Supercritical hydrothermal synthesis of Cu2O(SeO3): Structural characterization, thermal, spectroscopic and magnetic studies

Article abstract of DOI:10.1016/j.materresbull.2008.09.023

Cu₂O(SeO₃) has been synthesized in supercritical hydrothermal conditions, using an externally heated steel reactor with coupled hydraulic pump for the application of high pressure. The compound crystallizes in the P2₁3 cub

Full text of DOI:10.1016/j.materresbull.2008.09.023

Materials Research Bulletin 44 (2009) 1–5
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Materials Research Bulletin
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Supercritical hydrothermal synthesis of Cu2O(SeO3): Structural characterization,
thermal, spectroscopic and magnetic studies
a
b,
b
a
a
b
*
´
´
´
´
Aitor Larran˜aga , Jose L. Mesa , Luis Lezama , Jose L. Pizarro , Marıa I. Arriortua , Teofilo Rojo
a Departamento de Mineralogı´a y Petrologı´a, Facultad de Ciencia y Tecnologı´a, Universidad del Paı´s Vasco/EHU, Apdo. 644, E-48080 Bilbao, Spain
b
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Departamento de Quı´mica Inorganica, Facultad de Ciencia y Tecnologıa, Universidad del Paıs Vasco/EHU, Apdo. 644, E-48080 Bilbao, Spain
A R T I C L E I N F O
A B S T R A C T
Article history:
Cu2O(SeO3) has been synthesized in supercritical hydrothermal conditions, using an externally heated
steel reactor with coupled hydraulic pump for the application of high pressure. The compound
Received 23 March 2008
Received in revised form 11 September 2008
Accepted 17 September 2008
Available online 4 October 2008
˚
crystallizes in the P213 cubic space group. The unit cell parameter is a = 9.930(1) A with Z = 12. The crystal
structure has been refined by the Rietveld method. The limit of thermal stability is, approximately,
490 8C. Above this temperature the compound decomposes to SeO2(g) and CuO(s). The IR spectrum
shows the characteristic bands of the (SeO3)2ꢀ oxoanion. In the diffuse reflectance spectrum two intense
absorptions characteristic of the Cu(II) cations in five-coordination are observed. The ESR spectra are
isotropic from room temperature to 5 K, with g = 2.11(2). The thermal evolution of the intensity and line
width of the signals suggest a ferromagnetic transition in the 50–45 K range. Magnetic measurements, at
low temperatures, confirm the existence of a ferromagnetic transition with a critical temperature of 55 K.
ß 2008 Elsevier Ltd. All rights reserved.
Keywords:
A. Inorganic compounds
B. Chemical synthesis
C. High pressure
C. TGA
D. EPR
D. Magnetic properties
1. Introduction
selenium lone-pair electrons. Thus, it can be predicted that the
combination of the inherent asymmetry of (SeO3)2ꢀ with various
An important area in materials science is the design of
compounds with condensed frameworks, which can give rise to
original physical properties with potential practical applications
such as ion exchange, surface absorption chemistry, conductivity,
etc., due to the great number of different cations and arrangements
that they can exhibit [1,2]. In this way, the crystal chemistry of
selenium(IV) oxo compounds shows abundant structural versati-
lity expressed by the significant number of different compounds
[3]. The transition metal–selenium–oxygen system has been the
subject of many previous investigations. Several phases have been
reported in which selenium is found in an oxidation state IV.
Depending on the conditions in solution [4] transition-metal
compounds with (HSeO3)ꢀ, (SeO3)2ꢀ or (Se2O5)2ꢀ oxoanions are
known [5–7]. The electronic configuration of the selenium(IV)
oxoanion promotes the formation of three bonding electron pairs
with the oxygen atoms. Furthermore, the presence of an active
lone-pair of electron on the Se(IV) centers can play an important
role in the crystalline architectures of this family of compounds [3],
affected by the requirement for empty space to accommodate the
transition metals will result in a rich structural chemistry in
transition metal selenites. In this way, in the Mn4(H2O)3(SeO4)3
and Mn3(H2O)(SeO3) phases [8] which exhibit a three-dimensional
crystal structure constructed by chains, there exist cavities
delimited by the MnO6 octahedra, and SeO3 trigonal pyramids,
with the lone-pair, in an sp3 hybrid orbital, pointing towards the
center of the cavities. Similarly, the two Mn(SeO3) polymorps [9]
also exhibit a three-dimensional crystal structure formed by chains
that give rise to hexagonal channels with the lone electronic pair
directed towards the center of these channels. Recently, a few
amine-templated metal selenites have also been synthesized
under hydrothermal/solvothermal conditions in presence of
organic amines as templates. These systems have open-framework
structures [10].
Effenberger and Pertlik synthesized the Cu2O(SeO3) compound,
whose crystal structure was solved from single-crystal X-ray
diffraction [11]. The crystal structure of Cu2O(SeO3) consists of a
very compact three-dimensional framework constructed by two
CuO5 square pyramids and two SeO3 trigonal pyramids (Fig. 1).
Two kinds of structural units are observed in the structure, with
the square pyramid geometry habitually found in the copper(II)
compounds. The unit with greater connectivity is formed by link of
four square pyramids, one Cu(1)O5 and three Cu(2)O5, sharing the
* Corresponding author. Tel.: +34 946015523; fax: +34 946013500.
E-mail address: joseluis.mesa@ehu.es (J.L. Mesa).
0025-5408/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2008.09.023

