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

TL;DR

This study investigates the phase diagram of Na$_x$CoO$_2$ for x > 0.71, revealing phase separation, staging phenomena, and magnetic ordering, and proposes a stacking model to explain these behaviors.

Contribution

It introduces a staging model based on ordered stacking of vacancy layers to explain phase separation and magnetic properties in Na$_x$CoO$_2$ for x > 0.71.

Findings

Phase separation occurs in narrow x ranges (0.76-0.82 and 0.83-0.86).

Specific compositions (x=0.820, 0.833, 0.859) show distinct magnetic ordering temperatures.

Staging model explains hysteresis and magnetic correlation length behaviors.

Abstract

Phase diagram of Na$_x$CoO$_2$ (x $\gtrsim$ 0.71) has been reinvestigated using electrochemically fine tuned single crystals. Both phase separation and staging phenomena as a result of sodium multi-vacancy cluster ordering have been found. Phase separation phenomenon is observed in the narrow ranges of 0.76 $\lesssim$ x $\lesssim$ 0.82 and 0.83 $\lesssim$ x $\lesssim$ 0.86. While x = 0.820 shows A-type antiferromagnetic (A-AF) ordering below 22K, x = 0.833 is confirmed to have a magnetic ground state of A-AF ordering below $\sim$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 29K. Staging model based on ordered stacking of multi-vacancy layers is proposed to explain the hysteretic behavior and A-AF correlation length for x $\sim$ 0.82-0.86.