Anharmonicity and scissoring modes in the negative thermal expansion materials ScF3 and CaZrF6

We use a symmetry-motivated approach to analyzing X-ray pair distribution functions to study the mechanism of negative thermal expansion in two ReO3-like compounds: ScF3 and CaZrF6. Both average and local structures suggest that it is the flexibility of M-F-M linkages (M = Ca, Zr, Sc) due to dynamic rigid and semirigid "scissoring" modes that facilitates the observed negative thermal expansion (NTE) behavior. The amplitudes of these dynamic distortions are greater for CaZrF6 than for ScF3, which corresponds well with the larger magnitude of the thermal expansion reported in the literature for the former. We show that this flexibility is enhanced in CaZrF6 due to the rocksalt ordering mixing the characters of two of these scissoring modes. Additionally, we show that in ScF3 anharmonic coupling between the modes responsible for the structural flexibility and the rigid unit modes contributes to the unusually high NTE behavior in this material

Conversion of magnesium waste into complex magnesium hydride: Mg(NH2)2-LiH

The Mg(NH2)2–2LiH composite has been regarded as a promising on-board hydrogen storage material, due to its favorable thermodynamics and high H2 gravimetric capacity. In this paper, a new route to synthesize the Mg(NH2)2–2LiH composite, starting from hydrogenated Li2Mg(NH)2, is presented. Li2Mg(NH)2 is obtained from the decomposition of a mixture of a magnesium waste alloy (AZ91) and LiH, after a multi-step treatment. The as-prepared Mg(NH2)2–2LiH with Mg coming from AZ91 shows improved hydrogen storage properties, as compared with that synthesized from pure magnesium powder, with a lower dehydrogenation peak temperature of about 15 °C. The reaction pathways during the conversion of the AZ91 alloy to Mg(NH2)2–2LiH are investigated by ex situ and in situ XRD techniques. The obtained results confirm that this new approach is an effective way to recycle magnesium waste alloys into lightweight hydrogen storage materials. In addition, this new technology is easy to scale up and reduces the cost of Mg(NH2)2–2LiH, which has important significance for its practical applications

Spin crossover-induced colossal positive and negative thermal expansion in a nanoporous coordination framework material

Controlling mechanical motions in solid state devices is highly desirable for the development of nanoscale machines. Here, Kepert and colleagues exploit an ultra-flexible coordination framework in which thermally-controlled Fe(II) spin transitions result in remarkable flexing of the crystal lattice