Staging model of the ordered stacking of vacancy layers and phase separation in layeredNaxCoO2(x

Abstract

Phase diagram of Na x CoO 2 ͑x տ 0.71͒ has been reinvestigated using electrochemically fine-tuned single crystals. Both phase-separation and staging phenomena as a result of sodium multivacancy cluster ordering have been found. Phase-separation phenomenon is observed in the narrow ranges of 0.76Շ x Շ 0.82 and 0.83Շ x Շ 0.86. While x = 0.820 shows A-type antiferromagnetic ͑A-AF͒ ordering below 22 K, x = 0.833 is confirmed to have a magnetic ground state of A-AF ordering below ϳ8 K and is only reachable through slow cooling. In addition, x = 0.859 is found to be responsible for the highest A-AF transition temperature at about 29 K. Staging model based on ordered stacking of multivacancy layers is proposed to explain the hysteretic behavior and A-AF correlation length for x ϳ0.82-0.86.

Interplay between magnetism and sodium vacancy ordering inNaxCoO2

“…In particular the tri-vacancy layers follow the periodicity of the crystal (red and green arrows). On the other hand the location of the di-vacancy peaks suggests an additional modulation tripling the original unit cell height in accordance with previous findings (blue arrows) [10]. Sodium superstructures for x=0.75 are slightly different: there is no detectable change on crossing 290 K, possibly due to resolution limitations of our setup: the crystallographic transition at 290K is very subtle [14], thus the possible superstructure modification is expected to be miniscule.…”

“…A central question in the discussion of Na x CoO 2 is weather all these Fermi surface instabilities originate from the change of electron count only or whether, for a given value of x, the geometric arrangement of Na ions plays an essential role for electronic ordering in the CoO 2 layers [9][10][11].…”

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

“…23 This latter estimate of Na content introduced some confusion in the scientific community and required considering complicated staging structures with different Na contents on alternating planes, a situation which has been therefore proposed for most Na concentrations. 23 Here we present experimental work that resolves these data differences but also allows us to characterize the in plane Na atomic order in this T N = 22 K phase. We could synthesize both a single phase powder sample of this phase that has been aligned in the NMR field, and single crystals in which this phase could be isolated.…”

Floating-zone growth of Na0.8−yAyCoO2 (A=Ca, Sr) single crystals

“…Diffraction studies reveal complex long-range Na-ion superstructures, which re-order on cooling, and there is evidence that the nature of the Na order influences the magnetic order and charge distribution on the CoO 2 layers. [10][11][12][13][14][15]. However, at temperatures appropriate for practical applications (above room temperature) Na may escape from the layers and create vacancies.…”