A century of progress in the sciences due to atomic weight and isotopic composition measurements.

Even before the 20th century, a consistent set of internationally accepted atomic weights was an important objective of the scientific community because of the fundamental importance of these values to science, technology and trade. As the 20th century progressed, physicists, geoscientists, and metrologists collaborated with chemists to revolutionize the science of atomic weights. At the beginning of the century, atomic weights were determined from mass relationships between chemical reactants and products of known stoichiometry.

Towards Near Perfect Crystals With Only Well-Characterized Imperfections.

Throughout the past century NBS/NIST supported a great variety of research studies in crystallography where the aim was the highest attainable accuracy in measurements. While avoiding overlap with other papers in this volume, this article summarizes results from: 1) early work on crystallization as a method of purification, in contributions to sugar chemistry, and in solution growth of large crystals; 2) the NBS/ARPA Program of research on crystal growth and characterization; 3) the NBS Free-Radical Research Program; 4) the XRCD method as a direct path to relative atomic mass data and the fundamental physical constants; 5) the dynamical theory of x-ray diffraction; and 6) symmetry considerations such as are involved in the influence on crystals of mechanical stress or fields, and of point defect motion.

Letter to the Editor: New Values for Silicon Reference Materials, Certified for Isotope Abundance Ratios.

New isotope abundance and relative atomic mass (atomic weight) values - with low, hitherto unattained uncertainty - are reported for two previously described silicon reference materials using a well-known method with an improved isotope-ratio mass spectrometer. These new values are directly traceable to the SI, more specifically to the unit for amount of substance, the mole, and independent of the SI unit of mass and of the Avogadro constant. Besides the residual mass-spectrometric uncertainties, these new values depend in effect only on a recently published direct comparison of the cyclotron frequency in a Penning trap of Si with that of C.

Letter to the Editor: Silicon Reference Materials Certified for Isotope Abundances.

In a series of gas mass-spectrometric measurements performed near the highest attainable accuracy, samples from two highly homogeneous batches of silicon crystals and silica powder were compared directly with a synthetic mixture of the three stable isotopes of silicon. Thereby, this work not only established the "absolute" atomic weight of these batches, but also makes portions of these batches available as an Isotopie Reference Material for accurate isotopic abundance measurements in geochemical and other isotope-abundance studies of silicon.

Distorted Tetrahedra in Strontium Copper Åkermanite.

New analogs of åkermanite: SrCuSiO and SrCdSiO as well as many previously synthesized analogs were obtained by a simple technique consisting of heating in air of precipitated gels of the right composition. X-ray data revealed an anomaly of the Cu and (to a lesser extent) Fe åkermanites which is explained by a simple geometric argument in terms of a small (~3°) angular distortion of the oxygen tetrahedron surrounding the smaller divalent cation. This interpretation is consistent with previous theoretical discussions and Mössbauer data on the iron analog prepared for this study.