Pressure-induced structural deformations in SeO2

Article abstract of DOI:10.1006/jssc.2002.9708

The high-pressure behavior of low-dimensional selenium dioxide SeO₂ (P4₂/mbc, Z = 8) is studied with Raman scattering and synchrotron angle-dispersive X-ray powder diffraction in a diamond anvil cell up to 23 GPa at room temperature. Pressure-induced transformations in this material involve a sequence of structural distortions of the chain structure. The transformation occuring above 7.0 GPa is due to symmetry lowering to space group Pbam (Z = 8) without major changes of the crystal lattice dimensions and coordination around the Se atoms. Like in the ambient pressure polymorph, the structural unit is a SeO₃E polyhedron, where E is a Se non-bonded electron lone pair, or an irregular tetrahedron with the O atoms and Se lone pair at the vertices. Further structural transitions above 17 GPa are likely to be the result of additional distortions leading to monoclinic symmetry of the crystal structure. All transformations are reversible with little hysteresis.

Full text of DOI:10.1006/jssc.2002.9708

Journal of Solid State Chemistry 168, 184–191 (2002)
doi:10.1006/jssc.2002.9708
Pressure-Induced Structural Deformations in SeO2
n
,1
n
,2
n
n
n
A. Grzechnik, L. Farina, R. Lauck, K. Syassen, I. Loa, and P. Bouvierw
n
Max-Planck-Institut f u. r Festk o. rperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany; and wEuropean Synchrotron Radiation Facility,
B.P. 220, F-38000 Grenoble Cedex, France
Received March 15, 2002; in revised form June 4, 2002; accepted July 16, 2002
oxygen atoms of the adjacent chains are considered, the
The high-pressure behavior of low-dimensional selenium Se–O coordination can be described as a distorted
dioxide SeO2 (P4 /mbc, Z ¼ 8)is studied with Raman
octahedron.
2
scattering and synchrotron angle-dispersive X-ray powder
diffraction in a diamond anvil cell up to 23 GPa at room
temperature. Pressure-induced transformations in this material
involve a sequence of structural distortions of the chain
structure. The transformation occuring above 7.0 GPa is due
to symmetry lowering to space group Pbam (Z ¼ 8)without
major changes of the crystal lattice dimensions and coordination
around the Se atoms. Like in the ambient pressure polymorph,
Many oxide materials built of asymmetric coordination
units are non-centrosymmetric with nonlinear optical
properties (3). They are often synthesized from simple
oxides containing cations with non-bonded electron pairs.
In this respect, the best example is the system Te–Se–O2
(3–6), in which several phases have second-harmonic-
generating (SHG) properties. SHG efficiences of some of
the Te–Se oxides are two orders of magnitude higher than
the structural unit is a SeO E polyhedron, where E is a Se non-
3
bonded electron lone pair, or an irregular tetrahedron with the O that of SiO (6). In addition, low-dimensional solids are
2
atoms and Se lone pair at the vertices. Further structural obtained when oxides containing lone pair elements are
transitions above 17 GPa are likely to be the result of additional mixed with transition metal oxides, e.g., the SeO –V O
5
2
2
distortions leading to monoclinic symmetry of the crystal
structure. All transformations are reversible with little hyster-
esis. # 2002 Elsevier Science (USA)
Key Words: synchrotron angle-dispersive powder X-ray
diffraction; Raman spectroscopy; crystal structure; low-dimen-
sional solids; non-bonded electron lone pair; selenium dioxide.
system (7).
