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Radial distribution analysis of amorphous Al0.23Te0.77 by X-ray diffraction
Journal of Non-Crystalline Solids 28 (1978) 319-326 © North-Holland Publishing Company
R A D I A L DISTRIBUTION ANALYSIS OF AMORPHOUS Alo.23Teo.77 BY X-RAY D I F F R A C T I O N Alicia d ' A N J O U and Francisco SANZ Facultad de Fisieas, Universidad de Navarra, c/Urdaneta, 7 San Sebastian, Spain Received 7 July 1977 Revised manuscript received 21 December 1977 An atomic radial distribution analysis has been made on bulk AIo.23Te0.77 by X-ray diffraction techniques. The radial distribution function (RDF) shows peaks at r I = 2.72, r 2 = 3.51 and r 3 = 4.07 A. The second one is small but well resolved and is interpreted as exclusively due to Te-Te non-bonded separation as in the interchain distance in crystalline Te. The model, which agrees well with both RDF features and bonding conditions, consists of a three-dimensional covalent network of Te atoms around the Al atoms, under an identical bonding scheme as in crystalline A12Te3, the excess Te atoms forming chains which probably join together the groups with A1. The possibility that these chains are a separated phase of amorphous Te cannot be discarded. This model is in agreement with that postulated by Cornet for Te-X glasses with several X, including AI.
1. Introduction A model o f short-range structure in amorphous semiconductors o f type T e - X , for several X including A1, has been postulated on the basis of the knowledge o f the structure o f G e - T e and A s - T e glassy systems and density considerations. The model agrees with the observed glass-forming ability maxima in these materials [1], but more direct p r o o f is still lacking. On the other hand, in a study of the glass formation in G e - T e - X and A s - T e - X systems, it was found that the presence o f A1 as a third component enhances glass formation in these telluride systems [2]. Knowledge of the structural role that aluminium plays in the amorphous A1-Te might help in understanding the observed phenomena. Here, we report the results of a X-ray radial distribution analysis o f Alo.23Teo.77 which is the eutectic composition in the A1-Te system [ 1 ].
2. Experimental The Alo.gaTeo.77 sample used was obtained from commercial A1 and Te o f 99.99% purity. Appropriate proportions o f both elements were sealed in a quartz 319
320
A. d 'Anjou, F. Sanz / Radial distribution analysis of amorphous AIo.23Teo. 77
ampoule under a neutral Ar atmosphere and then melted. The molten mixture was rotated for four days inside a furnace at a temperature of about 700°C and then quenched in liquid nitrogen. The bright metallic solid obtained from the quenching was ground to a fine powder (<325 mesh) which was compacted by pressure into a brick of approximately 30 X 12 X 1 mm 3. No evidence of crystallinity was found in a conventional X-ray diffraction experiment. The diffraction intensities were measured on a Norelco diffractometer equipped with a bent graphite monochromator, scintillation counter and standard electronics. The radiation used was MoKa(k = 0.71069 A). Four series of data were collected in the interval 7 ° ~< 20 ~< 110 °, two at increasing angles in 20 and the other two in reverse. From 7 to 70 ° a step size of A(20) = 0.2 °, and from 70 to 110 ° of A(20) = 0.5 °, were used. Times were measured by keeping a fLxed number of counts. The intensity assigned to each observation point was the mean value of those measured at the point. Most of the averaged values lie within 3% of the average, with a maximum deviation of 4%. The density of the material measured with a pycnometer was 5.20 g/cm 3 with an estimated error of -+3%.
