Displacive Ordering Transitions in Perovskite-Like AgNb_1/2Ta_1/2O₃

Displacive phase transitions in perovskite-like solid solutions AgNb 1/2 Ta 1/2 O 3 were studied using several diffraction and spectroscopic techniques sensitive to average and local structures. The room-temperature phase of AgNb 1/2 Ta 1/2 O 3 (M 2 ) is analogous to that of the end-member AgNbO 3 and exhibits Pbcm orthorhombic symmetry with lattice parameters √2 a c × √2 a c × 4 a 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 AgNbO 3, 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 M 3 ↔ M 2 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 1/2 Ta 1/2 O 3 maintains residual 8-site disorder down to low temperatures as opposed to 2-site disorder in AgNbO 3 . Our results suggest that Ag cations also exhibit displacive disorder and, on cooling, undergo ordering coupled to that of the B-cations.