- Email: [email protected]
Raman active phonons in orthorhombic YMnO3 and LaMnO3
Pergamon
PII: S0022-3697(98)00161-9
J. Phys. Chem Solids Vol 59, No. 10–12, pp. 1982–1984, 1998 0022-3697/98/$ - see front matter 䉷 1998 Elsevier Science Ltd. All rights reserved
RAMAN ACTIVE PHONONS IN ORTHORHOMBIC YMnO 3 AND LaMnO 3 M. N. ILIEV †*, M. V. ABRASHEV ‡, H.-G. LEE †, V. N. POPOV ‡, Y. Y. SUN †, C. THOMSEN ن, R. L. MENG † and C. W. CHU † †
Texas Center for Superconductivity, University of Houston, Houston, TX 77204, USA ‡ Faculty of Physics, University of Sofia, 1126 Sofia, Bulgaria ن Institut fu¨r Festko¨rperphysik, Technische Universita¨t Berlin, 10623 Berlin, Germany
Abstract—The Raman-active phonons in orthorhombic perovskite-like YMnO 3 and LnMnO 3 are studied by measuring Raman spectra in various scattering configurations and by calculations of the lattice dynamics (LDC). The much larger linewidths and strong variation of the Raman spectra of LaMnO 3 with increasing laser power provide evidence for a structural instability. 䉷 1998 Elsevier Science Ltd. All rights reserved Keywords: A. oxides, A. magnetic materials, C. Raman spectroscopy, D. phonons, D. lattice dynamics
The properties of rotationally distorted perovskites RMnO 3 (R ¼ Y or rare earth element) are studied intensively since it was found that partial substitution of R by Ca, Sr or Ba results in structural changes and occurrence of colossal magnetoresistance near the temperatures of spin ordering of Mn ions [1]. It is plausible to expect that structural changes and magnetic ordering will also influence the phonon spectra. The latter, however, are not well known. In this work on the example of YMnO 3 and LaMnO 3 we analyze the Raman spectra and Raman-active phonon modes (7A g þ 5B 1g þ 7B 2g þ 5B 3g) in orthorhombic perovskite-like RMnO 3 (space group Pnma, Z ¼ 4). The spectra of orthorhombic YMnO 3 and LaMnO 3 as obtained at room temperature with several exact scattering configurations are shown in Figs 1 and 2, respectively. In Table 1 the wavenumbers of the experimentally observed Raman lines are compared to those obtained by calculations of lattice dynamics (LDC) within a shell model [2]. In Fig. 3 are compared spectra obtained at 4.2 K, 100 K and 300 K. It follows from Figs 1–3 and Table 1 that the most of the observed lines are of A g or B 2g symmetry. There is a relatively good agreement between the experimental and theoretical values for the high frequency modes involving mainly oxygen motions, while for the low frequency modes involving mainly Y (La) vibrations, the agreement is rather poor, in particular for YMnO 3. The Raman lines in the spectra of LaMnO 3 are broader than the corresponding lines for YMnO 3. Moreover, while with lowering temperature the Raman lines of YMnO 3 become significantly narrower, no significant changes of the line widths are observed for LaMnO 3. This indicates much shorter and
temperature-independent lifetime for the phonons in the latter compound. Such behavior is expected for a strongly distorted structure where the phonon lifetime is governed by scattering from lattice imperfections rather than from other phonons. Evidence for lattice instability of LaMnO 3 can be found in the strong variation of Raman spectra on the excitation laser power between 0.12 and 4.55 mW (the averaged laser power density varies between 2 ⫻ 10 7 W m ¹2 and 1 ⫻ 10 9 W m ¹2) as illustrated in Fig. 4. The irradiation with laser power above 1.5 mW results in irreversible changes and the Raman spectrum of LaMnO 3 becomes the typical one for the rhombohedral phase. A reasonable explanation for these changes
*Corresponding author. Tel: +1 713 743 8209; fax: +1 713 743 8201; e-mail: [email protected]
Fig. 1. Raman spectra of ortho-YMnO 3 as obtained at 300 K with various scattering configurations.
1982
Raman active phonons in orthorhombic YMnO 3 and LaMnO 3
1983
Fig. 2. Raman spectra of ortho-LaMnO 3 as obtained at 300 K with various scattering configurations.
Fig. 3. Unpolarized Raman spectra of YMnO 3 and LaMnO 3 at 4.2 K, 100 K, and 300 K.
could be given accounting that the structure of LaMnO 3 depends of method of preparation and is sensitive to actual oxygen content, oxygen partial pressure and temperature [3]. Laser annealing increases the temperature thus increasing the disorder and stimulating (at higher temperatures) in- and outdiffusion of oxygen. During irradiation of in air the
in-diffusion of oxygen prevails. As a result structural transformation toward oxygen-rich rhombohedral phase takes place in the irradiated spot at higher power densities. Acknowledgements—This work was supported in part by NSF Grant No DMR 95-10625, T. L. L. Temple Foundation, the John
Table 1. Measured and calculated wavenumbers of the Raman modes of orthorhombic YMnO 3 and LaMnO 3. The main atomic motions as obtained from LDC are also given; O1 is the apex oxygen Mode
Ag Ag Ag Ag Ag Ag Ag B 1g B 1g B 1g B 1g B 1g B 2g B 2g B 2g B 2g B 2g B 2g B 2g B 3g B 3g B 3g B 3g B 3g
YMnO 3
LaMnO 3
Assignment
Exp.
