The volume per molecular formula (V/Z) CH3NH3MI3 at room temperature is about 250 A^3 for M = Pb and about 242 A^3 for M = Sn. Since the volume of the Sn compound is smaller than of the Pb compound you may expect a decreasing volume of the unit cell (V) by doping with Sn. But decreasing of V would cause a shift of the peaks to higher values of 2theta. Only in the case of severe anisotropic behaviour some peaks might shift to lower peak positions while the others show no shift upto strong shift to higher peak positions.

But, you report a shift to lower 2theta after doping with Sn. Do you still have the same structure type after doping? Tetragonal space group I4/mcm? Or do you have the cubic polymorph?

Regards,

Robert

Paramita Sarkar

Yes, there will be shifting of XRD peaks during doping. The main reason in your case is the increase/decrease in the lattice parameters that is reflected by the shift of diffraction peaks. This effect is mainly due to the difference in ionic radii between the main element and the dopant ion. 

You can not generalize the value of maximum amount of peak shifting. Rather you can do checking the following, if the peak shifting arises from the instrument, i.e. by checking instrumental offset parameter.

Instrumental offset: If it is the case, the shift would be systematic i.e., all peaks will shift by almost similar magnitude. This shift can be determined easily by running a standard sample and the 2theta offset can be obtained.

Also, in plane and out of plane stress is also a reason for peak shift (you have to know residual stress of the sample).

Best Regards,

Dipayan

Ms. Sarkar,

Shift in the 2theta angle in XRD pattern strongly depends on ionic size and doping concentration in the sample, which directly affect the unit of the unit cell. It also depends on the site which doping ion occupies and other parameters as described by Mr. Dipayan.

Regards

Dinesh

Thanks Dipayan Pal Dinesh Kumar for the replies.

The volume per molecular formula (V/Z) CH3NH3MI3 at room temperature is about 250 A^3 for M = Pb and about 242 A^3 for M = Sn. Since the volume of the Sn compound is smaller than of the Pb compound you may expect a decreasing volume of the unit cell (V) by doping with Sn. But decreasing of V would cause a shift of the peaks to higher values of 2theta. Only in the case of severe anisotropic behaviour some peaks might shift to lower peak positions while the others show no shift upto strong shift to higher peak positions.

But, you report a shift to lower 2theta after doping with Sn. Do you still have the same structure type after doping? Tetragonal space group I4/mcm? Or do you have the cubic polymorph?

Regards,

Robert

Robert Haberkorn Thank you for your valuable comment Sir. I got the cubic structure.

It is much easier to take a real spectrum and calculate the cell parameter (using not the only one reflex, but the entire available set). If the parameter change is significant, i.e. more than an instrumental error, then it can be argued that the desired solid solution is formed. Of course, it is much better to make a series of samples (at least three, with different content of the doping element), then the dependence of the cell parameter on the concentration of the this element should be observed. Of course, in this series of solid solutions there should be no change in the structural type and the appearance of additional phases (i.e. additional XRD lines) [@Robert Habercom ]. Statistics starts with three samples, so it is better to do more, for example, five or even seven. It is better to do this at the same time (one-by-one) in order to minimize all human and instrumental factors.

It is worth remembering that XRD is a useful, but not the only way to detect the formation of solid solutions. By the way, the reasoning about the "ionic radii" is a rather weak basis.

As for the shift of the lines, it changes depending on the angle. It is easy to calculate this shift by the known formulas.

It is well discussed. In addition to the above discussed points, You can not expect peak shift always, even if you have successfully done the doping. Because the the peak shifts depend on more than one parameters(discussed earlier). Even if you didn't find peak shifts in the PXRD data of your doped sample, if you analyze the XRD data of your doped and pristine materials' sample using whole pattern powder fitting (WPPF) feature of the Rigaku PXDL software, you would be able to get the FWHM of particular facet of the materials used, d-spacing, and their lattice parameters. Then you can compare the values of doped sample with the pristine materials and would be able to see the parameters changes occurs in the doped materials.

Thanks a lot Shelton Jesuraj Varapragasam