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Fluctuating XRD Sapphire Intensityby maxxlr8
Tags: crystallography, diffraction, fluctuating, intensity, phase analysis, sapphire, xray diffractometer 
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#1
Dec913, 09:00 PM

P: 12

A few months back my colleagues and I are facing a problem where the intensity of our sapphire peak fluctuates every time an XRD phase analysis was done, even when the scan was done on the same sample.
By accident, I found that the orientation of the sapphire substrate with respect to phi axis somehow plays a part in the situation. So, I decided to do a scan following the procedure below:
Below is the result: It can be seen that although 2theta has been placed at the diffraction angle of the sapphire substrate, diffraction only occurs at two main regions in phi, which are approximately:
For now, only samples with sapphire substrates have this kind of problem. It is not known if other samples involving silicon or GaAs substrates have the same issue. Can anyone help explain the theory behind this observation? Thank you 


#2
Dec1113, 02:56 PM

P: 669

Sapphire peaks are very sharp, so even a slight misalignment will kill your intensity. It looks as if you tried to perform an azimuth scan, i.e. rotate the crystal about the Bragg planes while keeping the Bragg peak aligned. This will work only if the reciprocal lattice vector is aligned *exactly* along the phi axis of rotation. In practice that is a bit difficult to achieve. If there is some misalignment, then phi couples to the Bragg angle and you combine a rocking curve scan and an azimuth scan. If you carefully examine your UB matrix you can probably determine the angle of misalignment.
You might try to step phi in, say, steps of 5 degrees and perform a rocking curve scan at each value of phi. The integrated intensity of the rocking curve scans should not very  at least not strongly. 


#3
Dec1113, 07:13 PM

P: 12




#4
Dec1213, 03:19 AM

P: 669

Fluctuating XRD Sapphire Intensity
The UB matrix is actually the product of two matrices, U and B.
B performs the transformation from HKL coordinates to an orthonormal coordinate system of reciprocal space with the caxis along z. For cubic crystals B is just (2pi/a, 0,0; 0, 2pi/a, 0; 0,0,2pi/a). For hexagonal it is not diagonal because the angle between the a* and b* axes is not 90 deg, and so on. It depends only on the lattice parameters a,b,c and lattice angles alpha, beta, gamma. U is a rotation matrix that describes how the crystal is oriented relative to the innermost rotation axis of your diffractometer. If your sample is mounted on a goniometer head on the phi axis, then tweaking the goniometer head tilts will modify the matrix U. This is described in most books about xray diffraction, but I highly recommend reading the original article by Busing and Levy: Acta Cryst. (1967). 22, 457464 [ doi:10.1107/S0365110X67000970 ] Angle calculations for 3 and 4circle Xray and neutron diffractometers W. R. Busing and H. A. Levy http://scripts.iucr.org/cgibin/paper?a05492 


#5
Dec1213, 09:29 PM

P: 12

The literature is very interesting, although I may have to digest the contents slowly. 


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