Glass formation and properties of chalcogenide systems XXIII on glass formation in the system HgGeSeTe

Journal of Non-Crystalline Sofids 41 (1980) 301-317 © North-Holland Publishing Company

GLASS FORMATION AND PROPERTIES OF CHALCOGENIDE SYSTEMS XXIII ON GLASS FORMATION IN THE SYSTEM H g - G e - S e - T e A. FELTZ and W. BURCKHARDT Department of Chemistry, Friedrich-Schiller-University Jena, GDR Received 28 May 1980

Starting from the glass formation regions in the System Hg-Ge-Se it is possible to prepare by addition of Te as a fourth elemental component quaternary glasses in a wide range of composition. For selected series in the systems HgSe-GeSe-GeSe2, HgTe-GeSe-GeSe 2 and HgTe-GeTe-GeSe 2 the mole volume, glass transition temperature and the electrical direct current conductivity were investigated. Conclusions on structural changes in the glasses were drawn from their dependence on composition. There exist marked differences from glasses in the system Pb-Ge-Se-Te with analogous composition. The glass forming region HgSe-GeSe is extended by addition of HgTe and GeTe, respectively. Furthermore the glass formation range in the system Hg-Ge-Te is described for the first time.

1. Introduction In previous investigations it was shown that in GeS-GeS2 and GeSe-GeSe2 melts units of the general formula GenX(2n+2)/2 (X: S, Se or partial Te) are formed, favouring glass formation with different chalcogenides of heavy metals [ 1 - 7 ] . For instance it was possible to prepare glasses in the system PbSe-GeSe-GeSe2 with a content of PbSe up to 50 tool% and in the system HgSe-GeSe-GeSe2 with a content o f HgSe up to 50 mol% [8,9]. Even mixtures of HgSe/GeSe freeze glassily in a compact form in definite ranges o f compositions. Furthermore it was determined that the mercury containing glasses are distinguished by relatively unusual properties. The glass transition temperature and the mole volume decrease only slightly for instance by the insertion of HgSe, and the very low electrical conductivity of the GeSe-GeSe2 glasses as well as the optical gap is nearly constant even with a high HgSe content. On the other hand, additions of PbSe act as network-modifiers. The electrical conductivity decreases by six orders of magnitude if PbSe is added to glassy Ge2Se3. The glass forming region is considerably extended by the addition of Te as a fourth elemental component to the glass forming system P b - G e - S e or P b S e GeSe-GeSe 2, respectively, the specific electrical conductivity also increases [ 10]. In this paper we report on glass formation in the quaternary system H g - G e S e - T e and on some properties of the glasses in selected series. 301

302

A. Feltz, W. Burckhardt / Properties of chalcogenide systems

2. Experimental 2.1. Sample preparation Mixtures of 10-20 g of the semiconductor grade elements were placed in silica tubes, which were closed under vacuum, heated to 850-900°C and maintained at this temperature and rotated for 8 h. After homogenization the ampoules were cooled in air outside the furnace. Under these conditions the cooling rate in the range of the glass transition temperature corresponds to about 2 K s -1. As a result of the rapid decrease of the vapour pressure in the ampoule during quenching, melts with high mercury and a relatively low chalcogen content tended to boil over, so that a foam-like product was obtained to which elemental mercury was attached. By lowering the temperature of the furnace before removing the ampoule - it was possible to keep the melts at 450°C without crystallization - the formation of bubbles and the precipitation of mercury could be prevented. The homogeneity of the samples was tested by light microscopy and in some cases by electron microscopy. No phenomena of phase separation could be observed. Z2. Glass transition temperature The glass transition temperature was measured by differential thermal analysis in the manner described in previous earlier papers [11,12]. The applied heating rate was (4.7 + 0.1) K min -1. Multiple measurements yielded an average error of about 3K. 2.3. Density measurement The density of the glasses was estimated by measuring the buoyancy in CCl 4 of samples subjected to the same temperature treatment from different melts of identical composition. To minimize the error of the mole volume calculated from the density up to 40 single measurements were carried out. The average error of the mole volume was about 0.1%. 2.4. Electrical conductivity For measurement of electrical conductivity plane-parallel discs of about 0.4 mm thickness were used, which were provided by vapour deposition on both sides with circular Sb contacts of 0.5 cm diameter. To exclude surface conductivity, the measurements were carried out under high vacuum. The electrical conductivity was estimated by the use of the voltage drop on a resistor beginning with a temperature near the Tg value down to room temperature. Samples with A1 contacts yielded equal curves of electrical conductivity. The

A. Feltz, W. Burckhardt I Properties of chalcogenide systems

303

error of the specific electrical conductivity determined by multiple measurements of different samples, was about 0.2 logarithmic units and the accuracy of the activation energy which was derived from the equation o = Oo exp(-EA/kT), was -+0.02 eV.

