Point defects in BaS:Bi phosphors

Journal of Luminescence 33 (1985) 87—102 North-Holland, Amsterdam

87

POINT DEFECTS IN BaS : Bi PHOSPHORS

*

R.P. RAO Materials Science Centre, Indian Institute of Technology, Kharagpur—721 302, India

Original manuscript received 6 January 1984 Revised manuscript received 1 June 1984

For this study in phosphorescence, thermoluminescence(TL), thermally stimulated conductivity (TSC) and dielectric constant (k’), samples of polycrystalline barium sulphide phosphors doped with Bi were prepared. These samples were studied after exciting them with UV, X-rays, y-rays and low-energy electrons. It has been observed that the phosphorescence of these phosphors exhibits a two-stage exponential decay. Complex TL patterns with overlapping glow peaks are obtained and the magnitude of TL output for X-ray irradiated samples is the biggest when compared with that for other ionizing radiations. The TL emission in each peak observed in the 350 to 600 nm wavelength range consists of three overlapping bands. These samples exhibit high dielectric Constants in the frequency range 102_lOs Hz and at RT (30 °C). It is noticed that k’ decreases with increase in temperature up to 150°C, but increases thereafter with further increase in temperature. Trap depth (E) and frequency factor(S) are calculated from the experimental data. Attempts have been made to explain these results with impurity complex formation.

1. Introduction Luminescence and electronic properties of suiphides, like ZnS and CdS, belonging to the Il—VI group have been widely studied [1—5].However, systematic investigations have not been made in detail on alkaline earth sulphides of the same group (hA_VIA) which are traditionally known as ‘Lenard phosphors.’ In order to understand the role of impurities and their relationship with point defects, a comprehensive program [6] has been undertaken to study the luminescence and electronic properties of alkaline earth suiphides. In general, the alkaline earth suiphides, which have an f.c.c. (NaC1) crystal structure (except MgTe) show less ionic character than the alkali halides. Fractional ionic character of NaC1 is 0.94 and that of MgS is 0.79 [7]. It may be easy to analyze their properties in comparison with the available

~ This is part of the Ph.D. work carried India.

Out

by the author during 1976—80, financed by C.S.I.R.,

0022-2313/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

88

R. P. Rao

/ Point defects in

BaS.’ Bi phosphors

data and models for alkali halides. In different electronic processes, defects, both intrinsic and those associated with impurities, play major roles in the control of structure-sensitive properties like excitation, emission, absorption and electrical conductivity. Impurities present naturally as traces, or doped intentionally in the material (activators and coactivators) may act as trapping and/or recombination centres. Depending on the nature of the activation, the luminescence and electronic properties become modified significantly. Experimental techniques like afterglow decay, thermoluminescence(TL), thermally stimulated conductivity (TSC), and measurements like dielectric constant measurements have been used to obtain information regarding different trapping mechanisms and recombination phenomena in these materials. Most of the earlier work on these alkaline earth sulphides has been carried out on UV excited luminescence and only meagre information is available on the effect of X- and y-ray excitations [8—10].The luminescence properties like phosphorescence [11,12], photoluminescence [13—17], thermoluminescence [18—21],electroluminescence [22—24],cathodoluminescence [25—27]and radioluminescence [28],and electrical properties like conductivity [29—31],thermally stimulated conductivity [32—34]and dielectric constant [35] of alkaline earth suiphides have been studied by several researchers but little information is available on the nature of point defects [21,26].Of the alkaline earth sulphides BaS phosphors have been investigated the least systematically. This paper first presents the experimental procedure for the preparation of BaS doped with various concentrations of bismuth. The data on afterglow decay, TL, TSC and dielectric constant for these samples are then presented and discussed. Possible explanation for the formation of impurity complexes based on these experimental results is offered.