LDC
Exp.
LDC
151 188 288 323 396 497 518 205 284 383 – – 151 220 317 341 481 537 616 178 336 – – –
104 147 223 304 407 466 524 181 288 342 413 593 137 162 285 393 470 583 617 145 363 390 476 610
140 198 257 – 284 493 – 184 – – – – 109 170 – 308 481 – 611 – 320 – – –
81 162 246 263 326 480 582 182 254 347 575 693 123 150 218 369 464 509 669 158 343 462 603 692
R(x) R(z) In-phase rotations of MnO 6 octahedra around y O1(x) Out-of-phase rotations of MnO 6 around x Out-of-phase bending of MnO 6 octahedra In-phase stretching of MnO 6 in xz-planes R(y) Out-of-phase rotations of MnO 6 around y In-phase rotations of MnO 6 octahedra around x Out-of-phase stretching of MnO 6 in xz-planes Out-of-phase stretching of MnO 6 along y R(z) R(x) Out-of-phase rotations of MnO 6 around z O1(z) Out-of-phase bending of MnO 6 octahedra In-phase bending of MnO 6 octahedra In-phase stretching of MnO 6 in xz-planes R(y) In-phase rotations of MnO 6 around z Out-of-phase bending of MnO 6 octahedra Out-of-phase stretching of MnO 6 octahedra Out-of-phase breathing of MnO 6 octahedra
1984
M. N. ILIEV et al. J. and Rebecca Moores Endowment and by the State of Texas through the Texas Center for Superconductivity at the University of Houston.
REFERENCES 1. Khomskii, D. I. and Sawatzky, G. A., Solid State Commun., 1997, 102, 87. 2. Popov, V. N., J. Phys.: Condensed Matter, 1995, 7, 1625. 3. Norby, P., Krogh Andersen, I. G., Krogh Andersen, E. and Andersen, N. H., J. Solid State Chem., 1995, 119, 191.
Fig. 4. Variations of Raman spectra of LaMnO 3 with incident laser power.
PII: S0022-3697(98)00161-9
J. Phys. Chem Solids Vol 59, No. 10–12, pp. 1982–1984, 1998 0022-3697/98/$ - see front matter 䉷 1998 Elsevier Science Ltd. All rights reserved
RAMAN ACTIVE PHONONS IN ORTHORHOMBIC YMnO 3 AND LaMnO 3 M. N. ILIEV †*, M. V. ABRASHEV ‡, H.-G. LEE †, V. N. POPOV ‡, Y. Y. SUN †, C. THOMSEN ن, R. L. MENG † and C. W. CHU † †
Texas Center for Superconductivity, University of Houston, Houston, TX 77204, USA ‡ Faculty of Physics, University of Sofia, 1126 Sofia, Bulgaria ن Institut fu¨r Festko¨rperphysik, Technische Universita¨t Berlin, 10623 Berlin, Germany
Abstract—The Raman-active phonons in orthorhombic perovskite-like YMnO 3 and LnMnO 3 are studied by measuring Raman spectra in various scattering configurations and by calculations of the lattice dynamics (LDC). The much larger linewidths and strong variation of the Raman spectra of LaMnO 3 with increasing laser power provide evidence for a structural instability. 䉷 1998 Elsevier Science Ltd. All rights reserved Keywords: A. oxides, A. magnetic materials, C. Raman spectroscopy, D. phonons, D. lattice dynamics
The properties of rotationally distorted perovskites RMnO 3 (R ¼ Y or rare earth element) are studied intensively since it was found that partial substitution of R by Ca, Sr or Ba results in structural changes and occurrence of colossal magnetoresistance near the temperatures of spin ordering of Mn ions [1]. It is plausible to expect that structural changes and magnetic ordering will also influence the phonon spectra. The latter, however, are not well known. In this work on the example of YMnO 3 and LaMnO 3 we analyze the Raman spectra and Raman-active phonon modes (7A g þ 5B 1g þ 7B 2g þ 5B 3g) in orthorhombic perovskite-like RMnO 3 (space group Pnma, Z ¼ 4). The spectra of orthorhombic YMnO 3 and LaMnO 3 as obtained at room temperature with several exact scattering configurations are shown in Figs 1 and 2, respectively. In Table 1 the wavenumbers of the experimentally observed Raman lines are compared to those obtained by calculations of lattice dynamics (LDC) within a shell model [2]. In Fig. 3 are compared spectra obtained at 4.2 K, 100 K and 300 K. It follows from Figs 1–3 and Table 1 that the most of the observed lines are of A g or B 2g symmetry. There is a relatively good agreement between the experimental and theoretical values for the high frequency modes involving mainly oxygen motions, while for the low frequency modes involving mainly Y (La) vibrations, the agreement is rather poor, in particular for YMnO 3. The Raman lines in the spectra of LaMnO 3 are broader than the corresponding lines for YMnO 3. Moreover, while with lowering temperature the Raman lines of YMnO 3 become significantly narrower, no significant changes of the line widths are observed for LaMnO 3. This indicates much shorter and