3. Remits and discussion 3.1. Glass formation

Figure 1 shows the glass forming region in the quaternary system H g - G e Se-Te in the different planes of intersection investigated. For the Te-free glasses, i.e. in the front plane H g - G e - S e of the tetrahedron, there exist two separate glass forming regions, which can be described by the combinations HgSe-GeS%-Se and HgSe-GeSe-GeS%. In the basic plane G e - S e Te there also occur two separate glass forming ranges [13-15]. The H g - G e - S e - T e glasses exist in a cut out of the space between these planes. Quaternary glasses on the Se-rich side were not investigated. The contours of the glass forming region in the range of the binary compounds

Se

GeSez

E,

GeSe

F~. 1. Glass formation range in the system H g - G e - S e - T e between the Hg-Ge-Se front plane and the Ge-Se-Te basic plane of the tetrahedron. E1 and E 2 are eutectics (see text). The glass forming regions in the subsystems HgSe-C~Se-GeSe 2 (A), HgTe-GeSe-GeSe 2 (B) and

HgTe-GeTe-GeSe2 (C) are drawn.

304

A. Feltz, W. Burckhardt / Properties o f chalcogenide systems

are obvious from the position of the HgTe-GeSe-GeSe2 glasses in the plane B. This plane is arranged between the partial system A HgSe-GeSe-GeSe2 and the glass forming region, which extends along the line between the eutectics GeSeGeSe2(E1) and GeTe-Te(E2). The plane C HgTe-GeTe-GeSe2 represents an oblique cut through the quaternary glass formation range. The composition of the H g - G e - S e - T e glasses extends as far as the Se-free glasses HgTe-GeTe-Te or HgTe-GeTe2-Te, respectively. Although non-existent in a crystalline form, GeTe2 seems to be a glass forming compound with a definite composition and a specific short range order in amorphous systems [16,17]. The ternary H g - G e - T e glasses continue the series of glass forming systems, which derive from the GeTe-Te eutectic E2 by addition of third components, for instance G e - T e - J [18], G e - T e - S e [13-15], G e - T e - P [19], G e - T e - A s [20], G e - T e - S i [211. Furthermore, it is necessary to point out the possibility of preparing HgSe-GeSe glasses under conditions described in section 2.1., and the observation of glass forming in the region of the quadratic plane HgSe-HgTe-GeSe-GeTe. Their compositions can be described both by a combination of the components HgSe-HgTeGeSe and by a combination of HgSe-GeSe-GeTe. Obviously, redox-reactions take place in the melt, which lead at least partially to the formation of GeSe2 and GeTe2 or to an increase of the portion in the GenX(2n+2)/2 units (X: Se, Te) (decrease of n), respectively, and at the same time give rise to the creation of lower oxidation numbers of mercury. Altogether, the glass formation range is larger than in the analogous P b - G e - S e Te system [10]. In the latter, because of the broad immiscibility gap in the system PbSe-Se, it is not possible to prepare homogeneous samples on the Se-rich side. In the range of the binary compounds the region of the homogeneous P b - G e - S e Te glasses does show a similar position in the tetrahedron, but the content of lead does not exceed 20 mol%, while it is practicable to obtain glasses containing mercury with metal concentrations up to 25 mol%. With reference to the binary compounds, this corresponds to a content of 50 mol% HgSe or HgTe, respectively. The glasses adsorb in the visible part of the electromagnetic spectra and are therefore black, but because of the high concentration of heavy atoms, they possess a strong brightness. The boundaries of glass formation were investigated in the planes A, B, C in the series given in table 1. Both the series of system A and the series BV, BVI and CVI were selected for the detection of the properties mentioned in section 2 with regard to their composition dependence. The series AX, BX and CX describe the gradual introduction of HgSe or HgTe into the three-dimensional network of the glassy compound Ge2Se3, or the introduction of HgTe into a glass of the composition Ge2Se2Te, respectively. The compositions of the glasses in the system HgSeHgTe-GeSe or HgSe-GeSe-GeTe, respectively, are summarized in table 2. The glass formation range in the system H g - G e - T e is obvious from the data given in table 3.

A. Feltz, I¢. Burckhardt / Properties o f ehalcogenide systems

305

Table 1 System A Series AI AIII AV AVI AX System B Series BII Bill BIV BV BVI BX System C Series CIII CIV CV CVI CVII CVIII XIX CIX

(HgSe)a(GeSe)y(GeSe2) 1 - y - a y=0.8 y=0.7 y=0.6 y = 0.55

-a -a -a - a

0.2 < a < 0 . 4 7 5 0.09
0 < a < 0.35 0.15
y=0.7 -a y=0.65-a y=0.6 -a y = 0.55 - a y=0.5 -a y=0.4 -a y=0.3 -a

