   Exercise: Analysis of the spinel MgAl2O4
   ----------------------------------------
   Prepare an IRF file for the neutron powder diffractometer 3T2, determining
   cation distribution and analyse a sol-gel sample for obtaining the new
   cation distribution and microstructural parameters

- Analysis of the spinel MgAl2O4.
  Reference: Valerie Montouillout, PhD Thesis. Universite d'Orleans 1998.

  Information from literature:  Spinels AB2O4 may be prepared with a
  degree of inversion between octahedral and tetrahedral cation positions.
  Space Group: F d -3 m
          Setting 1:
               Tetrahedral A-positions (Mg,Al)  8a  (0,0,0)
               Octahedral  B-positions (Mg,Al)  16d (5/8,5/8,5/8)
               Oxygen positions        (O)      32e (x,x,x) x=0.386
          Setting 2:
               Tetrahedral A-positions (Mg,Al)  8a  (1/8,1/8,1/8)
               Octahedral  B-positions (Mg,Al)  16d (1/2,1/2,1/2)
               Oxygen positions        (O)      32e (x,x,x) x=0.261

  Unit cell  : a = 8.08 Angstroms


  Two neutron powder diffraction data files called:

       MgAl2O4s.dat  -> Sample obtained by conventional high temperature
                        solid state reaction
       MgAl2O4b.dat  -> Sample obtained by sol-gel reaction at low T.

  The instrument is the old 3T2 neutron powder diffractometer at the LLB.
  The wavelength is Lambda = 1.227 Angstroms, the approximate U,V,W parameters
  are: U=0.276,  V=-0.340,  W=0.147. The format of the data files corresponds
  to INSTRM=1 in FullProf.


(1) Start the analysis using the first file (well crystallized powder) in order
    to obtain the resolution function of the instrument. Start directly with
    WinPLOTR trying to determine "ab initio" the unit cell, background, etc.

(2) Refine the structure allowing the distribution of Mg and Al between the two
    available cationic sites.

(3) This sample contains a small parasitic phase of MgO (S.G. Fm3m,
    a = 4.22 Angstroms). Determine the percentage of this impurity.

(4) Obtain the best refinement of profile parameters using the function NPROF=7.
    Use the refined parameters to construct an Instrument Resolution File.

(5) Refine the second diffraction pattern obtained with a sample prepared at
    low temperature (not well crystallized). Determine the cation distribution
    and the microstructural parameters: domain crystallite size, microstrain,...