3. Radial distribution function (RDF) The observed intensities were corrected for background, polarization and multiple scattering. The last correction, for which Warren's method was used [3], had an influence on the data varying from 0.4 to 7%. Compton scattering was evaluated, taking in account the efficiency of the monochromator, and following the procedure described by Shevchik [4]. The intensities were put into electron units by the high-angle method and the incoherent scattering subtracted. Fig. 1 shows the normalized intensity curve as a function o f s (s = 47r sin 0/h). We calculated the functions
Io u i(s) -
;,
J
F(s) = s. i(s)
( G. xj. I
and 2 Smax
G(r) =-~ f
F(s)" sin(s" r) ds,
o
where x / i s the atomic fraction of the j element, being j = A1, Te, and Ie.u the normalized intensity minus the incoherent scattering. The individual scattering factors were taken from ref. [5] and corrected for anomalous dispersion.
A. d 'Anjou, F. Sanz / Radial distribution analysis o f amorphous Alo. 23Teo. 77
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A 123Te 77
1000 -
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i.
i
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I
I
I
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2
Z,
6
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10
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14
16
s(/,-1) Fig. 1. Normalized intensity curve as f u n c t i o n o f s (s = 4~r sin 0/X). The dashed line is the scattering which is i n d e p e n d e n t of the structural features.
The F(s) function was theoretically extended to Smax = 30 A -1, to avoid spurious oscillation in G(r) below the first significant peak due to the lack of high s data. This extension was made assuming, like Shevchik [4], that for high values of s, F(s) might be described by a function of the type F(s) = (c/a) sin(sa) e x p ( - o2s2/2), and where we considered c, a and a as parameters which were found by a leastsquares process as the set which makes the best fit between the postulate function and the last two weU-def'med oscillations of the experimental curve. Finally, the RDF was calculated from 47rr 2 p(r) = 47rr 2 Po + rG(r) , where Po is the mean atomic distribution over the sample and was directly deduced from the experimental density, and p(r) the local atomic density affected by the transform of the f d ~ / ( Z i x ~ ) 2 products.
A. d 'Anjou, F. Sanz / Radial distribution analysis of amorphous Alo.23Teo. 77
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A. d'An/ou, F. Sanz / Radial distribution analysis o f amorphous Alo.23Teo. 77
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The theoretical extension of the experimental data did not affect the significant features of the RDF, as may be seen by comparison of figs. 2 and 3 where the RDF curves from the data before and after the extension have been plotted. All the calculations were made on a HP 2100 S computer of the Computation Center of the Centro de Investigaciones T6cnicas de Guiptizcoa with programs written by us.
4. Results and discussion The analysis of the RDF may be summarized by the values given in table 1. The general features which show the RDF of the amorphous Alo.23Teo.77 are very similar to those found in the radial distribution analysis of the amorphous alloy systems G e - T e and As-Te [ 6 - 9 ] . The most noticeable difference is the small but well-resolved peak which in our case appears at r = 3.51 A and which we do not think is a spurious one. In this respect, Ichikawa [10] has discussed the possibility of the existence of a similar peak in his radial distribution study of amorphous Te films, and from the published RDF of Geo.llTeo.89 (fig. 2 in ref. [7]); the swell of the curve at about r = 3.5 A also suggests such a possibility. On the other hand the 3.5 A distance is precisely the smallest distance between two non-bonded Te atoms that occurs in crystalline Te [11 ], and given the high proportion of this element in the alloy is not surprising that this could occur here also. The first-neighbour peak is located at a distance of 2.72 A which is, as expected, between the 2.80 A T e - T e bond length in amorphous Te [10] and the sum of the covalent radii of the Te and AI of 2.53 A. We think that in this case the formation of metallic A1-A1 bonds is improbable, at least in a sufficient amount to influence the average. The peak at 4.07 A is at a distance a little shorter than the 4.35 A found for a similar distance in amorphous Te films [10]. This distance will be finally governed by the bond angle which may be smaller in our case. However, from the data of the crystal structure of A12Te3 [12] our value is in excellent agreement with the 4.08 A smallest T e - T e separation in the A12Te 3 crystal. The areas under the peaks calculated by the method proposed by Stetsiv [13], their values being listed in table 1. The errors are estimated. Table 1 Peak positions and area under the peaks of the RDF of AIo.23Te0.77. Errors are estimated Peak