0.3 0.2 0.1 0.05 0 0 0 0

y=l-y-a


Table 2 Compositions of glasses in the system G e S e - H g T e - G e S e or H g S e - G e S e - G e T e , respectively (10 g melts, cooling rate 2 k s -1 ) Mole fraction

Mole fraction

XHgSe

XHgTe

XGeSe

XHgSe

XGeSe

XGeTe

0.40 0.45 0.40 0.30 0.35 0.40 0.45 0.25 0.30 0.35

0.05 0.10 0.10 0.10 0.10 0.15 0.15 0.15

0.60 0.55 0.55 0.60 0.55 0.50 0.45 0.60 0.55 0.50

0.40 0.45 0.45 0.40 0.45 0.50 0.55 0.40 0.45 0.50

0.60 0.55 0.50 0.50 0.45 0.40 0.35 0.45 0.40 0.35

0.05 0.10 0.10 0.10 0.10 0.15 0.15 0.i15

306

A. Feltz, W. Burckhardt /Properties of chalcogenide systems

Table 3 Glass formation range in the system HgxGeyTe1- x - y (10 g melts, cooling rate 8 K s-l) Mole fraction

Mole fraction

x

y

x

y

0.030 0.080 0.010 0.030 0.060 0.070 0.090 0.025 0.040 0.055 0.070 0.085 0.060 0.060

0.170 0.120 0.200 0.180 0.150 0.140 0.120 0.190 0.175 0.160 0.145 0.130 0.160 0.180

0.090 0.100 0.120 0.070 0.085 0.100 0.115 0.130 0.150 0.115 0.130 0.145 0.120 0.150

0.150 0.140 0.120 0.175 0.160 0.145 0.130 0.140 0.120 0.160 0.145 0.130 0.160 0.160

3.2. Glass transition temperature and crystallization Figure 2 shows the dependence of the glass transition temperature on composition for glasses of the series AV and BV. With increasing HgSe or HgTe content a relatively uniform decrease can be seen. On the other hand, in an analogous series of the glass forming region PbSe-GeSe-GeSe2 even small additions of PbSe cause a drastic decrease of transition temperature [4]. Above the glass transition, the DTA diagrams show a complicated series of exothermic enthalpic effects, which indicate the crystallization of different phases and phase transformations. The melting of the different crystallization products is indicated by the following endothermic enthalpic effects. The difference between the temperature TR1 , which indicates the beginning of the crystallization, and the transition temperature Tg under normalized measuring conditions can be regarded as a standard for the degree of inhibition of nucleation and crystallization in the supercooled melt. That means that this difference is of relevance for estimating the glass forming tendency. These values for the series AV are also shown in fig. 2. Obviously the tendency to solidify in a glassy state is greatest in the range of medium compositions. In this range the DTA-digrams indicate the existence of several phases situated close together. In the course of the Tg-curve an irregularity is to be observed here. By X-ray investigations on samples, which were first annealed without reaching equilibrium (annealing time two days at a temperature of the highest exothermic

A. Feltz, W. Burckhardt /Properties of chalcogenide systems

307

l-rg

rg °c

K 120 -

100

320 80

300 60 280 40 260 2O 240 0,1 I

0,2

0,3

I

I

mole fraction

0,4 HgX

I

,. GeSe z

II

GeSe

Hg2GeSe~

|

I

Hg~GeSes phase Pt phase P2

Fig. 2. Dependence of Tg-values on composition in the series AV (HgSe)a(GeSe)o.6_ a(GeSe2)o.4 (e) and BV (HgTe)a(GeSe)o.6_a(GeSe2)o. 4 (=) and the glass forming tendency (TR1 - Tg) for AV (o). The bulk diagram in the lower part shows the crystalline phases of selected samples of the series AV, which are found by X-ray diffraction after annealing for 2 days.

enthalpic effect) it was concluded that a set of complicated solid state chemical reactions took place. As indicated by fig. 2, in the recrystallization products of the glasses of the series AV even with a HgSe content of 10 mol% there appears in addition to GeSe2, GeSe and the known compound Hg:GeSe4 a new substance, whose composition as far as we know is equal to the formula Hg2GeSe3. At 20 mol% HgSe the reflexions of a new phase P~ are also found, where only a negligibly increased concentration of HgSe the phase P2 occurs. It also vanishes for further additions of HgSe, i,e. in the medium range the powder diffraction diagrams can be described by a superimposition of reflexions of GeSe2, GeSe, Hg2GeSe4 and Hg2GeSea as for the sample with a content of 10 mol% HgSe. In the sample with 27.5 mol% HgSe the phase P2 appears again. It is replaced by phase P1 at 30 mol% HgSe, which remains in the polycrystalline mixtures. Now GeSe can no longer be

