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V. Vashook et al. / Journal of Alloys and Compounds 485 (2009) 73–81
expansion coefficients and crystal structure parameters of these
solid solutions depending on Ti concentration, the measurement
of the electrical conductivity (ꢀ) and oxygen non-stoichiometry at
high temperatures up to 1000 ◦C in air and inert gas atmospheres
with oxygen concentration down to 1 Pa, and the characterization
of the catalytic activity of selected compounds for hydrocarbon
oxidation and NO2 reduction.
measurements were collected during heating up to 900 ◦C with the rate 10 K/min.
The measurements were carried out in nitrogen flow (12 l/min) as well as in air
atmosphere.
2.5. Catalytic activity
The catalytic activity of the mixed oxide powders (surface area ∼0.4 m2) was
measured for propene oxidation and NO2 decomposition between 100–850 ◦C. Reac-
tive gas mixtures of 0.5%C3H6 + 2.5%O2 in nitrogen and 2%NO2 in N2 were used
in catalytic tests. Due to temperature change the gas velocity ranged between 0.4
and 0.9 cm s−1. The conversion was determined by change in the peak areas of the
CxHy absorption using a FTIR spectrometry at ꢂ = (2635–3225) cm−1 (R–CH3 peak)
or at ꢂ = (1782–1955) cm−1 (N–O peak). The FTIR-data were analyzed with program
MERLIN Version 1.
2. Experimental
2.1. Powder preparation and crystal structure investigation
CaRu1−xTixO3 (x = 0.0, 0.1, 0.2. . .1.0) specimens were prepared by solid state reac-
tions using RuO2−x (Fluka, 60% Ruthenium content), TiO2 (Riedel-de Haen, 99.9%
purity) and CaCO3 (Grüssing GmbH, 99% purity) powders. The stoichiometric mix-
tures of initial powders were milled in ethanol in an agate ball mill for 24 h. After
drying the mixtures were heated in air at different temperatures from 800 to 1300 ◦C
for10–130 h in alumina crucibles. After cooling to room temperature the products
were milled again and analyzed by powder X-ray diffractometry PXRD. The cooling
and heating temperature rates during syntheses at these stages were 5 ◦C/min.
Phase composition and crystal structure were investigated by means of the pow-
derdiffractiontechniqueusingaSiemensD5000powderX-raydiffractometer(CuK␣
radiation, ꢁ/2ꢁ—scanningmode, stepwidthof0.02◦, countingtimeperstep—7 s). The
crystal structures were refined by the full-profile Rietveld method, including refine-
ment of lattice parameters, positional and displacement parameters of atoms, site
occupancy, scaling factor, sample shift, background and Bragg-peak profile parame-
ters. The atomic displacement parameters were refined isotropically for all atoms. All
calculations were performed using the WinCSD (Crystal Structure Determination)
program package [14].
3. Results and discussions
3.1. Synthesis of CaRu1−xTixO3
X-ray diffraction examinations of the specimens from the
(1−x)CaRuO3−xCaTiO3 systems after treatment at 800 ◦C
(1 h) + 1000 ◦C (5 h) show that these samples consist of mix-
tures of CaRuO3 and CaTiO3 in the ratio corresponding to the
nominal composition. Remarkable broadening of the diffraction
maxima was observed, indicating a poor crystallinity of the
phases. No formation of CaRu1−xTixO3 solid solution could be
detected. Further heat treatment of the specimens at 1300 ◦C for
17 h led to significant changes in the phase compositions. X-ray
diffraction examination revealed perovskite-like structure for
all samples. No traces of non-perovskite phases were detected.
Crystal structures of the end members of the system—CaRuO3 and
CaTiO3—were successfully refined in space group Pnma (Fig. 1). The
main features of the diffraction profiles of all other compositions
within the CaRuO3–CaTiO3 system could be also satisfactorily
reproduced in the GdFeO3-type of structure, assuming formation
of CaRu1−xTixO3 solid solution. However, the examination of the
diffraction profiles revealed, that starting from the CaRu0.9Ti0.1O3
nominal composition, a broadening of the selected diffraction
maxima is observed at the patterns of the samples with “mixed”
compositions. Besides, weak satellite peaks are observed near the
(200), (301) and (321) reflections at the patterns of the samples
with x = 0.2–0.6 (Fig. 2, top). In contrast to the main diffraction
maxima, which are shifted to the higher angles with increasing Ti
content in nominal sample compositions, the positions of satellite
peaks remain unchanged. Besides, the intensities of satellite
maxima diminish with increasing Ti concentration, becoming
undetectable in the samples with x > 0.7. These observations could
be explained by a coexistence of two perovskite phases with the
same structure and slightly different lattice dimensions, mainly
a-parameter. Two-phase Rietveld refinement of the samples with
nominal composition x = 0.1–0.7 confirms this assumption. Signif-
2.2. Electrical conductivity
The electrical conductivity was measured using ceramic bars with rectangu-
lar cross section in gas atmospheres with defined oxygen partial pressures by a
DC four-point method described elsewhere [15]. The powders were pressed into
shapes of 1 mm × 3 mm × 10 mm together with 4 Pt wires (0.1 mm by diameter) and
sintered for 20–130 h in air at 1300 ◦C. Heating and cooling rates during sintering
were 5 K/min. The conductivity was measured at 20–1000 ◦C in air and in flowing
gas mixtures of Ar/O2 (1–100 Pa O2) and at heating rate 7 K/min and cooling rate
1.7 K/min. The temperature cycles were carried out in air, then in Ar gas flow at oxy-
gen concentration 46, 4.6, 1 Pa, and then finally in air again. Similar measurements
were repeated for all members of the CaRu1−xTixO3 solid solution.
2.3. Oxygen content
Changes in oxygen content were measured by solid electrolyte titration tech-
nique with PC-controlled Zirox-System device (Zirox, Greifswald, Germany). The
concept of a combined coulometric-potentiometric arrangement for the investiga-
tion of interactions between solids and the gaseous phases in the carrier gas mode
has been described earlier in [16,17]. The investigation of oxygen non-stoichiometry
was performed on ceramic samples prepared as described in Section 2.2. Air and
Ar/O2 (1 Pa) were used as the initial gas mixtures. The observed resolution of this
method is better than 0.005 of change of the oxygen atomic index.
2.4. Thermal expansion
The dilatometric analysis was performed by a L76/11C device (Linseis, Germany)
on the 3 mm × 3 mm × 20 mm bars sintered in air at 1300 ◦C for 20 h. The length
Fig. 1. Graphical results of the Rietveld refinement of CaRuO3 and CaTiO3 structures. Observed and calculated profiles, differential curves (in the bottom of figures), as well
as vertical bars indicating positions of diffraction maxima are shown.