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A. Larran˜aga et al. / Materials Research Bulletin 44 (2009) 1–5
autoclave. The reaction vessels were first heated to the final
reaction temperature, 300 8C, at 5 8C minꢀ1 with a constant
pressure of approximately 100 atm. The Cu(SeO3)ꢁ2H2O stating
reagent was maintained under those conditions for 72 h, followed
by slow cooling, approximately 50 8C hꢀ1, to room temperature.
Temperature and pressure conditions for the synthesis of the
compound were 300 8C and 100 atm. Attempts made to carry out
the synthesis of the Cu2O(SeO3) phase in the absence of high
pressure were unsuccessful. The well-formed, blue single-crystals
were isolated by filtration, washed with water and acetone and
dried over P2O5 for 1 h.
The percentage of the elements in the product was determined
by inductively coupled atomic emission spectroscopy (ICP-AES).
Observed: Cu, 22.2; Se, 28.9. Calculated for Cu2O(SeO3): Cu, 23.5;
Se, 29.2. The density, measured by picnometry in kerosene, was
5.0(1) g cmꢀ3
.
2.2. X-ray powder diffraction characterization
The X-ray powder diffraction characterization of Cu2O(SeO3)
has been carried out using a PHILIPS X’PERT automatic diffract-
ometer with the Cu-Ka radiation. Table 1 gives the experimental
conditions in which was recorded the powder diffraction pattern,
together with the final values of the residual indices of the
refinement.
The FULLPROF program was used to perform the profile
refinement of the diffractogram without structural model (pattern
matching routine) [13]. The structural refinement was carried out
using the refined unit-cell parameters and the atomic coordinates
given in the literature for the Cu2O(SeO3) phase [11]. The observed,
calculated and difference X-ray powder diffraction patterns are
shown in Fig. 2. The refined unit cell parameters, the atomic
coordinates and the bond distances and angles, obtained from the
Rietveld refinement, are very similar to those reported by
Effenberger and Pertlik [11] (see Supplementary Material).
2.3. Thermal characterization
Fig. 1. Polyhedral view of the of the crystal structure of Cu2O(SeO3).
O(1)–O(3) edge. The second unit has less connectivity, as it is a
trimer formed by three square pyramids, Cu(2)O5, sharing the
O(2)–O(4) edge.
The thermal decomposition curve of Cu2O(SeO3) shows
an experimental mass loss of, approximately, 40.3%, in the 450–
500 8C range, which has been assigned to the decomposition
of the (SeO3)2ꢀ anion in the form of SeO2 gas. The X-ray
powder diffraction pattern of the inorganic residue obtained at
800 8C shows the presence of CuO(s) [Space group, Fm-3m;
Furthermore, hydrothermal synthesis performed at high
temperature and pressure can be used in order to obtain new
condensed selenites. Supercritical hydrothermal synthetic tech-
niques can provide an adequate synthetic method to obtain novel
and interesting compounds that only exist over narrow stabiliza-
tion ranges [12] and to obtain mineral or complex selenites with
transition metals. In the present work, we report on the super-
critical hydrothermal synthesis, structural characterization, ther-
mal, spectroscopic and magnetic properties of the Cu2O(SeO3)
phase.
Table 1
Summary of crystallographic data and least-squares refinement for Cu2O(SeO3).
Compound
Cu2O(SeO3)
270.04
Cubic
M (g molꢀ1
)
Crystal system
Space group
P213 (no. 198)
8.930(1)
712.1(1)
273
˚
a (A)
3
˚
2. Experimental
Volume (A )
T (K)
l
Z
(Cu-K
a
)
1.5418
12
2.1. Synthesis and characterization
Dc (g cmꢀ3
)
5.04
Cu2O(SeO3) was synthesized in supercritical hydrothermal
conditions, using an externally heated steel reactor with coupled
hydraulic pump for the application of high pressure. The
Cu(SeO3)ꢁ2H2O phase was used as starting precursor in the
synthesis. It was prepared by reaction at room temperature of a
mixture containing SeO2 (9 mmol) and CuCl2ꢁ2H2O (3.0 mmol),
with a the of pH 6. Approximately 0.03 g of Cu(SeO3)ꢁ2H2O
dispersed in 2 ml of water, with a basic pH ffi 10, were sealed in a
small gold capsule, 9 mm  0.5 mm, which was loaded into a steel
2
u
range (8)
step-scan increment (8)
10–90
0.02
2u
Time per step (s)
10
No. of reflections, independent
No. of structural parameters
No. of profile parameters
No. of atoms refined
RBragg, Rf (%)
266, 124
14
12
8
3.88, 3.85
3.12, 4.00
1.94
Rp, Rwp (%)
x2 (%)

