Dear Shaopeng,

I suggest you download the GSAS manual from:

http://www.ccp14.ac.uk/ccp/ccp14/ftp-mirror/gsas/public/gsas/manual/GSASManual.pdf

and start reading it, since it contains all the information you are looking for.

If you cannot do this here is a short description of the parameter you ask about:

POLA is the polarization coefficient. For synchrotron radiation with 100% horizontal polarization and a vertical diffraction plane this number is 1, for sealed tube radiation without incident beam monochromator is 0.5, if you have a monochromator before the sample you have to check the monochromator angle to apply this correction. IPOLA is the polarization correction type. This is a bit more obscure but for all cases I know (conventional and synchrotron XRD) is 0.

GU, GV, GW and GP are the coeficients of Cagliotti's formula (modifyed with Scherrer broadening) of the gaussian component of the peak width.

LX and LY are coefficients of the lorentzian components of peak width. STEC and PTEC are also coefficients of the lorentzian width but they are used only in the case of anisotropic broadening. STEC is associated with the Y component and PTEC with X component.

S/L and H/L are geometrical terms that describe the low angle broadening of low angle peaks in general with 2theta165 deg).These terms can be calculated by dividing the width of the divergence and receiving slits by the diffractometer radius. These terms are in general equal and should be fixed for a specific (known) set of slits and diffractometer radius, but can be refined if sufficiently low angle peaks ara available. The two parameters are added and squared in the corresponding equation therefore they cannot be refined together.

TRNS is the correction for sample transparency in Bragg-Brentano geometry. It is the inverse of the absorption effect and in BB geometry is zero for very highly absorbing samples and different than zero for intermediate or low absorbing ones. The efect of this correction is a shift in peak positions that increases with sin(2theta). In general for short 2-theta range data it could correlate strongly with SHIFT, ZERO and the cell parameters.

SHFT is the correction in peak position for sample's vertical shift (respect to the tangent line of the focusing circle) in Bragg-Brentano geometry. For short 2theta range it correlates strongly with TRNS, ZERO and the cell parameters. It depends stronlgy on sample mounting so very special care must be taken when mounting the sample to avoid this error.

SFEC plays the role of PTEC in the lorentzian width when stacking faults are present and an aditional anisotropic broadening associated with this fenomenon is present. It relays on the definition of a vector that describes the stacking fault sub-lattice. This is not given in EXPGUI but you should look for it in EXPEDT.

Finally Lij are a set of terms of microstrain anisotropy associated with LY and STEC parameters but show a specific dependence with HKL.

You better check the literature cited in GSAS Manual if you want more details of the specific parameters.

You can also refer to several on-line courses or study materials available on the web on the Rietveld Method or several good books on the topic, may available on-line.

Best of luck.

Dear Shaopeng,

I suggest you download the GSAS manual from:

http://www.ccp14.ac.uk/ccp/ccp14/ftp-mirror/gsas/public/gsas/manual/GSASManual.pdf

and start reading it, since it contains all the information you are looking for.

If you cannot do this here is a short description of the parameter you ask about:

POLA is the polarization coefficient. For synchrotron radiation with 100% horizontal polarization and a vertical diffraction plane this number is 1, for sealed tube radiation without incident beam monochromator is 0.5, if you have a monochromator before the sample you have to check the monochromator angle to apply this correction. IPOLA is the polarization correction type. This is a bit more obscure but for all cases I know (conventional and synchrotron XRD) is 0.

GU, GV, GW and GP are the coeficients of Cagliotti's formula (modifyed with Scherrer broadening) of the gaussian component of the peak width.

LX and LY are coefficients of the lorentzian components of peak width. STEC and PTEC are also coefficients of the lorentzian width but they are used only in the case of anisotropic broadening. STEC is associated with the Y component and PTEC with X component.

S/L and H/L are geometrical terms that describe the low angle broadening of low angle peaks in general with 2theta165 deg).These terms can be calculated by dividing the width of the divergence and receiving slits by the diffractometer radius. These terms are in general equal and should be fixed for a specific (known) set of slits and diffractometer radius, but can be refined if sufficiently low angle peaks ara available. The two parameters are added and squared in the corresponding equation therefore they cannot be refined together.

TRNS is the correction for sample transparency in Bragg-Brentano geometry. It is the inverse of the absorption effect and in BB geometry is zero for very highly absorbing samples and different than zero for intermediate or low absorbing ones. The efect of this correction is a shift in peak positions that increases with sin(2theta). In general for short 2-theta range data it could correlate strongly with SHIFT, ZERO and the cell parameters.

SHFT is the correction in peak position for sample's vertical shift (respect to the tangent line of the focusing circle) in Bragg-Brentano geometry. For short 2theta range it correlates strongly with TRNS, ZERO and the cell parameters. It depends stronlgy on sample mounting so very special care must be taken when mounting the sample to avoid this error.

SFEC plays the role of PTEC in the lorentzian width when stacking faults are present and an aditional anisotropic broadening associated with this fenomenon is present. It relays on the definition of a vector that describes the stacking fault sub-lattice. This is not given in EXPGUI but you should look for it in EXPEDT.

Finally Lij are a set of terms of microstrain anisotropy associated with LY and STEC parameters but show a specific dependence with HKL.

You better check the literature cited in GSAS Manual if you want more details of the specific parameters.

You can also refer to several on-line courses or study materials available on the web on the Rietveld Method or several good books on the topic, may available on-line.

Best of luck.

Great Answer !!!