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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.

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.

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

By B. J. Campbell (et al.)

Abstract: 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

By Branton J. Campbell (et al.)

Abstract:

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].

The cation-vacancy ordering transition in dehydrated Na₆ sodalite

By Branton J. Campbell (et al.)

Abstract:

Variable-temperature synchrotron x-ray powder diffraction data from dehydrated Na-6[Al6Si6O24] reveal a structural phase transition that involves both the commensurate ordering of the extra-framework Na cations and a one-dimensional incommensurate modulation of the framework. Peak splittings and superlattice reflections implicate an orthorhombically-distorted volume-doubled supercell at room temperature (a=12.9432 Angstrom, b=12.8403 Angstrom, and c=9.1372 Angstrom) similar to(root(2) over bara x root(2) over bar axa). These data also included additional superlattice peaks associated with an incommensurate long-period modulation with tau=(3/2 1/2 1)/8.9. All unique orderings of the 12 Na cations among the 16 available sites of the commensurate supercell are enumerated. A unique solution is identified that minimizes the structural energy and accounts for the observed superlattice peaks. Rietveld analysis reveals a significant Pauling "partial-collapse" tilt angle in the cation-ordered phase, that appears to decrease with increasing temperature. (C) 2000 American Institute of Physics. [S0021-9606(00)00844-8].

Charge ordering and phase competition in the layered perovskite LaSr₂Mn₂O₇

By B. J. Campbell (et al.)

Abstract:

Charge-lattice fluctuations are observed in the layered perovskite manganite LaSr2Mn2O7 by Raman spectroscopy at temperatures as high as 340 K, and with decreasing temperature they become static, forming a charge-ordered (CO) phase below T-CO = 210 K. In the static regime, superlattice reflections are observed by neutron and x-ray diffraction with a propagation vector (1/4, - 1/4, 0). Crystallographic analysis of the CO state demonstrates that the degree of charge and orbital ordering in this manganite is weaker than that in the three-dimensional perovskite manganites. Below T-N= 170 K, type-ii antiferromagnetism (AF) develops and competes with the charge ordering, causing it to eventually melt below T* = 100 K. High-resolution diffraction measurements suggest that the CO and AF states do not coincide within the same region of material, but rather coexist as separate phases. The transition to typed antiferromagnetism at lower temperatures is characterized by the competition between these two phases.

The synthesis of zeolite ERS-7 and its structure determination using simulated annealing and synchrotron X-ray powder diffraction data

By B. J. Campbell (et al.)

Abstract:

ERS-7, a small-pore zeolite with a novel framework topology, was synthesized hydrothermally using N,N-dimethylpiperidinium hydroxide as a structure directing agent. Its structure was determined by using simulated annealing and refined from synchrotron X-ray powder diffraction data. It consists of 17-sided “picnic basket”-shaped cages connected by 8-membered-ring windows with 4.7 ´ 3.5 Å free dimensions. The calculated lattice energy and observed framework density are consistent with the empirical linear relationship obeyed by other zeolite frameworks.