xz with coupling strengths of 0.5, 0.0, 1.0, 1.5, and 1.0 cm -1 , and the spin system is modelled with D = -113 cm -1 . The resulting simulation reproduces the main features of the experimental spectrum, see Fig. and. We note in particular that the perturbation of the phonon at 234 cm -1 has an onset at about 6 T which is in line with the strongest expected Zeeman shift (for parallel field) of the -3/2 → -1/2 spin transition, see also Fig. ). In summary, the magnetic-field dependent FIR and Raman spectra provide us with a very accurate estimate for the axial zero-field splitting parameter of 1 of D = -113 cm -1 with a conservative error margin of ±2 cm -1 . Two further features were observed in the experimental Raman spectra (Fig. ). Feature 1B is located at 838 cm -1 in zero field and was attributed to the transition from the ground KD of the ground quartet to the ground KD of the excited quartet state of the cobalt ion. In field, it shows only a very small shift of less than 1 cm -1 to lower energies with increasing field. However, the experimental difference spectra in Fig. indicate that magnetic-field-induced changes do occur, especially at field strengths above 6 T. This behavior can only be explained in terms of a phonon transition that is perturbed by spin-phonon coupling. The computations predict a very small magnetic moment of the M = ± 1 2 KD of the excited quartet state (Fig. ), so the Zeeman shift of the spin transition is mainly due to the initial ground quartet M = -3 2 state, and, hence, the transitions to the excited quartet M = -1 2 and M = + 1 2 states have very similar transition frequencies. In order to explain that a stronger magnetic-field dependent distortion only occurs for fields above approximately 6 T, we have to assume that the zero-field origin of the spin transition is located at lower energies than the phonon transition, around 820 cm -1 , and that it only couples to the phonon at 838 cm -1 . In order to include the spin-phonon coupling, we have to modify the coupling term, Eq. 7 for the 'model m2'. The coupling can be either modeled by modulation of the xz or yz component of the D 0 contribution to the ZFS (D (a) 0,xz,yz ), see Eq. 3, or by modulation of the spin-orbit coupling between the two quartet states (in this case, a coupling via the xor y-component of spin-orbit coupling, γ