Structural imperfections in AsOx glasses

Journal of Non-Crystalline Solids 105 (1988) 69-77 North-Holland, Amsterdam

69

STRUCTURAL IMPERFECTIONS IN AsOx GLASSES M. KOHKETSU, H. KAWAZOE and M. YAMANE Department of Inorganic Materials, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Received 24 December 1987 Revised manuscript received 6 April 1988

Gamma- and ultraviolet (UV)-induced ESR at 77 K and Raman scattering of AsO x glasses with compositions of x = 1.42, 1.5 and 1.6 were examined. Two types of quartet absorption were observed in y-irradiated samples in the 1000-5000 G region. Their multiplicity and splitting width suggested that both absorptions can be assigned to a hyperfine structure due to single 75As (1 = a2) nucleus. One center, which was dominant in AsO1. 6 (O-rich), was assigned to the electron-trapped center on a pentavalent As. The other, which was dominant in ASO1.42 (As-rich), was assigned to an unpaired spin localized predominantly on an As 4p orbital. The precursors of these defects are considered to be O3As=O and A s - A s bond, respectively. The following disproportionation reaction was proposed as a model of formation of the defects under melting process:

(

2 =As-O-As=

)

O .

.

.

.

~=As-As=+=As-O-As=.

I. Introduction

In previous publications [1-3], detection and identification of phtoluminescence (PL) centers were reported in silica-based glasses prepared with varying preparation atmospheres from reducing to oxidizing. Chemical species and concentrations of the imperfections were strongly influenced by the redox condition of melting atmosphere. On the basis of the results of PL, ESR and vacuum ultraviolet (VUV) absorption measurements, a model of the formation mechanism of structural imperfections in silica-based glasses during the melting process was proposed. The essential point of the model is that a series of thermal decomposition reactions should exist in silica melt at a high temperature; the initial reaction is assumed to be a radical decomposition of the Si-O bond, = S i - O - ~ - S i ' + . O - , and there are secondary products, which include Si-Si and O-O homopolar bonds as an intermediate species, due to reactions among the initial products. The net reaction is simply expressed as 2SiO 2 ~ 2SiO 1' + 0 2 1'. 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

(1)

This reaction is accompanied by reduction of tetravalent Si(IV) to divalent Si(II). During the reaction trivalent Si(III) a n d / o r Si(II) would be produced in the silica network. In the subsequent cooling process the reactions become frozen and the reduced species of Si would be quenched in the glass as imperfections. The model can be extended to the other oxide glasses and the chalcogenide glasses, though it must be modified, conforming to their chemical properties. In the case of chalcogenide glasses, A s z S 3 , the following chemical reactions in the melt were suggested [4] as an origin of the formation of defects:

(=gs + 2(.S-) ~-=J~s- k s = and - S - S - .

(2)

This model is derived from the experimental resuits as given below: (1) A number of radical species (=As" and • S-) were present in the melt [4]; (2) From the results of Raman scattering measurements in the Asl_xSx alloy system from

70

M. Kohketsu et al. / Structural imperfections in AsO~ glasses"

S-rich (x > 0.6) to As-rich (x < 0.6), S-S bonds were present in the S-rich glasses and As-As in the As-rich glasses [5,6]. Even in the stoichiometric As2S 3 glasses a small number of As-As and S-S bonds was detected by resonance Raman scattering [7]. In the case of AsO~ glasses, it would also be true that the thermal decomposition reaction in the melt plays an important role in a formation of structural imperfections. As203 glass is regarded as a counter compound of As2S 3 glass in the oxide side. Similar chemical reactions with eq. (2) are expected to occur in the AszO3 melt because of the following reasons: (1) The structure of As203 glass is thought to be composed of macromolecular planes as in the case in As2S3 glass; (2) From Raman measurements [8,9] it is suggested that during the melting process dissociation of the A s - O bond and polymerization proceed thermally; (3) Both As203 and A s z S 3 vaporize as a small molecule such as As406 and As4S4 [8,10,11], respectively. It is noteworthy that in the A s - S system sulfur can easily make a sulfur polymer, S8 ring, whereas in the A s - O system oxygen cannot polymerize. Taylor et al. [12] reported that an unusually large amount ( - 5 X 10 a8 cm -3) of paramagnetic centers, which were stable at 300 K, were induced in As203 glass by X-irradiation at 77 K. The average spin-Hamiltonian parameters (gJl = 1.998, g~_ = 1.984, AIj = 243 G, A ± = 114 G) indicated that those centers were analogous to those previously observed in As2Se 3 and As2S 3 glasses similarly irradiated [12]. They consisted of unpaired electrons localized on non-bonding 4p orbitals of As atoms [13]. No hole-trapped centers on nonbonding oxygen p orbitals have been observed. The precursor of these centers should be formed in a melting process and be quenched in the glass through a cooling process. Therefore, the formation of these structural imperfections would be markedly influenced by the redox condition in the melt. A series of arsenic oxide glasses with compositions of AsO1.42 , AsO1. 5 and AsO1. 6 w e r e prepared and 7- or UV-induced ESR and Raman scattering were measured to detect and identify the defects. On the basis of the measurements the model of

