Searching for stable Na-ordered phases in single-crystal samples ofγ−NaxCoO2

Abstract

We report on the preparation and characterization of single crystal γ 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 xc ∼ 0.53, where the diffusion process above xc has a diffusion coefficient about five times larger than that below xc. 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.

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

“…In x = 0.71, the absence of the A-type magnetic ordering is also puzzling when taking into account that x = 0.71 is compositionally close to the magnetically ordered x = 0.75. 3,4,18,19 Moreover, x = 0.71 is the only material in the Curie-Weissmetal regime free from the A-type magnetic ordering, 3,4,18,19 and the existence of superlattice in x Ϸ 0.70 ͑close to x = 0.71 that we are considering͒ has also been suggested. 18,19 Indeed, it is generally believed that superlattices and their correlated sodium ordering should play an essential role in the various electronic ground states of the Curie-Weiss metal.…”

“…10 For 0.75Յ x Յ 0.84 in the Curie-Weiss metal regime, the onset of a long-range A-type antiferromagnetic ordering ͑Co-intraplane ferromagnetic coupling and Co-interplane antiferromagnetic coupling͒ at ϳ22 K has been indicated, [11][12][13] and the enhanced thermoelectric power due to large spin entropy has been found and explained for ϳ0.84. 14 Intriguingly, the other Curie-Weiss metal, x = 0.71, exhibits no magnetic ordering down to ϳ1.8 K. 3 The exactly half Na filled phase of x = 0.5 is the only compound that shows a metal-to-insulator transition ͑MIT͒ near ϳ51 K. 3,4,15,16 Indeed, the unique transport property for x = 0.5 had stimulated a remarkable number of associated studies, leading to the discovery that the room-temperature crystal structure of x = 0.5 shows an orthorhombic superlattice of ͱ 3a p ϫ 2a p ϫ c p ͑a p and c p , the hexagonal primitive cell parameters͒. 8,9,16,17 This orthorhombic supercell arises from the two-dimensional zigzag-chain ordering of Na1 and Na2 in the ab plane with the respective sodium sites fully occupied and is preserved down to 10 K without any further noticeable modifications.…”

“…17 The ab plane two-dimensional sodium ordering in the orthorhombic superlattice of x = 0.5 is, therefore, closely correlated with its various electronic instabilities. 3,4,17 For 0.75Յ x Յ 0.84 within the ionic context ͑charge localization 15 ͒, it has been suggested that the existence of A-type magnetic ordering near ϳ22 K is rather unexpectedly high considering the dilute spin-half Co 4+ in the lattices. 12,13 The inelastic neutron scattering further indicated that the ferromagnetic intraplane and antiferromagnetic interplane exchange constants are nearly comparable, whereas a much smaller interplane magnitude was expected due to the lamellar structure of ␥-Na x CoO 2 .…”

“…3,4 The most interesting material in the Pauli metal regime is x ϳ 1 / 3 that exhibits an unexpected superconductivity upon further intercalation with water. 10 For 0.75Յ x Յ 0.84 in the Curie-Weiss metal regime, the onset of a long-range A-type antiferromagnetic ordering ͑Co-intraplane ferromagnetic coupling and Co-interplane antiferromagnetic coupling͒ at ϳ22 K has been indicated, [11][12][13] and the enhanced thermoelectric power due to large spin entropy has been found and explained for ϳ0.84.…”

“…3,4 The Na ions in the structure are distinguished by two crystallographic sites, Na1 and Na2. In contrast, there is only one crystallographic site for the Co ions, which form a triangular sublattice in ab plane with the trigonal-coordinated oxygen above and below the Co planes.…”