Correlating A and B Site Co-Doping Effects on the Crystallographic Structure and Dielectric Properties of Sr2NaNb5O15 (SNN) Tetragonal Tungsten Bronze (TTB)

Tetragonal tungsten bronzes (TTBs) have emerged as a promising structure type for Pb- and Bi-free ferroics for high–temperature stable multilayer ceramic capacitors. In this work, A- and B-site co-doped Sr2-2zNaCazYzNb5-zZrzO15 (with z = 0 to 0.075; SNN) ceramics exhibited a flattening of relative permittivity across a wide temperature range covering two dielectric anomalies: T 2, a ferroelectric to paraelectric transition between 229 °C and 297 °C, and T 1, a relaxor-ferroelectric peak between -8 °C and -70 °C. Rietveld refinement of synchrotron X-ray powder diffraction of the SNN ceramics using an orthorhombic Ama2 model, identified changes in the temperature evolution with doping of lattice parameters a, b and c, plus symmetry–driven distortion and strain modes including the ferroelectric polar mode distortion amplitude along c ( Γ 3 − ) . A-site doping of smaller Ca2+ and Y3+ ions (for Sr2+), increases A-site disorder, working in combination with B-site doping of less polarisable Zr4+ ions (for Nb5+), disrupting long range commensurate ferroelectric ordering, via rigid unit-like octahedral tilting. This leads to an incommensurate modulation, whose associated strain is accommodated by an increasing stacking fault density, confirmed by TEM. This destabilises the ferroelectric polar mode ( Γ 3 − onset temperature decreases, lowering T 2), leading to increasing in orthorhombic strain in the ab plane (c axis compresses, inflection points of a and c at T 1 diverge). Resulting in an increase in relaxor-like frequency dispersion of permittivity at T 1 and a reduced negative thermal expansion of the c axis between T 2 and T 1, leading to a permittivity flattening.