Daniel Olds

Universal Dynamics of Molecular Reorientation in Hybrid Lead Iodide Perovskites

The role of organic molecular cations in the high-performance perovskite photovoltaic absorbers, methylammonium lead iodide (MAPbI3) and formamidinium lead iodide (FAPbI3), has been an enigmatic subject of great interest. Beyond aiding in the ease of processing of thin films for photovoltaic devices, there have been suggestions that many of the remarkable properties of the halide perovskites can be attributed to the dipolar nature and the dynamic behavior of these cations. Here, we establish the dynamics of the molecular cations in FAPbI3 between 4 K and 340 K and the nature of their interaction with the surrounding inorganic cage using a combination of solid state nuclear magnetic resonance and dielectric spectroscopies, neutron scattering, calorimetry, and ab initio calculations. Detailed comparisons with the reported temperature dependence of the dynamics of MAPbI3 are then carried out which reveal the molecular ions in the two different compounds to exhibit very similar rotation rates (≈8 ps) at room temperature, despite differences in other temperature regimes. For FA, rotation about the N···N axis, which reorients the molecular dipole, is the dominant motion in all phases, with an activation barrier of ≈21 meV in the ambient phase, compared to ≈110 meV for the analogous dipole reorientation of MA. Geometrical frustration of the molecule-cage interaction in FAPbI3 produces a disordered γ-phase and subsequent glassy freezing at yet lower temperatures. Hydrogen bonds suggested by atom-atom distances from neutron total scattering experiments imply a substantial role for the molecules in directing structure and dictating properties. The temperature dependence of reorientation of the dipolar molecular cations systematically described here can clarify various hypotheses including those of large-polaron charge transport and fugitive electron spin polarization that have been invoked in the context of these unusual materials.

Hydrogen adsorption on two catalysts for the ortho- to parahydrogen conversion: Cr-doped silica and ferric oxide gel

Molecular hydrogen exists in two spin-rotation coupled states: parahydrogen and orthohydrogen. Due to the variation of energy with rotational level, the occupation of ortho- and parahydrogen states is temperature dependent, with parahydrogen being the dominant species at low temperatures. The equilibrium at 20 K (99.8% parahydrogen) can be reached by natural conversion only after a lengthy process. With the use of a suitable catalyst, this process can be shortened significantly. Two types of commercial catalysts currently being used for ortho- to parahydrogen conversion are: iron(iii) oxide (Fe2O3, IONEX®), and chromium(ii) oxide doped silica catalyst (CrO·SiO2, OXISORB®). We investigate the interaction of ortho- and parahydrogen with the surfaces of these ortho-para conversion catalysts using neutron vibrational spectroscopy. The catalytic surfaces have been characterized using X-ray absorption fine structure (XAFS) and X-ray/neutron pair distribution function measurements.

A high precision gas flow cell for performing in situ neutron studies of local atomic structure in catalytic materials

Gas-solid interfaces enable a multitude of industrial processes, including heterogeneous catalysis; however, there are few methods available for studying the structure of this interface under operating conditions. Here, we present a new sample environment for interrogating materials under gas-flow conditions using time-of-flight neutron scattering under both constant and pulse probe gas flow. Outlined are descriptions of the gas flow cell and a commissioning example using the adsorption of N2 by Ca-exchanged zeolite-X (Na78-2xCaxAl78Si144O384,x ≈ 38). We demonstrate sensitivities to lattice contraction and N2 adsorption sites in the structure, with both static gas loading and gas flow. A steady-state isotope transient kinetic analysis of N2 adsorption measured simultaneously with mass spectrometry is also demonstrated. In the experiment, the gas flow through a plugged-flow gas-solid contactor is switched between N215 and N214 isotopes at a temperature of 300 K and a constant pressure of 1 atm; the gas flow and mass spectrum are correlated with the structure factor determined from event-based neutron total scattering. Available flow conditions, sample considerations, and future applications are discussed.

A high temperature gas flow environment for neutron total scattering studies of complex materials

We present the design and capabilities of a high temperature gas flow environment for neutron diffraction and pair distribution function studies available at the Nanoscale Ordered Materials Diffractometer instrument at the Spallation Neutron Source. Design considerations for successful total scattering studies are discussed, and guidance for planning experiments, preparing samples, and correcting and reducing data is defined. The new capabilities are demonstrated with an in situ decomposition study of a battery electrode material under inert gas flow and an in operando carbonation/decarbonation experiment under reactive gas flow. This capability will aid in identifying and quantifying the atomistic configurations of chemically reactive species and their influence on underlying crystal structures. Furthermore, studies of reaction kinetics and growth pathways in a wide variety of functional materials can be performed across a range of length scales spanning the atomic to the nanoscale.

