We present a rationalization of the Raman spectra of orthorhombic and rhombohedral, stoichiometric and doped, manganese perovskites. In particular, we study RMnO 3 (RϭLa, Pr, Nd, Tb, Ho, Er, Y, and Ca͒ and the different phases of Ca-or Sr-doped RMnO 3 compounds as well as cation deficient RMnO 3 . The spectra of manganites can be understood as combinations of two kinds of spectra corresponding to two structural configurations of MnO 6 octahedra and independently of the average structure obtained by diffraction techniques. One type of spectra corresponds to the orthorhombic Pbnm space group for octahedra with cooperative or dynamic Jahn-Teller distortions, with stretching modes as the main features and whose frequencies correlate to Mn-O distances. The other spectrum is associated to regular but tilted octahedra whose modes can be described in the rhombohedral R3c structure, where only bending and tilt modes are observed. The main peaks of compounds with regular MnO 6 octahedra, such as CaMnO 3 , highly Ca-doped LaMnO 3 , or the metallic phases of Ca-or Sr-doped LaMnO 3 , are bending and tilt MnO 6 octahedra modes which correlate to R-O(1) bonds and Mn-O-Mn angles, respectively. In low and optimally doped manganites, the intensity and width of the broad bands are related to the amplitude of the dynamic fluctuations produced by polaron hopping in the paramagnetic insulating regime. The activation energy, which is proportional to the polaron binding energy, is the measure of this amplitude. This study permits to detect and confirm the coexistence, in several compounds, of a paramagnetic matrix with lattice polaron together with regions without dynamic or static octahedron distortions, identical to the ferromagnetic metallic phase. We show that Raman spectroscopy is an excellent tool to obtain information on the local structure of the different microphases or macrophases present simultaneously in many manganites.

PACS. 63.20.-e Phonons in crystal lattices – 75.30.Vn Colossal magnetoresistance,

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Raman spectroscopy was used to seek out the orbital excitations in orbitally ordered RMnO3. The high energy Raman peaks are found to correspond to second and third order phonons which couple in some degree to the orbital order. Franck-Condon-type Raman resonance of multiphonons is observed but the cross sections are smaller than expected. A surging emission band at 1.5 eV, detected at high temperatures, disappears when the orbital order is established. The crossing of the configurational coordinate diagrams of the localized 3d Mn lowest levels, in the orbitally ordered state, originates the nonradiative relaxation and the quenching of the luminescence.

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Photoemission and x-ray absorption near edge spectroscopy measurements have been performed in Tl2Mn2O7 oxides doped with 10% different ions, Bi, Cd, or Sb, which produce colossal changes in the magnetoresistance values. The contributions to the valence band related to Mn, O, and the doping ions have been obtained. We found that the paramagnetic phase of doped Tl pyrochlores is charge-transfer insulator type with oxygen character of the upper edge of the valence band. Bi 6s and Cd 4d orbitals lie also at the upper edge of the valence band. Mn valence is identical for all samples while oxygen content varies to compensate for the charge introduced by doping. The density of carriers, which is correlated to the magnetoresistance values, is determined by the density of states near the Fermi level provided by Tl and O content.

A commercial Atomic Force Microscope (AFM) and a semi-home made Scanning Near-Field Optical Microscope (SNOM) have been used to characterize electrically, topographically and optically the domain walls among natural ferroelectric domains in a KNbO 3 crystal. The AFM measurements have been performed with a metallic coated tip in order to detect electrostatic forces between the polarization field at the ferroelectric surface and the tip. An external electric field has also been applied between the sample surface and the tip to tune this electrostatic interaction over the atomic forces. In optical transmission images, acquired under near field conditions, we observe a clear contrast of the signal at the domain walls between 180º spontaneous polarization domains; while the images of the surface topography, obtained simultaneously, show a reasonably flat surface of the crystal. The scanning probe microscopy techniques used in this work are valuable tools for the investigation of ferroelectric materials and, in particular, to characterize the domain walls, without needing a either especial preparation or damage of the sample surface. Microscopias de barrido aplicadas al estudio de los dominios y las paredes de dominio en un cristal ferroelectrico de KNbO 3. Hemos utilizado un Microscopio de Fuerzas Atómicas (AFM) comercial y un Microscopio Óptico de Campo Cercano (SNOM) semi-casero para caracterizar eléctrica, óptica y topográficamente las paredes de dominio presentes entre los dominios ferroeléctricos naturales de un cristal de KNbO 3. Las medidas de AFM las hemos realizado con una punta recubierta con metal, para detectar las fuerzas electrostáticas entre los campos de polarización de la superficie ferroeléctrica y la punta. Además, hemos aplicado campos eléctricos externos entre la superficie de la muestra y la punta, de manera que se pueda variar la fuerza electrostática en relación a las fuerzas atómicas. En imágenes de transmisión óptica, bajo condiciones de campo cercano, observamos un claro contraste de la señal en las fronteras entre los dominios ferroeléctricos con polarización espontánea a 180º, mientras que las imágenes de topografía muestran una superficie prácticamente plana. Las técnicas de microscopía de prueba usadas en este trabajo se revelan muy valiosas para la investigación de materiales ferroeléctricos y, en particular, para la caracterización de sus dominios y paredes de dominio, puesto que no es necesario preparar ni dañar la superficie de la muestra para ser investigada. Palabras clave: KNbO 3 , ferroeléctrico, pared de dominio, SNOM y AFM