Crystal growth and diffuse scattering of tungsten bronze Ba₂RFeNb₄O₁₅ferroelectrics

Tetragonal tungsten bronzes represent a particular, non-perovskite structural motif that can display relaxor ferroelectric properties, the most well-known example being Sr_1-xBa_xNb₂O₆, which has an unfilled (1-2-6) structure that contains random cation vacancies. More recently, a group of filled tungsten bronzes with the formula Ba₂RFeNb₄O₁₅ (R=Rare Earth) have also been shown to display relaxor ferroelectric behavior. These are of particular interest as a comparison to SBN because, lacking the random cation vacancies of the 1-2-6 structure, they may reveal which local structural features are common to both filled and unfilled structures and which are characteristic only of the cation vacancies. In this work, we carried out crystal growth experiments, frequency-dependent dielectric susceptibility measurements, and diffuse X-ray scattering experiments on Ba₂RFeNb₄O₁₅ (R=Pr, Nd) as well as Sr_1-xBa_xNb₂O₆ for a means of comparison. By identifying features that are common and disparate from these crystals, we attempt to correlate them with the dielectric properties.