the formation of imperfections in arsenic oxide glasses under a melting process is proposed.

2. Experimental AsO x glasses were made by melting the mixture of starting materials in an evacuated quartz ampoule. Chemical composition of AsOx glasses ranged from x = 1.42 to 1.6 and was changed by varying the mixing ratio of starting materials: for x = 1.42, polycrystalline As203 (arsenolite) and elemental As, and, for x = 1.6, arsenolite and polycrystalline As205, were used, respectively. The ampoule was held in a furnace at 600 ° C. The melt was homogenized for about 10 h. The ampoule was cooled slowly to 260 ° C in a furnace to avoid crystallization of arsenolite and then cooled to room temperature in air. Small pieces of AsO x glass were placed in an ESR sample tube of silica glass. It was sealed under vacuum, and subjected to UV- or 7-irradiation. During the irradiation the tube was immersed in liquid nitrogen. ,/-irradiation was performed using a 6°Co source and the total dose was 1 Mrad. A Xe lamp (300 W) was used for UV irradiations. X-band ESR was measured on a JEOL JES-FE-2XG spectrometer at 77 K. The frequency of microwave and magnetic field were calibrated with a cavity wavemeter and proton N M R field meter. The microwave power was less than 1 mW. The sample for optical measurements was cut and polished from the melt-quenched material in a dry box. Special care was taken to avoid exposing the sample to water vapor because surface devitrification is accelerated by moisture. The optical grade surface of the sample could be exposed to room air with an airconditioner during the experiment. Visible and UV absorption measurements were carried out using a JASCO UVIDEC-610C spectrophotometer at room temperature in air. Raman spectra of arsenic oxide glasses were measured using the 514.5 or 488.0 nm line of a 2 W A r + ion laser for excitation. The scattered light was collected in a 0.25 m double monochromator and detected by a photomultiplier tube. Tips of AsO x glass were placed in a quartz tube and

M, Kohketsu et al. / Structural imperfections in AsO X glasses

sealed under vacuum and subjected to Raman measurement. The Raman spectra were obtained at room temperature in a 90 ° configuration.

3. Results and discussion

3.1, Raman scattering Raman spectra of AsOx glasses whose composition ranged from O-rich (x = 1.6) to As-rich (x = 1.42) were measured to detect their specific structure, such as the As-As or As=O bond. Figure 1 shows the Raman spectra of AsO x glasses. The

4.a 03 c)

71

spectrum of AsOl.5 (b) was substantially the same as those reported in refs. [8,9]. The principal features of A s O I . 6 (c) and ASO1.42 (a) glasses were almost similar to those of (vitreous) v-AsO1. 5, except the small peaks whose assignment will be discussed below. The spectrum of AsO1. 6 included an additional weak peak at 940 cm-1, overlying the Raman mode of v-AsO~. 5. Comparison of the spectrum with that of crystalline As205 (d) or arsenic acid (e) clearly showed that the peak is an indication of the formation of the As=O double bond in the oxygen excess glass. Since an As=O double bond gives higher frequencies, the Raman band at 940 cm-1 was identified as being due to the symmetric stretching mode of the isolated AsO 4 tetrahedron, which included an As=O bond [14,15]. The observation suggests that a quasi-tetrahedral AsO 4 unit having an As=O bond exists in the As203 glass network, this including the fact that pentavalent arsenic atoms exist in the glasses. On the other hand, the spectrum of oxygen-deficient ASO1.42 glass showed additional features at 200, 250 and 350 c m - l . The three sharp peaks are superposed on the main v-AsO~, s Raman modes. These features are associated with a cluster having As-As bonds. It is based on a comparison with the Raman spectra of As-rich glasses in the alloy system AsS x [5,6] or molecular As4S 4 [16], which have direct A s - A s bonds. The dark brown color of ASO1.42 glass also supports the identification because the As clusters including direct As-As bonds make localized states in the band gap of v-AsOx. 5 and cause a shift of the absorption edge to the lower energy side [17].

