It depends on the technique that you use. For example, preparting thin films with reactive magnetron sputtering, you have to pay attention to the gas flows. With other techniques like spray pyrolysis, for example, the clue is the preparation of the precursor solution.
bismuth nitrate, lathanum nitrate and titanium butoxide are the my starting precursors, i already prepared la-doped bismuth titanate in bulk using oxides, now i am planning for thin film. In bulk it is easy to maintain stoichometry.
I'm sorry but I haven't worked with those precursors. The only thing that I can say is that maybe you should try it again and characterize it with XPS. I suppose that you have done it, but this is research! Good luck!
I only want, how to maintain stoichometry. Because if we are using oxides them it is easy to maintain stoichometry, when we use any nitrides, chlorides, it is difficult. Here we have to use stabilizers compulsorily.
Firstly you chemical reaction should be well understood.
As you are preparing a ferroelectric oxide thin film by a chemical process.
Secondly during the process you may be have chances to loose one of the element, most commonly Bismuth.
Excess bismuth is always useful. You must get the phase correct after making your film, which means a thorough analysis through XRD. Additional phases should not develop. Then you can do composition analysis. In your case the chemical reaction control the composition, chemical bonding and the final phase formation.