Phonons as a Probe of the Low Temperature Metal Insulator Transition in Na.75Co.95Ni.05O2

TL;DR

This study uses phonon measurements to investigate how nickel substitution induces a metal-insulator transition in Na0.75CoO2, revealing sodium ion ordering as a key feature of the low-temperature insulating state.

Contribution

It provides experimental evidence linking phonon behavior and sodium ion ordering to the metal-insulator transition in nickel-doped sodium cobaltate.

Findings

Splitting of Co-O mode with decreasing temperature

Sodium mode shows dramatic changes in Ni-doped sample

Sodium ion ordering observed in low-temperature insulating state

Abstract

Nickel substitution at the cobalt site in NaxCoO2 induces a Metal Insulator Transition (MIT) and the temperature at which this occurs (TMIT) increases with Ni content. Low temperature far infrared measurements on polycrystalline samples of Na0.75CoO2 and Na0.75Co0.95Ni0.05O2 (TMIT = ~ 150K) were carried out to look for signatures of this transition. While both the samples show an evident splitting of the high frequency Co-O mode on lowering the temperature, dramatic changes are observed in the low frequency sodium mode in Na0.75Co0.95Ni0.05O2 clearly pointing out to an ordering of the sodium ions in the low temperature insulating state.