Structural changes underlying the diffuse dielectric response inAgNbO3

Structural differences in the so-called M polymorphs of ${\text{AgNbO}}_{3}$ 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 $\ensuremath{\surd}2{a}_{c}\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}2{a}_{c}\ifmmode\times\else\texttimes\fi{}4{a}_{c}$ (where ${a}_{c}\ensuremath{\approx}4\text{ }\text{\AA{}}$ corresponds to the lattice parameter of an ideal cubic perovskite) which are determined by a complex octahedral tilt system $({a}^{\ensuremath{-}}{b}^{\ensuremath{-}}{c}^{\ensuremath{-}})/({a}^{\ensuremath{-}}{b}^{\ensuremath{-}}{c}^{+})$ involving a sequence of two in-phase and two antiphase rotations around the $c$ axis. Our results revealed that, similar to ${\text{KNbO}}_{3}$, the Nb cations in ${\text{AgNbO}}_{3}$ exhibit local off-center displacements correlated along Nb-Nb-Nb chains. The displacements appear to be present even in the high-temperature ${\text{AgNbO}}_{3}$ polymorphs where the Nb cations, on average, reside on the ideal fixed-coordinate sites. The onset of the $({a}^{\ensuremath{-}}{b}^{\ensuremath{-}}{c}^{\ensuremath{-}})/({a}^{\ensuremath{-}}{b}^{\ensuremath{-}}{c}^{+})$ 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 ${\text{M}}_{3}$ and ${\text{M}}_{2}$ 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 ${\text{AgNbO}}_{3}$ at $\ensuremath{\approx}250\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$.