Determination of the vapor pressure of Ga2 Te2

Materials Chemistry 2 (1977) 133 - !42 © CENFOR S.R.L. - Printed in Italy

DETERMINATION

OF THE VAPOR PRESSURE OF Ga2Te2

B.M. N A P P I , D. F E R R O , M. P E L I N O and V. P I A C E N T E Istituto

di Chimica

Fisica - Istituto

Chimico

- Universit~ degli Studi - ROMA

-

Italia.

Received 20 June 1977; revised 25 July 1977. Summary - The pressure of the vapor over Ga2 Te2 was determined by means of the mass-spectrometric, the thermo-baiance and the torsion.effusion techniques in the temperature range: 980-1135 K. The resulting expression for total pressure in atmosphere is: Ig P = (5.20 + 0.46) - (10.29 ± 0.57) 103/T The mass-spectrometric analysis showed that Ga2Te (g) and Te2 (g) are the predominant species in the vapor in equilibrium with Ga2Te2 (s). The enthaipy for the reaction: 2 GaTe (g)

~ Ga2Te (g) +

1./2

Te 2 (g)

has been also determined as AI-I~ = - 28 ± 2 Kcai/mol, average of values obtained by the second and the third-law method.

INTRODUCTION T h e large u t i l i z a t i o n o f b i n a r y c o m p o u n d s o f gallium w i t h the VI-B e l e m e n t s o f t h e p e r i o d i c s y s t e m as s e m i c o n d u c t o r m a t e r i a l s a n d the f a c t t h a t , a p p a r e n t l y , t h e v a p o r i z a t i o n p r o c e s s e s o f t h e s e c o m p o u n d s w i t h G a a n d t h e VI-B e l e m e n t in a t o m i c r a t i o 1 : 1 are n o t

134

known led us to study these compounds. The thermodynamic aspects of the vaporization of Ga2 $2 and Ga~ Se2 have been the object of a previous study 1. In order to complete the study of this series of compounds, the vaporization of Ga2 Te2 was investigated by using different techniques and the results obtained are reported in this paper.

EXPERIMENTAL The samples of Ga2 Te2 were used in this investigation: the sample A (99.9% pure) received from the Allusuisse, Zurich, Switzerland, and the Sample B syntetized by melting equimolecolar amounts of high-purity gallium (SN grade) and tellurium (SN grade) according to the Spandau and Klandberg procedure 2 . The stoichiometry of both samples was checked by atomic absorption and by X-ray methods 3. The vaporization of these samples was investigated by mass-spectrometric, thermogravimetric and torsion-effusion techniques. Torsion-effusion method The equilibrium vapor pressures over Ga2 Te2 have been measured essentially by torsion-effusion method 4. This technique and the apparatus used in the present study have been adequately described in previous publications 5. Pressures values were obtained from measurements of the torsion angles t~ of the effusion cell, by use the well known equation e : p = 2 a K/(a111 fl + a212f2 ) where K is the torsion constant of the tungsten wire (30 #m in diameter) to which the effusion cell is suspended, al and a2 the area of two effusion holes, 11 and 12 the respective distances from the rotation axis, and fl and f2 the corresponding geometrical correction factors 6. Two conventional ceils made of graphite and pyrophiUite

135

were used and their geometric constants are given in Table 1. Temperatures were measured by means of a calibrated Pt/Pt-Rh 10% thermocouple inserted in a blank cell placed below the torsion cell. To renew confidence in the results obtained, the vapor pressures of lead were determined for calibration purpose using the cells Table 1 -- Constants of the cells e m p l o y e d in t h e torsion-effusion technique.

orifice area (cm "2) x 103

Celt al

m o m e n t arms (cm) + 0.005

force correction factors*

a2

a graphite

7.85 + 0.3

7.85 + 0.3

0.825

0.900

0.57

0.57

b graphite

7.85 ± 0.3

7.85 +- 0.3

0.915

0.885

0.73

0,73

11.30 ± 0.4

11.30 ± 0.4

0.865

0.895

0.82

0.82

c pyrophillite

* Taken from Freeman and Searcy 6.

a and c in the temperature range 988-1121 K. From these measurements the second and third law enthalpies of vaporization of lead (AH0°(II) = 47.4 + 1.8 and AH0°(III) = 47.1 + 0.2 Kcal/mole) were calculated and compared with the value selected by Hultgren 7 (AH°gs = 46.8 + 0.3 Kcal/mole). Samples A and B of Ga2 Te2 (s) were outgassed for several hours at about 500 K before their use. Four series of vapor pressure determinations were carried out in the temperature range 980-1135 K. The pressure-temperature dependance equations obtained from x~hese measurements and reported in Table 2 allowed to obtain the following least-squares equation: IgP (Atm) = (5.26 + 0.43) - (10.44 + 0.54) 103/T where the associated errors are estimated on the basis of the standard deviations for each equation as reported in Table 2.

