A perfect general purpose standard specimen for high accuracy line-profile analysis is shown to be an illusion. Balancing the partly contradictory requirements, an optimum standard specimen for a parafocusing diffractometer is developed. To obtain the optimum standard specimen, a 5–10 μm particle size fraction is taken from the NIST certified Si powder SRM640a, about 1.5 mg/cm2 of this powder is uniformly deposited on a (510) oriented Si single-crystal wafer and the assembly is heat treated for 2 h at 1273 K to remove lattice imperfections. All procedures necessary are precisely given, easily applicable, and reproducing. For the present standard specimens, the random errors due to crystal statistics are quantified and shown to be acceptable for spinning specimens; the systematic errors due to residual size and transparency broadening are determined semi-empirically and can be eliminated, if desired. Thus the proposed optimum standard specimen allows the determination of instrumental line profiles free from systematic errors and with random errors in the line width of the order of 0.001 °2θ, allowing a full use of the capacities of modern diffractometers and data evaluation procedures.

Limitations in powder diffractometry imposed by scatter slits and/or a diffracted-beam monochromator, which have been ignored in the past, are discussed and mapped quantitatively. These limitations become manifest especially with ωoffsets, i.e.in stress measurements and with ωoscillation to improve the reproducibility of intensities in the presence of too coarse grains. The limitations can only be established with knowledge of slit sizes and with precise alignment of all slits, their holders and the diffracted-beam monochromator. To that end, concise, accurate procedures for obtaining these measurements in-situare proposed.