3.2. y-induced ESR

,

,

1

t

,

i

S00 Raman Shift(cm

,

I

100 -1

)

Fig. 1. Raman spectra of AsO x glasses with composition of (a) x = 1.42, (b) x = 1.5 and (c) x = 1.6. Spectra (d) and (e) show the Raman scattering derived from crystalline As205 powder and arsenic acid aqua solution, respectively.

The ESR derivative spectra observed in asquenched AsO x glasses are shown in fig. 2, (a) x = 1.42, (b) x = 1.5 and (c) x = 1.6. Complicated ESR absorptions exist in the x = 1,5 glass sample in the 3000-4000 G region. A broad absorption centered around 2800 G and a relatively sharp resonance at - 3 4 0 0 G were prominent in the glasses with smaller x. The intensities of these absorptions increased from x = 1.5 to x = 1.42. An As-rich condition seems to enhance the ESR absorption(s) but none of them could be identi-

M. Kohketsu et al. / Structural imperfections in AsO, glasses

72

AsOx g l a s s e s ( a s - q u e n c h e d )

(a)

F

(b}

x = 1.5.:_~..._

J ~

x=1.42 (As-rich)

XIO00

center is similar to the one which has been observed in optically excited and X-irradiated A s 2 S 3 or As2Se 3 [12] glasses or a thermally generated ESR center in amorphous As [18]. This is clearly demonstrated in the close agreement between the observed spectra and the calculated spectra (the upper dashed line in fig. 3). ESR parameters used in the calculation are given in table 1. The calculated trace was obtained by assuming that an unpaired spin is - 9 9 % localized on the As-4p orbital. In comparison with the spectral parameters of (C2Hs)2As" radicals [19], which is trapped in the X-irradiated single crystal of (C2Hs)2AsO, and R2As" radicals [20] in sulfuric acid glasses exposed to 6°Co y-rays at 77 K, the geometrical

e-

X1000

','~s~

0

o,-

-o

0K'~'0

(0-rich)

[-

-I.

0

I

'

r--FT--I I

2300

I

I

280'0 3300 3800 Magnetic Field (Gauss)

I

4300

Fig. 2. ESR spectra of as-quenched AsOx glasses. (a) x = 1.42, (b) x =1.5 and (c) x =1.6. The ESR measurements were carried out at 77 K. Amplifier gains are also given in the figure.

fled. In A s O 1 . 6 n o ESR absorption was observed except for six resolved features associated with Mn 2+ ions contaminating the glass and the narrow absorption at 3300 G. Figure 3 shows the ESR spectra of AsO x glasses y-irradiated at 77 K. The spectra were measured at 77 K without intervening warm-up. The intensity of the derivative spectra was one order of magnitude larger than that observed before the irradiation. Therefore, spectral contamination due to the absorption observed in the as-prepared glasses is slight, but discernible for x = 1.42 glass around 2800 G. Several and broad features are seen in the 1000-5000 G region in the spectra. The relatively sharp features in the central field are the absorptions characterized by a highly anisotropic hyperfine interaction due to 75As, which has nuclear spin of ~ (hereafter referred to as quartet-I). This

X63

~

X[260

J

Tz ' -~.~ X=I, 42

~

----

:JV

xoo

ol

--I O0

A"A

I0

I

I

I

20 0

3000

4000

5000

MAGNETIC FIELD (Gauss)

Fig. 3. ESR spectra of y-irradiated A s O x glasses, y-irradiation and ESR measurements were carried out at 77 K. - - : experimental trace; . . . . . . ; calculated. The ESR parameters employed in the simulation are summarized in table 1. The geometrical structures of two ESR centers are given. Amplifier gains are denoted in the figure.

