Structural characterization and electrical properties of NiNb2-xTaxO6 (0≤x≤2) and some Ti-substituted derivatives

Article abstract of DOI:10.1016/j.jssc.2009.04.038

A structural and electrical characterization of the system NiNb_2-xTa_xO₆ (0≤x≤2) is presented. For x≤0.25 materials with the columbite-type structure typical of NiNb₂O₆ have been obtained whereas for x

Full text of DOI:10.1016/j.jssc.2009.04.038

ARTICLE IN PRESS
Journal of Solid State Chemistry 182 (2009) 1944–1949
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Structural characterization and electrical properties of NiNb2 Ta O
ꢀx x 6
(
0rxr2) and some Ti-substituted derivatives
Ã
M. L o´ pez-Blanco, U. Amador, F. Garc ı´ a-Alvarado
Departamento de Qu ´ı mica, Universidad San Pablo-CEU, Urbanizaci o´ n Montepr ´ı ncipe, 28668 Boadilla del Monte, Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
A structural and electrical characterization of the system NiNb2 Ta O (0rxr2) is presented. For
ꢀx
x 6
Received 2 January 2009
Received in revised form
xr0.25 materials with the columbite-type structure typical of NiNb O have been obtained whereas for
2
6
xZ1 tri-rutile-like oxides were obtained. The electrical properties are similar in both cases; they are
semiconducting with very low electrical conductivity and very high activation energy, though slight
differences were found as a function of Ta content. Improvement of conductivity by reducing the
stoichiometric materials could not be achieved due to decomposition. In this connection, partial
substitution of Nb or Ta by Ti has been carried out in order to create oxygen vacancies. Tantalum was
partially replaced by Ti to a significant extent in the tri-rutile structure inducing a slight increasing of
conductivity. However, for the columbite case neither Nb nor Ta could be partially replaced. This
2
April 2009
Accepted 28 April 2009
Available online 13 May 2009
Keywords:
Columbite
Rutile
Electrical properties
Structural characterization
behavior is quite different from that reported for other similar columbites such as MnNb
exhibits high electrical conductivity upon substitution of niobium by titanium.
2 6ꢀd
O , which
&
2009 Elsevier Inc. All rights reserved.
1.
Introduction
AB (columbite/tri-rutile) type of oxides where B is either Nb
case of the columbite MnNb
quantity of oxygen vacancies that can be created is small (
As a novelty it was also found that MnNb is affected by an
2
O
6ꢀ
d
[10], it was found that the
dꢁ0.02).
2
O
6
2 6
O
or Ta are very interesting materials that have been investigated in
view of their dielectric properties and uses in a wide range of
related applications [1–5]. Besides, some of them have been
proposed as water splitting photocatalyst [6–8] as it is the case of
intrinsic instability and instead of the nominal composition two
close phases with either lower or higher Mn contents may
6
stabilize. The formation of the latter, Mn1.1Nb1.9O , is possible
because to the oxidation of Mn(II) to Mn(III). Interestingly, when
Nb(V) is partially replaced by Ti(IV) this charge-compensating
mechanism also operates [11]. The presence of trivalent manga-
nese is responsible of the p-type electronic conductivity observed
NiM
2
O
6
(M ¼ Nb, Ta). When M ¼ Nb the oxide crystallizes in the
columbite structure whereas for M ¼ Ta a tri-rutile-like material
is formed. To our best knowledge, the solubility range of both
phases is not known; therefore, we have first investigated the
2 2
at ambient pO . However, at much lower pO , Mn(III) is reduced
phases present in the system NiNb
columbite and tri-rutile structures are closely related [9] some
degree of solubility could be expected. One of the common
2
O
6
–NiTa
2
O
6
.
