The structural, electronic and magnetic properties of Fe 7 S 8 material have been studied within the framework of the ab-initio calculations, the mean field approximation (MFA) and Monte Carlo simulation (MCS). Our study shows that two forms of the iron atoms, Fe 2+ with spin S=2, and Fe 3+ with spin =5/2 are the most probable configurations. A mixed Ising model with ferromagnetic spin coupling between Fe 2+ and Fe 3+ ions and between Fe 3+ and Fe 3+ ions, and with antiferromagnetic spin coupling between Fe 2+ ions of adjacent layers has been used to study the magnetic properties of this compound. We demonstrated that the magnetic phase transition can be either of the first or of the second order, depending on the value of the exchange interaction and crystal field. The presence of vacancies in every second iron layer leads to incomplete cancellation of magnetic moments, hence to the emergence of the ferrimagnetism. Anomalies in the magnetization behavior have been found and compared with the experimental results.

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This work aims to study the optoelectronic and thermoelectric properties of the new perovskites LiSnX3 (X[Formula: see text]Br or I). This study is carried out by using the density functional theory (DFT) combined with the Boltzmann transport theory. First, the investigation of the structural, electronic and optical properties of such materials, using the Generalized Gradient Approximation (GGA), has been performed. This latter method has been performed under the Perdew–Burke–Ernzerhof (GGA–PBE) approximation applying the exchange correlation potential. It is found that the two compounds exhibit a p-type semiconductor characteristic and interesting optical properties. Moreover, the thermoelectric properties of the studied materials, such as the Seebeck coefficient, electrical conductivity, thermal conductivity, figure of merit and power factor, have been examined. It is found that the Seebeck coefficient takes positive values. This finding reveals and confirms the p-type semiconductor characteristic for two studied compounds. On the other hand, such materials display large values of the figure of merit (ZT) in comparison with the existing thermoelectric materials. Such large values of ZT are found for both small or large temperature values. The obtained results of the optoelectronic and thermoelectric properties show that these materials could be useful both for photovoltaic and thermoelectric applications.