Structural study of SnO at high pressure

Article abstract of DOI:10.1016/j.physb.2005.10.136

We have studied the structural behaviour of tin monoxide (SnO) by energy dispersive X-ray diffraction (EDXRD) and EXAFS in a pressure range up to 51 GPa. We found a shear stress induced phase transition to an orthorhombically distorted structure under non hydrostatic conditions. Besides this we observed no phase transition of SnO up to the highest pressure of this study. SnO shows strong anisotropy in compression. The a-axis is rather incompressible with a linear stiffness coefficient of Ka0=306(6)GPa whereas the stiffness of the c-axis is Kc0=43(2)GPa. The bulk modulus of SnO is K0=35(1)GPa and its derivative K0′=6.1(2). A possible disproportionation of SnO to SnO ₂ and Sn at ambient temperature under high pressure is also discussed.

Full text of DOI:10.1016/j.physb.2005.10.136

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Physica B 373 (2006) 76–81
www.elsevier.com/locate/physb
Structural study of SnO at high pressure
Ã
H. Giefersa,b, , F. Porschc, G. Wortmanna
aDepartment Physik, Universita¨t Paderborn, D-33095 Paderborn, Germany
bDepartment of Physics, University of Nevada Las Vegas, Las Vegas, NV 89154, USA
cMineralogisch-Petrologisches Institut, Universita¨t Bonn, D-53115 Bonn, Germany
Received 23 September 2005; received in revised form 26 October 2005; accepted 29 October 2005
Abstract
We have studied the structural behaviour of tin monoxide (SnO) by energy dispersive X-ray diffraction (EDXRD) and EXAFS in a
pressure range up to 51 GPa. We found a shear stress induced phase transition to an orthorhombically distorted structure under non
hydrostatic conditions. Besides this we observed no phase transition of SnO up to the highest pressure of this study. SnO shows strong
anisotropy in compression. The a-axis is rather incompressible with a linear stiffness coefficient of Ka0 ¼ 306ð6Þ GPa whereas the stiffness
of the c-axis is Kc0 ¼ 43ð2Þ GPa. The bulk modulus of SnO is K0 ¼ 35ð1Þ GPa and its derivative K00 ¼ 6:1ð2Þ. A possible
disproportionation of SnO to SnO2 and Sn at ambient temperature under high pressure is also discussed.
r 2005 Elsevier B.V. All rights reserved.
PACS: 61.50.Ks; 62.50.+p; 61.10.Nz; 64.30.+t
Keywords: Tin monoxide; High pressure; X-ray powder diffraction; Equation of state
1. Introduction
15 GPa but no phase transition up to 19 GPa. In contrast
to PbO, which transforms to the massicot structure (b-
PbO, Pbma, Z ¼ 4) at pressures above 2.5 GPa [6], such a
structure has not been observed for SnO up to 12 GPa
[4,6,7].
Infrared reflectivity measurements under pressure [7]
revealed a semiconductor to metal transition of SnO
around 5 GPa. The pressure of 5 GPa is stated to be a
lower bound for the transition pressure due to non-
hydrostatic conditions in those experiments. In a recent
theoretical study [8] a change of the slope of c/a vs.
pressure was found around 5 GPa which has been related
to that transition. The hybridization of the Sn–5s lone pair
with the O–p states together with the change of the c/a
ratio under pressure plays an important role in this
transition.
Tin monoxide (SnO) has the litharge-type (a-PbO)
structure (tP4, P4/nmm, SG No. 129, Z ¼ 2). This
structure is formed by Sn–O–Sn layers stacked along the
c-direction with weak Van der Waals interaction [1]
between the Sn–O–Sn layers. Beside this two metastable
structures of SnO are known [2,3].
There are only a few high-pressure studies on SnO with
controversal results. At 1.5 GPa a first-order transition to
an orthorhombic structure was observed in Ref. [4] which
can be described as a distorted litharge-type structure.
While Kapitanov and Yakovlev [5] could not confirm this
transition, Adams et al. [6] observed such a transition but
with a much smaller distortion and being of second-order
type. Wang et al. [7] observed only a small distortion above
In the course of our study of the thermal disproportio-
nation of SnO under high pressure [9] we have also
investigated the structural behaviour of SnO under high
pressure because this might affect the reaction mechanism.
The results of this structural investigation are presented
here.
Ã
Corresponding author. Department of Physics, University of Nevada
Las Vegas, Las Vegas, NV 89154-4002, USA. Tel.: +1 702 895 1718;
fax: +1 702 895 0804.
E-mail address: hubertus@physics.unlv.edu (H. Giefers).
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2005.10.136

