Lanthanum doped calcium titanates: Synthesis, crystal structure, thermal expansion and transport properties

Article abstract of DOI:10.1016/S0925-8388(02)01345-2

Two series of perovskite-type compounds La_1-xCa_xTiO₃ (x=0.2-1.0) and La_2(1-x)/3Ca_xTiO₃ (x=0, 0.1, 0.4 and 0.8) were synthesized by a ceramic technique in air (final heating 1350 °C). The crystal structures of the La_1-xCa_xTiO₃ compounds were characterised at room temperature as orthorhombic in space group Pbnm (samples with 0.7≤x≤1) and as rhombohedral in space group R3c (x=0.6 sample). At room temperature three different perovskite-like structures has been found for A-deficient La_2(1-x)/3Ca_xTiO₃ compound: orthorhombic Pbnm structure for the composition with x=0.8, orthorhombic Imma structure for x=0.4, and monoclinic P2/m (or possibly orthorhombic Cm2m) structure for x=0.1. The compositions were stable within the pO₂ range 21000 to 6·10^-16 Pa at 850-1000 °C. Oxygen stoichiometries and electrical conductivities of the solid solutions were investigated. At 900 °C within the oxygen partial pressure region 1·10^-15 to 0.21·10⁵ Pa the compounds are n-type semiconductors. For La_0.1Ca_0.9O₃ maximal conductivities were observed at 1000 °C of about 50 S/cm in Ar/H₂/H₂O (pH₂O/pH₂=0.01) and of 0.003 S/cm in air. No simple correlations between catalytic activity, electrical conductivity, and crystal structure parameters were found.

Full text of DOI:10.1016/S0925-8388(02)01345-2

Journal of Alloys and Compounds 354 (2003) 13–23
L
www.elsevier.com/locate/jallcom
Lanthanum doped calcium titanates: synthesis, crystal structure, thermal
expansion and transport properties
a ,
b
c
a
a
*
V. Vashook , L. Vasylechko , M. Knapp , H. Ullmann , U. Guth
a
Institute of Physical Chemistry and Electrochemistry, Dresden University of Technology, D-01062 Dresden, Germany
b
‘
Lviv Polytechnic’ National University, 12 Bandera Street, Lviv 79013, Ukraine
c
Institute for Materials Science, Darmstadt University of Technology, D-64287 Darmstadt, Germany
Received 7 November 2002; received in revised form 25 November 2002; accepted 25 November 2002
Abstract
Two series of perovskite-type compounds La12xCa TiO (x50.2–1.0) and La
Ca TiO (x50, 0.1, 0.4 and 0.8) were synthesized
x
3
2(12x) / 3
x
3
by a ceramic technique in air (final heating 1350 8C). The crystal structures of the La12xCa TiO compounds were characterised at room
x
3
]
temperature as orthorhombic in space group Pbnm (samples with 0.7#x#1) and as rhombohedral in space group R3c (x50.6 sample). At
room temperature three different perovskite-like structures has been found for A-deficient La2(12x) / 3Ca TiO compound: orthorhombic
x
3
Pbnm structure for the composition with x50.8, orthorhombic Imma structure for x50.4, and monoclinic P2/m (or possibly
2
16
orthorhombic Cm2m) structure for x50.1. The compositions were stable within the pO2 range 21000 to 6?10
Oxygen stoichiometries and electrical conductivities of the solid solutions were investigated. At 900 8C within the oxygen partial pressure
Pa at 850–1000 8C.
2
15
5
region 1?10
to 0.21?10 Pa the compounds are n-type semiconductors. For La0.1Ca0.9O3 maximal conductivities were observed at
1
000 8C of about 50 S/cm in Ar/H /H O (pH O/pH 50.01) and of 0.003 S/cm in air. No simple correlations between catalytic
2
2
2
2
activity, electrical conductivity, and crystal structure parameters were found.
