23Na-NMR Study in NaV6O11

A mixed valent vanadium compound NaV 6 O 11 shows a magnetic phase transition at T t =245 K. For the microscopic investigation of the transition at T t , 23 Na-NMR measurements from 70 K to 300 K were carried out on NaV 6 O 11 . The Knight shift, K , and the nuclear spin-lattice relaxation rate, 1/ T 1 , were measured. While 1/ T 1 above T t has an almost constant value, 1/ T 1 below T t shows a gap-type temperature dependence. This is the first direct evidence that the d -electrons at V(1) ions, which form a kagomé lattice in NaV 6 O 11 , exhibit a spin-singlet ground state below T t . The formation of the spin-singlet ground state of V(1) seems to be closely related to the frustration effect.