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H. Giefers et al. / Physica B 373 (2006) 76–81
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2. Experimental
The sample material SnO (99.9%) obtained from
Chempur was ground for the measurements. Upon
grinding the colour of the powder changes from black-
blue to brown [10]. The EDXRD measurements at room
temperature were carried out with diamond anvil cells
(DAC) in a first run up to 12.8 GPa and in a second run up
to 51 GPa. In both runs fine Au powder served as pressure
marker [11] and nitrogen (N2) was used as pressure-
transmitting medium. Diamond flat diameters were 1 mm
(first run) and 0.3 mm (second run), the sample hole
diameter were 0.4 and 0.12 mm, respectively, and the
synchrotron
beam
cross-section
0.2 ꢀ 0.2
and
0.08 ꢀ 0.08 mm2, respectively. Inconel was used as gasket
material. The EDXRD diffraction angle Y was 3.911. The
diffraction spectra were evaluated with the computer
program EDXPowd [12] which allows a direct refinement
of the lattice parameters by simultaneous fitting of the
diffraction spectra of sample and marker substance. This
program also has a special mode which allows the rapid
evaluation of a large amount of similar spectra (about 100
per minute) with only minimal manual intervention. This
mode is particular useful for kinetic investigations.
Fig. 1. EDXRD pattern in d-space of SnO at about 2 GPa with three
different pressure transmitting media: (a) nitrogen, (b) NaCl, and (c)
MgO. The pressure was determined with the diffraction lines of (a) Au, (b)
NaCl, and (c) MgO.
3. Results and discussion
3.1. The orthorhombic phase of SnO
Adams et al. [6] and Serebryanaya et al. [4] reported a
reversible phase transition of SnO under pressure to an
orthorhombically distorted structure. This transition could
not be confirmed by Kapitanov and Yakovlev [5] and
Wang et al. [7]. Instead, Kapitanov and Yakovlev [5]
proposed that the observed line broadening of the
diffraction lines (hkl) with hak is due to a phase transition
which is caused by the shear stress under nonhydrostatic
conditions. In order to prove this we performed experi-
ments with various pressure transmitting media under
otherwise identical conditions. At one hand we used N2
which is known as good quasi-hydrostatic pressure-
transmitting medium and on the other hand we used NaCl
and even MgO which are poor pressure transmitting media
which readily introduce shear stress. In the case of N2 gold
was used as pressure marker, otherwise NaCl [13] and
MgO [14] themselves served as pressure marker. The results
of these experiments are shown in Fig. 1 for a pressure of
about 2 GPa. The width of the diffraction lines from SnO
with hak increases in the sequence of the media N2, NaCl,
and MgO. This line broadening is not due to pressure
inhomogeneities in the cell because all the diffraction lines
of the pressure markers and also the lines of SnO with
h ¼ k are narrow. The ratio of the half width of the (1 0 1)
lines with hak is 1.0/1.8/4.0 (N2/NaCl/MgO) whilst that
for the (1 1 0) lines with h ¼ k is only 1.00/1.15/1.29.
Obviously, the strong broadening of the (1 0 1) lines is due
to shear stress. However, a pure mechanical deformation
by shear stress would cause a similar broadening of the
lines with h ¼ k which is not observed. Hence, the line
splitting is due to a shear stress induced phase transition as
proposed in [5]. The situation is somewhat complicated
because the sample is a powder sample and stress is
anisotropic. Therefore the splitting depends on the
orientation of the crystals with respect to the stress field.
This leads to an extra broadening of the lines that split with
hak but does not affect the lines with h ¼ k. Indeed this is
what we observe in our experiments and what has also been
observed in Refs. [4,6]. The lines with hak are not only
split but also broaden in such an extent that the doublet is
not well resolved. This specific extra line broadening
cannot be caused by a regular phase transition under
hydrostatic pressure. We conclude that there is no
orthorhombic phase under truly hydrostatic conditions.
In Ref. [4] LiF was used as pressure-transmitting
medium which is a poor pressure transmitter somewhere
between NaCl and MgO. In accord therewith the splitting
of the (1 0 1) line observed by Serebryanaya et al. [4] at
2 GPa is about half of that observed in our experiment with
MgO. However, at 12 GPa they observed a line splitting of
7.6% whereas in our experiment with N2 the line splitting is
less than 4% even at 51 GPa (cf. Fig. 2). This strongly
supports our conclusion that the orthorhombic phase is a
shear stress induced phase and at pressures of 50 GPa even
N2 introduces some shear stress.