r (A)
Area (atoms)
a
1st 2nd 3rd
2.72 3.51 4.07
2.0 1.8 8.0
0.10 0.15 0.20
324
A. d 'Anjou, F. Sanz / Radial distribution analysis of amorphous Alo.23Te o. 77
In this case the greatest structural information comes from data with s less than 6 A -1. We calculated the RDF using only these data, and it was found that the area under the peak at 2.7 A was 2.1 atoms, a value which differs only by 5% from that given for the same area in the final RDF. On the other hand, the calculation of the fi~/(Exj.fl.) 2 products showed that they were approximately linear throughout the entire range of 0 and within the interval 0 ~< s ~< 6 A -1 the greatest deviations of ft~/(Exi)~) 2 from the correspondent Z i Z j / ( E x / Z j ) 2 are about 12% in the A1-Te pairs and only about 5% in the T e - T e pairs. Thus, we made the approximation of treating these products as constant and equal t o Z i Z / / ( Z x j Z / ) 2 in the calculation of the first-neighbour peak area from the models. For this purpose the formula used was as follows: K"x
1
area -
Z.J x j nijZjZ i ,
jzj)
i, j = 1 , 2 ,
'J
I
where 1 means A1 and 2 means Te, xj is the atomic fraction of the jth element and nij is the averaged number of the/-type of atoms which are in the first coordination sphere of an atom of the type j. With the above approximation, the area under the first peak of the RDF is a kind of weighted average of the coordination numbers of the elements which form the alloy, and in terms of these coordination numbers, the structural models may be discussed first. The only crystalline compound of A1 and Te so far known is the unstable A12Te3 which has a wurtzite type of structure [12], with holes randomly distributed among the A1 positions. The A1-Te bond length is 2.54 A (calculated from ref. [12]. In this structure every A1 atom is tetrahedrally coordinated with four Te atoms, and the bonding situation may be formally regarded as that each A1 atom takes an electron from one of the Te atoms which are surrounding it, making a s p 3 tetrahedral hybridization. The Te ÷ which gave the electron will be threefold coordinated, while the rest of the Te atoms will be a twofold coordinated. In this structure an atom of threefold coordinated Te is produced for every A1 atom present in the compound and no A1-A1 bonds are formed. We tried a model assuming that around the A1 atoms the bonding scheme of the A12Te3 crystal is conserved, that the Te atoms in excess of the 2 : 3 composition are twofold coordinated and that all the bonds are saturated. Under these conditions the calculated area for the first-neighbour peak is 1.92 atoms, in agreement with the experimental results. The model is also consistent with all other features of the observed RDF and agrees perfectly with Cornet's hypothesis [1] on the local order of T e - X glasses. An average example of this model might be a complex threedimensional covalent random network where the 23% A1 atoms of the material are bonded to the 34.5% Te atoms, in the same way to build up a crystal structure; the
A. d'Anfou, F. Sanz / Radial distribution analysis o f amorphous Alo.23Teo. 77
325
rest (42.5%) are Te atoms forming chains which probably join the tetrahedral groups, but may be a separate phase of amorphous Te. Models made with a coordination number less than 4 for A1, should be discarded. Chain-like structures devised with coordination numbers of 1 or 2 for this element would fit the experimental RDF quite well, but such coordinations in A1 have been found only in a few gaseous short-life species and never in the solid state [14]. Threefold coordination for A1 from a planar sp 2 hybridization of the three outer electrons, probably unstable, gives a calculated area for the first RDF peak (1.74) atoms) far below the observed value. In Al-Group VI compounds A1 acting with a coordination number greater than 4, has been found in corundum (A1203) and ~/-A12Sa. In these compounds the metal atom has an octahedral coordination while the partner is fourfold coordinated. A model devised with such conditions, plus a coordination number of 2 for the excess Te, gave an area of 2.26 atoms, significantly larger than the observed one; also, such a configuration should produce a T e - T e peak at about 5 A that is clearly absent in our radial function. Higher coordination for A1 will increase the calculated area under the first peak and will disagree with experiment.
