Manipulation of polar order in the “empty” tetragonal tungsten bronzes: Ba_4-xSr_xDy_0.67_1.33Nb₁₀O₃₀, x = 0, 0.25, 0.5, 1, 2, 3

A series of “empty” tetragonal tungsten bronze (TTB) ferroelectrics, Ba_4-xSr_xDy_0.67□_1.33 Nb ₁₀O₃₀ (x = 0, 0.25, 0.5, 1, 2, 3; □ = vacancy), is reported. With increasing x the unit cell contracts in both the ab plane and c-axis; x ≤ 1 compounds are normal ferroelectrics (FE) with decreasing T_C as x increases, while x ≥ 2 are relaxor ferroelectrics (RFE) with associated frequency dependent permittivity peaks and with similar T_m and T_f (Vogel-Fulcher freezing temperatures) values. This observation is rationalised by differing cation occupancies: for x ≤ 1, Sr²⁺ principally occupies the A2-site (co-occupied by Ba²⁺ with the A1-site occupied by Dy³⁺ and vacancies); for x ≥ 2 significant Sr A1-site occupation leads to the observed RFE characteristics. This FE to RFE crossover is consistent with a previously proposed TTB crystal chemical framework where both a decrease in average A-site size and concurrent increase in A1-site tolerance factor (t_A1) favour destabilization of long range polar order and relaxor behaviour. The effect of increasing t_A1 as a result of Sr occupancy at the A1 site is dominant in the compounds reported here.