Fitting Nuclear and Magnetic Scattering in GSAS with a
Single Fmm'm' Nuclear and Magnetic Phase
Input model for Nuclear Scattering Phase
-
The first step in the refinement is to start a new refinement by creating a
new experiment file. Start EXPGUI and enter a new file name
then click on the "Create" button to then create a new, empty, experiment file
(screen image)
and enter a title.
The first step in creating an GSAS refinement is to create a phase. This can be done from this information:
Element | Label | x | y | z | Occupancy | Moment |
---|---|---|---|---|---|---|
Y | Y1 | 0.25 | 0.25 | 0.25 | 1 | |
Ba | Ba1 | 0.25 | 0.25 | 0.0835 | 1 | |
Fe+3 | Fe1a | 0 | 0 | 0 | 1 | 3.5 |
Fe+3 | Fe1b | 0 | 0 | 0.5 | 1 | -3.5 |
Fe+3 | Fe2a | 0 | 0 | 0.17 | 1 | -3.5 |
Fe+3 | Fe2b | 0 | 0 | 0.33 | 1 | 3.5 |
O | O1a | 0 | 0 | 0.0906 | 1 | |
O | O1b | 0 | 0 | -0.4094 | 1 | |
O | O2 | 0.25 | 0 | 0.192 | 1 | |
O | O3 | 0 | 0.25 | 0.190 | 1 | |
O | O4 | 0 | 0.25 | 0 | 0.853 | |
O | O5 | 0.25 | 0 | 0 | 0.978 |
YBaFeO_Fmmm.cif
in file
YBAFEO.zip
rather than by typing in the information shown in the screen images in the
following sections.
Go to the Phase panel
(screen image)
and press the "Add Phase" button. To input the information from the CIF,
click on the "PowderCell .CEL file" menu button to bring up a menu of choices
(screen image)
and select "Crystallographic Information File (CIF)"
which brings up a window where the file YBaFeO_Fmmm.cif
can be selected.
(screen image);
click on "Open" after the file is
selected.
Click on "Continue" on the add new phase
(screen image)
and on the Check symmetry windows
(screen image)
and then "Add Atoms" on the Adding atoms... window
(screen image)
and the phase has been added, just like that.
Finally, make sure the "Refine Cell" option is not selected,
(screen image)
so that the unit cell parameters
will be refined until later, after the background and scale factor
are first brought into the right range.
Input Diffraction Data
-
The second step in almost every GSAS refinement is to input diffraction data
(or select data parameters in a simulation). This is done from the Histogram
panel
(screen image).
Click on the "Add New Histogram" button
which brings up the add new histogram window
(screen image).
Press the Select file to select the appropriate GSAS data file
in the Open window
(screen image);
press the open button
and the file names for the both the data file and the instrument parameter files
are loaded (you might see messages about file conversions if the download
process has stripped non-printing characters needed from the files.). Finally,
change the data range to utilize a maximum two-theta of 152 degrees
on the add new histogram window, which
eliminates the last very wide peak from use in the refinement (the fit
would also progress well were this peak included)
(screen image).
Press "Add" on this window and the
diffraction data are now included in the refinement
Finally, change the background function to type 1 with 6 terms by pressing the
"Edit Background" button (this is the authors' preference -- but other
settings could be selected.)
(screen image).
Finally, confirm that the "Refine background" option is selected so that
the background will be fit
(screen image).
Start Initial Fit of Experimental Parameters
-
The fit can now be started, by pressing the "powpref"
(screen image)
and "genles" buttons
(screen image).
Note that pressing each button starts a program in another
window and then (unless the default settings have been changed) after
each run has been completed, the extra window is closed and the "Load
New" button is pressed on the Reload screen
(screen image).
Note that after the two cycles
of refinement, the chi2 value drops to approximately 44.
At this point the unit cell is refined, by selecting the the "Refine Cell" option on the Phase panel (screen image). Press the "genles" button and note that the fit improves to a chi2 value of approximately 30 (screen image). The two-theta zero is now refined using the "Refine zero" control on the Histogram panel (screen image). Press the "genles" button and note that the fit improves to a chi2 value of approximately 28 (screen image).
Constrain Structural Parameters
-
At this point an initial fit of the structure is performed (although some
researchers might delay this until the magnetic modeling is included). While
under normal conditions, it would be wise to add refinement of variables
over the course of several refinement runs, in this case the model is
quite stable with respect to refinement and this can
be done in a single run. Before starting the refinement, however,
Uiso values for like element
types are grouped together to reduce the number of parameters.
