X-ray and electron diffraction studies of superlattices and long-range three-dimensional Na ordering

Citation Huang, F.-T. et al. " X-ray and electron diffraction studies of superlattices and long-range three-dimensional Na ordering in gamma-Na[subscript x] CoO[subscript 2] (x=0.71 and 0.84). Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. 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. 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 spiral-like 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-shared-like 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 super-lattice 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.