Electrostatic energy of columbite/ixiolite

Abstract

A KNOWLEDGE of the crystal structure of an inorganic compound and the appropriate ionic charges is usually sufficient to allow the calculation of the electrostatic energy^1–3. A more difficult situation is found in crystals where some or all of the cations are disordered so that on the average a particular cation site is occupied by two or more cations with different charges. Very accurate crystal structure refinements, often including refinement of site occupancies, can give a good description of the nature of the disorder and the average composition of each cation site. The electrostatic energy can then be calculated using the average ionic charges on each of the disordered sites. Such an approach has been used to compare the stabilities of ordered and disordered structures of pyrrhotite^4,5 and some complex niobium and tantalum oxides⁶. There seems no theoretical justification for the use of average cation charges for the calculation of the potential energy of a disordered crystal so such calculations must be treated with caution.