2002 Elsevier Science B.V. All rights reserved.

Keywords: Rare earth compounds; Oxide materials; Crystal structure; Electrical transport
1
. Introduction
the B-position were prepared with regard to their applica-
tion as anodes in SOFCs [2]. A broad range of variation of
the physical properties such as electrical conductivity and
thermal expansion was observed. The oxygen partial
pressure dependence of the electrical conductivity and
oxygen ionic transport in La12xCa CrO was investigated
in Ref. [3]. Synthesis, crystal structure, oxygen stoichiome-
try, electrical conductivity, and catalytic activity for hydro-
carbon oxidation of compounds in the series
Recently, a lot of new compounds as alternative anode
materials for solid oxide fuel cells (SOFCs) have been
suggested: La12x(Sr,Ca) CrO , La0.8Sr0.2Cr0.97V0.03O3,
x
3
ZrO (Y O 1Nb O ),
(Sr12xBax)0.6Ti0.2Nb0.8O3,
La0.6Sr0.4Co0.2Fe0.8O3,
La12xBa Cr Ti O [1], selected compositions in the
32d
2
2
3
2
5
x
3
Gd (Ti Mo ) O ,
2
12y
y
2
7
x
12y
y
system La12xCa Cr Ti O [2]. Up to now not one of
x
12y
y
32d
the proposed compounds fulfil the requirements of chemi-
cal stability, electrocatalytic activity and thermal expansion
La12xCa Cr0.2Ti0.8O32d
,
La12xCa Cr0.8Ti0.2O32d and
x
x
La12xCa Cr0.5Ti0.5O32d were reported in our previous
x
[1] completely. Especially, for SOFCs with hydrocarbon
publications [4–6].
fuel and medium operation temperature the further search
for new anode materials continues.
One possible candidate as anode material for SOFCs
could be lanthanum doped calcium titanate, because it was
A series of lanthanum chromites partially substituted
with alkaline-earth elements in the A-position of the
perovskite and with transition metals like Ti, Co, or Fe in
found
La12xCa Cr Ti O
32d
increases of the electrical conductivity at reducing con-
ditions [4,5]. Recently, high electrical conductivity and
acceptable catalytic activity in SOFC anode conditions
that
increasing
Ti/Cr
ratios
in
are accompanied by corresponding
x
12y
y
*
Corresponding author. Tel.: 149-351-4633-4792; fax: 149-351-
were found for the analogous series La12xSr TiO [7].
4
633-7752.
x
32d
E-mail address: vladimir.vashook@chemie.tu-dresden.de (V. Vashook).
Several authors investigated the perovskite compounds
0
925-8388/02/$ – see front matter
 2002 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0925-8388(02)01345-2

1
4
V. Vashook et al. / Journal of Alloys and Compounds 354 (2003) 13–23
in the system La O –TiO . The stoichiometric compounds
Rietveld method, including refinement of lattice parame-
ters, positional and displacement parameters, scaling fac-
tor, zero shift, background and Bragg-peak profile parame-
ters. The atomic displacement parameters are refined
anisotropically for La(Ca) cations and isotropically for all
other atoms. All calculations were performed using the
WinCSD (Crystal Structure Determination) program pack-
age [16].
2
3
2
La O ?TiO , La O ?2TiO , 2La O ?9TiO were synthes-
2
3
2
2
3
2
2
3
2
ized in Ref. [8]. A-site deficient La12xTiO solid solutions
3
with x50–0.33 were prepared in Ref. [9]. For different
composition ranges three structures were found: Pnma
(x50–0.12), Imma (x50.20–0.25) and Pban (x50.30–
0
.33). La2 / 3TiO is an insulator, which has a wide metallic
3
31
range at concentrations of Ti
in the range 24–94%.
Slightly doped compositions in La2 / 31xTiO32d show
semiconducting behaviour, while heavy doped composi-
tions show metallic conductivity. Oxygen deficient perov-
skites La2 / 3TiO32d with oxygen vacancies d50.007–
2.3. Oxygen stoichiometry measurements
0
.079 were prepared in Ref. [10]. Ordering of cation
Oxygen stoichiometry changes were measured by solid-
electrolyte technique using the device OXYLYT [17]. The
initial oxygen contents of the samples were assumed as
3.00 after crystal structure investigations and after ex-
perimental results of phase stability of the compounds in
reducing atmosphere Ar/H /H O (pH O/pH 50.01).
vacancies and superstructure along the c-axis was ob-
served.
A-site
deficient
perovskite-type
titanates
A12xLa2x / 3TiO3 (A5Ba, Sr, Ca, Pb) are prepared and
intensively investigated in respect of their dielectric prop-
erties [11–14]. The system Ca12xLa2x / 3TiO3 was first
investigated in Ref. [13]. The solubility limit of La during
synthesis in air was found to be x50.96. Doped La2 / 3TiO3
compounds have high ionic conductivity, which makes
them useful as a component of SOFCs [15].
2
2
2
2
Air, Ar/O , and Ar/H /H O were used as flowing
2
2
2
reaction gas mixtures of defined oxygen partial pressures.
The oxygen partial pressures of the gas mixtures have been
controlled and measured by solid electrolyte poten-
tiometry. The oxygen content was investigated on powder
samples by solid electrolyte coulometry.
