P21 is a subgroup of Pna21, assuming the orientation of the lattice parameters is the same. Fully written it would be P_ _ 21 in this case. You should check your P21 structure for n and a symmetry elements missed; it might be Pna21 in reality. Or it might be an ordered form, with Pna21 disordered. Or something else. The actual possibilities depend a lot on things not entirely clear from your description: 1) Does different ratios of starting material also mean different composition of the educt? 2) By "structure", do you mean the atom positions or just the unit cell? 3) Does same PXRD refer only to peak positions or also to peak intensities? The conventions for all three are different in different scientific communities.

P21 is a subgroup of Pna21, assuming the orientation of the lattice parameters is the same. Fully written it would be P_ _ 21 in this case. You should check your P21 structure for n and a symmetry elements missed; it might be Pna21 in reality. Or it might be an ordered form, with Pna21 disordered. Or something else. The actual possibilities depend a lot on things not entirely clear from your description: 1) Does different ratios of starting material also mean different composition of the educt? 2) By "structure", do you mean the atom positions or just the unit cell? 3) Does same PXRD refer only to peak positions or also to peak intensities? The conventions for all three are different in different scientific communities.

If the powder diffraction diffractograms are similar (peak position and intensity) then it is very likely that the P21 structure is mistaken: a higher symmetry was missed and the identity with Pna21 structure was not recognized .

Piero Macchi

Thank you very much for your valuable suggestions. I had one general doubt that if any single prominent peak is missing in comarative results of Pna21 & P21 (For same composition), is there any refinement error or how i predid it