59Co, 23Na NMR and electric field gradient calculations in the layered cobalt oxides NaCoO 2 and HCoO 2

Abstract

59Co and 23Na NMR has been applied to the layered cobalt oxides NaCoO 2 and HCoO 2 at three different magnetic field strengths (4.7, 7.1 and 11.7 T). The 59Co and 23Na quadrupole and anisotropic shift tensors have been determined by iterative fitting of the NMR line shapes at the three magnetic field strengths. Due to the large 59Co quadrupole interaction in NaCoO 2, a frequency-swept irradiation procedure was used to alleviate the limited bandwidth of the excitation. While the 59Co and 23Na shift and quadrupole coupling tensors in NaCoO 2 are found to be coincident and axially symmetric in agreement with the crystal symmetry requirements, the fits of the 59Co NMR spectra clearly show the presence of structural disorder in HCoO 2. The 23Na chemical shift anisotropy can be reproduced by shift tensor calculations using a point dipole model and considering that the magnetic susceptibility in NaCoO 2 is due to Van Vleck paramagnetism for Co 3+. Electric field gradient calculations using either the empirical point charge model or the ab initio full potential-linearized augmented plane wave method are compared with the experimental NMR data.