2. Material preparation Alkaline earth sulphides in polycrystalline form have been prepared by researchers in the past. Most of these procedures involved the reduction of host sulphates using carbon or hydrogen. Recently, several papers have appeared reporting better yields on the preparation of these phosphors, [36—40]contradicting the general feeling [41] about the stability and the yield of these phosphors. It has been pointed out by Lehmann [38] and Kato et al. [46] that these materials when properly prepared and activated with suitable impurities become attractive phosphors for many possible applications. Since the investigations of Keller [42], many workers have also been trying to grow single crystals [43—46]with a view to replace the polycrystalline form of these sulphides. For the present study, barium sulphide in powder form was prepared by reducing pure Ba504 with carbon (spectroscopically pure) in an argon atmos-

R. P. Rao

/

Point defects in BaS.’ Bi phosphors

89

phere at 950 °Cfor two hours. Better yields (99 %) of BaS have been obtained at firing temperatures between 930 to 950 °C. X-ray powder diffraction analysis showed that these materials are polycrystalline with f.c.c. structure. Several samples were prepared by doping the BaS with different quantities of Bi, from 0.006 to 0.578 wt% taken in the form of Bi(S04)3 (0.O1N) in presence of NaCl flux [47,48]. The quantity of Bi present in the final product (BaS) was calculated from the volume of Bi(S04)3 added to the mixture before firing; the details of material preparation have already been described in an earlier paper [49]. After doping, the samples were quenched to room temperature (30°C), pulverized and brought to uniform particle size (~75 ~tm) [50]. Afterglow decay and TL measurements were then carried out by packing these powders in a thin brass well of 10 mm diameter and 1 mm depth, while TSC and dielectric measurements were made by compacting the powders into 2. circular Ohmic pellets (10 X 2) mm at 30 °C, applying a pressure of 780 kg/rn contacts were made by evaporating silver onto both surfaces of these pellets.

3. Experimental Before commencing the afterglow decay, TL and TSC measurements on these samples, UV, X-ray, y-ray and low-energy electrons were used for irradiation. The samples were X-irradiated for 10 mm at RT at a distance of 4 cm from the CuKa of an X-ray generator (30 kV, 10 mA) HPOH—I, USSR). For UV and y-excitations, a xenon lamp (250 W) with a 200 nm band pass filter and a Co6° y-ray source respectively were used. After irradiation, the samples were focused on a detector which was placed near the windows (exit) of the respective sources. The decay was measured using a 1P 28 photomultiplier (PM) tube and the photocurrent was recorded with a potentiometric recorder through an electrometer amplifier (ECIL, India, Model: EA 812). After recording the decay for a particular period (say 15 mm), the sample-well along with a heating arrangement was then transferred to a vacuum chamber (~10~ Torr) and the TL glow curves were recorded by heating the samples at a rate of 28 ±1°C/mm in the temparture range 30 to 350°C. The output currents due to TL from the electrometer amplifier and the temperature of the sample were recorded simultaneously. The spectral composition of TL emission at each glow maximum has been measured using a 0.25 M (Ebert Jarrell Ash) grating monochromator, a detector assembly (1140 B) and a strip chart recorder. The detector assembly contained a PM tube (R 456), an amplifier and a discriminator. Further details of the experimental set up used in this investigation have been described elsewhere [51]. The recorded TL spectra are plotted after normalizing the intensity with the response of the grating and the PM tube. The annealed pellets (at 250°Cfor 1 h in air) were irradiated by X-rays (30

90

R.P. Rao

/

Point defects in BaS.’Biphosphors

kV, 10 mA) and UV (200 nm) in darkness for 30 mm at RT. After irradiation, these samples were fixed on the sample holder of the experimental set up (for details see ref. [52]) and kept under vacuum for 30 mm before heating. The TSC curves have been recorded by heating the samples in the temperature range 30 to 250 °Cat a rate of 30 °C/min and applying 90 and 225 V (d.c.) across the pellets in the case of X- and UV irradiated samples, respectively. For the measurements of dielectric constant (k’) and loss (tan ~) at temperatures between 30 to 350 °Cand in the frequency range 102 to i0~Hz, a special sample holder having a teflon base with provision for cold water circulation was used. Measurements have been carried out on a capacitance bridge (CR 716); the details of the experimental setup have been reported elsewhere [53]. 4. Results 4.1. Afterglow decay Immediately after the excitation, the emission was recorded for about 15 mm. After this decay time elapsed, the glow was too weak for further

100

50 C

a >-

z 10 LU

z 5

2 4 6 8 10 TIME, mm. Fig. 1. Afterglow decay of BaSe Bi (0.231 wt%) phosphors.

