Workshop Jana2006

Powder option in Jana2006

LecturerJan Rohlíček

AuthorsVáclav Petříček, Michal Dušek andLukáš PalatinusInstitute of Physics ASCRPraha, Czech Republic

Workshop Jana2006

Powder sample

powder = polycrystalline material

Workshop Jana2006

Single crystal diffraction

Powder diffraction pattern is a combination of a large number single crystal diffraction patterns

Diffracted pattern

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Powder diffraction pattern of alaptide powder on the area detector

Diffracted pattern

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Diffracted pattern

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Problems and limitations

• Peak overlap – problem of powder data.

What is the intensity of overlapped reflections?

2 Theta

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Overview of the functionality for powdersJana2006 is a refinement program originally developed for single crystal data. Except for the refinement of basic structural parameters it allows also:

ADP parameters till 6th order – ionic conductors, positional disorder

e3=0.000

-3.0 -1.0 1.0 3.0e1-2.5

-1.5

-0.5

0.5

1.5

2.5

e2

Charge density studies - multipole refinement

Refinement of modulated and composite structures

E.Gaudin, F.Boucher, V.Petříček, F.Taulelle and M.Evain, Acta Cryst. (2000), B56, 402-408.

Melilite – Bindi L., Bonazzi P., Dušek M., V.Petříček and Chapuis G., (2001), Acta

Cryst., B57, 739-746.

Workshop Jana2006

Overview of the functionality for powders

magnetic structures

Jana2006 is a refinement program originally developed for single crystal data. Except for the refinement of basic structural parameters it allows also:

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Powder data of one or more phases

unit cell parameters, dimension

Le Bail refinement

Space group test

Structure determination (Superflip, Expo)

Rietveld refinement

JANA2006

Overview of the functionality for powders

Workshop Jana2006

Le Bail refinementSeparate intensities from powder pattern

in Jana2006 called as „Make only profile matching“

Structure solutionUse separated intensities to solve the

crystal structure

Rietveld refinementRefine the model from the structure

solution process by using powder data

Overview of the functionality for powders

Workshop Jana2006

The powder diffraction option (Rietveld refinement + leBail technique) was implemented in 2001: Dušek,M., Petříček,V., Wunschel,M., Dinnebier,R.,E. & Van Smaalen,S. (2001). J. Appl. Cryst. 34, 398-404.

„Edit Profile“:

Profile functions: Gauss, Lorentz and pseudo-Voigt

Background corrections: Legendre, Chebyshev polynomials, “cos” functions

Peak asymmetry: Simpson, Berar-Baldinozzi, Finger-Cox-Jephcoat

fundamental approach (Cheary-Coelho)

Preferred orientation: March-Dollase, Sasa-Uda

Roughness: Pitchke-Hermann-Matter, Suorti

Anisotropic line-broadening according to Stephens modified by Leineweber and Petricek to include modulated phases

„Profile Viewer“:

Edit manual background

checking calculated and measured data

Powder options in Jana2006

Workshop Jana2006

Gaussian:

where is Full-Width at Half-Maximum

22 2/exp2

1, G

GG bx

bxbG

GH

2ln8

22 GG

Hb

Lorentzian: 221

12,

LLL

bxbxbL

LL Hb

where is Full-Width at Half-MaximumLH

Powder profile parameters in Jana2006

Workshop Jana2006

Voigt function:

is a convolution of Gaussian and Lorentzian function. For powder profile we use a simpler analytical approximation of the Voigt function called pseudo-Voigt function:

xHGxHLxHpV ,1,,

543223455 07842.047163.442843.269269.2 LLGLGLGLGG HHHHHHHHHHH

the parameters and are functions of and H GH LH

32

11116.047719.036603.1

H

H

H

H

H

H LLL

xdxxbLxbGxbbV LGLG ,,,,

Powder profile parameters in Jana2006

Workshop Jana2006

For the Gaussian term the formula Cagliotti, Pauletti & Ricci, 1958 (Nucl.Instrum., 3, 223) is used:

2

22

costantan

PWVUbG

In the original formula only three first terms were used. The last one was introduced later as a Scherrer term and it is connected with crystalline size. But from the fact that:

2

2

cos

11tan

follows that only two of three coefficients can be refined simultaneously.

For the Lorentzian part we use the same terms as for pure crystal broadening but, as mentioned above, they cannot be used directly to find sample characteristics.

PWU ,,

tancos LLL YXb

Powder profile parameters in Jana2006

Workshop Jana2006

Powder profile parameters in Jana2006

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For modulated structures this method has been generalized by A. Leineweber and V. Petříček, (2007). J. Appl. Cryst., 40, 1027-1034.

Stephens model for anisotropic broadening

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Stephens model for anisotropic broadening

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Asymmetry options in Jana2006

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Simpson’s method – Peak is combined with several shifted peaks having the identical shape

Berar-Baldinozzi method - based on Hermite polynomials

By axial divergence – according to Finger, Cox and Jephcoat, (1994) J.Appl.Cryst. 27, 892-900.

Two parameters are used: “height” and “sample”

These parameters are strongly correlated. Our recommendation is to estimate their ratio and keep it as a restriction during the refinement.

LH LS

Fundamental approach – it follows the method introduced by Cheary and Coelho, (1998), J. Appl. Cryst. 31, 851-861. This method can estimate the profile asymmetry just on the base of experimental parameters. For Bragg-Brentano geometry it works very nicely.

Asymmetry options in Jana2006

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Asymmetry options in Jana2006

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Sample properties

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Background correction

Several possibilities of description: Legendre polynomials, Chebyshev polynomials, Cos-ortho, cos-GSAS

Manual background – the background is expressed as a set of background intensities over the diffraction interval. The actual value is calculated by a linear interpolation. This method is can very effectively describe even very complicated background profiles. But it need some user assistance to select it properly. Moreover this first background estimation can be combined with some of previous continuous functions.

Shift parametersShift – it defines the zero shift (again in units of 0.01 deg). This value is to be added to the

theoretical peak position to get a position in experimental profile

sycos – in analogy with the Fullprof:is connected with a specimen displacement

sysin – in analogy with the Fullprof:is connected with a transparency correction

Background, shift, displacement

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Background, shift, displacement

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Manual Background

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In Jana2006 is also used to predict symmetry by comparing of profile fits for different space groups:

Centering and space group test

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Profile viewer

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Profile viewer

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Profile viewer

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Practical notes•Refining of many profile parameters together causes endless process, which is not

converging. Note that only two of U, W and P can be refined simultaneously, also S/L and H/L cannot be refined together.

•It is true that using huge number of background points slightly improve agreement profile factors. The reasonable number of background points is up to 10 for flat sample (without amorphous content) and up to 30 for capillary samples (with amorphous content).

•Manual background makes Le Bail refinement more stable.

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•It is good idea to start Rietveld refinement with disabled unit cell and profile parameters which you got by previous Le Bail refinement. Enable these parameters after several cycles, when you are sure that your model is close enough to the correct position.

•Bond and angle restraints are very powerful but also very dangerous tools.

•Using small damping factor e.g. 0.1 at the beginning of the Rietveld refinement can protect you from repeating the last step again.

•Make backup copies

Practical notes

Workshop Jana2006

Workshop Jana2006

Powder pattern