5. Conclusions From the above, we conclude that the most probable short-range order structure of this material consists of a three-dimensional covalent network of Te atoms around tetrahedrally coordinate A1 atoms. The Te atoms not bonded to the A1 atoms can be either chain-forming, linking the tetrahedral groups, or a separated phase of amorphous Te.
Acknowledgment We thank Dr. Shevchik for providing us with a copy of his Ph.D. thesis. We also t h a n k the Departamento de Metalurgia del Centro de Investigaciones T6cnicas de Guipfizcoa for the use of their X-ray equipment, and the Computation Center of CIT de Guiptizcoa for calculation facilities. This work is a part of a Ph.D. thesis of one of us (A.d'A) who thanks the Ministerio de Educaci6n y Ciencia for a Fellowship of Formaci6n de Personal Investigador, that made it possible.
References [1] J. Cornet, Proc. 6th Int. Conf. on Amorphous and Liquid Semiconductors, Leningrad, USSR (1975).
326 [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
A. d'Anjou, F. Sanz / Radial distribution analysis o f amorphous Alo.23Teo. 77 J.A. Savage, J. Non-Crystalline Solids 11 (1972) 121. B.E. Warren, X-Ray Diffraction (Addison-Wesley, Reading, Mass., 1969). N.J. Shevchik, Ph.D. Thesis, Harvard University (1972). International Tables for X-Ray Crystallography, Vol. III, eds. C.H. MacGillavry, G.D. Rieck and K. Lousdale (Kynoch Press, Birmingham, 1962). A. Bienenstock, F. Betts and S.R. Ovshinsky, J. Non-Crystalline Solids 2 (1970) 347. F. Betts, A. Bienenstock and S.R. Ovshinsky, J. Non-Crystalline Solids 4 (1970) 554. F. Betts, A. Bienenstock, D.T. Keating and J.P. de Neufville, J. Non-Crystalline Solids 7 (1972) 417. J. Cornet and D. Rossier, J. Non-Crystalline Solids 12 (1973) 85. T. Ichikawa, Phys. Stat. Sol. (b) 56 (1973) 707. P. Unger and O.P. Cherin, in: The Physics of Selenium and Tellurium, ed. W. Cooper (Pergamon Press, New York, 1969) p. 223. M.S. Mirgalovskaja, E.V. Skudnova, Izv. Akad. Nauk. SSRR, Otd. Tekh. Nauk. Met., Topl. 4 (1959) 148. From Struct. Rep. 23 (1959) 14. Ya.I. Stetsiv, Soy. Phys. Crystallogr. 18 (1973) 306. F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 3rd ed. (Interscience, New York, 1972) p. 278.
R A D I A L DISTRIBUTION ANALYSIS OF AMORPHOUS Alo.23Teo.77 BY X-RAY D I F F R A C T I O N Alicia d ' A N J O U and Francisco SANZ Facultad de Fisieas, Universidad de Navarra, c/Urdaneta, 7 San Sebastian, Spain Received 7 July 1977 Revised manuscript received 21 December 1977 An atomic radial distribution analysis has been made on bulk AIo.23Te0.77 by X-ray diffraction techniques. The radial distribution function (RDF) shows peaks at r I = 2.72, r 2 = 3.51 and r 3 = 4.07 A. The second one is small but well resolved and is interpreted as exclusively due to Te-Te non-bonded separation as in the interchain distance in crystalline Te. The model, which agrees well with both RDF features and bonding conditions, consists of a three-dimensional covalent network of Te atoms around the Al atoms, under an identical bonding scheme as in crystalline A12Te3, the excess Te atoms forming chains which probably join together the groups with A1. The possibility that these chains are a separated phase of amorphous Te cannot be discarded. This model is in agreement with that postulated by Cornet for Te-X glasses with several X, including AI.
