Displacive Ordering Transitions in Perovskite-Like AgNb1/2Ta1/2O3

作者：Levin I^*; Woicik J C; Llobet A; Tucker M G; Krayzman V; Pokorny J; Reaney I M

来源：Chemistry of Materials, 2010, 22(17): 4987-4995.

DOI：10.1021/cm101263p

Displacive phase transitions in perovskite-like solid solutions AgNb1/2Ta1/2O3 were studied using several diffraction and spectroscopic techniques sensitive to average and local structures. The room-temperature phase of AgNb1/2Ta1/2O3 (M-2) is analogous to that of the end-member AgNbO3 and exhibits Pbcm orthorhombic symmetry with lattice parameters root 2a(c) x root 2a(c) x 4a(c) (a(c) approximate to 4 angstrom 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), 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. AgNb1/2Ta1/2O3 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.

• 出版日期2010-9-14