Effects of the atmosphere and Mn doping on the electrical properties of Sr₂Fe_1−xMn_xNbO₆ and their mechanisms

Abstract

Double-perovskite composite oxides (A 2 BB′O 6 ) usually have excellent functional properties and are widely studied and applied. Its electrical, magnetic or other properties can be optimized by doping transition metal elements at its B or B′ site. In this paper, the effects of Mn doping and testing atmosphere on the crystal structures and the electrical conductivities of Sr 2 Fe 1−x Mn x NbO 6 (SFMN) were systematically investigated. The experimental results demonstrate that the conductivities of SFMN will increase in air while decrease under H 2 /H 2 O(g) atmosphere with Mn content rise. Doping Mn can enlarge the interplanar spacing of SFMN which increases the resistance of electron conduction across the crystal. First-principles calculations and XPS analysis results reveal that the chemical adsorption of H 2 on the surface of SFMN is a key reason for its conductivity increase under H 2 /H 2 O(g) atmosphere. The forbidden band width of the reduced state of Sr 2 FeNbO 6 (1.177 eV) is much smaller than that of the oxidation state (1.426 eV). The analysis results suggest that Fe 3d electron transition and exchange interaction between Mn-O-Mn are mainly two possible mechanisms contribute to the electrical conductivity of SFMN.

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“…The authors concluded that through doping Mn in the B-site of the SFN double perovskite the conductivity increases in air but decreases under reducing atmospheres. 401 The observed increase in conductivity in air through doping the B-site of SFN was confirmed by other authors as well. 398 They achieved a maximum conductivity of 5.7 S cm −1 at 800 °C for a Co-doped Sr 2 Fe 0.1 Co 0.9 NbO 6 .…”

“…The chemical stability of Mn-doped SFN double perovskites after heat treatment in a reducing atmosphere (80% H 2 O + 20% H 2 /air, 900 °C, 24 h) was analyzed by XRD measurements. 401 The oxidized and reduced Mn-SFN powders showed no apparent differences in the XRD patterns for a low doping content of 0 ≤ x ≤ 0.2. Nevertheless, a shift from high-angle peaks to smaller angles was observed with increasing Mn content.…”

“…According to the authors, this shift correlates to an expansion of the crystal lattice. 401 The reactivity of Co-doped SFN (Sr 2 Fe 0.1 Co 0.9 NbO 6 ) with YSZ and GDC using powder mixtures with a weight ratio of 1 : 1 was tested by sintering at 1200 °C for 24 h. 398 The authors were not able to detect any new, additional peaks or peak shifting in the resulting XRD patterns. 398 For the simple perovskite SrFe 0.9 Nb 0.1 O 3− δ , its chemical stability in reducing atmospheres was confirmed after reducing the powder in 5H 2 + 95% Ar at 700 °C for 10 h. 399 After the reduction of SrFe 0.9 Nb 0.1 O 3− δ doped with Cu ( x = 0.1), the perovskite main phase in addition to a peak of metallic iron was detected in the XRD pattern.…”

“…398,399 The electrical conductivity of an SFN double perovskite at 850 °C in air is low with 0.05 S cm −1 . 400,401 The conductivity increases to 2.39 S cm −1 (@900 °C) 402 and 2.215 S cm −1 (@850 °C) 400 when the atmosphere is changed to the reducing conditions of 5% H 2 + 95% Ar and 80% H 2 O + 20% H 2 . The effect of Mn doping into the SFN double perovskite on the conductivity in air and in 80% H 2 O + 20% H 2 atmosphere was investigated.…”

“…The effect of Mn doping into the SFN double perovskite on the conductivity in air and in 80% H 2 O + 20% H 2 atmosphere was investigated. 401 Substituting 50% Fe with Mn leads to an electrical conductivity value of 0.5 S cm −1 in oxidizing and reducing atmospheres. With an increasing amount of Mn, a maximum conductivity of 1.37 S cm −1 in air was reached for Sr 2 MnNbO 6− δ .…”