Searching for Stable Na-ordered Phases in Single Crystal Samples of gamma-NaxCoO2

Abstract

We report on the preparation and characterization of single crystal gamma phase NaxCoO2 with 0.25 < x < 0.84 using a non-aqueous electrochemical chronoamperemetry technique. By carefully mapping the overpotential versus x (for x < 0.84), we find six distinct stable phases with Na levels corresponding to x ~ 0.75, 0.71, 0.50, 0.43, 0.33 and 0.25. The composition with x ~0.55 appears to have a critical Na concentration which separates samples with different magnetic behavior as well as different Na ion diffusion mechanisms. Chemical analysis of an aged crystal reveals different Na ion diffusion mechanisms above and below x_c ~ 0.53, where the diffusion process above x_c has a diffusion coefficient about five times larger than that below x_c. The series of crystals were studied with X-ray diffraction, susceptibility, and transport measurements. The crystal with x = 0.5 shows a weak ferromagnetic transition below T=27 K in addition to the usual transitions at T = 51 K and 88 K. The resistivity of the Curie-Weiss metallic Na0.71CoO2 composition has a very low residual resistivity, which attests to the high homogeneity of the crystals prepared by this improved electrochemical method. Our results on the various stable crystal compositions point to the importance of Na ion ordering across the phase diagram.

9 pages, 9 figures

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Cited by in corpus (22)

• Electronic phase diagram of the layered cobalt oxide system, LixCoO2 (0.0 <= x <= 1.0)
• Sodium vacancy ordering and the co-existence of localized spins and itinerant charges in NaxCoO2
• Spin correlations and cobalt charge states: A new phase diagram of sodium cobaltates
• Na ordering imprints a metallic kagome lattice onto the Co planes of Na2/3CoO2
• Local moment, itinerancy and deviation from Fermi liquid behavior in Na${}_x$CoO${}_2$ for $0.71\le x\le 0.84$
• Impact of lithium composition on the thermoelectric properties of the layered cobalt oxide system LixCoO2
• ^{59}Co NMR evidence for charge ordering below T_{CO}\sim 51 K in Na_{0.5}CoO_2
• 59Co NMR evidence for charge and orbital order in the kagome like structure of Na2/3CoO2
• Electronic phase diagram of LixCoO2 revisited with potentiostatically de-intercalated single crystals
• Strong correlations enhanced by charge-ordering in highly doped cobaltates
• Sodium Trimer Ordering on NaxCoO2 Surface
• Oxygen non-stoichiometry and the origin of Na ion ordering in P2-NaxCoO2
• Electron-phonon interaction in the lamellar cobaltate Na${}_x$CoO${}_2$
• Considerable non-local electronic correlations in strongly doped Na${}_x$CoO${}_2$
• Synthesis of sodium cobaltate Na${}_x$CoO${}_2$ single crystals with controlled Na ordering
• Phase segregation in NaxCoO2 for large Na contents
• Revisiting the crystal electric field of P2-NaxCoO2 for the intermediate spin state of Co3+
• Exploring the possibility of enhancing the figure-of-merit ( $>$ 2) of Na${}_{0.74}$CoO${}_2$: A combined experimental and theoretical study
• Superconducting to spin glass state transformation in ß-pyrochlore KxOs2O6
• Transition from a phase-segregated state to single-phase incommensurate sodium ordering in Na_xCoO_2 with x \approx 0.53
• Enhanced low energy fluctuations and increasing out-of-plane coherence in vacancy ordered NaxCoO2
• Sodium ion ordering of Na0.77CoO2 under competing multi-vacancy cluster, superlattice and domain formation