Structural Phase Transitions and Sodium Ordering in Na0.5CoO2 a Combined Electron Diffracti.docx
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1、Structural Phase Transitions and Sodium Ordering in Na0.5CoO2 a Combined Electron DiffractiStructural Phase Transitions and Sodium Ordering in Na0.5CoO2:a Combined Electron Diffraction and Raman Spectroscopy StudyH.X. Yang*, C.J. Nie, Y.G. Shi, H.C. Yu, S. Ding, Y.L. Liu, D. Wu, N.L. Wang and J.Q. L
2、iBeijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, ChinaAuthor to whom correspondence should be addressed: hxyanghttp:/ nonstoichiometric Na x CoO2 system exhibits extraordinary physical properties that correlate with tempera
3、ture and Na concentration in its layered lattice without evident long-range structure modification when conventional crystallographic techniques are applied. For instance, Na0.7CoO2, a thermodynamically stable phase, shows large thermoelectric power; water-intercalated Na0.33CoO21.3H2O is a newly di
4、scovered superconductor with T c 4K, and Na0.5CoO2 exhibits an unexpected charge ordering transition at around T co 55 K. Recent studies suggest that the transport and magnetic properties in the Na x CoO2 system strongly depend on the charge carrier density and local structural properties. Here we r
5、eport a combined variable temperature transmission electron microscopy and Raman scattering investigation on structural transformations in Na0.5CoO2 single crystals. A series of structural phase transitions in the temperature range from 80 K to 1000 K are directly identified and the observed superst
6、ructures and modulated phases can be interpreted by Na-ordering. The Raman scattering measurements reveal phase separation and a systematic evolution of active modes along with phase transitions. Our work demonstrates that the high mobility and ordering of sodium cations among the CoO2 layers are a
7、key factor for the presence of complex structural properties in Na x CoO2 materials, and also demonstrate that the combination of electron diffraction and Raman spectroscopy measurements is an efficient way for studying the cation ordering and phase transitions in related systems.PACS: 64.60.-i.; 64
8、.70.Rh.; 64.75.+g.Keywords: NaxCoO2; Structural phase transitions; Electron diffraction; Raman spectroscopy1.IntroductionLayered deintercalatable alkali metal oxides, such as Li x CoO2 and Na x CoO2, have been a subject of an intense research activity in the past years owing to their potential techn
9、ological applications as a battery electrodes and thermoelectric materials 1-11. The notable features of these materials, from both structural and chemical point of views, are that the cation content and crystal structure can vary over a wide range by deintercalation, and that the structural and phy
10、sical properties are profoundly affected by the cation concentration and cation and vacancy ordering 4-5, 11. Experimental measurements, however, indicate that it is very difficult to obtain comprehensive structural information using crystallographic techniques, owing to considerable positional diso
11、rder of the intercalated cations and phase segregations, as identified in our recent investigations 12. It is therefore necessary to perform systematic investigations using a variety of methods to gain insight into the local atomic arrangements, especially regarding the intercalated cations between
12、the metal oxides layers.Recently, extensive interest has been paid to structure and physical properties of Na0.5CoO2 which shows three low-temperature phase transitions at around 87 K, 53 K, and 25 K respectively. These phase transformations, possibly in connection with charge ordering, have been we
13、ll identified by the measurements of resistivity, thermal transport and magnetization. The 87 K transition arises partially from a structural change; the 53 K transition is found to be responsible for charge ordering associated with a magnetic ordering; and the 20 K transition is proposed to be a sp
14、in reorientation transformation 13.Neutron diffraction at 9K identified an orthorhombic structure for Na0.5CoO2 with the Na atoms completely ordered 13. TEM experimental investigations reveal that complicated Na-ordered states and phase segregations exist commonly in Na x CoO2 due to existence of tw
15、o crystallographic positions and high mobility of Na+ 12. It is well known that TEM is ideal for the study of crystal structure at the nano-scale and that Raman scattering is sensitive to the local atomic arrangement change induced by oxygen shifts and Na+ ordering. In this paper, a detailed Raman s
16、cattering study, revealing the changes of lattice vibrations with temperature, and TEM observations, revealing structural transitions, have been carried out to study the structural properties in Na0.5CoO2 material. A series of structural phase transitions, as observed in the temperature range from 7
17、0 K up to 1000 K has been analyzed in terms of Na+ ordering.2.ExperimentalSingle crystalline Na0.85CoO2 samples were grown using a traveling-solvent floating zone method, Na0.5CoO2 compounds were further prepared by sodium deintercalation of Na0.85CoO2 as describe in previous publications. The sodiu
18、m content of all samples was determined by the ICP method. Specimens for transmission-electron microscopy (TEM) observations were prepared simply by crushing the bulk material into fine fragments, which were then supported on a copper grid coated with a thin carbon film. The TEM investigations were
19、performed on an H-9000NA TEM operating at a voltage of 300 kV, and a TECNAI 20 operating at a voltage of 200 kV, both equipped with low and high temperature sample stages. Raman spectra were collected in back-scattering geometry from470 K down to 79 K using a Jobin-Yvon T64000 triple spectrometer eq
20、uipped with a cooled change-couple device. In the spectrometer an objective of100X-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 p
21、ower at the focus spot of 2-3 m in diameter was kept below 1 mW to prevent the samples from laser-induce damage during experiments.3.Results and discussionWe first consider the microstructure changes in Na0.5CoO2 and development of local Na+ order with temperature. We have performed in situ heating
22、TEM studies from 300 K to 1000 K and in situ cooling from 300 K to 100 K. A series of phase transitions related to Na+ order have been identified at different temperatures. The material has an incommensurate modulated structure at room temperature and upon in-situ heating, it transforms into a super
23、structure hexagonal phase with doubled cell parameters in a-b plane: this superstructure is stable in the temperature range between 410 K and 470 K where upon another structural transition appears at about 470 K and the Na0.5CoO2 material transforms into a high-temperature hexagonal structure. Upon
24、cooling, Na0.5CoO2 undergoes a transformation at 200 K towards an orthorhombic structure 13, 18 with space group Pnmm in which a charge-ordered state is observed at the lower temperature. Figs. 1 (a)-(d) show the typical selected-area electron diffraction patterns taken at different temperatures dem
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