A. Larran˜aga et al. / Materials Research Bulletin 44 (2009) 1–5
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2.4. Physicochemical characterization techniques
The thermogravimetric analysis was carried out under oxygen in
a SDC 2960 Simultaneous DSC-TGA TA Instrument, at 5 8C minꢀ1 in
the temperature range 30–800 8C. A crucible containing ca. 20 mg of
sample was used. The X-ray thermodiffractometry in air was
performed on a PHILIPS X’PERT automatic diffractometer (Cu-K
a
radiation)equippedwithavariable-temperaturestage(PaarPhysica
TCU2000) witha Ptsampleholder. The IRspectrum(KBrpellets) was
obtained with a Nicolet FT-IR 740 spectrophotometer in the 400–
4000 cmꢀ1 range. Diffuse reflectance spectrum was registered at
roomtemperatureona Cary5000spectrometer inthe 210–2000 nm
range. A Bruker ESP 300 spectrometer was used to record the
polycrystalline ESR spectra. The temperature was stabilized with an
Oxford Instrument (ITC 4) regulator. The magnetic field was
measured with a Bruker BNM 200 gaussmeter and the frequency
inside the cavity was determined using a Hewlett-packard 5352B
microwave frequency counter. Magnetic measurements on a
powdered sample were performed in the temperature range 5–
300 K, using a Quantum Desing MPMS-7 SQUID magnetometer. The
magnetic fields were 0.1, 0.05 and 0.01 T, values within the range of
linear dependence of magnetization vs. magnetic, field even at 5 K.
Fig. 2. Observed, calculated and difference X-ray powder diffraction patterns of
Cu2O(SeO3).
3. Results and discussion
3.1. Infrared and UV–vis spectroscopies
The infrared spectrum of Cu2O(SeO3) shows the bands
corresponding to the (SeO3)2ꢀ anion. The symmetric and
antisymmetric stretching vibrations,
ns(SeO3), nas(SeO3) and the
deformation mode, (SeO3), appear at 830; 755, 700 and 480,
d
450 cmꢀ1, respectively. The positions observed for these vibra-
tional modes are in good agreement with those expected for this
kind of compounds and are similar to those found in the literature
for other selenite phases [15].
In the diffuse reflectance spectrum of Cu2O(SeO3) are observed
two intense absorption bands at 800 y 1100 nm, which are
characteristic of five-coordinated environments with square
pyramidal geometry [16].
Fig. 3. Thermodifractograms of Cu2O(SeO3).
˚
a = 4.694 A] [14]. The proposed decomposition reaction is:
Cu2O(SeO3) (s) ! SeO2 (g) + 2CuO (s).
The thermal behavior of this selenite has also been studied by
time-resolved X-ray thermodiffractometry. The pattern were
recorded in 2u steps of 0.028 in the range 5–408, counting for 2 s
per step and increasing the temperature at 5 8C minꢀ1. The results
are given in Fig. 3. Cu2O(SeO3) is stable up to approximately 490 8C.
This step is characterized by small displacements in the diffraction
maxima, caused by the thermal expansion. Above approximately
490 8C starts the decomposition of the selenite, and subsequent
formation of CuO, whose crystallinity increases with increasing
temperature. This oxide is the final inorganic residue obtained at
the higher temperature of the experiment, 700 8C. The result is
similar to that observed in the thermogravimetric study.
3.2. Magnetic behavior
3.2.1. ESR and magnetic measurements
The ESR spectra of Cu2O(SeO3) were performed at the X-band on
a powdered sample from room temperature to 4.2 K (see Fig. 4).
The spectra exhibit isotropic signals centred at approximately
3200 G, that were fitted to Lorentzina curves obtaining a value for
g = 2.11(1). This value is in the characteristic range of the d9–Cu(II)
cation [17].
Fig. 4. Thermal evolution of the ESR spectra of Cu2O(SeO3).

