Relaxor-to-ferroelectric crossover in a family of tetragonal tungsten bronzes

The tetragonal tungsten bronze (TTB) structure, A12A24B12B28C4O30, consists of a cornersharing network of BO6 octahedra with 3 types of ‘tunnel’ sites, Figure 1. Distortion of the aristotype TTB structure is common due to... [ view full abstract ]

The tetragonal tungsten bronze (TTB) structure, A1₂A2₄B1₂B2₈C₄O₃₀, consists of a cornersharing network of BO₆ octahedra with 3 types of ‘tunnel’ sites, Figure 1. Distortion of the aristotype TTB structure is common due to octahedral tilting to relieve A-site-generated strain and results in superstructures and incommensurate modulations. These, often subtle, structural modifications influence whether ferroelectric (FE), relaxor-ferroelectric (RFE) or dipole glass behaviour is observed.¹ Altering the size of the A-site cations sizes, via doping, allows a degree of control over the structure and therefore the properties. A-site vacancies may be introduced while doping to maintain charge balance.²

This present work focuses on such a family of TTBs with an unusually large number of A-site vacancies (> 1/5 of A-sites are unoccupied).^3,4 which we have studied using an array of (variable temperature) diffraction techniques (high resolution neutron, synchrotron x-ray and selected area electron). The structural modulations are controlled by varying the A1-(perovskite)site cation size with different R, and allows manipulation of the nature of the dipolar ordering. For example Ba₄La_0.67_1.33Nb₁₀O₃₀ (R = La) is a commensurate RFE while R = Nd is incommensurate and FE.

The crossover from these behaviours has also been examined in more detail via the solid solution Ba₄(La_1-xNd_x)_0.67_1.33Nb₁₀O₃₀. La-rich compounds display an additional feature of electrical field-driven ordering between the RFE and paraelectric states, (demonstrated by ‘pinched’ polarisation-electric field (P-E) loops, Figure 1), which corresponds to a disordered superstructure. 

Figure 1: (left) TTB structure and (right) P-E loops obtained from Ba₄La_0.67_1.33Nb₁₀O₃₀ demonstrating a ‘normal’ ferroelectric switching and ‘pinched’ loops at different temperatures.

[1]. Zhu X., et al., Chem. Mater. 27, 3250, 2015

[2]. Masuno, K. J. Phys. Soc. Jpn., 19, 323, 1964

[3]. Gardner J. and Morrison F. D., Dalton Trans. 43, 1168, 2014

[4]. Gardner, J,  et al. Chem. Mater. 28, 4616, 2015