Raman Studies on Functional Perovskite Oxides

Venkata S. Bhadram Postdoctoral Research Associate

Geophysical Laboratory Carnegie Institution for Science

Jawaharlal Nehru Centre for Advanced Scientific Research

(JNCASR)

Bangalore, India

Bangalore

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

A

B

O

Cubic (Fm-3m)

ABO3 formula

BO6

AO12

5

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

7

Perovskite oxides

J. Phys. D: Appl. Phys. 45, 033001 (2012)

Perovskites

Perovskites

R-3c

Corundum

Cmmm

Orthorhombic

Science, 304, 855 (2004)

Perovskites

Photocatalytic splitting of H2O for H2 production

Energy Environ. Sci. 5, 9034 (2012)

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.

Pressure dependent Raman studies

Only in LaCrO3, distortions reduce with increase in pressure

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

Acknowledgements

Chandrabhas Narayana Sundaresan A Anil Kumar

JNCASR

IISc

Kaustuv Manna

Dhanya R

Thank you