Interaction studies in CaSGa2S3 system

Mat. Res. Bull., Vol. 25, pp. 271-276, 1990. Printed in the USA. 0025-5408/90 $3.00 + .00 Copyright (c) 1990 Pergamon Press plc.

INTERACTION STUDIES IN CaS-Ga2S 3 SYSTEM N.I. Yagubov, T.N. Guliev, P.G. Rustamov and E.T. Azizov S.M. Kirov Azerbaijan State University Baku, U.S.S.R.

(Received July 5, 1988; Refereed)

ABSTRACT: Chemical interactions in the quasibinary CaS-Ga2S 3 system have been studied by differential thermal, X-ray phase, and microstructural analyses as well as by the microhardness measurements, and its constitutional T-X diagram is plotted for the first time. The thiogallate formation is found as CaGa2S 4 compound which crystallizes into the orthorhombic system. MATERIALS INDEX:

calcium, gallium,

sulphur

Introduction Of particular practical interest is the study of AIIx VI BIIIx VI systems - 2 3 where A = alkaline earths; B = Ga, In; X = S, Se, Te. In particular, ternary compounds and solid solutions found in these systems have high luminescent properties and photosensitivity (i-3). The work (2) exhibits CaGa2S 4 compound and its crystal parameters but no constitutional diagram for CaS-Ga2S 3 is plotted, therefore the formational nature of ternary CaGa2S 4 is not found, which is required to develop its crystal growth technology for physical studies. This paper deals with the research results on phase equilibrium in ternary Ca-Ga-S system from the CaS-Ga2S 3 section.

Results and Discussion CaS-Ga2S 3 alloys were synthesized from binary sulphides CaS and Ga2S 3 previously synthesized from elements. Calcium sulphide was synthesized from calcium metal and sulphur at 500 K for 10-12 hrs, later the temperature was increased to 1300 K and after a 2 hr exposure the furnace was switched off. Calcium sulphide was later purified by the transport reaction technique with iodine as a carrier (5 mg per cu.cm). Our developed technology promoted preparation of a purer and more stoichiometric CaS. Gallium sesquisulphide Ga2S 3 was synthesized by direct pulse technique (1,4).

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The alloys of the CaS-Ga2S 3 system were synthesized from master alloys in double quartz ampoules at 1400 K with 4 hr conditioning and subsequent slow cooling in the switched-off furnace. Then the alloys were annealed at 975 K for 500 hrs, for homogenization. The system alloys were investigated within the entire concentration range by the techniques of DTA (binary system sections of 60% CaS were measured on the FRU 64 pyrometer, those with the melting point above 1400 K on the high-temperature VDTA 8M2 setup), XRPA (on the DRON 3 diffractometer with K s Cu radiation), and MSA (on the MIM 7 metallographic microscope), i:i K2Cr207 chromium mixture + concentrated H2SO 4 was used as an etchant, and the sample microhardnesses were determined on the PMT 3 device. Density was pycnometrically weighed with heptane as a filler; composition was also chemically analysed.

CaGa2S 4

A T-X constitutional diagram was plotted for the CaS-Ga2S 3 system on the basis of the data obtained (Fig. i). The section was seen to be a quasibinary section of the ternary Ca-Ga-S system. The section liquidus has 3 primary crystallization branches for CaS, CaGa2S 4, and Ga2S 3 phases. With component ratio of i:i a ternary CaGa2S4-type compound is formed which melts congruently at 1410 K.

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FIG. 1 Constitutional

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CaS-Ga2S 3 SYSTEM

273

To study calcium thiogallate CaGa2S 4 formation conditions a synthesis from parent CaS and Ga2S 4 sulphides was recorded. The D T A data showed that CaGa2S 3 formation from equimolar CaS and Ga2S 3 quantities was c h a r a c t e r i z e d by exothermal effect on the h e a t i n g curve at 1400 K. Calcium thiogallate, CaGa2S 4, forms an eutectic with CaS and Ga2S 3. An eutectic b e t w e e n Ga2S 3 and CaGa2S 4 has 20 mol.% of CaS and melts at ii00 K, whereas one between CaGa2S 4 and CaS has 55 mol.% of CaS and a m e l t i n g point of 1360 K. 5 mol.% CaS solid solutions are Ga2S3-based. B-Ga2S 3 transforms e u t e c t o i d a l l y into y-Ga2S 3 at 1220 K. The ~ - G a 2 S 3 - - B - G a 2 S 3 transition we have not found and therefore identified it at the plot by dot lines. CaGa2S 4 single crystals were grown by transport reaction technique with iodine as a carrier in d o u b l e - w a l l e d furnaces. O p t i m u m growth conditions were T I (cold zone) = i000 K, T 2 (hot zone) = 1250 K, C ~ = 5 mg/cu.cm, ~ = 120 h. Thus, w h i t i s h grey bulk crystals of 2 x 2 x i mm were obtained in the cold zone. D i f f r a c t i o n pattern study established that CaGa2S 4 crystallizes as an orthorhombic system with the unit cell parameters: a = 20.086, b = 20.086, c = 12.112, v = 4885.37 ~, and a space group of the orthorhombic system Fddd, z = 32. In CaGa2S 4 structure three c a l c i u m atoms occupy 16(c), 8(a), 8(b) positions of the Fddd group having x, y, z coordinates. Average interatomic Ca-S separation was 2.98 and 3.08 ~, that being rather close to the sum of Ca +2 a nd S -~ ionic radii (1.04 + 1 84 ~ = 2.88 ~). At the same time, average interatomic Ga-S s e p a r a t i o n is 2.30 ~, that c o r r e s p o n d i n g to the sum of covalent Ga +3 and S -2 radii (1.26 + 1.04 = 2.30 ~). Analysis of interatomic separations d e m o n s t r a t e s the ionic constituent p r e d o m i n a n c e in the m i x e d ion-covalent bond of c a l c i u m thiogallate. The ionicity is due to C a 4 S 2 - e l e c t r o n t r a n s m i s s i o n to sulphur atoms which tend to have the inert gas electron structure, whereas a covalent bond prevails between gallium and sulphur atoms. those