This is done with the Constraints panel
(screen image).
- To set up the constraint on the Fe atoms, Press on the "New Constraints" button, then select them by dragging over them with the mouse and select UISO in the Variable menu button (screen image) and press "Save".
- Repeat this with the O atoms, Press on the "New Constraints" buttton by dragging over those atoms with the mouse; then select UISO in the Variable menu button (screen image) and press "Save".
- Now create a constraint to link the positions of Fe2a and Fe2b, by pressing on the "New Constraints" button, then in the new window press the "New Column" button. In one column, select atom Fe2a, variable Z and multiplier 1.0 while in the column, select atom Fe2b, variable Z and multiplier -1.0 (screen image).
- For O1a and O1b, the z coordinates must move in the same
direction (since they are generated as z/2 and -0.5+z/2). This is
done by pressing on the "New Constraints" button, then
selecting atom O1a and O1b, variable Z and multiplier 1.0
(screen image).
The Constraints panel will then list the four constraints
(screen image).
As mentioned previously, the 0.25 values for O atom coordinates are not special values in Fmmm and must be constrained. This must be done with EXPEDT using the following commands:
- Y
- L A
- F
- i 9 x
- i 10 y
- i 11 y
- i 12 x
- L
- x x x x
The input to and output from EXPEDT are shown below. User-typed input is emphasized by display in this font: Input
|------------------------------------------| | Program EXPEDT Version MacOSX | | A menu driven routine to edit .EXP files | | Distributed on Thu Sep 30 14:58:36 2004 | |------------------------------------------| |---------------------------------------------------------------| | Allen C. Larson and Robert B. Von Dreele | | Manuel Lujan, Jr. Neutron Scattering Center, MS-H805 | | Los Alamos National Laboratory, Los Alamos, NM 87545 | | | | Copyright, 2000, The Regents of the University of California. | |---------------------------------------------------------------| The last history record is : HSTRY 13 EXPGUI 1.74 1.42 (4 changes) -- 07/30/05 20:46:38 Is this the file you wish to use? (<?>,D,K,Q,R,Y) >Y Experiment title: Model nuclear & magnetic scattering in Fmmm The last history record is : HSTRY 13 EXPGUI 1.74 1.42 (4 changes) -- 07/30/05 20:46:38 EXPEDT data setup option (<?>,D,F,K,L,P,R,S,X) >L A Phase No. 1; Phase has 12 atoms; Title: from /Users/toby/proj/ybafe/YBaFeO_ Give atom editing command (<?>,$,C,D,E,F,I,K,L,M,S,T,U,V,X,+,-,*,/) >F There currently no parameters being held for this phase Enter option (<?>,D,I,L,X) >i 9 x Enter option (<?>,D,I,L,X) >i 10 y Enter option (<?>,D,I,L,X) >i 11 y Enter option (<?>,D,I,L,X) >i 12 x Enter option (<?>,D,I,L,X) >l Parameters being held constant are 1(1 9X ) 2(1 10Y ) 3(1 11Y ) 4(1 12X ) Enter option (<?>,D,I,L,X) >x x x x Phase No. 1; Phase has 12 atoms; Title: from /Users/toby/proj/ybafe/YBaFeO_ EXPEDT data setup option (<?>,D,F,K,L,P,R,S,X) >x
Start Initial Fit of Structural Parameters
To refine the nuclear model, select all atoms on the phase panel (the right mouse button will do this) and click on the X and U "Refinement Flag" buttons (screen image) so that all atom positions can be optimized in addition to 4 Uiso values (1 for the Fe atoms, 1 for the O atoms, 1 for the Y atom and 1 for the Ba atom). Then set the F (fractional occupancy) flag for O4 and O5 only (screen image). Press the "genles" button and note that the fit improves to a chi2 value of approximately 21 (screen image). A view of the fit in LIVEPLOT shows the profile terms need some refinement (which will be done later) and there is significant missing intensity for several low angle reflections (screen image) -- not surprising, since magnetic scattering is not included.
Include Magnetic Scattering
-
At this time, EXPGUI does not implement many of the controls needed for
magnetic structure fitting. Nonetheless, as will be shown here. it is still
possible to use EXPGUI for many steps, and then use EXPEDT for steps that
cannot be done at present in EXPGUI.
- Y
- P
- P
- m 1
- b
- x x x x
To add magnetic scattering, EXPEDT is used to change a flag so the phase is recognized as both a magnetic as well as nuclear scatterer. The EXPEDT program is invoked from EXPGUI by pressing the EXPEDT button in the buttom bar or from the Powder and Xtal menus.
