Characterization and Phase Diagram of the Tetragonal Tungsten Bronze Type Ferroelectric Compounds Pb2(1−x)GdxK1+xNb5O15 for Energy Storage Applications

Abstract

The crystalline family of tetragonal tungsten bronze-type compounds, having the general formula of Pb2(1−x)GdxK1+xNb5O15 was elaborated by solid state reaction, and the complete composition-temperature phase diagram was investigated. Structural characterization and physical properties were highlighted based on dielectric spectrometer, x-ray diffraction, Raman spectroscopy and differential scanning calorimetry. The ferroelectric phases, their symmetry and the transition temperatures were found to be strongly dependent on the gadolinium concentration. At room temperature, the ferroelectric state is described by the symmetry group Cm2m (No. 38) for the low Gd concentrations (x<0.3) compounds, whereas the rich Gd-content compound (x>0.7) structure was refined in the tetragonal symmetry with the space group P4bm (No. 100). An intermediate phase between Gd concentrations of 0.3<x<0.7 was also observed that corresponding to a mixture of two phases, with Pba2 (No. 32) and P4bm (No. 100) space group symmetries. The ferroelectric/paraelectric phase transition temperature strongly changes with the Gd-content in a non-monotonic way. Two thermal phase transitions were evidenced in dielectric measurements for the compounds with rich Gd-content (x>0.7) showing an intermediate phase above room temperature, which was identified as antiferroelectric. Differential scanning calorimetry signal shows excess of specific heats in accordance with the dielectric permittivity anomalies. Scanning electron microscopy points out various morphologies connected with Gd concentration and ferroelectric behaviors connected to grain size and strain analysis. This family compound exhibits rich physical properties and constitutes a potential candidate for electrostatic energy storage applications.