Nature and conditions of formation of structural defects in zirconium(IV) oxide in the course of its preparation from zirconium hydroxide

Article abstract of DOI:10.1134/S1070363207040056

The nature of paramagnetic defects in nanosized ZrO₂ samples prepared by the sol-gel method was studied by ESR. Conditions of formation of different centers (Zr³⁺, F centers, O^-, O ₂ ^- ) in ZrO₂ were elucidated. The Zr³⁺ and O^- centers arise in the course of thermal dehydration of Zr(OH) ₄ under the conditions when the formation of a Zr-O-Zr bridging bond is hindered. The F centers are formed in ZrO₂ on heating in a reducing atmosphere or as a result of structural rearrangements accompanied by a decrease in the amount of lattice oxygen in the coordination surrounding of Zr(IV). Nauka/Interperiodica 2007.

Full text of DOI:10.1134/S1070363207040056

ISSN 1070-3632, Russian Journal of General Chemistry, 2007, Vol. 77, No. 4, pp. 524 531.
Pleiades Publishing, Ltd., 2007.
Original Russian Text M.I. Ivanovskaya, E.V. Frolova, 2007, published in Zhurnal Obshchei Khimii, 2007, Vol. 77, No. 4, pp. 564 571.
Nature and Conditions of Formation of Structural Defects
in Zirconium(IV) Oxide in the Course of Its Preparation
from Zirconium Hydroxide
M. I. Ivanovskaya and E. V. Frolova
Research Institute of Physicochemical Problems, Belarussian State University,
ul. Leningradskaya 14, Minsk, 220050 Belarus
e-mail: ivanovskaya@bsu.by
Received February 9, 2005
Abstract The nature of paramagnetic defects in nanosized ZrO samples prepared by the sol gel method
2
3
+
was studied by ESR. Conditions of formation of different centers (Zr , F centers, O , O ) in ZrO were
2
2
3
+
elucidated. The Zr and O centers arise in the course of thermal dehydration of Zr(OH) under the condi-
4
tions when the formation of a Zr O Zr bridging bond is hindered. The F centers are formed in ZrO on
2
heating in a reducing atmosphere or as a result of structural rearrangements accompanied by a decrease in the
amount of lattice oxygen in the coordination surrounding of Zr(IV).
DOI: 10.1134/S1070363207040056
Simulation of the ZrO structure and studies of its
properties are important for the development of mate-
structural defects in oxides (SnO , In O , ZrO ,
2 2 3 2
2
MoO ), primarily of anionic vacancies and of metal
3
rials for various purposes. ZrO doped with CaO,
ions in a lower (compared to the main substance)
oxidation state. However, structural defects caused by
the oxygen deficiency in the crystal lattice of some
oxides (MoO , ZrO ) are also formed in the course of
2
Y O , or other oxides is the most important oxygen-
2
3
ion conductor [1 4]. In addition, it is of interest as a
catalyst support [5 9] and as a component of complex
3
2
oxide catalytic systems based on CeO [10, 11]. It is
thermal dehydration of metal hydroxides without N-
and C-containing additives [27, 28].
2
also known as an active additive to some glass-form-
ing oxides (SiO , GeO ), strongly affecting the opti-
2
2
In most cases, ZrO obtained by thermal dehydra-
2
cal and mechanical properties of glasses [12 15], and
as a matrix for rare-earth elements, affecting the lumi-
nescence properties of the system [16 18].
tion of Zr(OH) xerogels is nanosized and nonstoi-
4
chiometric. Data of [29 37] show that, depending on
the formation conditions, ZrO and oxide systems
2
based on it can contain paramagnetic defects of the
The properties of materials containing ZrO differ
3+
2
following types: Zr , F centers (color centers V ),
O
essentially depending on the structural and phase state
of the material [19]. It is known that, depending on
the conditions of synthesis and heat treatment of ZrO2,
various structural modifications can be obtained:
stable (monoclinic) and metastable (amorphous, tet-
ragonal, cubic) [20]. The formation and stabilization
hole centers O , and adsorbed radical anions O2.
