The crystal chemistry of complex niobium and tantalum oxides: V. Electostaic energy calculations

Madelung energies and site potentials are used to show that ordered forms of columbite and wodginite are much more stable than the disordered forms, but that a mixture of the component oxides has an intermediate electrostatic energy. An ordering scheme for wodginite in which metal sites X₁, and X₂ are disordered is marginally unfavorable. Madelung energy of fergusonites is not sensitive to the niobium coordination. Site potentials in M-Nb₂O₅ showthat edge-shared octahedra at the corners of ReO₃ blocks are not ideal sites for Nb⁵⁺ and Ta⁵⁺, so that in many structures the corners are joined by tetrahedra.