AgNb₇O₁₈: an ergodic relaxor ferroelectric.

David I Woodward,Richard Beanland

doi:10.1021/ic5007346

Abstract

AgNb7O18 is a relaxor ferroelectric with a Burns temperature of ~490 K and an incipient transition to the nonergodic state. The short-range structure is shown by convergent-beam electron diffraction to have the polar space group Im2m, but refinements against powder X-ray diffraction find the long-range structure to have the centrosymmetric space group Immm. Relaxor behavior in AgNb7O18 appears to originate from the partial occupation of large interstices by Ag(+) cations. Both cations and oxygen anions are displaced away from zones where NbO6 octahedra are edge-sharing.

Highlights

Ferroelectric ceramics are used in an enormous variety of technological applications, and the majority of these materials are based on the perovskite structure
The Ag2O−Nb2O5 pseudobinary system is known to host an unusually large number of distinct crystal structures[5] with five confirmed structures and a number of additional, unconfirmed structures. Among the former are the perovskite-structured ferrielectric AgNbO3,6 the ferroelectric Ag2Nb4O11,7 which has the layered natrotantite structure containing edge-shared NbO7 polyhedra,[8] and the tungsten bronze-structured AgNb3O8.9 The system is rich with functional materials: AgNbO3 is a compound studied for its role in lead-free piezoelectrics,[10] reentrant relaxors,[11] and photocatalysis.[12]
AgNb7O18 has been fabricated as dense ceramics without Ag loss

Summary

Introduction

Ferroelectric ceramics are used in an enormous variety of technological applications, and the majority of these materials are based on the perovskite structure. The Ag2O−Nb2O5 pseudobinary system is known to host an unusually large number of distinct crystal structures[5] with five confirmed structures and a number of additional, unconfirmed structures Among the former are the perovskite-structured ferrielectric AgNbO3,6 the ferroelectric Ag2Nb4O11,7 which has the layered natrotantite structure containing edge-shared NbO7 polyhedra,[8] and the tungsten bronze-structured AgNb3O8.9 The system is rich with functional materials: AgNbO3 is a compound studied for its role in lead-free piezoelectrics,[10] reentrant relaxors,[11] and photocatalysis.[12] Ag2Nb4O11-structured materials are finding value in the study of tunable band gap materials for photocatalyzing the electrolysis of water[13] and in investigations of the anomalously large piezoelectric coefficients of Ta2O5 thin films.[14,15] Tungsten bronzes are studied for their ferroelectric properties[16] and for their potential use in thermoelectric applications.[17]

Methods

Results

Conclusion