Raman Studies on Functional Perovskite Oxides
Transcript of Raman Studies on Functional Perovskite Oxides
Raman Studies on Functional Perovskite Oxides
Venkata S. Bhadram Postdoctoral Research Associate
Geophysical Laboratory Carnegie Institution for Science
Parameters: Temperature /Pressure
Aim of the present studies
Structural distortions and their effect on the magnetic properties of rare-earth based oxides
3
RCrO3 LaMn0.5Co0.5O3
Multiferroic Dual magnetic phases
Euro. Phys. Lett. (2013)
Mater. Res. Exp. (2014) J. Phys. Condens. Matter (2010)
J. Appl. Phys. (2014)
Multifunctional RBO3
Raman scattering is sensitive to structural distortions and carry
information related to the physical properties.
Experimental facilities
35 GPa 4
55 mm
Membrane Diamond Anvil Cell (MDAC)
ELETTRA Synchrotron, Trieste
Heating/Cooling stage
77-873K
Raman set-up
Perovskites
Most common structures are “Rhombohedral (R-3c)” and “Orthorhombic (Pbnm)”
Cationic size mismatch
Tolarence factor (t) :
OB
OA
d
d
2
structural distortion t ≠1
cubic perovskite: t=1
Octahedral tilting Cation displacement
6
Orthorhombic distortion
BO6 tilts around [010] tilt angle “φ”
& [101] tilt angle “θ”
A-ion displacement
ABO
φ
θ
Cubic (Fm-3m) Orthorhombic (Pnma) 10
Cubic (Pm-3m)
Raman active modes are absent
Orthorhombic distortion
Orthorhombic (Pnma)
24 Raman active modes
LaMn1-xCoxO3
LaMnO3 Antiferromagnetic
LaCoO3 Diamagnetic
LaMn0.5Co0.5O3
Ferromagnetic
Tc ~ 240 K
Ferromagnetic
Tc ~ 142 K
Heat upto 800 0C Heat above 1300 0C
Distinct magnetic phases in LaMn0.5Co0.5O3
Two different TC behavior ???
12
Phys . Rev . B, 67, 014401 (2003)
(LMCO)
LTc HTc
B-site doped RMO3 Spintronic applications
TC =2nJex (J +1)
3KB
Jex= exchange energy J = total angular moment n = number of nearest neighbors KB = Boltzmann constant
180o - θ
θ
Jex Cos4(θavg)
Magnetic ordering temperature
Spin Exchange Interactions
Phys. Rev. Lett. 75, 914 (1995)
400 600 800
HTc
Raman shift (cm-1)
LTc
ωA,B
ωS
20 cm-1
Raman studies
φO-TM-O & <TM-O>
Octahedral self distortion
+
θavg & <La-O> Octahedral tilting
Lattice dynamical calculations J. Solid State Chem. 177, 2323 (2004)
14
TC =2nJex (J +1)
3KB
Phys. Rev. Lett. 75, 914 (1995)
400 600 800
HTc
Raman shift (cm-1)
LTc
ωA,B
ωS
20 cm-1
ωAB
Difference in Tc values
‘θavg'
15
θ ΔTC ≈ 100 K
Jex cos4(θavg)
Octahedral self distortion is same
Octahedral tilting distortion is different
The difference in Tc values can be understood qualitatively using Raman
Temperature dependent Raman scattering
In magnetic materials change in phonon frequency with temperature: Δω(T) = (Δω)latt
+ (Δω)anh + (Δω)ren
+ (Δω)s-ph
Exchange coupling between magnetic ions contribute as: Δωs-ph ≈ ± λ <Si.Sj
> J. Appl. Phys. 64, 5876 (1988)
In case of magnetic insulators: Δω(T) = (Δω)latt + (Δω)anh
+ (Δω)s-ph
In cubic anharmonic process, temperature dependence of phonon frequency: temperature dependence of Raman linewidth:
1e21 C(T)TK
0anhB0
Any deviation is considered as due to
spin-phonon coupling.