M. Kohketsu et al. / Structural imperfections in AsO x glasses Table 1 ESR parameters employed in the line shape simulation for quartet-I and I1

Quartet- I gtt =1.998, g± =1,984,

All = 243 G, A l = l 1 4 G

Quartet-ll a) g = 2.0

Aiso = 990 G (oi~o = 95 G)

a~ A Gaussian distribution was introduced for isotropic hyperfine coupling (A,~o) in order to fit the experimental line shape closely. Oiso is the assumed standard deviation of Aiso

[191.

structure of a center responsible for the quartet-I is given in the figure; arsenic is two-fold coordinated by oxygens and has one in-plane lone pair and an unpaired electron occupying the p. orbital perpendicular to the plane. The excess charge around the center is zero. Therefore, it is concluded that one of the -/-induced centers is that given in the figure. Figure 4(a) shows the composition- and annealing dependence of the intensity of quartet-I. It is noted that the signal is enhanced in the glasses with smaller x and decreases by thermal annealing. The structure of a precursor responsible for the ESR center giving quartet-I is considered to be As-As homopolar bonds present in as-quenched glasses on the basis of the following observations: (1) For the AsO~.42 glass the presence of As-As

(a} QUARTET- I

~.

"~

(h) QUARTET- I I

X=I ,5

=1,5

A n n e a l i n g Time(hr) Fig. 4. Change in relative intensity of y-induced ESR centers with varying composition of AsOx glasses. Thermal annealing was carried out at room temperature for 12 h. e: x = 1.42, o: x =1.5; z~: x =1.6.

73

bonds was expected from chemical composition and was experimentally confirmed by Raman scattering (fig. l(c)). (2) The intensity of quartet-I showed a similar composition dependence (fig. 4(a)) with that of Raman scattering due to As-As bonds (3) On y- or photo-irradiation As-As homobonds in As2Se 3 and As2S3 glasses have been known to dissociate to the center whose structure is given in fig. 3. This point will be further confirmed by measuring UV-induced ESR (see sect. 3,3). Besides the central absorptions discussed above several absorptions are noted in the 1000-5000 G region. Although the low field absorptions are overlapping with the absorption due to impurity Fe 3+ ( - 1 5 0 0 G), their intensity increased with increasing x; for x = 1.6 the highermost field absorption ( - 4700 G) was intense enough to be measurable simultaneously with the central absorptions. The absorptions were again the hyperfine quartet (quartet-II) due to 7SAs, but in this case the splitting is far larger (990 G) than the quartet-l. The quartet was assigned to the electron-trapped center on As by Hosono et al. [21] in 3,-irradiated 16Na20 • l l C a O • 73SIO 2 : 15AsO~ (tool%) glass melted under oxidizing conditions. The structure of the center giving the signal as well as the calculated spectrum (the bottom dashed trace) are shown in the figure. ESR parameters used in the calculation are given in table 1. The agreement between the observed and the calculated spectra is satisfactory. In this model, arsenic is fourfold coordinated by four oxygens tetrahedrally or trigonally. An unpaired electron occupies the o* level in the former case or one of the three equatorial orbitals in the latter case. In both cases there is a considerable contribution of the 4s atomic orbital of As to the singly occupied orbital, this giving rise to a large hyperfine coupling constant. The structure of the precursor of quartet-II is likely to be the O3As=O structural group. The assignment is derived from the following observations. (1) With changing glass composition the change in the intensity of quartet-II (fig. 4(b)) was in harmony with that of Raman scattering due to As=O. (2) Quartet-II was detected in oxidized 1 6 N a 2 0 • l l C a O • 73SIO 2 : 0.15AsO~ glass 3,-

74

M. Kohketsu et al. / Structural imperfections in AsO x glasses

Photon Energy(eV) 5.O

,

4.0

3.

4

E E

~3 ~,j, 2 <

1 275 nm

(4.5eV)

1 0 200

I 250

I 300

I 350

400

W a v e l e n g t h (nrn) Fig. 5. Optical absorption spectrum of AsO1. 5 glass measured at room temperature (0.5 mm thick).

irradiated at 77 K, in which most of the As is expected to be in the pentavalent state. A similar result was reported also for phosphate glasses

[22-241.

ments were carried out on AsO1. 5 and A s O 1 . 6 glasses UV-irradiated at 77 K. No ESR absorption was induced in the case of A s O 1 . 6 glass, while distinct peaks were recognized in the case of AsOl.5 glass, whose spectra are displayed in fig. 6. On UV irradiation at 77 K it is clearly seen that a quartet and a doublet were induced in the 2000-5000 G region (fig. 6(b)). The quartet is identical with quartet-I observed in 3,-irradiated glasses and is also assigned to an unpaired electron localized predominantly on a non-bonding As 4p orbital. Since its intensity was one order of magnitude smaller than that of y-irradiated glass, the shape of the absorption was probably influenced by the overlying absorptions which were present in asquenched glass. Similar results have also been observed in As2S 3 or As2Se 3 glasses when they

XlO00__..~_..~

o.is. 0

It is interesting to note in fig. 4 that A s - A s bonds (responsible for quartet-I) are present even in oxygen e x c e s s A s O ] . 6 glass and in contrast O3As--O (pentavalent As) is involved even in o x y g e n - d e f i c i e n t ASO1.42 glass. This fact will be discussed in more detail in relation to the melting process in sect. 3.4.

