Parameters

The sub-menu Parameters is shown on the left. It allows to visualize and modify:

· Global Parameters

· Phase Dependent Parameters

· Restrictions on Parameters

 

The sub-menu is available at different stages of the run:

1) when the message

Press "Parameters" to supply your starting value or "Continue" to go on

is displayed at the bottom of the window; at this stage is possible to supply starting parameter values to be used in the Preliminary procedure, before a total pattern is calculated;

2) when the message

Press "Refinement" to set your own strategy or "Continue" to go on

is displayed at the bottom of the window and the initial calculated pattern is plotted; at this stage is possible to modify the starting parameter values supplied by the Preliminary procedure, to be used in the Rietveld refinement. Click on Continue button to apply modifications;

3) when the message

Press "Refinement" to set your own strategy or "Exit" to stop

is displayed at the bottom of the window and the final pattern is plotted; at this stage is possible to modify the refined parameter values supplied by the automatic Rietveld or user refinement procedure. Click on Continue button to apply modifications;

 

 

Global

The Global option opens the dialog-box shown in Fig. 5.1.1

Fig. 5.1.1

 

It allows:

§ To supply or modify the 2q zero-shift correction

§ To select the background model function among:

o Young’s Polynomial

o Linear Interpolation

o Chebyshev approximation

§ To supply the number of background coefficients (for Young’s Polynomial and Chebyshev approximation)

 

Click OK to confirm or Cancel to leave the sub-menu without any change; click on Continue button to apply modifications.

 

 

Phase Dependent

The Phase Dependent option allows to manage all the parameters which are dependent on the selected phase. It is organized in 6 folders: Parameters, Prof. Shape, Anis. FWHM, Cell, Structure, Anis. Mix. Par.. Each folder opens a suitable dialog-box and displays the current values of the managed parameters for the phase selected at the top of the box. At the end of all parameters modifications and independently on which folder you are, click OK to confirm or Cancel to leave the Phase Dependent window without any change; click on Continue button to apply the modifications.

 

 

Parameters

The folder Parameters opens the dialog-box shown in Fig. 5.2.1

Fig. 5.2.1

 

It allows to manage the values of:

§ Scale

§ %w (phase weight fraction x100)

§ Radius (average particle size in micron for microabsorption Brindley correction)

§ Preferred Orientation (Miller indices of the PO Plane, March-Dollase G factor and corresponding fraction fi; max 3 plane can be supplied; the fraction values must satisfy the condition: åi fi = 1.0)

 

 

Prof. Shape

The folder Prof. Shape opens the dialog-box shown in Fig. 5.2.2

Fig. 5.2.2

 

It allows:

§ To select the Profile Shape function among

o Pearson VII (default choice)

o Pseudo-Voigt

o Gaussian

o Lorentzian

 

§ To manage the Mixing Parameters m0, m1 and m2 (for Pearson VII and Pseudo-Voigt)

§ To manage the Full Width at Half Maximum w, v and u parameters of the Caglioti formula for Pearson VII, Pseudo-Voigt and Gaussian functions; w and v parameters for Lorentzian function.

 

 

Anis. FWHM

The folder Anis. FWHM opens the dialog-box shown in Fig. 5.2.3

Fig. 5.2.3

 

It allows to manage the elements Wij, Vij and Uij of a second-rank symmetric tensor according to the empirical function proposed by Le Bail and Jounneaux (1997) to model anisotropic variations of the Full Width at Half Maximum of reflections versus 2q and hkl Miller indices.

Starting values of Wij, Vij and Uij are derived from the corresponding isotropic w, v and u parameters when the refinement of the anisotropic FWHM is required (Refinement > Anis. Peaks); to come back to the isotropic parameters, it is necessary to set at zero all the fields.

 

Cell

The folder Cell opens the dialog-box shown in Fig. 5.2.4

Fig. 5.2.4

 

It allows to manage direct cell parameters a, b, c (in Angstrom), a (alf), b (bet) and g (gam) (in degrees). Space Group symbol and cell volume are visualized.

 

 

Structure

The folder Structure opens the dialog-box shown in Fig. 5.2.5

Fig. 5.2.5

 

It allows to manage, for each atom of the structural model included in the .pha file:

§ Atomic specie symbol

§ Fractional coordinates x, y and z

§ Isotropic thermal factor B

§ Site occupancy Occ

 

 

Anis. Mix. Par.

The folder Anis. Mix. Par. opens the dialog-box shown in Fig. 5.2.6

Fig. 5.2.6

 

It allows to manage the elements m0ij, m1ij and m2ij of a second-rank symmetric tensor according to the empirical function proposed by Le Bail and Jounneaux (1997) to model anisotropic variations of the Pearson VII or Pseudo-Voigt mixing parameter of reflections versus 2q and hkl Miller indices.

Starting values of m0ij, m1ij and m2ij are derived from the corresponding isotropic m0, m1 and m2 parameters when the refinement of the anisotropic Mixing Parameters is required (Refinement > Anis. Peaks); to come back to the isotropic parameters, it is necessary to set at zero all the fields.

 

 

Restrictions

The folder Restrictions opens the dialog-box shown in Fig. 5.3.1

Fig. 5.3.1

 

It allows to manage, for the selected phase:

§ The minimum and maximum allowed values of the Full Width at Half Maximum and the Mixing Parameters of reflections. These values will be used as a threshold in the least squares refinement. Default choices are:

o FWHM min = 0.001 degrees.

o FWHM max = 2.0 degrees.

o Mix. Par. min = 0.95

o Mix. Par. max = 20.0

§ The maximum absolute % variations of the cell parameters. Default value is x = 10% for all the parameters. After each l.s.q. cycle, the new cell parameter is compared to the previous value (1) and to the starting value (2).

o A cell parameter variation > x is refused for case (1)

o A cell parameter variation > x /2 is refused for case (1) for phases with weigth fraction  < 10.0%

o A cell parameter variation > 2x is refused for case (2)