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Department of Mechanical, Materials and Aerospace Engineering Department of Mechanical, Materials and Aerospace Engineering Click to edit Master title style Raman Diagnostics of Temperature and Stress Induced Structural Modifications in LaCoO 3


  1. Department of Mechanical, Materials and Aerospace Engineering Department of Mechanical, Materials and Aerospace Engineering Click to edit Master title style Raman Diagnostics of Temperature and Stress Induced Structural Modifications in LaCoO 3 based Perovskites • Click to edit Master text styles • Second level • Third level Students: • Nina Gonzalez, Ethan Hackett, David Steinmetz • Fourth level • Fifth level This work was supported by the National Science Foundation under grant # 0201770 “Ferroelasticity and Hysteresis in Mixed Conducting Perovskites”

  2. Department of Mechanical, Materials and Aerospace Engineering Department of Mechanical, Materials and Aerospace Engineering Objectives Click to edit Master title style • Study lattice vibrations and optical phonons in • Click to edit Master text styles rhombohedral LaCoO 3 perovskite • Analyze • Second level the experimental Raman spectra as a function of a laser power • Third level • Use Stocks/anti-Stocks band ratio to determine the • Fourth level temperature of the perovskite surface under laser • Fifth level heating • Determine stress-induced changed in Raman spectra of LaCoO 3 perovskites using indentation technique

  3. Department of Mechanical, Materials and Aerospace Engineering Department of Mechanical, Materials and Aerospace Engineering Material T C  R =  1600 º C ; Grain size - 3-5  m Click to edit Master title style LaCoO 3 Pm m 3 High temperature - Cubic structure with the space group R c 3 Room temperature - Rhombohedral structure with the space group • Click to edit Master text styles Semiconductor to metallic conduction crossover exists at 230 o C Ferroelastic Phase Transition • Second level T C • Third level • Fourth level ά Lanthanum • Fifth level Cobalt Oxygen T C  R = LaCoO 3 at  1600 º C ; La 0.8 Ca 0.2 CoO 3 at 950 º C; La 0.6 Ca 0.4 CoO 3 at 700 º C High Temperature Low Temperature High symmetry prototypic phase Low symmetry phase a = b = c; a = b = g = 90° a = b = c ; a = b = g  90 ά = 60.78  ά = 60° Cubic Rhombohedral

  4. Department of Mechanical, Materials and Aerospace Engineering Department of Mechanical, Materials and Aerospace Engineering Classification of the  -point phonons Click to edit Master title style Pm m 3 The space group At perfect perovskite cubic structure all lattice sites have inversion symmetry and first order Raman scattering is forbidden • Click to edit Master text styles R c 3 The space group • Second level  La atoms occupy the 2 a (¼, ¼, ¼) positions and participate in • Third level    four  -point phonons modes ( ). A A E E 2 g 2 u g u • Fourth level  Co atoms occupy the 2 b (0,0,0) positions and also participate in   four  -point phonons modes ( ). A A 2 E • Fifth level 1 u 2 u u _ x x  Oxygen atoms occupy the 6 e ( , + ½, ¼) positions and take      A A 2 A 2 A 3 E 3 E part in twelve modes ( ). 1 g 1 u 2 g 2 u g u   A 4 E 3 A 5 E Raman active - Infrared active - 1 g g 2 u u M. Abrashev, A. Litvinchuk, M. Iliev, R. Meng, V. Popov, V. Ivanov, R. Chakalov, C. Thomsen, Phys. Rev. B , 59, 6, 4146-4153, 1999

  5. Department of Mechanical, Materials and Aerospace Engineering Department of Mechanical, Materials and Aerospace Engineering Experimental difficulties of Raman spectra collection Click to edit Master title style in the rhombohedral LaCoO 3 perovskite  Extremely low intensity of Raman lines and, therefore, a need for • Click to edit Master text styles a long time of spectra collections. The average time of collection varied from 200 to 500 s per point. • Second level  The small penetration depth of the excitation radiation also results • Third level in a decrease of the scattering intensity. • Fourth level  The micro-inhomogeneous areas exist at the perovskite surface. • Fifth level  Undesired/desired laser induced heating of the cobaltite surface occurs. To avoid overheating one needs to limit of the power of the laser excitation. To induce heating one needs to use a maximum of the laser power. The maximum power was 25mW with a possibility to reduce by 50, 75, 90, and 99%.

