3400-3600 (O-H stretch, Alcohols RCH2OH or R2CHOH or R3COH) or

3550-3600 (=NOH(OH stretch) =NOH oxime)

I agree with Ali about the probability it's an OH stretch. But it could be many things. The free OH stretch of water in the gas phase is about 3715 cm-1. But, it's pretty delicate and can easily red shift to 3600 cm-1 depending on what it's bonded to, or resting on. We work with TiO2 in our lab and we've studied how methanol binds to the surface using SFG. We've found it can be physisorbed, or chemisorbed. When chemisorbed, the R group and OH group are dissociated. Because of this, the two types of surface attachments have different red shifts.

Some questions might be: How do you make your measurement? ATR? How do you prepare your sample? Is it dry? Under vaccuum? A mull? Is there a possiblity an alcohol could be present? Is the sample exposed to the atmosphere? Is there water around? I don't expect you to answer these questions here. But you should consider them when you're trying to identify your "hump" at 3600cm-1.

Actually what we could understand from our FTIR spectra that OH stretch is appearing near about 3400 cm-1 but apart from that we are getting a hump near 3600 cm-1. Please see the attach file that would make the thing more clear. We have used the KBr method. Our samples were in powder form.

Shahid:

Very interesting famly of plots. You might want to do spectrum subtraction here to further emphasize how the spectrum is changing as you increase the Zn doping. This should bring out the very small hump you're seeing at 3600 cm-1 and other changes in the spectrum. An OH stretch appearing at about 3400cm-1 is typical of a hydrogen bonded water molecule where BOTH OH groups are H-bonded. That implies you have quite a bit of water in your KBr pellet. One at about 3600 cm-1 might be a OH bonded to one of the metals.

How about your synthesis? Combustion method folowed by sintering? If so, I don't think you've very much water in the original sample. On the other hand, you might have water in the KBr. Are you baking the KBr to insure it's also dry? You'll probably also want to pay attention to atmospheric water. If at all possible, do as much sample prep in dry nitrogen as you can. You might want to purge the FTIR sample chamber with dry nitrogen for some period of time before making the measurement. Most FTIR instruments allow this. You might also try letting the KBr pellet age in the open air over some period (perhaps days) and then taking periodic IR spectra and looking for changes with time. Doing spectrum subtraction works well here to track changes. That could shed some light on what's going on.

Patrick

What I could understand from your answer that hump near about 3600 cm-1 is related to water or moisture. Most probable cause in your view is OH bonded to one of the metal ion either La or Fe or Zn. Kindly correct me if i'm interpreting in wrong way.

I seen FTIR spectra and should to note that hump near about 3600 cm-1 also presence in undoped LaFeO3. For Zn-doped samples this hump is more clear. Therefore, in this case one should speak of a more clear manifestation of the hump as a result Zn doping.

Shahid:

Sorry for the late reply ... I guess what I'm implying is that since you're using a KBr pellet, and the fact that the hump is very weak, I suspect you might have water contamination in the KBr. I would note that it's very difficult to get vibrational modes up around 3600cm-1 for anything besides an X-H bond (X=just about anything). This is because any other combination has too much reduced mass to oscillate at such a high frequency. Of course, if the bond strength is extremely high, it's possible even with a heavier pair of atoms. But, in your system, that's unlikely.

Now, since the mode is at 3600cm-1, and assuming it's water, that means it's Hydrogen bonded (to something), and weakly bonded too. Strong hydrogen bonds can red shift the (free) OH frequency (@3715cm-1) as far down as 3000cm-1 (depends on who you talk to). A weak hydrogen bond will red shift the free OH peak less. If it's a red shift of only 100cm-1, almost any weak interaction with the water molecule could be at play. Even if a water molecule, introduced via contaminated KBr, is physisorbed on the surface, such a red shift is conceivable.

Because the peak is very weak, I'll say that there's not a lot of whatever is responsible for the mode (intensity proportional to number density). This again supports the idea of water as a contaminant.

My suggestion is to re-examine your preparation procedures. Take extra efforts to dry your samples and the KBr. I'd give it a couple of hours, even over night at 120C. Do the cool-down in a good quality desiccator. Don't forget to dry the mortar and pestle too. Wear gloves at all times. Be sure to purge the sample chamber in the FTIR instrument with dry N2 (e.g.: boil off from a LN2 tank) for several hours (really!) before you make the measurement. Then, after you insert your sample (you've opened the door now), purge the sample chamber some more before making your measurement.

Also, get used to doing spectral subtraction. It's a valuable technique in FTIR for removing background or monitoring small changes. For example, if you want to monitor the effect of Zn doping, do a measurement on an un-doped sample, then repeat with a doped sample. By subtracting the un-doped spectrum (possibly scaled), you'll more clearly see any changes in the doped sample. With this technique, be sure to preparation procedures are identical (insure the same mass of sample in the KBr pellet especially) between the two samples. You can scale the reference spectrum of course, but it helps to remove as much variation as possible.

@ Patrick : Shahid performed the measurements both Zn-doped and undoped samples. These spectra presented above. Indeed, subtracting of spectra provides information on the effect of doping on the spectrum. However, if you want to see the qualitive change of the spectra as a result of doping ii is first necessary to analyze each spectrum. In this case, the hump near about 3600 cm-1 also presence in undoped LaFeO3. For Zn-doped samples this hump is more clear.

And, if the KBr is "wet" in both cases, the hump will be there in both cases will it not? It's possible that the Zn doped ceramic interacts with the water in a slightly different way too. I still think drying the KBr and sample and purging the instrument is still in order. The hump is very small, after all. So whatever it is, it's not in high concentrations. If the hump goes away (even just partly) after thoroughly drying the KBr, sample, and purging the instrument, that says a lot.

I also think that a problem with the hump is caused with the preparation procedures of different samples.