temperature-independent lifetime for the phonons in the latter compound. Such behavior is expected for a strongly distorted structure where the phonon lifetime is governed by scattering from lattice imperfections rather than from other phonons. Evidence for lattice instability of LaMnO 3 can be found in the strong variation of Raman spectra on the excitation laser power between 0.12 and 4.55 mW (the averaged laser power density varies between 2 ⫻ 10 7 W m ¹2 and 1 ⫻ 10 9 W m ¹2) as illustrated in Fig. 4. The irradiation with laser power above 1.5 mW results in irreversible changes and the Raman spectrum of LaMnO 3 becomes the typical one for the rhombohedral phase. A reasonable explanation for these changes
*Corresponding author. Tel: +1 713 743 8209; fax: +1 713 743 8201; e-mail: [email protected]
Fig. 1. Raman spectra of ortho-YMnO 3 as obtained at 300 K with various scattering configurations.
1982
Raman active phonons in orthorhombic YMnO 3 and LaMnO 3
1983
Fig. 2. Raman spectra of ortho-LaMnO 3 as obtained at 300 K with various scattering configurations.
Fig. 3. Unpolarized Raman spectra of YMnO 3 and LaMnO 3 at 4.2 K, 100 K, and 300 K.
could be given accounting that the structure of LaMnO 3 depends of method of preparation and is sensitive to actual oxygen content, oxygen partial pressure and temperature [3]. Laser annealing increases the temperature thus increasing the disorder and stimulating (at higher temperatures) in- and outdiffusion of oxygen. During irradiation of in air the
in-diffusion of oxygen prevails. As a result structural transformation toward oxygen-rich rhombohedral phase takes place in the irradiated spot at higher power densities. Acknowledgements—This work was supported in part by NSF Grant No DMR 95-10625, T. L. L. Temple Foundation, the John
Table 1. Measured and calculated wavenumbers of the Raman modes of orthorhombic YMnO 3 and LaMnO 3. The main atomic motions as obtained from LDC are also given; O1 is the apex oxygen Mode
Ag Ag Ag Ag Ag Ag Ag B 1g B 1g B 1g B 1g B 1g B 2g B 2g B 2g B 2g B 2g B 2g B 2g B 3g B 3g B 3g B 3g B 3g
YMnO 3
LaMnO 3
Assignment
Exp.
LDC
Exp.
LDC
151 188 288 323 396 497 518 205 284 383 – – 151 220 317 341 481 537 616 178 336 – – –
104 147 223 304 407 466 524 181 288 342 413 593 137 162 285 393 470 583 617 145 363 390 476 610
140 198 257 – 284 493 – 184 – – – – 109 170 – 308 481 – 611 – 320 – – –
81 162 246 263 326 480 582 182 254 347 575 693 123 150 218 369 464 509 669 158 343 462 603 692
R(x) R(z) In-phase rotations of MnO 6 octahedra around y O1(x) Out-of-phase rotations of MnO 6 around x Out-of-phase bending of MnO 6 octahedra In-phase stretching of MnO 6 in xz-planes R(y) Out-of-phase rotations of MnO 6 around y In-phase rotations of MnO 6 octahedra around x Out-of-phase stretching of MnO 6 in xz-planes Out-of-phase stretching of MnO 6 along y R(z) R(x) Out-of-phase rotations of MnO 6 around z O1(z) Out-of-phase bending of MnO 6 octahedra In-phase bending of MnO 6 octahedra In-phase stretching of MnO 6 in xz-planes R(y) In-phase rotations of MnO 6 around z Out-of-phase bending of MnO 6 octahedra Out-of-phase stretching of MnO 6 octahedra Out-of-phase breathing of MnO 6 octahedra
1984
M. N. ILIEV et al. J. and Rebecca Moores Endowment and by the State of Texas through the Texas Center for Superconductivity at the University of Houston.
REFERENCES 1. Khomskii, D. I. and Sawatzky, G. A., Solid State Commun., 1997, 102, 87. 2. Popov, V. N., J. Phys.: Condensed Matter, 1995, 7, 1625. 3. Norby, P., Krogh Andersen, I. G., Krogh Andersen, E. and Andersen, N. H., J. Solid State Chem., 1995, 119, 191.
Fig. 4. Variations of Raman spectra of LaMnO 3 with incident laser power.