Local structure and order-disorder transitions in "empty" ferroelectric tetragonal tungsten bronzes

Abstract

The ‘empty’ tetragonal tungsten bronze Ba4La0.67 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 destabilised by reducing the average A-cation size (i.e. Nd-doping: Ba4(La1-xNdx)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 destabilisation of the disordering process with increasing Nd-substitution.