Structural changes among the so-called M-polymorphs of AgNbO3 were analyzed using combined high-resolution X-ray diffraction, neutron total scattering, electron diffraction, and X-ray absorption fine structure measurements. These polymorphs crystallize with the Pbcm symmetry and lattice parameters 2ac×2ac×4ac (ac≈4 Å is the lattice parameter of an ideal cubic perovskite) which are determined by a complex octahedral tilt system (a-b-c-)/(a-b-c+) that involves a sequence of two in-phase and two anti-phase rotations around the c-axis. Our results revealed that, similar to KNbO3, the Nb cations in AgNbO3 exhibit local off-center displacements correlated along the Nb-Nb-Nb chains. The displacements appear to be present even in the high-temperature AgNbO3 polymorphs where the Nb cations on average reside on the ideal fixed-coordinate sites. An onset of the (a-b-c-)/(a-b-c+) tilting lifts the symmetry restrictions on the Nb positions and promotes ordering of the local Nb displacements into a long-range antipolar-like array. This ordering preserves the average Pbcm symmetry but is manifested in electron diffuse scattering and corroborated by other local-structure sensitive techniques. Rietveld refinements indicated intimate coupling between the displacive behavior on the oxygen, Nb, and Ag sublattices. The Pbcm symmetry of the octahedral framework precludes a complete ordering of Nb displacements so that some positional disorder is still maintained in the structure. This disorder provides a likely source for the dielectric relaxation that, according to the previous spectroscopic studies, is at the origin of the diffuse dielectric response exhibited by the M-phase of AgNbO3 at ≈250 ºC.
Citation: Physical Review B (Condensed Matter and Materials Physics)
Pub Type: Journals
local structure, perovskites, phase transitions, total scattering, EXAFS, electron diffraction, diffuse scattering