Since the
and electrical conductivity decreases; further reduction affects
Nb(V) or Ti(IV) and n-type conductivity is then observed. Oxygen
ion conductivity in this material is low since in the best case a
ꢀ
5
ꢀ1
ꢀ1
features in both structures is that MO
6
octahedrons share edges
value of 6.0 ꢂ10
In the present paper we present the behavior of another
member of the columbite family, NiNb , for which oxidation of
the divalent metal, Ni(II), at ambient pO is not likely to occur. On
the other hand, we have replaced Nb by Ta to study the series
NiNb2ꢀxTa for which the limit members are the columbite
NiNb [12,13] and the tri-rutile NiTa [14], similarly to what
occurs for FeNb2ꢀxTa [15]. We aimed to analyze the effects of
O
cm at 9001C was found.
which makes more difficult the formation of oxygen vacancies
when compared to corner-sharing based structures (the perovs-
kite for instance). However, we have recently shown that a Nb-
2 6
O
2
based columbite, MnNb
or substitution in the Nb site, to yield good electronic conductors
10,11]. Even more, oxygen ion conductivity has been detected.
2 6
O , can be modified by either reduction
x 6
O
[
2
O
6
2 6
O
These results on columbites are interesting because oxides
presenting both electronic and oxygen ion conductivity may be
useful as highly efficient electrodes in solid oxide fuel cells. In the
x 6
O
aliovalent substitution in the pentavalent site of columbite and
tri-rutile looking for an improvement of electrical conductivity. In
4
+
5+
5+
particular, we have replaced Ti by either Nb or Ta , similarly
to what was made in the case of MnNb for which an increasing
2 6
O
Ã
of electrical conductivity by several orders of magnitude was
achieved [11].
Corresponding author.
E-mail address: flaga@ceu.es (F. Garc ı´ a-Alvarado).
0
022-4596/$ - see front matter & 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2009.04.038

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1945
2.
Experimental
11000
The synthesis of oxides with nominal composition NiNb2ꢀx
Ta
stoichiometric mixtures of Nb
Ni(NO O (20.43% in Ni). First, the blends were heated at
ꢂ H
00 1C for 12 h to fully decompose the nitrate; afterwards, they
x
O
6
(0rxr2) was carried out at high temperature by reaction of
9000
7000
5000
3000
2
O
5
(99.99%), Ta (99.99%) and
2 5
O
3
)
2
2
b
8
were ground, pelletized and heated up again at temperatures
ranging from 1250 1C (for x ¼ 0) to 1300 1C (for x ¼ 1). Pellets were
slowly cooled down to 800 1C and finally quenched in air from
that temperature. The same procedure was followed to obtain the
o
a
c
Ti-substituted derivatives: NiNb2ꢀyTi
Ti
(y ¼ 0.1, 0.15) and NiNb0.25Ta1.75ꢀyTi
Structural characterization of the samples was performed by
y
O
6
and NiNb1.75Ta0.25ꢀy
1
000
y
O
6
y
O
6
(y ¼ 0.15 and 0.30).
-
-
1000
3000
powder X-ray diffraction (XRD) on a Bruker D8 high-resolution
˚
diffractometer using
radiation obtained with a germanium primary monochromator;
diffracted beams were recorded using position-sensitive
1
a monochromatic CuKa (l ¼ 1.5406 A)
10
20
30
40
50
60
70
80
90
a
2θ (°)
detector (PSD) MBraun PSD-50 M. Diffraction patterns were
analyzed by the Rietveld method using the program FullProf [16].
Energy dispersive spectroscopy (EDS) has been carried out on a
Scanning Electron Microscope JEOL JSM 6400 equipped with an
OXFORD INCA EDS detector. Several spectra were recorded in
order to analyze the homogeneity of the samples and the eventual
presence of different types of particles. This equipment was also
used to analyze the morphology on as-prepared pellets obtained
by the sintering process described below.
Magnetic susceptibility measurements were made using a
SQUID magnetometer (Quantum Design, model MPMS-XL). The
temperature dependence of magnetization (M) was measured in
the temperature range 2–300 K at an applied magnetic field (H) of
9
7
5
3
1
000
b
a
000
000
000
000
c
0
.1 T upon heating samples under zero-field-cooled conditions
from 2 K (previously cooled at H ¼ 0 T). The experimental molar
magnetic susceptibility exp (M/H) was calculated on the basis of
the sample mass and the molecular weight. The molar para-
magnetic susceptibility was then obtained by subtraction of
-1000
3000
w
-
1
0
18
26
34
42
50
2θ (°)
58
66
74
82 90
w
m
the diamagnetic contribution from each ion.
