Calculation of bond distances and heats of formation for BeO, MgO, SiO2, TiO2, FeO, and ZnO using the ionic model

J. A. TossBlr

Department of Chemistry, University of Maryland College Park, Maryland 20742

Abstract 

The radius ratio model is criticized for its failure to accurately predict coordination number, its reliance upon an ambiguous entity (the ionic radius), and its inconsistency with modern quantum mechanical calculations. A purely theoretical version of the ionic model, without the assumption of fxed radii, was previously shown by Gordon and coworkers to accurately predict bond distances, heats of formation, and preferred structure type for the alkali halides. For BeO, MgO, and CaO the model also gives reasonable results, although it is somewhat less accurate due to ambiguity in the definition of the oxide ion wavefunction. For ZnO, SiOr, and TiO2, heats of formation calculated from this model are less negative than the experimental values by hundreds of kcal,/mole which suggests substantial covalency, although bond distancep are predicted with an average error ofless than 0.lA for SiO, and TiOr. For FeO, agreement with elperiment is similar to that observed for MgO. Applications of the method to the prediction of Fe,Mg ordering in orthopyroxenes and in ludwigite are described. Schemes for adding covalency effects to the ionic model calculations are considered.

http://www.minsocam.org/ammin/AM65/AM65_163.pdf