308

A. Feltz, W. Burekhardt / Properties of chalcogenicle systems

traced in the diffraction diagrams. GeSe2 tends strongly to disorder, so that it is sometimes difficult to detect the presence of this substance by X-ray methods. Hg2GeSe4 like /3-Ag2HgJ4 or In2CdSe4 crystallizes in a structure derived from the zinc-blende type, i.e. the sublattice of the cations is under-occupied [22]. Hg and Ge occupy the blanks of the tetrahedrons in an orderly manner. For the Se atoms the coordination number is equal to three. The X-ray reflexions allied to the compound Hg2GeSe3 are all the more prominent in the powder diffraction diagrams the more the composition of the samples approaches the edge HgSe-GeSe in the concentration diagram of fig. 1. From investigations of the system HgSe-GeSe the composition Hg2GeSe3 was concluded from the disappearance of the GeSe reflexions and the appearance of the HgSe reflexions [9]. The compound melts at 564°C, but splits off mercury earlier. Annealing experiments with excess mercury allow us to conclude, that the lattice can additionally absorb mercury in solid solution. Comparative ESCA-measurements provide leads, that the substance is a measury(I)-selenogermanate(IV). In this case the further bond of mercury would find its interpretation in the formation of polycations, identified for instance in the compounds Hg4(AsF6)2 [23] and Hg3(A1C14)2 [24]. It has not hitherto been possible to prepare useful single crystals of this material. The following net plane distances of powder diagrams can be indicated in different ways (CuKa, d values in pm): 449.28(27), 348.50(23), 317.99(100), 294.21(67), 259.67(23), 245.77(6), 235.00(19), 216.39(30), 201.28(19), 194.74(74), 189.31(6), 183.16(4), 178.80(17), 166.28(31), 156.12(9). 3.3. Mole volume o f the glasses

In fig. 3 the mole volume (normalized to one mole of atoms) is shown both for the series of system A and for the series BV, BVI and CVI. The values of the densities of the glasses are summarized in table 4. The decrease of the mole volume in the series of the Hg- and Te-free glasses (GeSe)y(GeSe2)l_y (0.419 < y < 0.60) is to be obtained from the values situated on the ordinate (a = 0). The greater the content of GeSe, the more the proportion of (GeSe4)-units recede in favour of the (Ge2Se6)-units corresponding to the general formula (Ge 2Se3)y(GeSe2) 1_2y. For the range 0.5 < y < 0.6 according to (Ge2Sea)2_ay(Ge3Se4)2y_1 an increasing co-participation of (Ge3Ses)-units was taken into consideration [4,9]. These structure groups can be understood as interpenetrating tetrahedrons, out of which there results a decrease of the mole volume. The partial substitution of Se atoms by Te leads to an increase of the mole volume as expected. From the sequence of the four series of system A it can be seen that the influence originating from the structural constitution of the germanium-selenide glasses predominates. In the series the mole volume decreases, i.e. the formal substitution of GeSe by HgSe results in a rather greater packing density in the glass.

A. Feltz, I¢. Burckhardt / Properties o f chalcogenide systems

cmJ.mold,

309

m-

18,0

17,6

-

- Bg

~

7~,~ " ~ i ~

17,2

16,8

\

~

o AE

\ \

16,6 ,

AZ I

0,1

.

~2

tZ3

mole fraction

0,4 FIgX

Fig. 3. Dependence of mole volume on composition in the series: AI (HgSe)a(GeSe)o.a_a(GeSe2)0.2 (®); AIII (HgSe)a(GeSe)o.7_a(GeSe2)o.3 (o); AV (HgSe)a(GeSe)o.6_a(GeSe2)o.4 (o); AV[ (HgSe)a(GeSe)o.ss_a(GeSe2)o.4s (e); BV (HgTe)a(GeSe)o.6_a(GeSe2)o.4 (A); BVI (HgTe)a(GeSe)o.5s -a(GeSe2)o.4 s (A); CVI (HgTe)a(GeTe)o.s s -a(GeSe2)o.4 s (')-

Corresponding

to

the

general formula, for instance for the series AV, (0 < a < 0 . 2 ) and (HgSe)a(Ge2Se3)0.6_a(GeSe2)o.2-a (0.2 < a < 0.45) the part of the network forming Ge-Se groups, exhibiting the greater need for space, increases with the growing degree of substitution, so that we would expect an increase of the mole volume caused by the greater size of the Hg particles. Such an influence is more than compensated for by the formation of Hg-Se coordination polyhedra which obviously give rise to a shrinkage of the structure. According to classical notions of glass structure we are inclined to reduce the observed decrease of the mole volume to a network-modifier function of HgSe. But it is necessary to note that the curves take an essentially flatter course than in the case of analogous series of germanium selenide glasses with other divalent cations of heavy metals, e.g. Sn II or Pb H instead of HgII [4,6]. A comparison o f ' series AIII with the mole volumina of glasses (PbSe)a(GeSe)o.7_a(GeSe2)o.3, also shown in fig. 3, shows that the steep decrease caused (HgSe)a(GeaSe4)on_a(Ge2Se3)o.2+ a

