ISSN 1087-6596, Glass Physics and Chemistry, 2007, Vol. 33, No. 5, pp. 467–470. © Pleiades Publishing, Ltd., 2007.Original Russian Text © G.A. Bordovskii, R.A. Castro, A.V. Marchenko, P.P. Seregin, 2007, published in Fizika i Khimiya Stekla.

467

INTRODUCTION

In our earlier work [1], we carried out the Möss-bauer investigation of the (As

2

Se

3

)

1 –

z

(SnSe)

z

–

x

(GeSe)

x

glasses and established that tin ions in the structure ofthese glasses can be stabilized in both the doublycharged Sn

2+

and quadruply charged Sn

4+

states. More-over, it was revealed that the physicochemical proper-ties of the glasses under investigation depend on theconcentration ratio of germanium selenide GeSe andtin selenide SnSe in the sample. The purpose of thepresent work was to investigate the thermal stability ofthe doubly charged Sn

2+

and quadruply charged Sn

4+

states in the (As

2

Se

3

)

0.4

(SnSe)

0.3

(GeSe)

0.3

glass.

SAMPLE PREPARATIONAND EXPERIMENTAL TECHNIQUE

Glasses of the composition(As

2

Se

3

)

0.4

(SnSe)

0.3

(GeSe)

0.3

were synthesized by melt-ing arsenic selenide As

2

Se

3

, tin selenide SnSe, and ger-manium selenide GeSe in evacuated (to a residual pres-sure of 10

–3

mmHg) thin-walled silica glass tubes at atemperature of 1250 K for 4 h with vibrational stirringof the melt. Then, the melt sample weighing 5 g wasquenched either in air, or into ice-cold water, or bypouring the melt onto a metal slab cooled with liquidnitrogen from the temperature of the melt (1250 or1350 K). The vitreous state of the samples was judgedfrom the X-ray amorphism and homogeneity in thecourse of the examination of the polished surfaces ofthe samples with the use of a metallurgical microscopeand during the examination of the samples with aninfrared microscope.

The binary compounds of tin with selenium andarsenic were synthesized by alloying elemental sub-stances Sn and Se (Sn and As) taken in the stoichiomet-ric ratios. The samples of the alloys thus prepared wereannealed at a temperature somewhat below the solidustemperature for 350 h. Then, the single-phase composi-tion of the samples was checked using the X-ray pow-der diffraction analysis on a DRON-6 diffractometer.

The

119

Sn Mössbauer spectra were recorded at atemperature of 80 K with a Ca

119

m

SnO

3

radiationsource. The isomer shifts in the Mössbauer spectra aregiven with respect to SnO

2

. The errors in the determina-tion of the isomer shifts, the quadrupole splittings, andthe line widths were equal to

±

0.02,

±

0.03, and

±

0.03 mm/s, respectively. The fraction of doublycharged Sn

2+

ions in the structure of the(As

2

Se

3

)

0.4

(SnSe)

0.3

(GeSe)

0.3

glass was determined

from the ratio

P

= , where

S

-

II

and

S

-

IV

are

the areas under the spectra of Sn

2+

and Sn

4+

, respec-tively. The error in the determination of the fraction ofSn

2+

ions in the structure of the glass was equal to

±

0.02. The photoelectron spectra were recorded on aHewlett Packard spectrometer. The error in the determi-nation of the electron binding energy was

±

0.1 eV. Theexperimental temperature dependence of the electricalconductivity was obtained by comparing with a refer-ence resistance. The optical band gap

E

0

was deter-mined at a temperature of 293 K from the location ofthe optical absorption edge with the use of the samplein the form of 20-

µ

m-thick films.

S-IIS-II S-IV+----------------------------

Thermal Stability of Tin Charge States in the Structure of the (As

2

Se

3

)

0.4

(SnSe)

0.3

(GeSe)

0.3

Glass

G. A. Bordovskii, R. A. Castro, A. V. Marchenko, and P. P. Seregin

Herzen Russian State Pedagogical University, nab. Reki Moiki 48, St. Petersburg, 191186 Russiae-mail: ppseregin@hotmail.ru

Received February 7, 2007

Abstract

—Two valence states of tin atoms (namely, the doubly charged Sn

2+

and quadruply charged Sn

4+

states) in the structure of the (As

2

Se

3

)

0.4

(SnSe)

