Topochemical ion-exchange reactions between solid micro-and nanostructured metal chalcogenides and aqueous salt solutions are generally used for formation of composite structures based on initial metal chalcogenides and products of their ion-exchange transformation.However,ion exchange has promises as a route to obtaining both composites and solid solutions based on the initial and the end chalcogenide phases.With the help of the ion-exchange technique,single-phase films of Pb_(1-x)Sn_xSe substitutional solid solutions with a tin content up to~2 at.%,which are promising for mid-and long-wavelength infrared radiation(IR)optoelectronics,have been obtained at the interface between PbSe polycrystalline thin films and SnCl_2 aqueous solutions containing sodium citrate.It has been shown that the pH value and temperature of the reaction system play an important role in the ion-exchange process.Incubation of lead selenide(PbSe)films in a tin(Ⅱ)salt aqueous solution also leads to their modification with oxygen-containing tin compounds to a depth of~3 nm.Differences in the film structure,such as changes in the coherent scattering region sizes and orientation of crystallites along the[220]direction,which arise during the contact with citrate-containing SnCl_2 solutions,have also been revealed.For the first time,an idea of the existence of a relatively wide reaction zone of an intragranular topochemical ion-exchange reaction in an aqueous solution,within which substitutional solid solutions can form in micro-and nanostructured systems,has been set forth. Topochemical ion-exchange reactions between solid micro-and nanostructured metal chalcogenides and aqueous salt solutions are generally used for formation of composite structures based on initial metal chalcogenides and products of their ion-exchange transformation. However, ion exchange has promises as a route to obtain both composites and solid solutions based on the initial and the end chalcogenide phases. The help of the ion-exchange technique, single-phase films of Pb - (1-x) Sn_xSe substitutional solid solutions with a tin content up to ~ 2 at. %, which are promising for mid-and long-wavelength infrared radiation (IR) optoelectronics, have been obtained at the interface between PbSe polycrystalline thin films and SnCl 2 aqueous solutions containing sodium citrate. It has been shown that the pH value and temperature of the reaction system play an important role in the ion-exchange process. Assembly of lead selenide (PbSe) films in a tin (II) salt aqueous solution also leads to their modifi cation with oxygen-containing tin compounds to a depth of ~ 3 nm. Differences in the film structure, such as changes in the coherent scattering region sizes and orientation of crystallites along the [220] direction, which arise during the contact with citrate-containing SnCl_2 solutions, have also been revealed. For the first time, an idea of ​​the existence of a relatively wide reaction zone of an intragranular topochemical ion-exchange reaction in an aqueous solution, among which substitutional solid solutions can form in micro-and nanostructured systems has been set forth.