I

r r

(b) UV(>3OOnm)i rrod, in)

H' t__

__J I~

3.3. UV-induced E S R

Prior to the ESR measurements optical absorption of AsOx glass was examined to determine the appropriate energy of the illumination. The optical gap of As203 glass has not been measured experimentally, but estimated to be in the region of 5 eV. Figure 5 shows the optical absorption spectrum of the A s O 1 . 5 sample. For x = 1.6 and 1.42 bulk samples required for optical measurement could not be obtained. The absorption edge was apparently at 4.5 eV (275 nm), at which energy, the absorption coefficient a exceeded the measurable range of the spectrometer ( - 60 c m - 1). Some of the absorption is thought to be that of Fe 3+ contaminating the glass. The ESR measure-

r

X790

I

(c) RT onnea](3Omin)

X/~__/~ !/~~ 1(30min) v /

I

I

I

1

I

I

2900 3300 3700 MAGNETIC fIELD (Gauss) Fig. 6. Change in UV-induced ESR absorption spectra by thermal annealing. UV-irradiation and ESR measurement were carried out at 77 K. (a) As-quenched; (b) UV-irradiated at 77 K. The brackets in the figure indicate the resonance fields of hfs due to O2As" and atomic hydrogen; (c) Annealed at room temperature for 30 min; (d) Annealed at 473 K for 30 min.

M. Kohketsu et aL / Structural imperfections in AsO x glasses,

weie irradiated with photons with sub-bandgap energy [25]. In both cases the paramagnetic center was expected to be generated by the photolysis of As-As homobonds, because the center was induced in the glasses with As-rich composition, in which many As-As homopolar bonds were included, and even in the glass of stoichiometric composition. The doublet having sharp features was characterized as a hyperfine structure due to atomic hydrogen on the basis of a spin multiplicity of proton (1 = ½) and a splitting (508 G), which has already been reported in a variety of materials [26]. One of the possible sources of photolytically generated atomic hydrogen may be the A s - H group in the glass, but there is no other experimental evidence of existence of A s - H groups• On annealing at room temperature, the doublet due to atomic hydrogen disappeared and the intensity of the quartet remained almost unchanged (fig. 6(c)). On heating at 473 K for 30 min the intensity of the photo-induced center decreased• The shape of spectrum changed to that of asquenched glass by annealing above 533 K (melting temperature of crystalline A s 2 0 3 ) . In the central region of the ESR spectra a single sharp absorption was present in as-quenched glass and the intensity increased in the case of

iadical -Fs"

la)

-]\s-OAs-O

3.4. Model of defect formation during the melting process From the Raman measurements of several investigators [27,8,9] it is revealed that the local structure of glassy A s 2 0 3 is thought to be closer to that which exists in the layered crystalline modification (claudetite I) than that in As203 molecular crystalline form (arsenolite). The local structure in the liquid is considered to be similar to that of the glass from the measurement of Raman scattering at melting temperature. These facts suggest that during the melting process the dissociation of A s - O bond and polymerization should occur thermally. The A s - O bond dissociates forming radical species, =As" and .O-. The large majority of radicals (=As', . O - )

= J ~ s - O - . The following equilibrium is expected in the melt:

-and \ -0-0-

(el

,0

• oxidation

Ascll[)

(1)

A small fraction of the radicals would bond to the identical species and form homobonds or a cluster as shown in eqs. (2) and (3):

homo[,ofar bond bond

would rebond to form

+ .o-A=.

.0~

[d)

normal

UV-irradiated glass at 77 K. On heating at RT the intensity decreased and again increased at 473 K. An additional feature was resolved on the low magnetic field side of this resonance. Since no other information for this absorption is present, identification cannot be made.

species

(bt

75

of

to

As[~.l

Fig. 7. Schematic energy diagram of equilibrium reactions in the glass melt of the AsO x system. Three states are in chemical equilibrium with each other and radical species are viewed as intermediate states between the other two. Process (a): dissociation of A s - O bond into =As" and - O - . (b): Recombination of the radical species to form an A s - O bond. (c): Formation of homopolar bonds or a cluster due to the reaction between two like radical species. (d): Dissociation of h o m o b o n d s into =As • and . O - . (e): As(III) can be easily oxidized to As(V) in the oxide system because peroxy linkage or oxygen cluster in the oxide system is relatively less stable than - S - S - in the A s - S system.

