New pressure-induced monoclinic β-Sb2Te3 phase with sevenfold symmetry
Sergio Michielon de Souza*, Daniela Menegon Trichês* and Claudio Michel Poffo
Departamento de Engenharia Mecânica, Universidade Federal de Santa Catarina, Campus
Universitário Trindade, S/N, C.P. 476, 88040-900 Florianópolis, Santa Catarina, Brazil
João Cardoso de Lima and Tarciso Antonio Grandi
Departamento de Física, Universidade Federal de Santa Catarina, Campus Universitário
Trindade, S/N, C.P. 476, 88040-900 Florianópolis, Santa Catarina, Brazil
Alain Polian and Michel Gauthier
Physique des Milieux Denses, IMPMC, CNRS-UMR 7590, Université Pierre et Marie
Curie-Paris 6, B115, 4 Place Jussieu, 75252 Paris Cedex 05, France
A nanometric Sb2Te3 rhombohedral phase was produced from Sb and Te by mechanical
alloying for 3 hours and its structural stability was studied by synchrotron X-ray diffraction
(XRD) and Raman spectroscopy (RS) measurements as a function of pressure. A phase
transformation from the ambient pressure rhombohedral phase into a β-Sb2Te3 monoclinic
structure between 9.8 and 13.2 GPa is observed by XRD. This phase transformation is
confirmed by the Raman spectroscopy measurements. The pressure dependence of the
volume fited to a Birch-Murnaghan equation of state gives a bulk modulus B0 = 40.6 1.5
GPa and B'0 = 5.1 0.6. The bulk modulus of the nano-Sb2Te3 seems to be slightly smaller
than that for its bulk counterpart (44.8 GPa).
PACS index: 61.05.cp, 61.50.Ks, 62.50.-p, 61.46.Hk
Compounds based on Bi, Sb and Te (V2-VI3 semiconductors) are the best known
materials for thermoelectric applications at room temperature or below. Sb2Te3 is one of
these compounds that has a great potential for technological applications [1-4]. Its figure of
merit ZT exhibits a maximum close to room temperature.
Sb2Te3 is a layered compound that at ambient conditions crystallizes in a
rhombohedral structure R3m (S.G. 166, Z = 3). One layer, i.e., one unit bonded through
iono-covalent bonds, consists of five alternating sheets of Sb and Te. The succession is
[Te(2)-Sb-Te(1)-Sb-Te(2)], where Sb and Te(2) atoms occupy the 6c Wyckoff sites (in the
hexagonal setting) and Te(1) atoms the 3a sites . The hexagonal c axis is perpendicular
to the plane of the layers and the layers are bonded by van der Waals bonds. The
elementary piece of this structure is a SbTe6 octahedron centered on a Sb atom. From this
point of view, the layers are formed by two planes of adjacent edge sharing SbTe6
octahedra with common Te(1) atoms.
In spite of its technological importance, the effect of high pressure on the Sb2Te3
compound has not been widely studied. Using X-ray diffraction (XRD) and electrical
resistance measurements, Sakai et al.  observed a phase transformation at 10.7 GPa,
while Jacobsen et al  using XRD measurements observed a phase transformation for
pressures between 7 and 10 GPa. In Ref.  the new phase is indexed to an orthorhombic
phase (I222, S.G.23), but this result is controversial.
The mechanical alloying (MA) is an efficient technique to synthesize many unique
materials, such as nanometric and amorphous alloys as well as metastable solid solutions
. It has many advantages including low temperature processing, easy composition
control, inexpensive equipment and the possibility of industrial production. Its main
disadvantage is the contamination by the milling media and/or the milling atmosphere.
Different techniques, including MA, have been used to produce nanometric phases . A
good review of the MA technique is found in Ref. 9, while the physical mechanisms
involved are discussed in Refs. 10-13.
Nanometric materials have two components: crystallites of nanometric dimensions
(2–100 nm) with the same structure as their crystalline counterparts, and an interfacial
component, which comprises the several types of defects (grain boundaries, interphase
boundaries, dislocations, etc.). Nanometric materials are metastable . Thermoelectric
Sb2Te3 has been prepared by several techniques, including MA .