310

A. Feltz, I¢. Burckhardt / Properties o f chalcogenide systems

Table 4 Density of the glasses in the series A, B, C Series

Mole fraction

p (g cm -3)

a

y

AI y + a = 0.8

0.20 0.25 0.30 0.335 0.35 0.40 0.45 0.475

0.60 0.55 0.50 0.465 0.45 0.40 0.35 0.325

5.242 5.425 5.606 5.734 5.784 5.969 6.160 6.255

AIII y+a=0.7

0.115 0.15 0.17 0.19 0.20 0.21 0.23 0.25 0.30 0.35 0.40

0.585 0.55 0.53 0.51 0.50 0.49 0.47 0.45 0.40 0.35 0.30

4.881 4.999 5.062 5.134 5.184 5.208 5.288 5.345 5.546 5.697 5.887

0 0.05 0.10 0.15 0.175 0.20 0.2075 0.2125 0.225 0.2375 0.25 0.275 0.30 0.35 0.40 0.45

0.60 0.55 0.50 0.45 0.425 0.40 0.3925 0.3875 0.375 0.3625 0.35 0.325 0.30 0.25 0.20 0.15

4.449 4.609 4.764 4.924 5.008 5.095 5.109 5.132 5.162 5.217 5.245 5.340 5.425 5.605 5.774 5.921

AV y+a=0.6

Series

Mole fraction

p (g c m-3)

a

y

AVI y + a = 0.55

0 0.05 0.10 0.15 0.20 0.22 0.24 0.25 0.26 0.28 0.30 0.35

0.55 0.50 0.45 0.40 0.35 0.33 0.31 0.30 0.29 0.27 0.25 0.20

4.403 4.563 4.719 4.877 5.042 5.115 5.170 5.209 5.255 5.301 5.376 5.539

BV y +a = 0.6

0.10 0.15 0.175 0.20 0.215 0.25 0.28 0.30 0.35

0.50 0.45 0.425 0.40 0.385 0.35 0.32 0.30 0.25

4.813 5.016 5.110 5.197 5.269 5.404 5.502 5.587 5.779

BVI y + a = 0.55

0.10 0.15 0.20 0.225 0.24 0.25 0.275 0.30 0.35

0.45 0.40 0.35 0.325 0.31 0.30 0.275 0.25 0.20

4.816 4.992 5.163 5.259 5.325 5.384 5.453 5.555 5.715

CVI y + a = 0.55

0.05 0.10 0.15 0.35 0.40

0.50 0.45 0.40 0.20 0.15

4.917 5.053 5.223 5.849 5.993

b y the relatively low a d d i t i o n o f P b S e , is n o t o b s e r v e d in g e r m a n i u m - s e l e n i d e glasses w i t h HgSe. C o m p a r a b l e values are o n l y o b t a i n e d w i t h very high c o n c e n t r a t i o n s o f PbSe or HgSe, respectively.

A. Feltz, 14/.Burckhardt /Properties o f chalcogenide systems

311

In the series of glasses AX with the general formula (MSe)a(Ge2Se3)o.s_o.sa a content of PbSe of a = 0.05 (M ---Pb) produces a decrease of the mole volume of 17.44 to 15.7 cm a mo1-1. On the other hand, the same concentration of HgSe only results in a decrease to 17.36 cm 3 mo1-1, as follows from fig. 4. The reason for such a suppression is doubtless to be seen in the different structural chemical properties of both cations, which is also demonstrated in the structures of crystalline Pb- and Hg-compounds of analogous composition, e.g. Pb2GeSe4 and HgzGeSe4 [22,25]. The reduction of HgII caused by the lower oxidation state of germanium where Hg~+ or higher polycations of mercury could be formed may increase the proportion of network-forming groups with a greater need for space [decrease of n in the GenSe(2n+z)12 units] but the attainable increase of the volume is still far too small to explain the small concentration dependence of the mole volume by comparison with the lead seleno-germanate glasses. The insertion of HgTe in the network structure of the Ge:Se3 glass leads, in the series BX of fig. 4, to a fiat increase of the mole volume. Obviously, the decrease is more than compensated for by the greater need for space of the Te atoms. This effect is to be observed more intensely in the series BV and BVI of fig. 3. The proportion of the network-forming groups with a greater need for space increases with the formal substitution of GeSe by HgTe as in the series of system A. However, in the series CVI the substitution of GeTe by HgTe gives rise to a decrease, and in the series CX a decrease of the mole volume is also observed. -¢ 18,2

_

•

C Z

8Z

16,6

o----o-o---

A,~( Pb)

15,8 I

I

o,t

0,2

mole fraction

i

I

0,3 HgX

Fig. 4. Dependence o f mole volume on composition i n the series: AX (MSe)a(Ge 2 Sea)o. s - o . s a ; M = Hg (e), M = Pb (o); BX (HgTe)a(Ge2Sea)o. s _ O . s a (A); CX (HgTe)a(Ge2Se2Te)o. s _ 0 . s a

(').

