Staging model of the ordered stacking of vacancy layers and phase separation in layered Nax CoO2 (x0.71) single crystals

Phase diagram of Nax CoO2 (x0.71) 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 0.76x0.82 and 0.83x0.86. 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 ∼8K 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.