Na Nmr Study of Layered Superconductor Na X Coo 2 · Yh 2 O

We measured 23 Na-NMR spectra and nuclear spin-lattice relaxation rates 23 (1/T1) of superconducting and non-superconducting bilayer hydrate NaxCoO2 · yH2O (x ∼ 0.3, y ∼ 1.3). The central resonance frequency shows a small but various shift due to the difference in the shielding effect by intercalated H2O molecules. The different shielding effect also gives a large difference in the magnitude of 23 (1/T1). Recently, a number of investigations for the super-conductivity in bilayer hydrate (BLH) sodium cobalt oxide NaxCoO2 · yH2O (x ∼ 0.3, y ∼ 1.3) have been done. Different properties were reported especially in NMR studies for different samples. In a report of chemical study, it is emphasized that some Na + ions are exchanged by H3O + ions during intercalation of H2O molecules[1]. Because of the nonstoichiometric properties in BLH, i. e. ambiguities of the amount of Na + , H2O, and H3O + , similar samples are hardly obtained. More chemical studies are needed to clarify the origin of the magnetism and the superconductivity of BLH. We prepared two BLH samples by following processes[2]. Powders of Co3O4 and Na2CO3 were well mixed and heated at 800 • C for 20 hours in O2 atmosphere. The obtained powder sample was characterized as in a single phase of Na0.7CoO2 by powder X-ray diffraction (XRD) measurements. Then Na0.7CoO2 was immersed in Br2/CH3CN solution for several days to deintercalate Na ions. The dried powder was immersed in distilled water for 1 day. The water-intercalated powder was put in a sealed chamber filled with 75 % of relative humidity (RH). We prepared several samples by changing the duration kept in the chamber, and frozen at about-10 • C for quenching further change. A sample named BLH1 was kept for one week in 75 % RH atmosphere, and BLH2 for 1 month. Another two samples named BLHs1 and BLHs2 were synthesized by similar process, but not at the same time with BLH1 and BLH2. BLHs1 has already been studied in [3]. BLHs2 was a derivative of BLHs1 after about 1 year in the 75 % RH atmosphere before frozen. Each sample was characterized to be in a single phase of the BLH by powder XRD result. The lattice constant c of BLHs1 and BLHs2 estimated by the XRD results are smaller than that of BLH1 and BLH2. This difference in c indicates that the content of H3O + is different between …