Contents lists available at ScienceDirect Thermochimica Acta journal homepage: www.elsevier.com/locate/tca Reaction mechanism of strontium cobaltite synthesis from equimolar mixture of Sr(NO 3 ) 2 and Co(NO 3 ) 2 ∙6H 2 O under air atmosphere Cemal Aslan, Sedat Ilhan, Ahmet Orkun Kalpakli, Cem Kahruman, Ibrahim Yusufoglu ⁎ Istanbul University-Cerrahpasa, Engineering Faculty, Metallurgical and Materials Engineering Department, 34320, Avcilar, Istanbul, Turkey ARTICLEINFO Keywords: Strontium cobaltite Brownmillerite Perovskite Characterization Thermal analysis ABSTRACT Strontium cobaltite was produced by heating the mixture containing equimolar amounts of Sr(NO 3 ) 2 and Co (NO 3 ) 2 ∙6H 2 O under dynamic air atmosphere. The reactions occurred during heating were determined and the intermediate and fnal products obtained at each reaction step were characterized using TG/DTA-MS, ICP-OES, XRD and FT-IR techniques. The oxygen stoichiometry of strontium cobaltite was determined using iodometric titration method and carbonation process. It was determined by thermal analysis results that Co(NO 3 ) 2 ∙6H 2 O was decomposed to CoO by forming the intermediate products Co(NO 3 ) 2 ∙4H 2 O, Co(NO 3 ) 2 ∙2H 2 O, Co(NO 3 ) 2 ∙H 2 O, Co(NO 3 ) 2 , CoOOH, Co 2 O 3 and Co 3 O 4 .Sr (NO 3 ) 2 was decomposed to SrO. Sr 6 Co 5 O 15 was formed by solid state reaction of SrO, Co 3 O 4 and O 2 . Sr 2 Co 2 O 5 was produced at elevated temperatures from Sr 6 Co 5 O 15 and Co 3 O 4 . Analyses carried out for the characterization of compounds obtained at diferent isothermal conditions showed that fnal product Sr 2 Co 2 O 5 was obtained via Sr 14 Co 11 O 33 and Sr 6 Co 5 O 15 .Sr 2 Co 2 O 5 was decomposed to a mixture of Sr 6 Co 5 O 15 and Co 3 O 4 with a mole ratio of 1 : 0.333 during slow cooling to the room temperature. 1. Introduction Strontium cobaltite (SrCoO 3-δ ), which is one of the oxides in the ABO 3-δδ perovskite crystal structure, has a wide range of oxygen stoi- chiometry depending on the temperature, oxygen partial pressure, and heat treatment method, and therefore has diferent crystalline struc- tures. Strontium cobaltite has oxygen permeability, ionic and electronic conductivity and electrochemical activity. Strontium cobaltite is used as gas sensor, gas detection probe, oxygen separator membrane, cathode material in solid oxide fuel cells, solid electrolyte and electrode mate- rial in super capacitors. Strontium cobaltite, which has mixed ionic and electronic conductivity (MIEC), has recently become increasingly im- portant [1–11]. Rodriguez et al. [12,13] indicated that Sr 2 Co 2 O 5 has rhombohedral structure at low temperatures while it has brownmillerite structure which is a modifed perovskite structure at temperatures higher than 1173 K and there is no diference between chemical composition of these two structures. While the high temperature structure of Sr 2 Co 2 O 5 remains stable when quenched, it undergoes changing during slow cooling. According to Takeda et al. [14], strontium cobaltite has brownmillerite or perovskite structure depending on oxygen defciency at temperatures above 1073 K. While the sample, quenched to room temperature from the temperatures between 1073 and 1273 K, has brownmillerite structure, sample decomposes into cubic perovskite (SrCoO 2.29 ) and brownmillerite structure (SrCoO 2.42 ) during slow cooling. The sample annealed at the temperatures below 1073 K con- sists of cobalt-defcient strontium cobaltite (SrCo 1-u O x , u ≈ 0.1) and Co 3 O 4 . Harrison et al. [15] reported that Sr 2 Co 2 O 5 is decomposed to Sr 6 Co 5 O 15 and Co 3 O 4 during cooling in air for the frst time. Vashook et al. [4,5] found that SrCoO x gains and gives of oxygen during cooling and heating, respectively. They emphasized that oxygen exchange is related to phase transformations explained by Rodriquez et al. [13] between 773 K and 1223 K and is related to the ordered-disordered mechanism of oxygen vacancies in the cubic phase between 1238 and 1273 K. When δ values for SrCoO 2.5-δ are less than 0.16, between 0.16 and 0.21 and greater than 0.21, rombohedral, cubic perovskite-like and disordered-cubic perovskite phases formed, respectively. While XRD and DTA results do not show any phase change up to 1173 K, above this temperature, transformation from rhombohedral phase to cubic phase is seen. According to the results of TG/DTA, the transformation from rhombohedral to cubic phase occurs at 1173 K in air and 1151 K in Ar during heating. DTA results show the transformation to low tempera- ture phase with exothermic peaks at 1051 K in air and 995 K at Ar during cooling. Ito et al. [16] determined the valence numbers of cobalt atoms in the lattice of the brownmillerite structure as 4+ for the oc- tahedral location and 2+ for the tetrahedral location. This charge https://doi.org/10.1016/j.tca.2019.03.030 Received 28 November 2018; Received in revised form 20 March 2019; Accepted 24 March 2019 ⁎ Corresponding author. E-mail address: yusufogl@istanbul.edu.tr (I. Yusufoglu). Thermochimica Acta 676 (2019) 52–63 Available online 01 April 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved. T