There are several studies describing the effect of high-pressure on nanometric
materials [15-20]. However, there are no results for Sb2Te3 in nanometric form. In this
work, we report the effect of high pressure on a nanometric Sb2Te3 rhombohedral phase
prepared by MA. The structural and vibrational changes with increasing pressure were
studied by XRD and Raman spectroscopy (RS).
II. EXPERIMENTAL PROCEDURE
A binary Sb-Te mixture of high-purity elemental powders of Sb (Aldrich 99.999%)
and Te (Alfa Aesar 99.999%, -100 meshes) in the proportion 2:3 atomic was sealed
together with several steel balls of 11.0 mm in diameter into a cylindrical steel vial under
argon atmosphere. The ball-to-powder weight ratio was 7:1. The vial was mounted on a
SPEX Mixer/mill, model 8000. The temperature was kept close to the ambient temperature
by a ventilation system. After 3 hours of milling, the measured XRD pattern was indexed to
the Sb2Te3 rhombohedral stable phase and milling process was interrupted.
A membrane diamond anvil cell (DAC)  with an opening that allowed probing
up to 28° of 2 was used. A small amount of Sb2Te3 powder was compacted between
diamonds to a final thickness of approximately 15 µm. A small chip of this preparation,
about 80 µm in diameter, was then loaded into a stainless-steel gasket hole of 150 m
diameter. Neon gas was used as a pressure-transmitting medium because (i) it is one of the
softests materials, (ii) it is chemically inert, and (iii) it has no luminescence and no Raman
activity. The pressure was determined through the fluorescence shift of a ruby sphere 
loaded in the sample chamber. The quasi-hydrostatic conditions were controled throughout
the experiments by monitoring the separation and widths of R1 and R2 lines. In situ XRD
patterns as a function of pressure were acquired at the XRD1 station of the ELETTRA
synchrotron radiation facility. This diffraction beamline is designed to provide a
monochromatized, high-flux, tunable x-ray source in the spectral range from 4 to 25 keV
. The present study was performed using a wavelength of 0.068881 nm. The detector
was a 345-mm imaging plate from MarResearch. The sample-to-detector distance was
calibrated by diffraction data from Si powder loaded in the diamond anvil cell. The data
were collected with a 10 min exposure time. The two-dimensional diffraction patterns were
converted to intensity versus 2 using the fit2D software  and analyzed by the Rietveld
method using the GSAS package .
For the Raman measurements as a function of pressure, one particle of
approximately 50 x 60 x 20 m2 was loaded in the DAC. The Raman spectra and ruby
luminescence were recorded in the backscattering geometry by means of a Jobin-Yvon
T64000 Raman triple spectrometer and a liquid-nitrogen-cooled charge coupled device
multichannel detector. An excitation line of = 514.5 nm of an Ar laser was used for
excitation and focused down to 5 µm with a power of about 20 mW at the entrance of the
DAC. The acquisition time was 1800 s. The Raman frequencies were determined from a fit
of the peaks to a Lorentzian profile. The frequency accuracy was better than 1 cm-1.
III. RESULTS AND DISCUSSION
The as-milled powder has a microstructure that consists of a Sb2Te3 rhombohedral
matrix with Te particles. This powder, in form of a pellet, was sealed in a quartz tube
evacuated at about 10-3 Torr and annealed at 583 K for 9 h, followed by cooling in air. As-
milled and annealed powders were characterized through XRD, RS, differential scanning
calorimetry (DSC) and photoacoustic absorption spectroscopy (PAS) measurements. The
results were reported in Ref. 26, and will be not repeated here, but the main difference
between them is that the annealed powder properties are very similar to that of the bulk
A. High pressure XRD measurements
In situ XRD measurements on as-milled nanometric Sb2Te3 rhombohedral (phase I)
powder were performed at increasing pressure up to 19.2 GPa. Fig. 1 shows some
representative XRD patterns. Up to 9.8 GPa, the XRD patterns correspond to the
rhombohedral phase I. With increasing pressure, the peaks are shifted toward higher 2
values, their intensity decreases and they broaden. At 9.8 GPa, a shoulder at about 2 =
13.8o is observed in the XRD pattern, indicating the nucleation and growth of a new phase
(phase II). This new phase is completely formed at 13.2 GPa and remains up to below 15.5
GPa where it disappears almost completely. Between 15.2 and 19.2 GPa, despite the
presence of diffraction peaks from the crystalline neon and the metallic gasket preventing
the correct interpretation of the XRD patterns, two new peaks located between 2 = 13.8o
and 18o indicate the emergence of a new Sb2Te3 phase (phase III).