R.P. Rao

/

Point defects in BaS: Bi phosphors

91

recording. Arrhenius plots between intensity and time of afterglow decay were drawn to investigate the nature of the decay and are shown in fig. 1. It is seen from this figure that the curves are nonlinear, this can be explained by considering either the exponential law [54] or a superposition of several exponentials [55]. For a sample containing one kind of traps and exhibiting an exponential decay at a given temperature (T), the intensity (I,) at any instant of time (t) is given by 1, = Jo exp(—pt) = 1o~exp( —p

1t)+

‘02

exp(—p21)+

...

+I~ exp(—p,,t),

where I~ is the phosphorescence intensity due to electrons in the traps of energy E~p~= S exp (—E~/kT) is the probability of an electron escaping from a trap, k is the Boltzmann constant and S is the attempt-to-escape frequency factor. In a system where by irradiation, shallow traps are also produced along with deeper traps, the decay of phosphorescence occurs generally in two or three stages, depending on the nature and concentration of the traps. In such cases, analysis of the decay curves would be usually split up by a ‘peeling-off’ procedure, where each stage of decay would be associated with an exponential law. From the slope of each linear portion of the curve, the Table I Trapping parameters of BaSe Ri phosphors from TL, TSC and after glow decay curves Source of excitation

Method of detection

Glow max. Tm (°C)

(s

Slope a ‘)

Frequency factor S4 (s

UV

TL TSC

102 173

—

2.8x10 1.9x10°

X-rays

Afterglow decay TL (thermal cleaning) TL (analytical)

y-rays Low-energy electrons *

2.2X10’2 4.8x103

1)

Trap depth E (eV) 0.49 0.55

3~5>< ~

*

l.9x103 I.2X106

*

0.31 0.35 0.36 0.61 1.04 0.38 0.63

1.8x 10° *

1.21

—

112 200 290 116 188

3.6x103 4.5X105 *

285

TSC (analytical)

150 198

6.5 x iO~ 3.9x iO~

0.60 0.80

TL (analytical)

213 297

1.5>< IO~* 1.1 x108 *

0.67

TL (analytical)

143 214 298

2.1 X iO~ 2.9X iO~* 3.5x107

0.43 0.66 1.10

Second-order kinetics

1.10

R. P. Rao

92

/

Point defects in BaS: Bi phosphors

trap depth (E) of the involved traps can be calculated by using the equation E = kTln(S/a), where a is the slope of each linear portion of the curve. From fig. 1, the decay process is observed to have occurred in two stages independent of the concentration of impurities. Utilizing the frequency factor (S), obtained from the TL glow curve, the activation energies associated with the traps corresponding to different stages of decay are calculated by using the above relations and are indicated in table 1. 4.2. Thermoluminescence glow curves and spectra

Undoped BaS exhibited practically no TL, and BaS doped with Bi has always yielded complex TL patterns, indicating overlapping glow maxima. TL glow curves of BaS: Bi recorded after irradiation with different sources of excitation, viz., UV (200 nm) for 2 mi y-ray (Co60) for 24 h, X-rays (30 kV, 10 mA) for 10 mm and low-energy electrons (10 kV, 10 ptA) for 2 mm are shown in fig. 2. These curves are saturation curves except for the low-energy electrons and the duration of time indicated above is the saturation time of respective excitations. Though a number of samples with different concentrations of Bi (in the range 0.006 to 0.578 wt%) have been prepared and studied,

180

1 50 z B

<

20

5IIn z 90 Lu I-

60

90

120

150

80

210

TEMPERATURE,

240

270

300

330

°C

Fig. 2. TL glow curves of BaSe Bi (0.231 wt%) phosphors excited by (A) UV (200 nm), (B) X-rays (30 kV, 10 mA), (C) y-rays (Co°°) and (D) low-energy electrons (10 kV, 10 jsA) at 30 °C.