The effect of pressure on the stereochemistry of electron
lone pair elements has not been extensively studied. One
could speculate on the possibility of pressure-induced
phase transitions in the relevant compounds leading to
changes of structure dimensionality, centrosymmetry-to-
non-centrosymmetry transformations, or to the synthesis
of new materials with compositions not obtainable at
atmospheric conditions. In this study, we are concerned
with the high-pressure behavior of one-dimensional SeO2
studied with Raman spectroscopy and angle-dispersive
powder X-ray diffraction in a diamond anvil cell above
INTRODUCTION
Selenium dioxide SeO2 belongs to a family of com-
pounds in which the stereochemical effect of non-bonded
electron lone pairs results in strong asymmetry of
coordination polyhedra (1). The structure of SeO2 at
2
0 GPa at room temperature. SeO is a basic component of
2
several oxide systems with low-dimensional structures and/
or with nonlinear optical properties. The knowledge of the
pressure effect on its bonding and dimensionality would be
helpful in understanding pressure-induced phenomena in
ambient conditions (P4 /mbc, Z ¼ 8) is built of infinite
2
puckered chains of corner-sharing SeO E structural units
3
more complex systems based on SeO . Recent investiga-
2
along the c-axis (Fig. 1), where E is a Se lone pair (2). The
Se atom is bonded to three oxygen atoms to form a SeO3
pyramid or an irregular tetrahedron with the O atoms
and Se lone pair at the vertices. When the next-nearest
tions of selenium dioxide with Raman spectroscopy up to
10 GPa showed no clear evidence for a structural phase
transition (8). Instead, we find a sequence of structural
distortions in the chain structure of SeO with an onset at
2
about 7.0–8.0 GPa. The new high-pressure phases are
structurally related to the one stable at ambient pressure
(2). We discuss our observations in relation to other
compounds containing cations with electron lone pairs.
1
To whom correspondence should be addressed. Fax: +49-711-689-
010. E-mail: andrzej@servix.mpi-stuttgart.mpg.de.
1
2
Permanent address: Dipartimento di Chimica Fisica e Inorganica,
Universit a" di Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy.
1
84
0022-4596/02 $35.00
r 2002 Elsevier Science (USA)
All rights reserved.

PRESSURE-INDUCED STRUCTURAL DEFORMATIONS
185
image plates. The images were integrated using the
program FIT2D (11) to yield intensity versus 2y diagrams.
The ruby luminescence method (12) was used for pressure
calibration.
RESULTS AND DISCUSSION
Raman Spectroscopy
SeO2 crystals are centrosymmetric, whose unit cell
1
3
h
contains eight formula units (D ; Z ¼ 8). The vibrational
4
modes are distributed as Gvibr: ¼ 5A1g þ 6A2g þ 6B1g þ
B2g þ 7Eg þ 3A1u þ 3A2u þ 4B1u þ 3B2u þ 10Eu; where
5
the A1g; B1g; B2g; and Eg modes are Raman active, while
the A2u and E modes are infrared active. The factor group
u
analysis and assignment of the vibrational modes are fully
described elsewhere (9). Selected Raman spectra of SeO2
upon compression up to 20 GPa are shown in Fig. 2. We
observed 20 of the 23 predicted Raman bands
(
5A1g þ 6B1g þ 5B2g þ 7Eg) in our spectra (Table 1). The
2
FIG. 1. Crystal structure of SeO at ambient pressure (2). Dark and
three missing modes are weak features that are only
observed at low temperatures and atmospheric pressure
light symbols represent the Se and O atoms.
(9).
EXPERIMENTAL
Crystals of SeO2 were grown by a vapor synthesis
reaction, in which selenium was sublimated in an oxygen
atmosphere at about 600 K at an initial pressure of
20.5 GPa
5
needle-like crystals of selenium and SeO with a length
00 mbar. With a temperature gradient of about 17 K/cm,
18.5 GPa
2
up to 5 mm were obtained at about 500 and 400 K,
respectively, after 16 days. The colorless crystals were
gently ground into a polycrystalline material for further
investigations at high pressures. Such a sample was
16.4 GPa
investigated with Raman spectroscopy in
a closed
14.1 G Pa
diamond anvil cell. It appeared to be of a pure SeO2
composition (9) without any traces of H SeO (10) that
2
3
1
2.0 GPa
forms instantly upon exposing the SeO2 sample to
moisture. During loading into a diamond cell, the sample
was always handled in a glove box in an atmosphere of dry
nitrogen. The sensitivity of SeO2 towards moisture
precluded any use of a pressure medium for all the
measurements.
