title: Nonmagnetic insulator state in Na1CoO2 and phase separation of na vacancies.
abstract: Crystallographic, magnetic, and NMR properties of a Na1CoO2 single crystal with x approximately = 1 are presented. We identify the stoichiometric Na1CoO2 phase, which is shown to be a nonmagnetic 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 approximately = 0.8 and the occurrence of a metal-insulator transition for x --> 1.
authors.affiliation: Laboratoire de Spectrométrie Physique, Université J. Fourier & UMR5588 CNRS, Saint Martin d'Hères, France.
authors.forename: C
authors.initials: C
authors.lastname: de Vaulx
authors.forename: M-H
authors.initials: MH
authors.lastname: Julien
authors.forename: C
authors.initials: C
authors.lastname: Berthier
authors.forename: M
authors.initials: M
authors.lastname: Horvatić
authors.forename: P
authors.initials: P
authors.lastname: Bordet
authors.forename: V
authors.initials: V
authors.lastname: Simonet
authors.forename: D P
authors.initials: DP
authors.lastname: Chen
authors.forename: C T
authors.initials: CT
authors.lastname: Lin