To designate the phase as both a nuclear and magnetic scatter, the following commands are used in EXPEDT:
EXPEDT prompts the user to supply a one-letter command, where inputting a blank line provides further information on what the options are (as demonstrated below). Note that commands can be combined onto the same line (usually) and can in many cases be spread over more than one line.
The input to and output from EXPEDT are shown below. User-typed input is emphasized by display in this font: Input
|------------------------------------------| | Program EXPEDT Version MacOSX | | A menu driven routine to edit .EXP files | | Distributed on Thu Sep 30 14:58:36 2004 | |------------------------------------------| |---------------------------------------------------------------| | Allen C. Larson and Robert B. Von Dreele | | Manuel Lujan, Jr. Neutron Scattering Center, MS-H805 | | Los Alamos National Laboratory, Los Alamos, NM 87545 | | | | Copyright, 2000, The Regents of the University of California. | |---------------------------------------------------------------| The last history record is : HSTRY 16 GENLES MacOSX Jul 30 20:53:43 2005 Sdsq= 0.615E+05 S/E= 4.63 Is this the file you wish to use? (<?>,D,K,Q,R,Y) >Y Experiment title: Model nuclear & magnetic scattering in Fmmm The last history record is : HSTRY 16 GENLES MacOSX Jul 30 20:53:43 2005 Sdsq= 0.615E+05 S/E= 4.63 EXPEDT data setup option (<?>,D,F,K,L,P,R,S,X) >P Select editing option for Powder data preparation (<?>,H,P,T,X) >P There is phase information present Name of phase no. 1 from /Users/toby/proj/ybafe/YBaFeO_Fmmm.cif The phase is non-magnetic Enter phase edit command(<?>,$,D,E,F,M,I,L,R,S,X) >M Give phase number for phase type flag toggle >1 The phase is non-magnetic Enter phase type (<?>,A,B,C,D,L,X,Z) > Selection of phase type: A - Nuclear structure only B - Nuclear and magnetic structure C - Magnetic structure only D - Macromolecular structure Z - Pawley extraction for all phases Enter phase type (<?>,A,B,C,D,L,X,Z) >B The phase is magnetic. Both nuclear and magnetic peaks will be included when generating reflections fo r powder patterns Enter phase type (<?>,A,B,C,L,X,Z) >x x x x EXPEDT data setup option (<?>,D,F,K,L,P,R,S,X) >
One more X command could now be entered to exit EXPEDT, but since the next step will continue in EXPEDT, this is not necessary. Note that the magnetic symmetry menu (as shown below) must be entered, even if no changes will be made, for GENLES to function once a magnetic phase has been added.
Setup Magnetic Symmetry and Moments
- L A
- M
- S
- c 2
- c 3
- x
- m 3 3.5
- m 4 -3.5
- m 5 -3.5
- m 6 3.5
- x
- L
- x x x x
In the previous step, the phase was flagged as magnetic, but before a magnetic scattering computation can be performed, the magnetic symmetry needs to be set. In this case, two operator colors must be changed to change the space group from Fmmm to Fmm'm'. Once the magnetic symmetry is established, the magnetic moments for each magnetic atom are supplied.