In this work two different series of perovskite-type
compounds
La12xCa TiO
(x50–1.0)
and
x
3
La2(12x) / 3Ca TiO (x50, 0.1, 0.4 and 0.8) were prepared,
x
3
their crystal structures, oxygen stoichiometries, electrical
conductivities, thermal expansion, and catalytic activity as
functions of calcium content, oxygen partial pressure and
temperature were investigated.
2.4. Electrical conductivity measurements
The electrical conductivities of the samples were mea-
sured by a DC four-point method described elsewhere [18].
Ceramic shapes approximately of the dimension of 103
3
3
31 mm equipped with fixed Pt wires were used for
2
. Experimental
electrical conductivity measurements in gas atmospheres of
defined oxygen partial pressures. The conductivity was
measured at 20–1000 8C in air and in the flows of Ar/O2
2
.1. Sample preparation
(
1–100 Pa O ) and Ar/H /H O with pH O/pH 50.01–
2 2 2 2 2
La12xCa TiO (x50.0 to 1.0) and La2(12x) / 3Ca TiO
3
x50, 0.1, 0.4 and 0.8) powders were prepared by a
0.53. At a rate of 3 to 5 K/min coincidence of the cooling
and heating curves of conductivity was observed indicating
equilibration.
x
3
x
(
standard ceramic solid state technique from the powders of
La O , CaCO , Cr O and TiO2 (99.9% purity). The
2
3
3
2
3
mixtures of initial oxides were ground with ethanol in an
agate ball mill for 24 h. After drying the mixtures were
heated in air at 1100 8C for 20 h in alumina crucibles.
After cooling to room temperature (RT) the products were
milled and finally heated in air at 1350 8C for 20 h. The
powders were pressed into pellets and sintered for 20 h at
temperatures of 1250 to 1400 8C depending on composi-
tion.
2.5. Catalytic activity
The catalytic activity of the oxide powders was mea-
sured in the temperature range 100 to 850 8C by means of
the catalyst system shown elsewhere [6]. As model re-
actions the oxidation reactions of hydrocarbons (1) CH 1
4
1.5O 5CO 12H O, (2) C H 14.5O 53CO 13H O,
2
2
2
3
6
2
2
2
and (3) C H 15O 53CO 14H O were used. Constant
3
8
2
2
2
2
2
.2. Crystal structure investigation
surface areas of 0.4 m of the samples were adjusted based
on previous BET results. Gas mixtures of N –C H O
2
x
y
2
The RT crystal structures were investigated by means of
(97:0.5:2.5, v/v) were used as reactants. The gas flow was
varied in the range of 0.4 to 0.9 cm/s. The turnover-
conversion was determined by the change of the peak area
of the C H -absorption using a Fourier transform (FT) IR
the powder diffraction technique using a Siemens D5000
powder X-ray diffractometer (Cu Ka radiation, u/2u
scanning mode, step width of 0.028, counting time per step
x
y
2
1
7
s). The crystal structures were refined by the full-profile
spectrometer at n5(2635–3225) cm (R–CH –Peak).
3

V. Vashook et al. / Journal of Alloys and Compounds 354 (2003) 13–23
15
2
.6. Thermal expansion
10–25% of the perovskite phase were registered in the
powder samples within x50.2 to 0.3 on the background of
Ca La Ti O [19] and CaLa Ti O [19] phases. No
perovskite phases were found in the powders within x50–
0.1.
High-resolution powder diffraction technique and
2
4
6
20
4
5
17
˚
synchrotron radiation with l50.70787 A were used for the
determination of thermal expansion in the temperature
range 293 to 1273 K in air. The diffraction experiments
were carried out at the Hamburg Synchrotron Radiation
Laboratory HASYLAB (beam line B2) by a powder
diffractometer equipped with STOE furnace. Lattice pa-
rameters and atomic coordinates were refined by the full
profile Rietveld method from the powder pattern, collected
using synchrotron radiation. On the basis of the results the
critical temperatures and the values of thermal expansion
coefficients were determined.
Structure refinement of the samples La12xCa TiO with
x
3
0.7#x#1 was performed in the GdFeO type of structure,
3
whereas the La0.4Ca0.6TiO structure has been refined in
3
the rhombohedral system (Table 1). Fig. 2 represents an
example of profile fitting for La0.1Ca0.9TiO . Site occupa-
3
tions, lattice parameters, interatomic distances, and ob-
served tolerance factors as functions of Ca content are
presented in Figs. 3 and 4.
