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I'm a student wanting to become an expert in quantitative X-Ray Diffraction (QXRD) to determine the wt%s of both crystalline and amorphous phases in a solid specimen. At my university I have access to a Bruker D8A X-Ray Diffractometer and use DiffracEVA for PhaseID and TOPAS5 for quantitative analysis.

I've recently asked a question on how to do QXRD on complex fly ash specimens but have been advised by some colleagues that I shouldn't start with quantifying such a complex sample so quickly as it's very difficult and likely not time-efficient. At present I have only done successful QXRD (i.e. with minimal error in Rietveld Refining) with the simple idealised files found in the TOPAS how-to guide.

I'm looking to build my skillset and want to sample prep, test and QXRD analyse some synthetic samples (i.e. where I make the samples myself and know the exact wt%s) in order to get better at QXRD. Does anyone have any advice around this and, more specifically, recommend any good starting points for which material feedstocks to use and sample prep methodologies?

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The easiest crystal to synthesize is probably alum, or potassium aluminium sulfate $\ce{KAl(SO4)2·12H2O}$ . It is not expensive and easily soluble in water. Evaporation produces nice colorless octahedral crystals, which may grow up to macroscopic dimensions.

To obtain nice crystals, a hot concentrated solution has to be done in hot water in some beaker. After cooling down, plenty of small crystals are to be found on the bottom of the beaker. Most of them have irregular shape. Select by hand the crystal which looks like a regular octahedron. Paste it at the tip of a cotton wire. Stir and reheat the alum solution so as to dissolve the crystals. Stop the heating when the last crystals is dissolved. Hang the wire from above, so that the small octahedron is in the middle of the solution. Let it cool down. When cold, the crystal dimension has increased. Repeat the operation heating - cooling, as often as required.

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  • $\begingroup$ This may be a silly question, but could I just mix a 50/50 wt% mixture of lab-grade CaF2 and corundum (Al2O3) together and go from there trying out some QXRD methods? $\endgroup$
    – Hendrix13
    Sep 5, 2022 at 8:22
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    $\begingroup$ Try ! But lab grade $\ce{CaF2}$ is obtained by precipitation in aqueous solution. So it may not be really crystallized. $\endgroup$
    – Maurice
    Sep 5, 2022 at 9:49
  • $\begingroup$ Could you please expand on the above comment? Why/how would CaF2 "not be really crystallised "? $\endgroup$
    – Hendrix13
    Nov 11, 2022 at 22:10
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    $\begingroup$ Usually, when preparing a substance in a test tube by a precipitation reaction, the obtained substance is made of micro crystals, which are too small to be analyzed by X-rays. $\endgroup$
    – Maurice
    Nov 12, 2022 at 9:32
  • $\begingroup$ Wow interesting, what other analytics would help here? NMR? $\endgroup$
    – Hendrix13
    Nov 12, 2022 at 14:36

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