Origin of the crossover from ferroelectric to relaxor in tetragonal tungsten bronzes

Abstract

The polar behavior of tetragonal tungsten bronze (TTB) (SrxBa1−x)Nb2O6 (SBN) is known to vary from normal ferroelectric to relaxor when x > 0.5. However, the mechanism triggering this crossover is still unclear. We study the role of the relative occupancy of the available interstices (A1, A2) in the TTB structure and use the residual entropy calculation approach, and experimental x-ray diffraction and dielectric data obtained on SBN and Ca-doped SBN, to reveal the key parameters at play in the polar response. We show that once half of Sr occupies the A2 sites, the crossover takes place. Concomitantly, the residual entropy from the randomness of the cationic arrangement exhibits a high entropy with an energy higher than 12 J K−1 mole−1 when x > 0.5 corresponding to the relaxor behavior occurrence. These findings provide comprehensive and quantitative tools to understand and predict the emergence of the relaxor state in other TTBs and other crystalline structures.