Three powder samples of the series La2(12x) / 3Ca TiO
x
3
Some of the ceramic samples were simultaneously
investigated in a LINSEIS dilatometer in air in the
temperature range of 293 to 1123 K.
with x50.1, 0.4 and 0.8 were found to be single phase
perovskite-type compounds. Less then 1% of TiO2 was
detected in La0.4Ca0.4TiO , only. The sample of the
3
nominal composition La0.67TiO3 was a mixture of
La Ti O (|40%), La Ti O (|35%), and La Ti O
2
2
7
4
2
24
4
2
24
3
. Results
(
|25%) phases. The oxidation state of titanium was 41.
At room temperature, La0.13Ca0.8TiO has the GdFeO -
3
3
3
.1. Synthesis and structure of compounds in the systems
˚
type structure [space group Pbnm, Z54, a55.4090(1) A,
La12xCa TiO and La
Ca TiO
x
3
2(12x) /3
x 3
˚
˚
b55.4446(1) A, c57.6695(1) A], La0.4Ca0.4TiO has a
3
body-centered orthorhombic structure [space group Imma,
Z54, a57.7115(2) A, b55.4624(6) A, c55.4623(5) A],
and La0.6Ca0.1TiO3 has a monoclinic structure [space
Phases with distorted perovskite-type structures were
observed in the La12xCa TiO system within the Ca
concentration range 0.2 to 1.0. The colour of the com-
˚
˚
˚
x
3
˚
˚
group P2/m, Z54, a55.4694(4) A, b57.7707(1) A,
pounds is light yellow, except for CaTiO which is light
3
˚
c55.4702(4) A, b590.196(1)8]. The latter structure could
violet. Single-phase compounds with perovskite-like struc-
tures were obtained for the samples with 0.7#x#1 (Fig.
also be described in an orthorhombic unit cell with 2a 3
p
2
7
ap 32a dimensions [space group Cm2m, Z58, a5
.7223(1) A, b57.7487(1) A, c57.7706(1) A]. The
1
). Noticeable amounts of foreign phases appeared in
p
˚
˚
˚
lanthanum-rich samples with x#0.7. Weak reflections of
monoclinic cell with the smaller volume looks more
preferable taking into account the same residuals achieved
Ca La Ti O phases [19] could be detected in the pat-
2
4
6
20
terns of powder compounds within x50.4 to 0.6. Only
in both structural models.
A
peculiarity of the
La0.6Ca0.1TiO structure is the ordering of La(Ca) cations
3
and A-cation vacancies between two (in the Cm2m struc-
ture—four) non-equivalent crystallographic positions, in
contrast to La0.13Ca0.8TiO and La0.4Ca0.4TiO3.
3
Full profile Rietveld refinement of the La0.4Ca0.4TiO3
data at elevated temperatures reveals a tetragonal structure
between 470 and 1070 K and cubic perovskite structure at
1
170 K. From the results obtained it may concluded that
two phase transitions occur in La0.4Ca0.4TiO within the
3
temperature range of 300 to 1170 K: Imma–I4/mcm near
6
50 K and I4/mcm–Pm3m near 1170 K. Similar phase
transformations (Pbnm–Imma at 1020 K, Imma–I4/mcm
at 1110 K and I4/mcm–Pm3m at 1340 K) were observed
in Ref. [20] for SrZrO . Fig. 5 demonstrates the changing
3
lattice parameters and cell volumes of this compound with
temperature. The monoclinic structure of La0.6Ca0.1TiO3
changes to tetragonal at temperatures higher than about
7
70 K.
A more detailed description of structural features in
A-site deficient compounds La2(12x) / 3Ca TiO will be
Fig. 1. Composition ranges of samples in the system La12xCa TiO
x
3
x
3
prepared at 1350 8C in air.
given in a separate paper.

