Sodium ion ordering of Na0.77CoO2 under competing multi-vacancy cluster, superlattice and domain formation

TL;DR

This study investigates sodium vacancy ordering in Na0.77CoO2, revealing complex superlattice structures, vacancy cluster sizes, and competing electronic correlations through synchrotron X-ray diffraction analysis.

Contribution

It provides detailed insights into sodium vacancy cluster sizes, superlattice formations, and domain behaviors in Na0.77CoO2, advancing understanding of its structural and electronic properties.

Findings

Large superlattice of sqrt{19}a with 4.5 vacancies per cluster

Competition between different vacancy cluster superlattices

Observation of spin glass-like magnetic hysteresis below 3K

Abstract

Hexagonal superlattice formed by sodium multi-vacancy cluster ordering in Na$_{0.77}$CoO$_2$ has been proposed based on synchrotron X-ray Laue diffraction study on electrochemically fine-tuned single crystals. The title compound sits closely to the proposed lower end of the miscibility gap of x ~ 0.77-0.82 phase separated range. The average sodium vacancy cluster size is estimated to be 4.5 Na vacancies per layer within a large superlattice size of sqrt{19}a*sqrt{19}a*3c. The exceptionally large Na vacancy cluster size favors large twinned simple hexagonal superlattice of sqrt{19}a, in competition with the smaller di-, tri- and quadri-vacancy clusters formed superlattices of sqrt{12}a and sqrt{13}a. Competing electronic correlations are revealed by the observed spin glass-like magnetic hysteresis below ~ 3K and the twin, triple and mono domain transformations during thermal cycling between 273-373K.