Dear Ravi Kumar,

You should look between 2.5 and 25µm. If you have the possibility, you can go until 65µm, there is a vibration mode around 50µm, corresponding to vibration of La or Sr relative to Mn-O lattice.

Usually, in a ABO3 perfect cubic perovskite, you have three vibration mode, at around 18, 24 and 55µm (approximately, this is the typical value for LaFeO3, but its depends of the constituents, for instance it is 15.5µm, 38 et 89 µm for CaTiO3), corresponding to B-O stretching, B-O-B bending, and A vibration relative to B-O lattice, respectively. When you break the symmetry, you increase the number of the mode, up to 22 modes for LaFeO3 for instance (Pnma group), but they are mainly grouped in three band at 18, 24 and 55µm. 

So, in your perovsktie, you should have mode at around 10-20µm, 22-27µm, and 45-65µm. If you substitute La by Sr, you must have two mode around 45-60µm, but the other mode should be less affected (due to B-O bound) compare to LaMnO3 or SrMnO3.

Regards,

Emile HAYE

PS: to see the effect of the RE on the FTIR spectra, you can look at figure 8 on the following paper "Properties of rare-earth orthoferrites perovskite driven by steric hindrance" Journal of Alloys and Compounds, Volume 657, 5 February 2016, Pages 631–638, a recent paper that I recently wrote.