1
6
V. Vashook et al. / Journal of Alloys and Compounds 354 (2003) 13–23
Table 1
Lattice parameters, positional and displacement parameters of atoms in La12xCa TiO structures
x
3
]
Atom
Parameters
R3c
Pbnm structure
structure,
x50.6
x50.7
x50.8
x50.9
x51
˚
a, A
5.4600(5)
5.4434(3)
5.4619(3)
7.7091(4)
229.20(4)
5.4302(3)
5.4499(3)
7.6911(5)
227.61(4)
5.40376(9)
5.45264(9)
7.6679(1)
225.93(1)
5.38052(7)
5.44251(6)
7.64096(9)
223.754(8)
˚
b, A
˚
c, A
13.341(2)
344.4(1)
3
˚
V, A
La/Ca
x
y
z
0
0
20.002(3)
0.0184(8)
1/4
20.003(4)
0.0201(9)
1/4
20.0058(7)
0.0317(3)
1/4
20.0064(5)
0.0348(2)
1/4
1/4
1.6(3)
B(eq)
1.1(3)
0.85(7)
0.82(7)
1.12(4)
Ti
x
0
1/2
1/2
1/2
1/2
y
0
0
0
0
0
z
0
0
0
0
0
B(eq)
0.7(3)
0.8(2)
0.7(2)
0.57(5)
0.86(4)
O1
x
y
z
0.556(2)
0
0.25
0.055(2)
0.493(2)
1/4
0.068(2)
0.490(2)
1/4
0.0720(10)
0.4868(9)
1/4
0.0719(8)
0.4856(7)
1/4
B(eq)
2.9(9)
1.2(5)
1.1(5)
1.1(2)
1.47(11)
O2
x
y
z
0.216(2)
0.218(2)
0.5333(10)
1.0(4)
0.221(2)
0.219(2)
0.5329(10)
1.1(4)
0.2137(8)
0.2117(8)
0.5365(5)
1.05(15)
0.2114(6)
0.2117(6)
0.5366(4)
1.36(7)
B(eq)
RI
RP
0.0783
0.2275
0.0680
0.1369
0.0683
0.1531
0.0449
0.0996
0.0576
0.1001
Fig. 2. Calculated (line) and observed (dots) XRD patterns and the difference spectrum for La0.1Ca0.9TiO3.

V. Vashook et al. / Journal of Alloys and Compounds 354 (2003) 13–23
17
Fig. 3. Site occupation of the elementary unit cell (a), lattice parameters (b) and average cation–oxygen and cation–cation distances in La12xCa TiO
x
32d
versus Ca content. Orthorhombic and rhombohedral lattice parameters were transformed into a perovskite-like cell according to the following relations:
a 5a /œ2, b 5b /œ2, c 5c /2; a 5b 5a /œ2, c 5c /œ12.
p
o
p
o
p
o
p
p
rh
p
rh
3
.2. Oxygen content
Two types of powders of each series were used in the
experiments. I and (32d ) correspond to the powders
1
1
Typical changes of titration current of the coulometric
cell and calculated oxygen content of the powders of both
series La12xCa TiO (La0.1Ca0.9TiO32d and
La2(12x) / 3Ca TiO (La0.13Ca0.8TiO32d ) during heating/
pre-treated in air at the temperature program specified
below (type 1), whereas I and (32d ) relate to powders
2
2
)
after synthesis at 1350 8C (type 2). During heating the
powders of type 1 the oxygen desorption started at 500 8C.
For equilibration at 1000 8C more than 1 h of exposure was
required. At shorter times the oxygen desorption continues.
Cooling was accompanied by a small increase of oxygen
x
32d
x
3
cooling in Ar/H /H O gas flow (pH O/pH 50.53) are
2
2
2
2
presented in Fig. 6a and b. Other single-phase powders of
each series show a similar behaviour.

1
8
V. Vashook et al. / Journal of Alloys and Compounds 354 (2003) 13–23
Fig. 4. Ca-content dependency of the observed tolerance factor, tobs, for
the series La12xCa TiO32d. Values of tobs were calculated according to
x
relation: tobs 5(RO) /œ2?(TiO) , where (RO) and (TiO) —average
9
6
9
6
R–O and Ti–O distances for CN 9 and 6, respectively.
content. Type 2 powders of both series show increasing
oxygen content in the temperature range 300 to 900 8C
during heating. Only a small oxygen desorption maximum
(
titration current minimum) was observed near 1000 8C.
After 1 h of exposure at 1000 8C, the powder samples did
not reach the equilibrium oxygen contents. Increasing
oxygen content during cooling was observed. After cooling
from 1000 8C in Ar/H /H O the samples show oxygen
2
2
content comparable with powders cooled in air from
1
350 8C.
For calculation of the oxygen content, the initial oxygen
indexes of the type 1 powders were assumed to be 3.0 and
Fig. 6. Oxygen desorption/sorption characteristics of the powder samples
La0.2Ca0.8TiO32d (a) and La0.13Ca0.8TiO32d (b) during heating–exposi-
tion–cooling in the Ar/H /H O gas flow with pH O/pH 50.53: I and
2
2
2
2
1
I2—titration currents (32d )1 and (32d ) —oxygen indexes of the pre-
2
treated in air and non pre-treated powder samples, respectively.
oxygen indexes of both powders after treatment were
suggested to be equal to each other. The oxygen contents
of La12xCa TiO
achieved at 1000 8C in Ar/H /H O ( pO 58.08?10
and La2(12x) / 3Ca TiO
compounds
32d
211
2
x
32d
x
2
2
Pa) are presented in Table 2. CaTiO did not change its
3
oxygen content in the whole temperature range from 20 to
1
1
000 8C in this gas. The highest oxygen deficiency at
000 8C, of about 0.011, were observed for the compound
with x50.9.
