Effect of A-Site Nonstoichiometry on Defect Chemistry and Electrical Conductivity of Undoped and Y-Doped SrZrO3

Abstract

The effect of Sr-nonstoichiometry on phase composition, microstructure, defect chemistry and electrical conductivity of SrxZrO3−δ and SrxZr0.95Y0.05O3−δ ceramics (SZx and SZYx, respectively; x = 0.94–1.02) was investigated via X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and impedance spectroscopy followed by distribution of relaxation times analysis of impedance data. It was shown that at low Sr deficiency (x > 0.96 and 0.98 for SZx and SZYx, respectively) a solid solution of strontium vacancies in strontium zirconate crystal structure forms, whereas at higher Sr deficiency the secondary phase, zirconium oxide or yttrium zirconium oxide, is precipitated. Yttrium solubility limit in strontium zirconate was found to be close to 2 mol%. Y-doped strontium zirconates possess up to two orders of magnitude higher total conductivity than SZx samples. A-site nonstoichiometry was shown to have a significant effect on the electrical conductivity of SZx and SZYx. The highest total and bulk conductivity were observed at x = 0.98 for both systems. Increasing the conductivity with a rise in humidity indicates that proton conduction appears in the oxides in wet conditions. A defect model based on consideration of different types of point defects, such as strontium vacancies, substitutional defects and oxygen vacancies, and assumption of Y ions partitioning over Zr and Sr sites was elaborated. The proposed model consistently describes the obtained data on conductivity.

Effect of Sr Deficiency on Electrical Conductivity of Yb-Doped Strontium Zirconate

“…The process of the secondary phases' precipitation might be enhanced by the evaporation of strontium during the sintering of the ceramics. The surface alkaline earth metal depletion has been previously reported for the doped SrZrO 3 and BaZrO 3 ceramics [17,19,21,23] According to the EDX mapping images shown in Figure S1 and the measured element concentrations summarized in Table S1, the light grey grains observed in the SEMimage of the sample with x = 0.94 contain mostly zirconium and ytterbium in the ratio close to 6:5. Taking into account the appearance of the X-ray reflection associated with the cubic ZrO2-based phase, these grains can be considered as the precipitated phase of Ybdoped ZrO2.…”

“…The study of the processes of hydration of the alkaline-earth zirconates [12,13], hydrogen isotope distribution, hydrogen solubility and diffusivity [14,15], the isotope effect on the electrical conductivity [16], and the effect of the water vapor partial pressure on the conductivity [4][5][6][17][18][19] regardless of certain differences, proved the proton incorporation and participation in the charge transport process in the perovskites. As demonstrated by multiple studies, the ionic (O 2− and H + ) conductivity of the zirconates can be improved by substitution of zirconium with trivalent cations that promote the generation of oxygen vacancies to maintain charge balance, see e.g., [4][5][6][7][8]18,19].…”

“…Along with the acceptor doping, A-site stoichiometry was shown to influence the conducting behavior of ABO 3 perovskites (A = Sr, Ba; B = Zr, Ce) [11,17,[19][20][21]. Recently, we have reported that Sr 0.98 Zr 0.95 Y 0.05 O 3-δ composition exhibited the highest ionic conductivity among Sr x Zr 0.95 Y 0.05 O 3-δ (x = 0.94-1.02) series [19], which was interpreted as follows.…”

“…Along with the acceptor doping, A-site stoichiometry was shown to influence the conducting behavior of ABO 3 perovskites (A = Sr, Ba; B = Zr, Ce) [11,17,[19][20][21]. Recently, we have reported that Sr 0.98 Zr 0.95 Y 0.05 O 3-δ composition exhibited the highest ionic conductivity among Sr x Zr 0.95 Y 0.05 O 3-δ (x = 0.94-1.02) series [19], which was interpreted as follows. Asite deficiency promotes the generation of additional oxygen vacancies that result in a rise of ionic conductivity; but on the other hand, it boosts the placement of the dopant ions into A-sites instead of B-sites.…”

Effects of Sn Doping and A‐Site Deficiency on the Phases and Electrical Conductivities of the High‐Temperature Proton Conductor LaNbO4

“…The state-of-the-art high-temperature ceramic proton conductors are the perovskite structure oxides with Ba or Sr as the main component, [7][8][9][10] such as acceptor-doped barium cerate [9,[11][12][13] or zirconate. [7,14,15] The former shows the highest-reported proton conductivity but suffers from low chemical stability and decomposing when exposed to acidic gases such as carbon oxides, hydrogen sulphide, or even water vapor.…”