OPTICAL PHONONS IN TYSONITE RARE EARTH FLUORIDES.

This work was supported in part by the Joint Services Electronics Program (Contract DA 28-043-AMC 02536(E)), by a N.A.S.A. grant NGR22-009-(237) and by the U.S. Air Force (ESD Contract AF19(628)-6066). Present address of R.P. Lowndes and C.H. Perry: Dept. of Physics, Northeastern University, Boston, Massachusetts 02115.