136

Table 2 -- P r e s s u r e * t e m p e r a t u r e e q u a t i o n s o b t a i n e d f r o m t h e t o r s i o n - e f f u s i o n experiments.

Exp.

Sample

Cell

1

B

a

2

A

3

A

4

A

n.

of

T e m p e r a t u r e range

points

IgP (Atm) = A -

B 103/T *

(K) A

B

15

991 -- 1083

5.10 + 0.32

10.23 + 0.61

c

I8

1010 - 1093

4.83 -+ 0.51

10.60 + 0.52

a

22

994 -- 1090

5.22 ± 0.46

10,17 ± 0.48

b

20

980-

5.89 ± 0.42

10.77 ± 0.55

1135

* The q u o t e d errors are standard deviations.

Mass-spectrometric method A Bendix time of flight, model 3015 equipped with a Knudsen cell assembly similar to that described previously 8 was used. Samples of A and B were vaporized in two different runs from a graphite crucible fitted with a molibdenum liner. The temperature of the crucible was measured by acalibrated Pt/Pt-Rh 10% thermocouple pressed into a hole in the bottom of the crucible itself. During the vaporization of the samples the observed ion species were identified by the usual procedure (mass/charge ratio, isotopic distribution, appearance potential and "shutter" effect). By heating the samples at 500-600 K, the vaporization of small amounts of Te~ ( ~ 1~) considered as impurity, was observed. At higher temperatures over the same temperature range covered in the torsion experiments, "the ion species observed were: Te~, Ga2Te +, GaTe +, Te +, Ga* and Ga~, in order of decreasing intensity. Their appearance potentials, relative to that of Hg (10.4 eV) as standard 9, were 8.2 + 0.2; 7.9 -+ 0.2; 8.5 + 0.2; 11.5 + 0.2; 8.7 + 0.4 and 9.2 -+ 0.4 eV respectively for Te~, Ga2Te +, GaTe +, Te ÷, Ga ÷ and Ga~; these values are in agreement with those measured by Uy et al. 10 when studying the vaporization of Ga2 Te3 (s) and show that Te 2, Ga2 Te and GaTe are the principal gaseous species in equilibrium over

137 Ga2 Te2 (s). According to Uy's results the species GaTe2 and Ga2 Te3, even if in minor concentration, are also present in the molecular beam, but under our experimental conditions we could not positively identify these species, because of the low resolving power of our instrument at high masses. The partial pressures of the observed species were calculated at each experimental temperature from the ion intensity measured at the m a x i m u m of the ionization efficiency (45 eV) and for isotopic abundances, and the fragmentation contribution by using the well k n o w n relation: Vi = I~ T / K o i The value of the instrument sensitivity factor K was determined through a quantitative vaporization of a k n o w n a m o u n t of gallium 11 ; the cross-sections o i, 10.0, 11.1 and 9.0 for Te 2, Ga2Te and GaTe respectively were estimated multiplying by an empirical factor 0.7512 the sum of the atomic cross-sections taken from Mann 13. In Fig. 1 are reported as lgP vs 1/T the partial pressures of each species determined in the course of the vaporization of the sample A. From the partial pressure values one can derive the total pressure of the vapor in equilibrium over Ga 2 Te 2 (s). The second law treatment of the data gives the following equations: Sample A

lgP (Atm) = (5.34 + 0.47) - (10.37 + 0.61) 103/T

Sample B

IgP (Atm) = (5.69 + 0.30) ~ (10.71 + 0.47) 103/T

where all the associated errors are standard deviations, The gas phase equilibrium 2GaTe(g)

• , Ga2Te(g) + 1/2 Te2 (g)

was also studied. The second law enthalpy change AH°o6o = - 27.3 -+ 1.9 or

,R

,,,,,, ~°

o~

0

C..O

.,,,

~

.y../

y

,///

I

0~

IX)

-IgP (Arm.)