R. P. Rao

/ Point defects

93

in BaS: Bi phosphors

only one typical concentration, which gave maximum TL output is presented here. An overview of all these TL results indicates that (i) the TL patterns are complex, exhibiting many overlapping glow maxima, (ii) the samples yield maximmum TL output by X-irradiation, compared to the other exciting radiations and (iii) the TL output increases with increase of impurity concentration up to a certain value (0.231 wt%) and then decreases with further increase of Bi content. The glow curves occur essentially in three temperature regions, viz., 90 to 130, 180 to 220 and 280 to 320 °C.For UV excitation, only a single well-defined glow peak is observed in the first region, while for other irradiations glow maxima are observed in all the three regions. After several exposures with ionizing radiations and heat cycles, these phosphors exhibited the same features and always showed the same number of glow peaks, with no significant changes in their relative intensities or glow maxima. The complex TL patterns have been resolved by using thermal cleaning [56] as well as analytical [57] techniques. As an example, the analysis of a typical case is shown in fig. 3. The parameters like trap depth (E) and frequency factor (5) are calculated from these glow peaks using Chen’s formulae [58].All the values along with those calculated from the decay curves are presented in table 1. Spectral compositions of the TL emission near the glow maxima have been recorded for samples of different concentrations of Bi, and are found to exhibit

14

—

Thoimal cltanrnt

—5——

Rio’s

technique

Ia A/

11B

~

U)

\

~Io’ (A-B) .5

\ lB-C) U)

z

~J6

\

4

2

I

60

90

120

I

I I

ISO

180

2)0

I

240

270

300

330

TEMPERATURE, ~C

Fig. 3. Analysis of TL glow curves of BaSe Bi (0.347 wt%) phosphors.

360

R.P. Rao

94

/

Point defects in BaS: Bi phosphors

similar features. The TL spectrum of a typical concentration is shown in fig. 4. The emission indicates overlapping bands in the wavelength range of 350 to 550 nm, which could be divided into three regions (350—400, 425—450 and 525—550 nm). A slight shift is found in the band maxima towards the longer wavelengths with an increase in temperature of glow maxima. 4.3. Thermally stimulated conductivity

TSC of all the samples has been studied in the temperature range 30 to 250 °C.The current beyond 250 °Ccould not be measured because of the rapid changes in the current values of these irradiated samples. In general, very poor TSC has been observed by UV irradiation, whereas by X-irradiation the TSC patterns become complex, showing overlapping TSC peaks in the region

400

450 500 550 WAVELENGTH )~~nm)

Fig. 4. TL spectra of BaSe Bi (0.405

600

wt%) phosphors.

R. P. Rao

/ Point defects in

BaS: Bi phosphors

95

150 to 220 °C. Some of the typical results obtained for UV and X-ray excitations are shown in fig. 5. A single peak has been observed in case of UV at 173 °C,while two peaks occur in case of X-rays, at 150 and 198 °C. Since the TSC was recorded 30 mm after irradiation, the effect of decay of the stored energy has to be taken into consideration to have a proper understanding of the TL and TSC results. Because of the overlapping in the high-temperature peaks, resolution could not be observed by thermal cleaning. The trapping parameters were calculated by using the approximate method [57] and are given in table 4. 4.4. Dielectric constant, loss and conductivity

Dielectric constant (k’) and dielectric loss (tan ~5)have been measured for these phosphors in the frequency range 102 to i05 Hz, employing a substitutional method [59]. The values of k’ and tan ~ for undoped BaS are observed to be independent of frequency and temperature up to 200 °C.The value of k’ for BaS: Bi has been observed to be very high (185 at 102 Hz) and frequency dependent at RT (30 °C). It has been further observed that the value of k’ decreases with increase of temperature up to 150 °C(k’ = 18) but does not

200

‘7’

60

90

120

50

TEMPERATURE,

180

210

240

‘C

Fig. 5. TSC of BaSe Ri (0.231 wt%) phosphors excited by (A) UV (200 nm) and (B) X-rays (30 kV, 10 mA) at 30 °C.

