Materials Sciences and Application, 2011, 2, 950-956
doi:10.4236/msa.2011.27127 Published Online July 2011 (http://www.SciRP.org/journal/msa)
Synthesis and Characterization of Sb65Se35-xGex
Alloys
Saleh Ahmed Saleh
Physics Department, Faculty of Science, Sohag University, Egypt; Physics Department, College of Science & Arts, Najran Unversity,
Saudi Ababia.
Received March 16th, 2011; revised April 14th, 2011; accepted April 26th, 2011.
ABSTRACT
Density, chemical, structural and vibrational studies of GexSb65Se35-x system with 0 ≤ x ≤ 20 produce by melt-quench
technique were carried out using Archimedes method, Energy Dispersive Spectroscopy (EDS), X-ray diffraction (XRD)
and Raman spectroscopy. All specimens are polycrystalline in nature as confirmed by XRD pattern. The compositional
dependence of the XRD and Raman spectra suggests the presence of two basic structural units, SbSe3 pyramids with
three-fold coordinated Sb atom at the apex and GeSe4 tetrahedrons. The compositional dependence of these physico-
chemical properties of the investigated samples are investigated and discussed in light of many models.
PACS: 68.55. Ln; 61.50. Ks; 68.55.Jk
Keywords: Ge Doping, Sb2Se, Structure, Raman Spectroscopy, Chalcogenides
1. Introduction
Chalcogenide alloys (which contain at least one of the
chalcogen elements: sulphur, selenium or tellurium) are of
special interest due to their wide application in modern
electronics, optoelectronics, integrated optics, electropho-
tography, solar cells, electrical and optical memory devices,
among others. One of the recent applications of these chal-
cogenides is in rewritable optical data recording (phase
change recording). This technology is based on reversible
phase transition between crystalline and amorphous state.
Currently, the primary materials for phase change re-
cording are based on Sb-Te alloys [1-9], but materials
research still continues due to the need for increased
storage capacity and data recording rates. Nowadays, the
attention is extended over Sb-Se system as possible candi-
dates for these applications. Recently the author group has
synthesized ternary selenide glasses based on Sb-Se system
with addition of Ge, and has considered the basic optical
parameters in dependence of glass composition [10]. How-
ever, normally the eutectic Sb-Se material system has poor
stability, which requires improvement of the stability by
doping other elements such as Ge. The higher coordina-
tion number of Ge is considered to be effective in form-
ing covalent bonds and reducing the atomic diffusivity,
which can provide sufficient amorphous stability i.e. the
addition of third element will create compositional and
configurational disorder in the material with respect to
the binary alloy, which will be useful in understanding the
structural properties of these materials. Therefore, the
structural studies of eutectic SbSe alloy doped by Ge with
systematic compositional variation can be advantageous for
gaining important insight in the structure-property rela-
tionships for these compounds.
Although, several reviews have appeared on various
physical properties and applications of chalcogenide gla-
sses [11-14], there is no thorough study of local atomic
structure and its modification for eutectic SbSe alloy doped
with Ge. Several experimental techniques such as X-ray
diffraction (XRD), scanning electron microscopy (SEM)
and Raman spectroscopy used to study the structure of
chalcogenide alloys. Raman scattering is a very powerful
experimental technique for providing information on the
constituent structural units in a given material [15].
This paper presents some new results of a systematic
study for eutectic Ge doped SbSe. XRD and Raman studies
have been carried out to test the structure. Characterization
of the basic parameters such as material density, average
atomic volume, average coordination number, number of
constrains and heat of atomization in dependence on com-
position, is also given.
2. Theoretical Basis
The densities of the pellets are determined from the rela-
tion,

