Ion Distribution Models for Defect Fluorite Zro 2 - AO 1.5 (a = Ln, Y) Solid Solutions: II. Thermodynamics of Mixing and Ordering

Thermodynamic mixing properties of A(x)B(1-x)O(2-0.5x)V(0.5x), fluorite-type solid solutions (B = Zr, A = {Nd-Yb, Y}, V = oxygen vacancy) are modelled as functions of four parameters, Delta H-1, Delta H-2, Delta H-3 and Delta H-4, which correspond to the enthalpy effects of the reactions (6)A + B-8 = (7)A + B-7 (1), (6)A + B-8 = (8)A + B-6 (2), B-6 + B-8 = B-7 + B-7 (3) and (6)A + (8)A = (7)A + (7)A (4), involving six cation species, (6)A, (7)A, (8)A, B-6, B-7 and B-8. The model predicts that the disordered configuration containing all cation species evolves with the decreasing temperature such that 6-fold coordinated cations tend to vanish within 0 <= x <= 1/2 domain, while 8-fold coordinated cations become extinct within 1/2 <= x <= 1 domain. The further evolution within the intervals of 0 <= x <= 1/3, 1/3 <= x <= 1/2, 1/2 <= x <= 2/3 and 2/3 <= x <= 1 favours the extinction of (7)A, B-8, B-7 and (6)A cation species, respectively. With the further decrease in the temperature 6-fold B and 8-fold A cations reappear within the domains of 1/3 <= x <= 1/2 and 1/2 <= x <= 2/3 via the reaction (7)A + B-7 = (8)A + B-6. The configurational entropy reduces along with these transformations. The model fits structural and calorimetric data on Zr-based A(x)B(1-x)O(2-0.5x)V(0.5x) systems and provides hints to understanding of ionic conductivity and radiation susceptibility data. (c) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.