In EXPEDT, use the following commands:
The input to and output from EXPEDT are shown below. User-typed input is emphasized by display in this font: Input
EXPEDT data setup option (<?>,D,F,K,L,P,R,S,X) >L A Phase No. 1; Phase has 12 atoms; Title: from /Users/toby/proj/ybafe/YBaFeO_ Give atom editing command (<?>,$,C,D,E,F,I,K,L,M,P,S,T,U,V,X,+,-,*,/) >M Enter magnetism editing option (<?>,A,C,L,M,S,X) >S These changes may alter the allowed moment orientations and constraints. All of the following data refer to the current space group symbol Spin flip editing commands are C n - Change spin flip "n" L - List the current spin flip info X - Exit to the atom magnetic data editing menu Enter spin flip editing command (<?>,C,L,X) >c 2 The unique symmetry operations and the associated spin colors are Mx My Mz Acen Bcen Ccen Black Red Black Black Black Black Enter spin flip editing command (<?>,C,L,X) >c 3 The unique symmetry operations and the associated spin colors are Mx My Mz Acen Bcen Ccen Black Red Red Black Black Black Enter spin flip editing command (<?>,C,L,X) >x Enter magnetism editing option (<?>,A,C,L,M,S,X) >m 3 3.5 Fe1a 0.000000 0.000000 0.000000 No magnetic moment is currently defined Constraints on the moment are 1 1.00 0 0.00 0 0.00 New moment = 3.50000 0.00000 0.00000 3.500 90.000 0.000 Enter magnetism editing option (<?>,A,C,L,M,S,X) >m 4 -3.5 Fe1b 0.000000 0.000000 0.500000 No magnetic moment is currently defined Constraints on the moment are 1 1.00 0 0.00 0 0.00 New moment =-3.50000 0.00000 0.00000 3.500 90.000 180.000 Enter magnetism editing option (<?>,A,C,L,M,S,X) >m 5 -3.5 Fe2a 0.000000 0.000000 0.169428 No magnetic moment is currently defined Constraints on the moment are 1 1.00 0 0.00 0 0.00 New moment =-3.50000 0.00000 0.00000 3.500 90.000 180.000 Enter magnetism editing option (<?>,A,C,L,M,S,X) >m 6 3.5 Fe2b 0.000000 0.000000 0.330573 No magnetic moment is currently defined Constraints on the moment are 1 1.00 0 0.00 0 0.00 New moment = 3.50000 0.00000 0.00000 3.500 90.000 0.000 Enter magnetism editing option (<?>,A,C,L,M,S,X) >x Phase No. 1; Phase has 12 atoms; Title: from /Users/toby/proj/ybafe/YBaFeO_ Give atom editing command (<?>,$,C,D,E,F,I,K,L,M,P,S,T,U,V,X,+,-,*,/) >L SER TYPE X Y Z FRAC NAME UISO CODE STSYM MULT FXU MX MY MZ MCODE 1 Y 0.25000 0.25000 0.25000 1.00000 Y 0.00874 I XU 222 8 000 2 BA 0.25000 0.25000 0.08598 1.00000 Ba 0.01200 I XU 2(001) 16 000 3 FE+3 0.00000 0.00000 0.00000 1.00000 Fe1a 0.01033 I XU MMM 4 000 3.500 0.000 0.000 4 FE+3 0.00000 0.00000 0.50000 1.00000 Fe1b 0.01033 I XU MMM 4 000 -3.500 0.000 0.000 5 FE+3 0.00000 0.00000 0.16943 1.00000 Fe2a 0.01033 I XU MM2(001) 8 000 -3.500 0.000 0.000 6 FE+3 0.00000 0.00000 0.33057 1.00000 Fe2b 0.01033 I XU MM2(001) 8 000 3.500 0.000 0.000 7 O 0.00000 0.00000 0.08755 1.00000 O1a 0.01789 I XU MM2(001) 8 000 8 O 0.00000 0.00000-0.41245 1.00000 O1b 0.01789 I XU MM2(001) 8 000 9 O 0.25000 0.00000 0.19336 1.00000 O2 0.01789 I XU M(010) 16 000 10 O 0.00000 0.25000 0.18982 1.00000 O3 0.01789 I XU M(100) 16 000 11 O 0.00000 0.25000 0.00000 0.86368 O4 0.01789 IFXU MM2(010) 8 000 12 O 0.25000 0.00000 0.00000 1.06416 O5 0.01789 IFXU MM2(100) 8 000 Phase No. 1; Phase has 12 atoms; Title: from /Users/toby/proj/ybafe/YBaFeO_ Give atom editing command (<?>,$,C,D,E,F,I,K,L,M,P,S,T,U,V,X,+,-,*,/) >x x x x EXPEDT data setup option (<?>,D,F,K,L,P,R,S,X) >
Check Magnetic Form Factor
- L F M
- FE+3
- C
- .3972 13.244 .6295 4.903 -.0313 .35 0 0 .0044
- N
- U
- x x x x
For this example, GSAS already has the correct magnetic form factor loaded for atom type FE+3, but it is always a good idea to check this. In some cases, the magnetic form factor for a problem will need to be loaded. Many magnetic form factors can be found in the International Tables of Crystallography, Volume C in section 4.4.5. Note that GSAS uses the terms A1, B1, A2, B2, A3, B3, and C for the terms labeled A, a, B, b, C, c and D in the International Tables. GSAS terms A4 and B4 are not used in the International Tables and should be specified as zero.
