59Co, 23Na NMR and electric field gradient calculations in the layered cobalt oxides NaCoO2 and HCoO2

Co-59 and Na-23 NMR has been applied to the layered cobalt oxides NaCoO2 and HCoO2 at three different magnetic field strengths (4.7, 7.1 and 11.7T). The Co-59 and Na-23 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 Co-59 quadrupole interaction in NaCoO2 a freqUency-swept irradiation procedure was used to alleviate the limited bandwidth of the excitation. While the Co-59 and Na-23 shift and quadrupole coupling tensors in NaCoO2 are found to be coincident and axially symmetric in agreement with the crystal symmetry requirements, the fits of the Co-59 NMR spectra clearly show the presence of structural disorder in FlCoO. The Na-23 chemical shift anisotropy can be reproduced by shift tensor calculations using a point dipole model and considering that the magnetic susceptibility in NaCoO2 is due to Van Vleck paramagnetism for Co3+. 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. (C) 2003 Elsevier Science (USA). All rights reserved.