At significant disturbance of the stoichiometry in the
course of mechanochemical activation, formation of
3+
crystallographic shear structures containing Zr with
4+
3+
ordering of [Zr Zr ] pairs is assumed; such struc-
tures are not detected by ESR [37].
of nonequilibrium phases in ZO in a wide tempera-
2
However, it is unclear from the available data what
factors favor formation of various kinds of structural
ture range are primarily favored by the high dispersity
of Zr(OH) and ZrO particles, and also by the non-
4
2
defects in ZrO . For example, it is unclear under what
stoichiometry in the ZrO structure caused by forma-
2
2
conditions the nonstoichiometry of ZrO , caused by
tion of oxygen vacancies [21, 22].
2
formation of oxygen vacancies in the course of ther-
A promising procedure for preparing finely dis-
mal dehydration of Zr(OH) , leads to the formation of
4
3
+
persed ZrO is the sol gel method based on the use of
Zr defects and under what conditions it causes the
formation of color centers (F centers), i.e., of electrons
localized in oxygen vacancies. Data on this matter are
contradictory. Furthermore, no data are available on
the formation of hole centers O in ZrO2.
2
inorganic precursors [23, 24]. It was shown [24 27]
that additives introduced for the stabilization of metal
hydroxide sols (HNO , C-containing compounds),
3
when decomposing on heating, promote formation of
524

NATURE AND CONDITIONS OF FORMATION OF STRUCTURAL DEFECTS
525
Thus, the nature of defects prevailing in ZrO2
depending on the conditions of synthesis and heat
treatment remains a matter of discussion.
Table 1. Phase composition of ZrO samples obtained at
2
different precipitant concentrations and calcination tem-
peratures ( C)a
The goal of this study was to elucidate the nature
of structural defects in finely dispersed samples of
T,
C
ZrO depending on the heat treatment conditions.
To this end, we varied the conditions of synthesis and
heat treatment to an extent ensuring significant effect
2
Precipitant and
concentration
3
00 500 700
950
on the structure of primary Zr(OH) particles and on
4
the process of their dehydration and crystallization in
the course of heat treatment.
1
9
9
8
8
4
0.0
NaOH, 2.8 M XA
T
XA
T
T
T
T
T
T
.5 10.0 NH OH, 5.5 M XA
4
Structural features of ZrO in relation to synthesis
2
.0 9.5
.5 9.0
.3 8.5
.2 4.5
NH OH, 3.0 M XA
T + M
M
M
4
conditions. Samples were prepared at high concentra-
tions of ammonia or NaOH. These conditions ensure
formation of the product from which, in the course of
NH OH, 1.5 M XA M+T M+T
4
NH OH, 0.5 M XA
M
M
M
M
4
M
M
heating, an amorphous ZrO phase is formed initially;
2
with increasing temperature, it gradually transforms
into tetragonal and then into stable monoclinic phase
a
(
XA) X-ray amorphous, (M) monoclinic, and (T) tetragonal
phase.
of ZrO . The formation and stabilization of the amor-
2
phous and tetragonal phases of ZrO are favored by
2
X-ray amorphous. Calcination at 500 C leads to the
formation of crystalline phases: tetragonal, monoclin-
ic, or their mixture with different extents of crystalli-
zation depending on the synthesis conditions. It
should be noted, however, that calcination at 500 C,
as a rule, does not eliminate the amorphous phase
completely, and the other phases are poorly crystal-
lized. Formation of fragments of crystalline structures
starts long before the phase becomes detectable by
X-ray phase analysis and DTA.
high concentration of OH groups in the course of the
synthesis (pH 9.5 10.0). The OH groups present in a
large amount on the surface of primary particles and
the bonds between them prevent the polymerization
and condensation. Crystallization processes in such
samples are observed at higher temperatures than in
samples prepared at lower pH values (pH 8.0 9.0).