Electrical characterization has been performed by impedance
spectroscopy (IS) using a FRA Solartron 1260 coupled with a 1290
Dielectric Interface in the 1 MHz–0.1 Hz range in the temperature
interval 25–800 1C. Measurements were carried out on pellets
whose polished faces were coated with platinum which acts as
electrodes.
To get dense pellets, powdery samples were mixed with
polyvinyl alcohol, as a binder, compacted into pellets and heated
up to 800 1C very slowly (2 1C/min) to decompose the binder.
Afterwards, pellets were fired at the synthesis temperature
1
1000
9
7
5
3
1
000
000
000
000
000
(1250–1300 1C) or slightly above it and slowly cooled down to
8
00 1C before being quenched in air. The density of pellets was
measured by the Archimedes method. Relative densities were find
to be the same (approximately 88%) for the whole composition
-
-
1000
3000
range explored, NiNb2ꢀxTa
x
O
6
(0rxr2). On the other hand
sintered Ti-substituted samples were found to have relative
densities higher than 95%.
10
20
30
40
50
2θ (°)
60
70
80
90
Fig. 1. Experimental X-ray diffraction patterns and calculated (profile matching) of
a) NiNb1.75Ta0.25 (columbite), (b) NiNb0.25Ta1.75 (tri-rutile) and (c) NiNb0.25
6ꢀ
Ta1.45Ti0.30 (tri-rutile). The insets show schematic representations of colum-
(
O
6
O
6
3. Results and discussion
O
d
bite (a) and tri-rutile (b).
Samples NiNb2ꢀxTa
.75 and 2 have been prepared. The columbite structure has a
narrow existence range since only the samples with x ¼ 0 and
.25 were found to be single-phase exhibiting this structure.
Fig. 1a shows the XRD pattern for NiNb1.75Ta0.25 ; all the
diffraction peaks can be interpreted with the cell and symmetry
of columbite [13]. On the other hand, single-phase samples with
tri-rutile structure were found for x ¼ 1, 1.5, 1.75 and 2. As an
x
O
6
with x ¼ 0, 0.25, 0.5, 0.75, 1, 1.25, 1.5,
1
6
example, Fig. 1b shows the XRD pattern for NiNb0.25Ta1.75O ; all
0
the Bragg peaks can be assigned to cells similar to that one of the
tri-rutile NiTa
O
6
2
O
6
[14]. On the other hand, for x ¼ 0.75 tri-rutile is
the main phase but an unidentified second phase is present. This
suggests that, in the phase-diagram, the stability region of tri-
rutile starts somewhere between x ¼ 0.75 and 1. Cell volume per

ARTICLE IN PRESS
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M. L o´ pez-Blanco et al. / Journal of Solid State Chemistry 182 (2009) 1944–1949
102
101
100
(
)*
NiNb1.75Ta0.25O6
tri-rutile
9
9
8
columbite
9
0
0.5
1
1.5
2
x in NiNb2-xTa O6
x
Fig. 2. Cell volume per formula unit as
a
function of tantalum content.
(
Z
columbite ¼ 4 f.u./unit cell, Ztri-rutile ¼ 2 f.u./unit cell). The asterisk indicates the
presence of a minor second phase for this composition. Dashed vertical line
between x ¼ 0.75 and 1 indicates that somewhere between these two composi-
tions the tri-rutile single-phase region starts.
60μm
Electron image1
NiNb0.25Ta1.75O6
Table 1
6
Lattice parameters for tri-rutile NiNb0.25Ta1.75O and some Ti-substituted samples.
