X-ray and electron diffraction studies of superlattices and long-range three-dimensional Na ordering inγ-NaxCoO2(x=

Abstract

We have recently demonstrated that x = 0.71 and 0.84 are the two most stable single-phase compounds above x = 0.5 in ␥-Na x CoO 2 ͓G. J. Shu et al., Phys. Rev. B 76, 184115 ͑2007͒; F. C. Chou et al., Phys. Rev. Lett. 101, 127404 ͑2008͔͒, and this structural investigation was performed on the single crystals and pulverized samples elaborately synthesized therein. Using the complementary techniques of x-ray and electron diffractions, we unambiguously established the existence of superlattices in x = 0.71 and 0.84, ͱ 12a p ϫ ͱ 12a p ϫ 3c p , and ͱ 13a p ϫ ͱ 13a p ϫ 3c p ͑a p and c p , the hexagonal primitive cell parameters͒, respectively. The exceptionally large superlattice for x = 0.71 arises from the long-range three-dimensional sodium ordering, consisting of the spirallike sodium-trimer chain screwing along c axis and being decorated with alternating truncated-triangle and honeycomb sodium sublattices in ab plane. The trimers in the neighboring sodium planes show corner-sharedlike characteristics along the chain direction. A larger interplane separation of the trimers that was expected to minimize the trimer interlayer Coulomb repulsion was, however, not observed, and all Co ions in the superlattice show similar charge characteristics. In x = 0.84, the large superlattice is rather a result of the long-range ordering of sodium monomers both in ab plane and along c axis. The differences in the superlattice structure and the corresponding long-range sodium ordering between x = 0.71 and 0.84 may provide the critical information in understanding their distinctly different physical properties from structural aspects.

Diffusion mechanism in the sodium-ion battery material sodium cobaltate

“…The low-temperature superstructure was obtained via a model-independent reverse Monte Carlo fit to the superlattice reflections. An elegant alternative model based on di-vacancy clusters and with different periodicity 37 can be ruled out by the data for this composition. Furthermore, the formation of tri-vacancy clusters at high x is supported by first-principles computer simulations 24 .…”

59Co NMR evidence for charge and orbital order in the kagome-like structure of Na2/3CoOMukhamedshin1, Alloul2 2011Phys. Rev. B194290View full textAdd to dashboardCite

“…7 In the cobaltates, NMR experiments and structural investigations have given evidence that for x 0.5, a large interplay occurs between atomic arrangements and electronic properties, as the Na atoms are found to be ordered. [8][9][10][11][12][13] It has been shown earlier that this ordering is associated quite systematically with cobalt charge disproportionation into 3 + and ≈3.5 + Co charge states. 10,[14][15][16] As the cobalt ions in sodium cobaltates are in low-spin configurations, the Co 3+ has an electronic spin S = 0 and appears to be inert magnetically, when compared to the other Co sites with higher-charge state on which holes delocalize (formally Co 4+ should have S = 1/2).…”

1D to 2DNa+Ion Diffusion Inherently Linked to Structural Transitions inNa0.7CoO2

“…The corresponding supercells have been reported to be commensurate with the underlying P6 3 =mmc lattice, which according to Refs. [8,14,15] remains metrically hexagonal for T ¼ 1:5-350 K.…”

“…For this purpose, we have chosen Na 0:7 CoO 2 , whose Na-vacancy ordered array is amongst the most stable of the Na x CoO 2 family [12]. The symmetry of this pattern has been investigated in the past [8,[13][14][15], but the actual periodicity as well as the size and arrangement of Na-vacancy clusters are still a subject of debate. The temperature dependence of this array has been much less investigated, but a few groups have predicted [16] as well as reported [11,14,17] the existence of a first-order transition at T 1 $290K.…”