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Table 1
Lattice parameters of SnO at different pressures determined with EDXRD
3
V (A )
˚
a (A)
˚
c (A)
˚
p (GPa)
0.0
0.4
3.800 (2)
3.796 (4)
3.781 (5)
3.773 (5)
3.764 (5)
3.749 (5)
3.747 (5)
3.735 (5)
3.727 (5)
3.713 (6)
3.703 (5)
3.674 (6)
3.670 (6)
3.663 (6)
3.657 (8)
3.634 (7)
3.618 (8)
3.601 (9)
3.575 (9)
3.556 (9)
3.531 (8)
3.525 (8)
3.507 (8)
3.499 (8)
3.486 (8)
3.478 (9)
3.475 (8)
4.836 (2)
69.8 (1)
69.1 (4)
66.9 (4)
65.7 (4)
64.6 (4)
63.0 (4)
62.5 (4)
61.9 (4)
61.1 (4)
60.2 (4)
59.3 (4)
57.6 (4)
57.2 (5)
56.9 (5)
56.2 (6)
54.9 (6)
54.0 (6)
53.2 (7)
51.6 (8)
50.3 (7)
48.9 (7)
48.5 (7)
47.6 (7)
47.1 (7)
46.4 (7)
46.1 (7)
46.0 (8)
4.796 (13)
4.682 (14)
4.614 (13)
4.558 (14)
4.483 (14)
4.455 (13)
4.439 (14)
4.400 (13)
4.363 (15)
4.326 (14)
4.264 (14)
4.249 (15)
4.241 (15)
4.202 (20)
4.159 (19)
4.127 (20)
4.103 (21)
4.035 (23)
3.975 (20)
3.925 (19)
3.905 (19)
3.867 (19)
3.848 (19)
3.819 (18)
3.813 (20)
3.805 (23)
2.0
2.6
3.3
4.7
5.1
5.7
6.6
7.6
8.7
11.5
12.1
12.8
14.3
17.4
20.0
21.9
27.1
30.8
35.0
37.8
41.2
44.2
47.3
49.5
50.9
Fig. 2. EDXRD spectra of SnO under high pressure with a diffraction
angle Y of 3.911. Nitrogen was used as pressure transmitting medium and
gold as pressure marker. The diffraction lines (hkl) with h¼k become
broader by nonhydrostatic stress due to the solidification of nitrogen.
Arrow up: pressure increase; arrow down: after decompression.
observed no additional phase transition for SnO up to the
highest pressure applied.
At ambient conditions the unit cell parameters of SnO
Adams et al. [6] used fluorinert and nujol as pressure
transmitting media which have to our experience pressure
transmitting properties better than LiF and similar to
NaCl. This explains why these authors observed a weak
line splitting and an almost continuous onset of the
transition. Adams et al. [6] not only studied SnO but also
PbO which has the same crystal structure. They found for
PbO the same kind of orthorhombic distortion under
pressure as for SnO but to a much larger extend. The line
splitting has the same pattern as that we observed for SnO,
namely lines with hak are split and broadened and lines
with h ¼ k remain narrow. Hence, we conclude that the
observed phase transition to orthorhombic g-PbO is also
induced by shear stress as has already been assumed by
Dashille and Roy [15].
˚
˚
are a ¼ 3.800(2) A and c ¼ 4.836(2) A. The lattice para-
meter a shows an extremely small compressibility (cf. Table
1 and Fig. 3(a)) resulting in a strong decrease of the c/a
ratio under pressure (cf. Fig. 3(b)). The standard errors of
the lattice parameters given in Fig. 3(a) are estimates from
the half width of the diffraction lines. On pressure release
the lattice parameters relax to the starting values within the
experimental errors but some broadening of the lines with
h¼k remains (cf. Fig. 2(d)).
The pressure dependence of the volume V is shown in
Fig. 3(c). A fit of the third-order Birch–Murnaghan
equation of state to the p–V data of SnO up to 51 GPa
provides a bulk modulus of K0 ¼ 35(1) GPa and a first
derivative of K00 ¼ 6.1(2). These values are in good
agreement with the data K0 ¼ 38(2) GPa and K00 ¼ 6.0(6)
obtained by Wang et al. [7] for the smaller pressure range
0–19 GPa.
3.2. Compressibility of SnO
We studied the variation of the lattice parameter of SnO
under pressure up to 51 GPa. Although we used N2 in this
experiment an extra broadening of the diffraction lines
with h¼k was observed at very high pressures (Fig. 2). We
attribute this extra broadening to the shear stress induced
phase transition as described above. For the data evalua-
tion we have neglected this orthorhombic distortion and
assumed a tetragonal lattice. In contrast to PbO [6] we
While the decrease of the volume V can be well described
by a conventional equation of state this is not true for the
lattice parameters a and c. Therefore we calculated the
linear stiffness Kl ¼ ꢁldp/dl for a and c by numerical
differentiation. The result is shown in Fig. 4(a). At ambient
pressure the linear stiffness for a has the large value of
Ka0 ¼ 306(6) GPa and is seven times larger than that for c
which is about Kc0 ¼ 43(2) GPa. In Refs. [7,16] the mean

