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CFBr3 Rietveld

CFBr3 A Simple Organic Molecule

Aim of Tutorial: This tutorial concerns a simple molecular solid. CFBr3 was measured on D1A at the ILL, Grenoble, with =1.911Å at T=1.5K. The data file is cfbr3_d1a_2k.gsas. Use inst file cfbr3.inst. The powder pattern indexes as orthorhombic, space group Pnam, with a = 8.50 Å, b = 9.65 Å, and c = 6.25 Å.
Molecular modeling suggests that the molecule lies on the mirror plane with the central C atom at (0.42,3/4,0.1) with one of the Br atoms on the mirror plane at (0.3,3/4,-0.15) and the other Br atom off the mirror plane at (0.42,0.6,1/4). The C-F and C-Br bond lengths are 1.33 Å and 1.93 Å, respectively, with the molecule having a tetrahedral geometry (109.5° inter-bond angles). The structure is fully ordered. This is an ideal problem for the use of molecular restraints functions since the F atom can be “added in” by this method.

Instructions
1. Read in the data and enter the partial atomic coordinates.

2. Set up bond distance restraints as you think necessary.

3. Try and deduce the position of the F by either geometric or other means.

4. Refine the structure to completion.

5. Output a .cif file and look at the molecule.

Outcome
A high precision structural determination of a freon.