Raman Studies on Functional Perovskite Oxides

Transcription

1 Raman Studies on Functional Perovskite Oxides Venkata S. Bhadram Postdoctoral Research Associate Geophysical Laboratory Carnegie Institution for Science

2 Bangalore Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) Bangalore, India

3 Aim of the present studies Structural distortions and their effect on the magnetic properties of rare-earth based oxides Parameters: Temperature /Pressure Raman scattering is sensitive to structural distortions and carry information related to the physical properties. Multifunctional RBO 3 LaMn 0.5 Co 0.5 O 3 Dual magnetic phases J. Phys. Condens. Matter (2010) J. Appl. Phys. (2014) RCrO 3 Multiferroic Euro. Phys. Lett. (2013) Mater. Res. Exp. (2014) 3

4 Experimental facilities Raman set-up ELETTRA Synchrotron, Trieste Membrane Diamond Anvil Cell (MDAC) Heating/Cooling stage K 55 mm 35 GPa 4

5 Perovskites ABO 3 formula A B O BO 6 AO 12 Cubic (Fm-3m) 5

6 Perovskites Cationic size mismatch Octahedral tilting Cation displacement Tolarence factor (t) : d AO 2d BO cubic perovskite: t=1 structural distortion t 1 Most common structures are Rhombohedral (R-3c) and Orthorhombic (Pbnm) 6

7 Perovskites Perovskite oxides J. Phys. D: Appl. Phys. 45, (2012) 7

8 Perovskites Corundum R-3c Orthorhombic Cmmm Science, 304, 855 (2004)

9 Perovskites Photocatalytic splitting of H 2 O for H 2 production Energy Environ. Sci. 5, 9034 (2012)

10 Orthorhombic distortion BO 6 tilts around [010] tilt angle φ & [101] tilt angle θ A-ion displacement θ A B O φ Cubic (Fm-3m) Orthorhombic (Pnma) 10

11 Orthorhombic distortion Cubic (Pm-3m) Raman active modes are absent Orthorhombic (Pnma) 24 Raman active modes

12 Distinct magnetic phases in LaMn 0.5 Co 0.5 O 3 B-site doped RMO 3 Spintronic applications LaMn 1-x Co x O 3 LaMnO 3 Antiferromagnetic LaCoO 3 Diamagnetic LaMn 0.5 Co 0.5 O 3 (LMCO) Heat above C Heat upto C LTc Ferromagnetic T c ~ 142 K Ferromagnetic T c ~ 240 K HTc Two different T C behavior??? Phys. Rev. B, 67, (2003) 12

13 Magnetic ordering temperature Spin Exchange Interactions T C = 2nJ ex(j +1) 3K B J ex = exchange energy J = total angular moment n = number of nearest neighbors K B = Boltzmann constant θ 180 o - θ J ex Cos 4 (θ avg ) Phys. Rev. Lett. 75, 914 (1995)

14 Raman studies ω S Lattice dynamical calculations J. Solid State Chem. 177, 2323 (2004) φ O-TM-O & <TM-O> Octahedral self distortion LTc HTc ω A,B 20 cm Raman shift (cm -1 ) θ avg & <La-O> Octahedral tilting 14

15 Difference in T c values ω S Octahedral self distortion is same ΔT C 100 K Octahedral tilting distortion is different θ ω A,B ω AB θ avg ' LTc 20 cm -1 J ex cos 4 (θ avg ) Phys. Rev. Lett. 75, 914 (1995) HTc T C = 2nJ ex(j +1) 3K B Raman shift (cm -1 ) The difference in T c values can be understood qualitatively using Raman 15

16 Temperature dependent Raman scattering LTc 300K 250K 210K 160K 130K 80K In magnetic materials change in phonon frequency with temperature: Δω(T) = (Δω) latt + (Δω) anh + (Δω) ren + (Δω) s-ph Phys. Rev. B 60, (1999) In case of magnetic insulators: Δω(T) = (Δω) latt + (Δω) anh + (Δω) s-ph Raman shift (cm -1 ) Exchange coupling between magnetic ions contribute as: Δω s-ph ± λ <S i.s j > J. Appl. Phys. 64, 5876 (1988) ћω o S In cubic anharmonic process, temperature dependence of phonon frequency: anh temperature dependence of Raman linewidth: anh (T) C 1 (T) 0 2 e 0 KBT Any deviation is considered as due to spin-phonon coupling. ћω P or 16 0 C 1 2 e 1 0 KBT 1

17 Temperature dependent Raman scattering ω LTc LTc T c HTc T c HTc ω HTc HTc LTc T c LTc T c Co-existance of two magnetic phases is clearly seen from temperature dependent Raman 17

18 Conclusions Distortion dependent Raman modes in LMCO could give a qualitative understanding of the two T C behaviour. Temperature dependence of Raman modes through spin-phonon coupling could show the coexistence of two magnetic phases in LMCO.

19 Multiferroicity in RCrO 3 Transport studies Multiferroicity below Néel temperature (T N ) is observed in RCrO 3 only in the case of magnetic R 3+ ion. GdCrO 3, SmCrO 3, NdCrO 3 etc. Multiferroic LaCrO 3, LuCrO 3, TbCrO 3 etc. Non-multiferroic Rajeswaran et.al. Phys. Rev. B (2012) Applications in spintronics and magnetic memory devices The mechanism for the occurrence of ferroelectricity in RCrO 3

20 Multiferroicity in RCrO 3 Raman studies Magnetic R-Cr interactions mediated spin-phonon coupling could be leading to multiferroicity.

