NMR studies of Successive Phase Transitions in Na0.5CoO2 and K0.5CoO2

TL;DR

This study uses NMR techniques to investigate the successive magnetic and structural phase transitions in Na0.5CoO2 and K0.5CoO2, revealing detailed changes in local magnetic environments and internal fields across transition temperatures.

Contribution

It provides detailed NMR analysis of the successive phase transitions in Na0.5CoO2 and K0.5CoO2, elucidating the local magnetic structures and internal fields associated with these transitions.

Findings

Observation of two sets of 23Na-NMR lines indicating quadrupolar frequencies.

Characteristic variation of NMR spectra through transition temperatures.

Comparison of internal fields and relaxation curves between Na0.5CoO2 and K0.5CoO2.

Abstract

59Co- and 23Na-NMR measurements have been carried out on polycrystalline and c-axis aligned samples of Na0.5CoO2, which exhibits successive transitions at temperatures T = 87 K (= Tc1) and T = 53 K (= Tc2). 59Co-NMR has also been carried out on c-axis aligned crystallites of K0.5CoO2 with similar successive transitions at Tc1 ~ 60 K and Tc2 ~ 20 K. For Na0.5CoO2, two sets of three NMR lines of 23Na nuclei explained by considering the quadrupolar frequencies nuQ ~1.32 and 1.40 MHz have been observed above Tc1, as is expected from the crystalline structure. Rather complicated but characteristic variation of the 23Na-NMR spectra has been observed with varying T through the transition temperatures, and the internal fields at two crystallographically distinct Na sites are discussed on the basis of the magnetic structures reported previously. The internal fields at two distinct Co sites observed below Tc1 and the 591/T1-T curves of Na0.5CoO2 and K0.5CoO2 are also discussed in a comparative way.