Spatial distribution of rare-earth ions in Se-based chalcogenide glasses with or without Ga

Measured lifetime of the (3F3, 3F4) level of Pr3+ ions doped in representative Se-based chalcogenide Ge–Sb–Se glasses peaked at the mean coordination number of ∼2.67, indicating that inter-ionic distance between Pr3+ dopants became maximized on the average at such particular host compositions. However, this phenomenon was not observed when Ga was further introduced. It has been verified from Ga K-edge EXAFS spectroscopic analysis that a Ga atom has four Se atoms as its nearest neighbors in the Ge–Sb–Se glasses doped with Pr. The Ga–Se bonds are less covalent than other heteropolar chemical bonds, and thereby positively ionized rare-earth elements would be located preferentially next to the GaSe4 units, acting as charge compensators or network modifiers. We presume that there is a negligible structural and chemical correlation between a rare-earth ion and its next-nearest neighbors on their sites in the typical covalent chalcogenide glasses without Ga, and as a result spatial distribution of rare-earth ions would depend on changes of the free volume accompanied with the structural phase transitions on a medium-range scale from two-dimensional layers to three-dimensional networks. In selenide glasses with Ga, however, the empirical connection disappears due to the structural correlation between rare-earth ions and the GaSe4 tetrahedra.