10.0 GPa
7.0 GPa
Raman spectra at high pressures were collected
using a triple spectrograph (1800 groove/mm gratings)
with CCD signal detection. Raman scattering was
3.6 GPa
+
excited with an Ar laser at a wavelength of 514.5 nm.
The laser power was less than 10 mW to avoid any
thermal damage of the sample. Angle-dispersive
powder X-ray diffractograms were measured at the ID30
beamline of the European Synchrotron Radiation
Facility, Grenoble. Monochromatic radiation at a wave-
0
.7 GPa
0
200
400
Raman shift (cm_1)
FIG. 2. Selected Raman spectra of SeO upon compression.
600
800 1000 1200
(
length of 0.3738 A was used for pattern collection on
2

1
86
GRZECHNIK ET AL.
TABLE 1
Observed Raman Frequencies x in cmÀ1 in SeO at 15 K and 10 GPa (9)and at 298 K and 0.05 GPa
À4
2
Mode
o (15 K) Ref. (9)
Type of mode
o (298 K, 0.05 GPa) This work
do=P This work
do=dP Ref. (8)
B2g
Eg
49.4
53.4
60.7
Chain translation
55.2
58.3
62.3
1.6
1.6
4.4
1.1
A1g
101.3
Chain deformation
B1g
118.0
131.8
197.5
208.4
251.8
286.6
289.6
119.1
126.4
199.5
206.5
255.8
7.1
7.1
2.9
6.9
1.7
B2g
4.0
2.3
4.3
1.4
Eg
Intrachain O–Se–O bending
A1g
Eg
B1g
A1g þ B2g
O–Se–O bendings (branches)
288.7
291.7
304.2
360.9
524.5
532.3
599.4
1.7
4.5
2.6
1.9
4.8
6.7
5.8
0.7
2.6
4.1
3.5
3.0
292.9
Eg
300.5
360.0
523.0
533.5
599.6
B2g
Eg
Se–O stretching (bridging oxygens)
Eg
A1g
6
17.3
713.4
66.6
B2g
710.2
888.2
889.3
912.7
944.1
5.4
À3.9
À4.1
À2.0
À0.7
8
Se–O stretching (terminal oxygens)
A1g
B2g
B1g
879.5
904.5
936.7
À4.0
À1.0
À0.8
The pressure dependencies of Raman modes related to around the Se atoms. However, the stretching modes of the
the high-frequency Se–O stretching vibrations do not show Se–O bonds in side branches have negative pressure shifts.
any discontinuities or splittings in the entire pressure range The weakening of these bands could result from a charge
studied here (Table 1, Figs. 2 and 3), nor are there any new transfer to neighboring interchain bonds (8). As shown in
Se–O bands that would signal a coordination change Figs. 2 and 3, several new features appear in the spectra in
À1
the bending mode region from 190 to 500 cm at pressures
5
4
4
3
3
2
2
1
1
00
50
00
50
00
50
00
50
00
1000
950
900
850
800
750
700
650
600
550
500
between 9 and 11 GPa. These new features are not the
missing modes at room temperature and near ambient
pressure (Table 1). The band appearing on the low-energy
À1
side of the 199.5 cm mode has a negative shift up to
1
3
8 GPa. In addition, two new modes occur at about 320–
À1
40 cm between 10 and 11 GPa. Above 7.0 GPa, a new
À1
band is visible at 75.8 cm in the region of the chain
translations. Above about 14 GPa, additional Raman
bands are observed in the energy region of the chain
deformation and interchain translation modes. All the
pressure-induced changes in the Raman spectra are
reversible with little hysteresis (Fig. 4).
The appearance of new Raman bands in the 7–11 GPa
pressure range indicates a pressure-induced phase transi-
tion involving a structural distortion of the low-dimen-
sional SeO structure due to deformations of the chains
2
without changes in both the coordination of the Se atoms
and the number of molecular units in the cell. Accordingly,
it is possible that the degeneracy of the Eg modes is lifted
5
0
0
0
5
10 15 20
0
5
10 15 20
because the crystal symmetry lowers. Therefore, the
À1
(
a)
Pressure (GPa)
(b) Pressure (GPa)
splitting of the E bands at 53.4, 197.5 and 300.5 cm
g
into doublets would imply an orthorhombic distortion of
the chain structure, e.g., D -D ðE -B þ B Þ (17). In
2
FIG. 3. Pressure dependence of Raman frequencies in SeO upon
compression. Lines are guides to the eye.
