WHY POWDERS SHOULD ALWAYS BE MEASURED IN TRANSMISSION-/ DEBYE-SCHERRER GEOMETRY
THE STOE TRANSMISSION GEOMETRY
The STOE Transmission geometry is a hybrid of the Scherrer (left) and the Guinier camera (right) using the sample position centered in the detector circle from the first and the focusing optic from the latter.
A (111)-cut Ge monochromator (Johann-type) provides pure Ka1-radiation. It focuses primary and diffracted beam on the detector circle yielding highest resolution in 2q (FWHM<0.03° for a LaB6 (110) reflection and Mo Ka1-radiation).
CONSTANT SAMPLE VOLUME
With a constant sample volume in the beam, the Transmission-/Debye-Scherrer geometry provides reliable intensities over the full 2q scale (appr. 0.3 to 140°) while the variable amount of unaffected beam as a function of the q-value in a reflection setup yields false intensities up to at least 10° 2q if not corrected by variable slits!
NO HEIGHT DISPLACEMENT
Reflection data often yields a zero shift in 2q if the sample thickness varies and cannot be corrected by an automated z-translation. An aligned capillary is always in the center of the goniometer.
NO LINE BROADENING FOR WEAK ABSORBERS
In reflection geometry the difference in the depth penetration as a function of the absorption factor yields a remarkable peak broadening for weak absorbers. Powder diffraction in Transmission-/ Debye-Scherrer geometry avoids this phenomenon.
LESS AFFECTED BY PREFERED ORIENTATION
The statistical distribution of the particles in a capillary yields a pattern less affected by the effects of preferred orientation than the periodic stacking sequence of the planes in reflection mode.
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