Hello Everyone,
While carrying out the refinement process using Fullprof, at a certain stage the input information regarding the space group, cell parameters raise. My doubt is that if we know space group of the material how to find out the Wyckoff positions, occupancy parameters if not available in the literature. Please suggest me the details to get through.
I don't know how you get the space group w/o Wyckoff positions, but for the following I assume you have a certain space group (e.g. 139) and a material XY_2Z_2 wiht XYZ any sort of atom and the stoichometric ratio 122. Additionally, you have some idea about the density of your material (this is an important check in your fullprof output file; the density is calculated on the occupancy and volume of the unit cell).
For Spgrp 139 you would start putting X on either 2a or 2b, and Y and Z on 4c, 4d, 4e; you start with fully occupied sites and determine the Wyckoff position.If the density is too small (say only half) you move to the Wyckoff layers 4,8. If you suspect that Z maybe only 1.9 instead of 2 you can refine the occupancy afterwards.
Greetings Sir,
Firstly Thank you for your kind explanation. My material is A2B2O6 (double perovskite structure AA'0.5A'0.5B0.5B1.5O5.85) with space group P4/mmm (SP -123). The geometrical density of material after sintering is app 5.11. The volume from XRD is 115.8. Then sir, how should I determine the atomic positions and occupancy parameters while refining
Start with a closely related material, oxygen sits normally on 2g and 4i;
depending on you material A and A' can sit on the same site or on different sites; start with A on 1a and A' on 2h B on 1d and B' on 2g (with a z somewhere ~0.3); I would start with fully occupied sites and figure out first if they are on the same site or different sites; positions should make sense (check fst-file); then its a bit of checking different possibilities.
With fullprof you can knowing the specials positions.
- open WinPLOTOR
- Tools ---space groups info
- Window was opened--- write or insertion space groups number (123)
-Enter
I think, you can use Bilbao Crystallographic server (Assignment of Wyckoff Positions)