Synthesis and Characterization of SbSe Ge Alloys951
65 35-x x

lair air l
WWW   (1)
where Wair, Wl and
l are, respectively, the weight of
the sample in air, the weight of the sample in the liquid,
and the density of the immersion liquid. Some physi-
chemical properties related to density were calculated in
terms of the density and literature data [16-22] for each
constituent element, summarized in Table 1.
The average atomic volume (Va) was determined by
the equation,
1,
a
i
VC

ii A (2)
where Ai is the atomic weight of the ith component and
Ci is the atomic concentration of the same element.
Structural properties of both amorphous and crystal-
line solids can be explained with topological models [23],
chain crossing model (CCM) [24], random covalent net-
work model (RCNM) [25] and chemical ordered network
model (CONM) [26]. In these models, some of the prop-
erties can be discussed in terms of the average coordina-
tion number, which is indiscriminate of the species or
valence bond. Average coordination number Z of a ter-
nary GeαSbβSeγ system is defined by the expression [27]:
432
Z





 (3)
where α, β, and γ are the atomic percentages of Ge, Sb
and Se, while 4, 3, and 2 their coordination number. The
coordination number Z characterizes the electronic prop-
erties of semiconducting materials, and shows the bond-
ing character in the nearest-neighbor region [28].
The parameter R, which determines the deviation of
stoichiometry and is expressed by the ratio of the cova-
lent bonding possibilities of chalcogen atoms to that of
non-chalcogen atoms, was calculated for a ternary
GeαSbβSeγ system using the following relation [29]:
2
43
R



 (4)
The bond energies of various possible heteropolar
Sb-Se, Se-Ge, and Sb-Ge bonds have been calculated on
the basis of the relation postulated by Pauling [30]:

0.5 2
30
ABAA BBAB
EEE XX



(5)
Table 1. Literature data of the constituent elements in the
synthesized alloys.
Property Sb Se Ge
Density (g/cm3) 5.30 4.28 5.00
Coordination number 3 2 4
Hs (kcal/g atom) 62.0 49.4 90.0
Bond energy (kcal/mol) 30.22 44.04 37.60
Electronegativity 2.05 2.55 2.01
where EA–A and EB–B are the single-bond energies and XA
and XB are the electronegativities of atoms A and B, re-
spectively.
For a ternary system Ge Sb Seγ, the average heat of
atomization, Hs, can be determined as [31]:



GeSb Se
SSSS
HHHH




 
(6)
where Hs is the heat of atomization of constituent atoms,
and corresponds to the average nonpolar bond energy of
the Ge-Ge, Sb-Sb and Se-Se chemical bonds [30]; α, β,
and γ are the atomic percent of the corresponding ele-
ments. Calculations of these parameters are made on the
basis of the density and composition measurements on
these compounds.
3. Experimental
3.1. Sample Synthesis
The Sb65Se35-xGex (x = 0, 5, 10, 15 and 20 at%) bulk mate-
rials have been prepared according to the well established
melt-quench technique. Appropriate atomic percentages of
high purity elements (5 N) are vacuum sealed (10–5 Torr)
into fused silica tubes of length 75 mm and internal diame-
ter 8 mm. The sealed tubes are then heated in an electric
furnace up to 850˚C for 5 h. After complete melting and
homogenization, the tubes are quenched in an ice-water
mixture. The ingots were then ground into a fine powder
with average particle size <63 μm. The main part of the
as-quenched powder was used for preparation of powder
compact disc shape samples (pellets) using the cold press-
ing technique. For this purpose a stainless steal die was
used, where the forming pressure was adjusted to be 5
ton/cm2 for all the prepared samples. The residual part of
this powder was used for both SEM and XRD analyses.
3.2. The Characterization Properties of the
Synthesized Samples
The characterization properties including structural,
chemical compositions and density of Sb-Se-Ge system
were measured. Various experimental techniques such as
XRD, Raman, EDS and Archimedes methods were em-
ployed to study the samples at room temperature. The
crystal structure of the synthesized alloys was determined
by X-ray diffraction (XRD) using PANalytical X’PertP-
RO with Cu K (λ = 1.5406 Å) X-ray tube (V = 40 kV, I =
30 mA) with scanning speed 0.4˚/S. The Raman spectra
were obtained at room temperature using Perkin Elmer
(Raman station 400) Raman spectrometer in the
wavenumber region 500 - 80 cm–1 at 4 cm–1 resolution
and excitation wavelengths were provided by an Ar+
Spectra -Physics Laser with exciting wavelength of 514.5
nm. The microstructure of the investigated samples were
observed by field emission scanning electron microscope

