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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 (through the tools of the ISOTROPY Software Suite) 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.

Slab-Stacking in Gd5(Si,Bi)4

Structures based on stacking sequences of Gd5T4 slabs are determined by the directionality of interslab T-T dimers. These include giant-magnetocaloric compounds like Gd5Si2Ge2. In the T = (Si,Bi) system, substituting Si for Bi not only leads to the complete electronic/geometric cleavage of the interslab dimers, it removes the directionality of residual nearest-slab interactions, which facilitates novel stacking sequences and extensive stacking faults.

Selected Publications

Linear framework defects in zeolite mordenite

B. J. Campbell (et al.)

Framework defect correlations are identified in zeolite mordenite that explain both X-ray powder diffraction anomalies and diffuse intensity features in previously reported electron diffraction data. The c/2 displacement of linear "chains" of framework material parallel to the main [001] channel are shown to selectively suppress and broaden the l = 2n + 1 Bragg reflections, thereby linking these framework defects to observed trends in a number of diffracted intensity ratios. At the same time, these defects are shown to produce thin sheets of diffuse scattering that are restricted to the l = 2n + 1 planes of reciprocal space, features that have been reported in electron diffraction studies of mordenite. An empirical relationship between the defect concentration and the framework aluminum content is also established.

The Structure of Jahn-Teller Polarons in the Colossal Magnetoresistive Manganites

Branton J. Campbell (et al.)

The observation of large negative magnetoresistive effects has generated tremendous scientific and technical interest in the perovskite manganites and related materials. These phenomena are commonly classified under the heading “colossal magnetoresistance” (CMR) to distinguish them from magnetoresistive effects in metals and metal-multilayers. CMR effects are observed in conjunction with the formation of a ferromagnetic metallic (FM) state at low temperatures and/or high magnetic fields. The close connection between the magnetism and the charge transport in these mixed-valence Mn3+n4+ materials is commonly believed to have its origin in the magnetic double-exchange interaction in which itinerant e_aelectrons mediate the ferromagnetic coupling between neighboring Mn core spins. Recent works have shown that double-exchange alone is insufficient to explain the extraordinary magnetoresistance values observed experimentally, so that electron-phonon interactions must also be invoked. Alternatively, a direct Mn-Mn magnetic exchange interaction has been reported to better explain the insensitivity of the Curie temperature (T_c) to the carrier concentration in oxygen-depleted samples. This model also relies on strong electron-lattice coupling, which occurs via the Jahn-Teller (JT) mechanism in which an Mn³⁺O₆octahedron experiences a spontaneous distortion designed to break the degeneracy of its e₅orbital, and thus lower the energy of its extra electron. If the coupling is strong, the electron may induce a significant local distortion of the lattice, which then follows the electron as it hops from site to site, thus forming a polaron.

Structure of nanoscale polaron correlations in La_1.2Sr_1.8Mn₂O₇

B. J. Campbell (et al.)

A system of strongly interacting electron-lattice polarons can exhibit charge and orbital order at sufficiently high polaron concentrations. In this study, the structure of short-range polaron correlations in the layered colossal magnetoresistive perovskite manganite La1.2Sr1.8Mn2O7 has been determined by a cystallographic analysis of broad satellite maxima observed in diffuse x-ray and neutron-scattering data, The resulting q approximate to (0.3,0, +/-1) modulation is a longitudinal octahedral-stretch mode, consistent with incommensurate Jahn-Teller-coupled charge-density-wave fluctuations, that implies an unusual orbital-stripe pattern parallel to the (100) directions.

The determination of Bronsted acid sites in zeolite ERS-7 by neutron and X-ray powder diffraction

B. J. Campbell (et al.)

The deuterated acid form of zeolite ERS-7 (Si/Al = 8.4) has been prepared by repeated cycles of D2O exposure and calcination. Al-27 NMR data show that the relative proportions of tetrahedral and octahedral Al in the resulting material are 70 and 30%. respectively. Three Bronsted sites were identified by a combined Rietveld refinement that simultaneously employed synchrotron X-ray powder diffraction data and neutron powder diffraction data. Final lattice parameters were a = 9.7843(1) Angstrom, b = 12.3675(1) Angstrom, and c = 22.8336(2) Angstrom, the combined R-wp factor was 6.3%. and x(2)(reduced) was 1.48. Acid deuterons were located approximately 1 A from oxygens O3, O5, and O7, with occupancy factors of 0.16(2), 0.16(3), and 0.18(2), respectively. These correpsond to 1.28(16), 0.64(12), and 1.44(16) deuterons per unit cell, respectively, for a total of 3.36(26) acid sites per unit cell, which is consistent with the tetrahedral Al content. Only deuteron D5 lies on a special position. T-O bond lengths suggest a site preference for Al at T4, T1, and possibly T2, each of which is bonded to at least one of the observed acid-site oxygens. A bridging framework oxygen site was identified in both the PND and PXD difference Fourier maps that appears to be disordered over at least two distinct sites; this corresponds to different local arrangements of Al at the neighboring T-sites. The out-of-plane framework hydroxyl tilt angles at each acid site were comparable to those of H+ SSZ-13 and D+ RHO. Acid site D7 demonstrated a significant in-plane hydroxyl tilt angle (7.5 degrees), consistent with ab initio quantum mechanical calculations. This tilt clearly indicates an Al preference for site T4 over T6.

Cation-vacancy ordering in dehydrated Na₆[AlSiO₄]₆

Branton J. Campbell (et al.)

The low-temperature cation-ordered superstructure of anhydrous sodium sodalite, a zeolite with composition Na-6[AlSiO4](6), has been determined through the use of both density functional theory (DFT) and classical force-field lattice energy minimizations. The charge-balancing Na+ cations are assumed to occupy their characteristic locations within the cubic alumino-silicate framework near the centers of the 6-ring windows. Within the constraints of the volume-doubled pseudotetragonal supercell reported in a previous x-ray diffraction study [B. Campbell, S. R. Shannon, H. Metiu, and N. P. Blake (submitted)], all possible arrangements of cations and vacancies amongst the 6-ring window sites were considered. Force-field calculations employing the ab initio based potential energy function derived by Blake, Weakliem, and Metiu [J. Phys. Chem. B 102, 67 (1998)] and the empirical shell-model potential of Catlow [J. Chem. Soc. Commun. 1984, 1271; Mol. Simul. 1, 207 (1988)], were used to perform full lattice-energy minimizations of each configuration, and to assess their relative stabilities both before and after minimization. The most stable configurations were then examined in more detail via ab initio density functional calculations in the generalized gradient approximation. The lowest-energy supercell ordering proved more stable than the lowest-energy parent cell ordering, and also yielded a pseudotetragonal distortion (space group Pnc2) and a calculated diffraction pattern that qualitatively match experimental results. The structural influences that contribute to the low energy of the correct vacancy ordering are described in detail. (C) 2000 American Institute of Physics. [S0021-9606(00)00744-3].