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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.

Selected Publications

An Exhaustive Symmetry Approach to Structure Determination: Phase Transitions in Bi₂Sn₂O₇

BYU Authors: Harold T. Stokes and Branton J Campbell, published in J. Am. Chem. Soc.

The exploitable properties of many materials are intimately linked to symmetry-lowering structural phase transitions. We present an automated and exhaustive symmetry-mode method for systematically exploring and solving such structures which will be widely applicable to a range of functional materials. We exemplify the method with an investigation of the Bi₂Sn₂O₇ pyrochlore, which has been shown to undergo transitions from a parent γ- cubic phase to β- and α- structures on cooling. The results include the first reliable structural model for β-Bi₂Sn₂O₇ (orthorhombic Aba2, a = 7.571833(8), b = 21.41262(2), c = 15.132459(14)) and a much simpler description of α-Bi₂Sn₂O₇ (monoclinic Cc, a = 13.15493(6), b = 7.54118(4), c = 15.07672(7), β = 125.0120(3)) than has been presented previously. We use the symmetry-mode basis to describe the phase transition in terms of coupled rotations of the Bi₂O’ anti-cristobalite framework which allow Bi atoms to adopt low-symmetry coordination environments favoured by lone pair cations.

Structural state and magnetic properties of multilayer-graphene/Fe composites

BYU Authors: B. J. Campbell, published in Phys. Metals Metallogr.

Results are given of measurements of X-ray and neutron diffraction, small-angle neutron scattering, and field dependences of the magnetization for nanocomposites formed by multilayer graphene and iron particles. Four samples have been investigated that differed in the content of Fe as follows: 0.75, 30, 45, and 70 wt %. The calculations of the X-ray diffraction and neutron diffraction patterns show that the size of graphene particles is about 4 nm, and the size of the iron domains of coherent scattering is no less than 50 nm. The content of the Fe particles in the composites has been estimated based on the data of the measurements of neutron diffraction, small-angle neutron scattering, and magnetization. The values of the iron concentration obtained by different methods differ quite significantly. The difference is caused by the specific character of the methods of measurements.

La-Dopant Location in La-Doped γ-Al₂O₃ Nanoparticles Synthesized Using a Novel One-Pot Process

BYU Authors: Stacey J. Smith, Baiyu Huang, Calvin H. Bartholomew, Branton J Campbell, Juliana Boerio-Goates, and Brian F. Woodfield, published in J. Phys. Chem. C

We have recently developed a ‘solvent-deficient’ method of synthesizing high surface area γ-Al2O3 nanoparticles that show promise for catalyst support applications. Here, we investigate doping the alumina with La3+ to stabilize the gamma phase to higher temperatures. The 1-pot method for synthesizing La-doped γ-Al2O3 nanoparticles developed here has several advantages over conventional methods including requiring only 3 hours instead of 3 days and wasting no lanthanum. TEM, X-ray PDF, and BET analyses as a function of calcination temperature show that the La stabilizes the gamma phase by 100°C. In order to begin understanding the mechanism of stabilization, the location of the La3+ atoms in our doped γ-Al2O3 nanoparticles is investigated via X-ray PDF and EXAFS analyses which indicate that the La dopant adsorbs as single, isolated atoms on the γ-Al2O3 surface. As calcination temperature increases, the immediate oxygen coordination shell of the La becomes increasingly like La2O3 though an extended La2O3 lattice is not formed due to the sparse (3 wt%) concentration of La atoms. During the γ-to-α transition, the La environment changes drastically to become more like LaAlO3 than La2O3, suggesting that the La is enveloped by the alumina lattice during the alpha phase transition though an extended LaAlO3 lattice is not formed.

Long-range two-dimensional superstructure in the superconducting electron-doped cuprate Pr_0.88LaCe_0.12CuO₄

BYU Authors: B. J. Campbell and H. T. Stokes, published in Phys. Rev. B

Utilizing single-crystal synchrotron x-ray scattering, we observe distorted CuO 2 planes in the electron-doped superconductor Pr 1−x LaCe x CuO 4+δ , x = 0.12. Resolution-limited rods of scattering are indicative of a long-range two-dimensional 22 √ ×22 √ superstructure in the a−b plane, adhering to planar space-group symmetry p4gm , which is subject to stacking disorder perpendicular to the planes. This superstructure is present only in annealed, superconducting samples, but not in the as-grown, nonsuperconducting samples. These long-range distortions of the CuO 2 planes, which are generally considered to be detrimental to superconductivity, have avoided detection to date due to the challenges of observing and interpreting subtle diffuse-scattering features.

Revisiting the CuPt₃ prototype and the L1₃ structure

BYU Authors: 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, published in Acta Mater.

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

X_{1}^{+} (a, 0, 0)

order parameter of the L1₃ phase is unexpectedly weak relative to the primary

L_{1}^{+} (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.