Synthesis and Characterization of Sb65Se35-xGex Alloys 952
(Joel JSM 7600F) combined with an energy dispersive
spectroscopy (EDS) was used to do chemical analysis for
specimens was measured using Archimedes balance
method. The pellets used for density measurement were
carefully chosen to be free from crakes and any surface
contamination was removed by cleaning in methanol. For
each composition the measurement was repeated four
times and the obtained values were averaged. The accu-
racy of the density measurement, and consequently in
average atomic volume (Va) is estimated to be better than
±0.01. On the basis of the results obtained from the den-
sity and composition measurements, for each compound
the average atomic volume (Va), average coordination
number (Z), parameter R and average heat of atomization
(Hs) were determined using the above equations and pre-
sented in Table 2.
4. Results and Discussion
The compositional variation of density and average atomic
volume (ρ and Va versus x) of Sb65Se35-xGex alloys is shown
in Figure 1. ρ and Va show clear changes in slope at x = 5
(Sb65Se30Ge5 composition). A sharp rise or decline from x =
0 to nearly 5 is followed by slightly changes above that
composition. A similar fashion is also observed in the opti-
cal gap as Ge content increases [10]. Therefore, the effect
of germanium on SbSe appears to be limited to composi-
tions with less than x = 5. This can be largely explained
on the basis of chemical ordered network model (CONM)
proposed by Biecerano and Ovshinsky [26]. In CONM,
the glass structure is assumed to be composed of
cross-linked structural units of the stable chemical com-
pounds (heteropolar bonds) of the system and excess if
any, of the elements (homopolar bonds). Due to the
chemical ordering, features like extremum, a change in
slope or kink [32], cur for the various properties at the tie
line composition or the chemical threshold of the system.
At this composition, the system structure is made up of
cross-linked pyramidal-like SbSe3 and tetrahedral-like
GeSe4 structural units which consist of the energetically
favored heteropolar bonds only. Heteropolar bonds thus
have pre-eminence over homopolar bonds and bonds are
formed in the sequences of decreasing bond energy until
all the available valences of the atoms are saturated. Each
constituent is coordinated by 8-N atoms, where N is the
number of electrons in outer shell and this is equivalent
to neglecting the dangling bonds and the other valence
defects. As can be seen from Table 2, a maximum in the
compositional dependence of Va is attained at Z = 2.75
which can be attributed to a change from two-dimension-
nal (2D) layered structure to a three-dimensional (3D)
network arrangement due to cross-linking. This Z value
lies in the region near to Tanaka's threshold (Z = 2.67).
Table 2. Some physical parameters as a function of Ge content for Sb65Se35-xGex (where x = 0 - 20 at%) specimens.
Composition g·cm–3 Va Z
Parameter R Hs (kcal/
g atom) Hs/Z
Sb65Se35 5.96 17.92 2.65 0.36 57.59 21.73
Sb65Se30Ge5 5.57 19.11 2.75 0.28 59.62 21.68
Sb65Se25Ge10 5.95 17.84 2.85 0.21 61.65
21.63
Sb65Se20Ge15 5.91 17.91 2.95 0.16 63.68
21.59
Sb65Se15Ge20 6.08 17.35 3.05 0.11 65.71
21.54
17.2
17.6
18
18.4
18.8
19.2
-14914 19
Ge c ontent
Va (cm-3)
5.4
5.6
5.8
6
6.2
Density (gm cm -3)
Figure 1. ρ and Va versus Ge conent

