Structure of Te-rich Te-Ge-X (X = I, Se, Ga) glasses

The structure of glassy Te₇₈Ge₁₁Ga₁₁, Te₇₉Ge₁₆Ga₅, Te₇₀Ge ₂₀Se₁₀ and Te₇₃Ge₂₀I ₇-promising materials for far infrared applications-was investigated by means of x-ray and neutron diffraction as well as extended x-ray absorption fine structure measurements at various edges. Experimental data sets were fitted simultaneously in the framework of the reverse Monte Carlo simulation technique. Short range order in Te₈₅Ge₁₅ was reinvestigated by fitting a new x-ray diffraction measurement together with available neutron diffraction and extended x-ray absorption fine structure data. It was found that Te₈₅Ge₁₅ consists mostly of GeTe ₄ structural units linked together directly or via bridging Te atoms. Te is predominantly twofold coordinated in Te₈₅Ge₁₅, Te₇₀Ge₂₀Se₁₀ and Te₇₃Ge ₂₀I₇ while in Te₇₈Ge₁₁Ga ₁₁ and Te₇₉Ge₁₆Ga₅ the Te coordination number is significantly higher than 2. The Te-Te bond length is 2.80 ± 0.02 Å in Te₇₈Ge₁₁Ga₁₁ while it is as short as 2.70 ± 0.02 Å and 2.73 ± 0.02 Å in Te₇₃Ge₂₀I₇ and Te ₇₀Ge₂₀Se₁₀, respectively. Our results show that the strengths of GeTe₄ (GeTe₃I, GeTe₃Se) 'units' are very similar in all glasses investigated but the connection between these units depends on the third component. Differences in the Te coordination number suggest that unlike Se or I, Ga does not build into the Ge-Te covalent network. Instead, it forms a covalent bond with the non-bonding p electrons of Te, which results in an increase in the average Te coordination number.