96

R. P. Rao

/

Point defects in BaS: Bi phosphors

change till the temperature rises beyond 230°C. The variation of k’ above 230°C,however, is similar as in the other cases (BaS, BaS : Cu and BaS : Cu, Bi [35]). The variation of tan 6 with temperature and frequency is the same as that of k’. The conductivity (aac) which follows from the values of k’ and tan 8 has been calculated using the relation = 2irJlc’

tan 6,

where f is the frequency. All the results, viz., k’, tan 6 and 0ac for a concentration of Bi are shown in figs. 6 and 7. The activation energy (E~)calculated from the slope using the relation 0uc

=

a 0 exp(—E~/kT)

is found to be 1.7 eV for temperatures above 200°C.

5. Discussion The results of this investigation (which deals essentially with thermoluminescence, afterglow decay and other electronic properties of BaS phosphors, excited by ionizing radiations like UV, X-rays, y-rays, etc.) are to be understood and explained in the light of similar information and models available on radiation produced traps in similar materials. Though several compounds with rock salt structure appear to be typically ionic where the (CI 00

/7

(bI

/2°

~

002

I TEMPERATURE, I I I I I TEMPERATURE, I I ~C 50I 120 180 240 ‘C 360 120 180 240 Fig. 6. Dielectric constant (k’) 300 and loss (tan 6) of BaS e Bi 60 (0.231 wt%) phosphors.

I 300

I

R.P. Rao

/

Point defects in BaS: Bi phosphors

97

atoms have octahedral coordination, in the materials having band gaps between insulators and semiconductors (band gap of BaS being 3.8 eV), the bonding is observed to be a mixture of ionic and covalent. Luminescence, color centres and related phenomena in ionic crystals having f.c.c. structure are fairly well understood, and models for some of the trapped electron (F-type) and trapped hole (V-type) centres produced by irradiation are also well established [60]. Though the ionicity of this system of phosphors (alkaline earth sulphides) is small compared to the alkali halides (for example NaC1), most of the defect controlled electronic and luminescence properties have been explained by such ionic models (divalent [4]). The complete absence of TL with UV and low TL yield with X-rays for the pure BaS samples suggest that the storageability of excitation energy is poor,

l0~6

10°

E ~

(0

x ió° .

(Os (.4

(.8

2.2

(O~/T (‘K ‘

Fig. 7. Conductivity (°*c)of BaS e Bi (0.231 wt%) phosphors.

98

R.P. Rao

/ Point

defects in BaS: Bi phosphors

indicating a low concentration of traps. The intense TL glow curves are observed in BaS phosphors only after they have been doped with impurities like Bi. The glow peaks, independent of concentration of the activating impurity, occur nearly in the same region of temperature and give emission covering the same wavelength regions. These results indicate that the TL in activated BaS phosphors is connected with the native defects of the material, and the associated ‘IL traps bear a close relationship with the impurities in these phosphors. The number of such traps is observed to be greater under X-irradiation than under UV and low-energy electron irradiations. This is explained by the fact that X-rays produce ionization in a large volume (penetration depth for 30 kV X-ray is = 0.4 mm) of the material and the TL output obtained is therefore greater compared with that of the low-energy electrons (penetration depth = 1.5 ~tm [61]) In order to identify the electron and hole traps responsible for the TL emission, one has to look for the centres or/and regions which possess effective positive or negative charge, since they in turn act as places for stabilizing the trapped electron or/and hole centres. It has been established that anion (sulphur ion) vacancies (associated with impurities like Cu) act as effective electron traps in the Il—VI compounds of ZnS [62] and CaS [26]. To explain the results of this investigation one may consider the defects like [V~]2~ and [VBa_B~Ba], the sulphur ion vacancies and their complexes involving the Bi impurities respectively, which may be the possible centres for trapping electrons liberated during irradiations *~ The possible types of charge compensation in the case of BaS: Bi phosphors are shown in fig. 8. As shown here, these impurities and the related lattice vacancies (for charge compensation), either remain isolated and randomly distributed or form complexes, depending upon the concentration of the impurity (bismuth) in the material. In all the samples, the trap depth values calculated from the afterglow decay are found to be in good agreement with those obtained from the low temperature TL peaks (first-order kinetics), as mentioned already. This result means that the traps emptied during the later part of the afterglow decay and those related with the first glow peak of this system of phosphors are similar. Further, the observed monomolecular process indicates that the involved electron and hole traps are located near each other. The trap depth values obtained from the analysis are found to be in the range 0.7 to 1.2 eV, indicating a secnd-order kinetics for the high-temperature peaks (exhibited by BaS excited by X- or y-rays or low-energy electrons) and may be attributed to the distribution of trapped hole and electron centres. Considering the alternatives for the formation of complexes in BaS : Bi, it is seen that the impurity *

