Displacive ordering transitions in perovskite-like AgNb _1/2Ta_1/2O₃

Displacive phase transitions in perovskite-like solid solutions AgNb _1/2Ta_1/2O₃ were studied using several diffraction and spectroscopic techniques sensitive to average and local structures. The room-temperature phase of AgNb_1/2Ta _1/2O₃ (M₂) is analogous to that of the end-member AgNbO₃ and exhibits Pbcm orthorhombic symmetry with lattice parameters √2a_c×√2a_c× 4a_c (a_c ≈ 4 Å refers to an ideal cubic perovskite cell). This structure combines complex octahedral tilting and average antipolar B-cation (Nb, Ta) displacements. Similar to AgNbO₃, at higher temperatures, B-cations are disordered among multiple sites displaced along 〈111〉_c directions. Partial ordering of local B-cation displacements is manifested in the so-called M₃ → M₂ transition, which preserves the average Pbcm symmetry determined by the tilted octahedral framework; the transition is accompanied by a broad exploitable maximum of dielectric constant. Ta substitution suppresses this ordering because of the dissimilar off-centering trends for Ta and Nb. According to the extended X-ray absorption fine structure (EXAFS) measurements, Nb cations exhibit much larger local off-center displacements than Ta, consistent with larger dielectric constants typically displayed by niobates compared to tantalates. AgNb _1/2Ta_1/2O₃ maintains residual 8-site disorder down to low temperatures as opposed to 2-site disorder in AgNbO₃. Our results suggest that Ag cations also exhibit displacive disorder and, on cooling, undergo ordering coupled to that of the B-cations.