>hello all, I have a little question concerning preferred orientation and GSAS.
>GSAS allows you to insert "preferred orientation axes". Well, how does one
>know which axes to insert? In one of my samples, I used a high purity lead
>oxide. Its pattern matches that of 38-1477 and it is orthorhombic. The only
>problem is that the intensities for the 100% and 24% peaks are switched. I am
>assuming that the processing method which the company used gave some kind of
>preferred orientation to the lead oxide which caused this sort of intensity
>mismatch. NOw, how should I account for this using the preferred orientation
>option in GSAS? Any suggestions?
I can say how I might do it using a vertical goniometer (horizontal planar
sample holder), theta-2theta reflection mode, X-rays :
1- disregard the 38-1477 file.
2- make at least two patterns.
a- with a maximum of preferred orientation by pressing the sample.
Do not use this pattern for refinement.
b- with a minimum of preferred orientation either by :
b-1 preparing the sample by dusting the powder on the holder
through a 63 micrometers sieve (or less). Do not press. After
me, this is the best inexpensive way. The problem is that the
surface will not be planar, then you will observe enlarged
FWHMs. You may have difficulties to put your sample in the
reflecting plane and consequently observe strongly shifted
positions (zeroshift). Do not forget that your grain size should
be ideally 1-2 micrometers. If some large crystals are present
in the "powder", then they may generate strong and narrow peaks
somewhere on your pattern.
b-2 using the famous McMurdie et al sample holder which is loaded
vertically. If your sample is easy oriented, this method is
not perfect. However it allows to obtain better surfaces than
in the b-1 method, so that this pattern may be used as the
final pattern for refinement.
b-3 using a spray dryer (expensive).
3- Compare the two patterns,
a- if there is only one preferred orientation direction, it will
become evident. March-Dollase parameter < 1.0
b- if you have needles, you may have one non-preferred orientation
direction. March-Dollase parameter > 1.0
c- more complicated than a and b is possible.
4- Now look at the 38-1477 file.
5- Best wishes.
reference : McMurdie et al (1986). Powder Diffraction 1, 40-43.
armel@one.univ-lemans.fr