312

A. Feltz, W. Burckhardt / Properties of chalcogenide systems

In the range of medium composition, the curves of fig. 3 have a non-monotonous course. Thermal and X-ray investigations in the same range indicated a complicated sequence of phases in the recrystallized glasses. The extreme values shift with increasing GeSe content in direction of lower mole fractions of HgSe. The lower the average oxidation number of germanium, the smaller the concentration of HgSe at which the irregularity begins. Both the minima and the small maximum situated between these become all the more prominent the higher the proportion of GeX2 in the formal composition of the glasses. Obviously the differences between the structure polyhedra, which alternate in rapid succession, attain greater importance the lower the packing density in the glass at the outset. On both sides of the extremes the curves of fig. 3 show opposite curvature. Approaching the range of medium HgSe concentrations the volume change coming from both sides first inverts, and then the structure "tips" to the other configuration. Obviously in the glass forming region there exist two ranges with different structures. Such a separation is also evident from the dielectric properties of the glasses [9]. 3.4. Electrical conductivity

Figure 5 demonstrates the conductivity parameters for glasses of the series AX, BX and CX together with values for containing glasses lead, whose composition corresponds to series AX. While the activation energy in the system PbSe-Ge2Se3 decreases with increasing content of PbSe from 1.0-0.7 eV and the specific electrical conductivity increases by four logarithmic units, the corresponding values in the succession of the glasses HgSe-Ge2Se3 are nearly constant. For the series AV and BV a largely corresponding picture (fig. 6) emerges. The values are summarized in table 5. The conductivity first drops more than one unit in the series AV. It again reaches at 23.75 mol% HgSe and at 35 mol% HgSe a minimum of about -lgty29s = 14.8 to 14.9. Even a glass of the composition (HgSe)0.4(GeSe)o.2(GeSe2)o.4 or (HgSe)o.4(Ge2Se3)o.2(GeS%)o. 2 possesses only a negligibly higher conductivity than the sample (GeSe)0.6(GeSe2)o.4 at the starting point of the series. The thermal activation energy varies between 1.0 and about 0.9 eV. Against this a glass of composition (PbSe)0.4(GeSe)o.2s(GeS%)o.3s possesses a specific conductivity of - - l g o 2 9 8 = 9.9 and an activation energy of 0.67 eV [4]. As follows from fig. 6, -lgo298 and EA change to a certain degree in opposition. No qualitatively detectable correlation as with the lead-seleno-germanate glasses [4] was observed. The course of the lga and E A curve varies with the concentration dependence of the mole volume indicating two ranges of different structure in the glass forming region. There are also two parts of the curve to be seen here in whose range of intersection a minimum is situated in the case of the activation energy and a maximum in the case of the specific conductivity.

A. Feltz, W. Burckhardt /Properties of chalcogenide systems

eV 1,0

O,8

~

~

313

AZ

~.

BZ

'

- Ig~b.,i

/ /

--

i.

•,

Y

,

A

_

T

,

0,1 0,2 mole fraction HgX

l

,

0,3

Fig. 5. Dependence of activation energy E A and the electrical conductivity at 298 K on composition in the series: AX (MSe)a(G%Se3)o.s_o.sa; M = Hg (e), M = Pb (o); BX (HgTe)a(Ge2S%)o.s_o.sa (A);CX (HgTe)a(G%S%Te)o.s_o.sa (=).

In the series AX, BX and CX (fig. 5) the differences in compositional change between samples were too great, so that the range of the structural change could not be expressed in measurable values. The dependence of the specific conductivity on the HgSe concentration and on the structure of the glasses is, by several orders of magnitude, smaller than in the glass system PbSe-GeSe-GeSe2~ The conductivity parameters remain approximately in the range of the values characteristic for the GeSe-GeS% and GeSe-GeTe-GeSe2 glasses, respectively. The high conductivity of HgSe (0298 about 10 a [2-1 cm-l) and HgTe (0298 about 102.3 [2 -1 cm -1) is bound to the zinc-blende lattice. At high pressure HgSe can be transformed in a modification analogous to red cinnabar [26]. HgSe in this state possesses a conductivity reduced by six orders of magnitude. In the melt of HgSe

314

A. Feltz, I¢. Burckhardt / Properties o f chalcogenide systems

EA 1,1 eV

B.E lo

I

0,9

12 O,7

14 O,5 ,

I

0,1

,

I

L

I

0,2

mole fraction

0,3 HgX

~

15

.