An increase in the crystallization temperature favors
formation of tetragonal, and not monoclinic, ZrO2
phase in a wider temperature range. Transformation
of the precipitate into a sol leads to further inhibition
of the crystallization of the amorphous phase and to
higher temperature of formation of the tetragonal
and/or monoclinic phase. It is presumed that xerogels
of such samples have a developed surface layer con-
sisting of polymeric cationic zirconium species, which
provides high content of water in the sols dried at
Zr3+ centers. The Zr3+ signal is observed in the
ESR spectra of samples synthesized in alkaline solu-
tions and containing both amorphous and crystalline
(tetragonal, monoclinic) phases of ZrO . Figure 1
2
shows the spectra of ZrO samples differing (accord-
2
ing to X-ray phase analysis) in the phase composition.
The Zr3 signal has an axial shape with a weakly
pronounced g component, which is particularly char-
+
1
00 C; the water molecules stabilize the nondissoci-
|
|
ated OH groups on the surface. The monoclinic ZrO2
phase can be formed directly from the amorphous
xerogel, without intermediate formation of the tetrag-
onal phase, if zirconium hydroxide was precipitated
at pH 8.3 8.5.
acteristic of samples having an amorphous structure
(Fig. 1, spectrum 1). It is known that such a shape
(broadened signal with a weakly pronounced parallel
component) is caused by the facts that the coordina-
tion surrounding of Zr3 is nonuniform and the struc-
ture as a whole is amorphous. The parameters of the
+
Hydrolysis in an acidic ZrO(NO ) solution (pH
.2 4.5) leads to the formation of the monoclinic
3
2
ESR spectra of the ZrO samples meet the condition
2
4
1
3+
g , g < g (d state of Zr ) and, depending on the
|
|
e
phase at low temperatrue. It is noted [19] that the
monoclinic structure is formed under such conditions
even in solution in the course of precipitation (with-
out formation of the amorphous phase). The phase
composition of the main examined samples of ZrO2
in relation to the pH of precipitation and calcination
temperature is given in Table 1.
sample history, can be somewhat different (g 1.977
.979, g 1.958 1.963). With an increase in the degree
1
||
of crystallinity of the samples in going from amor-
phous to tetragonal and then monoclinic structure, the
3+
anisotropy of the Zr spectrum ( g = g
g||) de-
creases from 0.020 to 0.016.
The Zr3+ signal appears in the ESR spectra of
All the samples synthesized in an alkaline solution
(pH 8 10), dried at 100 C, and calcined at 300 C are
ZrO samples prepared by low-temperature calcina-
2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 4 2007

526
IVANOVSKAYA, FROLOVA
log c
1
1
8.0
7.5
[
g 1]
1
1
7.0
2
1
1
1
1
6.5
6.0
5.5
2
3
2
00
400
600
800
1000
T, C
3
170
3270
3370
3470
H, G
3
+
Fig. 1. ESR spectra of ZrO samples of various phase
Fig. 2. Logarithm of the Zr
concentration in ZrO
2
2
compositions (300 K): (1) amorphous, (2) monoclinic,
(c) as a function of the calcination temperature: (1) xero-
and (3) tetragonal.
gel and (2) precipitate.
tion of both the precipitates prepared at high pH
values (9.0 10.0) and of xerogels (air-dry sols pre-
pared by converting the precipitates into a colloid
The sample synthesized using ammonia and cal-
cined at 700 C gives two signals from Zr3 centers.
These signals have the same shape and somewhat
differ in the g-factors, suggesting a certain difference
in the coordination surrounding of Zr3 (Fig. 3). The
spectrum of the sample calcined at a lower tempera-
ture (500 C) contains only the signal with the larger
anisotropy, and that of the sample calcined at 950 C,
only the signal with a smaller anisotropy (Table 2).
