J O U R N A L OF M A T E R I A L S

SCIENCE LETTERS 1 (1982) 1"/-18

Electrical conductivity of BaS phosphors R. P. RAO, D. R. RAO Materials Science Centre, Indian Institute of Technology, Kharagpur 721 302, India

Luminescence and allied phenomena of alkalineearth sulphide phosphors activated with impurities such as Cu, Zn and Bi have been studied in the past by several workers [1-3] and the energy storage properties of this system have been attributed to the presence of native defects and their associated impurity complexes [4-6]. In order to "understand further the nature of the point defects in this system, work has been conducted [6] on BaS phosphors (a comparatively little studied alkaline-earth sulphide phospho 0 prepared in our laboratory using commercially available chemicals. From the studies on thermoluminescence (TL), after-glow decay and bleaching characteristics of ultra-violet, as well as X-ray, excited BaS phosphors (activated with impurities like Cu and Bi) it was concluded that a large number of impurity vacancy complexes, formed during the preparation of the phosphors, were responsible for the different features observed in the energy storage properties. The results reported in the present paper on the direct current (d.c.) conductivity of the pellets of these phosphors also gave supporting evidence for the existence of such complexes. Polycrystalline BaS powders were prepared by reducing pure BaSO4 with spectroscopic grade purity carbon conducting the reaction at 950°C for 2h. Doping with Bi was carried out in a manner similar to that of Cu in BaS : Cu, as given in an earlier paper [5]. The circular pellets (of diameter 10ram and depth 2ram) were made by compactix~g the powders (75/~m size) at room temperature applying a pressure of 825kgcm -2. Ohmic contacts were made by evaporating silver on to both the surfaces of the pellet. A suitable sample holder with spring-loaded silver electrodes was fabricated to hold the sample [6] in a vacuum chamber (--~ 10-3tort) and the d.c. conductivity was measured in the temperature range of 30 to 500 ° C using a high sensitivity EA812 electrometer amplifier.

Several samples of BaS were prepared, activating with different concentrations of Cu and Bi, and in each series conductivity measurements were made on at least four pellets. Some typical results on the electrical conductivity, ~r, of BaS, BaS0.0119wt%Cu, BaS-0.2324wt%Bi and BaS0 . 0 1 1 9 w t % C u - 0 . 1 1 5 7 w t % B i measured in the temperature range 30 to 500°C are shown in Fig. 1. The value of ~r at room temperature varies between 4 and 6 x 10 -13 ohrn -1 cm -1 depending on the sample and approaches 10-7ohm-lcm -1 at 500 ° C. The conductivity of doubly-doped samples (BaS-Cu, Bi) is high compared to the other compounds and increases rapidly with temperature. While in case of BaS-Cu phosphors the Arrhenius plot between a and T -~ exhibits two distinct regions, the a - T of BaS-Bi phosphors remains practically unchanged up to 200°C. The variation of a with T above 250°C is, however, similar in both the cases. The thermal activation energy, q, calculated using the relation a = ~roe-q/~T is found, depending upon the sample, to vary between 0.39 and 0.45 eV in the temperature region 30 to 260 ° C and from 0.81 to 1.18 eV in the temperature region 250 to 500 ° C. Since, in this study, the highest temperature to which the pellets are heated is much lower than the melting-point temperature (-~ 1200°C), the intrinsic region has not been reached and, thus, the observed conductivity may be associated with extrinsic region. In order to, explain the results of the TL decay and bleaching characteristics we have proposed "the formation of complexes such as [BiBa--VBa--BiBa ], [BiBa--CUBa ] and [Vs-CuBa]* in BaS-Bi, BaS--Cu, Bi and BaS-Cu phosphors, respectively. Considering the above proposed complex formation the variation of d.c. conductivity in the extrinsic region could also be understood. Essentially, as the temperature increases, breaking out of the complexes occurs facilitating the availability of charge carriers in the corre-

*BiBa,CUBa -~ Bi and Cu on Ba-sites and Vs, Vt~a--"anion and cation vacancies.

0261--8028/82/010017--02502.20/0

©1982 Chapman andHallLtd.

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Figure 1 Temperature variation of d.c. conductivity of (A) BaS, (B) BaS-Cu, (C) BaS-Bi and (D) BaS-Cu, Bi phosphors.

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sponding regions. In the case of doubly-doped samples the formation of neutral pairs like [BiBa-CuBa]° leads to the availability of a large number of anion as well as cation vacancies in the system according to the principle of charge compensation, and this may be made responsible for the high conductivities exhibited by these samples. The results of dielectric constant, K, and dielectric loss, tan 6, studied in the same temperature region for these samples could also be explained using the above proposed models [7].

References 1. 2. 3. 4. 5. 6. 7.

R.C. SAXENA and J. D. RANADE, Ind. J. Pure AppL Phys. 7 (1969) 817. W. LEHMANN, J. Luminescence 5 (1972) 87. H.I. VOOLAID, J. Appl. Spectroscopy 24 (1976) 595. S.H. PAWAR and A.V. NARLIKAR, MaL Res. Bull. l l (1976) 821. R.P. RAO, D. R. RAO and H. D. BANERJEE, ibid. 13 (1978) 491. R.P. RAO, PhD thesis, Indian Institute of Technology, Kharagpur (1980). R.P. RAO and D. R. RAO, unpublished work.

Acknowledgements

One of the authors (RPR) is indebted to CSIR, India, for the award of a senior research fellow-

ship. 18

Received 11 May and accepted 18 June 1981