Throughout the discussion, the nomenclature

“[]“

indicates the effective charge of the centre.

For example the real charge of Bi ion on the cation site is negative while its effective charge [Bi 6aI is positive.

R. P. Rao

/ Point

99

defects in BaS: Bi phosphors

ions [VBa]2~ and [B~Ba]~ create donor levels and [VBa_B~Ba] the acceptor level. Since these phosphors are prepared with NaC1 as a flux, it may become necessary to understand the role played by the C1 and Na ± ions in the emission phenomena. In the case of ZnS: Cu, Cl, the complex [Cu~~—Cl 5]°is known to create associated donor—acceptor (DA) pairs [4]. In a similar manner, the presence of sodium ions in the present case is likely to create the DA pairs such as [BiBa—NaBa]°.However, the DA pair emission may be expected beyond the red region (>700 nm), which falls well outside the detection limit of the system used in the present study in recording the IL spectra, as in case of CaS : Ga, Cu [63]. The IL spectra near different glow maxima in these phosphors, may be thought to be due to the transitions involving radiative recombination between the released electrons from donor-like centres and the trapped hole centres, as supported by TSC results. The trap depths calculated from the TL and TSC curves give the values nearly in the same range. The absence of negative TSC in all the samples indicates the destruction of the corresponding traps involving liberation of electrons from these traps to the conduction band, followed by a radiative transition, destroying the related trapped hole centres in the process of recombination.

=*=*=*~=*=*=* ~+f. = * = = +~-

* = * = * = *

=

=

~l_~

=*=*=*=*=*=* =

2° —

[V

2 51

,

=

__

—

=

—

* =

Vacancy,

[ ]

= *

—

Effective

charge,

Ba ,

tV~ i2&

[Bi 85 1°

Fig. 8. Possible types of charge compensation in BaS e Bi phosphors prepared in presence of NaCI flux.

100

R. P. Rao

/

Point defects in BaS: Bi phosphors

The variation of dielectric constant (k’) with temperature in the frequency range 102_1105 Hz can also be understood by the models proposed above for the impurity-defect complexes. Considering the usual contributions, viz., electronic, ionic and space charge polarizations to the dielectric constant, a satisfactory explanation cannot be offered for understanding the high values of k’. However, the existence of neutral complexes like [B~Ba_VBa~~B~Ba]° as suggested in this system (shown in fig. 8) (these may exist in the material during preparation as dipolar complexes possessing a large dipole moment) may be responsible for the high dielectric constant. On the basis of this concept, the dipole moment associated with these complexes can be calculated by using the geometrical configuration shown in fig. 8. From this figure one can visualize the alignment of the dipoles in two ways, either in the same direction as of the applied field or its opposite (antiparallel). Thus, the dipolar relaxation effects do not appear to be possible in this system in view of the configuration of this complex as well as the nature of the variations of k’ and tan 6. Such breaking-up of impurity complexes in these phosphors is supported by the data of activation energies obtained from ln aac vs 1/T and d.c. conductivity where the mechanism of breaking-up complexes was proposed [52]. Hence from all these experimental results, it may be concluded that the complexes are formed by the presence of impurities and the native defects are responsible for the energy storage properties like ‘IL, TSC in the activated phosphors. Such studies on defects generally give valuable information for assessing the suitability of a given phosphor material for different applications.