+
3
+
state). In the case of xerogels, Zr appears in ZrO2
at a lower temperature (150 C) than in heat treatment
of the precipitates (200 C). With increasing calcina-
+
3
+
tion temperature, the concentration of Zr in ZrO2
passes through a maximum (500 C, Fig. 2). Under
3
+
similar conditions of heat treatment, the Zr concen-
tration in the samples prepared from sols is higher
than in the samples prepared from the Zr(OH) pre-
The observed difference in the anisotropy of the
Zr3+ ESR signals may be due to the fact that one of
them (with a smaller anisotropy) belongs to bulk Zr
4
3
+
4+
cipitate. Calculation shows that the Zr : Zr ratio in
3
+
3+
the ZrO sample with the highest content of Zr
(
2
1
8
1
3+
10 g ) is about 1 : 4000. This content of Zr is
centers, and the other (with a larger anisotropy), to
surface Zr3 centers. Morterra et al. [33] also assigned
+
sufficiently high for affecting the adsorption and
3
+
catalytic properties of ZrO , taking into account the
the two observed signals to the surface and bulk Zr
centers. In this case, an increase in the concentration
2
3
+
arrangement of the Zr centers on the surface.
Table 2. Parameters of the ESR spectra of some ZrO samples differing in the conditions of the synthesis and heat
2
treatment
Zr3+
O
O2
Precipitant
T, C)
Zr3+ concentration,
1
6
1
(
10 , g
g
g||
g
g||
g1
g2
g3
NH OH (500)
1.979
1.979
1.958
1.958
1.965
1.964
1.963
1.961
100
20
2.055
2.055
2.044
2.044
2.033
2.008
4
NH OH (700)
2.033
2.003
4
1
.977
NH OH (950)
1.978
1.977
1.976
4
40
3
2.045
2.044
2.033
2.033
2.033
2.008
2.008
2.003
2.003
2.003
4
NaOH (500)
NaOH (700)
2.055
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 4 2007

NATURE AND CONDITIONS OF FORMATION OF STRUCTURAL DEFECTS
O
527
2
g
O2
||
1
1
2
1
0 G
g
g
||
|
|
||
1
2
2
5
2
1
0 G
1
0
|
|
g2
g2
Zr3+
3
3
170
3270
3370
3470
H, G
1
,2 2
1
g
g|
| g||
Fig. 3. ESR spectra of ZrO (77 K) synthesized with NH OH and calcined at (1) 500, (2) 700, and (3) 950 C. Insert: fragments
2
4
3
+
of ESR spectra of Zr
centers in ZrO samples 1 and 2.
2
of bulk centers with increasing calcination temperature
is attributable to coalescence of the primary particles;
in the process, the initially surface centers become
bulk centers. However, it is not improbable that the
change in the anisotropy is due to the fact that the
Color centers (F centers). In the ESR spectra of
ZrO samples prepared in strongly alkaline solutions
2
and calcined in air at 300 950 C, in which the amor-
3
+
phous and/or tetragonal phase is formed and Zr
signals are observed, the signal of F centers is absent.
A weak signal of F centers is observed only in the
ESR spectra of the samples containing the monoclinic
phase. Calcination in H (350 C, 0.5 h) of ZrO sam-
crystalline structure of ZrO becomes more perfect.
2
In the spectra of samples containing the better crystal-
lized monoclinic phase, g is lower than in the spectra
of the amorphous and tetragonal samples. However,
the interpretation based on the surface and bulk cen-
ters seems preferable, because structural rearrange-
ments occurring in the course of formation of the
monoclinic ZrO2 structure lead to a considerable
decrase in the surface area of the dioxide compared to
the samples having amorphous or tetragonal structure.
Under similar conditions of heat treatment, the Zr3
concentration and signal anisotropy are lower for the
sample prepared using NaOH, compared to the sample
prepared with ammonia (Table 2). In this case, the
2
2
ples calcined in air (500 C) leads to appearance of a
strong signal of F centers at g 2.003 and to a decrease
3
+
in the Zr signal (Fig. 4). A decrease in the intensity
of the Zr3 signal is accompanied by an increase in its
anisotropy due to a decrease in g (g 1.979, g||
+
|
|
1
.956). Also, weak ill-resolved lines of the hyperfine
structure of the signal of F centers are observed in the
+
ESR spectrum of the ZrO sample reduced in H
2
2
(
Fig. 4, spectrum 2). The hyperfine structure of this
signal probably originates from partial delocalization
of the electron density of the unpaired electron bet-
ween the orbitals of adjacent zirconium cations with
11.2 at. % abundance of the 91Zr isotope (nuclear spin
J 5/2).