A2B3 (A = Bi, and Sb) and (B = Te and Se) form an isomorphous family of
compounds. In an unpublished study, we investigated the effect of high-pressure up to 31.7
GPa on the mechanically alloyed Bi2Te3 rhombohedral powder. Besides the ambient
rhombohedral phase (phase I), at least three new high-pressure phases (phases II, III and
IV) were observed. To date, only the results concerning the effect of high pressure on the
phase I were published . Comparison between the XRD patterns for the phase II of
Sb2Te3 and Bi2Te3 shows a resemblance (Fig. 2).
Einaga et al.  and Zhu et al.  investigated the effect of high pressure on the
bulk Bi2Te3 rhombohedral phase. The new high pressure phases are similar to those
observed by us for nanometric Bi2Te3. Zhu et al.  reproduced the XRD pattern
measured for the phase II and III assuming seven- (C2/m, S.G. 12) β-Bi2Te3 and eightfold
(C2/c, S.G. 15) γ-Bi2Te3 monoclinic structures, while Einaga et al.  reproduced the
XRD pattern measured for the phase IV assuming a structural model analogous to a
substitutional Bi-Te binary alloy (60 atomic % tellurium), where the Bi and Te atoms are
distributed in the bcc lattice sites (
, S.G. 229).
The XRD patterns for the nanometric Sb2Te3 rhombohedral phase (phase I) at
various pressures were refined using the Rietveld method . The lattice parameters a and
c, the c/a ratio, and the volume V as a function of pressure are shown and compared with
the literature data of Ref. 6 and 7 in Figs. 3 (a), (b) and (c), respectively. At low pressures,
the c parameter decreases faster than a as demonstrated by the initial slope of the c/a ratio
versus pressure. This anisotropic compressibility is often observed in low dimensionality
compounds [31, 32], where atoms in adjacent layers are linked through van der Waals
forces. Above ~4 GPa, the c/a ratio slope changes sign, in qualitative agreement with the
results reported in Refs. 6 for Sb2Te3 and 27 and 33 for Bi2Te3. This is an indication that
the repulsive part of the Van der Waals bonds starts to play an important role. The volume
as a function of pressure V(p) obtained from the Rietveld refinement was fitted to a Birch-
Murnaghan equation of state (BM EOS) :
where X = (V0/V)1/3. A fit of the experimental V(p) in the stability range of rhombohedral
phase to a BM EOS (Fig. 3 (c)) gives B0 = 40.6 1.5 GPa and B'0 = 5.1 0.6. The literature
reports a value of B0 = 44.823 GPa for bulk Sb2Te3 compound . Jacobsen et al. 
reported B0 = 60.8 GPa with B'0 = 3.4 using a Vinet EOS.
Jacobsen et al.  also reported the existence of an electronic topological transition
(ETT) or Lifschitz transition  around 3 GPa. Such a transition is due a modification of
the Fermi surface topology due to hydrostatic and non-hydrostatic compression and has
been shown to influence strongly the thermoelectrical properties of compounds ,
particularly in the case of Bi2Te3 . The insufficient number of data points in the XRD
measurements prevents us to evidence the ETT in Sb2Te3.
B. β-Sb2Te3 (phase II)
The structural model proposed by Zhu et al.  for the sevenfold β-Bi2Te3
monoclinic (C2/m S.G. 12) was used as initial data to simulate the measured XRD pattern
at 13.2 GPa (phase II) of as-milled Sb2Te3 powder. The best simulation was achieved for
the structural parameters listed in Table I. Fig. 4 shows the excellent agreement between the
experimental and simulated patterns.