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A. Larran˜aga et al. / Materials Research Bulletin 44 (2009) 1–5
Fig. 5. Thermal variation of the xm and
x
mT curves and the derivative of xm for
Fig. 6. Thermal evolution of the xm at 1000, 500 and 100 G for Cu2O(SeO3).
Cu2O(SeO3).
The intensity of the signals is very small from room
temperature to approximately 70 K (Fig. 4(a)). Below this
temperature, the signals increase vigorously and disappear at
approximately 50 K (Fig. 4(a) and (b)). In the 45.0–4.2 K range the
intensity of the signals again increases (Fig. 4(b)). These results
suggest the possible existence of a ferromagnetic transition in the
50–45 K range.
different multiplicity, present in the crystal structure. The Curie
temperature, Tc = 55 K, has been calculated from the derivative of
the magnetic susceptibility curve (see inset in Fig. 5). These results
are in good agreement with that obtained from the ESR
measurements.
From the measurements performed at 500 and 100 G, similar
Curie temperatures were obtained. Furthermore, the irreversibility
observed between the measurements performed in the ZFC and FC
modes is greater when the magnetic field decreases (Fig. 6). These
facts corroborate the existence of ferromagnetic couplings at low
temperature.
Magnetization measurements at 5, 45 and 80 K, temperatures
located below and above of that of the magnetic transition, were
also carried out. Below 50 K, the magnetization curve shows
saturation at 3000 G, with a value for the imanation of 5500 emu/
mol, equivalent to a value for M/NAb = 0.90 electrons (Fig. 7). This
value indicates a ferromagnetic contribution of two copper(II)
cations [18].
3.2.2. Magnetic measurements
Magnetic susceptibility measurements were carried out on
powdered sample from 5.0 K to room temperature at magnetic
fields of 1000, 500 and 100 G in Zero Field Cooling (ZFC) and Field
Cooling (FC) modes. The molar magnetic susceptibility measured
at 1000 G shows a strong increase from approximately 70 to 5 K,
due to a magnetic transition between a paramagnetic state and
other ferromagnetic one, showing a ferrimagnetic behavior at low
temperature, probably due to a non-collinear ordering of the
magnetic moments of the two independent Cu(II) cations with
Fig. 7. Hysteresis loops carried out below and above of the magnetic transition for Cu2O(SeO3).

A. Larran˜aga et al. / Materials Research Bulletin 44 (2009) 1–5
5
Europeo’’ (FSE), for the magnetic measurements and the X-ray
diffraction measurement respectively.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.materresbull.2008.09.023.
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The Cu2O(SeO3) phase has been synthesized by using super-
critical hydrothermal conditions. Its crystal structure has been
refined in the P213 cubic space group. The structure consists of a
three-dimensional framework constructed by two CuO5 square
pyramids and two SeO3 trigonal pyramids. This phase is stable
until 400 8C. In the IR spectrum the vibrational bands of the
selenite group are observed. The diffuse reflectance spectroscopy is
consistent with the existence of copper(II) cations with square
pyramidal geometry. The ESR and magnetization measurements
confirm the existence of a magnetic transition from a paramagnetic
to a ferromagnetic state below 55 K, which gives rise to a
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This work has been financially supported by the ‘‘Ministerio de
´
Educacion y Ciencia’’ (MAT2007-60400/66737-C02-01) and the
‘‘Gobierno Vasco’’ (IT-177-07/312-07). The authors thank the
technicians of SGIker, Dr. I. Orue and Dr. F.J. Sangu¨esa, financed by
the National Program for the Promotion of Human Resources
within the National Plan of Scientific Research, Development and
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