Capabilities of the POWDER Instrument

The scientific areas that may benefit from using the HB-2A instrument are condensed matter physics, chemistry, geology, and material science. Due to its versatility, this instrument can be employed for a large variety of experiments, but it is particularly adapted for determining crystal structures with relatively large unit cells (dmax ≈ 28 Å), as well as complex magnetic structures. Furthermore, studies of phase transitions, thermal expansion, quantitative analysis, and ab-initio structure solution from powder data can be undertaken. A full range of ancillary sample environments can be used to provide a complete control of thermodynamic variables such as temperature, magnetic field, and pressure.

The following examples highlight some of the basic features and capabilities of this instrument:

  1. Neutron data collected at HB-2A can be used to carry out accurate crystal structure refinements to determine atomic positions, atomic displacement parameters, and site occupancies. By using a 1.54 A neutron wavelength selected by the vertically focusing Ge(115) monochromator, a “Rietveld quality” data can be obtained in about 2 hours. Collection of such data has been recently used to explore the detailed structural changes of the crystal structure of the clathrate-type thermoelectric material Ba8AlxSi46-x as a function of aluminum content. The barium atomic displacements were found to increase with increasing cage size but appear to be primarily dependent on the host framework site occupancies. Differences in the physical properties, specifically thermal conductivity, have been linked to the displacement of the atom in the large cage.

    Roudebush, J. H., de la Cruz, C., Chakoumakos, B. C., and Kauzlarich, S. M., "Neutron diffraction study of the type I clathrate Ba8AlxSi46–x: site occupancies, cage volumes, and the interaction between the guest and the host framework," Inorganic Chemistry 51, 1805-1812 (2012). (5 days of beam time to investigate 5 samples)

  2. Studies of crystal and magnetic phase transitions in strongly correlated electron systems. Measurements performed using a 2.41 A wavelength provide an accurate determination of the magnetic commensurate or incommensurate wave vectors and allow the determination of the spin arrangement within the nuclear unit cell. Depending on the amount of sample, the collection time for solving a magnetic structure may take from 1 to 6 hours. Order parameters of the phase transition can also be measured to extract the critical exponent revealing the universality class of the studied system. For such studies, a cryostat with 3He insert capable of reaching temperatures of 275 mK is available.

    McGuire, M. A., Gout, D. J., Garlea, V. O., Sefat, A. S., Sales, B. C., and Mandrus, D., "Magnetic phase transitions in NdCoAsO," Physical Review B 81, 104405 (2010). (4 days using several sample environments).

  3. Magnetoelastic effects in geometrically frustrated spin systems can also be investigated. Due to the good signal to noise ratio at low angles, detailed experimental studies of the development of short-range magnetic correlations can be undertaken. Scattering from two-dimensional (2D) spin ordered systems or spin glasses can be accurately measured.

    Garlea, V. O., Savici, A. T., and Jin R., "Tuning the magnetic ground state of a triangular lattice system Cu(Mn1-xCux)O2," Physical Review B 83, 172407 (2011)  (4 days).

  4. Studies of magnetic phase separation in nanocrystalline materials. Dramatic changes in the properties of spin systems are known to appear in materials reduced to the nanoscale, where finite size, grain boundary, and surface strain effects can play an important role. The HB-2A diffractometer has been used to demonstrate the strain-induced magnetic phase separation within LCMO nanocrystallites. By mechanically stressing nominally ferromagnetic La5/8Ca3/8MnO3 nanocrystallites through high-energy ball-milling techniques, the appearance of an anisotropically enhanced strain field coupled to the emergence phase-separated antiferromagnetic order has been observed.

    Dhital, C., de la Cruz, C., Opeil, C., Treat, A., Wang, K. F., Liu, J. M., Ren, Z. F., and Wilson, S. D., "Neutron scattering study of magnetic phase separation in nanocrystalline La5/8Ca3/8MnO3," Physical Review B 84, 144401 (2011). (2 days)

  5. HB-2A data have also been used to solve crystallographic structures via the ab initio method and Rietveld refinement. Neutron powder diffraction data can have a marked impact on the ability to accurately determine the positions of light atoms within the crystal lattice. For instance, the neutron diffraction data were successfully used to quantitatively “see” the lithium location, which is crucial to understanding the microscopic behavior of lithium battery materials.