0.3

(GeSe)

0.3

glasses are identified by

119

Sn Mössbauer spectros-copy. It is demonstrated that the concentration ratio of the doubly charged Sn

2+

and quadruply charged Sn

4+

states in the glass of this composition depends on the rate of quenching of the melt and on the initial temperatureof the melt before quenching. The optical band gap and the activation energy for electrical conduction of thestudied glass do not depend on the concentration ratio of the Sn

2+

and Sn

4+

ions. This behavior of the opticalband gap and the activation energy is explained within the model according to which the structure of the glassesunder investigation is built up of the structural units AsS

3/2

, As

2/2

Se

4/4

, GeSe

4/2

, SnSe

4/2

, and SnSe

3/3

, which cor-respond to the compounds AsSe

3

, AsSe, GeSe

2

, SnSe

2

, and SnSe, respectively.

DOI:

10.1134/S1087659607050069

468

GLASS PHYSICS AND CHEMISTRY

Vol. 33

No. 5

2007

BORDOVSKII et al.

EXPERIMENTAL RESULTS AND DISCUSSION

The Mössbauer spectra of the glasses (Fig. 1) repre-sent a superposition of the single broadened line (thespectrum of Sn

4+

with the isomer shift

δ

= 1.70 mm/sand the line width at half-maximum

G

= 1.20 mm/s)and the quadrupole doublet (the spectrum of Sn

2+

withthe isomer shift

δ

= 3.55 mm/s, the quadrupole splitting

∆

= 0.74 mm/s, and the width of the components of thequadrupole doublet

G

= 0.95 mm/s (Fig. 1, spectrum

a

)). The parameters of the Sn

4+

and Sn

2+

spectra do notdepend on the composition of the glasses, the rate ofquenching of the melt, or the temperature of the melt.

The structure of the glass under investigation, inprinciple, can contain structural units that are formedby tin atoms and are characteristic of binary com-pounds of tin with selenium or arsenic. For this reason,

we measured the Mössbauer spectra of all the knowncompounds of tin with selenium and arsenic.

As was shown by Nasredinov et al. [2], the tin–arsenic system includes the individual compoundsSnAs and Sn

4

As

3

. The Mössbauer spectra of these com-pounds represent single lines (see Fig. 2, spectrum

a

with

δ

= 2.70 mm/s and

G

= 1.40 mm/s for SnAs andspectrum

b

with

δ

= 2.80 mm/s and

G

= 1.55 mm/s forSn

4

As

3

). The formulas SnAs and Sn

4

As

3

can be rewrit-

ten as As

3–

and , respectively.It can be seen from the latter two formulas that part ofthe tin atoms in these compounds have a free pair oftheir associated bound charge electrons. Furthermore,all tin atoms in these compounds are located in the octa-hedral environment formed by arsenic atoms and, as aconsequence, the crystal structure does not prevent theoccurrence of free electron exchange between the Sn

4+

and Sn

2+

ions. Since the Mössbauer spectra of thebinary compounds of tin with arsenic do not exhibitlines with the chemical shift corresponding to different

Sn0.52+ Sn0.5

4+ Sn3.52+ Sn0.5

4+ As33–

6420–2Velocity, mm/s

Relative counting rate

a

b

c

d

Sn4+

Sn2+

Sn2+

Sn2+

Sn2+

Sn4+

Sn4+

Sn4+

SnO2

Fig. 1. 119Sn Mössbauer spectra of the(As2Se3)0.4(SnSe)0.3(GeSe)0.3 glasses prepared (a–c) byquenching the melt from a temperature of 1250 K (a) in air,(b) into ice-cold water, and (c) onto a metal slab cooled withliquid nitrogen and (d) by quenching the melt from a tem-perature of 1350 K onto a metal slab cooled with liquidnitrogen. Shown are the spectra assigned to the doublycharged Sn2+ and quadruply charged Sn4+ ions in the struc-ture of the glasses and the spectrum attributed to the SnO2phase (formed as a result of the oxidation of the glass uponpouring the melt onto the metal slab).