Nominal composition
a ( A˚ )
c ( A˚ )
v ( A˚ 3
)
NiNb0.25Ta1.75
NiNb0.25Ta1.6Ti0.15
NiNb0.25Ta1.45Ti0.3
O
6
4.7184(1)
4.7127(1)
4.7055(5)
9.1205(1)
9.1163(3)
9.1091(5)
203.05(1)
202.47(1)
201.69(3)
O
5.925
5.85
O
formula unit for each composition is graphically presented in
Fig. 2; the variation for each phase is within the experimental
error, indicating that the cell-volume does not change when Nb/Ta
ratio does. This agrees with the fact that the ionic radii of both
˚
Nb(V) and Ta(V) are the same (0.64 A in octahedral coordination
17]).
Regarding the Ti-substituted derivatives, the tri-rutile
accepts the partial replacement of Ta by Ti;
[
y 6
NiNb0.25Ta1.75ꢀyTi O
both y ¼ 0.15 and 0.30 samples have been found to be single-
phase as it is shown for example in Fig. 1c. Beyond y ¼ 0.3
secondary phases have detected. Lattice parameters decrease as Ti
replaces Ta, as it can be seen in Table 1. This variation is expected
taking into account the ionic radii of Ta(V) and Ti(IV) (0.64 and
Electron image1
6
0μm
NiNb0.25Ta1.6Ti0.15O6
˚
0
.605 A, respectively, in octahedral coordination [17]).
Interestingly, it has not been possible to replace either Nb or Ta
by Ti in the columbites NiNb and NiNb1.75Ta0.25 even though
synthesis of samples with small amount of Ti (yo0.15) were also
attempted. Therefore, for NiNb a behavior different from the
one observed in the case MnNb [11] has been found. A
2
O
6
O
6
2 6
O
2 6
O
tentative explanation of this difference is given below.
Fig. 3 shows representative scanning micrographs of the faces
of sintered pellets with nominal composition NiNb2ꢀxTa
x ¼ 0.25 (columbite), x ¼ 1.75 (tri-rutile) and NiNb0.25Ta1.60
(tri-rutile). A significant difference can be seen in
x 6
O
Ti0.15O
6
particle size. In the former (Fig. 3a) particle size is larger than
for the tantalum-rich tri-rutile (Fig. 3b). However, we have not
found any influence of particle size in the sintering process since
all samples sintered to 88% relative density. A better sintering of
the tri-rutiles has been obtained after partial substitution of Ta by
6ꢀd
Ti. For NiNb0.25Ta1.60Ti0.15O a relative density of 95% has been
6
0μm
reached. The corresponding SEM image (Fig. 3c) shows a typical
image of high dense ceramics. On the other hand, the metallic
ratios determined by EDS in all the samples are in agreement with
the corresponding nominal compositions. For example, for
Fig. 3. SEM images of pellet face of (a) NiNb1.75Ta0.25
O
6
sintered at 1250 1C, (b)
sintered at
NiNb0.25Ta1.75O6 sintered at 1300 1C and (c) NiNb0.25Ta1.6Ti0.15
O
6ꢀ
d
1300 1C.

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M. L o´ pez-Blanco et al. / Journal of Solid State Chemistry 182 (2009) 1944–1949
1947
ꢀ
12
ꢀ9
ꢀ1
nominal NiNb1.75Ta0.25
O
6
,
the formula estimated by EDS is
2.3 ꢂ10
and 7.8 ꢂ 10 F cm , respectively. Consequently, the
NiNb1.72(2)Ta0.28(1) assuming an oxygen content close to the
O
6
high frequency semicircle is assigned to the response of the bulk
resistance whereas the low frequency semicircle is assigned to the
grain boundary response. The corresponding values obtained for
ideal stoichiometry.
Fig. 4a shows a typical Nyquist plot depicting the electrical
response of NiNb2ꢀxTa with x ¼ 0 in the temperature range
50–840 1C. For x ¼ 0.25 the same characteristics are found in the
x
O
6
NiNb2ꢀxTa
x
O
6
with x ¼ 0.25 are similar. Fig. 5a shows the
5
Arrhenius behavior of the bulk conductivity for both samples,
corresponding plot. The spectra were fitted to equivalent circuits
comprising a resistor in parallel with a constant phase element
that are in series with another similar parallel combination. At
lower temperature only the high frequency semicircle (see Fig. 4b)
is detected. Besides, it can be seen that the real part of impedance
is zero at the highest frequency reached in the experiment. Above
x ¼ 0 and 0.25. It can be seen that increasing the Ta content in the
2 6 2 6
columbite side of the system NiNb O –NiTa O has a slight
deleterious effect on the bulk conductivity. For x ¼ 0.25
ꢀ5
ꢀ1
ꢀ1 ꢀ1
conductivity is as low as 10
O
cm
K
at 8001C; besides,
the activation energies are very high (ca. 1.3 eV). On the other
hand, grain boundaries seem to be more sensitive to Ta
substitution in the columbite-like materials as it can be seen in
Fig. 5b.
