Ana Ly Sis of Possi Bl E Sca Tte Ri Ng Proce Sses for Bloc H Positr on I Um in Ion Ic Cry Sta Ls at Ele Vate D Te M Pera Tu Re S

The role of non polar optic phonon scatterin g and of umklapp acoustic phonon scattering is analy zed for a delocali zed p ositroni um atom in ionic crystals at elev ated temp eratures. A t temp eratures above w hich they become essential both of these scattering mechanisms contribute to the imagina ry self-energy of p ositroni um renormalizi ng its acoustic deformation potential constant. I t w as such a renormalizati on that w as recently observed exp erimentally for delo cali zed positroni um in Mg F 2 crystal. W e put forw ard the theoretical explanati on of these experiments based upon the mo del of nonp olar optic phonon scattering of positroniu m. A n e ̃ect analogous to that observed in MgF 2 is predicted for crystalli ne quartz above the temp erature of ( ̃ À Ù )phase transitio n.