Synthesis and Characterization of SbSe Ge Alloys 953
65 35-x x
According to the constrain theory [33], the investigated
compositions are over-coordinated, stressed-rigid and with
lower connectivity, as the values of Z are larger than 2.4.
These observations indicate that the effects of chemical
ordering are also present in this system along with the
overall topological effects. In other words, the dependence
of Va on Ge content and Z has been examined in light of
topological and chemical ordered network models.
Other parameters, such as parameter R, also play an im-
portant role in the analysis of the results. Depending on R
values, the chalcogenide systems can be organized into
three different categories [29]:
1) For R = 1, the system reaches the stoichiometric
composition since only hetero polar bonds are present.
2) For R > 1, the system is chalcogen-rich. There are
hetero-polar bonds and chalcogen–chalcogen bonds pre-
sent.
3) For R < 1, the system is chalcogen-poor. There are
only hetero-polar bonds and metal–metal bonds present.
As shown in Table 2, values of R were found to be
smaller than unity of the prepared system indicating
Se-poor materials. There are only hetero-polar bonds and
metal-metal (Sb-Sb) bonds present.
From the bonding energy values (Table 3) it follows that
Ge-Se bonds with the highest possible energy are expected
to be formed first, followed by Sb-Se bonds till saturation
of all available valence of Se is achieved. There are still
some unsaturated bonds of Sb, which must be bonded
through formation of homopolar Sb-Sb bonds, being de-
fects in the composition structure. In other words,the vari-
ous bonds energies of expected bonds in the Sb-Se-Ge sys-
tem are listed in Table 3. CONM could be applied to the
present system. It allows the determination of the number
of possible bonds and their type (heteropolar and homopo-
lar). Therefore, only Sb-Se and Sb-Sb exist in the binary
system consistent with CONM and R-values. Increasing
the germanium content the amount of GeSe4 tetrahedral
unit increases at the expense of SbSe3 pyramidal units, re-
placing the weaker Sb-Se bonds with the stronger Ge-Se
ones.
The average heat of atomization (Hs) is a measure of the
cohesive energy and it represents the relative bond strength,
which in turn is correlated with the energy gap- of
isostructural semiconductors. According to [34], for over-
Table 3. Bond energies of various bonds in Ge-Se-Sb alloys.
Bond Bond energy
(kcal/mol)
Ge-Se 49.44
Sb-Se 43.98
Se-Se 44.04
Ge-Ge 37.60
Ge-Sb 33.76
Sb-Sb 30.22
constrained materials with higher connectivity (3 ≤ Z ≤ 4)
the energy band gap depends much more strongly on Hs
than for alloys with lower connectivity (2 ≤ Z ≤ 3). From
Table 2, it can be seen that these alloys except
Sb65Se15Ge20 are with lower connectivity (2 ≤ Z ≤ 3) and
the parameter Hs/Z is almost constant independently on
composition. Therefore, the average heat of atomization Hs
would have a negligible effect on the band gap energy
values.
To identify the crystalline phases of the samples, X-ray
diffraction was carried out for binary and ternary samples
as shown in Figure (2). Analysis of the X-ray diffraction
pattern reveals that the diffraction peaks for the binary
system (Sb65Se35- composition) correspond to the Sb2Se
rehombohedral phase. The unit cell contains nine layers
stacked along the c-axis, five-layer stacks of Sb2Se3 and
two-layer stacks of Sb2 [35]. In the ternary composition,
with increase in the Ge content, there is observed an up-
shift to a higher value in the diffraction peaks with respect
to those of the Sb2Se crystalline phase. This indicates
that the Ge is incorporated in lattice sites, perhaps substi-
tuting the Sb atoms as previously reported [36]. Also, the
increase in the intensity of the main peak and the position
of the peak shift can be attributed to the formation of
GeSe4 structural units with increasing Ge concentration.
Raman spectra were used to obtain basic structural in-
formation. For the vibrational bands reported in the litera-
ture for their crystalline accompaniment were taken as
reference for the discussion of the spectrum [37-40]. The
Raman spectra of Sb65Se35-xGex with 0 ≤ x ≤ 20, depending
on composition, are depicted in Figure 3. With no germa-
nium substitution (Sb65Se35 composition), three character-
istic vibrations were found at 98, 148 and 190 cm–1, respec-
tively. The first two bands are assigned to symmetrical
pyramidal SbSe3 bending modes. The band positioned at
190 cm–1 has been related to homopolar Sb-Sb in
Se2Sb-SbSe2 structural units [37], due to the excess of an-
timony and Se-deficient system. This result is in excellent
agreement with the value of R and CONM. It is interesting
to note that the 192 cm–1 band is present, albeit quite
weakly, even in the Raman spectrum of Sb65Se30Ge5. This
result is indicative of violation of the chemical order and
the presence of a small concentration of Sb-Sb homopolar
bonds in the structure of this composition. Moreover, the
lowest frequency band and band at around 148 cm–1, have
intensities that strongly decrease with the progressive in-
troduction of Ge to 5 at%, corresponding to GeSe4 tetra-
hedra and SbSe3 pyramids which are weakly coupled
through two atomic -Se-Se bridging groups. This means
that two bands could be the combined effect of the bend-
ing modes of Sb-pyramidal units and/or Ge-tetrahedral
and addition of germanium until 5 at% causes a sharply

