Structural, dielectric, and conduction behaviour of A-site deficient SrxNa1−2xNbO3 ceramics

Abstract

The crystallographic and electrical properties of A-site deficient Sr_xNa_1−2xNbO₃ (x = 0.00, 0.05, 0.10, 0.15, 0.20, and 0.25) perovskites have been studied by X-ray diffraction, scanning electron microscopy, dielectric spectroscopy, impedance spectroscopy and polarisation against electric field loops. Despite complexities in the microstructural evolution due to the formation of a NaNb₃O₈ liquid phase, the crystallographic and dielectric properties show much clearer trends. The introduction of A-site vacancies shows an almost linear expansion of the pseudo-cubic unit cell volume of ∼0.05 ų per at%, stabilising the ferroelectric P2₁ma Q-phase. Compositions x > 0.10 show increasingly relaxor-like behaviour in the dielectric properties and impedance spectroscopy shows increasing conductivity and tan δ values associated with predominantly electronic conduction from x = 0.05 to 0.25. Measurements in N₂ at 600 °C show progressively n-type behaviour with increasing levels of A-site vacancies due to the increased tendency to lose under-bonded O²⁻ ions and therefore partially reduce some Nb⁵⁺ to Nb⁴⁺ ions. Although it is not possible to fully establish the electrical properties of our undoped NaNbO₃ ceramics, we propose they contain mixed ionic–electronic behaviour with the clear presence of a weak n-type dependent grain boundary contribution to the total conductivity.