Displacive Ordering Transitions in Perovskite-Like AgNb0.5Ta0.5O3

Journal Article · Fri Jan 01 04:00:00 EST 2010 · Chemistry of Materials

DOI:https://doi.org/10.1021/cm101263p· OSTI ID:1019728

Levin, I; Woicik, J; Llobet, A; Tucker, M; Krayzman, V; Pokorny, J; Reaney, I

Displacive phase transitions in perovskite-like solid solutions AgNb{sub 1/2}Ta{sub 1/2}O{sub 3} were studied using several diffraction and spectroscopic techniques sensitive to average and local structures. The room-temperature phase of AgNb{sub 1/2}Ta{sub 1/2}O{sub 3} (M{sub 2}) is analogous to that of the end-member AgNbO{sub 3} and exhibits Pbcm orthorhombic symmetry with lattice parameters {radical}2a{sub c} x {radical}2a{sub c} x 4a{sub c} (a{sub c} {approx} 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 AgNbO{sub 3}, at higher temperatures, B-cations are disordered among multiple sites displaced along <111>{sub c} directions. Partial ordering of local B-cation displacements is manifested in the so-called M{sub 3} {leftrightarrow} M{sub 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{sub 1/2}Ta{sub 1/2}O{sub 3} maintains residual 8-site disorder down to low temperatures as opposed to 2-site disorder in AgNbO{sub 3}. Our results suggest that Ag cations also exhibit displacive disorder and, on cooling, undergo ordering coupled to that of the B-cations.

• Research Organization: Brookhaven National Laboratory (BNL) National Synchrotron Light Source
• Sponsoring Organization: DOE - OFFICE OF SCIENCE
• DOE Contract Number: AC02-98CH10886
• OSTI ID: 1019728
• Report Number(s): BNL--95574-2011-JA
• Journal Information: Chemistry of Materials, Journal Name: Chemistry of Materials Journal Issue: 17 Vol. 22; ISSN CMATEX; ISSN 0897-4756
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ABSORPTION
CATIONS
DIFFRACTION
FINE STRUCTURE
LATTICE PARAMETERS
NIOBATES
PERMITTIVITY
PEROVSKITE
SOLID SOLUTIONS
SYMMETRY
TANTALATES
national synchrotron light source