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-FeSe0.96-Ag2Se (I) of the Ag-Fe-Pb-Se system were obtained from the melt. Their annealing at T < 600 K lead to the 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 separate regions of (I) was established by the electromotive force (EMF) measurements. The equilibrium phase formations in (I) at T < 600 K is characterized by the presence in the T-x space of the Ag2FeSe2 compound and of low-temperature modifications of AgFeSe2 and Ag2FePbSe4. The compounds were obtained by non-activation reconstruction of the metastable alloys of the positive electrodes in electrochemical cells (ECCs): (-) IE vertical bar Ag vertical bar SE vertical bar R (Ag+) vertical bar PE vertical bar 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 of Ag+ ions into PE. The formation of the equilibrium set of phases is facilitated by Ag+ that shifted from the left to the right electrode of ECCs. Silver cations act as the nucleation centers for new compounds. Formation of the three- and four-element compounds were established by the temperature dependence results of the EMF of ECCs with positive electrodes composed of different parts of the phase space (I). The AgFeSe2 and Ag2FePbSe4 compounds differ in thermal stability when obtained from the melt and by the synthesis under the conditions of the potential-forming process at T < 600 K. This is due to the difference in the crystal structures of high- and low-temperature modifications of the compounds. The reliability of the division of the equilibrium phase space (I) involving the AgFeSe2, Ag2FeSe2, and Ag2FePbSe4 compounds was confirmed by the calculated thermodynamic properties of these compounds. Non-activation synthesis of magnetic semiconductors in the potential-forming processes at relatively low temperatures expands the list of compounds and their solid solutions that may be of interest in spintronics applications.