–2 0

SnO2

Velocity, mm/s

Relative counting rate

2 4 6

Sn4+

Sn4+

Sn2+

Sn2+

Sn2+

Sn4+

a

b

c

d

e

f

Fig. 2. 119Sn Mössbauer spectra of (a) SnAs, (b) Sn4As3,(c) SnSe, and (d) SnSe2 compounds and (e, f) the SnSe2alloys prepared (e) by quenching the melt into ice-coldwater and (f) by quenching the melt onto a metal slab cooledwith liquid nitrogen. Shown are the spectra assigned to thedoubly charged Sn2+ and quadruply charged Sn4+ ions inthe structure of the glasses and the spectrum attributed tothe SnO2 phase (formed as a result of the oxidation of theglass upon pouring the melt onto the metal slab).

GLASS PHYSICS AND CHEMISTRY Vol. 33 No. 5 2007

THERMAL STABILITY OF TIN CHARGE STATES 469

charge states of tin atoms, the lifetime of particularcharge states of tin atoms is considerable shorter thanthe lifetime of the 119mSn Mössbauer level. It is for thisreason that the Mössbauer spectra contain a single linewith the chemical shift corresponding to an averagedcharge number of tin atoms in these compounds.

The tin–selenium system includes the compoundscontaining doubly charged (SnSe) and quadruplycharged (SnSe2) tin ions [2]. Figure 2 shows the Möss-bauer spectrum of tin monoselenide (spectrum c in theform of a quadrupole doublet with δ = 3.55 mm/s, ∆ =0.65 mm/s, and G = 0.80 mm/s) and the Mössbauerspectrum of tin diselenide (spectrum d in the form of asingle broadened line with δ = 1.55 mm/s and G =1.10 mm/s).

A comparison of the Mössbauer spectra of glassesand binary compounds of tin with selenium and arsenicdemonstrates that tin ions in the structure of the glassesunder investigation can be stabilized in two oxidationstates, namely, Sn4+ and Sn2+. It is worth noting that, inboth states, the nearest environment of tin atomsinvolves selenium atoms. The fraction of doublycharged Sn2+ ions (P) in the structure of the glassesincreases both with an increase in the rate of quenchingof the melt (as is clearly seen from spectra a, b, and c inFig. 1, for which the fraction of doubly charged tin ionsP = 0.60, 0.69, and 0.75, respectively) and with anincrease in the temperature of the melt (spectra c and din Fig. 1, for which P = 0.75 and 0.81, respectively). Itshould also be noted that even samples of the glasseswith the maximum amount of Sn2+ ions exhibit all fea-tures characteristic of glass (X-ray amorphism, trans-parency in the IR spectral range, absence of microinclu-sions, conchoidal fracture), and, hence, the possibilityof precipitating doubly charged tin in the form ofmicrocrystalline inclusions must be ruled out.

The charge state of germanium atoms in the struc-ture of the glasses was also identified by photoelectronspectroscopy. It was established that the binding energyof the 3d3/3 and 3d5/2 electrons in the germanium atomsdoes not depend on the conditions of quenching of themelt and is close to the binding energy of the Ge 3d3/3and Ge 3d5/2 electrons in the GeSe2 compounds(31.3 eV). Apparently, these findings indicate that, inthe structure of the glasses under investigation, germa-nium occurs only in the form of quadruply chargedGe4+ state, whereas tin can reside in the form of both thequadruply charged Sn4+ and doubly charged Sn2+ states.

Since the composition of the batch involves germa-nium and tin monoselenides, the charge states of ger-manium and tin atoms can change during the synthesisof the glasses only as a result of the occurrence of thefollowing processes: As2Se3 + SnSe 2AsSe +SnSe2 and As2Se3 + GeSe 2AsSe + GeSe2. In thiscase, an increase in the fraction of doubly charged Sn2+

ions in the structure of the glass with an increase in therate of quenching of the melt, as well as with an

increase in the temperature of the melt, is determinedby the thermal instability of the quadruply charged Sn4+

state. In particular, we carried out an investigation intothe thermal stability of the SnSe and SnSe2 compounds.It turned out that the quenching of the melt from themelting temperature does not affect the Mössbauerspectrum of the SnSe alloy. However, after the samequenching, the Mössbauer spectrum of the SnSe2 alloyexhibits an additional quadrupole doublet, which corre-sponds to the SnSe phase (Fig. 2, spectra e, f) (δ =3.55 mm/s, ∆ = 0.65 mm/s, and G = 0.90 mm/s). (Notethat, in the case when the quenching is performed bycooling the melt into ice-cold water, the relative areaunder the Mössbauer spectrum of the SnSe compoundamounts to 0.16; however, when the melt sample isquenched by pouring the melt onto a metal slab cooledwith liquid nitrogen, the relative area under the Möss-bauer spectrum of the SnSe compound increases to0.52.) This can be explained by the occurrence of thereaction SnSe2 SnSe + Se. It is obvious that a sim-ilar reaction occurs in the course of the rapid quenchingof the melt during the synthesis of the glass, eventhough excess selenium in this case is bound accordingto the reaction 2AsSe + Se As2Se3.