Low conductivities and high activation energies are also found
for NiNb2ꢀxTa
8
40 1C only the low frequency semicircle is detected. In such cases
only total conductivity could be obtained from fitting. Constant
phase elements (CPE) were used instead of pure capacitor due to
the slight depression observed in the semicircles. In particular,
this is significant in the low frequency semicircle. The capacitance
values associated to the semicircles have been obtained from the
x
O
6
with x ¼ 1, 1.5, 1.75 and 2.0, all of them
exhibiting a tri-rutile-like structure. From the corresponding
Nyquist plots (see for example Fig. 6) capacitances and
resistances are extracted for the different observed phenomena.
Thus, high-frequency semicircles are assigned to the bulk
response in view of their capacitances (for example
(
1ꢀn)/n 1/n
relationship C ¼ R
Q
, where R is the resistance, Q is the
pseudo capacitance of the CPE and n indicates the degree of
deviation with respect to the value of the pure capacitor. As an
example, in the case of NiNb
values obtained for the high and the low frequency semicircles are
ꢀ
12
ꢀ1
2
O
6
the corresponding capacitance
3.12 ꢂ10
F cm
6
in the case of NiNb0.5Ta1.5O ). The low
frequency semicircles could be only reliably fitted in some cases
ꢀ
9
ꢀ1
for which capacities ca. 10 F cm were calculated. These values
suggest that this semicircle is due to grain boundaries response.
The Arrhenius behavior of the bulks are shown in Fig. 7 for two
selected samples (x ¼ 1.5 and 2.0). It can be seen that conductivity
is very low and activation energies are very high (greater than 1 eV
in all cases). On the other hand, poor information was obtained for
.5 105
1
1
7
3
NiNb2O6
577ºC/air
.1 105
.5 104
.7 104
0
-
-
-
-
1.5
bulk
-2
100KHz
2.5
-3
x=0
1.2
1
KHz
100Hz
x=0.25
1MHz
3.5
0
3.7 104 7.5 104 1.1 105 1.5 105
Z' /ohms
-4
NiNb2-xTa O6
x
4.5
0.9
106
1
1.1
1.3
1.4
4
3
2
1
-1
NiNb2O6
1000/T (K )
427ºC/air
106
-0.5
grain boundary
-1
106
106
0
-1.5
x=0
-2
10KHz
-2.5
x=0.25
1
1KHz
-3
1MHz
-
3.5
0
1 106
2 106
3 106
4 106
0
.9
1.1
1.2
1.3
1.4
-
1
Z' / ohms
1000/T (K )
00
0
Fig. 4. Impedance data presented in the Z –Z plane for pelletized NiNb
sintered at 1250 1C. Spectrum recorded at (a) 577 1C and (b) 427 1C.
2
O
6
Fig. 5. Arrhenius plot showing the temperature dependence of conductivity of
NiNb2ꢀxTa (x ¼ 0 and 0.25): (a) bulk and (b) grain boundary.
x 6
O

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M. L o´ pez-Blanco et al. / Journal of Solid State Chemistry 182 (2009) 1944–1949
104
-4.2
4
3
2
1
bulk
NiNb Ta1.5O6
0
.5
-4.4
-4.6
-4.8
7
46ºC/air
104
104
-5
104
1MHz
-5.2
0
0.5
1
1.5
2
1
KHz
x in NiNb2-xTa O6
x
1
00Hz
0
ꢀx
Fig. 8. Bulk conductivity of NiNb2 Ta O (0rxr2) as a function of tantalum
x 6
1 104
2 104
3 104 4 104
content.