4
h
2h
g
2g
3g

PRESSURE-INDUCED STRUCTURAL DEFORMATIONS
187
SeO2 phases at these conditions. In fact, most of the
observed reflections at the highest pressures can be
associated to the peaks already seen at lower pressures.
In order to obtain more detailed information on the
1
1
5.9 GPa
1.2 GPa
high-pressure structural behavior of SeO , we refined
2
selected patterns with the Rietveld method included in
the GSAS program (13) (Tables 2–5 and Fig. 6). The
refined parameters were: the atomic positions, Chebyshev
polynomial background, Stephens profile function (14), an
overall intensity scaling factor, cell parameters, and
March–Dollase preferred orientation correction. The use
of the Stephens function (14) for X-ray powder patterns
collected at high pressures under non-hydrostatic condi-
tions is discussed in Refs. (15, 16). A similar strategy was
employed for all refinements reported in this study.
6
1
.5 GPa
.7 GPa
Isotropic thermal parameters (U =Ue 100) for the Se,
i
*
O(1), and O(2) atoms in the P4 /mbc structural model (2)
2
were fixed at 0.5, 0.7, and 1.0, respectively. These values
could be derived from the thermal parameters at
ambient pressure and room temperature as given by St a( hl
et al. (2).
0
.1 GPa
2
3.2
0
200
400
600
800
1000 1200
-1
Raman shift (cm )
2
0.1
FIG. 4. Selected Raman spectra of SeO
2
upon decompression.
1
1
7.5
4.0
the case of the E modes due to Se–O stretchings of the
g
À1
bridging bonds at 523.0 and 533.5 cm and to the O–Se–O
À1
bending at about 251.8 cm
(Table 1), splittings are
expected. However, they are not resolved as the three
bands broaden with increasing pressure.
1
2.8
1.0
9.0
7.0
1
X-Ray Diffraction
X-ray powder diffraction patterns of SeO2 upon
compression to 23.2 GPa are shown in Fig. 5. Up to about
7
.0 GPa, the diagrams can be explained by the P4 /mbc
2
structural model of selenium dioxide at 286 K and 1 atm
2). At higher pressures, several peaks, initially asymmetric,
eventually become well-resolved doublets. Such peak
splittings and the appearance of weak additional reflections
indicate a structural phase transition with its onset above
(
4
2
0
.9
.9
.9
7
GPa. It is worth noting that up to about 17 GPa there is
only one reflection observed at about 2y ¼ 41: It is the
110) peak of the P4 /mbc space group. Upon a further
(
2
GPa
pressure increase, two weak peaks appear in this 2y range
and, overall, the diagrams exhibit significant peak broad-
ening. Prolonged annealing at 425 K did not improve the
pattern quality. The reflection broadening could be due not
only to pressure gradients in the diamond cell during our
2
4
6
8
10 12 14 16 18 20 22
2θ (˚) [λ = 0.3738 Å]
FIG. 5. Selected X-ray powder diffraction patterns of SeO
2
upon
compression. Numbers stand for pressures in GPa. The patterns at 14.0
experiments, but also to a possible mixture of different and 23.2 GPa were collected after annealing the sample at 423 K for 8 h.

1
88
GRZECHNIK ET AL.
