Synthesis of a 12R-type hexagonal perovskite solid solution Sr3NdNb3−xTixO12−δ and the influence of acceptor doping on electrical properties

A solid solution forms for Sr 3 NdNb 3− x Ti x O 12− δ with approximate limits 0 ≤ x ≤ 0.06. The system crystallizes with a 12R-type hexagonal perovskite structure in the space group R 3&cmb.macr;, as determined by neutron diffraction and selected area electron diffraction. The electrical properties of the end members have been investigated by impedance spectroscopy in the temperature range 550-800 °C under various gas atmospheres and as a function of oxygen and water-vapour partial pressure. Proton transport dominates under wet oxidising conditions in the temperature range 550-700 °C, as confirmed by the H + /D + isotope effect. Acceptor doping considerably enhances proton conductivity with a value of 3.3 × 10 −6 S cm −1 for the bulk response of x = 0.06 at 700 °C in moistened air. The presence of a −¼ slope for both doped and undoped samples in the range 10 −19 ≤ p O 2 ≤ 10 −8 atm at 900 °C indicates n-type transport under reducing conditions following the extrinsic model attributable to acceptor centres. The conductivity is essentially independent of p O 2 at 600 °C under dry oxidising conditions, consistent with oxide-ion transport; a positive power-law dependence at higher temperature indicates extrinsic behaviour and a significant electron-hole contribution. The dielectric constant at RT of nominally stoichiometric Sr 3 NdNb 3 O 12 is r ∼ 37, with a moderately high quality factor of Q × f ∼ 16 400 GHz at f r ∼ 6.4 GHz. The temperature coefficient of resonant frequency of x = 0 is τ f ∼ 12 ppm °C −1 , which lowers to −3 ppm °C −1 for the Ti-doped phase x = 0.06. The solid solution Sr 3 NdNb 3− x Ti x O 12− δ (0 ≤ x ≤ 0.06) crystallises with a 12R-type hexagonal perovskite structure and exhibits increasing proton conductivity with x .