Raman spectroscopy study of sodium–lithium cobaltite

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Abstract

We report on a Raman scattering study of the LiCoO2, γ-Na0.7CoO2 and Li0.42Na0.36CoO2 phases. The Raman spectrum of the ordered cobaltite Li0.42Na0.36CoO2 consists of a set of seven distinct peaks with the wave numbers around 190, 445, 467, 513, 572, 609 and 674 cm−1. It exhibits features of the spectra for LiCoO2 and γ-Na0.7CoO2 and therefore can be considered as a superposition of these two spectra. It may be supposed that in the structure of the double cobaltite the bonding between sodium- and lithium-containing blocks is rather weak.

Graphical abstract

The Raman spectrum measured for the lithium-sodium cobaltite Li0.42Na0.36 CoO2 can be considered as a superposition of spectra for LiCoO2 and γ-Na0.7CoO2 phases.

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Introduction

The layered cobaltites of alkaline metals AxCoO2 (A – Li, Na) attract much attention first of all because their diverse and sometimes unique properties can be used in practice. So, the lithium cobaltite LiCoO2, which possesses high stability of crystal structure to reversible deintercalation of alkaline metal, is used in electrochemical devices as an efficient cathode material [1]. Combination of high thermoelectric power and low electrical resistivity characteristic of NaxCoO2 makes sodium cobaltites promising thermoelectric materials [2], [3], [4]. Besides, the discovery of superconductivity at 4.5 K in the hydrated sodium cobaltite with the composition Na0.3CoO2·1.4H2O [5] has generated significant interest in the research community. Both magnetic and metal–insulator transitions [6], [7], [8], [9], [10], [11] as well as charge ordering [12], [13], [14] and phase segregation effects [15], [16], [17] in NaxCoO2 and LixCoO2 have been extensively examined too. The majority of authors explain the unusual combination of properties of these materials by strongly correlated behavior of 3d electrons [18], [19], [20] and by the 2D character of exchange interactions [21] caused by the cobaltite laminated structure.

All alkaline metal (A) cobaltites AxCoO2 crystallize in a layered structure, in which A+ triangular planes are positioned between edge-sharing CoO6 octahedra layers [22], but individual phases have a different stacking of alternating layers. As a result, only octahedral oxygen environment is observed for lithium ions [23], whereas sodium ions are more often found within trigonal oxygen prisms [24], [25], [26]. Obviously, that is why LiCoO2 and NaxCoO2 cobaltites do not form solid solutions in spite of the similarity of their structures.

However, reported by Balsys and Davis [27], the ordered phase Li0.43Na0.36CoO1.96 (space group P63cmc; a = 2.839(4) Ǻ; c = 2.36(3) Ǻ) forms in the NaxCoO2–LixCoO2 system near the equiatomic concentration of alkaline metals. In the structure of the new phase, sodium ions are located within trigonal prismatic oxygen environment, whereas lithium ions occupy octahedral positions. The alternating lithium and sodium layers are intercalated between octahedral sheets of CoO2 layers in the same way as in the end members of the considered system. It should be mentioned that the formation of a long-period ordered phase is an exceptional case for the AMeO2-based structures. In addition, it was established that the lithium–sodium intergrowth cobaltite LixNayCoO2 (x  y  0.5) possesses high thermoelectric properties [28]. Furthermore, data on magnetic transitions in these compounds are available [29]. This makes the double lithium–sodium cobaltite an extremely interesting object for research. In this Letter we report for the first time the results of Raman scattering study.

Section snippets

Synthesis of materials and experimental methods

LiCoO2 and γ-Na0.7CoO2 used as parent compounds were synthesized as described previously [30]. The unit-cell parameters were a = 2.817(2) Å, с = 14.052(3) Å, and a = 2.821(1) Å, с = 10.92(3) Å for LiCoO2 and Na0.7CoO2, respectively. An appropriate mixture of LiCoO2 and Na0.7CoO2 (1:1) was pressed in pellets, annealed in oxygen flow at 1050 °C for 8 h and quenched onto a thick copper plate.

Powder X-ray diffraction measurements showed that the obtained sample was a single phase. The hexagonal structure of the

Results and discussion

Fig. 1 shows the Raman spectrum of the double cobaltite Li0.42Na0.36CoO2, which consists of a set of seven distinct peaks with the wave numbers around 190, 445, 467, 513, 572, 609 and 674 cm−1. According to the classical group factor analysis carried out for the P63mc (No. 186) space group with the use of software [31], there are 27 Raman active phonon modes described as A1 + E2 + E1. This does not allow us to attribute the observed bands to the vibrational modes unambiguously. Table 1 lists the

Conclusion

In may be concluded from comparison of Raman spectra of Li0.42Na0.36CoO2 with those of γ-Na0.7CoO2 and LiCoO2 that the ordered long-period phase under consideration contains weakly coupled ‘γ-Na0.7CoO2’ and ‘LiCoO2’ blocks. It should be noted that compounds with such ‘nanoblock’ structures show much promise as thermoelectric materials since they contain blocks with different properties. Such compounds may possess weak thermal conduction like a glass and good electric conduction like a crystal.

Acknowledgement

The present authors acknowledge the financial support of Russian Foundation for Basic Research (Grant No. 10-03-00203-a).

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