3
+
+
Zr formation can be affected by the presence of Na
ions in the ZrO structure. The sample prepared in
2
acidic solution and calcined at 500 C (optimal tem-
3
+
3+
perature for the formation of Zr ) gives no Zr
signal in the ESR spectrum.
Reoxidation of the samples in air (500 C, 1 h)
leads to disappearance of signals of F centers and
restoration of the Zr3 signal of the initial shape
These data show that the Zr3+ centers were formed
+
in ZrO samples obtained by thermal dehydration at
2
(Fig. 4). Thus, in ZrO prepared by the sol gel meth-
2
3
+
2
00 950 C of both xerogels and precipitates prepared
od, the main defects (Zr , F centers) undergo revers-
ible transformations depending on the conditions of
redox treatment. Free electrons are stabilized either on
zirconium cations or on oxygen vacancies. The spec-
3
+
in a strongly alkaline solution. No Zr was detected
upon dehydration of the precipitate prepared in acidic
solution.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 4 2007

528
IVANOVSKAYA, FROLOVA
lows us to distinguish two signals of paramagnetic
oxygen centers (Fig. 5): a signal with triaxial anisot-
ropy of the g-factor (g 2.033, g 2.008, g 2.003) and
1
2
3
an axial signal (g 2.055, g 2.044). The first of them
|
|
prevails in the ESR spectrum of ZrO prepared using
1
2
ammonia, and the second, in the spectrum of ZrO2
prepared with NaOH. The triplet signal is undoubtedly
due to the presence of adsorbed oxygen species (O2)
F
on the ZrO surface. Such paramagnetic centers on the
2
surface of ZrO and other oxides (TiO , ZnO, SnO ,
2
2
2
2
Al O ) are well known; they are formed on the par-
x 1/2
2
3
tially reduced surface of metal oxides upon oxygen
adsorption [38]. In most cases, oxygen adsorption
3
gives rise to several forms of O differing in the coor-
2
dination and effective charge of the cation near which
Zr3+
O is stabilized. For the coordination of O , the g
2
2
value depends on the cation charge. The g value we
1
obtained from the ESR spectra (2.033) suggests the
4
+
stabilization of O on Zr , which is consistent with
2
3
170
3270 3370 3470
H, G
the data of [32, 35] on fixation of O on ZrO with
2
2
the same parameters. Formation of such centers is due
3
+
3+
4+
to adsorption of O on Zr : Zr + O2
Zr O .
2
2
Fig. 4. ESR spectra (300 K) of ZrO samples heat-
2
treated under various conditions: (1) 500 C, 1 h;
The broadened signal of the pseudoaxial shape with
the above-indicated (or very close) g-factors was
observed in the spectra of TiO , SnO , and MoO and
(
2) 350 C, 1 h; and (3) 350 C, 1 h + 500 C.
2
2
3
was attributed to thermo- and photostimulated forma-
tion of hole centers O [38 41].
g1
Mechanism of formation of paramagnetic centers in
O2
ZrO . It is common knowledge that thermal dehydra-
g3
2
1
tion of hydroxides inevitably disturbs the oxide stoi-
chiometry [22, 42]. The results agree with the concept
that ZrO prepared from Zr(OH) is oxygen-deficient.
g2
g||
2
4
g
Some authors gave for amorphous ZrO the composi-
tion ZrO1.95 [34].
2
2
Experimental data show that appearance of Zr3+ in
ZrO simultaneously with O is observed in the course
of themal dehydration of the hydroxide, leading to the
g 2.0036
2
(DPPH)
formation of the amorphous ZrO phase. The temper-
2
3
+
Fig. 5. ESR spectra (300 K) of ZrO samples prepared
ature range of the appearance of Zr and growth of
its concentration coincides (according to DTA data)
with the onset of intense elimination of OH groups.
