Metal--insulator transition in Ni-doped Na_0.75CoO₂:

Insights from infrared studies

M PREMILA, A BHARATHI, N GAYATHRI, P YASODHA,

Y HARIHARAN and C S SUNDAR^*

Materials Science Division, Indira Gandhi Centre for Atomic Research,

Kalpakkam 603 102, India

^*E-mail: css@igcar.gov.in

Abstract. Nickel substitution at the cobalt site in Na$_{0.75}$CoO$_2$

induces an upturn in the resistivity on lowering the temperature, with the

metal-to-insulator transition temperature ($T_{\rm MIT}$) increasing with

the Ni content. Low temperature far infrared measurements on

polycrystalline samples of Na$_{0.75}$CoO$_2$ and

Na$_{0.75}$Co$_{0.95}$Ni$_{0.05}$O$_2$, the latter having $T_{\rm MIT} \sim

175$ K, have been carried out. Dramatic changes in the Na mode

frequencies, and relative intensities of the out-of-plane modes

corresponding to the two Na sites are observed, coincident with the MIT in

Na$_{0.75}$Co$_{0.95}$Ni$_{0.05}$O$_2$. It is argued that these changes are

associated with a charge ordering of the CoO$_2$ layer, associated with

the metal--insulator transition.

Keywords. Sodium cobaltate; metal--insulator transition; charge

ordering; sodium modes; infrared spectroscopy.

PACS Nos 71.27.+a; 74.25.Fy; 78.30.-j; 71.30.+h