  6. Department of Mechanical, Materials and Aerospace Engineering Department of Mechanical, Materials and Aerospace Engineering Raman spectra of LaCoO 3 Click to edit Master title style 100% • Click to edit Master text styles 50% • Second level • Third level 25% • Fourth level 10% • Fifth level Raman Shift, cm -1  155 cm -1 – rare earth internal vibration mode  430 cm -1 – O-O octahedra rotation;  557 cm -1 – Co-O bending vibration  650 cm -1 – Co-O stretching vibration Raman spectra taken at the different laser intensity, 25mW, 514,5 nm Ar Ion

  7. Department of Mechanical, Materials and Aerospace Engineering Department of Mechanical, Materials and Aerospace Engineering Temperature determination using Raman scattering Click to edit Master title style Rayleigh scattering 9000 4 e         h kT I I [( ) ( )] 417cm -1 s St aSt I s I s 8000 • Click to edit Master text styles T = 250 o C 7000 6000 Intensity • Second level 5000 Stocks • Third level Anti-Stocks 4000 • Fourth level 3000 -417cm -1 2000 • Fifth level 1000 0 -800 -600 -400 -200 0 200 400 600 800 Raman shift, cm -1

  8. Department of Mechanical, Materials and Aerospace Engineering Department of Mechanical, Materials and Aerospace Engineering Optical Micrographs and Raman Spectra of Vickers Impression of LaCoO 3 Ceramics Click to edit Master title style • Click to edit Master text styles • Second level • Third level Mapping point outside of Vickers impression Sketch of Vickers • Fourth level indentations showing contact stresses, that • Fifth level arise at the interface between the indenter and material. The maximum contact stresses occur in the center of indentation zone and decrease according to the stress isobars Mapping point inside of Vickers impression

  9. Department of Mechanical, Materials and Aerospace Engineering Department of Mechanical, Materials and Aerospace Engineering Raman Spectra of La 0.8 Sr 0.2 CoO 3 ceramics from fracture, machined and scratched surfaces Click to edit Master title style 3 Scratched Surface, 1 Scratched Surface,2 • Click to edit Master text styles • Second level 500  m • Third level Machined Surface, 3 • Fourth level • Fifth level 2 1 Fracture Surface 500  m Scratch on a machined surface

  10. Department of Mechanical, Materials and Aerospace Engineering Department of Mechanical, Materials and Aerospace Engineering 2-D Raman map of a crack from Vickers indentation Click to edit Master title style Point I Point III Indentation 422 405 • Click to edit Master text styles O-O I Octahedra Rotation 416 • Second level • Third level II No stress III • Fourth level • Fifth level 400 450 350 Raman Shift (cm -1 ) Point III Point II Point I 2-D position map was created | | | using the 416 cm -1 O-O | | | | | | octahedra rotation band. 400 405 410 416 422 430 Tensile Stresses No stress Compressive Stresses

  11. Department of Mechanical, Materials and Aerospace Engineering Department of Mechanical, Materials and Aerospace Engineering Scheme of Indentation Induced Deformation Click to edit Master title style Zone Around Vickers Impression The large deformation zones • Click to edit Master text styles of compressive stresses around Vickers impression • Second level can be detected by Raman • Third level mapping and atomic force I microscopy (AFM). Tensile • Fourth level stresses exist along the II III • Fifth level cracks, originating from the Deformation corners of the impression. Zone Upshifted Raman Peak (compressive stress) Vickers crack Downshifted Raman Peak (due to tensile stress or bond stretching lattice deformation)

  12. Department of Mechanical, Materials and Aerospace Engineering Department of Mechanical, Materials and Aerospace Engineering Conclusions Click to edit Master title style  Rhombohedral LaCoO 3 perovskite is Raman active material and • Click to edit Master text styles can be studied by Raman spectroscopy.  The semiconductor/metal transition along with a decrease in • Second level rhombohedral distortion of the LaCoO 3 could be driven by laser • Third level overheating during a collection of Raman signal.  There • Fourth level are significant stress-induced changes in Raman spectrum of LaCoO 3 after indentation. The significant growth of • Fifth level two 557 and 670cm -1 bands could be possibly explained by the reversible semiconductor-metal-semiconductor transition upon loading and further unloading of the perovskite.  416cm -1 band of LaCoO 3 is a stress sensitive and, therefore, can be used for mapping residual stresses induced in the material after indentation.

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