Tetragonal tungsten bronzes (TTBs) have emerged as a promising structure type for Pb- and Bi- free ferroelectrics for temperature stable ceramic capacitors, with filled Sr2NaNb5O15 (SNN1) and unfilled Sr1-xBaxNb2O6 (SBN) attracting recent interest. In this work, A and B site co-doped Sr2-2zNaCazYzNb5-zZrzO15 (with z = 0; SNN1 to 0.075) and Na+ vacancy containing Sr2+wNay☐wNb5O15 (w = 0, [y = 1]; SNN1 to w = 0.15, [y = 0.7]) ceramics were synthesized, and shown to exhibit a flattening of relative permittivity (εr) across a wide temperature range covering two dielectric anomalies: a relaxor ferroelectric peak at T1, between -8 °C and -70 °C and a ferroelectric to paraelectric transition at T2, between 229 °C and 297 °C.
Rietveld refinement of variable temperature synchrotron X-ray powder diffraction of the SNN ceramics was performed using an orthorhombic Ama2 structural model. Doping smaller A site cations (Ca2+, Y3+) for Sr2+ and less polarisable B site cations (Zr4+) for Nb5+ in SNN reduces the c lattice parameter, reducing negative thermal expansion below the ferroelectric to paraelectric transition temperature, T2. The reduction of c destabilises the ferroelectric polar mode, lowering the temperature at which the amplitude of this polar distortion mode rises above zero, ultimately decreasing T2 and εr at T2 with doping. Doping also reduces the negative thermal expansion of the a lattice parameter below the low temperature, relaxor-like peak (T1), onset to which there is sufficient orthorhombic strain to drive the Ama2 to Aa phase transition, ultimately decreasing T1 with doping. Transmission electron microscopy shows that increased doping, increased octahedral tilt, leading to a higher density of stacking faults to accommodate the associated strain increase. Thus, the ferroelastic domain structure is disrupted, contributing to a relaxor ferroelectric (RFE)-like frequency dispersion at T1 and flattening the permittivity (εr) – temperature profile. A flattening of permittivity with addition of Na+ vacancies in Sr2+wNay☐wNb5O15, was attributed to similar effects. Na+ vacancies can be considered to have an effective size at the A-site and thus play a similar role to A-site dopants, which facilitate tilting of the octahedral framework, through an incommensurate modulation. In turn, this breaks down the ferroelastic domain structure, disrupting long-range ferroelectric ordering decreasing T2 and by extension contributing to the relaxor-like nature of T1.
Introduction of A- and B-site co-dopants into Sr0.53Ba0.47Nb2O6 (SBN) to form Sr(0.53–0.15x)Ba0.47R0.1xNb2-xTaxO6 (SBRTN) where x = 0.2 and R3+ = Y3+, Dy3+, Gd3+, Sm3+, Nd3+, and La3+ is also reported here because this lowers and broadens the relaxor ferroelectric permittivity anomaly Tm. A large decrease in tetragonality (c/a) with R3+ and Ta5+ doping, is followed by a minor decrease in tetragonality with increasing R3+ cationic radii, coinciding with little change of Tm and Pmax from SBYTN to SBLTN. The expansion of the a lattice parameter reflects that reported in A- and B-site co-doped SNN1 (Sr2-2zNaCazYzNb5-zZrzO15) and Na+ A-site vacancy containing SNN1 (Sr2+wNay☐wNb5O15).
Taken together, the work in this thesis demonstrates that an A- and B-site co-doping strategy for an unfilled TTB can lead to a flattened, high and wide temperature ranging permittivity (εr) – temperature response ideal for capacitor applications.
