T.V. Aksenova et al. / Journal of Alloys and Compounds 590 (2014) 474–478
477
copper (r
¼ 0:87=0:68 Å; CN ¼ 6) are larger than ionic radius
¼ 0:75=0:67 Å; CN ¼ 6Þ [16]. The anisotropy in
2þ=Cu3þ
of cobaltCuðrCo3þ
=Co4þ
the unit cell parameter changes with increasing of the dopant con-
tent is more obvious for the Cu-substituted samples. One can be per-
plexed why the substitution in the B site influences the values of a
and c parameters differently. The probable reason of such anisotropy
can be related with the peculiarity of oxygen vacancies location in
the structure of double perovskites LnBaCo2O5+d. It is well recognized
that oxygen vacancies are not randomly distributed in the lattice, but
located in the Ln–O planes [5]. As it will be shown below, the substi-
tution of Co ions by Ni or Cu leads to the increase of oxygen vacancies
concentration and as a result can strongly affect the distances be-
tween the layers along c-axis while the values of a parameter change
in a smaller extent. Similar variations of the unit cell parameters
while cobalt was substituted by nickel were also detected by Kim
and Manthiram [12]. Simultaneous influence of different although
linked parameters (such as, oxidation states of 3d-transition metals,
oxygen content in particular crystallographic planes) causes a nonlin-
ear change of the unit cell parameters with the dopant concentration
changes. However the substitution of Fe for Co in the rare earth and
barium cobaltates that leads to the increase of oxygen content also
yields same anisotropic increase of the
a and c parameters
[10,17,18]. Therefore the explanation of this anisotropy effect is more
complicated and not so obvious.
The results of TGA measurements for NdBaCo2ꢀxMxO5+d (M = Ni,
Cu) versus temperature in air are shown in Fig. 3. Solid lines repre-
sent the data obtained in dynamic regime (cooling rate 2 K/min),
points correspond to the values measured in static regime (isother-
mal dwells for 10 h). It can be seen that oxygen exchange between
the solid and gaseous phases has started at about 550 K. An excel-
lent agreement between the data, obtained in dynamic and static
regimes suggests that the oxygen exchange process and, hence,
equilibration between the solid oxides and gaseous phase is fast
enough. The substitution of nickel or copper for cobalt sites results
in a decrease of oxygen content. Such behavior can be explained by
the fact that nickel and copper are more electronegative elements
in comparison to cobalt (vCo = 1.88; vNi = 1.91; vCu = 2.00 in the
Pauling scale [19]) and therefore they act partially or completely
as the acceptors of electrons (Ni0Co and CuC0 o). The negatively
charged acceptor defects in the oxide are balanced by the corre-
sponding amount of positively charged oxygen vacancies (VꢃOꢃ),
and/or electron holes. Similar behavior was observed in the ordin-
ary perovskite system Ln1ꢀxMxCo1ꢀyNiyO3ꢀd (M = alkali earth met-
als) [20,21]. It is interesting to note that as expected the increase
of acceptor dopant (NiC0 o and CuC0 o) content yields to the decrease
of mean oxidation state of 3d-transition metals, however if we as-
sume that all acceptor impurities (Ni or Cu) possesses the oxida-
tion state equal to 2+, the values of oxidation state of the Co ions
remain practically (at least for the Cu containing phases) constant
(Table 2). In other words, the substitution of cobalt ions with oxi-
dation state larger than 2+ by the Cu2+ is accompanied with the re-
lease of oxygen from the lattice in the equivalent amount
according to the electroneutrality condition that allows maintain-
ing the oxidation state of cobalt at the constant value.
The dependencies of relative linear expansion for the NdBaCo2-
ꢀxMxO5+d (M = Ni, Cu with x = 0; 0.2; 0.4) versus temperature with-
in the range 298–1273 K in air, obtained during heating and
cooling sufficiently coincide with each other (Fig. 4). The average
values of thermal expansion coefficients (TECs), calculated from
experimental results are listed in Table 3. It can be seen that the
average values of TEC remains practically unchanged for the 0.2
nickel or copper substituted cobaltites, while further increase of
3d-transition metal content leads to the decrease of TEC values.
In order to check possible chemical interactions the
NdBaCo2ꢀxMxO5+d samples (M = Ni with x = 0; 0.1; 0.3; 0.5 and
M = Cu with x = 0.2; 0.8) were mixed with the solid electrolyte
Fig. 5. XRD pattern of the NdBaCo2ꢀxNixO5+d samples in contact with the
Ce0.8Sm0.2O2ꢀd fired at 1373 K in air (a) and NdBaCo2ꢀxCuxO5+d samples in contact
with the Zr0.85Y0.15O2ꢀd fired at 1173 K in air (b).
sisted of the two saturated solid solution: NdBaCo0.9Cu1.1O5+d and
Nd3Ba3(Cu1.9Co0.1)3O14ꢀd
.
Similar to undoped NdBaCo2O5+d, the crystal structure of both
solid solutions NdBaCo2ꢀxNixO5+d with 0.0 6 [ 6 0.5 and
NdBaCo2ꢀxCuxO5+d with 0.0 6 [ 6 1.1 can be described within the
tetragonal unit cell ap ꢁ ap ꢁ 2ap (sp. gr. P4/mmm). Fig. 1 illustrates
X-ray diffraction patterns for the NdBaCo1.6M0.4O5+d (M = Ni, Cu)
refined by the Rietveld analysis. The structural parameters for all
single phase samples NdBaCo2ꢀxMxO5+d are listed in Table 1. The
crystal structure of complex oxides Nd3Ba3Cu6O14ꢀd (x = 2.0) and
Nd3Ba3Cu5.7Co0.3O14ꢀd (x = 1.9) can be described within the tetrag-
onal unit cell ap ꢁ ap ꢁ 3ap (sp. gr. P4/mmm) with the unit cell
parameters a = 3.882(1) Å, c = 11.629(1) Å for Nd3Ba3Cu6O14ꢀd
and a = 3.895(1) Å, c = 11.640(2) (1) Å for Nd3Ba3Cu5.7Co0.3O14ꢀd
,
that agrees with the data reported in [14,15].
The dependencies of the unit cell parameters for a series of NdB-
aCo2ꢀxMxO5+d (M = Ni, Cu) solid solutions versus nickel and copper
content (x) are shown in Fig. 2. The substitution of nickel or copper
into the cobalt sublattice leads to a slight variation of the a parame-
ters and a gradual increase of the c parameters and the unit cell
volume. Such behavior can be explained by the size factor since
the ionic radius of nickel (rNi2þ
¼ 0:79=0:72 Å; CN ¼ 6) and
=Ni3þ
Aksenova
