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iFit/rietveld

PURPOSE ^

model=rietveld(sample, instrument, ....) Rietveld refinement of powder/single crystal

SYNOPSIS ^

function y = rietveld(sample, instr, varargin)

DESCRIPTION ^

 model=rietveld(sample, instrument, ....) Rietveld refinement of powder/single crystal

 This function builds a fit model from:
   * a sample structure
   * a McCode instrument model (usually a diffractometer)
 The model can then be used for refinement as a usual fit model:
   fits(model, data_set, parameters, options, constraints)

 MODEL CREATION

 Any powder structure can be entered, and there is no limitation on the number
   of parameters/atoms in the cell. The powder/crystal model takes into account 
     Biso (thermal motions), charge, occupancy, spin
   Equally, the instrument model can be of any complexity, including sample
   environments, other sample phases (incl. amorphous), multi-dimensional PSD 
   detectors, ... It is possible to specify which monitor file to use from 
   the McCode instrument. The instrument resolution function is not computed
   using the legacy Caglioti formalism, but is fully convoluted with the
   sample component from the McStas instrument description.
   The default, and recommended, instrument is templateDIFF, but others are possible
   including TOF-diffractometers, PSD-diffractometers, and even Laue.

 Once set, the fit parameters may be constraint with either the usual 'constraints'
   argument to 'fits', or by setting the model constraints such as:
     model.Sample_a     = 'fix';     % fix 'a' lattice parameter
     model.Sample_alpha = [80 110];  % restrict alpha lattice angle in degrees
 
 Parameters of the model can be entered as any of:
 * a structure with one field 'structure.<atom>' per atom in the cell, named from the atom, e.g.
     p.structure.Al1=[x y z {Biso occ spin charge}] where {Biso occ spin charge} are optional
     p.Al1          =[...] can also be used for a compact specification (see exemple below).
   plus additional fields
     p.cell=[a b c alpha beta gamma] lattice cell parameters (Angs and deg)
     p.Spgr='space group' as Number, Hall or Hermman-Mauguin

   In addition, the 'CFML_write' field allows to set the name of the intermediate
     reflection file written by the CrysFML routines, and usable by the PowderN
     and Isotropic_Sqw McStas components. The default intermediate file is
     'reflections.laz'.
   The optional 'mode' argument to the sample description can be 'p' for powders
     (default) or 'x' for single crystal.

 * a vector array where Z is the atomic number
     p=[ a b c alpha beta gamma (Z x y z Biso occ spin charge) (...) ... ]

 * a CIF/CFL/PCR/shellX file name from which structure is extracted.

 * a McCode instrument description file name (.instr extension) to specify
     the instrument model to use. The default instrument is 'templateDIFF.instr'

 * a char with label/value pairs separated by ';' to build a structure.

 * the string 'defaults' or 'gui' to use a default model, or pop-up dialogue box.

 * All additional arguments are sent to the instrument model.

 MODEL EVALUATION

 The dimensionality and axes of the model is that of the monitor in the instrument, 
 and its parameters are that from the sample with that of the instrument.

 Example: refine a NaCaAlF powder structure with the templateDIFF McStas instrument
    Sample.title = 'Na2Ca3Al2F14';
    Sample.cell  = [10.242696  10.242696  10.242696  90.000  90.000  90.000];
    Sample.Spgr  = 'I 21 3';
    Sample.Ca1   = [0.46737  0.00000  0.25000  0.60046  0.50000   0.0   2.0];
    Sample.Al1   = [0.24994  0.24994  0.24994  0.18277  0.33333   0.0   3.0];
    Sample.Na1   = [0.08468  0.08468  0.08468  2.25442  0.33330   0.0   1.0];
    Sample.F1    = [0.13782  0.30639  0.12023  0.56669  1.00000   0.0  -1.0];
    Sample.F2    = [0.36209  0.36363  0.18726  1.42140  1.00000   0.0  -1.0];
    Sample.F3    = [0.46123  0.46123  0.46123  0.88899  0.33333   0.0  -1.0];
 The resulting hklF2 file 'reflections.laz' will be used by the instrument Powder 
 parameter and the model is computed at constant wavelength lambda=2.36 (given as char)
    f = rietveld(Sample, 'templateDIFF.instr',' Powder=reflections.laz; lambda="2.36"');
 you may plot the model (using McStas) in the [10 170] angular range:
    plot(f, f.Guess, linspace(-.2,.2,50),linspace(10,170,300));
 Then 'f' is an iFunc Rietveld model. Then perform the Rietveld refinement:
    p = fits(measurement, f); % where measurement holds a measured powder diffractogram

 See also: mccode, iFunc, iData, iFunc/fits
 (c) E.Farhi, ILL. License: EUPL.

CROSS-REFERENCE INFORMATION ^

This function calls: This function is called by:
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