12 research outputs found
AC - conductivity studies on Y1-xBixCrO3 solid solution
YCrO3 and the substitution of Bi+3(6s2) in the Y-site were synthesized by the sol gel process. The conductivity measurements using dielectric spectroscopy reveals that conductivity increases with increase of composition. Dielectric loss were used to understand the nature of conducting species and it reveals that chromates exhibits the usual reciprocal behaviour of charge carriers to motion of oxygen vacancies as we increase the Bi composition and temperature. Impedance and microstructural studies revealed that conductivity of the chromates was dominated by the grain contribution
Tailoring the Band gap and magnetic properties by Bismuth substitution in Neodymium Chromite
The intrinsic distortions present in r
are earth
ortho
chromites (RCrO
3
)
observed from lanthanum to lutetium
(in
R
-
site) can
influence the magnetic properties
like Neel Transition and weak
ferromagnetic coupling
.
A n
on
-
magnetic
cation with
similar ionic radius
would
be
a
suitable candidate to
engineer
the inherent distortions of particular
orthochromite. In
this study
Bismuth (Bi
3+
)
with a 6s
2
lone pair was chosen to substitute
in Neodymium
(Nd
3+
)
site
of
NdCrO
3
(NCO)
to
tailor
the int
rinsic
structural distortions.
Th
e variation of optical absorption edge evidently
suggests that
Bi (
6
s
2
)
substituted in the m
agnetic rare earth Nd
+3
influences the Cr
-
O overlap integral.
Th
e
interaction of Bi cation with oxygen bonds influences the structural distortions through Cr
-
O polyhedra which are
evident from
Raman scattering studies
. The observed structural and magnetic properties of similar ionic radius of
Bi
3+
in Nd
3+
reveals that intrinsic structural distortions are interrelated to enhanced weak ferromagnetic component
and change in Neel and spin reorientation temperatures in our compou
nds
.
In addition a reduction in the optical band
gap of NCO from 3.1 eV to 2.
6
eV w
as observed
Studies on Local Structural Inhomogeneity and Origin of Ferroelectricity in Yttrium chromite Ceramics
YCrO3 (YCO) is known to be a multiferroic with orthorhombic, Pnma, structure with center of inversion. However, the local structural inhomogeneity in this compound is believed to give rise to ferroelectric behavior. In this study we explore high temperature Raman investigations of YCO and observed that one of its Raman mode B3g (3) (CrO6 Octahedral tilt mode) softens around the structural phase transition which could be the origin of ferroelectric nature in YCO. In addition, we substitute bismuth (Bi) in YCO to understand the structural distortions that lead to local structural inhomogeneity. Besides B3g (3) mode softening with composition and high temperature studies in Y1-xBixCrO3 the study reveals the structural distortions and the structural tunability Bi offers in such systems
Structural and electrical transport studies in Bi-substituted Yttrium Chromite
Perovskite type chromites are studied recently for their fascinating structure and inter-related functional properties. YCrO3 (YCO) is one among the chromites which possess structural heterogeneity. The structural studies on YCrO3 (YCO) and Y0.9Bi0.1CrO3 (YBiCO-10) reveal that the powder diffraction patterns exhibits a single phase with orthorhombic Pnma space group. Compositional studies on YCO and YBiCO-10 indicates the cation ratios of (Y, Bi): Cr (1: 1) is preserved. Microstructural studies reveal that there is an increment in grain growth in YBiCO-10 ceramics. The microstructural studies also evidently show that YBiCO-10 has more of faceted orthorhombic type of grains, whereas, YCO has a spherical grain type of features. Impedance spectroscopy studies were performed on both YCO and YBiCO ceramics and the studies reveal a significant change in grain resistance. The decrease of resistance could be due to increase of grain size which offers the conductive path of hopping of charge carriers or inherent electrical transitions of Cr3+ in YBiCO-10 sample. These chromites can be used as potential candidates for the thermistor applications whose thermistor parameter (beta) and sensitivity (alpha) values are 2873 K, -5 x 10(-3)K(-1) (YCO) and 3102 K,-5.44 x 10(-3)K(-1) (YBiCO-10) at 755 K
Structural, Electrical, Optical and Magnetic properties of Y1-xBixCrO3and Nd1-xBixCrO3 (x=0 to 0.15) Ceramics
Multiferroics exhibit the unusual combination of ferro/Antiferro electricity and ferro/Antiferro
magnetism in the single phase material.