3
.3. Electrical conductivity
The electrical conductivities of La12xCa TiO
in air
32d
x
Fig. 5. Thermal behaviour of the lattice parameters of the La0.4Ca0.4TiO3
structure. The orthorhombic (Imma) and tetragonal (I4/mcm) lattice
parameters are transformed into perovskite-like cell parameters according
to the following relations: a 5a /2, b 5b /œ2, c 5c /œ2; a 5b 5
and in Ar/H /H O with pH O/pH 50.01 versus tem-
2
2
2
2
perature are given in Fig. 7. The dependencies of con-
ductivity on Ca content at 900 8C are shown in Fig. 8.
p
o
p
o
p
o
p
p
a /œ2, c 5c /2.
Maximum conductivities of about 50 S/cm in Ar/H gas
t
p
t
2

V. Vashook et al. / Journal of Alloys and Compounds 354 (2003) 13–23
19
Table 2
Oxygen stoichiometry achieved in powder, after 1 h exposition at 1000 8C in gas flow Ar/H O/H with pO 58.08?10
2
11
Pa by assumption that initial
2
2
2
powders were characterised by oxygen index 3.0
4
1
31
41
31
41
31
NN
Composition
32d
Ti
Ti
[Ti ]*[Ti ]/([Ti ]1[Ti ])
La12xCa TiO
x
32d
1
2
3
4
La0.3Ca0.7TiO32d
La0.2Ca0.8TiO32d
La0.1Ca0.9TiO32d
CaTiO3
2.991
2.990
2.989
2.999
0.682
0.78
0.878
1
0.318
0.22
0.122
0
0.21688
0.1716
0.10712
0
La2(12x) / 3Ca TiO
x
3
5
6
7
La0.6Ca0.1TiO32d
La0.4Ca0.4TiO32d
La0.13Ca0.8TiO32d
2.998
2.997
2.995
0.996
0.994
1
0.004
0.006
0
0.003984
0.005964
0
Fig. 9. Electrical conductivity of some La12xCa TiO
ceramic samples
32d
x
versus pO2 at 900 8C.
Fig. 7. Electrical conductivity versus temperature for some compounds in
the system La12xCa TiO32d.
x
flow and of about 0.002 S/cm in air were observed at
x50.9.
Fig.
La12xCa TiO
pO2 range from 10
9
demonstrates the n-type conductivity of
samples with x50.6 to 1.0 in the whole
x
32d
2
13
to 21000 Pa. The slope (k) of
conductivity versus log pO dependence is approximately
2
2
1/4 for all compounds, except CaTiO32d for which
k521/6.
Compounds of the series La2(12x) / 3Ca TiO with x5
x
3
0
.1, 0.4, and 0.8 have n-type conductivity, characterized by
a slope of k521/4 versus pO (Fig. 10). The conduc-
2
tivities of samples of this series are lower than those of the
series La12xCa TiO
(Fig. 9).
x
32d
Equilibrium conductivity values of the samples were
achieved only after some days of exposure at high tem-
peratures in gas flows with constant pO . As an example,
2
the conductivity change of La0 Ca0.9TiO32d after a pO2
.1
2
13
jump from air to pO 51.9?10
Pa is shown in Fig. 11.
2
La2(12x) / 3Ca TiO samples were gas dense and show
lower equilibration kinetics in comparison with the porous
Fig. 8. Electrical conductivity of the ceramic samples in the system
x
3
La12xCa TiO at 1000 8C in air and in Ar/H /H O gas flow with
x
3
2
2
pH O/pH 50.01 versus Ca content.
samples of La12xCa TiO32d. Because of the very slow
2
2
x

2
0
V. Vashook et al. / Journal of Alloys and Compounds 354 (2003) 13–23
Fig. 12. Linear thermal expansions of the ceramic samples
La0.4Ca0.4TiO32d and YSZ in air.
Fig. 10. Electrical conductivity of some La2(12x) / 3Ca TiO
ceramics
32d
x
versus pO2 at 900 8C.
of the thermal expansion coefficient, obtained by di-
latometric method (Fig. 12).
Fig. 12 represents the linear thermal expansion of the
La0.4Ca0.4TiO32d ceramic sample measured in air by
dilatometry and XRD in comparison with YSZ ceramics.
