Staging model of the ordered stacking of vacancy layers and phase separation in layered NaxCoO2 (x≳0.71) single crystals

Abstract

Phase diagram of Na[subscript x]CoO[subscript 2](x≳0.71) 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 0.76≲x≲0.82 and 0.83≲x≲0.86. While x=0.820 shows A-type antiferromagnetic (A-AF) ordering below 22 K, x=0.833 is confirmed to have a magnetic ground state of A-AF ordering below ∼8 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 ∼0.82–0.86.