Local Structure and Order–Disorder Transitions in “Empty” Ferroelectric Tetragonal Tungsten Bronzes

The “empty” tetragonal tungsten bronze Ba₄La_0.67□_1.33Nb₁₀O₃₀ displays both relaxor-like and normal dielectric anomalies as a function of temperature; the former is associated with loss of ferroelectricity and was proposed to originate from anion disordering [Chem. Mater. 2016, 28, 4616–4627]. Here we present total neutron scattering and pair distribution function (PDF) analysis, which shows an increase in the distribution of oxygen–oxygen distances at the relaxor transition and which supports the proposed anion disordering mechanism. The disordering process can be destabilized by reducing the average A-cation size (i.e., Nd-doping: Ba₄(La_1–xNd_x)_0.67Nb₁₀O₃₀); this introduces a more strongly propagating tilt system in line with the previously reported crystal-chemical framework model [Chem. Mater. 2015, 27, 3250–3261]. Mechanical loss data obtained using resonant ultrasound spectroscopy also indicate destabilization of the disordering process with increasing Nd-substitution.