Advances in utilizing event based data structures for neutron scattering experiments

This article strives to expand on existing work to demonstrate advancements in data processing made available using event mode measurements. Most spallation neutron sources in the world have data acquisition systems that provide event recording. The new science that is enabled by utilizing event mode has only begun to be explored. In the past, these studies were difficult to perform because histograms forced dealing with either large chunks of time or a large number of files. With event based data collection, data can be explored and rebinned long after the measurement has completed. This article will review some of the principles of event data and how the method opens up new possibilities for in situ measurements, highlighting techniques that can be used to explore changes in the data. We also demonstrate the statistical basis for determining data quality and address the challenge of determining how long to measure mid-measurement. Finally, we demonstrate a model independent method of grouping data via hierarchical clustering methods that can be used to improve calibration, reduction, and data exploration.

A uniaxial load frame for in situ neutron studies of stress-induced changes in cementitious materials and related systems

For in situ neutron scattering experiments on cementitious materials, it is of great interest to have access to a robust device which can induce uniaxial load on a given solid sample. Challenges involve selection of materials making up the apparatus that are both weak neutron scatterers and yet adequately strong to induce loads of up to a few kilonewtons on the sample. Here, the design and experimental commissioning of a novel load frame is provided with the intended use as a neutron scattering sample environment enabling time-dependent stress-induced changes to be probed in an engineering material under compression. The frame is a scaled down version of a creep apparatus, which is typically used in the laboratory to measure deformation due to creep in concrete. Components were optimized to enable 22 MPa of compressive stress to be exerted on a 1 cm diameter cement cylinder. To minimize secondary scattering signals from the load frame, careful selection of the metal components was needed. Furthermore, due to the need to maximize the wide angular detector coverage and signal to noise for neutron total scattering measurements, the frame was designed specifically to minimize the size and required number of support posts while matching sample dimensions to the available neutron beam size.

ADDIE: ADvanced DIffraction Environment – a software environment for analyzing neutron diffraction data

ADDIE is a software environment that aims to provide an intuitive graphical user interface for executing, managing, and visualizing total scattering neutron powder diffraction data. ADDIE is the current data reduction software being developed and used at the Nanoscale-Ordered Materials Diffractometer (NOMAD)[1], a time-of-flight powder diffractometer at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. In ADDIE, the user is provided with a workflow that begins with selection of standard, normalization, and background datasets for sample data corrections. Options are available to perform inelastic (Placzek), absorption, and multiple scattering data corrections. After initial data reduction of individual runs, the user can analyze datasets and perform summing of selected runs to provide datasets of better statistical significance. From the summed spectrum, the user can visualize the by-bank Bragg peak data to determine the average structure of materials while simultaneously visualizing the S(Q) and G(r) data. Finally, the user can output the optimized spectra to different file formats that feed into multiple data modelling programs. One of the major goals of ADDIE is to deliver an accessible, simultaneous view of Bragg data, the total scattering structure factor S(Q), and the pair distribution function G(r) for data analysis. Another major goal is ease of optimization of the Fourier transform from S(Q) to G(r) via pre-defined filter functions and adjustable limits on Qmin and Qmax. A Python interface is integrated into the visualization window with the Mantid framework[2] to provide the ability for user-defined extensions and manipulations of the datasets. The current development of ADDIE is directed towards the ability to access metadata of datasets from multiple databases at the SNS; filtering neutron events by certain sample conditions to aid in in-situ experiments; provide input files for other data reduction software for benchmarking and comparison; and increase the portfolio of output formats to support different data modelling software. Overall, ADDIE aims to deliver an optimal user experience, increase the connectivity between data reduction and modelling software, and to optimize the data analysis process in neutron diffraction experiments. [1] J. Neuefeind, M. Feygenson, J. Carruth, R. Hoffman, and K. K. Chipley. The Nanoscale Ordered MAterials Diffractometer NOMAD at the Spallation Neutron Source SNS. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms. 287:68-75, 2012. [2] O. Arnolda, b, J.C. Bilheuxc, J.M. Borregueroc, A. Butsa, S.I. Campbellc, L. Chapona, d, M. Doucetc, N. Drapera, b, R. Ferraz Leald, M.A. Gigga, b, V.E. Lynchc, A. Markvardsena, D.J. Mikkelsone, c, R.L. Mikkelsone, c, R. Millerf, K. Palmena, P. Parkera, G. Passosa, T.G. Perringa, P.F. Petersonc, S. Renc, M.A. Reuterc, A.T. Savicic, , , J.W. Taylora, R.J. Taylorc, g, R. Tolchenova, b, W. Zhouc, J. Zikovskyc.. Mantid—Data analysis and visualization package for neutron scattering and μSR experiments. Equipment. 764:156-166, 2014. Acta Cryst. (2017). A73, a377