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 √2a_c×√2a_c×4a_c (where a_c≈4 Å 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.