Although the concentration ratio of the Sn2+ andSn4+ ions in the structure of the glass of the given com-position depends substantially both on the rate ofquenching of the melt and on the initial temperature ofthe melt before quenching, the optical band gap E0 isvirtually independent of the conditions used for coolingof the melt (Fig. 3b). The temperature dependences ofthe electrical conductivity of the glasses exhibit an acti-

vation behavior: σ = σ0exp . It is worth noting

that the activation energy for electrical conduction ofthe studied glass is likewise independent of the coolingconditions of the melt. These findings can be explainedin the framework of the model according to which the

Eσ

kT------–

1.35

1.250.6

EO

, eV

PSn2+0.7 0.8

1.55

1.65

Eσ,

eV

Fig. 3. Dependences of (a) the activation energy for electri-cal conduction Eσ and (b) the optical band gap E0 on thefraction of doubly charged tin ions in the structure of the(As2Se3)0.4(SnSe)0.3(GeSe)0.3 glass.

(a)

(b)

470

GLASS PHYSICS AND CHEMISTRY Vol. 33 No. 5 2007

BORDOVSKII et al.

structure of the (As2Se3)1 – z(SnSe)z – x(GeSe)x glasses isbuilt up of the structural units AsSe3/2, As2/2Se4/4,GeSe4/2, SnSe4/2, and SnSe3/2, which correspond to thecompounds As2Se3, AsSe, GeSe2, SnSe2, and SnSe,respectively. The replacement of tin diselenide (E0 =1.0 eV) in the structure of the glasses by tin monose-lenide (E0 = 0.8 eV) and the replacement of AsSe (E0 =1.67 eV) by As2Se3 (E0 = 1.65 eV) due to the occur-rence of processes of the types SnSe2 SnSe + Seand 2AsSe + Se As2Se3 have no effect on the opti-cal band gap of the glasses, because the optical bandgaps E0 of the aforementioned binary compounds areclose to each other.

CONCLUSIONS

Thus, the above investigation has revealed twovalence states of tin atoms (namely, the doubly chargedSn2+ and quadruply charged Sn4+ states) in the structureof the (As2Se3)0.4(SnSe)0.3(GeSe)0.3 glass. It has beendemonstrated that the concentration ratio of thesecharge states in the glass is determined by the rate ofquenching of the melt and the initial temperature of the

melt before quenching. The optical band gap and theactivation energy for electrical conduction in the glassof this composition do not depend on the concentrationratio of the Sn2+ and Sn4+ ions. This behavior of theoptical band gap and the activation energy for electricalconduction is explained within the model according towhich the structure of the glasses under investigation isbuilt up of the structural units AsSe3/2, As2/2Se4/4,GeSe4/2, SnSe4/2, and SnSe3/3, which correspond to thecompounds AsSe3, AsSe, GeSe2, SnSe2, and SnSe,respectively.

REFERENCES1. Bordovskii, G.A., Castro, R.A., Seregin, P.P., and

Dobrodub, A.A., Properties and Structure of(As2Se3)1 − z(SnSe)z – x(GeSe)x and(As2Se3)1 − z(SnSe2)z – x(GeSe2)x Glasses, Fiz. Khim.Stekla, 2006, vol. 32, no. 3, pp. 438–445 [Glass Phys.Chem. (Engl. transl.), 2006, vol. 32, no. 3, pp. 320–325].

2. Nasredinov, F.S., Nemov, S.A., Masterov, V.F., and Sere-gin, P.P., Mössbauer Studies of Negative-U Tin Centersin Lead Chalcogenides, Fiz. Tverd. Tela (St. Petersburg),1999, vol. 41, no. 11, pp. 1897–1917 [Phys. Solid State(Engl. transl.), 1999, vol. 41, no. 11, pp. 1741–1758].