Structure of As2Te3 glass, influence thermal processing

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ELSEVIER

Journal of Non-CrystallineSolids 223 (1998) 86-90

Structure of As2Te 3 glass, influence thermal processing A. Tverjanovich a,*, M. Yagodkina b, V. Strykanov b a

Chemical Department, St. Petersburg State University, Universitypr. 2, 198904 St. Petersburg, Russia b Mekhanobr-Analyt Co., 199026 St. Petersburg, Russia

Received 10 February 1997; revised 10 September 1997

Abstract A comparison of atom radial density distribution curves for quenched and annealed A s 2 X 3 (X = Se, Te) glasses shows that quenched and annealed As2Se 3 and annealed As2Te 3 have a similar structure, but the local order in quenched As2Te 3 glass differs. This result is explained by a semiconductor-metal transition in As2Te 3 liquid. © 1998 Elsevier Science B.V.

1. Introduction The local structure of As2Te 3 glass and film has been examined by X-ray and electron diffraction methods [1-6], but the sample preparation regime affects on the local structure have not been reported, to our knowledge. This work will investigate the effects of annealing on local order in As2Te 3 glass as detected by X-ray diffraction.

2. Experimental As2Te 3 was synthesized by reaction of the requisite elements in an evacuated quartz ampoule. A glass was produced by quenching the liquid in a thin wall quartz ampoule (wall thickness = 0.6 m m and diameter = 5 mm) in cold water. AszTe 3 glass was annealed at 85°C for 370 h under vacuum. It is difficult to produce bulk As2Te 3 glass for diffraction measurements because it crystallizes. Therefore the

samples for diffraction measurement were prepared by pressing tablets from the powdered glass. The experiments were done at the 'Geigerflex' D / m a x - R C (Rigaku) diffraction system using Cu K s monochromatized radiation. Measurement details were: scan step = 0.01 °, rate---2°/min. Radial distribution functions were calculated with original Rigaku software [13,14]; measured intensities were corrected for air scattering, the polarization and the absorption, converted to absolute units and corrected for inelastic scattering, followed by Fourier transformation. Fourier transformation gives the radial distribution function 4 ~ r r 2 p ( r ) = 4~r r2p0

2rf]q[S(q)

+ --

where q = 4~r sin O/A, r is the distance, p ( r ) is the average number density function, Po is the average number density of the sample and S ( q ) is the average structure factor, ( S ( q ) = [ ,cOh(q) _ ( ( f 2 )

* Corresponding author. Tel.: +7-812 428 4084; fax: +7-812 428 6939; e-mail: [email protected].

- 1]sin o r d q ,

'IT

_ (f)z)l/(f)2),

where Ic°h(q) is the coherent scattering intensity in

0022-3093/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0022- 3093(97)00433-X

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A. Tverjanovich et aL / Journal of Non-Crystalline Solids 223 (1998) 86-90

electron units per atom, (f2) the mean square scattering factor and ( f ) 2 the square of the mean scattering factor.

50

4O

3°

3. Results

The radial distribution function (RDF) of an annealed As2Te 3 sample (Fig. 1(1)) is similar to results of previous works [2,3] and is typical for all arsenic sesquichalcogenide (Fig. 1(2) shows the RDF of As2Se 3 glass measured for comparison). The difference between RDF of ASETe3 and ASESe 3 glasses in peak positions (interatomic distances) is due to an increase of atomic radius from Se to Te. A peculiarity of this displacement is the constant ratio of peak position for at least the first five peaks ( R As2Te3" R A s 2 S e 3 = 1.09 + 0.02, 1 < n < 5). This ra--n "='n ---tio agrees with data of Cervinka and Hruby [2]. Moreover, average atomic coordination number is larger in the As2Te 3 glass compared with the As2Se 3 glass. The RDF of a quenched sample differs from the RDF of an annealed sample (Fig. 2(2, 1), respecAs2Te

As2Se

3

a

C

10

0

010

o12

oi,

610

018

110

Radius, r (nm)

Fig. 2. Comparison between the radial distribution function of annealed (curve 1) and quenched(curve 2) As2Te3 glasses. tively). Two new peaks at 0.46 and 0.55 nm( + 0.1305 nm) appear instead of the peak with maximum at 0.49 nm. Partly, their formation takes place in response to the decreasing atomic density in the second coordination sphere. Moreover, the peak width is smaller for the quenched glass. That means the location of atoms is more correlated in quenched ASETe3 compared to an annealed glass sample. In the case of ASESe 3 a different situation arises. Annealing of a quenched sample orders its structure, as shown by the prominent RDF peaks (RDF of quenched and annealed glass shown in Fig. 3(1, 2), respectively). At the same time annealing does not change the position of As2Se 3 RDF maxima. This lack of change means that the most probable arrangement of atoms remains the same after annealing.