1. Introduction A model o f short-range structure in amorphous semiconductors o f type T e - X , for several X including A1, has been postulated on the basis of the knowledge o f the structure o f G e - T e and A s - T e glassy systems and density considerations. The model agrees with the observed glass-forming ability maxima in these materials [1], but more direct p r o o f is still lacking. On the other hand, in a study of the glass formation in G e - T e - X and A s - T e - X systems, it was found that the presence o f A1 as a third component enhances glass formation in these telluride systems [2]. Knowledge of the structural role that aluminium plays in the amorphous A1-Te might help in understanding the observed phenomena. Here, we report the results of a X-ray radial distribution analysis o f Alo.23Teo.77 which is the eutectic composition in the A1-Te system [ 1 ].
2. Experimental The Alo.gaTeo.77 sample used was obtained from commercial A1 and Te o f 99.99% purity. Appropriate proportions o f both elements were sealed in a quartz 319
320
A. d 'Anjou, F. Sanz / Radial distribution analysis of amorphous AIo.23Teo. 77
ampoule under a neutral Ar atmosphere and then melted. The molten mixture was rotated for four days inside a furnace at a temperature of about 700°C and then quenched in liquid nitrogen. The bright metallic solid obtained from the quenching was ground to a fine powder (<325 mesh) which was compacted by pressure into a brick of approximately 30 X 12 X 1 mm 3. No evidence of crystallinity was found in a conventional X-ray diffraction experiment. The diffraction intensities were measured on a Norelco diffractometer equipped with a bent graphite monochromator, scintillation counter and standard electronics. The radiation used was MoKa(k = 0.71069 A). Four series of data were collected in the interval 7 ° ~< 20 ~< 110 °, two at increasing angles in 20 and the other two in reverse. From 7 to 70 ° a step size of A(20) = 0.2 °, and from 70 to 110 ° of A(20) = 0.5 °, were used. Times were measured by keeping a fLxed number of counts. The intensity assigned to each observation point was the mean value of those measured at the point. Most of the averaged values lie within 3% of the average, with a maximum deviation of 4%. The density of the material measured with a pycnometer was 5.20 g/cm 3 with an estimated error of -+3%.
3. Radial distribution function (RDF) The observed intensities were corrected for background, polarization and multiple scattering. The last correction, for which Warren's method was used [3], had an influence on the data varying from 0.4 to 7%. Compton scattering was evaluated, taking in account the efficiency of the monochromator, and following the procedure described by Shevchik [4]. The intensities were put into electron units by the high-angle method and the incoherent scattering subtracted. Fig. 1 shows the normalized intensity curve as a function o f s (s = 47r sin 0/h). We calculated the functions
Io u i(s) -
;,
J
F(s) = s. i(s)
( G. xj. I
and 2 Smax
G(r) =-~ f
F(s)" sin(s" r) ds,
o
where x / i s the atomic fraction of the j element, being j = A1, Te, and Ie.u the normalized intensity minus the incoherent scattering. The individual scattering factors were taken from ref. [5] and corrected for anomalous dispersion.
A. d 'Anjou, F. Sanz / Radial distribution analysis o f amorphous Alo. 23Teo. 77
321
3000-
I(¢.u) 2000'
\ ',
A 123Te 77
1000 -
0 0
i.
i
I
I
I
I
I
I
2
Z,
6
8
10
12
14
16
s(/,-1) Fig. 1. Normalized intensity curve as f u n c t i o n o f s (s = 4~r sin 0/X). The dashed line is the scattering which is i n d e p e n d e n t of the structural features.