There are constraints that the proposed sevenfold β-Sb2Te3 monoclinic structure
must satisfy: i) the density must be larger than or equal to that of the lower pressure
structure, and ii) the smallest Sb-Te interatomic distance must not be smaller than roughly
the sum of the atomic radii of Sb and Te atoms. If the covalent atomic radii of Sb (0.141
nm) and Te (0.137 nm)  are considered, the smallest Sb-Te interatomic distances
should not be less than approximately 0.278 nm. At 9.8 GPa, from the Rietveld simulation
a density value of 7.639 g/cm3 was obtained for the rhombohedral phase (phase I) while,
after the transition, a value of 8.224 g/cm3 was obtained for the sevenfold monoclinic phase
(phase II). This value is 8% larger than before phase transition. Using the structural data
listed in Table I in the Crystal Office 98 software , the smallest calculated Sb-Te
interatomic distance is 0.275 nm, which is compatible with the estimated Sb-Te interatomic
C. Raman spectroscopy under pressure
The Sb2Te3 rhombohedral phase crystallizes in R3m symmetry, and the normal
modes at the point of the Brillouin zone are classified according to the irreducible
representations of this point group 
) ( 3
Because of the inversion symmetry, there is exclusion between the Raman and infrared
activity, and the g modes are Raman active whereas the u modes are IR active (one Au and
one Eu are acoustic modes). Richter et al.  measured the Raman spectra at ambient
conditions (except the low frequency Eg1) and more recently Sosso et al.  calculated the
frequency shift of the Raman and IR active modes at ambient conditions for the
rhombohedral phase. The calculated  (experimental ) wave number of the Raman
active modes are: σ(Eg1) = 46 cm-1 (-), σ(Eg2) = 113 cm-1 (112 cm-1), σ(A1g1) = 69 cm-1 (69
cm-1) and σ(A1g2) = 166 cm-1 (165 cm-1). The IR modes are listed in Ref. 41, where each
Raman irreducible representation is correlated with a set of displacement pattern in the a–b
plane (E modes) and along the c-axis (A modes).
Figure 5 shows the measured RS spectra for the nanometric Sb2Te3 powder at
several pressures. The RS spectrum measured at ambient pressure agrees quite well with
that measured out of the DAC and shown in Ref. 26. The as-milled powder has a
microstructure formed by a Sb2Te3 rhombohedral matrix and Te particles , and their
Raman active modes are marked in Fig. 5. At ambient pressure, the wave number of the Te
Raman active modes are A1 = 122 cm-1 and E = 141.3 cm-1 (E). The effect of high pressure
on the structure and on the Raman active modes of trigonal Te was investigated by
Partharasathy and Holzapfel  and Richter et al , respectively. All the Te Raman
active modes decrease with increasing pressure  as shown by the red solid lines (online)
in Fig. 5 and the first structural phase transition occurs at about 4 GPa . For the
nanometric Sb2Te3 rhombohedral powder, all the Raman active modes originating from
phase I become weaker, broader, shift to higher wave numbers with increasing pressure up
to 10.6 GPa, and disappear completely at 13.8 GPa. The behavior of these modes suggests a
gradual transformation of nanometric Sb2Te3 phase I into the β-Sb2Te3 phase II, which is
completed at 13.8 GPa.
The sevenfold β-Sb2Te3 monoclinic structure (C2/m) has 6 Raman active modes at
the point of the Brillouin zone, which are given by the irreducible representation 
Figure 6 shows the Raman spectrum measured for the sevenfold β-Sb2Te3
monoclinic phase for 13.8 and 25.8 GPa, where one can see that between 50 and 650 cm-1
there are three broad bands labeled A, B and C in Fig. 7, which could be followed up to the
maximum pressure reached in this experiment, although a second phase transformation was
observed by XRD around 15.8 GPa (see supra). Nevertheless, the intensity of the two
highest frequency modes decrease with increasing pressure, which may be an indication of
a metallization of the compound.