400 500 600 700
Raman shift (cm-1)
300K
250K
210K
160K
130K
80KLTc
ћωo
S
ћωP or 16
Phys. Rev. B 60, 11879 (1999)
1e21 C(T)TK
0anhB0
S @ LTc
Tc
ωS @ LTc
Tc
Tc
ωS @ HTc
HTc
HTc
Temperature dependent Raman scattering
17
LTc
S @ HTc
Tc
LTc
Co-existance of two magnetic phases is clearly seen from temperature dependent Raman
Conclusions
Distortion dependent Raman modes in LMCO could give a qualitative
understanding of the two TC behaviour.
Temperature dependence of Raman modes through spin-phonon
coupling could show the coexistence of two magnetic phases in LMCO.
Multiferroicity in RCrO3
Multiferroicity below Néel temperature (TN ) is observed in RCrO3
only in the case of magnetic R3+ ion.
Rajeswaran et.al. Phys. Rev. B (2012)
GdCrO3, SmCrO3, NdCrO3 etc. – Multiferroic
LaCrO3, LuCrO3, TbCrO3 etc. – Non-multiferroic
Transport studies
Applications in spintronics and magnetic memory devices
The mechanism for the occurrence of ferroelectricity in RCrO3
Multiferroicity in RCrO3
Raman studies
Magnetic R-Cr interactions mediated spin-phonon coupling could be leading
to multiferroicity.
Ionic Radii
21
Reduce ‘θavg’& ‘l’ to increase TN , but how???
Phys. Rev. B 86, 214409 (2012)
Phys. Rev. B, 8, 054303 (2012)
Orthorhombic RCrO3
74 )(cos lT avg
Cr
N
Motivation for high pressure studies
is constant for all RCrO3
ABO
φ
θ
Cubic (Fm-3m) Orthorhombic (Pnma)
θavg = (θ+2φ)/2;
CrO6 tilts around [010] θ [101] ϕ
General Rule: with increasing pressure, A+3 B+3O3 : distortions increase A+2 B+4O3 : distortions decrease
High Pressure
Low symmetry (Pbnm) High symmetry (R-3C)
In case of LaCrO3
In case of YCrO3 High Pressure
Phys. Rev. Lett. 95, 025503 (2005)
Contrasting Reports!
Phys. Rev. Lett. 106, 057201 (2011)
Phys. Rev. B 82, 064109 (2010)
22
Role of R-ion size!
Motivation for high pressure studies
100 200 300 400 500 600
*
*
*
*
*
*
*
LaCrO3
PrCrO3
NdCrO3
SmCrO3
EuCrO3
GdCrO3
TbCrO3
LuCrO3
B1g
(2) Ag(2) B
1g(4) A
g(4) A
g(1)A
g(7)
Raman Shift (cm-1)
Inte
ns
ity
(a
rb.u
nit
s)
*
CrO6 bending
R-ion motion
Ionic R
adii
‘R – O’ vib. y
z
Raman spectra of RCrO3
Orthorhombic Pnma
CrO6 tilts around [010] & [101]
c
b a
R
Cr O
“φ” “θ”
Ag(2) and Ag(4) are soft modes whose frequency vary linearly with “φ” and “θ” respectively.
Ag(7)
Pressure in GPa
Pressure dependent Raman studies
Only in LaCrO3, distortions reduce with increase in pressure
Compressibility : CrO6 vs RO12
26
R
Cr Tilt angle
Difference in compressions at Cr and R sites alters the octahedral tilting
Pressure
100 200 300 400 500 600
*
*
*
*
*
*
*
LaCrO3
PrCrO3
NdCrO3
SmCrO3
EuCrO3
GdCrO3
TbCrO3
LuCrO3
B1g
(2) Ag(2) B
1g(4) A
g(4) A
g(1)A
g(7)
Raman Shift (cm-1)
Inte
ns
ity
(a
rb.u
nit
s)
*
Compressibility : CrO6 vs RO12
R-O vib. Cr-O str.
