Compositional dependences of some optical and electrical properties of theGexAs40-xSe60 system

The compositional dependences of the Urbach energy E-U, dark conductivity a nd its activation energy have been measured in the GexAs40-xS60 system. Two regions of E-U increase with different steepness and a shoulder at x = 20 to 27 have been observed. A well expressed minimum of the dark conductivity at x = 27 to 30 corresponding to an average coordination number Z approxim ate to 2.7 has also been found. The observed peculiarities in the compositi onal dependences studied are connected with the topological phase transitio n expected at Z = 2.67 as well as with compositionally induced changes in t he short-range order.