6. Conclusions (i) The IL traps with intense IL glow peaks observed in BaS phosphors doped with Bi are associated with the native defects, which have a close relationship with impurities. Large IL output in magnitude by X-irradiation is attributed to the breakup of complexes [B~~a_VBa_B~Ba]° during irradiation. (ii) Considering the trap depth values obtained from low-temperature IL and decay curves, it is concluded that the electron and hole traps are located near each other. (iii) The TSC results indicate that distribution of the corresponding traps must involve the liberation of electrons from the traps to the conduction band, destroying the related trapped hole centres in the process of recombination. (iv) High dielectric constant (k’) at room temperature has been attributed to the formation of dipoles like [B~Ba~VBa~B~Ba]° during the preparation of the material. The formation of impurity complexes involving the native defects explains some of the properties of these phosphors.

R.P. Rao

/

Point defects in BaS:Bi phosphors

101

Acknowledgements The author expresses his gratefulness to the referee of this paper for many valulable suggestions. He also wishes to thank Prof. DR. Rao, Prof. K.V. Rao, Prof. D. Sen, Dr. A. Subhramanyam and Dr. Y.S. Sarma for helpful discussions.

References [1] RH. Bube, Photoconductivity of Solids (Wiley, New York, 1960). [21F.E. Williams, Luminescence of Organic and Inorganic Materials, eds., H.P. Kallmann and G.M. Spruch (Wiley, New York, 1962). [31D. Curie, Luminescence in Crystals (Methuen, London, 1963). [4] Physics and Chemistry of II-.VI compounds, eds., M. Aven and J.S. Prener (North-Holland, Amsterdam, 1967). [51S. Shionoya, J. Lumin. 1, 2 (1970) 17. [61 R.P. Rao, Ph. D. thesis, Indian Institute of Tech., Kharagpur (1980). [7] J.C. Philips and J.A. Vanvechten, Phys. Rev. Lett. 22 (1969) 705. [8] R.P. Rao, Rad. Effects 77 (1983) 159. [9] R.P. Rao and DR. Rao, Health Phys. 45(1983) 1001. [10] R.P. Rao, J. Gasiot and M. de Murcia, Radiation Protection Dosimetry 6 (1983) 64. [11] V.K. Khare and J.D. Ranade, md. J. Pure and AppI. Phys. 8 (1970) 649. [12] R.D. Lawanger and A.V. Narlikar, J. Lumin. 11(1975)135. [13] W. Lehmann, J. Lumin. 6 (1973) 455. [141 B.B. Laud and V.W. Kulkarni, Phys. Status Solidi (a) 51(1979) 269. [15] S.N. Gour, Ind. J. Pure and App). Phys. 20 (1982) 177. [16] N. Singh, G.L. Marwaha, S.P. Garg and V.K. Mathur, Rad. Effects 60 (1982) 155. [171 N. Yamashita, T. Ohira, H. Mizuochi and S. Asano J. Phys. Soc. Japan 53 (1984) 419. [18] R.P. Rao, D.R. Rao and H.D. Banerjee, Mat. Res. Bull. 13 (1978) 491. [19] MG. Patel and RD. Lawanger, Mat. Res. Bull. 16 (1981) 109. [20] G.L. Marwaha, N. Singh and V.K. Mathur, J. Phys. & Chem. Solids 43 (1982) 271. [21] P.K. Ghosh and R. Pandey, J. Phys. C (Sol. Stat. Phys.) 15 (1982) 5875. [221 D. Theis, 3. Lumin. 23 (1981) 191. [23] A. Vecht and M. Waite, J. Lumin. 24, 25(1981) 917. [24] M.G. Pate), Ind. J. Pure and AppI. Phys. 20 (1982) 139. [251 W. Lehmann and F.M. Rayan, J. Electro. Chem. Soc. 118 (1971) 477. [26] P.K. Ghosh and V. Shankar, J. Lumin. 20 (1979) 139. [27] F. Okamoto and K. Kato, J. Electro. Chem. Soc. 130 (1983) 432. [281 R.P. Rao and D.R. Rao, J. Mater. Sci. Lett. I (1982) 435. [29] K. Nagata and K.S. Goto, Metall. Trans. (USA) 5 (1974) 899. [30] A. Egami, T. Onoye and K. Narita, Trans. Jpn. Inst. Met. 22 (1981) 399. [311 H. Nakamura, and K. Gunji, J. Jpn. Inst. Met. (Japan) 47 (1983) 21. [32] D.R. Vij and V.K. Mathur, Brit. J. Appl. Phys. 2 (1969) 624. [33] S.C. Jam and D.R. Bhawalkar, md. J. Pure and Appi. Phys. 13 (1975) 78. [34] R.P. Rao and D.R. Rao, Physica Scripta 25 (1982) 592. [351R.P. Rao and D.R. Rao, Physics Scripta 26 (1982) 467. [36] A. Wachtel, J. Electro. Chem. Soc. 107 (1960) 199. [37] S. Asono, N. Yamashita, M. Oishi and K. Omori, 3. Phys. Soc. Jpn. 25 (1968) 789.

