Solid-state electrochemical synthesis and thermodynamic properties of selected compounds in the Ag–Fe–Pb–Se system

The AgFeSe₂ and Ag₂FePbSe₄ compounds within the phase region Ag₂Se–PbSe–Se–FeSe_0.96–Ag₂Se (I) of the Ag–Fe–Pb–Se system wereobtained from the melt. Their annealing at T<600 K lead tothe decomposition into binary phases of the Ag–Se, Pb–Se, and Fe–Se systems. The metastable state,for kinetic reasons, of alloys of mixtures of the binary compounds in separateregions of (I) was established by theelectromotive force (EMF) measurements. The equilibrium phase formations in (I) at T<600 K is characterized by the presence in the Т–х space of the Ag₂FeSe₂ compound and of low-temperaturemodifications of AgFeSe₂ and Ag₂FePbSe₄. The compoundswere obtained by non-activation reconstruction of the metastable alloys of the positiveelectrodes in electrochemical cells (ECCs): (−)IE | Ag | SE | R (Ag⁺) | PE | IE(+),where IE is the inert electrode (graphite), SE is the solid-state Ag⁺ ion-conducting electrolyte, PE is the positive (right) electrode,R (Ag⁺) is the region of the penetration ofAg⁺ ions into PE. The formation of the equilibrium set of phasesis facilitated by Ag⁺ that shifted from the left to the rightelectrode of ECCs. Silver cations act as the nucleation centers for newcompounds. Formation of the three- and four-element compounds were establishedby the temperature dependence results of the EMF of ECCs with positiveelectrodes composed of different parts of the phase space (I). The AgFeSe₂ and Ag₂FePbSe₄compounds differ in thermal stability when obtained from the melt and by thesynthesis under the conditions of the potential-forming process at T<600 K. This is due to thedifference in the crystal structures of high- and low-temperature modificationsof the compounds. The reliability of the division of the equilibrium phasespace (I) involving the AgFeSe₂, Ag₂FeSe₂, and Ag₂FePbSe₄ compounds was confirmed by thecalculated thermodynamic properties of these compounds. Non-activationsynthesis of magnetic semiconductors in the potential-forming processes atrelatively low temperatures expands the list of compounds and their solid solutionsthat may be of interest in spintronics applications.