Table I lists some p h y s i c o - c h e m i c a l properties of initial constituents, CaS and Ca2S 3 TABLE Physico-Chemical

Compound

CaS

Properties

of CaGa2S 4 as well

I

of CaGa2S 4, CaS,

H (Pa)

2700

Ga2S 3

1390

52.8x106

CaGa2S 4

1410

31.8xi06

as

and Ga2S 3 d kg/m 3

410

3.0

3.1

480

2.5

2.85

575

2.16

2.95

108 2x1013

Conduction type

Reference

2.59xi03

n

2; 6

3.68xi03

n

i; 5

13.37xi03

n

-

To study p h o t o e l e c t r i c properties we made specimens of p o l y c r y s t a l l i n e CaGa2S 4 in the form of 2 x 2 3 m m rectangular wafers. Aquadag was used as an ohmic contact for the study of e l e c t r i c a l and p h o t o e l e c t r i c a l CaGa2S 4 properties. Figure 2 shows c u r r e n t - v o l t a g e c h a r a c t e r i s t i c (CVC) of A q u a d a g - C a G a 2 S 4A q u a d a g structures at room temperature. The CVC is seen to be symmetrical relative to the applied voltage polarity, and c u r r e n t - v o l t a g e dependence is linear up to 550 V. CaGa2S 4 r e s i s t i v i t y is d e t e r m i n e d from the CVC linear

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FIG. 2 Current-voltage characteristic of Aquadag-CaGa2S4-Aquadag structure at 293 K. portion. Dark resistance varies within 0.5-4.5 x 1013 ohm for different specimens. To study photoelectric properties, an electric field where the Ohm's law holds is applied to the specimens. Figure 3 presents lux-ampere response. The plot shows power dependence; photocurrent vs. illuminancy factor is also calculated. At low intensities n = 2.53, and at high ones n = 0.88. Moreover, photocurrent spectral distribution is found and shown in Fig. 4. The photocurrent spectrum covers 400-1300 nm wavelength range and has a maximum at 575 nm. Bandgap calculated from 1.239 m

is 2.16 eV.

=

-

-

Total sensitivity of the crystals is i x i0 -I0

A__ mV

at low intensities and 2 x 10-10 A mV at high ones.

Conclusions The CaS-Ga2S 3 system was first studied by physico-chemical techniques and the T-X constitutional diagram was plotted. A ternary CaGa2S 4 compound was found which melts congruently. Synthesis and growth techniques have been developed for CaGa2S 4 single crystals. CaGa2S 4 was found to crystallize into the orthorhombic system with lattice periods of: a = 20.086, b = 20.086,

i

275

CaS-Ga2S3 SYSTEM

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FIG. 3 Lux-ampere c = 12.112 ~. resistivities was found that broad spectral manufacture.

response

for CaGa2S 4.

The densities (3.37xi03 kg/m), microhardness (31.8 x 106 Pa), (2 x 1013 ohm'm), and bandgaps (2.95 eV) were determined and it semiconductor CaGa2S 4 compounds have photosensitivities in a region (400-1300 nm) and can be employed for the photoresistor

References i.

P.G. Rustamov,

Izv. An Azerb.

SSR, 46 (1967).

2.

T.E. Peters and J.A. Baglio, Technol. 119, 230 (1972).

3.

U.S Patent N-3-801-702

4.

K.P. Thakur and J.D. Pandey,

5.

A.V. Novosyolov et al. (eds.), Physico-Chemical Properties of Semiconductor Materials. A Handbook. Nauka Publishing House, Moscow (1979) (in Russian).

6.

T.L. von Wen, W. Weppner and A. Rabenau, (1983).

J. Electrochem.

Soc. Solid State Sci. and

(1974). J. Inorg. Nucl.

Chem. 37, 645 (1975).

Z. anorg, allg. Chem. 497, 93

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Vol. 25, No. 3

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FIG. 4 Photocurrent

spectral distribution in CaGa2S 4.

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