In the present paper we have focus our study on relaxation of photodarkening in α-As₂Se₃ doped with Sn and rare-earth (RE) ions (Dy³⁺, Pr³⁺, Ho³⁺, Sm ³⁺, Nd³⁺, Er³⁺). The relaxation process of photodarkening is described by a stretched exponential function with the dispersion parameter 0<α<1.0, and the time constant increasing with doping and heat treatment. On the basis of the structure-property correlation, the role of metal impurity in reformation of molecular cluster structure was studied with particular attention to retardation of relative "slip motion" of clusters by the impurity. The case of Sn in As ₂Se₃ glass was probed by a local probe like ¹¹⁹Sn Mössbauer spectroscopy, and in conjunction with Temperature Differential Scanning Calorimetry (TMDSC) some aspects of the molecular glass structure were elucidate. Recommendation concerning the stabilization of the glass structure by metal impurity against photostructural transformation have been suggested, and which are very important for chalcogenide glasses doped with rare-earth ions as perspective materials for fibre optics amplifiers operating at the 1.3 μm and 1.5 μm telecommunication windows.