Characterization of the spin gap nature in Na3Cu2SbO6 using 23Na NMR

► NMR results provide microscopic evidence for the existence of a spin gap in Na3Cu2SbO6. ► The NMR shift data can be fitted well to the AF-F alternating chain model. ► Analysis of the NMR spin-lattice relaxation rate yields a spin gap of 105 K.