Synthesis and Characterization of SbSe Ge Alloys 954 65 35-x x
0
50
100
150
200
20 30 40 50 60
degree)
I nt ensity (Ar b. Un.
)
S1
S2
S3
S4
S5
Figure 2. XRD patterns of S1) Sb65Se35, S2) Sb65Se30Ge5, S3) Sb65Se25Ge10, S4) Sb65Se20Ge15, and S5) Sb65Se15Ge20 alloys.
1000
11000
21000
31000
41000
80120160 200 240
Raman Shift cm
-1
Intensity
Ge=0 Ge=5 Ge=10 Ge=15 Ge=20
Figure 3. Raman spectra of all samples.
reduction in the intensity of the bands. However, the in-
tensity of these bands decreases strongly when 5 at% Ge
added to the binary system showing that the introduction
of germanium leads to the decrease of the number of
homopolar Sb-Sb bonds and increase the number of het-
eropolar Ge-Se bonds. Moreover, in addition to the main
bands which appear at 96, and 150 cm–1, a very broad,
low intensity peak around 168 cm–1 is also observed for
sample contained 10 at%. Band 168 cm–1 is assigned to
Sb-Sb vibrations in Se2Sb-SbSe2 units [41]. At 15 at%
Ge substitution, there are three bands located at 96, 152
and 168 cm–1. In Sb65Se15Ge20 alloy, bands 96, 154 and
184 cm–1 could be assumed to GeSe4 tetrahedral and
SbSe3 pyramidal structural units.
Some important observations emerge from Figure 3:
1) For the binary system (Sb65Se35 composition), the
vibrational spectra must correspond primarily to vibra-
tional modes involving Sb-Se bonds as Se-Se bonds
would be highly unexpected in these Se-poor materials
and stretching modes of Sb-Sb homopolar bonds are lo-
cated at significantly higher frequencies.
2) The vibrational spectra for Sb-Se-Ge system has
been discussed by taking the formation of two basic
structural units, SbSe3 pyramids with three-fold coordi-
nated Sb atom at the apex and GeSe4 tetrahedrons with
Ge in the center.
3) A decrease in the peak height and an upshift to
higher values may attribute to an increase in structural
randomness [15].
4) By comparing peak position and Raman intensity in
the range of bond modes, it is derived that the changes
occur non-monotonically with increasing Ge content.
5. Conclusions
The chalcogenide bulk alloys with composition
Sb65Se35-xGex (x = 0 - 20 at%) were prepared and charac-
terized. From the measured composition and density of
these materials the physical parameters (namely, 
 , Va, Z,
parameter R and Hs) have been evaluated. These pa-
rameters are well-correlated with topological and chemi-
cal ordered network models. All specimens are polycrys-
talline in nature as confirmed by XRD pattern. XRD and
Raman spectroscopy were useful tools in the study of
structural change induced by the progressive incorpora-
tion of Ge. The compositional dependence of the XRD
and Raman spectra suggests the presence of two basic
structural units, SbSe3 pyramids with three-fold coordi-
nated Sb atom at the apex and GeSe4 tetrahedrons.

Synthesis and Characterization of SbSe Ge Alloys955
65 35-x x
6. Acknowledgements
The authors express appreciation for the financial support
to this work from the Deanship of Scientific Research,
Najran University, Najran, Saudi Arabia, in the form of
research project (Project No. NU 09/10).
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