Can the CsAlSiO4 and CsAlSi2O6 materials be grown as stoichiometric (mm sized) single crystals? Can they be grown in large enough quantities suitable for global distribution (~100 grams)? Is this an expensive synthesis?
I suggest that the compounds CsCoPO4 and TlZnPO4 could be preferable for use as Cs and Tl standards, respectively.
The former has more Cs than pollucite, is insoluble and beam-stable, has no X-ray overlaps among its elements, and can be synthesized as reasonably-sized crystals at <1375 K.
See: Kawaji, H., Ishihara, Y., Nidaira, A., Tojo, T., Atake, T. and Kuroiwa, Y., 2008. Particle size effect on the new phase transition in a tridymite compound, CsCoPO4. Journal of Thermal Analysis and Calorimetry, 92(2), pp.451-455.
The latter has high Tl content, and is (very probably) insoluble and beam-stable, with no X-ray overlaps among its elements, and can be synthesized as reasonably-sized crystals at <1350 K,
See: Wallez, G., Xuriguera, A.M., Elfakir, A. and Quarton, M., 1993. Crystal growth and crystallographic data of five TIB II XO 4 compounds. Powder Diffraction, 8(3), pp.145-148.
Best regards, Andrew