A neutron diffraction and molecular dynamics investigation of the environment of Dy3+ ions in a fluoroberyllate glass

A combined neutron diffraction and molecular dynamics study is reported of the Dy3+ ion environment in vitreous NaF–DyF3–BeF2, using a special version of the isotopic substitution procedure known as the null technique. To investigate the effects of both NaF and DyF3 on the basic beryllium fluoride glass network, complementary measurements and simulations have been performed for vitreous NaF–BeF2, with the same NaF:BeF2 ratio as the DyF3 containing glass, and also for pure vitreous BeF2. The neutron diffraction data indicate that for glasses simulated using pair potentials the BeF4 tetrahedra are much more distorted than those in the real materials and the mean Be–F–Be angle is too high. Both of these deficiencies in the simulations are attributed to the use of simple ionic potentials with no bond angle restoring forces. Experimentally, the distribution of Dy–F first neighbor distances is found to be narrow, having a root mean square deviation of 0.110±0.003 Å about the mean value of 2.290±0.003 Å. The Dy(F) coordination number is 7.3±0.2 which is slightly less than that in crystalline DyF3.