The anomalies in expansion behaviour observed by the
equilibration the conductivity values of some compounds
presented in Figs. 9 and 10 were collected at certain pO2
values only.
dilatometric method at 800–900 K [curve d(DL/L )/dT]
3
.4. Thermal expansion
0
correspond to changing of slope of a-parameter and cell
volume. The relative average linear thermal expansion
calculated from XRD data shows changing slopes near 800
Thermal expansions of La0.4Ca0.4TiO3 in a- and c-
directions are practically the same up to |900 K (Fig. 5).
A slope change of the a-parameter is observed at tempera-
tures higher than 900 K. The difference between a- and
c-parameters at temperatures higher than 900 K decreases
and at 1170 K they become equal indicating a transition to
the cubic structure. Changes of the slope of the a-parame-
ter and cell volume correspond to the observed anomalies
K for La0.4Ca0.4TiO . La0.6Ca TiO shows near linear
3
0.1
3
expansion in the whole temperature range investigated
Fig. 13). Shifts to lower temperature region of dilatomet-
(
ric curves relatively XRD curves are observed (Figs. 12
and 13). The thermal expansions of La0.4Ca0.4TiO32d and
Fig. 13. Relative average linear thermal expansion of La0.4Ca0.4TiO3 and
La0.6Ca0.1TiO3 in the temperature range 300–1170 K measured by XRD
and dilatometry.
Fig. 11. Time dependence of the conductivity of La0.1Ca0.9TiO32d at
4
213
9
00 8C after jump of pO2 from 2.1?10 to 1.9?10
Pa.

V. Vashook et al. / Journal of Alloys and Compounds 354 (2003) 13–23
21
and Pb [21], Li [22–24], Al [25–27], Nb and Na [28,29] in
the corresponding doped titanates.
4
.2. Point defects and transport properties
Lanthanum–calcium titanates prepared in air seem to be
characterised by a constant oxygen index of 3.0 that is
independent of their cationic compositions as it follows
from the crystal structure analysis (Figs. 3–5). Electrical
neutrality of the compounds can be achieved by the
changing oxidation states of titanium, or by formation of
oxygen or cation vacancies. The oxidation states of the
titanium cations under oxidizing conditions is assumed to
be fixed as 41, whereas under reducing conditions Ti
cations can change their oxidation states between 31 and
41.
Fig. 14. Propene conversion for two compounds of the systems
La12xCa TiO and La2(12x) / 3Ca TiO32d.
?
??
x
32d
x
As the predominant point defects La , Ti
9
and V are
Ca
Ti
O
taken into consideration for La12xCa TiO
and
x
32d
La2(12x) / 3Ca TiO , analogous with La Ca Cr0.2Ti0.8O3
x
3
12x
x
(
x,0.8) [4] and La12xCa Cr0.5Ti0.5O (x,0.5) [6]. The
x 3
YSZ samples measured by dilatometric method coincide
almost fully.
n-type conduction of the compounds can be explained by
41
defect creation, associated with the reduction of Ti
3
1
3
Ti
3
O
cations to Ti by removal of oxygen 2Ti 1O ⇔ 2Ti 1
9
Ti
?
?
V 10.5O . Small changes of the oxygen content at
O
2
3
.5. Catalytic activity
relatively high oxidizing conditions lead to the relations
?
O
?
?
Ca
?
Ca
[
V ],,[La ] and [Ti
9
]5[La ]. That means, the con-
Ti
Catalytic activities of compounds of both series
centration of electronic carriers is determined by the
concentration of lanthanum cations on calcium sites, but
does not depend on oxygen partial pressure. For
La0.2Ca0.8TiO32d (Fig. 9) and for La2(12x) / 3Ca TiO (Fig.
La12xCa TiO
and La2(12x) / 3Ca TiO were character-
x
32d
x 3
ized as extremely low, being comparable with the activity
of the empty quartz reactor (Fig. 14).
x
3
1
0) no deviation from the linear dependence of con-
ductivity versus oxygen partial pressure with slope 21/4
was observed. That means, the conductivity region in-
dependent of pO is very narrow, or exists at higher pO .
4
. Discussion of the results
2
2
4
.1. Crystal structure
The slope log(s)/log( pO ) 21/4 in the whole pO region
2 2
213
from 10
La12xCa TiO
to 21000 Pa experimentally observed for
and La2(12x) / 3Ca TiO points at the
From the analyses of the concentration dependencies of
x
32d
x
3
interatomic distances (Fig. 3c) it follows that the oxidation
state of titanium in La12xCa TiO compounds is 41
validity
of
( pO2)
the
.
relation
[e9]5[Ti ]5k1?
9
Ti
?
?
