Magnetic transition in Na0.5CoO2 at 88 K

Abstract Previous investigations of the compound Na 0.5 CoO 2 gave evidence for a metal–insulator transition at T MI = 53 K upon cooling, and another transition at T X = 88 K whose nature has not been clarified yet. We report the results of NMR measurements on a powder sample of Na 0.5 CoO 2 . They include the mapping of 23 Na and 59 Co spectra and the evaluation of the 23 Na NMR spin-lattice relaxation rate T 1 - 1 ( T ) in the temperature range between 10 and 305 K. The NMR data reflect the transition at T X very well but give little evidence of the metal–insulator transition at T MI . A prominent peak in T 1 - 1 ( T ) at T X is accompanied by an abrupt broadening of both the 23 Na and 59 Co spectra upon decreasing temperature. The shape of the spectra below T X implies the formation of a staggered internal field below T X .