As Single Crystals*

We present our NMR study of over-doped pnictide NaFe_1-xCu_xAs, which has been demonstrated to be a possible Mott insulator near x ≈ 0.5 [1,2]. Our NMR spectra show two inequivalent ²³Na sites due to Cu dopants. Both ²³Na and ⁷⁵As NMR spectra show a possible formation of stripe-like ordering of Fe and Cu in the 35% and 50% Cu-doped samples. The large Curie-Weiss-like magnetization due to impurity effects results in strongly inhomogeneous field distribution through out the heavily Cu-doped samples, significantly broadening both ²³Na and ⁷⁵As spectra. Our ²³Na spin-lattice relaxation data shows an enhanced relaxation rate in regions close to Cu dopants.