21 / 2
21 / 4
[V ]
*
x
32d
O
and the corresponding perovskite phases except CaTiO3
are A-site deficient. The average Ti–O distances remain
constant in the Ca concentration range 0.7#x#1 (Fig. 3c),
indicating a constant oxidation state of the titanium cations
by partial substitution of Ca for La . Refinement of the
site occupations confirmed the A-site deficiency of the
For La12xCa TiO
and La2(12x) / 3Ca TiO , the
x 32d
x
3
changes of the oxygen contents during treatment up to
1000 8C in a reducing atmosphere are comparable with that
during treatment in air up to 1350 8C (Section 3.2).
Increasing oxygen content of powders of the type 2 during
heating in the temperature range from 300 to 900 8C (Fig.
6) could be understood by assuming a very low diffusion
mobility of oxygen. The sub-stoichiometry of oxygen
adjusted after solid-state synthesis at 1350 8C in air does
not relax to 3.0 during cooling of the samples in air with a
rate of 3 to 5 8C/min, because of the relatively large grains
formed at 1350 8C. The ground small grain powders pre-
treated in air at 1000 8C and cooled to room temperature
adjust to an oxygen content near 3.0. No grain growth
takes place at 1000 8C and therefore oxygen absorption
during cooling in air is not limited by oxygen diffusion.
2
1
31
La12xCa TiO
compounds (Fig. 3a). Substitution of La
x
32d
for Ca in La12xCa TiO
structure led to a decreasing
32d
x
deformation of the perovskite structure as illustrated by the
increasing tolerance factors (Fig. 4). As a result, the
orthorhombic GdFeO -type structure of the compounds
3
La12xCa TiO
with 0.7#x#1 transforms into a rhom-
x
32d
bohedral one for La0.4Ca0.6TiO3.
The perovskite structure
La2(12x) / 3Ca TiO is stabilized by small amounts of
of
the
samples
x
3
calcium as it was found before for small additions of Sr

2
2
V. Vashook et al. / Journal of Alloys and Compounds 354 (2003) 13–23
For
La12xCa TiO32d,
in
contrast
to
prepared from simple oxides in air conditions at tempera-
tures of maximal 1350 8C. The more Ca reach composi-
tions were heterogeneous. The single-phase compounds
La12xCa TiO crystallised in an orthorhombic structure
x
La12xCa Cr0.8Ti0.2O32d [5], La12xCa Cr0.5Ti0.5O32d [6],
and La12xCa Cr0.2Ti0.8O32d [4] no clear correlation be-
x
x
x
tween oxygen content achieved at 1000 8C under reducing
conditions and Ca content was found. The non-equilibrium
oxygen contents adjusted at 1000 8C under reducing con-
ditions for titanates after 1 h of exposure explains that
phenomenon (Table 2).
x
3
(Pbnm). The crystal structure, oxygen stoichiometry, and
electrical conductivity investigations consider the existence
of A-site deficiencies in the La2(12x) / 3Ca TiO and
x
3
La12xCa TiO
900 8C in the whole pO region between air and 10
compounds. An n-type conductivity at
x
32d
215
The electrical conductivities of La12xCa TiO
and
Pa
x
32d
2
La2(12x) / 3Ca TiO compounds under reducing and oxi-
was registered for all compounds. The highest electrical
conductivity, about 50 S/cm at 1000 8C and pO 55?10
2
x
3
2
14
dizing conditions differ minimally (Figs. 9 and 10),
probably caused by identical electron transport mecha-
nisms. Varying A-site compositions influence the oxygen
Pa, was found for La0.1Ca0.8TiO . The relatively low
3
catalytic activity in comparison with chromites-titanates
points to importance of nature of B-site cations for
catalytic efficiency in the perovskite type compounds
investigated.
2
11
content at 1000 8C and pO 58?10
Pa only weakly
2
(
Table 2, Fig. 6). The oxygen content correlates with the
4
1
31
concentrations of Ti and Ti cations, which influence
the conductivity level. The closer this ratio to 1, the higher
the conductivity.
The negative temperature coefficient of the electrical
conductivity observed for La0.2Ca0.8TiO32d in Ar/H2 /
Acknowledgements
H O (Fig. 7) can be explained by assuming a metallic type
of conductivity or by decreasing oxygen content during
2
The authors gratefully acknowledge the financial support
of Deutsche Forschungsgemeinschaft (project GU 484/1-
3) and of WTZ (project UKR-01/12).
cooling. The equilibrium of the exothermic reaction 2H 1
2
O ←→ H O 2DH is shifted to the right at decreasing
2
2
temperature which decreases the oxygen concentration in
the gas flow.
The coincidence of the thermal expansions of
La0.4Ca0.4TiO32d and YSZ in air within the temperature
range 300 to 1200 K is typical of all others compounds of
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