102 [38] [39] [40] [41] [42] [43] [44] [45] [46] [47]

R.P. Ran

/ Point defects in

BaS: Bi phosphors

W. Lehmann, J. Electro. Chem. Soc. 117 (1970) 1389. D.C. Lynch, Met. Trans. B (USA) IIB (1980) 415. J. Gasiot, P. Braunlich and J.P. Fillard, AppI. Phys. Lett. 40 (1982) 376. W. Mellor, A Comprehensive Treatise of Inorganic and Theoretical Chemistry VI 3 (1948). G. Cheroff, M. Okrasinski and S.P. Keller, J. Chem. Phys. 27 (1957) 330. M. Drofenik and A. Azman, J. Phys. and Chem. Solids; 33 (1972) 761. Y. Kaneko, K. Morimoto and T. Koda, J. Phys. Soc. Jpn. 51(1982) 2247. J.W. Brightwell, B. Ray and C.N. Buckly, J. Crystal Growth 59 (1982) 210. K. Kato and F. Okamoto, Jpn. J. AppI. Phys. 22 (1983) 76. E.P. Efanova, Malasela Du, V.V. Mikhalilu, A.A. Placher, M-L, Yu. Allsalu and ER. Kil’k, Moscow Univ. Phys. Bull. (USA) 35 (1980) 90. [48] R.P. Rao and D.R. Rao, Bull. Mater. Sci. 5 (1983) 29. [49] R.P. Rao, J. Mater. Sci. Lett. 2 (1983) 106. [50] R.P. Ran and D.R. Rao, Solid State Commun. 30 (1979) 315. [51] R.P. Rao and D.R. Rao, Chem. and Phys. Mat. 9 (1983) 501. [52] R.P. Rao and DR. Rao, J. Mater. Sci. LeIt. 1(1982)17. [53] Westphal WB, Tech. Rep. 182 (Laboratory for Insulation Research, MIT, USA) 1963. [54] J.T. Randall and M.H.F. Wilkins, Proc. Roy. Soc. A184 (1945) 366. [55] RH. Rube, Phys. Rev. 80 (1950) 655. [56] H. Gobrecht and D. Hofman, J. Phys. Chem. Solids 27 (1966) 509. [57] D.R. Rao, Phys. Status Solidi (a) 22 (1974) 337. [58] R. Chen, J. AppI. Phys. 40 (1969) 570. [59] Ky. Rao and A. Smakula, 3. Appl. Phys. 37 (1966) 319. [60] J.H. Schulman and W.D. Compton, Color Centres in Solids (Pergaman Press, Oxford, 1963). [61] C. Feldmann, Phys. Rev. 117 (1960) 455. [62] J. Dieleman, S.H. de Bruin, C.Z. van Doom and J.1-J. Haanstra, Philips Res. Rept. 19 (1964) 311. [63] W. Lehmann, J. Lumin. 5 (1972) 87.