I have a graph that have a kink in it and I found the reason of this kink in two resources ,there are two reasons accure this kink ;I will put them here in a braket .one has to keep in your mind we have to say “we think this is a reason of the kink since we are not sure ”

these are the reasons
{{A splitting (kink) is seen in =240 cm-1 mode.
first reasons
The splitting occurs in this mode because this mode is expected to be sensitive to disorder in La sublattice, for instance that resulting from the partial replacement of La by Sr.The substitution of Sr on the La sites may be the direct origin of the splitting of this mode observed for LSCO samples[article11]

second reasons
Essential splitting of the longitudinal optical (LO) and transverse optical (TO) modes in the zone center was observed, which is caused by long-range Coulomb forces. Upon the transition to metallic state, this splitting disappears due to the screening of the Coulomb forces in the CuO2 planes by free charge carriers. On the other hand, vibrations of ions perpendicular to the CuO2 planes preserve their ionic nature the in view of the small screening of these displacements. In most cases, quite satisfactory theoretical description of phonon modes was obtained within the framework of the conventional shell model with a short- range repulsive potential and an anisotropic screening in the metallic phase .[book 22]

as you see I gave you the citation of them just but what i put and i will know which one you choose