In EXPEDT, use the following commands:
Note that if EXPGUI were to be started at this point, the first command would be "Y" to indicate that the correct file has been selected to be edited. Command 1) [L] enters the least-squares menu; command 2) [F] moves from the least-squares to the form factor submenu; command 3) [M FE+3] opens editing of the magnetic form factor for Fe3+ command 4) [C] specifies that the saved values will be changed, the values [.3972 13.244 .6295 4.903 -.0313 .35 0 0 .0044] specify the new <j0> coefficients. command 5) [n] indicates that the <j2> terms will not be changed. command 6) [U] specifies that the entered coefficients are to be saved. If this U is not entered, the original values will not be changed and the new values are ignored -- use care to remember to enter this command. Command 7) [x x x x] returns to the main menu. The input to and output from EXPEDT are shown below. User-typed input is emphasized by display in this font: Input
EXPEDT data setup option (<?>,D,F,K,L,P,R,S,X) >L Select editing option for Least Squares calculation (<?>,A,B,F,H,L,O,R,S,T,X) >F Form factor editing options - (<?>,M,N,R,S,X) >M Enter atom type >FE+3F-factor is expressed in the form : A(I)*EXP(-B(I)*SQ) + C , I=1 to 4 F-factor is expressed in the form : [A(I)*EXP(-B(I)*SQ) + C]*SQ , I=1 to 4 where SQ is (Sin theta/lambda)**2 Magnetic form-factor coeffs. for FE+3 are A1,B1,A2,B2 = 0.39720 13.24400 0.62950 4.90300 A3,B3,A4,B4,C= -0.03140 0.35000 0.00000 0.00000 0.00440 mff(0) = 0.99970 Magnetic form-factor coeffs. for FE+3 are A1,B1,A2,B2 = 1.36020 11.99800 1.51880 5.00300 A3,B3,A4,B4,C= 0.47050 1.99100 0.00000 0.00000 0.00380 Lande g factor = 2.00 Magnetic form factor editing options - (<?>,A,C,E,G,L,P,R,U,X) >C Enter A(1) & B(1) >.3972 13.244 Enter A(2) & B(2) >.6295 4.903 Enter A(3) & B(3) >-.0313 .35 Enter A(4) & B(4) >0 0 Enter C >.0044 Do you want to modify the values (Y/ )? >n F-factor is expressed in the form : A(I)*EXP(-B(I)*SQ) + C , I=1 to 4 F-factor is expressed in the form : [A(I)*EXP(-B(I)*SQ) + C]*SQ , I=1 to 4 where SQ is (Sin theta/lambda)**2 Magnetic form-factor coeffs. for FE+3 are A1,B1,A2,B2 = 0.39720 13.24400 0.62950 4.90300 A3,B3,A4,B4,C= -0.03130 0.35000 0.00000 0.00000 0.00440 mff(0) = 0.99980 Magnetic form-factor coeffs. for FE+3 are A1,B1,A2,B2 = 1.36020 11.99800 1.51880 5.00300 A3,B3,A4,B4,C= 0.47050 1.99100 0.00000 0.00000 0.00380 Lande g factor = 2.00 Magnetic form factor editing options - (<?>,A,C,E,G,L,P,R,U,X) >U Magnetic form factor editing options - (<?>,A,C,E,G,L,P,R,U,X) >x x x x EXPEDT data setup option (<?>,D,F,K,L,P,R,S,X) >x STOP EXPEDT terminated successfully statement executed Press Enter to continue
After the EXPEDT window is closed since EXPEDT has changed the experiment file, (unless the Autoload EXP option is set) there is a prompt to load the information from this changed file: press the "Load new" button to continue.
Refine with Magnetic Phase
Since the phase type has been changed, it is necessary to run POWPREF (screen image). and then GENLES (screen image). The fit improves from the previous chi2 value of approximately 21 to a new value of approximately 15, as the low-angle magnetic lines are now computed with more intensity, as can now be seen in LIVEPLOT (screen image).
Refine Profile Constraints
From the previous LIVEPLOT result, it is clear that the profile is a major source of differences between the observed and computed diffraction data. While it is possible to have different ordering ranges for the nuclear and magnetic ordering, this cannot be incorporated into a refinement with a single nuclear and magnetic phase.
Select the check buttons for GU, GV & GW on the Profile panel (screen image) and then start GENLES (screen image). The fit improves from the previous chi2 value of approximately 15 to a new value of approximately 3.4. Running POWPREF and then GENLES again causes chi2 to drop from 3.4 to 2.8 just due to the expansion of the peak shape range from POWPREF. Subsequent refinement cycles improves chi2 slightly, to 2.6.
