Short range order in Ge-Ga-Se glasses

• The coordination number of Se is significantly higher than 2.
• Ga–Ge bonds can be found even in Se-rich compositions.
• The formation of Se–Se bonds increases the solubility of rare earth ions.
• The coordination number of Ga is 4.

Short range order of glassy Ge20Ga10Se70 and Ge20Ga5Se75 was investigated by neutron diffraction and extended X-ray absorption fine structure spectroscopy (EXAFS) at Ge, Ga and Se K-edges. For each composition large scale structural models were obtained by fitting simultaneously the four experimental datasets in the framework of the reverse Monte Carlo simulation technique. It was found that both Ge and Ga are predominantly fourfold coordinated. The quality of the fits was strongly improved by introducing Ge–Ga bonding. Models giving the best agreement with experimental data show that Ga has a complex effect on the Ge–Se host matrix: i) it enters the covalent network by forming Ga–Ge bonds ii) by decreasing the number of Se atoms around Ge, it contributes to the formation of Se–Se bonds, which may explain the higher solubility of lanthanide ions iii) the average coordination number of Se increases due to the Ga–Se ‘extra’ bonds. The higher average coordination of the network may be responsible for the increase of Tg upon adding Ga to Ge–Se glasses.