0
Z' / ohms
0
0
0
some other phases. Another possible approach to improve the
electrical conductivity of NiNb2ꢀxTa may be to replace Nb(V)
by Ti(IV). Upon substitution two charge compensating mechan-
isms could operate: oxidation of Ni to Ni and loss of oxygen.
The former would produce holes and the latter oxygen vacancies.
Fig. 6. Impedance data presented in the Z –Z plane for pelletized NiNb0.5Ta1.5
O
6
sintered at 1300 1C. The spectrum shown was recorded at 746 1C.
x 6
O
2
+
3+
-
-
-
-
1.5
x 6ꢀd
Note that in the case of MnNb2ꢀxTi O both charge carriers
bulk
contribute to an increase of conductivity of several orders of
magnitude [11]. Therefore, the substitution of either Ta(V) or
Nb(V) by Ti(IV) have been tried in some members of the
-2
2.5
x 6
NiNb2ꢀxTa O series.
x=1.5
1.2
In the case of the columbite-like oxides (x ¼ 0 and 0.25)
substitution of Ta by Ti has not been achieved in the temperature
range attempted in the synthesis process (up to 1400 1C). A
-3
x=2.0
3.5
comparison to the case of MnNb2ꢀxTi
some light about the immiscibility of TiO
is replaced by Ti in NiNb both of the above mentioned charge-
x
O
6ꢀ
d
(xmax ¼ 0.02) sheds
2
and NiNb . When Nb
2 6
O
-4
NiNb2-xTa O6
x
2 6
O
compensating mechanisms might operate. However, oxidation of
Ni(II) to Ni(III) is not likely to occur as easy as oxidation of Mn(II)
4.5
0
.9
1
1.1
000/T (K )
1.3
1.4
-1
to Mn(III) (the case of MnNb
vacancies would be the only way to balance the loss of positive
charge. The example of MnNb gives again an idea of the
readiness of Nb-columbites to lose oxygen. The maximum value of
in MnNb has been found to be ca. 0.02 when prepared at
150 1C under Ar/H
2 6
O ). Thus, the formation of oxygen
1
2 6
O
Fig. 7. Arrhenius plot showing the temperature dependence of bulk conductivity
of NiNb2ꢀxTa (x ¼ 1.5 and 2).
x 6
O
d
2 6ꢀd
O
1
2
(95:5) [10]. Therefore, if inducing oxygen
is also difficult, none of the two possible
the dependence of grain boundary with temperature due to the
flat response obtained in most of cases.
Fig. 8 shows the variation of bulk conductivity with the Ta
content for the whole series. The observed trend is that the higher
the Ta content is, the lower the conductivity found, though it is
quite low in all cases. To explain this we have thought firstly of the
vacancies in NiNb
2
O
6
charge-compensating mechanisms is likely to favor substitution
of Nb(V) by Ti(IV) and hence the reaction does not take place.
In the case of tri-rutiles, the sample NiNb0.25Ta1.75
selected to replace Ta by Ti. Substitution took place up to a very
high extent: NiNb0.25Ta1.75ꢀyTi (ymax ¼ 0.3). Note that in this
case the structure seems to be more suited to accept oxygen
6
O was
y 6
O
origin of electrical conductivity in NiNb
2 6
O . Besides intrinsic
defects, the semiconducting behavior of the columbite NiNb
2
O
6
vacancies, as we have already found for reduced CrNbO
rutile-like structure.
To determine the actual charge-compensating mechanism
operating these tri-rutile-like oxides upon Ti-doping we looked
for an evidence of the oxidation state of nickel by analyzing the
4
[18] with
may be due to the slight reduction of Nb(V) at high temperature.
This may also explain the high activation energy observed. The
reduction process is expected to become more difficult if it is
Ta(V) the cation that has to be reduced, explaining the
dependence of conductivity with the Ta content (see Fig. 8).
