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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.

Selected Publications

Supercolossal Uniaxial Negative Thermal Expansion in Chloranilic Acid Pyrazine, CA-Pyz

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

There has been significant recent interest in exploiting the large dimension changes that can occur in molecular materials as a function of temperature, stress, or under optical illumination. Here, we report the remarkable thermal expansion properties of chloranilic acid pyrazine co-crystals. We show that the compound shows uniaxial negative thermal expansion over a wide temperature range with a linear contraction coefficient as low as (−)1500 × 10^–6 K^–1 at 250 K. The corresponding 10% contraction between 200 and 300 K is an order of magnitude larger than in the so-called colossal contraction materials. We adopt a symmetry-inspired approach to describe both the structural changes that occur (using rotational symmetry modes) and the thermal expansion (using strain modes). This allows an extremely compact description of the phase transition responsible for this unusual behavior and gives detailed understanding of its atomic origins. We show how the coupling of primary and secondary strain modes in materials showing extreme expansion and contraction can lead to unusual reversals in the temperature dependence of cell parameters.

Understanding the behaviour of the room-temperature molecular ferroelectric 5,6-dichloro-2-methylbenzimidazole using symmetry adapted distortion mode analysis

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

The exploitable properties of many functional materials are intimately linked with symmetry-changing phase transitions. These include properties such as ferroelectricity, second harmonic generation, conductivity, magnetism and many others. We describe a new symmetry-inspired method for systematic and exhaustive evaluation of the symmetry changes possible in molecular systems using molecular distortion modes and how different models can be automatically tested against diffraction data. The method produces a quantitative structural landscape from which the most appropriate structural description of a child phase can be chosen. It can be applied to any molecular of molecular-fragment containing material where a (semi) rigid molecule description is appropriate. We exemplify the method on 5,6-dichloro-2-methylbenzimidazole (DC-MBI), an important molecular ferroelectric. We show that DC-MBI under-goes an unusual symmetry-lowering transition on warming from orthorhombic Pca21 (T ≲ 400 K) to monoclinic Pc. Contrary to expectations, the high temperature phase of DC-MBI remains polar.

An algebraic approach to cooperative rotations in networks of interconnected rigid units

BYU Authors: Branton Campbell, Tyler B. Averett, Christopher Yost, and Harold T. Stokes, published in Acta Crystallogr. Sect. A

Crystalline solids consisting of three-dim ensional networks of interconnected

rigid units are ubiquitous amongst functional materials. In many cases,

application-critica l properties are sensitive to rigid-unit rotations at low

temperature, high pressure or specific stoichiometry. The shared atoms that

connect rigid units impose severe constraints on any rotational degrees of

freedom, which must then be coop erative throughout the entire network.

Successful efforts to identify coop erative-rotational rigid-unit modes (RUMs) in

crystals have employed split-atom harmo nic potentials, exhaustive testing of the

rotational symmetry modes allowed by group representation theory, and even

simple geometric considerations. This article presents a purely algebraic

approach to RUM identification wherein the conditions of connectedness are

used to constru ct a linear system of equations in the rotational symmetry-mode

amplitud es.

Revisiting the revised Ag-Pt phase diagram

BYU Authors: Gus L.W. Hart, Lance J. Nelson, Richard R. Vanfleet, and Branton J. Campbell, published in Acta Mater.

Abstract Because of the important applications of platinum alloys and related platinum-group-metals phases, complete phase diagrams for these systems are important for materials engineering. The currently accepted phase diagram for the Ag-Pt system is questionable because of its disagreement with earlier experiments and because of its claim for a lone ordered structure at 53%-Pt which was not characterized and which contradicts both computational predictions and analogy to the isoelectronic system Cu-Pt. A complete re-examination of the Ag-Pt system by computational and experimental means suggests a phase diagram similar to the isoelectronic system Cu-Pt. The unknown compound, claimed to be 53%-Pt, is found to be the \{L11\} structure at 50%-Pt.

A general algorithm for generating isotropy subgroups in superspace

BYU Authors: Harold T. Stokes and Branton J. Campbell, published in Acta Crystallogr. Sect. A

This paper presents a general algorithm for generating the isotropy subgroups of superspace extensions of crystallographic space groups involving arbitrary superpositions of multi-k order parameters from incommensurate and commensurate k vectors. Several examples are presented in detail in order to illuminate each step of the algorithm. The practical outcome is that one can now start with any commensurate parent crystal structure and generate a structure model for any conceivable incommensurate modulation of that parent, fully parameterized in terms of order parameters of irreducible representations at the relevant wavevectors. The resulting modulated structures have (3 + d)-dimensional superspace-group symmetry. Because incommensurate structures are now commonly encountered in the context of many scientifically and technologically important functional materials, the opportunity to apply the powerful methods of group representation theory to this broader class of structural distortions is very timely.