The larger the extent to which the crystallization with
the formation of the monoclinic phase is inhibited, the
2
using (1) NaOH and (2) NH OH and calcined at 700 C.
4
trum of the sample prepared in acidic solution exhibits
a broad and strong signal at g 2.003 assignable to
iso
3
+
3+
the F centers, with no Zr signal as mentioned above.
higher the Zr content. The factors inhibiting the
crystallization are high dispersity of Zr(OH) samples
4
Oxygen paramagnetic centers. Along with the well-
resolved Zr3+ signal, the ESR spectra of the samples
and presence of a large amount of OH groups on the
surface, hindering the particle aggregation and the
structural rearrangements in the course of the dehydra-
tion [22].
prepared in alkaline solutions and calcined at 500
9
4
50 C contain broadened signals at g > g (Figs. 1, 3,
e
), which can be assigned, taking into account the
signal shapes and parameters, to oxygen paramagnetic
centers. In the room-temperature ESR spectra of some
samples calcined at 700 950 C, the structure of the
broadened signal becomes well-resolved, which al-
At low temperatures of formation of monoclinic
ZrO from coarsely dispersed ZrO nH O precipi-
2
2
2
tates, F centers with a high degree of disordering are
stabilized, and no Zr3 centers are formed.
+
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 4 2007

NATURE AND CONDITIONS OF FORMATION OF STRUCTURAL DEFECTS
529
3
+
Heating of ZrO in H also leads to appearance of
F centers and to a decrease in the content of Zr .
Thus, those defects are realized (Zr ; V or F
2
2
O
3
+
centers) whose formation is the most favorable ener-
getically depending on the dispersity and structure
These results are fully consistent with the regular
trends in formation of paramagnetic centers, reported
by Bobricheva et al. [43]. They showed that heating
in a vacuum of ZrO samples modified with H SO
1
of ZrO samples. Metal ions in the d state readily
2
change the oxidation state depending on the symmetry
of the oxygen surrounding. In a tetrahedrally distorted
octahedral field characteristic of amorphous and tet-
2
2
4
leads to formation of F centers only. However, heat
treatment in a vacuum of samples prepared by treat-
ment of Zr(OH) (and not ZrO ) in H SO gives rise
3
+
ragonal ZrO , stabilization of Zr with an oxygen
2
vacancy in the coordination surrounding is energeti-
cally favorable. With a decrease in the symmetry, the
4
2
2
4
to both Zr3+ and F centers simultaneously. It is known
3
+
Zr state is no longer stabilized, which leads to the
localization of a free electron in an oxygen vacancy
with the formation of an F center.
that treatment in H SO , along with surface activation,
2
4
inhibits the dehydration and crystallization of the
samples.
The results obtained, in combination with pub-
A decrease in the symmetry of the Zr3+ surround-
ing under the action of hydrogen, due to formation of
additional oxygen vacancies, leads to disappearance of
3
+
lished data [22, 29, 30], suggest that Zr appears in
intermediate steps of redox transformations accompa-
nying the cleavage of the Zr OH bonds and transfor-
mation of Zr(OH) into ZrO . According to the exist-
3
+
Zr centers and appearance of F centers. This phe-
nomenon can be accounted for as follows. Appearance
of additional oxygen vacancies in the surrounding of
the [Zr3 VO] defect upon removal of lattice oxygen
under the action of hydrogen promotes the migration
of Zr3 into the interstice, with the shift of the elec-
tron density to the oxygen vacancy. As follows from
4
2
ing views, Zr(OH) can be considered as a polymer
4
+
with cubic-lattice-type ordering and significant local
distortions of the structure. The polymerization and
condensation processes occurring in the course of
+
transformation of Zr(OH) into amorphous and then
4
+
4+
tetragonal ZrO are not accompanied by significant
the experimental data, the transition Zr3 VO Zr
2
changes in the coordination surrounding of zirconium.