The F(s) function was theoretically extended to Smax = 30 A -1, to avoid spurious oscillation in G(r) below the first significant peak due to the lack of high s data. This extension was made assuming, like Shevchik [4], that for high values of s, F(s) might be described by a function of the type F(s) = (c/a) sin(sa) e x p ( - o2s2/2), and where we considered c, a and a as parameters which were found by a leastsquares process as the set which makes the best fit between the postulate function and the last two weU-def'med oscillations of the experimental curve. Finally, the RDF was calculated from 47rr 2 p(r) = 47rr 2 Po + rG(r) , where Po is the mean atomic distribution over the sample and was directly deduced from the experimental density, and p(r) the local atomic density affected by the transform of the f d ~ / ( Z i x ~ ) 2 products.
A. d 'Anjou, F. Sanz / Radial distribution analysis of amorphous Alo.23Teo. 77
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A. d'An/ou, F. Sanz / Radial distribution analysis o f amorphous Alo.23Teo. 77
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The theoretical extension of the experimental data did not affect the significant features of the RDF, as may be seen by comparison of figs. 2 and 3 where the RDF curves from the data before and after the extension have been plotted. All the calculations were made on a HP 2100 S computer of the Computation Center of the Centro de Investigaciones T6cnicas de Guiptizcoa with programs written by us.
4. Results and discussion The analysis of the RDF may be summarized by the values given in table 1. The general features which show the RDF of the amorphous Alo.23Teo.77 are very similar to those found in the radial distribution analysis of the amorphous alloy systems G e - T e and As-Te [ 6 - 9 ] . The most noticeable difference is the small but well-resolved peak which in our case appears at r = 3.51 A and which we do not think is a spurious one. In this respect, Ichikawa [10] has discussed the possibility of the existence of a similar peak in his radial distribution study of amorphous Te films, and from the published RDF of Geo.llTeo.89 (fig. 2 in ref. [7]); the swell of the curve at about r = 3.5 A also suggests such a possibility. On the other hand the 3.5 A distance is precisely the smallest distance between two non-bonded Te atoms that occurs in crystalline Te [11 ], and given the high proportion of this element in the alloy is not surprising that this could occur here also. The first-neighbour peak is located at a distance of 2.72 A which is, as expected, between the 2.80 A T e - T e bond length in amorphous Te [10] and the sum of the covalent radii of the Te and AI of 2.53 A. We think that in this case the formation of metallic A1-A1 bonds is improbable, at least in a sufficient amount to influence the average. The peak at 4.07 A is at a distance a little shorter than the 4.35 A found for a similar distance in amorphous Te films [10]. This distance will be finally governed by the bond angle which may be smaller in our case. However, from the data of the crystal structure of A12Te3 [12] our value is in excellent agreement with the 4.08 A smallest T e - T e separation in the A12Te 3 crystal. The areas under the peaks calculated by the method proposed by Stetsiv [13], their values being listed in table 1. The errors are estimated. Table 1 Peak positions and area under the peaks of the RDF of AIo.23Te0.77. Errors are estimated Peak
r (A)
Area (atoms)
a
1st 2nd 3rd
2.72 3.51 4.07
2.0 1.8 8.0
0.10 0.15 0.20
324
A. d 'Anjou, F. Sanz / Radial distribution analysis of amorphous Alo.23Te o. 77
In this case the greatest structural information comes from data with s less than 6 A -1. We calculated the RDF using only these data, and it was found that the area under the peak at 2.7 A was 2.1 atoms, a value which differs only by 5% from that given for the same area in the final RDF. On the other hand, the calculation of the fi~/(Exj.fl.) 2 products showed that they were approximately linear throughout the entire range of 0 and within the interval 0 ~< s ~< 6 A -1 the greatest deviations of ft~/(Exi)~) 2 from the correspondent Z i Z j / ( E x / Z j ) 2 are about 12% in the A1-Te pairs and only about 5% in the T e - T e pairs. Thus, we made the approximation of treating these products as constant and equal t o Z i Z / / ( Z x j Z / ) 2 in the calculation of the first-neighbour peak area from the models. For this purpose the formula used was as follows: K"x
