X-ray study and structure simulation of amorphous tungsten oxide.

In this work, X-ray studies of the amorphous oxide films obtained by thermal evaporation of WO(3) powder in a vacuum and by anodic oxidation were carried out. X-ray diffraction patterns were obtained in the symmetric reflection geometry on a DRON-4 diffractometer (Mo K(alpha) radiation, LiF monochromator) in automatic mode. Molecular dynamics simulation of amorphous tungsten oxide atomic configurations has been carried out in the micro-canonical ensemble (NVE) for N = 208 atoms and N = 624 atoms, in a cubic cell, using pairwise Born-Mayer interaction potentials and periodic boundary conditions. One of the purposes of the present work is to analyze the influence of the parameters and the cutoff of the interaction potentials on the interatomic distances. The values obtained in the molecular dynamics simulation for the pair functions D(r) are compared with the experimental data for amorphous oxides in order to choose the most convenient aforesaid values. The values of the average interatomic distances and the coordination numbers obtained by both methods are also compared. The result shows that the tungsten subsystem can be well reproduced using the potential cutoff radius of about 4 A, but the oxygen subsystem can be well reproduced when the cutoff of the potential for the W-O pairs is equal to 2.8 A. The configuration built during the molecular dynamics experiment consists of distorted octahedra. These octahedra form chains, as in the WO(3) phases of type ReO(3), and hexagonal rings, of the same type as in the WO(3)(1/3)H(2)O phase, when we extract (1/3)O at every WO(3) unit. The pair function D(r) and scattering intensity I(s) distribution curves calculated for simulation configurations show a satisfactory agreement with experiment.