23Na NMR study of sodium order in NaxCoO2with 22 K Néel temperature

Abstract

We report a systematic study of the c lattice parameter in the NaxCoO2 phases versus Na content x > 0.5, in which sodium always displays ordered arrangements. This allows us to single out the first phase which exhibits an antiferromagnetic order at a Néel temperature TN =22 K which is found to occur for x ≈ 0.77(1). Pure samples of this phase have been studied both as aligned powders and single crystals. They exhibit identical 23 Na NMR spectra in which three sets of Na sites could be fully resolved, and are found to display T dependencies of their NMR shifts which scale with each other. This allows us to establish that the T variation of the shifts is due to the paramagnetism of the Co sites with formal charge state larger than 3 + . The existence of a sodium site with axial charge symmetry and the intensity ratio between the sets of 23 Na lines permits us to reveal that the two-dimensional structure of the Na order corresponds to 10 Na sites on top of a 13 Co sites unit cell, that is with x = 10/13 ≈ 0.77. This structure fits with that determined from local density calculations and involves triangles of 3 Na sites located on top of Co sites (so called Na1 sites). The associated ordering of the Na vacancies is quite distinct from that found for x < 0.75.

Evolution of Co charge disproportionation with Na order inNaxCoO2

“…Similarly for the x = 0.77 phases we calculated ν latt Q for different stacking along c-axis of 2D Na order pattern established in Ref. 16 Despite the uncertainty in the 3D stacking the trend in the variation of ν latt Q (x) remains similar.…”

“…The detailed description of the preparation methods and main characteristics of the powder sample and single crystal SC1 have been given in Ref. 16. Also a series of sodium cobaltates Na x CoO 2 with x ≈ 0.8 were grown independently by the floating zone technique in Kazan Federal University.…”

Pinball liquid phase from Hund's coupling in frustrated transition-metal oxides

“…Geometrical frustration can play a similar role in charge-ordering phenomena, leading to puzzling unconventional metallic and superconducting states. Remarkable examples are found in the quarter-filled organic salts θ -(BEDT-TTF) 2 X [2,3], the layered cobaltates Na x CoO 2 [4][5][6][7], and the transition-metal dichalcogenide 1T -TaS 2 [8].…”

Multiorbital kinetic effects on charge ordering of frustrated electrons on the triangular lattice

“…The second type of multiband effect, which can also lead to novel and original properties, is of kinetic origin and has to do with the microscopic form and symmetry of the interatomic electron transfers. One remarkable example is the hidden kagome symmetry and flat bands which have been pointed out in the layered cobaltates 25 , and which could be related with the experimentally observed Kagome order in these compounds 14,26 . The purpose of this paper is to study how such multiorbital kinetic effects influence the frustrated charge order on the triangular lattice.…”

“…These include transition-metal oxides such as the layered cobaltates Na x CoO 2 , which exhibit complex electronic patterns which can be tuned by electron doping [11][12][13][14] , and the triangular nickelates AgNiO 2 15,16 and Ag 2 NiO 2 17 , which show a threefold ordered metallic phase with anomalous metallic properties. Another interesting class is that of transition-metal dichalcogenides.…”