Thermodynamic properties of nonstoichiometric H-Nb2 Osub(5-x) derived from a statistical model of its defect structure

A statistical method for the treatment of the defect structure of oxides is applied to H-Nb2 Osub(5-x) and its thermodynamic properties are derived as a function of x and temperature. The results based on a model of Nb O3 vacancy clusters located at the tetrahedral columns of the structure presented very good agreement with experimental data in the literature [2]. Further, the predicted arrangement of the clusters of vacancies along the columns at the limiting composition of the H-Nb2 O5 phase indicates, according to recent electron microscopy experiments [18, 19], that the initial step of the transformation is the collapse of the structure around rows of defective sites along the columns, involving Andersson and Wadsley's [20] cooperative migration of atoms. The limiting compositions of the H-Nb2 O5 and Nb53 O132 phases are also correctly predicted on the basis of electrostatic interactions among defect units only. Thus elastic interactions among planar defects appear to affect only the arrangement of such defects, and not the compositions of the initial and final compounds. (author)