TABLE 2
Lattice Parameters, Unit Cell Volumes, and Final Atomic
Coordinates for SeO2 at 0.9 and 4.9 GPaFSpace Group Group Pbam, Z ¼ 8; a ¼ 8:134ð1Þ A, b ¼ 7:928ð2Þ A,
TABLE 4
Final Atomic Coordinates for SeO2 at 9.0 GPaFSpace
(
(
(
(
3
P4 /mbc, Z ¼ 8: Estimated Standard Deviations Are Given in c ¼ 4:9491ð3Þ A, V ¼ 319:16ð8Þ A . Estimated Standard Devia-
2
Brackets
tions Are Given in Brackets
Z0.9 GPa
4.9 GPa
Atom
Site
x
y
z
(
a (A)
8.2626(2)
5.0328(1)
343.59(1)
8.1463(4) Se(1)
4.9913(2) Se(2)
331.23(2) O(1)
O(2)
4g
4h
8i
0.6316(5)
0.2884(4)
0.155(1)
0.441(2)
0.295(2)
0.2891(5)
0.3607(6)
0.405(2)
0.384(2)
0.548(2)
0.0
(
c (A)
1
2
(
3
V (A )
0.225(2)
Se
Site
x
8h
8g
8h
4g
4h
0.0
1
2
0.1345(2)
0.2099(2)
0.0
0.1337(2) O(3)
0.2094(2)
y
Note. Further details of the crystal structure investigation may be
obtained from the Fachinformationzentrum Energie, Physik, Mathema-
tik, D-76344 Eggenstein-Leopoldshafen 2, Germany, http://www.fiz-
karlsruhe.de, on quoting the depository number CSD-412234, the names
of the authors, and the journal citation.
z
0.0
O(1)
O(2)
Site
x
0.1400(6)
0.3600(6)
1
4
0.1372(6)
0.3628(6)
1
4
y
z
Site
x
0.9441(8)
0.1710(8)
0.0
0.9401(9)
0.1668(9)
0.0
y
range 7–17 GPa (Fig. 5) are indicative of structural
distortions in which the number of formula units in the
cell is not changed, as seen in the Raman spectra (Figs. 2
and 3). There is no doubling of any axis at the transition.
z
Note. Further details of the crystal structure investigation for the phase
at 4.9 GPa may be obtained from the Fachinformationzentrum Energie,
Physik, Mathematik, D-76344 Eggenstein-Leopoldshafen 2, Germany,
http://www.fiz-karlsruhe.de, on quoting the depository number CSD- The diagrams collected at this pressure range can no longer
412233, the names of the authors, and the journal citation.
be Rietveld refined with the ambient pressure structural
model (2). All the new features in the X-ray powder
patterns are consistent with the pressure-induced phase
For the patterns at 0.9 and 4.9 GPa, the input structural transition involving the P4 /mbc-Pbam subgroup rela-
2
model was the one of selenium dioxide at 1 atm and 286 K tionship, with Pbam space group being the translationen-
2). In this initial model, when adding the second nearest gleiche subgroup of P4 /mbc. Accordingly, in the new
(
2
oxygen atoms to the covalently bonded ones in the chains structure there are two non-equivalent sites for the Se
along the c-axis, the Se–O coordination can be described as atoms (4g and 4h) and three sites for oxygens (8i, 4g, and
a distorted octahedron. As can be inferred from a 4h). The sites 4 g and 4 h of Pbam are correlated with the
comparison of interatomic distances given in Table 3, site 8h of P4 /mbc, while the site 8i (Pbam) is correlated
2
compression of SeO2 does not result in a significant with the 8g one (P4 /mbc). The oxygen atoms at the 4 g and
2
decrease in a distortion of the octahedron below 7 GPa. 4 h positions are terminal ones while the one at the 8i
Under the same conditions, the intrachain Se–O distances
are independent of pressure within the estimated errors.
The splitting of several reflections and the appearance of
weak peaks in the X-ray powder patterns in the pressure
TABLE 5
Selected Distances (A)and Angles (deg)in SeO at 9.0 GPa
(
2
Bond distances
TABLE 3
Selected Distances (A)and Angles (deg)in SeO at 0.9
and 4.9 Gpa
(
Se(1)–O(1)
Se(1)–O(2)
Se(2)–O(1)
Se(2)–O(3)
(2x)
(2x)
1.91(1)
1.72(2)
1.77(1)
1.49(2)
2
0
.9 GPa
4.9 Gpa
Bond distances
Se–O(1)
Se–O(2)
Shortest interchain distances
(2x)
1.767(3)
1.605(7)
1.766(4) Se(1)–O(2)
1.615(7) Se(1)–O(3)
Se(2)–O(3)
2.66(2)
2.85
(2x)
(2x)
Shortest interchain distances
Se–O(2)
2.57(2)
2.78
2.74
2.84
2.69
2.79
Se(2)–O(2)
Se–O(2)
(2x)
Bridging angles within chains
Se–O(1)–Se
Bridging angles within chains
121.5(4) Se(1)–O(1)–Se(2)
122.9(4)
110.4
Note. Estimated standard deviations (ESD) are given in brackets. The
distances with no ESD are calculated using the data in Table 1.
