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59Co- and 23Na-NMR measurements have been carried out on polycrystalline andc-axis aligned samples of Na0.5CoO2, which exhibits successive transitions attemperatures T = 87 K (= Tc1) and T = 53 K (= Tc2). 59Co-NMR has also beencarried out on c-axis aligned crystallites of K0.5CoO2 with similar successivetransitions at Tc1 ~ 60 K and Tc2 ~ 20 K. For Na0.5CoO2, two sets of three NMRlines 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 thecrystalline structure. Rather complicated but characteristic variation of the23Na-NMR spectra has been observed with varying T through the transitiontemperatures, and the internal fields at two crystallographically distinct Nasites are discussed on the basis of the magnetic structures reportedpreviously. The internal fields at two distinct Co sites observed below Tc1 andthe 591/T1-T curves of Na0.5CoO2 and K0.5CoO2 are also discussed in acomparative way.

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