OTHER DESIGNATIONS: X-ray diffraction (XRD), powder diffraction, single-crystal diffraction, surface diffraction, x-ray standing wave (XSW).

PURPOSE: Essentially everything we know about the atomic structure of materials is based on results from x-ray and neutron diffraction. From advanced ceramics to catalysts, from semiconductor technology to the frontiers of medicine, and from new magnetic materials and devices to framework compounds used to sequester radioactive waste, crystallography using hard x-ray diffraction techniques at synchrotron radiation facilities plays a crucial role in our ability to understand and control the world in which we live. Research problems that this technique can address are:

• Structural studies of crystalline materials
• Drug design by pharmaceutical industry
• Biomineralization
• New microporous materials including natrolites, phosphates, and titanates
• Novel complex oxides: structure–property relationships, phase transitions
• Residual stress determination in situ

HOW THE TECHNIQUE WORKS: Hard x-rays have wavelengths comparable to the distance between atoms. When a crystalline sample is illuminated with x-rays, the x-rays are scattered (diffracted) into very specific directions with various strengths. Detectors are used to measure this "diffraction pattern," which is then processed by computers to deduce the arrangement of atoms within the crystal. There are two principal modes. In "Bragg" diffraction, the incident x-rays are monochromatic (single wavelength) and the sample is an oriented single crystal. In "Laue" diffraction, the incident x-ray beam is white (all wavelengths) so all the possible diffraction routes are detected.

UNIQUENESS: Many materials that can be obtained in single-crystal form are impossible to investigate with laboratory x-ray diffraction equipment because the crystals are too small. Synchrotron hard x-rays provide significant advantages over conventional laboratory sources: increased peak-to-background ratios, angular resolution, and the ability to select photon energies.

EXAMPLES:

A Zeolite "Molecular Sponge" to Trap Pollutants?
Axiotaxy — Discovery of a New Texture in Thin Films
Graphite Acts Like Diamond Under Pressure
Looking into Glass’s Secrets
Levitated Droplets Reveal Origin of Undercooling

Polyhedral representations of two zeolites before and after pressure-induced hydration.

Zeolites are solids containing aluminum, silicon, and oxygen with regularly spaced pores within the molecular framework. These nanopores make zeolites very useful for trapping small molecules, ions, or gases. It has been previously shown that certain zeolites can expand under pressure and take up more water to become superhydrated. This extra volume allows slightly larger molecules or atoms, such as pollutants, to enter the expanded pores. When the pressure is released and the material contracts, the pollutants would become trapped. However, in a reversible system, half of the water would be expelled again, making the sponges somewhat leaky. Using hard x-ray powder diffraction and single-crystal diffraction, scientists solved the structure of a zeolite (potassium gallosilicate) that exhibits irreversible pressure-induced hydration: when the pressure is released, the material stays superhydrated. The structures obtained show that the irreversible hydration is associated with a rearrangement of the charge-balancing cations contained in the nanopores. By understanding these cation migrations and rearrangements under pressure, scientists may reduce the pressure at which the pressure-induced hydration occurs, and thereby open up new ways to use zeolites as "molecular sponges" for pollutants such as tritiated water or as transport vessels for medical applications.

Y. Lee, T. Vogt, J. Hriljac, J. Parise, J. Hanson, and S. Kim, "Non-framework cation migration and irreversible pressure-induced hydration in a zeolite," Nature 420, 485 (2002).

At the microscopic level, most materials are made of crystalline grains, and the way these grains fit together is referred to as the material’s "texture." The grains can be randomly oriented in three dimensions ("random" texture), randomly oriented within flat layers ("fiber" texture), or neatly arranged and stacked ("epitaxy" texture). The effect of texture on the material’s electrical, magnetic, and mechanical properties can be exploited to produce specific characteristics. As the miniaturization of silicon-based devices continues, understanding and controlling texture, which controls the properties of silicides, becomes critical. X-ray diffraction was used to analyze the texture of a nickel silicide (NiSi) thin film on a silicon substrate. The resulting image, called a "pole figure," represents the distribution of the grain orientations in the film. The pole figure obtained for NiSi did not resemble those produced by any of the three known textures or any combination of them. The new texture, named "axiotaxy," is similar to the fiber (layered) texture. However, instead of sitting flat on the substrate, each grain is tilted upwards at the same angle. This causes a special relationship to form between the orientation of the grains in the thin film and the substrate. The mechanism that causes the formation of this new type of texture is unique, and helps us to better understand the physics of thin film growth and phase transformations in thin films.

C. Detavernier, A.S. Ozcan, J. Jordan-Sweet, E.A. Stach, J. Tersoff, F.M. Ross, and C. Lavoie, "An off-normal fibre-like texture in thin films on single-crystal substrates," Nature 426, 641 (2003).

