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

Abstract

A solid solution forms for Sr₃NdNb_3−xTi_xO_12−δ with approximate limits 0 ≤ x ≤ 0.06. The system crystallizes with a 12R-type hexagonal perovskite structure in the space group R, 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 ≤ pO₂ ≤ 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 pO₂ 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 Sr₃NdNb₃O₁₂ is ε_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.