Refinement
The sub-menu Refinement allows to handle a non automatic run or to modify some default choices of the automatic run. The window is subdived in 3 part: Top, Middle and Bottom
Top (See Fig. 6.1). It displays the "eps" field (value used as convergence criterium in l.s.q.) and the "Automatic Refinement" field (cross the field for the automatic run and viceversa).
Fig. 6.1
Middle (See Fig. 6.2). It allows to set the refinement (cross the fields) of the zero-shift parameter and of the Background model (single parameters cannot be managed), and to select
the number of least squares cycles (max 10)
the weighting scheme number [to select the weights wi for the observations yo(i) in l.s.q.] among 3 possible choices:
n = 1, wi = 1.0
n = 2, wi = 1.0/ yo(i)
n = 3, wi = 1.0/ [yo(i)]2
The default value is n = 2.
the damping factors. The supplied values will be applied to the l.s.q. shifts of groups of parameters according to:
(1) profile shape, preferred orientation and anisotropic peak model parameters;
(2) Cell parameters
(3) Structural parameters
(4) is not used
Fig. 6.2
Bottom. It allows to select a phase and to set the refinement (by crossing the fields) of all the Phase dependent parameters refinable by the program; they are organized in 5 folders: Profile, Pref. Orient., Cell, Anis. Peaks, Struct. Par.. When all the desired fields have been setted and independently on which folder you are, click OK to confirm, then click on Continue button of the main menu to start the l.s.q. cycles. Click Cancel to leave the Refinement window without any change.
The sub-menu is available at two different stages of the run:
1) when the message
Press "Refinement" to set your own strategy or "Continue" to go onis displayed at the bottom of the window and the initial calculated pattern is plotted; only the "eps" field and the "Automatic Refinement" field are active and setted to the default values, as it is shown in Fig. 6.3
Fig. 6.3
At this stage is possible:
to modify the "eps" value (the default is 0.3), click Ok to confirm, then click on Continue button of the main menu to carry out the automatic run with the new "eps" value
to require a non automatic refinement by removing the cross from the corresponding field; the non active fields of the bottom part become active (see Fig. 6.4) and a user strategy must be supplied by setting the parameters to refine.
Fig.6. 4
Click Cancel to come back to the main menu, then click on Continue button to carry out the automatic run with the default "eps" value.
2) when the automatic run stops and the message
Press "Refinement" to set your own strategy or "Exit" to stopis displayed at the bottom of the window; at this stage is possible to continue the refinement in the interactive way, if necessary.
The folder Profile opens the dialog-box shown in Fig. 6.5.1
Fig. 6.5.1
It allows to set the refinement of the following parameters:
Scale
w, v, u (FWHM Parameters)
m0, m1 and m2 (mixing Parameters for Pearson VII and Pseudo-Voigt)
asymmetry
The folder Pref. Orient. opens the dialog-box shown in Fig. 6.5.2
Fig. 6.5.2
It allows to set the refinement of the Preferred Orientation parameters: the March-Dollase G factor of each plane (max 3) and the fraction f2 and f3.
The folder Anis.Peaks opens the dialog-box shown in Fig. 6.5.3
Fig. 6.5.3
It allows to set the refinement of the elements of the two second-rank symmetric tensors proposed by Le Bail and Jounneaux (1997) to model, respectively, the anisotropic variations of the Full Width at Half Maximum (Wij, Vij and Uij ) and mixing parameters (m0ij, m1ij and m2ij ) of reflections versus 2q and hkl Miller indices. The number of the elements ij depends on the crystal symmetry (max 6 for each tensor) and it is automatically controlled by the program, so one field is displayed for each parameter.
Starting values of Wij, Vij, Uij , m0ij, m1ij and m2ij are derived from the corresponding isotropic w, v, u, m0, m1 and m2 parameters when the refinement of the anisotropic parameters is required for the first time; to come back to the isotropic parameters, it is necessary to set at zero all the fields in the folders "Parameters > Anis. FWHM" and "Parameters > Anis.Mix.Par."
The folder Cell opens the dialog-box shown in Fig. 6.5.4
Fig. 6.5.4
It allows to set the refinement of the direct cell parameters a, b, c , a (alf), b (bet) and g (gam). Crystal System restrictions are automatically controlled by the program.
The folder Struc. Par. opens the dialog-box shown in Fig. 6.5.5
Fig. 6.5.5
For each atom in the asymmetric unit (the structural model included in the .pha file), it allows to set the refinement of:
§
Fractional coordinates x, y and z§
Isotropic thermal factor B§
Site occupancy Occ (max 2 atomic species in the same site can be managed)
Atomic special positions are automatically controlled by the program; special coordinates are blocked.