Diamond-anvil cell.

Researchers have discovered that, under extreme pressure, graphite, among the softest of materials, becomes as hard as diamond, the hardest known material. While graphite is made of loosely bound carbon atoms, diamonds are made of tightly bound carbon atoms, resulting in extreme hardness. These diamond-type bonds are difficult to achieve; natural diamonds are made in the Earth by great pressures and intense heat over geological time scales. The researchers in this study used a diamond-anvil cell to produce pressures approximately 170,000 times the atmosphere at sea level or equal to that more than 300 miles beneath the Earth's surface. The atomic bonds were probed using inelastic x-ray scattering, and structural information was obtained using x-ray diffraction. The findings showed that half of the weak, widely spaced graphite bonds were forced closer together, converting them to stronger, diamond-like bonds. In fact, the graphite became so hard that it cracked the diamond anvil. Moreover, the graphite became optically transparent, a superhard insulator, much like diamond. But, while the known forms of naturally produced diamond retain their hardness, the graphite in this experiment reverted back to its original softness once the pressure was removed. This reversible change in strength offers the possibility of intriguing applications as a pressure-dependent structural component (for instance, a composite gasket for a high-pressure apparatus).

W.L. Mao, H.-K. Mao, P.J. Eng, T.P. Trainor, M. Newville, C.?]C. Kao, D.L. Heinz, J. Shu, Y. Meng, and R.J. Hemley, "Bonding changes in compressed superhard graphite," Science 302, 425 (2003).

Octahedral structure of germania glass at 100,000 times atmospheric pressure.

Little is known about the structure of glass under pressure, even though we use it in our cars, our homes, and industry. It is difficult to study because, on the atomic level, glass is disordered; it lacks a periodic crystal structure. Researchers used a combination of tools, including neutron and x-ray diffraction, to study how the structure of glass changes under high pressures. Germania, a structural analogue to the more common form of silica glass, was used because it transforms structurally at much lower pressures and provides a greater contrast in neutron and x-ray techniques. At ambient pressure, glass is made up of tetrahedral units: four oxygen atoms enclose a germanium atom to create cages that are only a nanometer across. While neutron studies are sensitive to lighter elements such as oxygen, x-ray studies reveal germanium atoms more clearly and can test smaller samples, allowing studies at higher pressures. The researchers found that, as the sample was pushed from 60,000 to 150,000 times ambient pressure, the tetrahedral cages collapsed, and an average of five oxygen atoms organized around the germanium atom before the final, dense, purely octahedral form that has eluded scientists for decades was observed. This provided evidence that germania glass undergoes a continuous structural transformation, disproving the theory that tetrahedral glasses go through a distinct transition between low- and high-density phases. In addition, they found that the angles of the structures were not the 90 and 180 degrees of a perfect octahedron; instead, the angles were near 90 and 165 degrees.

M. Guthrie, C.A. Tulk, C.J. Benmore, J. Xu, J.L. Yarger, D.D. Klug, J.S. Tse, H-K. Mao, and R.J. Hemley, "Formation and structure of a dense octahedral glass," Phys. Rev. Lett. 93, 115502 (2004).

For over 50 years, it has been known that, under carefully controlled conditions, metallic liquids can be cooled far below their melting temperatures (undercooled) before crystallizing. This suggests that the mechanism responsible for forming the solid phase must present a large barrier to phase change. To explain this surprising result, F.C. Frank theorized that as metallic liquids cool, local structures with icosahedral (20-sided) symmetry develop that are incompatible with the long-range periodicity of the crystalline phase. Several experimental studies have supported this hypothesis but have shopped short of providing direct proof. Using an electrostatic levitation technique that allows in situ x-ray diffraction of a liquid-metal droplet in a containerless environment, researchers for the first time directly confirmed Frank’s hypotheses by studying the undercooling behavior of the liquid metal Ti-Zr-Ni. High-energy x-rays were required to penetrate the droplet to collect x-ray diffraction data in a transmission mode. The researchers measured increasing icosahedral short-range order (ISRO) in the liquid as it was undercooled, finding that this was responsible for the nucleation of a metastable icosahedral quasicrystal phase instead of the stable polytetrahedral phase. This verifies Frank’s hypothesis and demonstrates that local order in the liquid phase strongly influences the nucleation of specific phases, even in metallic liquids, where atomic interactions are weak and relatively isotropic.

K.F. Kelton, G.W. Lee, A.K. Gangopadhyay, R.W. Hyers, T.J. Rathz, J.R. Rogers, M.B. Robinson, and D.S. Robinson, "First x-ray scattering studies on electrostatically levitated metallic liquids: Demonstrated influence of local icosahedral order on the nucleation barrier," Phys. Rev. Lett. 90, 195504 (2003).