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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.

Crystal Distortions

Crystal transformations that reduce symmetry are called "distortions". ISODISTORT (iso.byu.edu) is a powerful group-theoretical tool that can generate, parameterize, and interactively visualize virtually any crystal distortion involving atomic displacements, magnetic moments, occupational orderings, or lattice strains. Figure based on La2CoRuO6 (J. Mater. Chem. 15, 715-720, 2005).

Size-dependent crystalline and magnetic properties of 5 to 100 nm Fe₃O₄ nanoparticles: superparamagnetism, Verwey transition and FeO-Fe₃O₄ core-shell formation

Shelby Klomp, Colby Walker, Mason Christiansen, Brittni Newbold, Dalton Griner, Yanping Cai, Paul Minson, Jeffrey Farrer, Stacey Smith, Branton J. Campbell, Roger G. Harrison, and Karine Chesnel

Due to their non-toxicity and their ability to be functionalized, magnetite (Fe3O4) nanoparticles (NP) are good candidates for a variety of biomedical applications. To better implement their applications, it is crucial to well understand the basic structural and magnetic properties of the NPs in correlation with their synthesis method. Here, we show interesting properties of Fe3O4 NPs of various sizes ranging from 5 to 100 nm and the dependence of these properties on particle size and preparation method. One synthetic method based on heating Fe(acac)3 with oleic acid consistently gives 5 ± 1 nm NPs. A second method using the thermal decomposition of Fe(oleate)3 in oleic acid led to larger NPs, greater than 8 nm in size. Increasing the amount of oleic acid caused the average NP size to slightly increase, from 8 to 10 nm. Increasing both the reaction temperature and the reaction time caused the NP size to drastically increase from 10 to 100 nm. Powder x-ray diffraction and electron-microscopy imaging show a pure single crystalline Fe3O4 phase for all NPs smaller than 50 nm and spherical in shape. When the NPs get larger than 50 nm, they notably tend to form faceted, FeO core-Fe3O4 shell structures. Magnetometry data collected in various field-cooling conditions show a pure superparamagnetic (SPM) behavior for all NPs smaller than 20 nm. The observed blocking temperature, TB, gradually increases with NP size from about 25 K to 150 K. In addition, the Verwey transition is observed with the emergence of a strong narrow peak at 125 K in the magnetization curves when larger NPs are present. Our data confirm the vanishing of the Verwey transition in smaller NPs. Magnetization loops indicate that the saturating field drastically decreases with NP size. While larger NPs show some coercivity (Hc ) up to 30 mT at 400 K, NPs smaller than 20 nm show no coercivity (Hc =0), confirming their pure SPM behavior at high temperature. Upon cooling below TB, some of the SPM NPs gradually show some coercivity, with Hc reaching 50 mT at 5 K for the 10 nm NPs, indicating emergent interparticle couplings in the blocked state.

Generalized regular k-point grid generation on the fly

Wiley S. Morgan, John E. Christensen, Parker K. Hamilton, Jeremy J. Jorgensen, Branton J. Campbell, Gus L.W. Hart, and Rodney W. Forcade

In the DFT community, it is common practice to use regular k-point grids (Monkhorst-Pack, MP) for Brillioun zone integration. Recently Wisesa et al. (2016) and Morgan et al. (2018) demonstrated that generalized regular (GR) grids offer an advantage over traditional MP grids. The difference is simple but effective. At the same k-point density, GR grids have greater symmetry and 60% fewer irreducible k-points. GR grids have not been widely adopted because one must search through a large number of candidate grids; in many cases, a brute force search could take hours. This work describes an algorithm that can quickly search over GR grids for those that have the most uniform distribution of points and the best symmetry reduction. The grids are ∼60% more efficient, on average, than MP grids and can now be generated on the fly in seconds.

