Figure 2:

The NRVS spectrum of (et₄N)⁵⁷FeCl₄. The low energy intensity is related to lattice phonons and the features at 378 and 139 cm⁻¹ can be approximated as T₂ modes in T_d symmetry. The energies of these modes are used to calibrate NRVS spectra. Noticeably absent is the A₁ symmetric stretch as that mode does not involve ⁵⁷Fe motion.