Note. Estimated standard deviations (ESD) are given in brackets. The
distances with no ESD are calculated from the data in Table 3.

PRESSURE-INDUCED STRUCTURAL DEFORMATIONS
189
Pbam, Z = 8
9
.0 GPa
P4 /mbc, Z = 8
2
4
.9 GPa
P4 /mbc, Z = 8
2
0
.9 GPa
2θ (˚) [λ = 0.3738 Å]
2
FIG. 6. Observed, calculated, and difference X-ray powder patterns for SeO at selected pressures. Vertical markers indicate Bragg reflections.
2
2
The residuals Rwp; Rp; RðF Þ; and w ; whose definitions are given in the GSAS manual (13), areFfor the diagram at 0.9 GPa: Rwp ¼ 6:4%;
2
2
2
2
Rp ¼ 4:8%; w ¼ 1:58; RðF Þ ¼ 6:3%; for the diagram at 4.9 GPa: Rwp ¼ 7:4%; Rp ¼ 4:6%; w ¼ 1:63; RðF Þ ¼ 8:8%; for the diagram at 9.0 GPa:
2
2
Rwp ¼ 7:7%; Rp ¼ 5:0%; w ¼ 2:08; RðF Þ ¼ 13:0%: All the residuals Rwp and Rp are calculated only for the Bragg contribution to the diffraction
patterns.
position bridges the chain along the c-axis. The starting phase above 7 GPa is made in analogy to the assignment
atomic parameters for the Rietveld refinement of the for the ambient pressure structure, when considering
pattern at 9.0 GPa with the Pbam (Z ¼ 8) model (Fig. 6) separate energy regions for modes due to the Se–O
were derived from the final ones in space group P4 /mbc at stretchings, O–Se–O bendings, chain deformations and
2
4
.9 GPa (Table 2). The thermal parameters for the Se(1) translations (Table 1).
and Se(2) atoms were fixed at 0.5. The O(1), O(2), and O(3) Further structural changes above 17 GPa, involving the
atoms of the same model had their U =U 100 parameters appearance of new peaks at low 2y angles among other new
i
e *
fixed at 0.7, 1.0, and 1.0, respectively. As can be seen from features (Fig. 5), is obscured by broadening of the observed
the data given in Table 5, the octahedral coordination X-ray reflections. However, based on the absence of major
around the Se atoms is still largely distorted in the high- overall changes in the X-ray diagrams and Raman spectra,
pressure phase. In other words, the nearest and next- as discussed above, we tend to suggest that there could be a
nearest oxygen atom neighbors are still distinct. On the sequence of pressure-induced structural distortions in
other hand, the intrachain Se–O distances vary largely. The selenium dioxide up to about 23 GPa, leading to mono-
magnitude of interchain interactions and chain deforma- clinic symmetry of the structure.
tions at high pressures could be evaluated by a comparison
The pressure dependences of lattice parameters, axial
of the Se–O–Se intrachain angles (Tables 3 and 5). These ratios, and unit cell volumes across the P4 /mbc-Pbam
2
angles are much smaller for the distorted Pbam structure. It phase transition up to about 11 GPa are shown in Fig. 7.