Fig. 6. Dependence of activation energy EA and the electrical conductivity at 298 K on composition in the series: AV (HgSe)a(GeSe)o.6_a(GeSe2)o.4 (., o); BV (HgTe)a(GeSe)o.6_ a(GeSe2)o.4 (A, ~).

there also exists a short range order, which can be described chiefly by the coordination number 2 or 2 + 4, respectively. At the melting point of 690°C the conductivity drops by one logarithmic unit to about 10 ~2-l cm -1 [27]. 2.3 eV are assigned to the bandgap or mobility gap. The assumption of a small coordination number is in good agreement with the relatively low decrease of the mole volume (fig. 3). Small coordination numbers, for instance 2 for the Hg 2÷ ions, indicate a great restriction of the network-modifier function of HgSe or HgTe in germanium chalcogenide glasses. The possibility of the preparation of homogeneous glasses in the range of HgSe-GeSe2-Se (fig. 1) baring in mind the marked immiscibility behavior of melts of the combination PbSe-GeSe2-Se [28] also refers to the fact that mercury chalcogenides act in such systems, structure-chemically, in an entirely different manner than for instance lead chalcogenides. The generally low dependence on concentration of conductivity parameters in the series gives rise to the assumption that the density and the distribution of the charged defect centres (GeaSe) ÷ and (SeaGe)-, which are presumbaly characteristic of germanium chalcogenide glasses are only slightly changed by the insertion of HgSe. The formation of such centres by partial dispropotionation of the GenSe(2n+2)/2 units is assumed to be fairly probable on the basis of previous investigations [29]. While PbSe strongly reduces the presence of the (SeaGe)--centres by formation of (SeaPb)--units or as a network-modifier largely removes the (GeaSe) ÷centres with the three-fold bonded Se atoms, respectively, and thus cancels the compensation of the glass semiconductor, HgSe and HgTe, respectively, fit into the

A. Feltz, W. Burckhardt /Properties of chalcogenide systems

315

Table 5 Electrical conductivity and activation energy of the glasses in the series AV, BV and AX, BX, CX at 298 K Series

AV y + a = 0.6

AX y = (1 - a)/2

BV y + a = 0.6

BX y = (1 - a)/2

CX y = (1 - a)/2

Mole fraction a

y

0 0.10 0.20 0.2075 0.2125 0.225 0.2375 0.25 0.30 0.35 0.40 0 0.10 0.20 0.30 0.40 0.125 0.175 0.20 0.215 0.25 0.30 0 0.10 0.20 0.30 0 0.10 0.20 0.30 0.40

0.60 0.50 0.40 0.3925 0.3875 0.375 0.3625 0.35 0.30 0.25 0.20 0.50 0.45 0.40 0.35 0.30 0.475 0.425 0.40 0.385 0.35 0.30 0.50 0.45 0.40 0.35 0.50 0.45 0.40 0.35 0.30

-lgo298 (s2-1 cm-1 )

E A (eV)

13.6 14.2 14.7 14.8 14.8 14.4 14.6 14.6 14.8 14.9 13.1 14.2 14.8 14.7 14.5 13.8 12.6 12.4 12.2 11.4 11.1 9.8 14.2 13.9 12.2 11.6 11.3 10.1 9.5 8.9 8.6

0.95 0.98 1.00 0.94 0.97 0.92 0.87 0.99 1.01 0.95 0.88 1.00 1.04 1.00 0.99 0.99 0.81 0.82 0.87 "0.81 0.77 0.63 1.00 0.95 0.88 0.81 0.56 0.60 0.72 0.66 0.51

glass s t r u c t u r e w i t h u n i t s , w h i c h act p r e d o m i n a n t l y as n e t w o r k - f o r m e r s . T h e patt e r n o f d e f e c t c e n t r e s r e m a i n s w i d e l y u n c h a n g e d . T h e valence e l e c t r o n s are localized in t h e o r i e n t e d b o n d s o f t h e s e u n i t s , so t h a t e v e n at h i g h c o n c e n t r a t i o n s o f HgTe t h e c o o r d i n a t i o n p o l y h e d r a o f t h e glass s t r u c t u r e s k e l e t o n r e m a i n r e s p o n s i b l e for t h e charge carrier t r a n s p o r t . T h e c h a n g e s in t h e s t r u c t u r e w h i c h are o b v i o u s f r o m t h e c o m p o s i t i o n a l d e p e n d e n c e o f t h e d i f f e r e n t p r o p e r t i e s , h a v e a c o m p a r a t i v e l y small i n f l u e n c e o n t h e c h a r a c t e r o f t h e b o n d i n g r e l a t i o n s in t h e glass, u p to n o w it h a s n o t b e e n possible

316

A. Feltz, W. Burckhardt /Properties o f chaleogenide systems

Hgre

1o.,/ #

°

0,1

GeTe

\o.,.h o

~o

0,4

\

oo

0,3 0,2 mote fraction Ge

0,1

Te

Fig. 7. Glass formation range in the system H g - G e - T e near the eutectic GeTe/Te

Table 6 Properties of glasses in the system HgxGeyTe 1 - x - y Molenbruch

P (g em -a)