.,

•,

=As" + - J~s= ~ = A s - A s = ,

(2)

= A - o + . O-A =

(3)

A schematic energy diagram of the proposed reactions is displayed in fig. 7. The concentrations of these chemical species in the melt would be determined according to their stability at a given temperature. The important characteristic of the picture is that the three states are in chemical equilibria with each other and radical species are viewed as the intermediate state between the other two.

76

M. Kohketsu et al. / Structural imperfections in AsO x glasses

In the succeeding cooling process these species are quenched in the glass as an imperfection. The reaction model shown in fig. 7 is in essence identical with that proposed for the As2S 3 melt [4]. In the case of AsOx, however, one additional characteristic must be taken into consideration; the - O - O - peroxy linkage in the A s - O system is relatively less stable than - S - S - in the As-S system and As can easily be oxidized to As(V) possibly by the local excess oxygens ( - O - O - ) formed by eq. (3) in the oxide system: O

II

= J ~ s - O - O - J~s= ~ = A s - O - J~'s=.

(4)

This fact indicates that the stability of As(V) is considered to be large in the A s - O system. The net reaction of eqs. (1)-(4) is written as O

2(:x;-o-

x;: +

II

(5) This model can explain the fact that the A s - A s bond is present even in oxygen excess AsO1. 6 glass and in contrast pentavalent As is involved even in o x y g e n - d e f i c i e n t AsO1.42 glass. Since the stability of the pentavalent state of arsenic is considered to be large, eq. (5) proceeds to the right hand side, a small amount of the As(V)-related center (quartetII) can be detected in ASOl.42 glass (As-rich; reducing condition) and As-As bonds can be induced in the AsO1. 6 glass (O-rich; oxidizing condition). The same reactions are observed in the case of phosphate glass melted under reducing conditions [23,24]. As the stability of the pentavalent state of phosphorus is much larger than that of arsenic, the marked disproportionation reactions occurred and clusters of atomic P's were generated in the phosphate glass melted under reducing conditions.

4. Summary We focused on the structural imperfections formed in the melting process in arsenic oxide glass with compositions of ASO1.42 , msO1. 5 and

A s O I . 6. Gamma- and UV-induced ESR at 77 K

and Raman scattering were measured to detect and identify the defects. From the results of Raman scattering measurements, principal features of all three glasses were those of As203 glass. It was found that A s O I A 2 and AsO1. 6 involved additional features such as A s - A s bonds and As=O bonds. Their concentrations were estimated in the order of 1% from the sensitivity of the Raman measurements. We detected the superposition of two types of quartet absorptions in all of the glasses in the 1000-5000 G region by ~,-irradiation at 77 K. One, which was dominant in AsOI.6, was assigned to the electron-trapped center on a pentavalent As. Another, which was dominant in AsOI.4Z , was assigned to an unpaired spin localized predominantly on an As 4p orbital. These assignments were supported by th result of UV-induced ESR. The precursors of each ~,-induced defect were thought to be As(V) and the As-As bond, respectively. From the compositional trends of defect formation, we proposed the model of the formation of defects during the melting process; a series of thermal decomposition reactions in an arsenic oxide melt at a high temperature is a possible mode of formation of the defects. Disproportionation reactions in the melt may facilitate the formation of structural imperfections, which is written as

2

(

As =

)

O

I,

=

A; = + = As-O-

A; =

This reaction was considered to form pentavalent As and As-As.

References [1] M. Kohketsu, H. Kawazoe, A. Kashiwazaki and K. Muta, Proc. MRS 1986 Fall Meeting, Boston, Vol. 88 (1986) p. 61. [2] M. Kohketsu, K. Awazu, H. Kawazoe and M. Yamane, Jpn. J. Appl. Phys. (1988) in press. [3] A. Kashiwazaki, K. Muta, M. Kohketsu and H. Kawazoe, Proc. MRS 1986 Fall Meeting, Boston, Vol. 88 (1986) p. 217. [4] H. Kawazoe, H. Yanagita, Y. Watanabe and M. Yamane, Phys. Rev. B (1988) in press.

M. Kohketsu et aL / Structural imperfections in AsO x glasses

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