Multiferroics are known
to be potential candidate for various
applications such as memory,
data storage
, sensors etc.
Rare earth orthochromites (RCrO
3
) are interesting
group of compounds that were studied in search of multiferroic features in recent years. Some of them
possess a
structural heterogeneity at local scale which is crucial for the observed macroscopic
electrical
properties such as dielectric and ferroelectric hysteresis behavior. The Neutron diffraction and phonon
studies on RCrO
3
confirm the structural
distortions whi
ch lead to
non
-
centrosymmetry at local scale
irrespective
of possessing a
global
/average
orthorhombic
(
i.e.
centrosymmetric) structure. The
pyroelectric measurements on RCrO
3
ceramics confirm
structure
-
electrical property correlation at
magnetic ordering temperature. The essential point to be noted is that such local
non
-
centrosymmetry or
the structural heterogeneity
in chromites is due to presence of magnetic rare earth and its interaction with
Cr
3+
ion. Thus the nonmagnetic rare earth
in RCrO
3
leads to non
-
ferroelectric conclusion i.e. local
structural distortions are absent in such chromites
as
observed
from pyroelectric measurements.
However
the neutron and phonon studies on YCrO
3
(Y
-
nonmagnetic) exhibits the structural distortions a
s well as
hysterisis loops indicative of existence of ferroelectricity.
It is worth to remember that RCrO
3
samples are
associated with high dc
-
leakage currents which inhibit the polarization hysteresis loops irrespective of
magnetic or nonmagnetic nature o
f R
-
ion. Hence, the structural studies give more insight on inherent
structural distortions
rather than electrical studies
on RCrO
3
samples
,
where electrical measurements
mislead the existence of possible ferroelectricity. Several
structural
studies are ca
rried
out such
as
Neutron
diffraction;
Transmission electron microscopy (TEM)
and Phonon scattering to understand local
structural distortions in RCrO
3
samples.
Neutron studies confirm
the existence of global
Pnma
orthorhombic
(centrosymmetric)
and local
P2
1
monoclinc
(non
-
centrosymmetric)
structure at different
temperatures
.
Phonon studies revealed the presence of spin
-
phonon coupling (R
-
ion is magnetic) and
magnetostriction (R
-
ion is non
-
magnetic) in RCrO
3
samples. The Pyroelectric and phonon
studies
at
magnetic ordering temperature indicate
that nonmagnetic R
-
ion is unlikely to exhibit polar behavior.
However, the local
non
centrosymmetry
is observed from neutron studies on nonmagnetic Y
-
ion in YCrO
3
(YCO) raises a question that whether such local structu
ral changes are really associated with magnetic
nature of R
-
ion? In literature, there are no extensive studies related to phonon studies on nonmagnetic rare
earth RCrO
3
which can rule out possible polar nature in these samples.
Hence,
the detailed investigations
on phonon studies
are carried out in
nonmagnetic rare earth RCrO
3
sample i.e.YCrO
3
(YCO).
vii
YCO possess a
global
orthorhombic structure with
Pnma
space group at RT observed from
neutron and
x
-
ray
diffraction measurements. YCO ex
hibits the antiferromagnetic phase transition
(T
N
~140K), as well as ferroelectric curie transition temperature (T
c
) around 473K, reveals the
presence of
multiferroic nature.
The weak ferroelectric nature of YCO understood from the dielectric and hysteresis
behavior at RT
and the magnetic transition in YCO is due to the presence of Cr
3+
ion.
High temperature x
-
ray diffraction studies reveal that the global
Pnma
orthorhombic structure throughout the measured
temperature range.