Tg (°C)

-lgo298

E A (eV)

(~2-1 cm -1) x

y

0.025 0.04 0.055 0.07 0.085 0.07 0.085 0.10 0,115 0.115 0.13 0.145

0.19 0.175 0.16 0.145 0.13 0.175 0.16 0.145 0.13 0.16 0.145 0.13

5.713 5.814 5.930 6.036 6.144 5.982 6.116 6.192 6.333 6.289 6.398 6.488

138

4.7

0.39

133

4.3

0.39

125 150 140

3.9 4.6 4.3

0.38 0.38 0.38

138 142

4.1 4.5

0.35 0.37

148

A. Feltz, W. Burckhardt / Properties of chalcogenide systems

317

to say with certainty h o w far a change o f the o x i d a t i o n n u m b e r o f m e r c u r y is c o m bined w i t h structural change.

3.5. Properties o f r i g - Ge-Ge- Te glasses F o r the characterization o f H g - G e - T e glasses, m a r k e d in fig. 7, the density, the glass transition t e m p e r a t u r e and the electrical c o n d u c t i v i t y were measured. These values are summarized in table 6.

References [1] A. Feltz, B. Voigt and E. Schlenzig, Proc. 5th Int. Conf. Amorph. Liquid Semicond., Garmisch-Partenkirchen, Vol. 1 (1973) p. 261. [2] A. Feltz, W. Burckhardt, L. Senf, B. Voigt and K. Zickmiiiler, Z. Anorg. AUg. Chem. 435 (1977) 172. [3] A. Feltz and G. Pfaff, Z. Anorg. Allg. Chem. 442 (1978) 41. [4] A. Feltz and L. Senf, Z. Anorg. Allg. Chem. 444 (1978) 195. [5] A. Feltz and B. Voigt, Z. Anorg. Allg. Chem. 403 (1974) 61. [6] A. Feltz, E. Schlenzig and D. Arnold, Z. Anorg. Allg. Chem. 403 (1974) 243. [7] A. Feltz, D. Linke and B. Voigt, Z. Chem. 20 (1980) 81. [8] A. Feltz, W. Burckhardt, L. Senf and B. Ktinzel, Proc. Vlth Int. Conf. Amorph. Liquid Semicond. (Verlag Wissenschaft, Leningrad 1976) p. 24. [9] A. Feltz and W. Burckhardt, Z. Anorg. AUg. Chem. 461 (1980) 35. [ 10] A. Feltz, G. Kley and I. Linke, J. Non-Crystalline Solids 33 (1979) 299. [11] A. Feltz and F.J. Lippmann, Z. Anorg. Allg. Chem. 398 (1973) 157. [12] W. Ludwig, J. Thermal. Anal. 8 (1975) 75. [13] A.V. Pazin, A.A. Obrazcov and Z.U. Borisova, Izv. Akad. Nauk SSSR, Neorg. Mater. 8 (1972) 247. [141 M. No da and S. Maruno, Y ogyo-Kyokai-Shi 82 (1974) 234. [ 15] J. Sarrach and J.P. De Neufville, J. Non-Crystalline Solids 22 (1976) 245. [16] J.P. De Neufville, Fourth Semi-Annual Technical Report, DAHC 15-70-C-0187 (1973). [17] G. Lucovsky, Proc. 5th Int. Conf. Amorph. Liquid Semicond., Vol. 2 (Taylor and Francis, London, 1974) p. 1099. [18] A. Feltz and H.J. Biitter, Z. Chem. 12 (1972) 393. [19] A.R. Hilton, C.E. Jones and M. Brau, Infrared Phys. 6 (1966) 183. [20] H. Krebs and P. Fischer, Discuss. Farad. Soc. 50 (1970) 35. [21] A. Feltz, W. Maul and J. Sch/Snfeld, Z. Anorg. Allg. Chem. 396 (1973) 103. [22] H. Hahn and Ch. Delorent, Naturwissenschaften 45 (1958) 621. [23] B.D. Cutforth, R.J. Gillespie and P.R. Ireland, J. Chem. Soc., Chem. Commun. (1973) 723. [24] G. Torsi, K.W. Fung, G.M. Begun and G. Mamontov, Inorg. Chem. 10 (1971) 2285. [25] A. Feltz, W. Ludwig, L. Senf and G. Simon, Krist. Techn. im Druck. [26] J.A. Kafalas, H.C. Gatos, M.-C. Lavine and M.D. Banns, J. Phys. Chem. Solids 23 (1962) 1541. [27] V.A. Joffe and A.R. Regel, Progr. Semicond. IV (1960) 239. [28] D. Linke, M. Gitter and F. Krug, Z. Anorg. AUg. Chem. 444 (1978) 217. [29] A. Feltz, F. Schixrmeister, H. Kahnt, J. Non-Cryst- Solids 35/36 (1980) 865.