Neutron Powder Diffraction

The first powder diffraction studies on simple materials such as iron metal using x rays were done independently by Debye and Scherrer (1916) in Germany and by Hull (1917) in the United States. For a long time x-ray powder diffraction was primarily used for qualitative purposes such as phase identification and the assessment of crystallinity. The use of neutrons for such powder diffraction work has advanced significantly over the past 50 years. The continuing efforts made in unraveling the structural details of superconducting oxides and the recently discovered oxides exhibiting colossal magnetoresistance have demonstrated the indisputable usefulness of high-resolution neutron powder diffraction. Modern powder diffraction using neutrons, x rays, and synchrotron x-ray radiation has developed from a qualitative method some 20 years ago to a quantitative method now used to detect new phases and determine their atomic, magnetic, and mesoscopic structure as well as their volume fractions if they are present in a mixture. Neutron powder diffraction is a complementary technique to x-ray powder diffraction and electron diffraction. The greater penetration depth of neutrons, the fact that the neutron-nucleus interaction is a point scattering process implying no variation of the nuclear scattering length with scattering angle, the independence of the scattering cross-section from the number of electrons (Z) of an element, and therefore the stronger interaction of neutrons with “light” elements such as oxygen and hydrogen, and its isotope specificity as well as its interaction with unpaired electrons (“magnetic scattering”) make neutrons a unique and indispensable probe for structural condensed matter physics and chemistry. However, x-ray powder diffraction and, in particular, synchrotron x-ray powder diffraction can investigate samples many thousand times smaller and have an intrinsic resolution at least one order of magnitude better than the best neutron powder diffractometer. A major drawback for neutron scattering is that it can presently only be done at large facilities (research reactors and spallation sources), whereas laboratory-based x-ray scattering equipment provides the same flux much more conveniently. Current governmental policies will not permit the construction or upgrade of present-day research reactor sources in the United States. The future of U.S. neutron scattering therefore relies solely on accelerator-based spallation sources. Electron diffraction is a very powerful tool to obtain both real and reciprocal space images of minute amounts of powders. In combination with other supplementary techniques such as solid-state nuclear magnetic resonance (NMR), which allows, e.g., the determination of Wyckoff multiplicities and interatomic distances by exploiting the nuclear Overhauser effect (NOE), physicists, chemists, and materials scientists have a powerful arsenal to elucidate condensed matter structures. Keywords: neutron powder diffraction; neutron; principles; practical aspects; time-of-flight diffractometers; sample preparation; data analysis; initial interpretation; peak shape; rietveld method; mesoscopic properties; quantitative phase analysis; structure determination; powder diffraction; problems; crystallographic refinements; constraints; restraints