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

The structure of glassy Te(78)Ge(11)Ga(11), Te(79)Ge(16)Ga(5), Te(70)Ge(20)Se(10) and Te(73)Ge(20)I(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 Te(85)Ge(15) 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 Te(85)Ge(15) consists mostly of GeTe(4) structural units linked together directly or via bridging Te atoms. Te is predominantly twofold coordinated in Te(85)Ge(15), Te(70)Ge(20)Se(10) and Te(73)Ge(20)I(7) while in Te(78)Ge(11)Ga(11) and Te(79)Ge(16)Ga(5) the Te coordination number is significantly higher than 2. The Te-Te bond length is 2.80 ± 0.02 Å in Te(78)Ge(11)Ga(11) while it is as short as 2.70 ± 0.02 Å and 2.73 ± 0.02 Å in Te(73)Ge(20)I(7) and Te(70)Ge(20)Se(10), respectively. Our results show that the strengths of GeTe(4) (GeTe(3)I, GeTe(3)Se) '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.