The preservation of the surrounding characteristic of
cubic symmetry in products with different degrees of
dehydration is provided by high dispersity of the par-
ticles and formation from them of loose framework
structures in which structural rearrangements leading
to the monoclinic phase are hindered.
VO under the conditions of redox treatments of ZrO2
is reversible, which is important for the preservation
of the constancy of the adsorption-catalytic properties
of ZrO prepared by this procedure. Amorphous ZrO
2
2
has an enormous internal surface to which the access
of atmospheric oxygen is hindered and hence oxida-
tion of the Zr3 and F centers is hindered also. Oxida-
tion of these centers becomes possible only on heating
to 700 950 C when the rate of oxygen diffusion
becomes noticeable, the particles get consolidated, and
their structure becomes more perfect.
+
Garvie [21] showed in small ZrO particles the
2
formation of the tetragonal and cubic structures is
more favorable than the formation of the monoclinic
structure characteristic of bulk ZrO . The stability of
2
these structures (in the absence of admixtures) is pro-
vided by lattice defects. The stabilizing factor is the
decreased number of ligands (oxygen atoms) in the
coordination surrounding of zirconium atoms. Forma-
It is not fully clear under what conditions the sur-
face and bulk O centers appear in metal oxides and
whether these centers are detected by ESR. Cornoz
et al. [45] showed that the ESR parameters of ad-
sorbed O radicals coincide with the parameters of
Olat (V centers), and these centers cannot be distin-
guished without using oxygen isotopes. In oxide sys-
tems with the symmetry close to C (SnO ), the ESR
tion of oxygen vacancies (V ) is necessary for in-
O
creasing the volume of the free space in the surround-
ing of zirconium atom, in order to decrease the extent
of deformation of oxygen ions relative to the eight-
coordinate surrounding [44]. Thus, small crystalline
4
v
2
spectrum of adsorbed and photoinduced O centers is
and amorphous ZrO particles with defects in the form
2
not observed, because O is a surface hole and cap-
2
of oxygen vacancies are thermodynamically stable,
because high symmetry of the surrounding in them is
favorable.
tures an electron at a high rate: O + e
O .
lat
The rate of this reaction can be decreased by creat-
ing electron traps, e.g., by introducing admixtures. For
example, the presence of GeO in ZrO enhances the
Formation of the monoclinic ZrO structure in-
2
2
2
volves major structural rearrangements with changes
in the symmetry of the oxygen surrounding, decrease
in the coordination number (from 8 to 7), and changes
in the length and directions of Zr O bonds. With
small particles, this is energetically unfavorable.
intensity of the O signal in the ESR spectra [46].
In oxides with a low conductivity (MgO) and in
amorphous nonconducting oxides (SiO , GeO ), the
2
2
ESR signal from O can be observed [47 49]. This
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 4 2007

530
IVANOVSKAYA, FROLOVA
is, probably, the reason why the O signal is observed
3. West, A.R., Solid State Chemistry and Its Applica-
tions, Chichester: Wiley, 1984. Translated under the
title Khimiya tverdogo tela. Teoriya i prilozheniya,
Moscow: Mir, 1988, vol. 2, p. 36.
in the ESR spectrum of ZrO whose electronic con-
2
ductivity is low. High dispersity of samples, giving
rise to a large number of energy barriers on intergrain
boundaries, also hinders recombination of the electron
4
. Subbarao, E.G. and Maiti, H.S., Solid State Ionics,
and hole centers and allows these centers in ZrO to
2
1984, vol. 11, no. 2, p. 317.
be detected simultaneously by ESR.
5
. Kuznetsov, P.N., Kuznetsova, L.I., Zhizhaev, A.M.,
Kolesnikova, S.M., Pashkov, G.L., and Boldyrev, V.V.,
Zh. Neorg. Khim., 2002, vol. 47, no. 3, p. 450.
