Changeset 4413 for trunk/GSASIIobj.py


Ignore:
Timestamp:
Apr 24, 2020 9:52:46 PM (19 months ago)
Author:
toby
Message:

add texture docs and fix build problem

File:
1 edited

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  • trunk/GSASIIobj.py

    r4399 r4413  
    10001000letter or string flag value (such as I or A for iso/anisotropic).
    10011001
     1002Texture implementation
     1003------------------------------
     1004
     1005There are two different places where texture can be treated in GSAS-II.
     1006One is for mitigating the effects of texture in a structural refinement.
     1007The other is for texture characterization.
     1008
     1009For reducing the effect of texture in a structural refinement
     1010there are entries labeled preferred orientation in each phase's
     1011data tab. Two different
     1012approaches can be used for this, the March-Dollase model and
     1013spherical harmonics.
     1014For the March-Dollase model, one axis in reciprocal space is designated as
     1015unique (defaulting to the 001 axis) and reflections are corrected
     1016according to the angle they make with this axis depending on
     1017the March-Dollase ratio. (If unity, no correction is made).
     1018The ratio can be greater than one or less than one depending on if
     1019crystallites oriented along the designated axis are
     1020overrepresented or underrepresented. For most crystal systems there is an
     1021obvious choice for the direction of the unique axis and then only a single
     1022term needs to be refined. If the number is close to 1, then the correction
     1023is not needed.
     1024
     1025The second method for reducing the effect of texture in a structural
     1026refinement is to create a probability surface as an expansion in
     1027terms spherical harmonic functions. Only functions consistent with
     1028cylindrical diffraction suymmetry and having texture symmetry
     1029consistent with the Laue class of phase are used and are allowed,
     1030so the higher the symmetry
     1031the fewer terms that are available for a given spherical harmonics order.
     1032For use of this correction, select the lowest order that provides
     1033refinable terms and perform a refinement. If the texture index remains close to
     1034one, then the correction is not needed. If a significant improvement is
     1035noted in the profile Rwp, one may wish to see if a higher order expansion
     1036gives an even larger improvement.
     1037
     1038To characterize texture in a material, one needs data collected with the
     1039sample at multiple orientations or, for TOF, with detectors at multiple
     1040locations around the sample. In this case the detector orientation is given in
     1041each histogram's Sample Parameters and the sample's orientation is described
     1042with the Euler angles specifed on the phase's Texture tab, which is also
     1043where the texture type (cylindrical, rolling,...) and the sherical
     1044harmonic order is selected. This should not be used with a single dataset and
     1045should not be used if the preferred orientations corrections are used.
     1046
     1047The coordinate system used for texture characterization is defined where
     1048the sample coordinates (Psi, gamma) are defined with an instrument coordinate
     1049system (I, J, K) such that I is normal to the diffraction plane and J is coincident with the
     1050direction of the incident radiation beam pointing away from the source. We further define
     1051a standard set of right-handed goniometer eulerian angles (Omega, Chi, Phi) so that Omega and Phi are
     1052rotations about I and Chi is a rotation about J when Omega, Chi, Phi = 0. Finally, as the sample
     1053may be mounted so that the sample coordinate system (Is, Js, Ks) does not coincide with
     1054the instrument coordinate system (I, J, K), we define three eulerian sample rotation angles
     1055(Omega-s, Chi-s, Phi-s) that describe the rotation from (I, J, K) to (Is, Js, Ks). The sample rotation
     1056angles are defined so that with the goniometer angles at zero Omega-s and Phi-s are rotations
     1057about I and Chi-s is a rotation about J.
     1058
    10021059ISODISTORT implementation
    10031060------------------------------
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