Thus diffraction studies did not reveal
global
structural changes with temperature but
involve minor changes in lattice parameters and volume. In order to understand the local structural
changes in
YCO
,
phonon studies
were
performed
across the dielectric p
hase transition temperature (
i.e.
303
-
573K). From phonon
studies
,
the Raman active
B
3g
(3)
(
out
-
of
-
phase
O2
scissor
like)
mode (SM)
showed anomalous behavior with temperature.
The phonon mode
B
3g
(3)
softens
across the proposed
ferroelectric to paraelectric transition temperature and
hardens
while
at low temperatures
. The
experime
ntal results on disappearance of
B
3g
(3) mode with temperature reveal that it is not merely from
the volume expansion effect but du
e to inherent order
–
disorder process within YCO i.e.
possibl
e local
structural distortions. To
get
into
deeper insights of microscopic origin, Density functional theory (DFT)
calculations were performed. The combined experimental and DFT studies reveal tha
t
disappearance of
Raman active B
3g
(3) mode above the dielectric transition temperature is due to the transformation of
B
3g
(3) (out of phase O2 scissors like motion)
into Raman inactive mode of B
1u
symmetry
.
The change in
B
3g
(3) symmetry is associated w
ith the local structural distortions such as Y
-
displacement along with
octahedra distortions in the unit cell of YCO
. Our calculations also suggest that the isolated and
anomalous behavior of this B
3g
(3) mode at lower temperatures could be due to the conv
ersion of scissor
mode to anti
-
stretching mode at extreme Y
cation displacement.
Further to confirm the Y
-
displacement
effect
,
we replaced the Y
-
site with Bismuth and compared the composition dependent phonon studies
with temperature dependent phonon studi
es.
The observed intensity
behavior
of B
3g
(3) indicates that
replacement of Y
-
site by
a
small amount of Bismuth exhibit the similar effect of temperature dependent
YCO. The
composition dependent
Raman shift of B
3g
(3) from 486 (x=0) to 479 cm
-
1
(x=0.15) is
comparable to the 486 (303K) to
479
cm
-
1
(
523K
) in temperature dependent YCO. These inherent
distortions are seen as
a change in morphology
with
the
composition which is direct experimental
confirmation
of
the
local structural distortions pres
ent in YCO
and is indeed due to Y
-
site
distortions/displacements observed from phonon studies
.
Hence,
the out of phase scissor like B
3g
(3)
mode seems to play a crucial role in the polar nature of YCO ceramics.
Magnetic and optical properties were studied
to understand the influence of structural distortions
on magnetic rare earth chromite, Nd
1
-
x
Bi
x
CrO
3
(
NBiCO
)
as well as nonmagnetic rare earth chromite, Y
1
-
viii
x
Bi
x
CrO
3
(YBiCO)
ceramics.
The diffraction studies on NBiCO and YBiCO reveals
an averaged
orthorhombic
Pnma
structure. The solubility limit
of Bi
is
observed to be
15at% and 10at% in YBiCO
and NBiCO respectively
, when synthesized through
sol
-
gel method
. The phonon studies show that local
structural distortions are getting stronger in NBiCO samp
les whereas the distortions are suppressed in
YBiCO samples.
T
he out o
f phase O2 scissor
-
like mode (B
3g
(3)) is shifting towards the blue frequency
regime (hardening of this mode) which indicates the structural distortions are getting stronger
in NBiCO
.
Fro
m the observations it is inferred that, that the structural distortions are associated with B
3g
(3) phonon
mode irrespective of magnetic or nonmagnetic nature of A
-
site in orthochromites. And t
he major changes
involved
in
phonon
modes
of NBiCO
suggests that the
collective behaviour
of CrO
6
rotation
al and
Nd
-
displacement
modes
create the
structural
distortions
.
On the other hand, the phonon studies on YBiCO
compounds suppress the possible structural distortions in YCO by Bismuth substitution esp
ecially the
phonon mode B
3g
(3) (out of phase scissor mode which is responsible for structural distortions) with the
composition.
