Vol.:(0123456789) 1 3 Journal of Materials Science: Materials in Electronics https://doi.org/10.1007/s10854-018-9471-8 Disparity in electrical and magnetic properties of isostructural oxygen-defcient perovskites BaSrCo 2 O 6−δ and BaSrCoFeO 6−δ Ram Krishna Hona 1  · Farshid Ramezanipour 1 Received: 23 April 2018 / Accepted: 11 June 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018 Abstract Despite having the same crystal structure, the two oxygen-defcient perovskites, BaSrCo 2 O 6−δ and BaSrCoFeO 6−δ have signifcantly diferent electrical conductivity and magnetic properties, highlighting the important efect of oxygen stoichi- ometry. These two materials have been investigated by X-ray difraction, scanning electron microscopy, X-ray photoelectron spectroscopy, variable-temperature electrical conductivity and magnetization studies. BaSrCo 2 O 6−δ shows magnetic prop- erties consistent with antiferromagnetic order, whereas BaSrCoFeO 6−δ exhibits the presence of uncompensated moments and short-range ferrimagnetism. XPS and iodometric titrations show a greater degree of oxygen vacancies in BaSrCo 2 O 6−δ compared to BaSrCoFeO 6−δ . Cobalt is in di- and trivalent states in both compounds and iron is in tri- and tetravalent states in BaSrCoFeO 6−δ . The diferences in electrical conductivity are remarkable, where BaSrCo 2 O 6−δ shows nearly temperature- independent conductivity up to 400 °C, above which a semiconducting behavior sets in and persists up to 900 °C. However, BaSrCoFeO 6−δ exhibits semiconductivity up to 200 °C, followed by a downturn in conductivity from 200 to 900 °C, similar to the behavior observed in metallic systems. 1 Introduction We have recently studied [1–3] the structure–property rela- tionships in a series of oxygen defcient perovskites (ODPs). These materials have general formula ABO 3−x where A is usually an alkaline earth metal or lanthanide, and B is a smaller ion, typically a transition metal, although some main group elements could also reside on the B-site. The ODPs feature interesting properties, from superconductivity [4], to magnetoresistant, [5] and have been considered for dif- ferent applications including gas sensors [6], gas difusion membranes for gas separation [7], and electrodes in solid oxide fuel cells [8]. In typical perovskites, oxygen atoms form octahedral geometry around the B-site metal, forming BO 6 octahedra. In ODPs, the absence of some oxygen atoms from the structure creates vacancies which may lead to the formation of diferent coordination geometries, such as tetra- hedral (BO 4 ) or square pyramidal (BO 5 ) [1]. Considering the various coordination geometries that can be formed due to the presence of oxygen vacancies, a high degree of structural diversity is observed in ODPs [9–15]. Structural changes can sometimes occur through substi- tution on the A or B-sites. For example, the coordination around the B-site cation can change when the A site cation is substituted in Sr 2−x Ca x Fe 2 O 6−δ [1]. Here, Sr 2 Fe 2 O 6−δ has a tetragonal structure and contains alternating octahedral and square pyramidal coordination of the B-site cations. When one Sr is substituted by Ca, i.e., SrCaFe 2 O 6−δ , the structure changes into orthorhombic, and the coordination geometry transforms into alternating tetrahedral and octa- hedral [1]. These structural changes lead to the transforma- tion of electrical properties from metallic to semiconductor. The magnetic structure also changes, where the spin-density wave state in Sr 2 Fe 2 O 6−δ is converted into long-range G-type antiferromagnetic order in SrCaFe 2 O 6−δ [1]. Similar changes in structure and electrical conductivity as a result of substitu- tion on the A-site have been observed for Sr 2−x Ca x FeCoO 6−δ [1]. Substitution on the B-site can also lead to changes in crys- tal structure and material properties. For example, the above mentioned tetragonal compound, Sr 2 Fe 2 O 6−δ , which has magnetic moments in spin-density wave state, can be modi- fed by replacing one of the Fe atoms with Mn. The resulting material, Sr 2 FeMnO 6−δ , has a cubic Pm-3m structure and * Farshid Ramezanipour farshid.ramezanipour@louisville.edu 1 Department of Chemistry, University of Louisville, Louisville, KY 40292, USA