Normally supportive sublattices of crystallographic space groups

Miles A. Clemens, Branton J. Campbell, and Stephen P. Humphries

The tabulation of normal subgroups of 3D crystallographic space groups that are themselves 3D crystallographic space groups (csg's) is an ambitious goal, but would have a variety of applications. For convenience, such subgroups are referred to as `csg-normal' while normal subgroups of the crystallographic point group (cpg) of a crystallographic space group are referred to as `cpg-normal'. The point group of a csg-normal subgroup must be a cpg-normal subgroup. The present work takes a significant step towards that goal by tabulating the translational subgroups (a.k.a. sublattices) that are capable of supporting csg-normal subgroups. Two necessary conditions are identified on the relative sublattice basis that must be met in order for the sublattice to support csg-normal subgroups: one depends on the operations of the point group of the space group, while the other depends on the operations of the cpg-normal subgroup. Sublattices that meet these conditions are referred to as `normally supportive'. For each cpg-normal subgroup (excluding the identity subgroup 1) of each of the arithmetic crystal classes of 3D space groups, all of the normally supportive sublattices have been tabulated in symbolic form, such that most of the entries in the table contain one or more integer variables of infinite range; thus it could be more accurately described as a table of the infinite families of normally supportive sublattices. For a given pair of cpg-normal subgroup and normally supportive sublattice, csg-normal subgroups of the space groups of the parent arithmetic crystal class can be constructed via group extension, though in general such a pair does not guarantee the existence of a corresponding csg-normal subgroup.

High-pressure polymorphism in pyridine

Branton J. Campbell (et al.)

Single crystals of the high-pressure phases II and III of pyridine have been obtained by in situ crystallization at 1.09 and 1.69 GPa, revealing the crystal structure of phase III for the first time using X-ray diffraction. Phase II crystallizes in P212121 with Z′ = 1 and phase III in P41212 with Z′ = ½. Neutron powder diffraction experiments using pyridine-d5 establish approximate equations of state of both phases. The space group and unit-cell dimensions of phase III are similar to the structures of other simple compounds with C2v molecular symmetry, and the phase becomes stable at high pressure because it is topologically close-packed, resulting in a lower molar volume than the topologically body-centred cubic phase II. Phases II and III have been observed previously by Raman spectroscopy, but have been mis-identified or inconsistently named. Raman spectra collected on the same samples as used in the X-ray experiments establish the vibrational characteristics of both phases unambiguously. The pyridine molecules interact in both phases through CH⋯π and CH⋯N interactions. The nature of individual contacts is preserved through the phase transition between phases III and II, which occurs on decompression. A combination of rigid-body symmetry mode analysis and density functional theory calculations enables the soft vibrational lattice mode which governs the transformation to be identified.

Topotactic, pressure-driven, diffusion-less phase transition of layered CsCoO₂ to a stuffed cristobalite-type configuration

Branton J. Campbell (et al.)

CsCoO₂, featuring a two-dimensional layered architecture of edge- and vertex-linked CoO₄ tetrahedra, is subjected to a temperature-driven reversible second-order phase transformation (α → β) at 100 K, which corresponds to a structural relaxation with concurrent tilting and breathing modes of edge-sharing CoO₄ tetrahedra. In the present investigation, it was found that pressure induces a phase transition, which encompasses a dramatic change in the connectivity of the tetrahedra. At 923 K and 2 GPa, β-CsCoO₂ undergoes a first-order phase transition to a new quenchable high-pressure polymorph, γ-CsCoO₂. It is built up of a three-dimensional cristobalite-type network of vertex-sharing CoO₄ tetrahedra. According to a Rietveld refinement of high-resolution powder diffraction data, the new high-pressure polymorph γ-CsCoO₂ crystallizes in the tetragonal space group I4₁/amd:2 (Z = 4) with the lattice constants a = 5.8711 (1) and c = 8.3214 (2) Å, corresponding to a shrinkage in volume by 5.7% compared with the ambient-temperature and atmospheric pressure β-CsCoO₂ polymorph. The pressure-induced transition (β → γ) is reversible; γ-CsCoO₂ stays metastable under ambient conditions, but transforms back to the β-CsCoO₂ structure upon heating to 573 K. The transformation pathway revealed is remarkable in that it is topotactic, as is demonstrated through a clean displacive transformation track between the two phases that employs the symmetry of their common subgroup Pb2₁a (alternative setting of space group No. 29 that matches the conventional β-phase cell).