This evidently shows that Bi subs
titut
ion exhibits an enhanced structural distortion in case
of NBiCO and a suppressed distort
ion in case of YBiCO.
The influence of structural distortions on magnetic properties of NBiCO and YBiCO compounds
has also been studied where;
Chromites with Cr
3+
configuration, only π bonds of
t
3
-
o
-
t
3
are present and
eventually lead t
o an antiferromagnetic ordering.
It’s worth remembering that the structural distortions
due to the Bi substitution in NCO lattice was evident from the
raman
studies such as change in phonon
behavior related to CrO
6
rotations
as well as Nd
-
O modes
.
In add
ition, p
resence of such distortion
(decrease the Cr
-
O
-
Cr bond angle and consequently two facing oxygens are now close enough to each
other)
facilitates
hybridization of
t
-
e
orbitals of Cr
3+
which provides the path for transfer of electrons from
filled orbi
tal (
t
3
) to empty orbital (
e
0
) i.e.
t
3
-
O
-
e
0
, effectively leads to observed weak ferromagnetic
coupling. Hence
,
with
Bi
substitution
which
is similar ionic radii, but with a 6s
2
lone pair configuration
plausibly creates the local structural distortions due to rotations of CrO
6
further leads to
t
-
e
hybridization
which in turn changes the values of T
N
in our chromites.
The observed increase of 40 Oe to 830Oe above
spin reorientatio
n temperature (
T
SR
)
and 184 Oe(x=0) to 445 Oe(x=0.1) below T
SR
gives a clear indication
that Bi
3+
can influence greater in Cr
3+
-
Cr
3+
interaction than that of Nd
3+
-
Cr
3+
interaction
.
In case of
YBiCO the structural distortions are suppressed as is evident
from the phonon studies and from increase
of Cr
-
O
-
Cr bond angles towards 180
º
. This further will inhibit the virtual charge transfer of electrons
across
t
3
-
O
-
e
0
in YBiCO samples and results the observed decrease of coercive and remanent magnetic
values.
Th
e decrease in band gap value
s
from 3.1eV
to 2.6 eV in
case of
N
Bi
CO
and from 3.3 to 2.9 eV
in case of YBiCO was observed
with increase of Bi content
.
The variation of optical absorption edge
evidently suggests that Bi (
6s
2
) substituted in the magnetic rare
earth Nd
+3
and nonmagnetic Y
3+
influences the Cr
-
O overlap integral.
ix
The electrical studies such as dielectric studies are performed on Y
1
-
x
Bi
x
CrO
3
(YBiCO) samples.
The dielectric studies reveals that YCrO
3
(YCO) possess single dielectric anomaly and YBi
CO samples
exhibiting two dielectric anomalies.
The
observed
dielectric
anomalie
s in YBiCO samples are analyzed
through structural as well as conductive mechanisms.
YCO shows local structural distortions across the
dielectric transition whereas higher
losses in the sample might mask the dipolar/structural transitions.
Thus
the inherent combined effect of dipolar and conductivity in YCO sample mislead the existence of
ferroelectricity. However, the conductivity due to grain and grain boundary contributio
ns and hopping of
localized electron transitions might lead to the existence of Maxwell Wagner effect and eventually
concludes the non
-
ferroelectric nature in YCO. In YBiCO, t
he observed
two dielectric
anomalies are not
related to the structural phase tran
sitions
which are
confirmed from high temperature diffraction as well
as phonon studies.
Hence, the observed dielectric anomalies in YBiCO might be due to the Maxwell
Wagner relaxations and/or due to the hopping mechanism. The loss behavior at different fr
equencies and
temperatures reveal that dielectric relaxations are of different conductive origin
. The impedance
measurements reveal
that
the grain and grain boundary contributions
are very prominent in YBiCO
samples
. The huge difference of total resistance
compared with the grain resistance in
YBiCO
samples
reveals the electrical heterogeneity across the grain and grain boundary might cause low temper
ature
dielectric relaxation. Moreover, the
frequency dependent as well as temperature depende
nt conductivity
studies confirm
the observed
low temperature
dielectric relaxation is due to
hopping of charge carriers
(Cr
3+
/Cr
4+
) across the grain and grain boundary interface in YBiCO samples
.