In other related Nb-based columbites and rutiles conductivity
increases when Nb(V) ions are partially reduced to Nb(IV) ions as
magnetic behavior of both NiNb0.25Ta1.75
. The effective magnetic moment of NiNb0.25Ta1.75
temperature is 3.2 which is within the usual range for Ni [19]
if one takes into account that incomplete quenching of the orbital
6
O and NiNb0.25Ta1.45Ti0.3
O
6ꢀ
d
6
O at room
2+
m
B
2 6 4
it has been observed for MnNb O and CrNbO [10,18]. In these
2
+
cases, electronic conductivity changes by several orders of
magnitude because electrons are injected in the conduction band
upon reduction. Similarly, we tried to improve the electronic
momentum of Ni electrons is found very often in nickel oxides
[20,21]. On the other hand, the effective magnetic moment for
NiNb0.25Ta1.45Ti0.3O is only slightly higher, 3.4m , indicating
6ꢀd B
3+
conductivity of materials in the system NiNb2ꢀxTa
these oxides under 5% H /Ar at 1100 1C. However, they decom-
posed under this reducing atmosphere to form metallic Ni and
x
O
6
by reducing
that if high-spin Ni has formed upon substitution its amount is
very small. Therefore, the main charge compensating mechanism
proposed is the formation of anion vacancies.
2

ARTICLE IN PRESS
M. L o´ pez-Blanco et al. / Journal of Solid State Chemistry 182 (2009) 1944–1949
1949
inducing an increasing of conductivity of almost two orders of
magnitude. However, neither Nb nor Ta could be partially
replaced in the columbites samples. This behavior is different
from that reported for other similar columbites, such as
-
-
-
-
-
-
-
1
2
3
4
5
6
7
NiNb0.25Ta1.75-yTi O
y
6-δ
MnNb
substitution. In MnNb
2
O
6ꢀ
d
, that exhibited high electrical conductivity upon
oxidation of Mn(II) to Mn(III) provides
2 6
O
y=0
holes that improve conductivity by four orders of magnitude. On
the contrary, for the Ni-based columbites and tri-rutiles herein
investigated it seems that the main charge compensating
mechanism is the formation of oxygen vacancies. This operates
effectively in the Ti-substituted tri-rutiles, as for example
y=0.30
y=0.15
1.4
y 6ꢀd
NiNb0.25Ta1.75ꢀyTi O . The values of conductivity in these
materials suggest that the contribution to total electrical con-
ductivity of oxygen-vacancies in edge-sharing structures can be
noticeable. However, in the present case the contribution could
not be quantified as Ti also benefits the sintering process likely
reducing the grain boundary resistance significantly.
0
.8
1
1.2
1.6
1.8
-1
1
000/T (K )
Fig. 9. Arrhenius plot showing the temperature dependence of total conductivity
of NiNb0.25Ta1.75ꢀyTa (y ¼ 0, 0.15 and 0.30).
y 6ꢀd
O
Acknowledgments
Although substitution produces a beneficial effect on sinter-
ization, the electrical measurements did not allow a deep analysis
of bulk and grain boundary conductivities because the different
electrical responses could not be separated. Typical Nyquist (not
shown) present two overlapped semicircles that could not be
resolved. Therefore, only total conductivity for some selected
temperatures were determined. Nevertheless, the obtained results
deserve some comments. Fig. 9 shows the Arrhenius behavior of
We thank ‘‘Ministerio de Ciencia e Innovaci o´ n’’ and ‘‘Comuni-
dad de Madrid’’ for funding the projects MAT2007-64486-C07-01
and S-0505/PPQ/0358, respectively. Financial support from Uni-
versidad San Pablo CEU is also acknowledged. European Commis-
sion has partially funded a grant for M. L o´ pez-Blanco through the
ESF. Finally, we are also indebted to J. Romero (UCM) for magnetic
measurements.
NiNb0.25Ta1.75ꢀyTi
y
O
6ꢀd
with y ¼ 0, 0.15 and 0.30 and dꢁy/2; it can
be seen that conductivity increases upon substitution albeit it
remains low. For example, the conductivity at 800 1C for the
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ꢀ1
it increases up to 3 ꢂ10
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