Here is what I know about making “insoluble” halide salts to be used as standards for microanalysis (SfM hereafter).
First of all these are not perfectly insoluble in water. Good crystals are possible in many cases because there is a slight solubility.
These salts are all truly insoluble in 100% lab (not pharmacy) isopropanol from a glass bottle. Not too toxic, compatible with epoxy and plastics, and was found to be safe for metal mounts.
Second critical point is the matter of mounting perfect crystals. Most crystal growing yields excellent octahedral or cubic crystals and then some residue of hoppered garbage. The hoppers are worse than powder because they can retain microlayers of solvent that outgas for far too long.
Buyer Beware Inspect loose grains that you buy as “SfM” and return any with hoppers for a full refund. Inspect any natural “SfM” for inclusions by using a few drops of isopropanol on a slide, and rotate to see any second phase under magnification.
Fluorine Commercial LiF and other synthetics including NaMgF3, K2SiF6, Cs2SiF6. My A2SiF6 never developed colour centres in full scan XRF.
I have been told (never checked) that colourless natural topaz is free from OH. If you need an O bearing F standard then use SiO2 and Al2O3 and LiF to calibrate, because both O and F should be resolved in a good EDS. Pieces at my local rock shop would cost less than SEM time…
DO NOT use fluoroapatite as an F standard because the crystal structure guarantees F migration under the electron beam, that has been shown to be permanent. Please read Pyle et al, Reviews in Mineralogy and Geochemistry (2002) vol 48 pp 337-362 for the details. The F migration occurs in all F-Ap regardless of origin.
Chlorine I made Rb2PtCl6 from both Pt-aqua regia and Alfa H2PtCl6, adding RbCl. Nice raft of octahedral crystals grew at the top of a 1 litre Erlenmeyer flask over the space of 2 months using water with 5mL HCl and 1mL HNO3. Cover with watch glass. Lowest setting on hotplate, airflow at front corner of fume hood made an excellent thermal gradient. No hoppers formed. You want a good supply of yellow solid at the bottom of the flask to feed the crystals at the top. Green solid means you need more HNO3.
Rb2SnCl6 can be made by mixing saturated solution of 2RbCl into solution of SnCl4-5H2O and a trace of HCl. Slow evaporation while excluding air gives the white hexoctahedral product. It is more soluble but OK in isopropanol. Success depends on the quality of SnCl4-5H2O which degrades with age.
Thallium chloride TlCl is extremely soft and difficult to polish. Ugly mess.
End-member chlorapatite Ca5(PO4)3Cl2 can be made under hydrothermal conditions. Difficult to prepare in open crucible because there is a transition on cooling below 1300°C.
Spodiosite Ca2PO4F is easier to prepare from commercial calcium orthophosphate and CaCl2 flux in a crucible at 800-900°C. Good project to start learning about high temperature synthesis.
Bromine Thallium bromide is insoluble. It is commercially available in good single crystal form. Add this after polishing all other materials in a mount to minimize exposure to ground particles.
KPbBr3 might be useful, I have not tried. See KPbI3 below.
Iodine Several insoluble options
There is one caveat to using thallium bromide-iodide—make sure of exact details of manufacture. Certified KRS-5 is a dark orange-red single crystal optical material. KRS-5 has 58% Br ions and 42% I ions. KRS-5 should be seller certified as the eutectic (lowest melting point mixture of thallium bromide and thallium iodide). It should be zone refined to ensure absence of other phases. Check the loose piece by IR rather than in polarized light because all phases are cubic.
NB Some Taylor material was the 60Br-40I mix prepared by simple mixing and melting. To my eyes it was slightly lighter orange.
In theory it should be possible to melt thallium iodide that has been vacuum sealed in quartz without significant decomposition.
I found it was easier to grow good-sized crystals of KPbI3 than PbI2. Simple slow cooling of a saturated solution (beaker in large water bath) gave flakes. Prebonding in epoxy allowed the flakes to be mounted sideways. NB you get a hydrated form by evaporative growth.
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