MEDIUM-RANGE ORDER AND COHESIVE ENERGY IN GEXSE1-X GLASSES

The study of the electronic structure of GexSe1-x glasses (x ≤ 0.33) is presented using the tight-binding approximation with a self-consistent Hartree model. The results indicate that the leading term in cohesive energy, when the possible structures of a glass with a given composition are compared, is due to the transfer of electronic charges from Ge atoms to Se atoms. Excluding Ge-Ge bonds from Raman scattering measurements, the calculation of the cohesive energy versus medium-range order confirms the tendency of Ge atoms to diffuse into the selenium matrix so preventing, as long as the germanium concentration makes this possible, the formation of Ge-Se-Ge sequences. For example GeSe4 glasses (x = 0.2) are mainly built from Ge-Se-Se-Ge sequences i.e., from GeSe4 tetrahedra. Edge-sharing GeSe4/2 tetrahedra are shown to enhance cohesive energy in glassy GeSe2; on the contrary edge-sharing GeSe4 tetrahedra do not maximise cohesive energy in glassy GeSe4.