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conventional equation of state fail for the linear compres-
sibility of SnO. Ka has a large value but its first derivative
K0a ¼ dKa/dp is close to zero at low pressure. With
increasing pressure the stiffness shows a strong increases
to a value of K0aE16 a0bove 12 GPa. Likewise, the pressure
dependence of Kc is Kc 0 ¼ 15 at low pressure but less than
11 at a pressures of 12 GPa and above. Therefore, the ratio
Kc/Ka (Fig. 4(b)) strongly increases with pressure up to
12 GPa and remains at a constant value of 0.5 at pressures
above.
It should be mentioned that at 12 GPa the cell volume is
3
57 A . This is just the volume where the theoretical studies
˚
[8] give a discontinuity in c/a. Unfortunately, no explana-
tion is given for these anomaly, although it is found with
both calculation methods used (LDA and GGA). The
discontinuity of the calculation for c/a [8] at 12 GPa maybe
connected with a transition, which we observe in the Kc/Ka
ratio at 12 GPa.
3.3. Structural variation under pressure
The strong anisotropy of the compressibility of SnO
raises the question for the variation of the free atomic
positional parameter z(Sn) under pressure. Our attempts to
obtain this parameter from the EDXRD spectra failed.
While structure refinements with EDXRD are difficult in
general in a DAC under high pressure we encountered an
additional problem. Due to the crystal shape SnO shows
strong preferred orientation effects which is shown for SnO
powder prepared on an adhesive tape, measured with the
tape oriented perpendicular (Fig. 5(a)) and almost parallel
(Fig. 5(b)) to the synchrotron radiation beam, respectively.
This sample exhibits a strong (0 0 1)-texture with preferred
orientation of the c-axis perpendicular to the tape plane.
In order to overcome the problem of preferred orienta-
tion we determined the pressure dependence of the Sn–O
distance by EXAFS measurements [17]. In combination
with the lattice parameters determined in this study we can
calculate the variation of the free atomic positional
parameter z(Sn) of SnO. The Sn–O distance decreases
Fig. 3. Normalized lattice parameter a and c from EDXRD measure-
ments and derived values of the c/a ratio and V/V0. The straight line in (c)
is a fit of the Birch–Murnaghan equation to our data (solid symbols) with
K0 ¼ 35(1) GPa and K00 ¼ 6.1(2). The open symbols are the experimental
results from Ref. [7].
Fig. 4. (a) Linear stiffness Ka ¼ ꢁadp=da and Kc ¼ ꢁcdp=dc of SnO
derived from the curves in Fig. 3(a). (b) The ratio of the linear stiffnesses.
values given for the pressure range from 0 to 4 GPa are 303
and 357 GPa, respectively, for Ka, and 78 and 56 GPa,
respectively, for Kc. These are in very good agreement with
our results Ka ¼ 312 GPa and Kc ¼ 63 GPa for the mean
values up to 4 GPa. From Fig. 4(a) it is clear why fits to
Fig. 5. EDXRD pattern of SnO powder measured (a) perpendicular and
(b) nearly parallel to the sample plane reveal the strong texture of SnO.

