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Branton Campbell in the lab

Branton J. Campbell

Professor

Department of Physics & Astronomy

Brigham Young University

Provo, UT 84602, USA

Tel: 801-422-5758, Fax: 801-422-0553

Email: branton_campbell[at]byu.edu

//physics.byu.edu/faculty/campbell/

Research Interests

I apply state-of-the-art x-ray and neutron scattering techniques to study local and long-range structures in a variety of complex solids, including fast-ion conductors, ferroelectric relaxors, high-temperature superconductors, and colossal magnetoresistive manganites, where nanoscale structural features influence macroscopic physical properties. This includes the development of symmetry-mode analysis as a tool for the determination, refinement and interpretation of distorted structures involving lattice strains, atomic displacements, magnetic moments and occupational orderings at both commensurate and incommensurate wavevectors.

Stellar Diffuse X-ray Scattering

Not astronomy! This is actually single-crystal diffuse x-ray scattering from an important industrial isomerization catalyst called mordenite, where the L = 0 plane of reciprocal space was reconstructed using portions from over 1000 CCD X-ray camera images. The broad patches, open diamonds, and star-shaped distributions are clues that reveal a complex architecture of framework defects with implications for this zeolite’s unusual adsorptive and catalytic properties.

Revisiting the CuPt₃ prototype and the L1₃ structure

By Lauren R. Richey, K.C. Erb, Conrad W. Rosenbrock, Lance J. Nelson, Richard R. Vanfleet, Harold T. Stokes, Branton J. Campbell, and Gus L. W. Hart (et al.)

Abstract:

Experimentally and computationally, the structure of Pt–Cu at 1:3 stoichiometry has a convoluted history. The L1₃ structure has been predicted to occur in binary alloy systems, but has not been linked to experimental observations. Using a combination of electron diffraction, synchrotron X-ray powder diffraction, and Monte Carlo simulations, we demonstrate that it is present in the Cu–Pt system at 1:3 stoichiometry. We also find that the 4-atom, fcc superstructure L1₃ is equivalent to the large 32-atom orthorhombic superstructure reported in older literature, resolving much of the confusion surrounding this composition. Quantitative Rietveld analysis of the X-ray data and qualitative trends in the electron-diffraction patterns reveal that the secondary X1+(a,0,0) order parameter of the L1₃ phase is unexpectedly weak relative to the primary L1+(a,a,0,0) order parameter, resulting in a partially-ordered L1₃ ordering, which we conclude to be the result of kinetic limitations. Monte Carlo simulations confirm the formation of a large cubic superstructure at high temperatures, and its eventual transformation to the L1₃ structure at lower temperature, but also provide evidence of other transitional orderings.

A symmetry-mode description of rigid-body rotations in crystalline solids: a case study of Mg(H₂O)₆RbBr₃

By Harold T. Stokes and Branton J. Campbell (et al.)

Abstract:

The application of rotational symmetry modes to quantitative rigid-body analysis is demonstrated for octahedral rotations in Mg(H₂O)₆RbBr₃. Rigid-body rotations are treated as axial-vector order parameters and projected using group-theoretical methods. The high-temperature crystal structure of the Mg(H₂O)₆RbBr₃ double salt consists of a cubic perovskite-like corner-sharing network of RbBr₆ octahedra with isolated MgO₆ octahedra at the perovskite A sites. A phase transition occurs at 411 K upon cooling, whereupon the MgO₆ octahedra experience a substantial rigid-body rotation, the RbBr₆ octahedra are translated but not rotated, and both types of octahedra become slightly distorted. The MgO₆ rotation has three orthogonal components associated with the X₅^-, Γ₄⁺ and X₁^- irreducible representations of the parent Pm3m space-group symmetry which, given the weakly first-order character of the transition, appear to be strongly coupled. Parametric and sequential refinements of the temperature-dependent structure were conducted using four model types: (1) traditional atomic xyz coordinates for each atom, (2) traditional rigid-body parameters, (3) purely displacive symmetry modes and (4) rigid-body rotational symmetry modes. We demonstrate that rigid-body rotational symmetry modes are an especially effective parameter set for the Rietveld characterization of phase transitions involving polyhedral rotations.