was earlier demonstrated that the effective volume of the E The ambient pressure lattice parameters from our powder
(
(
pair is approximately the same as the volume taken up by X-ray diffraction data are a ¼ 8:310ð1Þ A, c ¼ 5:045ð2Þ A,
2
À
(
3
the ‘‘O ion’’ (1). The differences in Se–O distances and and V ¼ 348:39ð6Þ A . They are smaller than those
changes in the angles are the consequence of increased non- determined from single crystal data (2) by 0.6%, 0.3%,
bonding interactions between the oxygen atoms and the and 1.6% relative, respectively. There is no major anomaly
non-compressible electron lone pair (1) that has to be in the c parameter evolution between 7 and 8 GPa, while
accommodated in the high-pressure phase.
the a parameter of the low-pressure structure discontinu-
The symmetry lowering to D9 results in two non- ously splits into a and b parameters of the Pbam space
2h
equivalent Se positions (two Cs sites) and three non- group. The c=a ratio (P4 /mbc) slightly increases indicating
2
equivalent O positions (two Cs sites and one C1 site). the anisotropic axial compression of the chain structure,
Accordingly, the vibrational modes are distributed (17, 18) with the direction along the chain being slightly less
as Gvibr: ¼11Agþ11B1gþ7B2gþ7B3g þ7Au þ6B1u þ10B2u+ compressible. At pressures above 7 GPa it splits into the
1
0B ; where the A ; B ; B ; and B modes are Raman c=a and c=b ratios of the Pbam structure. A change in the
3u g 1g 2g 3g
active, while the B ; B ; and B modes are infrared pressure dependence of the unit cell volume is clearly
1
u
2u
3u
active. The assignment of the Raman bands in the Pbam noticeable at about 8 GPa at the structural transformation.

1
90
GRZECHNIK ET AL.
2
1
2
by the soft acoustic mode rV ¼ ðC11 À C12Þ propagating
5
.05
8
8
8
8
7
.3
.2
.1
.0
.9
along /110S. The high-pressure phase is a slight distortion
of the paratellurite structure. Whether there occur further
pressure-induced phase transitions in this material is still a
matter of discussion. Based on X-ray powder diffraction
data, Liu (21) indicates that there are no additional
transformations up to about 25 GPa. On the other hand,
Raman scattering data seem to provide evidence for a
sequence of phases closely related to the P2 2 2 one
c
a
a
5.00
4
.95
1
1 1
b
occuring near 4.5 and 11.0 GPa, while the phase forming at
about 22.0 GPa is argued to be of the PbCl type (22).
0
4
8
12
0
4
8
12
2
However, when crystal chemistry of the AX compounds
2
3
3
50
40
under pressure (23) is considered, the transformation to the
cotunnite phase (Pnam, Z ¼ 4) from rutile-related a-TeO2
seems to be quite unlikely. The coordination of the cations
0
0
0
0
.620
.615
.610
.605
c/b
in the PbCl structure is nine and there are several known
2
V
pressure-induced structures with intermediate coordination
numbers from 6 (rutile) to 9, besides the fact that the Te–O
coordination unit in a-TeO is in fact a distorted TeO E
c/a
330
2
4
bipyramid.
3
20
c/a
0
4
8
12
0
4
8
12
CONCLUDING REMARKS
Pressure (GPa)
Pressure (GPa)
The Raman scattering and synchrotron angle-dispersive
X-ray diffraction data collected in a diamond cell at room
FIG. 7. Pressure dependence of lattice parameters, axial ratios, and
unit cell volumes of SeO up at 12 GPa. Open and solid symbols represent
the data for the P4
2
2
/mbc (Z ¼ 8) and Pbam (Z ¼ 8) phases, respectively. temperature provide evidence for a sequence of pressure-
induced phase transitions in SeO involving a structural
2
We calculated various Birch–Murnaghan equations of distortion of the low-dimensional structure due to chain
state (EOS) for the compression curve up to 7.0 GPa. deformations without changes in the coordination of the Se
The V volume at ambient pressure, K bulk modulus, and atoms. The transformation occuring above 7.0 GPa is due
0
0
0
(
3
K its first derivative at zero pressure are 348.0(6) A , 75(7) to a decrease of symmetry to space group Pbam (Z ¼ 8).
