Prebiotic NH₃ Formation: Insights from Simulations

Simulations of prebiotic NH₃ synthesis from NO₃^– and NO₂^– on pyrite surfaces under hydrothermal conditions are reported. Ab initio metadynamics calculations have successfully explored the full reaction path which explains earlier experimental observations. We have found that the reaction mechanism can be constructed from stepwise single atom transfers which are compatible with the expected reaction time scales. The roles of the hot-pressurized water and of the pyrite surfaces have been addressed. The mechanistic picture that emerged from the simulations strengthens the theory of chemoautotrophic origin of life by providing plausible reaction pathways for the formation of ammonia within the iron–sulfur-world scenario

Dynamical Heterogeneity in the Supercooled Liquid State of the Phase Change Material GeTe

A contending technology for nonvolatile memories of the next generation is based on a remarkable property of chalcogenide alloys known as phase change materials, namely their ability to undergo a fast and reversible transition between the amorphous and crystalline phases upon heating. The fast crystallization has been ascribed to the persistence of a high atomic mobility in the supercooled liquid phase, down to temperatures close to the glass transition. In this work we unravel the atomistic, structural origin of this feature in the supercooled liquid state of GeTe, a prototypical phase change compound, by means of molecular dynamic simulations. To this end, we employed an interatomic potential based on a neural network framework, which allows simulating thousands of atoms for tens of ns by keeping an accuracy close to that of the underlying first-principles framework. Our findings demonstrate that the high atomic mobility is related to the presence of clusters of slow and fast moving atoms. The latter contain a large fraction of chains of homopolar Ge–Ge bonds, which at low temperatures have a tendency to move by discontinuous cage-jump rearrangements. This structural fingerprint of dynamical heterogeneity provides an explanation of the breakdown of the Stokes–Einstein relation in GeTe, which is the ultimate origin of the fast crystallization of phase change materials exploited in the devices

A Novel Sb₂Te₃ Polymorph Stable at the Nanoscale

We report on the MOCVD synthesis of Sb₂Te₃ nanowires that self-assemble in a novel metastable polymorph. The nanowires crystallize in a primitive trigonal lattice (<i>P</i>3̅<i>m</i>1 SG #164) with lattice parameters <i>a</i> = <i>b</i> = 0.422 nm, and <i>c</i> = 1.06 nm. The stability of the polymorph has been studied by first principle calculations: it has been demonstrated that the stabilization is due to the particular side-wall faceting, finding excellent agreement with the experimental observations