X-ray and neutron scattering

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

Abstract:

Methylammonium lead iodide crystallises in three phases. The high-temperature phase is cubic with a high degree of orientational disorder of the molecular ions. The intermediate phase shows progressive alignment of the molecules, but still with disorder.

Abstract:

In the rapidly expanding field of topological materials there is growing interest in systems whose topological electronic band features can be induced or controlled by magnetism. Magnetic Weyl semimetals, which contain linear band crossings near the Fermi level, are of particular interest owing to their exotic charge and spin transport properties. Up to now, the majority of magnetic Weyl semimetals have been realized in ferro- or ferrimagnetically ordered compounds, but a disadvantage of these materials for practical use is their stray magnetic field which limits the minimum size of devices. Here we show that Weyl nodes can be induced by a helical spin configuration, in which the magnetization is fully compensated. Using a combination of neutron diffraction and resonant elastic x-ray scattering, we find that below TN = 14.5 K the Eu spins in EuCuAs develop a planar helical structure which induces two quadratic Weyl nodes with Chern numbers C = ±2 at the A point in the Brillouin zone.