Lattice and molecular dynamics in (C2H5)4NFeX4(X=Cl,Br)

We have measured the thermal dependence of thef factor and the line shift of the title compounds, in order to determine the (−2) and (+2) moments of the Fe frequency spectrum. The latter is approximated by a superposition of an acoustic-wave band and an intramolecular optical mode. Our results show that both frequency bands are well separated, owing to a relatively weak interaction between tetrahedral units. Optical frequencies thus determined are consistent with Raman data. Debye temperatures were obtained for the low-frequency vibrations of both crystals.