NMR Studies of Successive Phase Transitions in Na0.5CoO2and K0.5CoO2

Abstract

59 Co- and 23 Na-NMR measurements have been carried out on polycrystalline and c -axis aligned samples of Na 0.5 CoO 2 , which exhibits successive transitions at temperatures T = 87 K (= T c1 ) and T = 53 K (= T c2 ). 59 Co-NMR has also been carried out on c -axis aligned crystallites of K 0.5 CoO 2 with similar successive transitions at T c1 ∼60 K and T c2 ∼20 K. For Na 0.5 CoO 2 , two sets of three NMR lines of 23 Na nuclei, explained by considering the quadrupolar frequencies ν Q ∼1.32 and 1.40 MHz, have been observed above T c1 , as is expected from the crystalline structure. A rather complicated but characteristic variation of the 23 Na-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 T c1 and the 59 1/ T 1 – T curves of Na 0.5 CoO 2 and K 0.5 CoO 2 are also discusse...