Raman spectroscopy study ofNaxCoO2and superconductingNax

Abstract

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...

Electron energy loss spectra ofNa0.33CoO2∙yH2O(Yang1, Li2, Xiao3 et al. 2005Phys. Rev. BSelf Cite8180View full textAdd to dashboardCite

“…The sample syntheses and characterizations have been reported in ref. [12,13]. Specimens for transmission-electron microscopy (TEM) observations were prepared by gently crushing the powder material into fine fragments, which were then supported by a copper grid coated with a thin carbon film.…”

Optical properties of Na0.68CoO2 thin film

“…As Na content increasing from x = 0.3, the electron ground state changes from a paramagnetic metal to a charge-ordered insulator for x = 0.5, and to a spin ordered phase for x > 0.75 [3]. A strong electron-phonon interaction has been found in Na x CoO 2 systems with x = 0.18, 0.32, 0.36, 0.57, 0.7, 0.82, and 0.85 single crystals [4][5][6][7][8][9][10]. In order to gain additional insight into the novel properties of Na x CoO 2 systems, we present the results of the optical reflectance investigations of Na 0.68 CoO 2 thin film.…”

Metal-insulator transition in Ni-doped Na0.75CoO2: Insights from infrared studies

“…It is of interest to see if the transition to the insulating phase is associated with a charge ordering transition, since charge ordering in the CoO 2 layer is another major instability in these materials [7,8]. Charge ordering has been addressed using various techniques like neutron diffraction [9,10], electron diffraction [3,11], NMR [12], PES [13] and optical studies including both Raman [14,15] and infrared spectroscopy [7,[16][17][18][19][20].…”