Standard Reference Material (SRM 1990) for single crystal diffractometer alignment

An international project was successfully completed which involved two major undertakings: (1) a round-robin to demonstrate the viability of the selected standard and ( 2) the certification of the lattice parameters of the SRM 1990, a Standard Reference Material(R) for single crystal diffractometer alignment. This SRM is a set of approximate to 3500 units of Cr-doped Al2O3, or ruby spheres [ (0.420.011 mole fraction % C ( expanded uncertainty)]. The round-robin consisted of determination of lattice parameters of a pair of crystals: the ruby sphere as a standard, and a zeolite reference to serve as an unknown. Fifty pairs of crystals were dispatched from Hauptman-Woodward Medical Research Institute to volunteers in x-ray laboratories world-wide. A total of 45 sets of data was received from 32 laboratories. The mean unit cell parameters of the ruby spheres was found to be a = 4.7608 Angstrom +/- 0.0062 Angstrom, and c = 12.9979 Angstrom +/- 0.020 Angstrom ( 95 % intervals of the laboratory means). The source of errors of outlier data was identified. The SRM project involved the certification of lattice parameters using four well-aligned single crystal diffractometers at ( Bell Laboratories) Lucent Technologies and at NRC of Canada ( 39 ruby spheres), the quantification of the Cr content using a combined microprobe and SEM/EDS technique, and the evaluation of the mosaicity of the ruby spheres using a double-crystal spectrometry method. A confirmation of the lattice parameters was also conducted using a Guinier-Hagg camera. Systematic corrections of thermal expansion and refraction corrections were applied. These rubies are rhombohedral, with space group R 3 c. The certified mean unit cell parameters are a = 4.76080 +/- 0.00029 Angstrom, and c = 12.99568 Angstrom +/- 0.00087 Angstrom ( expanded uncertainty). These certified lattice parameters fall well within the results of those obtained from the international round-robin study. The Guinier-Hagg transmission measurements on five samples of powdered rubies ( a = 4.7610 Angstrom +/- 0.0013 Angstrom, and c = 12.9954 Angstrom +/- 0.0034 Angstrom) agreed well with the values obtained from the single crystal spheres.