I have been taking IR spectra of nitrate compounds for my dissertation. I am aware that ionic bonds will not generate an absorption band on the spectra and therefore any absorption band must be generated by the nitrate ion.

I have found a journal article from 1961, which shows that in a nitrate there will be a medium sharp peak between 800-850 cm-1 and a strong peak around 1350. Which I am presuming is N=O stretch and N-O stretch from left to right.

I have attached an IR of both sodium nitrate, which shows the two peaks within the wavenumbers mentioned above, with the addition of three further peaks. Can anybody provide any insight into what may have caused these peaks? Thank you.