I'm studying a deposited thin film, in powder form. I see both (110) and (220) peaks. However in what appears to be an epitaxial film, I only see a very strong (220) peak but not a (110) peak.. is that possible? I am aware of plane "Extinctions" between parallel planes, i.e. FCC systems (100) peaks are cancelled by (200) peaks due to destructive interference. But if some crystal is proven to show both (110) and (220), can a single crystal only show (220)?
In bcc system (h+k+l) should be even and for fcc system(h,k,l) should be either odd or even. Accordingly, if your system is bcc (110) and ( 220). In bcc system, in case of epitaxial system, there is a possibility for ( 22o) to be strong. But in powder pattern one will notice (both 110) and ( 220 ) reflections.
The systematic extinctions conditions for a FCC lattice are h+k, h+l, k+l = odd. In other words, in a FCC lattice the indices of a given reflection must be either all even or all odd to result in a nonzero intensity. This implies that in a FCC lattice the 110 reflection is necessarily forbidden. Note that this has nothing to do with the actual existence of real lattice planes in your bulk material. Actually, the real plane family (110) certainly exists even in a FCC lattice, but when it is Bragg-excited it simply gives rise to null intensity due to the destructive interference that arises from the translational symmetry elements present in the cell. Moreover, the specific symmetry of your space group may add some other extinction conditions.
Therefore, if you observe a non-zero intensity for the 110 reflection some possible explanations could be:
(i) your actual symmetry is lower than you expect;
(ii) your sample contains some amount of another crystal phase (in other words, the '110' peak you observe is spurious);
(iii) your sample is monophasic, but some other material is interfering with your measurements.
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