139 AH ° = - 28 + 2 Kcal]mole for the above reaction is in agreement with the value AH°0 = - 28.7 + 1.1 Kcal/mole obtained from the third-law treatment of the data. The necessary free. energy functions, - (G~ - H°)/T, and the heat-content function, H~ - H0°, for Te2 (g) were taken from the literature7 ; those for Ga 2 Te (g) and'GaTe (g) were calculated by the usual statistical mechanical formulae using the same molecular parameter estimated by U y l ° : Ga2Te (g): e l = 240, co2 = 107, co3 = 267 cm "1 , a symmetric bent structure with an angle of 110 ° and a G a - T e distance equal to 2.9 A; GaTe (g) ~ = 2 5 0 c m -1 and the equilibrium interatomic distance 2.9 A. Thermogravimetric m e t h o d The vaporization of Ga2Te 2 (sample A) was been studied b y employing a thermobalance Setaram (Mod. 1360) Ugine Eyraud coupled with a graphite Knudsen cell, with an effusion hole 1 mm in diameter, suspended in the isothermal zone of the furnace. Details of the m e t h o d and of the apparatus were described in a previous work 14. The measurements were performed in the temperature range 1005-1126 K and the temperatures were measured with a Pt/Pt-Rh 10% calibrated thermocouple inserted in a second cell placed immediately below the effusion cell. The vapor pressure values P in Atm at each experimental temperature T were derived from the rate of mass-loss (dm/dt) of the sample, b y using the equation: P = 2.26 10 -2 (T/M)'/2 (dm/dt)/SK

where S (cm2) is the area of the effusion hole, K its geometrical correction factor Is and M the vapor molar mass (M = 241) obtained as average of M (Ga 2 Te) and M (Te2), in accord with the results of the mass-spectrometric analysis. The second law treatment of the obtained data gives the following vapor pressure-temperature equation:

140

lgP = 4.32 + 0.71 - (9.21 -+ 0.74) 103/T where, once again, the reported errors are standard deviations.

CONCLUSION By a least square treatment of the experimental tensimetric data obtained by torsion, mass spectrometric and gravimetric methods, we selected the following vapor pressure for the vaporization of Ga2 Te2 : lgP -- (5.20 + 0.46 ) - (10.29 + 0.57) 103/T The slope and the intercept are derived from those obtained with the different techniques weighting them proportionally to the number of the experimental points. The associated errors have been estimated on the basis of the respective standard deviations. The mass-spectrometric analysis shows that in the investigated temperature range Ga2 Te2 vaporizes predominantly to Te 2 (g) and Ga2 Te (g). The species GaTe (g) and other probably minor species of higher tellurium content (GaTe2 and Ga2 Te2 ) not observed with our instrument because of its low sensitivity, originate from the gas phase reaction between Ga 2 Te and Te2; therefore we are led to conclude that in the temperature range explored Ga2Te2 (s) vaporizes according to the principal reaction: Ga2 Te2 (s)

> Ga2 Te(g) + 1/2 Te2 (g)

Since the partial pressure of Te2 (g) is higher than the partial pressure of Ga2Te(g), it is necessary to admit the coexistence of other equilibria as: Ga2Te2(s) +'-'7 2 Ga(1) + Te2(g)

141 T h e t h e r m o d y n a m i c equilibrium c o n d i t i o n s have been c h e c k e d b y d e t e r m i n i n g the e n t h a l p y change associated w i t h the reaction: 2 GaTe(g)

) G a 2 T e ( g ) + 1/2 Te 2(g)

f r o m the mass s p e c t r o m e t r i c m e a s u r e m e n t s . T h e value AH ° = - 28 + 2 K c a l / m o l e o b t a i n e d as average Of the second and third law values, results in satisfactory a g r e e m e n t with the value p r o p o s e d b y U y et al. ~° (AH ° = - 26 + 4 K c a l / m o l e ) for the same reaction. This could be c o n s i d e r e d an i n d i c a t i o n that t h e r m o d y n a m i c equilibrium c o n d i t i o n s actually exist w i t h i n the cell. Acknowledgements Thanks are due to Dott. R. Rizzo for the synthesis of the samples, Dott. P.L Cignini and Miss M. Urbani for the X-ray analysis and for technical assistence in the course of the experiments. This work has been carried out with the financial support of the C.N.R. REFERENCES

1. 2.

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