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

Measured lifetime of the ( 3 F 3 , 3 F 4 ) level of Pr 3+ 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 Pr 3+ 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 GaSe 4 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 GaSe 4 tetrahedra.