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Fig. 7. Comparison between the volume of SnO and that of 12(SnO2+Sn).
The volume for SnO2 and Sn are from Refs. [18] and [19], respectively.
Fig. 6. Pressure dependence of (a) the Sn–O distance determined from
EXAFS measurements [17], (b) the Sn–O–Sn bond angle in the a–c plane,
and (c) the position parameter z(Sn) in SnO determined from the
combination of EXAFS and EDXRD studies. The inset shows the
tetragonal unit cell of SnO. Large light spheres are O2ꢁ at the 2a site
same at 20 GPa [7]. But our high temperature high-pressure
disproportionation studies [9] reveal that at pressures of
14 GPa and above, SnO2–IV (modified fluorite-type) [18] is
the stable phase of SnO2. The specific volume of this phase
is about 10% less than that of SnO2–I and SnO2–II.
Therefore even pressures above 20 GPa may favour the
decomposition of SnO at room temperature (Fig. 7).
However, in our high-pressure study we observed no
disproportionation of SnO at room temperature up to the
highest pressure of 51 GPa and even not in preparatory
experiments up to 60 GPa.
(0,0,0); small dark spheres are Sn2+ at the 2c site ð2; 0; zÞ.
1
linearly with pressure by about 1.8% from ambient
pressure to 10 GPa (Fig. 6(a)), the highest pressure in the
EXAFS study. In combination with the variation of the
lattice parameter we obtain a continuous increase of the
position from z(Sn) ¼ 0.2369 at ambient pressure to 0.2692
at 10 GPa. These findings agree with full Rietveld refine-
ments of angle-dispersive X-ray diffraction spectra up to
19 GPa [7]. The Sn–O–Sn bond angel of 117.81 at ambient
pressure decreases to 1161 at 10 GPa (Fig. 6(b)).
4. Summary
We have shown that the orthorhombic distorted high-
pressure structure of SnO is a real shear stress induced
phase. The orthorhombic distortion does not appear under
hydrostatic conditions but it is also not a simple (dissim-
ilar) mechanical deformation by the shear stress.
SnO has a strong anisotropy of the compressibility which
leads to a decrease of the axial ratio from c/a ¼ 1.27 at
ambient pressure to c/a ¼ 1.1 at 50 GPa. There is also a
remarkable change in the ratio of the linear stiffnesses Kc/Ka
from 0.15 at ambient pressure up to 0.5 at 12 GPa. However,
at higher pressures this ratio remains constant at Kc/Ka ¼ 0.5.
The volume compressibility of SnO shows no anomalies and
can be well described by a Birch–Murnaghan equation of
3.4. Disproportionation of SnO under high pressure
SnO is metastable at ambient conditions and dispropor-
tionates at elevated temperatures to SnO2 and Sn. We
found that the disproportionation rate under pressure is
higher than at ambient pressure [9]. Since the total volume
strongly decreases on the disproportionation reaction by
about 12% at ambient pressure, high pressure might favour
the disproportionation of SnO already at room tempera-
ture. However, SnO is much more compressible than the
disproportionation products Sn and SnO2 and the specific
volumes of SnO and the products would become almost the

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81
state with a bulk modulus of K0 ¼ 35(1) GPa and the first
derivative of K00 ¼ 6.1(2).
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We could also determine the variation of the atomic
positional parameter z(Sn) under pressure up to 10 GPa
from the variation of the lattice parameters and the
pressure dependence of the Sn-O distance determined by
separate EXAFS measurements.
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There is no high-pressure phase of SnO up to 51 GPa
except for the orthorhombic distorted phase under non-
hydrostatic conditions. Finally, SnO does not decompose
to SnO2 and Sn under high pressure at room temperature
despite the fact that the specific volume of the products is
up to 40 GPa smaller than that of SnO.
(2005) 014109.
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Acknowledgement
[12] F. Porsch, EDXPowd 3.13, Windows Version, RTI GmbH Pader-
born, Germany (1996-2004).
The authors acknowledge the financial support by the
Bundesministerium fur Bildung und Forschung (BMBF)
¨
under grant No. 05 KS1PPA5 and by the Deutsches
Elektronen Synchrotron (DESY), Hamburg (II-01-084 and
II-01-085). We thank K. Syassen for making preprints
available prior to publication.
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