Solution of the p(2 x 2) NiO(111) surface structure using direct methods

A solution for the p(2 x 2) NiO(1 1 1) surface reconstruction was obtained using direct methods applied to X-ray diffraction data. The solution was re fined with 296 data points and 21 parameters using chi (2) minimization (ch i (2) = 1.82, R = 0.17). The surface atoms showed very small relaxation fro m the bulk interatomic distances (Ni-Ni distances are 2.9 +/- 0.1 Angstrom; Ni-O -2.0 +/- 0.1 Angstrom). The solution can be characterized by alternat ing close-packed layers of oxygen and nickel atoms: the top surface layer i s nickel terminated with 3/4 of the nickel atoms missing, the next oxygen l ayer is completely full, and the third, nickel layer, has 1/4 of the nickel atoms missing. The structure is consistent with theoretical predictions of octopolar termination of the surface and exhibits the features observed by previous STM studies. In addition, local density functional calculations h ave been carried out in this work in order to gain insights into the surfac e charge distribution and electronic structure of the proposed reconstructi on. Calculated partial atomic charges and magnetic moments as well as densi ties of state are reported. The cation deficient nature of the surface requ ires the presence of electron holes for charge compensation, which we find mainly located on second layer oxygen atoms. The structure differs from tha t recently reported for the same surface, and we are not able to reproduce the reported good fit to the (same) experimental data. (C) 2000 Elsevier Sc ience B.V. All rights reserved.