The Raman spectra of the parent compound Na x CoO 2 (x=0.75) and the superconducting were collected in back-scattering geometry at room temperature using a Jobin-Yvon T64000 triple spectrometer equipped with a cooled change-couple device. In the spectrometer an objective of 100X-magnification was used to focus the laser beam on the sample surface and to collect the scattered light. Two excitation wavelengths at 488.0nm and 514.5 nm of an Ar + ion laser were used in our experiments. The laser power at the focus spot of 2-3 µm in diameter was kept below 1 mW to prevent laser-induce damage to the samples. parallel to the hexagon surfaces. All diffraction peaks in the corresponding XRD pattern can be indexed by a hexagonal cell with lattice parameters a =2.834Å, and c=10.94Å. The a-axis parameter agrees with the data for the ceramic samples as reported in our previous paper [3], while the c-axis parameter is slightly longer.The Raman spectra are very sensitive to properties of the sample surface. Fig. 2(b) shows a typical result obtained from the as-grown sample without pre-cleaning. It is noted 3 that this spectrum shows up certain similarities with those reported in Ref. 16. The EDAX analysis (see the inset of Fig. 2(b)) suggests that the surface of the as-made sample is covered by impurity phases mainly identified as Na 2 CO 3 . The formation of Na 2 CO 3 on the sample surface can be explained as following; when the Na x CoO 2 compound is stored under ambient condition, it reacts with water in air, decomposes into Na 2 O, and then further react with the CO 2 to form a more stable phase Na 2 CO 3 . On the other hand, the migration of Na + ions in this kind of materials also contributes to this phenomenon. As Ronald et al discussed in Ref.17, the structure of Na x CoO 2 consists of CoO 6 sheets and Na + ions intercalated within a trigonal prismatic site between the CoO 6 sheets. The distance between the faces of oxygen ions above and below the sodium is 0.81Å, which is wide enough to allow the sodium ion moving freely through this material, as there is a tunnel available for motion between oxygen sheets. Hence, it is indeed necessary to clean the surface of Na x CoO 2 samples thoroughly before Raman scattering measurements to ensure precise data to be obtained.The hexagonal crystal structure of Na x CoO 2 consisting of CoO 2 and Na layers parallel to the ab planes belongs to space group D 6h ( P6 3 /mmc, Z=2 ). Its vibrations at a wave vector of are classified following the irreducible representation of the factor group. There are five Raman active phonon modes: A 0 q ≅ 1g + E 1g + 3E 2g , which can be identified unambiguously by specific polarization configurations. Their second-order susceptibilities are restricted by the symmetric properties:where the absence of an entry in the matrix position ij implies a zero component, the letters Polarized Raman spectra were measured from ab and ac single crystal surfaces of Na x CoO 2 . In order to insure that the incident and scattered polarizations are parallel to the cry...

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Electronic structural features of the charge ordered (CO) state in Fe 2 OBO 3 have been theoretically calculated by using the ab initio method and analyzed in comparison with the experimental results of electron energy-loss spectroscopy (EELS). Structural relaxations using GGA + U reveal that the CO structure has a supercell of 2a × b × c with the CO modulation along the a-axis direction. The theoretical investigations suggest that both lattice distortions and electrostatic repulsion are essentially important for understanding the properties of the CO state in the present system. This conclusion is also supported by the agreement between experimental and theoretical data for the O-K and Fe-L 2,3 edges in the electron energy-loss spectra. Moreover, both bond-valencesum and spectral analysis demonstrate that the CO state adopts a quasi-ionic nature in Fe 2 OBO 3 .Charge ordering phenomenon has been extensively studied in numerous transition metal oxides associated with important physical phenomena, such as the colossal magnetoresistance 1 and the high-temperature superconductivity. 2 Recently it was also found that the charge ordering could give rise to multiferroicity in some transition metal oxides. 3,4 Take magnetite (Fe 3 O 4 ) for example, the CO state on two octahedral Fe sites in the inverted spinel AB 2 O 4 structure is responsible for the remarkable anomaly in the transport properties at about 120 K. 5 However, recent studies [6][7][8][9] show that an ideal ionic model for Fe 3 O 4 is too simple to well explain the experimental results, and further studies suggested that the strong interplay between the Jahn-Teller effect and electrostatic repulsion should be considered. 10 Iron oxyborate (Fe 2 OBO 3 ) is an isostructural compound of magnetite in which the A-site Fe atoms in Fe 3 O 4 are substituted by boron atoms, and Fe atoms are octahedrally coordinated with oxygen atoms. The edge-sharing FeO 6 octahedra form four infinite ribbons along the a-axis direction, which are linked by the trigonal BO 3 planes, as shown in Fig. 1. Fe 2 OBO 3 undergoes a CO transition at T CO = 317 K accompanied by a structural phase transition from orthorhombic Pmcn to monoclinic P 2 1 /c, and an antiferromagnetic phase transition at T N = 155 K. 11 The x-ray diffraction studies indicated that this CO phase has integer valence states for Fe ions. 12 However, certain essential issues for this CO state are still in debate. [13][14][15] In this paper we will report on the electronic structure of the CO state in Fe 2 OBO 3 via the first-principles calculations and electron energy-loss spectroscopy (EELS) measurements. In order to understand the essential properties of the CO state, the effects of lattice distortions and on-site Coulomb repulsion are accounted for, respectively, in our investigations. Based on the theoretical and experimental data, we suggest that both the electron-lattice coupling and electrostatic interaction play important roles for the formation of the CO state.The first-principles calculations were carried out ...