Constrain & Refine Magnetic Moment
-
In most magnetic refinements, one wishes to refine magnetic moments.
It should be noted that the Fe1a and Fe1b atoms are symmetry related,
as is the case for Fe2a and Fe2b.
One could consider treating the two Fe1 atoms as having
different moments from the two Fe2 atoms, but this makes little
chemical sense, since there is no reason to expect either site to
have an accumulation of off-valence Fe atoms. Further, due to the
pseudo-symmmetry of the cell, there are few reflections that distinguish
the two non-equivalent Fe scatterers. Thus, it is best
to treat all Fe atoms as having a single magnetic moment. EXPGUI
can be used to set up constraints on the magnetic moments,
by creating a new atom constraint on the Constraints panel, where a
shift multiplier of 1.0 is applied to MX of atoms 3 & 6
and a shift multiplier of -1.0 is applied to MX of atoms 4 & 5
(screen image).
Press the "Save" button and the constraint appears
(screen image).
- Y L
- A
- v 3:6 M
- x x x x
- x
Changing the refinement flag for the magnetic moments must currently be done from EXPEDT, using the following commands:
The input to and output from EXPEDT are shown below. User-typed input is emphasized by display in this font: Input
|------------------------------------------| | Program EXPEDT Version MacOSX | | A menu driven routine to edit .EXP files | | Distributed on Thu Sep 30 14:58:36 2004 | |------------------------------------------| |---------------------------------------------------------------| | Allen C. Larson and Robert B. Von Dreele | | Manuel Lujan, Jr. Neutron Scattering Center, MS-H805 | | Los Alamos National Laboratory, Los Alamos, NM 87545 | | | | Copyright, 2000, The Regents of the University of California. | |---------------------------------------------------------------| The last history record is : HSTRY 24 EXPGUI 1.74 1.42 (1 changes) -- 07/30/05 22:57:33 Is this the file you wish to use? (<?>,D,K,Q,R,Y) >Y Experiment title: Model nuclear & magnetic scattering in Fmmm The last history record is : HSTRY 24 EXPGUI 1.74 1.42 (1 changes) -- 07/30/05 22:57:33 EXPEDT data setup option (<?>,D,F,K,L,P,R,S,X) >l a Phase No. 1; Phase has 12 atoms; Title: from /Users/toby/proj/ybafe/YBaFeO_ Give atom editing command (<?>,$,C,D,E,F,I,K,L,M,P,S,T,U,V,X,+,-,*,/) >v 3:6 M SER TYPE X Y Z FRAC NAME UISO CODE STSYM MULT FXU MX MY MZ MCODE 3 FE 0.00000 0.00000 0.00000 1.00000 Fe1a 0.00655 I XU MMM 4 000 3.500 0.000 0.000 M 4 FE 0.00000 0.00000 0.50000 1.00000 Fe1b 0.00655 I XU MMM 4 000 -3.500 0.000 0.000 M 5 FE 0.00000 0.00000 0.17002 1.00000 Fe2a 0.00655 I XU MM2(001) 8 000 -3.500 0.000 0.000 M 6 FE 0.00000 0.00000 0.32998 1.00000 Fe2b 0.00655 I XU MM2(001) 8 000 3.500 0.000 0.000 M Phase No. 1; Phase has 12 atoms; Title: from /Users/toby/proj/ybafe/YBaFeO_ Give atom editing command (<?>,$,C,D,E,F,I,K,L,M,P,S,T,U,V,X,+,-,*,/) >x x x x EXPEDT data setup option (<?>,D,F,K,L,P,R,S,X) >x
The refinement is then started with GENLES. The magnetic moments shift only slightly (as can be seen from the LSTVIEW output) and the chi2 value drops only slightly (screen image).
Further Model Improvements
-
At this point the refinement has progressed significantly, but for publication
one might wish to obtain a slightly better fit. To do this, the number of
background parameters can be increased to the point where little improvement
is seen by adding more terms, say to around 12-14 terms. The profile can be
improved further by switching to profile type 2 or 3 and allowing the LX term
to refine.
There is a minor impurity phase, BaFeO3, that can be
included (Pm3m, a=4.0875, Ba: 0,0,0; Fe 1/2,1/2,1/2; O 1/2,1/2,0).
Note that the
phase fraction for this phase can be refined, and later the unit cell,
but the Uiso should be fixed at a reasonable level, such as the default of
0.025. With these additions, the fit can be improved to a reasonable
chi2 = 2.1 and very good R(F2) = 0.044.