The above facts show that the Zr3+ and O centers
in ZrO are formed by thermal dehydration of the
2
6
. Chen, F.R., Goudurier, G., Joly, J.-F., and Ved-
hydroxide under the conditions when formation of a
bridging oxygen bond Zr O Zr is hindered. Such
hindrance arises when surface terminal hydroxy
groups Zr OH are removed from finely dispersed
rine, J.C., J. Catal., 1993, vol. 143, no. 4, p. 616.
7. Spilbauer, D., Mekhemer, G.A.H., Bosch, E., and
Knozinger, H., Catal. Lett., 1996, vol. 36, no. 5, p. 59.
Zr(OH) samples characterized by loose structure and
4
8. Mastikhin, V.V., Nosov, A.V., Filimonova, S.V.,
Terskikh, V.V., Kotsarenko, N.S., Shmachkova, V.P.,
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p. 81.
strongly hydrated surface. Reduction of ZrO and
2
removal of lattice oxygen, and also structural rear-
rangements with cleavage of the Zr O bond are ac-
companied by formation of F centers.
9
. Vasil’ev, M.A., Ishchenko, E.V., Yatsimirskii, A.V.,
and Bakuntseva, M.V., Zh. Fiz. Khim., 2002, vol. 76,
no. 11, p. 2090.
EXPERIMENTAL
1
1
0. Bozo, S., Gaillard, F., and Guilhaume, N., Appl. Catal.
A, 2001, vol. 220, no. 1, p. 69.
Nanosized ZrO samples were prepared by the sol
2
gel method involving precipitation of Zr(OH) , its
4
1. Craciun, R., Daniell, W., and Knozinger, H., Appl.
Catal. (A), 2002, vol. 230, no. 2, p. 153.
conversion into sol (with adding a peptizing agent or
without it), drying with the formation of a xerogel,
and heat treatment at various temperatures (100
12. Lee, S.W. and Condrate, R.A., J. Mater. Sci., 1988,
9
50 C). For comparison, we also studied Zr(OH)4
vol. 23, no. 8, p. 2951.
precipitates heat-treated without conversion to sol. As
precipitants we used NH and NaOH solutions.
1
1
1
3. Simhan, R.G., J. Non-Cryst. Solids, 1983, vol. 54,
3
no. 4, p. 335.
4. Ikryannikova, L., Aksenov, A., and Markaryan, G.,
The structural and phase transitions were studied
by X-ray phase analysis, and the nature of defects, by
ESR. X-ray diffraction patterns were recorded with an
Appl. Catal. (A), 2001, vol. 210, no. 2, p. 225.
5. Karlin, S. and Colomban, P., J. Am. Ceram. Soc.,
1
999, vol. 82, no. 3, p. 735.
HZG 4A diffractometer (CoK radiation, MnO fil-
2
ter). The ESR spectra were recorded at 77 and 300 K
on a Varian E112 spectrometer at a frequency of
16. Gutzov, S., Ponahlo, J., Lengauer, C.L., and Beran, A.,
J. Am. Ceram. Soc., 1994, vol. 77, no. 5, p. 1649.
9
.35 GHz. The g-factors and spin concentration were
1
1
1
7. Petrik, N.G., Taylor, D.P., and Orlando, T.M., J. Appl.
Phys., 1999, vol. 85, no. 12, p. 6770.
determined relative to references: diphenylpicrylhy-
drazyl (DPPH, g 2.0036) and lines of the hyperfine
structure of Mn in MgO.
8. Assefa, Z., Haire, R.G., and Raison, P.E., J. Nucl. Sci.
Technol., 2002, vol. 3, no. 1, p. 82.
2
+
9. Physical and Chemical Aspects of Adsorbents and
Catalysts, Linsen, B.G., Ed., London: Academic,
ACKNOWLEDGMENTS
1
970. Translated under the title Stroenie i svoistva
The study was financially supported by the INTAS
program (project no. 2001-2162).
adsorbentov i katalizatorov, Moscow: Mir, 1973,
p. 215.
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