30

20 10

I 0.0

i

I

=

0.2

I

0.4

i

I

0.6

,

I

0.8

i

I

,

1.0

Radius, r (rim) F i g . l. R a d i a l a t o m i c d e n s i t y d i s t r i b u t i o n f u n c t i o n f o r a n n e a l e d As2Se 3 and As2Te 3 glasses.

o

010

o12

Z,

016

o16

110

Radius, r (nm)

Fig. 3. Comparisonbetween the RDF of quenched(curve 1) and annealed (curve 2) As2Se3 glasses.

A. Tverjanovich et al. / Journal of Non-Crystalline Solids 223 (1998) 86-90

88

4. Discussion The facts are as follows: the annealing of an As2Se 3 glass leads to structure stabilization without structure change, while the annealing of an As2Te 3 glass results in structure reconstruction and decreasing structure order. Vaipolin and Poral-Koshits [5] measured quenched As2Te 3 glass sample (Fig. 4, curve 3). The RDF peaks are not well resolved but great density near 0.45 nm and minimum density near 0.5 nm testify to similarity of these data and RDF data measured by us for quenched sample (Fig. 4, curve 2). The tendency of the RDF curve shape alteration from annealed glass to quenched (increasing of nonequilibrium degree) is traced at the next step (from the sample prepared in the hardest regime to the film) also. Fig. 4 shows data of several authors ordered in accordance with sample preparation regime: annealed glass (curve 1 (this work))-quenched glass (curve 2 (this work) and 3 [5])-film (curve 4 [6]). Two explanations of RDF alteration on changing of cooling rate are possible. First, As2Te 3 dissociates in the liquid under heating. When the liquid is cooled

~2 1

L,.

u2

I

0.0

n

I

0.2

n

I

0.4

*

I

0.6

*

I

0.8

n

I

i

1.0

Radius, r (rim)

Fig. 4. Radial atomic distribution functions for As2Te3: (1) annealed glass (this work), (2) quenched glass (this work), (3) quenched glass [5], (4) film [6].

Table 1 Position (R), measured area (S), FWHM of the first RDF peak and bond angle for quenched and annealed As2Te 3 glasses R l (nm) Annealed 0.272+0.0005 Quenched 0.273+0.0005

S

FWHM (nm)

/ a (°)

2.71+0.05 2.82+0.05

0.050+0.001 0.046+0.001

94+ 1 89+1

so quickly that the cooling rate and rate of back chemical reaction (compound formation) is approximately the same, the vitrified liquid consists of structure units of stoichiometric compounds and products of its dissociation. Indeed, As-As and T e Te bonds were found in the As2Te 3 glass [1]. As2Te 3 dissociates in a vapor according to [7] 2As2Te 3 ¢0 As 4 + 3Te 2 . The rate of subsequent vapor condensation is so fast, that the fraction of homobonds (As-As and Te-Te) in the film must be greater than in the quenched sample. The bond length difference between the As-As bond (0.24 nm) and T e - T e bond (0.28 nm) is essential and hence the high concentration of homobonds must broaden the RDF peak of first coordination sphere, but we observe an opposite effect. Full width on half maximum (FWHM) of the first RDF peak in the diffractogram of a quenched sample is less than that of an annealed sample (see Fig. 2, Table 1). The second explanation is the existence of an alternative stoichiometric structure (polymorphic modification) of As2Te 3 forming under rapid cooling only. There are some data consistent with this supposition. X-ray diffraction investigations of the As-Te binary system carried out by several authors [8-11] determined X-ray reflections sets which did not belong to As, Te, or monoclinic ot-As2Te 3. To explain these reflections some metastable compound were proposed: AsTe [8], AsTe 2 [9] and metastable f3modification of As2Te 3 [10,11]. An important note is that all these metastable compounds form from liquid phase under quenching only, and then transform into stable a-As2Te 3 under annealing [8-11]. For example, 13-modification forms from the liquid when quenched with a cooling rate up to the critical cooling rate i.e. the minimum cooling rate of the melt sufficient to prevent crystallization [10]. Besides all these metastable compounds have a more symmetrical structure compared to ct-As2Te3: AsTe is face-

