X-ray and neutron scattering

Abstract:

We use a combination of X-ray pair distribution function (PDF) measurements, lattice dynamical calculations, and ab initio density functional theory (DFT) calculations to study the local structure and dynamics in various MPt­(CN)6 Prussian blue analogues. In order to link directly the local distortions captured by the PDF with the lattice dynamics of this family, we develop and apply a new “interaction-space” PDF refinement approach. This approach yields effective harmonic force constants, from which the (experiment-derived) low-energy phonon dispersion relations can be approximated. Calculation of the corresponding Grüneisen parameters allows us to identify the key modes responsible for negative thermal expansion (NTE) as arising from correlated tilts of coordination octahedra. We compare our results against the phonon dispersion relations determined using DFT calculations, which identify the same NTE mechanism.

Abstract:

X-ray free electron laser (XFEL) sources coupled to high-power laser systems offer an avenue to study the structural dynamics of materials at extreme pressures and temperatures. The recent commissioning of the DiPOLE 100-X laser on the high energy density (HED) instrument at the European XFEL represents the state-of-the-art in combining x-ray diffraction with laser compression, allowing for compressed materials to be probed in unprecedented detail. Here, we report quantitative structural measurements of molten Sn compressed to 85(5) GPa and ∼ 3500 K. The capabilities of the HED instrument enable liquid density measurements with an uncertainty of ∼ 1 % at conditions which are extremely challenging to reach via static compression methods. We discuss best practices for conducting liquid diffraction dynamic compression experiments and the necessary intensity corrections which allow for accurate quantitative analysis. We also provide a polyimide ablation pressure vs input laser energy for the DiPOLE 100-X drive laser which will serve future users of the HED instrument.

Abstract:

The field of metal–organic frameworks (MOFs) has rapidly evolved from disconnected examples of multi-dimensional coordination polymers into an expansive collection of functional materials containing over 100,000 structures. Because any material’s chemical structure dictates its function, a robust structural understanding is a crucial prerequisite for the optimization of a material’s physical properties and its effective deployment towards an application. This is especially true for MOFs, which frequently demonstrate multiple possible structures from a single metal-ligand combination. Without proper identification, these phase distributions confuse analysis and hinder applicability of the material(s). Conversely, when well-understood, phase multiplicity serves to increase the number of available structures (and functions) within an established chemical window. Thus, control over phase distributions in MOF systems is a critical aspect of their analysis and utility.In this work, a luminescent ligand based on the tetraphenylethene (TPE) molecule is used as a foundation to generate three distinct MOF systems, which are analyzed for their essential structures and functional properties. All three TPE-based systems are found to demonstrate various degrees of phase multiplicity; while some distributions depend solely on the initial synthetic conditions to generate distinct framework isomers, others undergo dynamic and reversible phase transformations according to their post-synthetic treatments. These structural aspects are characterized in detail, and mechanistic explanations are presented that can be applied to broad design principles towards phase-controlled MOF systems