21 Motivation for high pressure studies Orthorhombic RCrO 3 Ionic Radii T Cr N cos 4 ( avg ) l 7 θ avg = (θ+2φ)/2; is constant for all RCrO 3 θ A B O φ CrO 6 tilts around [010] θ [101] ϕ Phys. Rev. B 86, (2012) Phys. Rev. B, 8, (2012) Cubic (Fm-3m) Orthorhombic (Pnma) Reduce θ avg & l to increase T N, but how??? 21

22 Motivation for high pressure studies General Rule: with increasing pressure, A +3 B +3 O 3 : distortions increase A +2 B +4 O 3 : distortions decrease Phys. Rev. Lett. 95, (2005) Low symmetry (Pbnm) High symmetry (R-3C) High Pressure In case of LaCrO 3 Phys. Rev. Lett. 106, (2011) Contrasting Reports! Role of R-ion size! High Pressure In case of YCrO 3 Phys. Rev. B 82, (2010) 22

23 Intensity (arb.units) Raman spectra of RCrO 3 Raman Shift (cm -1 ) R-ion motion * * * * * CrO 6 bending * * B 1g (2) A g (2) B 1g (4) A g (4) A g (1) A g (7) * LaCrO 3 PrCrO 3 NdCrO 3 SmCrO 3 EuCrO 3 GdCrO 3 TbCrO 3 LuCrO 3 Ionic Radii Orthorhombic Pnma a R Cr O c b y z R O vib. θ φ CrO 6 tilts around [010] & [101] A g (2) and A g (4) are soft modes whose frequency vary linearly with φ and θ respectively.

24 Pressure dependent Raman studies Only in LaCrO 3, distortions reduce with increase in pressure

25 Pressure dependent Raman studies Pressure in GPa A g (7) Only in LaCrO 3, distortions reduce with increase in pressure

26 Compressibility : CrO 6 vs RO 12 Pressure Tilt angle Cr R Difference in compressions at Cr and R sites alters the octahedral tilting 26

27 Intensity (arb.units) Compressibility : CrO 6 vs RO 12 Raman Shift (cm -1 ) R-O vib. Cr-O str. * * * * * * * B 1g (2) A g (2) B 1g (4) A g (4) A g (1) A g (7) * LaCrO 3 PrCrO 3 NdCrO 3 SmCrO 3 EuCrO 3 GdCrO 3 TbCrO 3 LuCrO 3 RO 12 CrO 6

28 Compressibility : CrO 6 vs RO 12 La La LaCrO 3 Tb Eu Gd Sm Pr Nd Less compressible at La More compressible at Cr P Lu La Unique analysis of Raman modes Role of R-ion size

29 Intensity (arb.units) Intensity (arb.units) Intensity (arb.units) XRD pattern 17.7 GPa GdCrO 3 R Bragg = 0.07 R P = 1.70 R wp = 2.50 Profile matching with Le Bail fit I obs -I cal 0 GPa I obs -I cal R Bragg = 0.03 R P = 2.32 R wp = 3.27 a= (1)Å b=5.5279(1) Å C=7.6184(1)Å a= (2) Å b=5.5255(2)å C=7.6372(3)Å 2θ ( o ) 16.1 GPa I obs -I cal 0 GPa EuCrO 3 R Bragg = 0.14 R P = 0.72 R wp = 1.25 R Bragg = 0.05 R P = 1.45 R wp = 2.51 Orthorhombic (Pbnm) I obs -I cal Monochromatic λ = 0.7 Å 2θ ( o ) a= (3) Å b=5.5143(1)å C=7.6512(1)Å 15.5 GPa I obs -I cal 0 GPa I obs -I cal SmCrO 3 R Bragg = 0.10 R P = 1.40 R wp = 2.18 R Bragg = 0.05 R P = 1.80 R wp = θ ( o )

30 Lattice parameters No structural transition is observed 2 nd order Birch-Murnagham EOS B 0 = bulk modulus 30

31 Cell distortion factor Where Am. Mineral., 68, 1189 (1983) θ A B O φ Support for Raman results Cubic (Fm-3m) Orthorhombic (Pnma) 31

32 External pressure Softmode vs Tilt angle By neglecting the self distortion of the CrO 6 cos 1 a 2c cos 1 a b 16.1 cm -1 /degree Phys. Earth. Planet. Inter. 76, 1 (1993) 32

33 Softmode vs Tilt angle External pressure Chemical pressure 16.1 cm -1 /degree 23.4 cm -1 /degree Phys. Rev. B 85, (2012) Behavior of octahedral distortions w.r.t. chemical pressure (R-ion size) and external pressure is different.

34 Pressure effects on the Néel temperature Bloch s rule: Based on the change in lattice volume with pressure, κ is the compressibility J. Phys. Chem. Solids, 27, 881 (1965) dt N dp = (1.561) Gd < (2.543) Eu < (2.691) Sm degree/gpa Phys. Rev. B 81, (2010) where and Explicit relation Negative quantity Positive/negative quantity (Raman data) 34

35 -dl/dp (Å/GPa) dt Cr N /dp (K/GPa) Pressure effects on the Néel temperature (a) La (b) Bloch s rule La Sm Eu Gd Gd Sm Eu r R 3+ (Å) r R 3+ (Å) dt N dp is more sensitive to compression of Cr-O under external pressure 35

36 Increasing the pressure Conclusions Y Y La La Pressure effects on perovskite distortions Role of R-ion size Effect on the magnetic properties Chemical pressure vs external pressure Increasing R-ion size V. S. Bhadram et al, Mater. Res. Express 1, (2014) V. S. Bhadram et al (unpublished results) 36

37 Acknowledgements Chandrabhas Narayana Sundaresan A Anil Kumar Kaustuv Manna IISc Dhanya R JNCASR

38 Thank you