Non-magnetic insulator state in Na1CoO2 and phase separation of Na vacancies

TL;DR

This study characterizes Na1CoO2 as a non-magnetic insulator and reveals phase separation due to Na vacancies, affecting magnetic properties and metal-insulator transitions in NaxCoO2.

Contribution

It identifies the non-magnetic insulating Na1CoO2 phase and demonstrates phase separation caused by Na vacancies in NaxCoO2.

Findings

Na1CoO2 is a non-magnetic insulator.

Na vacancies lead to phase separation into magnetic and non-magnetic regions.

Phase separation influences magnetic order and metal-insulator transition.

Abstract

Crystallographic, magnetic and NMR properties of a NaxCoO2 single crystal with x~1 are presented. We identify the stoichiometric Na1CoO2 phase, which is shown to be a non-magnetic insulator, as expected for homogeneous planes of Co3+ ions with S=0. In addition, we present evidence that, because of slight average Na deficiency, chemical and electronic phase separation leads to a segregation of Na vacancies into the well-defined, magnetic, Na0.8CoO2 phase. The importance of phase separation is discussed in the context of magnetic order for x~0.8 and the occurrence of a metal-insulator transition for x->1.