Changeset 4960 for trunk/GSASIIobj.py
 Timestamp:
 Jun 15, 2021 1:36:25 PM (3 months ago)
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trunk/GSASIIobj.py
r4924 r4960 1084 1084 For reducing the effect of texture in a structural refinement 1085 1085 there are entries labeled preferred orientation in each phase's 1086 data tab. Two different 1087 approaches can be used for this, the MarchDollase model and 1088 spherical harmonics. 1086 data tab. Two different approaches can be used for this, the MarchDollase 1087 model and spherical harmonics. 1088 1089 1089 For the MarchDollase model, one axis in reciprocal space is designated as 1090 1090 unique (defaulting to the 001 axis) and reflections are corrected … … 1099 1099 1100 1100 The second method for reducing the effect of texture in a structural 1101 refinement is to create a probability surface as an expansion in1102 terms spherical harmonic functions. Only functions consistent with1101 refinement is to create a crystallite orientation probability surface as an 1102 expansion in terms spherical harmonic functions. Only functions consistent with 1103 1103 cylindrical diffraction suymmetry and having texture symmetry 1104 1104 consistent with the Laue class of phase are used and are allowed, 1105 so the higher the symmetry 1106 the fewer terms that are available for a given spherical harmonics order. 1107 For use of this correction, select the lowest order that provides 1105 so the higher the symmetry the fewer terms that are available for a given spherical harmonics order. 1106 To use this correction, select the lowest order that provides 1108 1107 refinable terms and perform a refinement. If the texture index remains close to 1109 1108 one, then the correction is not needed. If a significant improvement is … … 1111 1110 gives an even larger improvement. 1112 1111 1113 To characterize texture in a material, one needs data collected with the1112 To characterize texture in a material, generally one needs data collected with the 1114 1113 sample at multiple orientations or, for TOF, with detectors at multiple 1115 1114 locations around the sample. In this case the detector orientation is given in 1116 1115 each histogram's Sample Parameters and the sample's orientation is described 1117 1116 with the Euler angles specifed on the phase's Texture tab, which is also 1118 where the texture type (cylindrical, rolling,...) and the s herical1117 where the texture type (cylindrical, rolling,...) and the spherical 1119 1118 harmonic order is selected. This should not be used with a single dataset and 1120 1119 should not be used if the preferred orientations corrections are used. … … 1132 1131 about K and Chis is a rotation about J. 1133 1132 1133 Three typical examples: 1134 1) BraggBrentano laboratory diffractometer: Chi=0 1135 2) DebyeScherrer counter detector; sample capillary axis perpendicular to diffraction plane: Chi=90 1136 3) DebyeScherrer 2D area detector positioned directly behind sample; sample capillary axis horizontal; Chi=0 1137 NB: The area detector azimuthal angle = 0 in horizontal plane to right as viewed from xray source & 90 at vertical "up" direction 1138 1134 1139 ISODISTORT implementation 1135 1140 
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