In phase I, the pressure dependence of the wave number may be approximated with
a standard second order polynomial:
where 0 is the wave number in cm-1 at zero pressure and P in GPa. The wave number of
the Raman active modes were obtained through a fitting procedure using Lorentzian
profiles. The obtained dependences are:
P 13 . 0P 42 . 458. 70A)P(
P04. 0P75. 2 35.166A ) P(
P 88. 0 P 86 . 220.114 E)P(
Figure 7 shows this pressure dependence.
The effect of high pressure on the Raman active modes can be better understood by
considering the derivative of Eq. (4): d/dP = A + 2BP
Figure 8 shows the derivative of analytical expressions (5) obtained from fits for Eg2, A1g1
and A1g2 modes. From this figure one can see that with increasing pressure, the derivative
of A1g1 mode varies faster than that of the Eg2 and A1g2 modes. This is not surprising
considering the displacement pattern of these different modes [41,44]. Indeed, the A1g1
mode is a "respiratory" mode of the layer parallel to the c-axis, and hence, the rapidly
varying interlayer van der Waals interaction is one of the main restoring forces for this
mode. On the contrary, for the other two vibrations, this interaction is only implied at the
second order. This behavior is very similar to that of the vibrational modes of GaS .
From another point of view, Eg2 and A1g2 imply the same interatomic interactions although
the displacement is along the c-axis for the A1g mode and in the layer plane for the Eg one,
and hence similar pressure dependence is not surprising. The A1g2 mode is slightly less
sensitive to pressure increase of than the Eg2 mode. Above 9 GPa, the effect of pressure is
stronger in the A1g1 mode than in the Eg2 and A1g2 modes, as seen in Fig. 5.
The Grüneisen parameter
describes the effect of high pressure on the volume of
structural lattice of the material, and consequently, on the phonons vibrations. The zero-
pressure mode Grüneisen parameters
are determined using the equation [47, 48]
B and 0 are the bulk modulus in GPa and wave number in cm-1 at zero pressure.
By XRD, we obtained (see preceding paragraph)
B = 40.6 ± 1.5 GPa. This value is ≈ 9%
smaller than that reported in the literature for bulk rhombohedral Sb2Te3 . Using
and 0 values in Eq. (6), the
values for the A1g1, Eg2 and A1g2 modes are 2.55 (2.82), 0.67
(0.74), and 1.04 (1.15), respectively. The numbers between parentheses were calculated
B = 44.823 GPa. Again, one observe here that the pressure affect much more the
A1g1 mode than the other.
Nanometric Sb2Te3 rhombohedral powder was produced by MA. Its structural and
vibrational properties were investigated through in situ high-pressure XRD and RS
measurements. With increasing pressure, several structural transformations were seen, but
due to the diffraction peaks associated with the crystallization of neon gas (used as a
pressure transmitting medium) and those associated to the gasket, only the data of phase I
and phase II could be used. The XRD pattern measured at 13.2 GPa (phase II) for the
Sb2Te3 was well reproduced assuming a sevenfold β-Bi2Te3 monoclinic structure (C2/m).
From the analysis of RS spectra measured with increasing pressure for the as-milled
nanometric Sb2Te3 powder, the pressure dependence of the Raman active modes were
established. The weak van der Waals interlayer interaction translates more on the A1g1
mode variation than in that of the Eg2 and A1g2 ones. This is because this interaction is more
involved in the displacement pattern of the former than in that of the later.
This study was part of the PhD thesis of one of the authors (S.M.S) and it was
financially supported by the Brazilian-French CAPES/COFECUB Program (Project No.
559/7). We thank ELETTRA synchrotron (Trieste, Italy) for the XRD measurements as a
function of pressure. We are indebted to Jean-Claude Chervin, Pascal Munch and Gilles Le
Marchand for technical support.
* Present adress: Departamento de Física, Universidade Federal do Amazonas, 3000 Japiim,
69077-000 Manaus, Amazonas, Brazil
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