Crystal and magnetic structures of hexagonal YMnO₃

By Branton J. Campbell and Harold T. Stokes (et al.)

Abstract:

The available data on the structural and magnetic transitions in multiferroic hexagonal YMnO3 have been reviewed, first making use of the computer programs from the group theoretical ISOTROPY software suite to list possible crystal and magnetic structures, then taking into account the capability of neutron diffraction and other physical methods to distinguish them. This leads to a clear view of the transformation sequence, as follows. Hexagonal YMnO3 is paraelectric in P63/mmc at elevated temperatures, and undergoes a single structural transition on cooling through 1250 K to a ferrielectric phase in P6(3)cm that is retained through room temperature. At a much lower temperature, 70 K, there is a magnetic transition from paramagnetic to a triangular antiferromagnetic arrangement, most likely with symmetry P63'cm'. Comment is made on the unusual coupling of ferroelectric and magnetic domains reported to occur in this material, as well as on the so-called 'giant magneto-elastic' effect.

Tabulation of irreducible representations of the crystallographic space groups and their superspace extensions

By Harold T. Stokes, Branton J. Campbell, and Ryan Cordes

Abstract: New tables of irreducible representations (IRs) are introduced for the 230 crystallographic space groups (SGs) in three-dimensional space, at both special and non-special k vectors, and for their extensions to (3 + d)-dimensional superspace ('superspace-extended SGs' or SSESGs). Neither a tabulation of SG IR matrices for non-special k vectors nor a tabulation of SSESG IR matrices for d > 1 have been previously published. These tabulations are made possible by a new form in which the IR matrices of SGs are separated as a product of a translation part T and a point-operation part P, and where the IR matrices of SSESGs are separated as a product of a phase-shift part Q and a point-operation part P-s. Both T and Q have a simple prescribed form that does not need to be tabulated. Also, the new IR matrices are in a convenient block form which allows one to see by inspection which parts of the matrices and the associated order parameters belong to which arm of the star of k. In addition to complex IR matrices, real physically irreducible representation (PIR) matrices are tabulated. The new IR and PIR tables are available on the ISO-IR website (http://stokes.byu.edu/iso/irtables.php) in both convenient human-readable and computer-readable forms.

Phase Progression of γ-Al₂O₃ Nanoparticles Synthesized in a Solvent-Deficient Environment

By Stacey J. Smith, Brian F. Woodfield, Juliana Boerio-Goates, and Branton J. Campbell (et al.)

Abstract:

Our simple and uniquely cost-effective solvent-deficient synthetic method produces 3-5 nm Al2O3 nanoparticles which show promise as improved industrial catalyst supports. While catalytic applications are sensitive to the details of the atomic structure, a diffraction analysis of alumina nanoparticles is challenging because of extreme size/rnicrostrain-related peak broadening and the similarity of the diffraction patterns of various transitional Al2O3 phases. Here, we employ a combination of X-ray pair-distribution function (PDF) and Rietveld methods, together with solid-state NMR and thermogravimetry/differential thermal analysis-mass spectrometry (TG/DTA-MS), to characterize the alumina phase-progression in our nanoparticles as a function of calcination temperature between 300 and 1200 degrees C. In the solvent-deficient synthetic environment, a boehmite precursor phase forms which transitions to gamma-Al2O3 at an extraordinarily low temperature (below 300 degrees C), but this gamma-Al2O3 is initially riddled with boehmite-like stacking-fault defects that steadily disappear during calcination in the range from 300 to 950 degrees C. The healing of these defects accounts for many of the most interesting and widely reported properties of the gamma-phase.