Evidence of nanophase separation in Ge–Se glasses

Nanophase separation of two structural units, ethane-like Ge2(Se1/2)6 and Ge(Se1/2)4 tetrahedra, in GexSe1−x (x=0.30–0.37) glasses has been investigated by Raman scattering measurements in a temperature range 300–1000 K. For x<0.34, randomly distributed Ge2(Se1/2)6 ethane-like units induce an additional stress to the backbone of Ge(Se1/2)4 tetrahedral units. For x>0.34, a self-organization mechanism occurs in large regions of the nanoscaled-separated parts owing to: (i) a considerable concentration of ethane-like units, (ii) stresses within the regions, and (iii) lower constraints on the backbone network. For samples with x=0.36 and 0.37, each structural unit is stable below the glass-transition temperature, and crystalline regions of GeSe2 and GeSe appear simultaneously. In the lower Ge-concentration sample, with increasing temperature the local stressed structure relaxes through breaking of Ge–Ge bonds (constituting the ethane-like unit) and acquisition of a nearby Se atom (belonging to Se–Se wrong bonds) to construct corner-shared Ge(Se1/2)4-tetrahedral units, and thereafter various crystalline phases of GeSe2 results depending on the temperature.