In YBiCO, t
he
high
temperature dielectric anomaly understood from the
frequ
ency and temperature dependent ac conductivity
measurements
. The conductivity studies reveal that
polaron hopping is the main conductive mechanism
that is responsible for
high temperature
dielectric anomaly
. dc
-
conductivity measurements
were used to
under
stand the
polaron
hopping model in
YBiCO
samples
. The
nonlinear
variation of the resistivity with
temperature reveals
that
chromites can be used as potential candidates for the negative thermistor
applications. The observed activation energies 0.2
-
0.35eV,
the thermistor constant (2872.7K and 3102K
for YCO and YBiCO
-
10 at 755K) and sensitivity values (
-
5x10
-
3
K
-
1
and
-
5.44x10
-
3
K
-
1
for YCO and
YBiCO
-
10 at 755K) are in the comparable range
for the
presently employed oxide materials for NTC
thermistor applicati
on
Local structural distortion and interrelated phonon mode studies in yttrium chromite
YCrO3 (YCO) perovskite has been originally reported to be a biferroic with antiferromagnetic and ferroelectric (FE) properties, in which the origin of FE in YCO remains ambiguous. However, further studies reveal the presence of a global orthorhombic Pnma structure with a local structural heterogeneity. In this study, we discuss the high temperature phonon modes and their inter-relation to local structural distortions in YCO perovskite through Raman spectroscopy experiments and density functional theory (DFT) calculations. We observe that the Raman active B3g(3) out of phase scissor mode (SM) disappears above the dielectric transition temperature (Tc) commensurate with the local structural distortions. DFT calculations show that the transformation of a room temperature Y-cation distorted orthorhombic structure to a perfect orthorhombic structure above the dielectric transition temperature in which the Y cation is undisplaced could lead to the conversion of SM with symmetry B3g to Raman inactive B1u mode
Tailoring the bandgap and magnetic properties by bismuth substitution in neodymium chromite
Local structural distortion and interrelated phonon mode studies in yttrium chromite
International audienceYCrO3 (YCO) perovskite has been originally reported to be a biferroic with antiferromagnetic and ferroelectric (FE) properties, in which the origin of FE in YCO remains ambiguous. However, further studies reveal the presence of a global orthorhombic Pnma structure with a local structural heterogeneity. In this study, we discuss the high temperature phonon modes and their inter-relation to local structural distortions in YCO perovskite through Raman spectroscopy experiments and density functional theory (DFT) calculations. We observe that the Raman active B3g(3) out of phase scissor mode (SM) disappears above the dielectric transition temperature (T c) commensurate with the local structural distortions. DFT calculations show that the transformation of a room temperature Y-cation distorted orthorhombic structure to a perfect orthorhombic structure above the dielectric transition temperature in which the Y cation is undisplaced could lead to the conversion of SM with symmetry B3g to Raman inactive B1u mode
Local structural distortion and interrelated phonon mode studies in yttrium chromite
YCrO3 (YCO) perovskite has been originally reported to be a biferroic with antiferromagnetic and ferroelectric (FE) properties, in which the origin of FE in YCO remains ambiguous. However, further studies reveal the presence of a global orthorhombic Pnma structure with a local structural heterogeneity. In this study, we discuss the high temperature phonon modes and their inter-relation to local structural distortions in YCO perovskite through Raman spectroscopy experiments and density functional theory (DFT) calculations. We observe that the Raman active B(3)g(3) out of phase scissor mode (SM) disappears above the dielectric transition temperature (Tc) commensurate with the local structural distortions. DFT calculations show that the transformation of a room temperature Y-cation distorted orthorhombic structure to a perfect orthorhombic structure above the dielectric transition temperature in which the Y cation is undisplaced could lead to the conversion of SM with symmetry B-3g to Raman inactive B-1u mode