Structural changes underlying the diffuse dielectric response in AgNbO3

Structural differences in 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 all crystallize with Pbcm symmetry and lattice parameters 2 ac × 2 ac ×4 ac (where ac 4 corresponds to the lattice parameter of an ideal cubic perovskite) which are determined by a complex octahedral tilt system (a- b- c-) / (a- b- c+) involving a sequence of two in-phase and two antiphase rotations around the c axis. Our results revealed that, similar to KNbO3, the Nb cations in AgNbO3 exhibit local off-center displacements correlated along 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. The onset of the (a- b- c-) / (a- b- c+) tilting in the M polymorphs lifts the symmetry restrictions on the Nb positions and promotes ordering of the local Nb displacements into a long-range antipolarlike array. This ordering preserves the average Pbcm symmetry but is manifested in electron diffuse scattering and corroborated by other local-structure sensitive techniques. Structural states previously identified as the M3 and M2 phases represent different stages of displacive ordering rather than distinct thermodynamic phases. 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 retained. This partial disorder likely gives a source to the dielectric relaxation which, according to previous spectroscopic studies, is the origin of the diffuse dielectric response exhibited by M-type AgNbO3 at 250°C.