Structure And Properties Of （Sr,Ba）Nb₂O₆ Unfilled Tungsten Bronze Ceramics

ceramics as the typical representatives of unfilled tungsten bronze ferroelectrics have received much attention due to their excellent ferroelectric, pyroelectric, piezoelectric and non-linear optical properties. However, the understanding on the dielectric characteristics, especially the ferroelectric transition and the low-temperature dielectric relaxations is still insufficient. Based on the understandings of the filled tungsten bronze niobates, the dielectric and ferroelectric characteristics of (SrxBa1-x)Nb206 unfilled tungsten bronze ceramics have been investigated together with the structure.The dielectric and ferroelectric characteristics of the present ceramics vary significantly with x, and the A1/A2-sites occupancy has been determined as the primary parameter governing this variation tendency. When the ratio of Sr/Ba is close to 1:4 (where four A2-sites are just occupied by Ba ions, and one A1-site is occupied by Sr ion while another A1-site is empty), the normal ferroelectric transition is observed with one anomaly in the tanδ-T curve (x=0.25). When the ratio of Sr/Ba is far away from 1:4, the typical relaxor behavior is indicated together with three anomalies in the tanδ-T curve (x=0.75). The incommensurate oxygen octahedral tilting and A-site random distribution are considered to be the structure origins for the relaxor ferroelectricity and low temperature dielectric relaxations.Studies on domain structure by piezoresponse force microscopy (PFM) well confirm the evolution from normal ferroelectric to relaxor ferroelectric. Though the P-E loop becomes slimmer and the maximum and remanent polarizations tend to decrease and vanish at low temperatures under a lower electric field, these phenomena are just due to the increase of coercive field rather than the re-entrant relaxor behavior. The saturated P-E loop can be obtained with increasing electric field even at low temperatures for all composition in the present ceramics.