$^{11}B$ NMR spin-lattice relaxation study in $Nd^{3+}$-doped binary alkali borate glasses: A possible method to look at the environment of rare-earth sites in glasses

Rare-earth (RE) doped glasses are widely used for laser and optoelectronic applications and their optical properties depend on the host glass matrix to a large extent. RE doped alkali borate glasses are interesting to observe the effect of network modifiers on the RE environment. In spite of several studies on RE doped glasses, a consensus on the RE environment is still elusive. In this communication we are able to show the effect of the modifier ion on the RE environment in $30R_{2}O-69.5B_{2}O_{3}-0.5Nd_{2}O_{3}$ (R=Li, Na, K, Rb and the composition is in mol %) by measuring $^{11}B$ Nuclear Spin Lattice(NSLR) time over a temperature range 100-4.2 K. These measurements allow us to evaluate the crystal field splitting (\Delta) of the two lowest lying multiplets of the ground state $(4I_{9/2})$ of $Nd^{3+}$. We showhere that a systematic variation of \Delta with the type of alkali can be correlated to the findings from optical and EXAFS measurements. Our results suggest that the next nearest neighbour of RE is the modifierion itself. We also show that this method has a potential in probing the Medium Range Order (MRO) in glasses which is otherwise difficult to probe by optical or EXAFS measurements