A geometric rationale for hydrogenated Pd 3P 1− x

Abstract

Hole sizes and intersite distances are calculated for all the types of interstices in Pd 3P 1− x H n which has the structure of cementite (Fe 3C) and belongs to space group Pnma. In Pd 3P 0.80D 0.15 hole radii range from 0.87 to 0.43 Å for the following interstices in descending order: 4c (phosphorus vacancies in triangular prisms); 8d(1) (distorted square pyramids); 4c(1) (distorted square pyramids); 4a (distorted octahedra); 4c(2), 8d(2) and 8d(3) (tetrahedra). A repulsive interaction between hydrogen and phosphorus atoms has been invoked in order to rationalize the observed failure of hydrogen to occupy any of these large interstices except 8d(1) which is associated with a vacant phosphorus site. Hydrogen occupation of one 8d(1) site blocks its “sister” site which is at a distance of only 0.84 Å and is located symmetrically at the other side of the same vacant phosphorus site. No other 8d(1) site can block that same site, however, because of the great distance (almost 3.3 Å) to the next-nearest neighbor. For this reason, a model based on blocking with no overlap can be used to express the relationship between equilibrium hydrogen pressure and hydrogen concentration for any value of x consistent with stability of the structure.