Lead mercaptides: materials useful as powder secondary standards and internal references for calibration of X-ray diffractometers at small and medium angles

Heavy-atom alkanethiolates (mercaptides), especially of lead, provide a series of sharp first-order lines for calibrating both wide- and small-angle X-ray diffractometers (WAXD and SAXS), out to at least 60 Å d spacing. There is no reliance on published data or on externally validated substances other than silicon standard reference material (SRM); any preparation showing a large number of higher-order lines is a perfect standard. Well resolved lines extending beyond the 30th order are observed and provide an excellent statistical basis for calculating a reliable first-order spacing, and then for diffractometer calibration at all 2θ values. Owing to their very low solubility, the lead mercaptides may often be mixed into a sample without altering it, thus providing one or more internal references to bracket a line of interest, enabling interpolation. Simple high-yield preparations of finely divided long-chain mercaptides of lead (C₈ to C₂₂), mercury, thallium and silver are reported, with first-order d spacings. The mercaptides react readily with carbon disulfide to generate the corresponding metal alkyltrithiocarbonates in good yield; these also serve for WAXD/SAXS calibration, though the shorter-chain trithiocarbonates lack long-term stability.