    Ellis, B. L., Ramesh, T. N., Rowan-Weetaluktuk, W. N., Ryan, D. H., and Nazar, L. F., "Solvothermal synthesis of electroactive lithium iron tavorites and structure of Li2FePO4F," Journal of Materials Chemistry 22, 4759-4766 (2012) (1 day).

  6. Mapping out the Pressure-Temperature phase diagrams of hydrates and inorganic systems. In-situ neutron diffraction studies on pressure-induced structural and magnetic phases transformation are especially attractive for microscopic modeling since the chemistry of the system remains untouched. High-pressure neutron diffraction studies of neutron diffraction measurements performed at the HB-2A on the negative thermal expansion system (NTEs) ScF3 show that the stability of the cubic structure against a rhombohedral distortion becomes increasingly marginal upon cooling.

    Greve, B. K., Martin, K. L., Lee, P. L., Chupas, P. J., Chapman, K. W., and Wilkinson, A. P., "Pronounced negative thermal expansion from a simple structure: cubic ScF3," Journal of the American Chemical Society 132, 15496-15498 (2010). (3 days)

  7. Studies of magnetization density distribution in complex materials. By using an in-house–developed 3He cell, the HB-2A neutron incident beam can be polarized and used to perform polarized studies for high sensitivity to weak ferromagnetic/ferrimagnetic signals. A successful test of polarized measurements has already been performed on a thermally quenched K-Co-Fe Prussian blue analogue photomagnet. The material has been studied down to 4 K with both unpolarized and polarized neutron powder diffraction as a function of applied magnetic field, and analysis of the data have allowed the on-site coherent magnetization of the cobalt and iron spins to be established.

    Pajerowski, D. M., Garlea, V. O., Knowles, E. S., Andrus, M. J., Dumont, M. F., Calm, Y. M., Nagler, S. E., Tong, X., Talham, D. R., and Meisel, M. W. “Magnetic Neutron Scattering of Thermally Quenched K-Co-Fe Prussian Blue Analogue Photomagnet,” Physical Review B - submitted (2012) (5 days).

  8. Microstrain studies as a function of temperature can be performed to elevated temperatures, up to 1200°C, in both vacuum and air environment. As an example, the mechanisms of microcracking and stress release has recently been investigated in b-eucryptite ceramics by applying a combination of neutron diffraction, dilatometry, and integrity factor modeling. It has been observed that the macroscopic thermal expansion of solid samples closely follows the lattice thermal expansion as a function of temperature, and both are dominated by microcracks closing (during heating) and opening (during cooling). By means of Rietveld refinement of the neutron diffraction data, the evolution with temperature of peak width parameters linked to inter- and intra-granular strain along the basal, pyramidal and axial planes could also be extracted. They correlate well with the microstrains calculated by peak shift and with the model results.

    Bruno, G., Garlea, V. O., Muth, J., Efremov, A. M., Watkins, T. R., and Shya, A., “Microstrain temperature evolution in b-eucryptite ceramics: measurement and model,”  Acta Materialia 60, 4982–4996 (2012). (6 days for 6 samples)

  9. In-situ neutron diffraction studies of solidification processes of advanced alloy systems. A solidification cell was designed and built to carry out controlled melting and solidification experiments under the simultaneous exposure to neutron radiation. D. Sediako et al. demonstrated the applicability of neutron diffraction to solidification analysis of a binary hypereutectic Al-19%Si alloy using step-wise cooling. Besides qualitative analysis, the authors showed the potential to quantify the volume fraction of primary silicon and aluminum as well as silicon in the eutectic phase using diffraction signals from liquid and solid phases as they evolved during solidification.

    Sediako, D., Kasprzak, W., Swainson, I., and Garlea, O., "Solidification analysis of Al-Si alloys modified with addition of Cu using in-situ neutron diffraction," Materials Fabrication, Properties, Characterization, and Modeling 2, 279-289 (2011). (4 days)