51V-NMR study of charge order induced by cation order in δ-Ag2/3V2O5

51V-NMR experiments have been performed to investigate the local magnetic and electronic properties of the mixed-valence oxide δ-Ag2/3V2O5, which shows the novel V4+/V5+ charge ordering trigged by Ag ion ordering. We have observed the abrupt loss of 51V-NMR signal coming from the non-magnetic V5+-like ions above Tc = 225 K. It indicates that the majority of V ions are magnetic above Tc and that the phase transition is accompanied by the charge separation and the charge ordering of 3d electrons on V sites. In the low-temperature phase below Tc, the nuclear spin-lattice relaxation rate 1/T1 shows the thermal activation-type temperature dependence with the activation energy of about 130 K. It has been clarified that the ground state of this material is the charge ordered one with the spin singlet of 3d electrons on V4+-V4+ pair from a microscopic point of view.