Analysis of possible scattering processes for bloch positronium in ionic crystals at elevated temperatures

The role of nonpolar optic phonon scattering and of umklapp acoustic phonon scattering is analyzed for a delocalized positronium atom in ionic crystal s at elevated temperatures. At temperatures above which they become essenti al both of these scattering mechanisms contribute to the imaginary self-ene rgy of positronium renormalizing its acoustic deformation potential constan t. It was such a renormalization that was recently observed experimentally for delocalized positronium in MgF2 crystal. We put forward the theoretical explanation of these experiments based upon the model of nonpolar optic ph onon scattering of positronium. An effect analogous to that observed in MgF 2 is predicted for crystalline quartz above the temperature of (alpha-beta) -phase transition.