CHARGE-DISTRIBUTION AND ELECTRIC-FIELD GRADIENTS IN YBA2CU3O7-X

The electric-field gradients (EFGs) of YBa2Cu3O7, YBa2Cu3O6.5, and YBa2Cu3O6 are calculated on a first-principles basis using the full-potential linear augmented-plane-wave (LAPW) method in which exchange and correlation effects are treated by the local-density approximation (LDA). Good agreement with experimental EFGs and their anisotropies is found for the Cu(1) position in all three compounds. For YBa2Cu3O7 the same is true for all oxygen positions, while at Cu(2) the direction of the EFG is predicted correctly, but a transfer of 0.07 electrons from dx2-y2 to dz2 symmetry would be needed to bring the theoretical EFG into agreement with the experimental value. The EFG calculations on YBa2Cu3O6 and YBa2Cu3O6.5 [assuming an ordered structure in which Cu(1) is threefold coordinated by oxygens] confirm the experimental assignment and strengthen the confidence in our results. The origin of the EFG is discussed and the relation to the anisotropy of the electronic charge distribution is illustrated in connection with symmetry-decomposed partial charges and difference-electron-density maps. It is shown that the LDA calculations yield reliable charge distributions to which the EFG is so sensitive. © 1990 The American Physical Society.