Subject:

**Stephens anistropic strain parameters**
Dear all,

I'm refining a structure in the space group R3m described with an hexagonal cell.

Since TCHZ peak shape is not optimal ( some peaks are broad and some are narrow) I tried the Stephens anistropic strain

I adpated the macro for the Trigonal for hexagonal indeces following the article

P. W. Stephens, J. Appl. Crystallogr., 1999, 32, 281–289

the macro I wrote is this one:

macro Stephens_trigonal_R3 (s400,s202,s004,s301,eta)

{

prm mhkl = H^4 s400 + K^4 s400 + L^4 s004 +

3 ( H^2 K^2 3 s400 + H^2 L^2 s202 + K^2 L^2 3 s400) +

2 ( H K^3 2 s400 + H^3 L s301 + K L^3 2 s400)+

3 ( H^2 K L 3/2 s301 - H K^2 L 3/2 s301 + H K L^2 s202);

prm pp = D_spacing^2 * Sqrt(Max(mhkl,0)) / 1000;

gauss_fwhm = 1.8/3.1415927 pp (1-eta) Tan(Th) + 0.0001;

lor_fwhm = 1.8/3.1415927 pp eta Tan(Th) + 0.0001;

}

I hope it is correct ! it works ( no syntax errors)

But I have problem to define the parameter s400,s202,s004,s301

I started with an equal value for all parameters (prm=100) and let them refining or should I get the starting value from somewhere?

are negative parameters acceptables?

Thank you

Lucia

I'm refining a structure in the space group R3m described with an hexagonal cell.

Since TCHZ peak shape is not optimal ( some peaks are broad and some are narrow) I tried the Stephens anistropic strain

I adpated the macro for the Trigonal for hexagonal indeces following the article

P. W. Stephens, J. Appl. Crystallogr., 1999, 32, 281–289

the macro I wrote is this one:

macro Stephens_trigonal_R3 (s400,s202,s004,s301,eta)

{

prm mhkl = H^4 s400 + K^4 s400 + L^4 s004 +

3 ( H^2 K^2 3 s400 + H^2 L^2 s202 + K^2 L^2 3 s400) +

2 ( H K^3 2 s400 + H^3 L s301 + K L^3 2 s400)+

3 ( H^2 K L 3/2 s301 - H K^2 L 3/2 s301 + H K L^2 s202);

prm pp = D_spacing^2 * Sqrt(Max(mhkl,0)) / 1000;

gauss_fwhm = 1.8/3.1415927 pp (1-eta) Tan(Th) + 0.0001;

lor_fwhm = 1.8/3.1415927 pp eta Tan(Th) + 0.0001;

}

I hope it is correct ! it works ( no syntax errors)

But I have problem to define the parameter s400,s202,s004,s301

I started with an equal value for all parameters (prm=100) and let them refining or should I get the starting value from somewhere?

are negative parameters acceptables?

Thank you

Lucia