RO12 CrO6
La
Nd Pr
Sm
Gd
Eu
Tb
Lu
La
La
P
Compressibility : CrO6 vs RO12
Less compressible at ‘La’ More compressible at ‘Cr’
LaCrO3
‘Unique’ analysis of Raman modes Role of R-ion size
2θ (o)
Inte
nsi
ty (
arb
.un
its)
EuCrO3
0 GPa
16.1 GPa
RBragg = 0.14
RP = 0.72
Rwp = 1.25
RBragg = 0.05
RP = 1.45
Rwp = 2.51
Iobs-Ical
Iobs-Ical
Inte
nsi
ty (
arb
.un
its)
2θ (o)
SmCrO3
0 GPa
15.5 GPa
RBragg = 0.10
RP = 1.40
Rwp = 2.18
RBragg = 0.05
RP = 1.80
Rwp = 2.90
Iobs-Ical
Iobs-Ical
GdCrO3
2θ (o)
0 GPa
17.7 GPa
RBragg = 0.07
RP = 1.70
Rwp = 2.50
RBragg = 0.03
RP = 2.32
Rwp = 3.27
Iobs-Ical
Iobs-Ical
Inte
nsi
ty (
arb
.un
its)
XRD pattern
Profile matching with Le Bail fit
λ = 0.7 Å
Monochromatic
Orthorhombic (Pbnm)
29
a= 5.3238(1)Å b=5.5279(1) Å C=7.6184(1)Å
a= 5.3714(2) Å b=5.5255(2)Å C=7.6372(3)Å
a= 5.3698(3) Å b=5.5143(1)Å C=7.6512(1)Å
2nd order Birch-Murnagham EOS
B0= bulk modulus
Lattice parameters
No structural transition is observed
30
Where
Cell distortion factor
ABO
φ
θ
Cubic (Fm-3m) Orthorhombic (Pnma) 31
Am. Mineral., 68, 1189 (1983)
Support for Raman results
32
Softmode vs Tilt angle
b
a1cos
By neglecting the self distortion of the CrO6
c
a
2cos 1
Phys. Earth. Planet. Inter. 76, 1 (1993) 16.1 cm-1 /degree
External pressure
Phys. Rev. B 85, 054303 (2012)
Behavior of octahedral distortions w.r.t. chemical pressure (R-ion size) and external pressure is different.
23.4 cm-1/degree
Softmode vs Tilt angle
Chemical pressure
16.1 cm-1 /degree
External pressure
34
where and
Pressure effects on the Néel temperature
Bloch’s rule: Based on the change in lattice volume with pressure,
‘κ ‘ is the compressibility
Negative quantity Positive/negative quantity
(Raman data)
Explicit relation
dP
dTN = (1.561)Gd < (2.543)Eu < (2.691)Sm degree/GPa
J. Phys. Chem. Solids, 27, 881 (1965)
Phys. Rev. B 81, 214115 (2010)
35
1.04 1.06 1.08 1.10 1.12 1.14 1.161
2
3
4
5
6
1.04 1.06 1.08 1.10 1.12 1.14 1.16
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
0.0035
0.0040
La
Sm
Eu
dT
Cr
N/d
P (
K/G
Pa
)
rR
3+ (Å)
Gd
(b)
La
Sm
Eu
-d
l/d
P (
Å/G
Pa
)
rR
3+ (Å)
Gd
(a)
Bloch’s rule
is more sensitive to compression of Cr-O under external pressure dP
dTN
Pressure effects on the Néel temperature
Conclusions
36
V. S. Bhadram et al, Mater. Res. Express 1, 026111 (2014)
V. S. Bhadram et al (unpublished results)
Increasing R-ion size
Incr
easi
ng
th
e p
ress
ure
Y La
La Y
Role of R-ion size Effect on the magnetic properties Chemical pressure vs external pressure
Pressure effects on perovskite distortions