Eu 2+, Mn 2+ sample for 445 nm and 530 nm emissions,

Vol. 69, No. 11

sample comprises a broad line with (peak to peak derivative width) AH = 48 G and g value 2.0588, and a relatively narrow line with AH = 28G and g value 2.0024. The absence [3] of resolved fine structure is indicative of impurity aggregates and the relatively narrow Lorentzian line shape, further, indicates the existence of strong exchange narrowing for some of the ions. The 445nm emission band could be further resolved into two bands masked under it. The peak positions of the resolved bands are at ~ 434 nm and ,-, 456 nm wavelengths. The earlier data reported in other Eu 2÷ doped alkali halides [8] confirms that the observed 445 nm emission band in the present system is due to Eu 2+ ions. Further, the structure of 365 nm excitation band for this emission resembles quite well with the absorption and excitation spectra of E u 2+ ions around 365nm in other alkali halide hosts [8]. This suggests that the 365nm wavelength is exciting the Eu 2÷ ions of the system under study and the 445nm emission is associated with the [4f6(5d) 4f7(8S7/2)] transition in Eu 2+ emission [8]. The two bands at 434nm and 456nm enveloped under the 445 nm band may now be attributed to several metastable or stable phases of aggregates and precipitates of Eu 2+ similar to the ones observed in other alkali halides [8, 9]. The detailed comparison of emission band shown in Fig. 1 with the emission bands of Eu 2+ in other alkali halides [1, 8, 9] allow us to assign the emission band peaking at 434 nm to the Suzuki phase aggregate of Eu 2+ vacancy dipoles and 456 nm band to a meta-stable precipitate of EuCl2-type. The 530 nm band could not be accounted for in terms of allowed transitions obtained from the energy levels of Eu 2+ ion [8]. Therefore, this emission should be attributed to Mn 2+ ion emission, and it is seen that this agrees well with 4 T I ( G ) ~ 6 A I ( S ) transition as per the energy level scheme of Mn 2+ reported by Mehra [10]. The EPR spectrum of Fig. 3 confirms the presence of precipitates and indicates that the impurity divalent ions are in close proximity to each other aggregated in the bulk of the crystal sample. The absence of fine structure in the EPR spectrum of as grown sample indicates that isolated ion-vacancy dipoles are not present in this sample. Absence of fine structure even in samples quenched from 240°C, further, indicates that aggregates were strong enough not to be broken upto this temperature. It may be mentioned here that the precipitation of impurities [3, 11] is also indicated through observed enhanced hardness in our doubly doped crystals of CsCI. The observance of two broad lines in the EPR spectrum, further, shows aggregation of both Eu 2+ and Mn 2+ impurities.

Vol. 69, No. 11

T H E S U Z U K I - P H A S E I N CsCI:Eu 2÷, Mn 2÷ C R Y S T A L S

1111

( (a)

(c)

(c)

16mT

I

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Fig. 3. EPR spectra at RT of CsCI : Eu 2+ , Mn 2+ samples: (a) as grown sample, (b) with 5 times increased gain than to case (a), (c) sample heated at 240°C for 5 min then quenched into cold air. It is seen that the 530 nm emission is being excited by both 365 nm and 445 nm wavelengths, the latter being more efficient (Fig. 2) and incidentally matching with the Eu 2÷ emission band. This emission can, therefore, be enhanced either on direct excitation by 4 4 5 n m or through indirect 365nm Eu 2+ excitation. The comparison of excitation spectra (Fig. 2) for 445nm and 530nm emissions suggests that some of the Mn 2+ centres are also being excited directly (thickline) by 365 nm radiation. It means that 530 nm emission is a combined effect of excitation of both Eu 2+ and Mn 2+ ions. However, contribution of Eu 2+ centres is more than Mn 2+ centres directly excited by 365 nm because excitation by 445 nm (Fig. 2) which is the Eu 2+ emission is quite strong. The europium emission is thus transferred to manganese and the system appears to provide a simple and efficient way of wavelength conversion. The energy transfer from Eu 2÷ to Mn 2÷ is of radiative type in the present system. It cannot be non-radiative, because for such an energy transfer from the sensitiser (donor) Eu 2+ to activator (acceptor) Mn z+ there should be no direct excitation of acceptor by the radiation in the emission band of the sensitiser, contrary to the present observations.

Therefore, in the present case the simplest mechanism for the energy transfer may be explained as follows. The Eu 2÷ ions are being mainly excited by 365nm wavelength to the 4f6(5d)configuration which decays to the 4f7(8S7/2) ground state [8] giving rise to 445 nm emission. This radiation subsequently excites the Mn 2+ ions to 4T~(G) state, the 530 nm emission being obtained [I0] through their decay to 6AI(S ) ground state. This 530 nm emission very nearly matches with the energy difference for the transition 4T~(G) 6A I (S) for Mn 2÷ ions and is thus related to the transfer of energy without any involvement of the host lattice. It may be due to an aggregated phase of Mn 2+ -dipoles most likely the Suzuki-phase reported for other doped alkali halides. In this work the Suzuki-phases of both Eu 2+ and Mn 2÷ vacancy dipoles are identified in the as grown samples through optical measurements. The emissions at 434 nm and 530 nm wavelengths are attributed to the Suzuki phases of Eu 2÷ and Mn 2÷ dipoles respectively. The parallel EPR measurements also indicate the presence of aggregates in our as grown sample. The emission and excitation spectra together reveal that the 445 nm europium emission is being transferred to manganese through radiative energy transfer. The

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THE SUZUKI-PHASE IN CsCI:Eu 2+, Mn 2+ CRYSTALS

system could thus also provide a simple way for an efficient wavelength conversion from 365 nm to 530 nm.

4. 5.

Acknowledgement - T h a n k s are due to Prof. D.D. Pant, Kumaon University, Nainital, for giving the facility of optical work.

6.

REFERENCES

8.

1. 2. 3.

J.A. Hernandez, W.K. Cory & J. Rubio O, J. Chem. Phys. 72, 198 (1980). F. Rodriguez, M. Moreno, F. Jaque & F.J. Lopez, J. Chem. Phys. 78, 73 (1983). G.D. Watkins, Phys. Rev. 113, 79 (1959).

7.

9. 10. 11.

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C. Bucci & R. Fieschi, Phys. Rev. 148, 816 (1966). J.A. Chapman & E. Lilley, J. De Physique Suppl. C9, 341 (1973). J. Rubio O, H.S. Murrieta, R.C. Powell & W.A. Sibley, Phys. Rev. B-31, 59 (1985). F. Jaque, C. Zaldo, F. Cusso & F.A. Lopez, Solid State Commun. 43, 123 (1982). H.S. Murrieta, J.A. Hernandez & J. Rubio O, Kinam 5, 75 (1983). F.J. Lopez, H.S. Murrieta, J.A. Hernandez & J. Rubio O, J. Lumin. 26, 129 (1981). A. Mehra, Phys. Status Solidi 29, 847 (1968). J.A. Chapman & E. Lilley, J. Mater. Sci. 10, 1154 (1975).