Structural Phase Transitions and Sodium Ordering in Na0.5CoO2 a Combined Electron Diffracti.docx

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. LiBeijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, ChinaAuthor to whom correspondence should be addressed: hxyanghttp:/www.doczj.com/doc/36334a737fd5360cba1adb90.htmlAbstractThe nonstoichiometric Na x CoO2 system exhibits extraordinary physical properties that correlate with temperature 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 discovered 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 report 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 superstructures 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 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.; 64.70.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 technological 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 physical 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 disorder 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 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 well 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 spin 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 two 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 scattering 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 70 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 sodium 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 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 equipped 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 power 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 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 superstructure 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 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 demonstrating the presence of a series of remarkable phase transitions in the Na0.5CoO2 material.We now proceed to a detailed discussion of the structures observed at different temperatures. TEM structural analyses suggest that the high temperature hexagonal phase (above 470K) has the similar average structure to the conventional hexagonal phase in which no any ordered states are observed (see Fig.1 (a). This ideal (average) structural homogeneity is perhaps in connection with the high mobility of Na ions at high temperatures. Moreover, the thin TEM samples are found to gradually decompose into small polycrystalline particles above 850 K.The notable superstructure appearing between 410 K and 470 K (see Fig.1 (b) has doubled cell parameters within the a-b plane and this superstructure is interpreted as arising from partial Na+ ordering between CoO2 layers. The Na+ ions in reported average structures can occupy two possible positions (Na1 and Na2) governed by the space between the two adjacent O planes. Both of the sites are trigonal prismatic sites, but one trigonal prism shares edges with adjacent CoO6 octahedral (Na2), while the other trigonal prism shares faces with adjacent CoO6 octahedral (Na1) 13. Fig. 2 (a) displays a simple structural model based on the symmetric occupation of a fraction of the Na2 sites. Fig. 2 (b) shows a theoretical simulated diffraction pattern along the 001 zone axis direction, which is in good agreement with the experimental results as exhibited in Fig.1 (b). It is also noted that Na0.5CoO2 in this temperature range also has perfect structural homogeneity. We have checked the crystal structure from one area to another and no other kind of additional ordering or structural changes are observed.Structural data obtained between 200 K and 410 K, especially close to room temperature, reveal a variety of interesting structure phenomena, such as phase separationand complex ordered states corresponding to different Na-ordered states 12. It is likely that Na0.5CoO2 has an intermediate (or metastable) structure within this large temperature range. Detailed TEM structural analyses suggest that the major structural feature of this intermediate phase can be described by a structural modulation along the According to theoretical results for structural modulations, the incommensurate state consists of small commensurate domains separated by discommensurations within whichthe phase of structural modulation varies rapidly 17. Fig. 3 (b) shows a dark-field TEM image revealing the presence of a lamella structural feature where the average periodicity changes slightly from area to another ( 5nm). This kind of domain structure is often seen in incommensurate or nearly commensurate modulated systems. The white stripes are considered as commensurate regions where the modulation phase factor remains constant. The average periodicity (L) of this type of domain structure is related to the incommensuratability through the relationship L = 2/p , where p is the order of commensurability and, in the present case, p = 4. In Na0.5CoO2, we have =1/15, this gives rise to a periodical domain structure with L 5 nm. This result is in good agreement with the periodicity observed in the dark field image.The mechanism for Na+ ordering over the available sites has been discussed previosuly using the model that the Na atoms are spaced as far apart as possible to minimize the Na+- Na+ Coulombic repulsions 13. Occupancy of the Na2 site is favored as demonstrated by neutron diffraction for Na0.61CoO2 19. A structural model for explaining the incommensurate modulated structure is reported in ref. 18, in which only one of the two Na1 and Na2 sites is partially occupied. The incommensurability is proposed to originate from an insertion of an extra vacancy plane. Fig. 3 (c) and (d) display two structural models which could be used to explain the two incommensurate modulations in Fig 3 (a). These models give a reasonable fit with the observed electron diffraction patterns, and both orientation and space anomalies can be explained.Now we move on to discuss the structure properties of the low-temperature structural phase transition. TEM observations reveal that the incommensurate structural modulationshows up as systematic changes in wavelength, intensity and incommensurability with the decrease of temperature. The satellite reflections at temperatures above 200 K in general emerge as weak spots and under in situ cooling their intensity increases rapidly at the temperature just below 200 K. The incommensurate structural modulation undergoes a lock-in phase transition to a well-defined orthorhombic structure as demonstrated by the diffraction observations (see Fig. 1 (d). A structural model for this phase is proposed with Na ordering 13, in which one out of four Na1as well as Na2 sites are occupied and zig-zag Na chains are formed between two neighboring CoO2 layers. Figs. 3 (e) and (f) qualitatively illustrate the intensity and the incommensurability of modulation, q, as a function of the temperature obtained from several single crystal samples. These results directly suggest that the structural changes occur just below 200 K for the Na0.5CoO2 system.Raman scattering measurements on single crystalline samples of Na0.5CoO2 indicate that structural inhomogeneity commonly appears at room temperature and this result is consistent with our previous TEM data that showed the presence of phase separation in Na0.5CoO2 materials 12. Fig. 4 (a) shows three typical Raman spectra obtained from different areas on a single crystal at room temperature. The focus spot of the laser beam in our measurements is about 2-3 m in diameter. Notable distinctions among these Raman spectra from different areas can be clearly recognized in both peak positions and intensities: these data directly demonstrate the presence of structural inhomogeneity (phase separation) in the -Na0.5CoO2 single crystals. The top spectrum in Fig. 4 (a), containing five clear Raman peaks, can be easily matched to the hexagonal average structure as reported in ourprevious paper 20. These five peaks are assigned respectively as A1g at 681cm-1, E1g at 193cm-1,E2g at 476cm-1, 520cm-1, and 614 cm-1 for the space group of P63/mmc. The bottom spectrum in Fig. 4 (a) shows very different Raman active modes from that of the hexagonal phase, and contains three distinct phonon modes at 439, 476, and 573 cm-1. Careful analysis (see below) suggests that this spectrum is taken from an area dominated by the presence of an orthorhombic phase. The middle spectrum in Fig. 4 (a) shows recognizable features arising from the combination of the other two spectra. This last of spectrum is frequently observed in our experiments and is considered to originate from structural inhomogeneities characterized by an incommensurate structural modulation along the In accordance with previous study of the Na x CoO2 hexagonal average structure 20-21, the A1g and E1g modes only involve motions from oxygen atoms; the E1g mode is an in-plane oxygen mode with diagonal displacements while the A1g mode is an out-of-plane mode. The energy of A1g mode strongly depends on the Na site occupancy and on the Na occupation in the layer that divides the oxygen octahedral in the c-axis direction, Hence, the A1g mode is a sensitive factor in the analysis of Na distribution/ordering in Na x CoO2 system.In order to further understand the variation of the Raman active modes in relation tothe structural transitions in the high-temperature range, we have carried out in situ heating Raman scattering experiments on