ELECTRICAL CONDUCTION OF LANTHANUM METANIOBATE
A. P. Pivovarova,
V. I Strakhov,
V. P. Popov,
and P. A. Tikhonov
Translated from Ogneupory i Tekhnicheskaya Keramika, No. 2, pp.2–4,February, 2002.
The electrical conduction of LaNb
is studied in the temperature range of 100 – 1100°C in air. It is shown,
using a polarization method, that the electric transfer above 470°C is of electronic nature (DE = 1.40 eV), and
below 470°C — predominantly of anionic nature (DE = 0.65 eV). Electric conduction of the nonstoichio
metric lanthanum metaniobate involved the reduction of Nb
. Depending on the synthesis conditions,
can behave as an electronic, cationic, or anionic conductor, which provides a way for obtaining a
material with tailored properties.
As is well known, lanthanum metaniobate with the struc-
ture of a cation-deficient perovskite holds promise for use as
an electret; furthermore, it exhibits a high permittivity (about
200) at room temperature . The electric conduction of
has been studied in the temperature range of 100 –
1100°C in air . It was established that this compound has a
mixed electron-ionic conduction. In , the specimens stu-
died were preliminarily calcined at 330°C for 100 h in air so
as to keep the reduced Nb
as low as possible and to bring
the composition close to the stoichiometric LaNb
However, in actual service conditions where the material
is repeatedly subjected to high temperatures, one always has
to deal with a partially reduced specimen, that is, with an
anion-deficient lanthanum metaniobate LaNb
, where x
depends on temperature, gas medium, and heating time.
Therefore, our goal in this study was to consider the elec
tric behavior of LaNb
under different conditions of syn
thesis in the temperature range of 100 – 1100°C. The con
ductivity of nonstoichiometric specimens and specimens cal
cined at 350°C for 100 h in air was measured and resolved
into ionic and electronic components.
The specimens for measuring conductivity were pre
pared as pellets 10 – 12 mm in diameter with a thickness of
1–2mmusing a ceramic technology by which high-purity
were sintered in air at 900°C for
15 h and at 1250°C for 10 h; between the two heatings, the
specimens were ground and air-hardened from the final tem
perature. The synthesis was checked for completeness by
x-ray diffractometry using a DRON-3.0 instrument and
-radiation. The open porosity of specimens was about
1%. For electrical contact, a silver paste was burnt into the
specimen’s surface at 780°C.
The conductivity measurements were carried out in air in
the temperature range from 100 to 1100°C using a double-
contact technique under stepwise heating (cooking) in steps
of 40 – 50°C, each time allowing a state of equilibrium to be
reached; the measurement were made in direct current and
alternating current (103 Hz) using an E-63 teraohmmeter, a
VK-72E digital ohmmeter, an E-94 Q-meter, and an E-82 ac
bridge were used. The dc conductivity was measured in weak
(0.5 V) fields after a prolonged (up to 30 min) current decay.
The use of pore-free silver electrodes made it possible to
block mobile oxygen ions and allow thereby the use of a po
larization technique for resolving ionic and electronic con
ductivities as proposed in . By this technique, the total
conductivity is measured using alternating current, whereas
the electronic components is measured using direct current.
The three temperature curves log s versus 1/T for
specimens are shown in Fig. 1.
The conductivity curve of a calcined LaNb
(curve 1 ) is similar to that obtained by George and Virkar
. Those authors, based on an extensive experimental data,
provided evidence for the occurrence of cationic conduction
ions) at temperatures below 850 K (for the
activation energy DE = 0.14 eV). Above 850 K, electronic
conduction with DE = 1.58 eV (our data, DE = 1.60 eV) be
comes the prevailing process.
Refractories and Industrial Ceramics Vol. 43, Nos.1–2, 2002
1083-4877/02/0102-0043$27.00 © 2002 Plenum Publishing Corporation
St. Petersburg State Technological Institute (Technical Univer
sity), St. Petersburg, Russia; Institute of Chemistry of Silicates,
Russian Academy of Sciences, St. Petersburg, Russia.