A. Tverjanovich et al. / Journal of Non-Crystalline Solids 223 (1998) 86-90

centered cubic [8], [3-AszTe 3 is rhombohedral [10]. The density and electroconductivity of the As2Te 3 metastable modification are greater than the stable structure [11]. In the same time there is a reversible structural transition from a semiconductor to a metal state for liquid AszTe 3 in the 600-1050 K temperature range [12]. Formation of a more dense and more conductive phase can be explained by freezing of the metal-semiconductor transition when cooling from the liquid state. The shape of the RDF presented in Fig. 1 is defined by trigonal structure units, AsTe3/2, where arsenic is threefold coordinated [3]. The structure of the As 2Se(S) 3 crystal compounds is formed by trigonal pyramids, too [16,17], but in tx-As2Te 3 there are octahedral coordinated arsenic together with threefold coordinated As [15]. Supposedly, in the case of metastable AsTe and [3-AszTe 3 compounds all arsenic are sixfold coordinated [8,10], the octahedral environment being symmetrical in AsTe [8] and distorted in [3-As2Te 3 [10]. Increasing arsenic coordination from 3 to 6 must be accompanied by a decreasing bond angle to 90 ° for the case of a symmetrical octahedron and some increase of bond length. The

89

parameters of RDF for quenched and annealed As2Te 3 glasses are presented in Table 1. The bonds angle is about 90 ° in quenched glass, the bond length and coordination number are slightly larger than in annealed glass (the density of quenched glass is unknown therefore it was taken equal to the density of annealed glass). The RDF of an hypothetical NaCI type crystal structure is shown in Fig. 5(1) (for calculation the following data were used: composition - AsTe, bond length = 0.27 nm, peak area weighing factor for interference between atoms of As and Te = 0.9). The shape of this RDF is similar to the RDF of the quenched sample (Fig. 5(2)), although the atomic coordination in the quenched sample is smaller. Hence we assume that part of the atoms in the quenched glass have octahedral coordination. A direct comparison between the glass and crystal structure of As2Te 3 polymorphic modifications cannot be made, because the average atom coordination number in annealed and in quenched glasses is less than in the monoclinic or the rombohedral modification of As2Te 3.

5. Conclusion Radial distribution functions indicate that the structure of quenched As2Te 3 glass differs from the structure of annealed glass which is typical among other glassy arsenic sesquichalcogenides. In comparison with the structure of annealed glass, the structure of quenched glass is more ordered, and a fraction of the atoms are sixfold coordinated.

1

e-I

Acknowledgements u2

This work was partly supported by Competitive Center for Basic Natural Science of the St. Petersburg State University, grant No. 95-0-9.2-194 and Competitive Center of the Moscow Institute of Electronic Engineering, grant No. 95-1-83.

r~ tw

I 0.0

,

l 0.2

,

I 0.4

*

I 0.6

,

I 0.8

i

I

,

1.0

Radius, r (nm)

Fig. 5. RDF of quenched AszTe3 and calculated RDF of AsTe (NaC1 structureassumption).

References [l] Q. Ma, S. Benazeth, D. Raoux, in: S. Samar Hasnain (Ed.), X-ray Absorption Fine Structure, Ellis Horwood, New York, 1991 p. 715.

90 [2] [3] [4] [5] [6] [7] [8] [9] [10]

A. Tverjanovich et al. / Journal of Non-Crystalline Solids 223 (1998) 86-90

L. (~ervinka, A. Hruby, J. Non-Cryst. Solids 48 (1982) 231. J. Cornet, D. Rossier, J. Non-Cryst. Solids 12 (1973) 85. J. Chang, D.B. Dove, J. Non-Cryst. Solids 16 (1) (1974) 72. A.A. Vaipolin, E.A. Porai-Koshits, Sov. Phys.-Solid State 2 (1960) 1500. Yu.G. Poltavtsev, V.P. Zakharov, V.I. Shvets, Ukr. Fiz. Zh. 18 (4) (1973) 663. A.V. Novoselova, A.S. Pashinkin, Davlenie Para Letuchikh Khal'kogenidov Metallov, Nanka, Moscow, 1978, p. 83. R.K. Quinn, Mater. Res. Bull. 9 (6) (1974) 803. V.A. Kirkinskiy, V.G. Yakushev, Neorg. Mater. 10 (8) (1974) 1431. H.V. Shu, S. Jaulmes, J. Flahaut, Mater. Res. Bull. 21 (12) (1986) 1509.

[11] S. Toscani, J. Dugue, R. Ollitrault, R. Ceolin, Thermochim. Acta 186 (2) (1991) 247. [12] Y.S. Tver'yanovich, V.M. Ushakov, A. Tverjanovich, J. Non-Cryst. Solids 197 (1996) 235. [13] Y. Waseda, The Structure of Non-Crystalline Materials Liquids and Amorphous Solids, McGraw-Hill, 1980. [14] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2rid Ed., Wiley, New York. [15] G.J. Carron, Acta Cryst. 16 (1963) 338. [16] K. Kotaro, P. Munori, S. Hitoshi, S. Syuro, Rep. Progr. Polym. Phys. Japan 12 (1969) 253. [17] A.A. Vaipolin, E.A. Porai-Koshits, Sov. Phys.-Solid State 5 (1963) 186.