Optical properties of SeO43− centres in NH4H2PO4 single crystals

0038-1098/88 $3.00 + .00 Pergamon Press plc

Solicl State Communications, Vol. 65, No. 12, pp. 1621-1623, 1988. "Printed in Great Britain.

OPTICAL PROPERTIES OF SeO~- CENTRES IN NH4H2PO4 SINGLE CRYSTALS J.K. Rath Department of Optics, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700032, India and S. Radhakrishna Department of Physics, Indian Institute of Technology, Madras 600036, India

(Received 21 May 1987 by C.W. McCombie) The optical absorption of the y-irradiated SeO2- doped KDP crystal is studied. EPR and optical absorption studies are correlated to identify the optical absorption band of SeO3- radical. A strong coupling of the electronic states and the totally symmetry stretching mode of vibration is found to be present.

1. INTRODUCTION THE NON-TRANSITION metal oxyanions are generally colourless. Hence, not many reports are available on the optical absorption of these complexes. SeO2- ion is colourless as well as diamagnetic. However, earlier EPR studies on the y-irradiated SeO42- doped NH4HzPO4 (ADP) crystals [1] have showed the formation of paramagnetic centres of SeO43- and SeO3 on irradiation. In the present study the optical properties of the above paramagnetic centres are investigated. 2. EXPERIMENTAL Single crystals of SeO2- doped ADP were grown by the slow evaporation of the saturated solution. K2SeO4 was used as dopant. The presence of the

impurity was detected by using EPR technique. EPR spectra were recorded using AEG 20 spectrometer. The optical absorption spectra were recorded with Cary-14 spectrophotometer. For recording optical absorption spectra at liquid nitrogen temperatures, a bath type cryostat was used. 3. RESULTS AND DISCUSSION EPR technique was used to confirm the presence of SeO42- in the ADP lattice. According to earlier report [1], when the SeO~- doped ADP crystals were subjected to y-irradiation, paramagnetic centres SeO43- and SeOf were produced in the lattice. The EPR spectra showed the presence of these radicals. In the present investigation the EPR spectra of the y-irradiated SeO2- doped ADP crystals were recorded and

HIIC

D

C

A

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Fig. 1. EPR spectrum of y-irradiated ADP: SeO2- single crystals at RT [H,C]. A: hyperfine lines of SeOlcenbtre; B,C: hyperfine lines of SeO; centre; D: EPR lines of the spinless isotopes of selenium radicals SeO~and SeO3. 1621

NH4H2PO4 SINGLE CRYSTALS

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compared with the reported spectrum [1]. Figure 1 shows the EPR spectrum of the 7-irradiated crystal when the magnetic field is along c-axis of the crystal. Comparing with the the reported spectrum [1], the group of line A are attributed to the hyperfine lines of SeO]- radical and the other sets B and C are attributed to the hyperfine lines of SeO~- radical. Each spectrum of SeO~- and SeO~- splits into two lines by the hyperfine interaction with 775e ( l = 1/3 and nat-

1

.

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25 1

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2600

3000

3400 3800 WAVELENGTH (~)

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Fig. 2. Optical absorption spectra of 7-irradiated ADP: SeO4z- single crystals. Curve a: spectrum recorded at room temperature. Curve b: spectrum recorded at 80 K.

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ural abundance is 7.5%). The intense spectrum observed in the central part of the figure contains the spectra of A, B and C with the spinless isotopes of Se. The above observation confirms the presence of SeO42- in ADP lattice before irradiation. The optical absorption spectrum of the SeO42doped ADP crystal before irradiation does not show any absorption band. Figure 2(a) shows the optical absorption of the crystal after irradiation with ),-rays. It is noticed that after irradiation with 7-rays, there is a growth of a new optical absorption band at 3000 A. In order to establish the species responsible for the optical absorption band, the correlation o the optical and EPR spectra has been carried out. Figure 3(a) and (b) show the growth characteristics of the EPR lines and the 3000/~ optical absorption band respectively at different dosages of irradiation with 7-rays. It is observed that the intensity of the EPR line due to SeO~- radical increases with the increasing dosages of irradiation reaching a maximum after 4 h. Thereafter, any further irradiation causes decrease in the intensity of this line so that after 18 h of irradiation the intensity almost drops to zero. But the optical band and SeO43- EPR line grow in identical manner and reach a maximum at 8 h of irradiation. Thereafter the intensities decrease slowly compared to the decay of the SeO3 EPR line. From the above study the optical absorption band at 3000 A may be attributed to the SeO43- radical. At liquid nitrogen temperature (80 K) the optical band was found to show a considerable vibration structure with an average spacing of 681 cm- l between

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4 8 12 16 Time of irradiation (Hours) (b)

I 20

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Ti'ne of

I 12 irradiation

I

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16

20

(Hours)

(a)

Fig. 3. Growth characteristics of the optical absorption band and the EPR lines of y-irradiated ADP: SeO~single crystals (a) for optical band; (b) for EPR lines.

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the successive maxima (Fig. 2(b)). The tetrahedral electronic states of the SeO~- molecules involved in molecule SeO~- has four normal modes of vibration the transition. [2], Aj (883 cm-l), E (335cm-1), T1 (875 cm -~) and T2 (432 cm -~). The optical band at 3000 A is an intense REFERENCES one, a characteristic of an orbitally allowed transition. The average spacing between the successive maxima T. Kawano, J. Phys. Soc. Japan 37, 848 (1974). of the vibrational structure is of the same order as the 2. K. Nakamoto, InfraredSpectra of Inorganic and totally symmetric stretching vibrational frequency. Co-ordination Compounds Wiley-Interscience, This may be attributed to the coupling of the totalll New York (1970). symmetric stretching mode of the vibration with th .