Complex magnetic behaviour of Sr₂CoNb_1−xTi_xO₆(0 ≤ x ≤ 0.5) as a result of a flexible microstructure

Abstract

We report the rich magnetic behaviour of Sr2CoNb1-xTixO6 (0 ≤ x ≤ 0.5) oxides as a result of their complex microstructure. Although these oxides show an average simple-cubic perovskite structure, they present a flexible microstructure due to short-range ordering between Co/Ti and Nb cations in the perovskite B-sites. The microstructure consists of double-cubic perovskite domains grown in a simple-cubic perovskite matrix. The size and number of the double-cubic perovskite domains decrease as the Ti content increases. As a result of aliovalent substitution of Nb(5+) by Ti(4+) in the parent Sr2CoNbO6 mixed-valence Co(3+)/Co(4+) oxides are obtained. A spin glass-like state has been observed at low temperatures for all the series, with freezing temperatures increasing with the Ti-content in the range 22 to 33 K. Furthermore, the x = 0.3 and x = 0.5 samples show non-interacting superparamagnetic particle-like dynamics associated with relatively high amounts of Co(4+), with "blocking temperatures" of 13 and ∼16 K, respectively. The complex magnetic behaviour of the title oxides seems to be connected with the clustering of magnetic Co(3+) and the distribution of Co(4+) as a result of the microstructure.

Novel Perovskite Materials for Thermal Water Splitting at Moderate Temperature

“…In recent papers, we have studied the structure and microstructure as well as the performance as SOFC cathodes of materials with compositions Sr 2 CoNb 1− x Ti x O 6− δ (0≤ x ≤1; δ =number of oxygen vacancies), which fulfil all mentioned requirements. Along the whole series, the transport properties and catalytic activity vary with the titanium content.…”

“…The metal compositions of both materials correspond to the nominal ones, within the experimental errors, whereas the resulting refined oxygen contents clearly indicate the presence of non‐negligible amounts of oxygen vacancies. Despite this simple average structure, this family of materials presents a complex microstructure . Some of these oxides display performances as SOFC cathodes that put them among the best materials for this application .…”

“…Reduction and oxidation of the title materials are highly reversible processes and can be properly described by Equation (6). Finally, the thermal factors’ values increase with the concentration of oxygen vacancies; this is most likely a signature of local disorder (not dynamical vibrations), mainly associated with oxygen ions because this family of oxides presents a complex microstructure in which vacancies can be accommodated.…”

“…The title materials were synthesized by a modified Pechini method, previously reported by Azcondo et al . Reduced samples were obtained by treating the as‐prepared samples at 900 °C in a continuous flow of pure Ar for 12 h.…”

“…Lowering the Ti/Nb ratio decreases the material's catalytic activity and the amount of released/recovered oxygen as well; the less extensive the reduction/oxidation processes are, the more reversible they are expected to be. In addition, the material's microstructure may play a relevant role in reduction/oxidation because oxygen vacancies can be accommodated in the domain walls . Neither Ti 4+ nor Nb 5+ experience any oxidation/reduction processes; the couple Co 4+ /Co 3+ is solely responsible for the redox behavior of these materials …”