GPa, and 11(2), respectively. The large value of K can be The octahedral coordination around the Se atoms in the
0
explained by the low-dimensional character of the crystal new phase remains strongly distorted. Like in the ambient
0
structure. When the K parameter is restricted to 4, K0 and pressure polymorph (P4 /mbc, Z ¼ 8) (2), the coordinated
2
(
3
V0 are equal to 96(7) GPa and 347.2(9) A , respectively. polyhedron is a SeO E structural unit, where E is a Se lone
3
The extrapolated relative difference of the unit cell volumes pair, or an irregular tetrahedron with the O atoms and the
between the P4 /mbc and Pbam phases is about 0.3% at Se lone pair at the vertices. A discontinuous change in the
2
8
GPa.
pressure dependence of unit cell volumes is observed at this
structural transformation. Further structural changes
above 17 GPa are likely to be a result of additional
distortions in selenium dioxide, resulting in monoclinic
Comparison with TeO2
It is useful to briefly consider the structure and pressure- symmetry of the structure. All the transformations are
induced phase transformations in a-TeO . In the three- reversible with little hysteresis.
2
dimensional paratellurite form of TeO (P4 2 2, Z ¼ 4),
The results of this study, evidencing distortions of the
tellurium atoms have four neighboring oxygen atoms in a chain structure in SeO under pressure without any major
2
1 1
2
disphenoidal coordination. When the Te electron lone pair structural changes, could be rationalized by recalling the
(E) or the next-nearest O neighbors are taken into account, previous work on stereochemistry of elements with electron
the Te–O coordination unit is a distorted TeO E bipyramid lone E pairs at atmospheric pressure (1). Galy et al. already
4
or a very deformed TeO octahedron, respectively (19). The demonstrated that the effective volume of the E pair is
6
latter establishes a close structural relationship of a-TeO2 approximately the same as the volume taken up by the
to the rutile type. Paratellurite undergoes a second-order ‘‘O ion’’. They subsequently discussed the plausible effect
2
À
phase transition to an orthorhombic phase (P2 2 2 , of pressure on relevant compounds and suggested that the
1
1 1
Z ¼ 4) at 0.9 GPa and room temperature (19, 20). This incomplete coordination spheres around the cations are not
reversible transition, involving no volume change, is driven condensed at high pressures as a consequence of the

PRESSURE-INDUCED STRUCTURAL DEFORMATIONS
191
presence of the non-bonded E pairs. The behavior of SeO2
investigated here up to 23 GPa and that of TeO up to
25 GPa, as observed by Liu (21), add much credence to this
postulate.
8. D. Stanila, W. Smith, and A. Anderson, Spectrosc. Lett. 33, 555
2000).
. A. Anderson, A. Sanders, and W. Smith, J. Raman Spectrosc. 31,
03 (2000).
(
2
9
4
1
1
0. B. H. Torrie, Can. J. Phys. 51, 610 (1973).
1. A. P. Hammersley, S. O. Svensson, M. Hanfland, A. N. Fitch,
and D. H a. usermann, High Pressure Res. 14, 235 (1996).
ACKNOWLEDGMENTS
12. H. K. Mao, J. Xu, and P. M. Bell, J. Geophys. Res. 91, 4673
1986).
(
1
3. A. C. Larson and R. B. von Dreele, GSAS: General Structure
Analysis System, Los Alamos National Laboratory, 2000.
4. P. W. Stephens, J. Appl. Crystallogr. 32, 281 (1999).
We thank A. Brillante for several fruitful discussions about the Raman
scattering experiment and results. L. Farina acknowledges financial
support from Max Planck Gesellschaft.
1
1
5. A. Grzechnik, A. Vegas, K. Syassen, I. Loa, M. Hanfland, and
M. Jansen, J. Solid State Chem. 154, 603 (2000).
1
6. A. Grzechnik, V. V. Ursaki, K. Syassen, I. Loa, I. M. Tiginyanu,
and M. Hanfland, J. Solid State Chem. 160, 205 (2001).
7. E. B. Wilson, J. C. Decius, and P. C. Cross, ‘‘Molecular Vibrations.’’
Dover Publications Inc., New York, 1980.
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(
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