1
area -
Z.J x j nijZjZ i ,
jzj)
i, j = 1 , 2 ,
'J
I
where 1 means A1 and 2 means Te, xj is the atomic fraction of the jth element and nij is the averaged number of the/-type of atoms which are in the first coordination sphere of an atom of the type j. With the above approximation, the area under the first peak of the RDF is a kind of weighted average of the coordination numbers of the elements which form the alloy, and in terms of these coordination numbers, the structural models may be discussed first. The only crystalline compound of A1 and Te so far known is the unstable A12Te3 which has a wurtzite type of structure [12], with holes randomly distributed among the A1 positions. The A1-Te bond length is 2.54 A (calculated from ref. [12]. In this structure every A1 atom is tetrahedrally coordinated with four Te atoms, and the bonding situation may be formally regarded as that each A1 atom takes an electron from one of the Te atoms which are surrounding it, making a s p 3 tetrahedral hybridization. The Te ÷ which gave the electron will be threefold coordinated, while the rest of the Te atoms will be a twofold coordinated. In this structure an atom of threefold coordinated Te is produced for every A1 atom present in the compound and no A1-A1 bonds are formed. We tried a model assuming that around the A1 atoms the bonding scheme of the A12Te3 crystal is conserved, that the Te atoms in excess of the 2 : 3 composition are twofold coordinated and that all the bonds are saturated. Under these conditions the calculated area for the first-neighbour peak is 1.92 atoms, in agreement with the experimental results. The model is also consistent with all other features of the observed RDF and agrees perfectly with Cornet's hypothesis [1] on the local order of T e - X glasses. An average example of this model might be a complex threedimensional covalent random network where the 23% A1 atoms of the material are bonded to the 34.5% Te atoms, in the same way to build up a crystal structure; the
A. d'Anfou, F. Sanz / Radial distribution analysis o f amorphous Alo.23Teo. 77
325
rest (42.5%) are Te atoms forming chains which probably join the tetrahedral groups, but may be a separate phase of amorphous Te. Models made with a coordination number less than 4 for A1, should be discarded. Chain-like structures devised with coordination numbers of 1 or 2 for this element would fit the experimental RDF quite well, but such coordinations in A1 have been found only in a few gaseous short-life species and never in the solid state [14]. Threefold coordination for A1 from a planar sp 2 hybridization of the three outer electrons, probably unstable, gives a calculated area for the first RDF peak (1.74) atoms) far below the observed value. In Al-Group VI compounds A1 acting with a coordination number greater than 4, has been found in corundum (A1203) and ~/-A12Sa. In these compounds the metal atom has an octahedral coordination while the partner is fourfold coordinated. A model devised with such conditions, plus a coordination number of 2 for the excess Te, gave an area of 2.26 atoms, significantly larger than the observed one; also, such a configuration should produce a T e - T e peak at about 5 A that is clearly absent in our radial function. Higher coordination for A1 will increase the calculated area under the first peak and will disagree with experiment.
5. Conclusions From the above, we conclude that the most probable short-range order structure of this material consists of a three-dimensional covalent network of Te atoms around tetrahedrally coordinate A1 atoms. The Te atoms not bonded to the A1 atoms can be either chain-forming, linking the tetrahedral groups, or a separated phase of amorphous Te.
Acknowledgment We thank Dr. Shevchik for providing us with a copy of his Ph.D. thesis. We also t h a n k the Departamento de Metalurgia del Centro de Investigaciones T6cnicas de Guipfizcoa for the use of their X-ray equipment, and the Computation Center of CIT de Guiptizcoa for calculation facilities. This work is a part of a Ph.D. thesis of one of us (A.d'A) who thanks the Ministerio de Educaci6n y Ciencia for a Fellowship of Formaci6n de Personal Investigador, that made it possible.
References [1] J. Cornet, Proc. 6th Int. Conf. on Amorphous and Liquid Semiconductors, Leningrad, USSR (1975).
326 [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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