Anomalous homogeneous optical absorption line broadening of Eu3+ in superionic PbF2

Homogeneous broadening of one of the ⁵D₁←⁷F₀ absorption lines of Eu³⁺ in PbF₂ was measured in the temperature range 300–800 K. Below 625 K the linewidth depends linearly on T; above 625 K it increases rapidly followed by a plateau beginning near 700 K. This behavior is quite distinct from that expected for the normal Raman process in crystalline materials and appears to be correlated with F^- ion motion responsible for the superionic conductivity of PbF₂.