arXiv subjects

Reetam Paul

Publications and source records attributed to Reetam Paul.

4 recordsLinked to original sources

arXiv2026

High pressure melt dynamics in shock-compressed titanium

We study the high-pressure melting behavior of titanium using laser-driven shock compression with in situ femtosecond x-ray diffraction and molecular-dynamics simulations based on a machine-learned interatomic potential. The MD simulations predict the solid-liquid coexistence on the Hugoniot in the $\sim$$111-124$ GPa range. Experimentally, we observe the first evidence of liquid at 86 GPa. We also observe pronounced microstructural changes with pressure with strong grain refinement associated with the emergence of liquid, within the solid-liquid coexistence ($\sim$$110-126$ GPa). Above 126 GPa, we observe the persistence of residual levels of highly textured crystalline Ti to $\sim$$180$ GPa, well above the expected melt completion pressure. We discuss the accuracy that current laser-shock experimental platforms have at determining the melt onset and completion pressures.

cond-mat.mtrl-sci↗

arXiv2025

Nonadiabaticity under compression in metastable carbon monoxide-nitroxide mixtures

Carbon monoxide (CO) and nitrous oxide (N2O) both undergo profound structural and chemical transformations when compressed. While their individual high-P/T phase diagrams have been mapped in considerable detail, comparatively little attention has been paid to the mixtures in which the two species can couple through oxygen transfer, charge redistribution, and nonadiabatic dissociation pathways. Here we use comprehensive ab initio adiabatic/nonadiabatic molecular dynamics simulations, essentially a diabatic trajectory stitching approach, that chart the evolution of CO-N2O mixtures from van-der-Waals fluids to extended amorphous network solids over the range 0-160 GPa and 300-1500 K. We emphasize on (i) the sequence of gas to molecular crystal to polymerized amorphous solid reactive transitions that arise from an interplay between thermal and compression effects in metastable C-N-O mixtures, (ii) the role of N2O unimolecular dissociation in lowering the onset pressure for CO polymerization, and (iii) the emergence of nonadiabatic pathways, via thermal unimolecular dissociation of N2O, accompanied by spin-transition in oxygen atoms that can make C-N-O systems deviate from Born-Oppenheimer dynamics. This dominates the chemistry once the mixture enters the regime of bond-breaking temperatures (T>900 K).

cond-mat.mtrl-sci↗

arXiv2024

Prediction of an alternative route to polymeric carbon dioxide: A metastable energetic material

The use of pressure to obtain new materials that can be recovered under ambient conditions is a central problem in high-pressure physics. Despite decades of research, this goal has only been achieved in the laboratory for a few notable examples, such as diamond and cubic boron nitride. An area of significant interest is the transformation under compression of light-element molecular compounds to extended covalent-bonded (polymeric) solids. Among them, CO$_2$ has been extensively studied because of its status as a prototypical simple molecular system with a rich phase diagram and due to its fundamental role in Earth's physics and chemistry. One of its polymeric crystalline phases, accessible at extreme pressures and temperatures, has been recently quenched to ambient pressure, but below room temperature. Here we report ab initio calculations predicting that isothermal compression of a carbon monoxide and oxygen mixture (CO+O$_2$), rather than the compound CO$_2$, lowers the onset of C-polymerization at room temperature from ~118 GPa to ~7 GPa (complete by ~23 GPa). Moreover, it leads to the formation of an intrinsically different polymer with enhanced metastability. We predict that this dense phase is an energetic material which can potentially be recovered to ambient pressure and temperature.

cond-mat.mtrl-sci↗

arXiv2023

Toward an accurate equation of state and B1-B2 phase boundary for magnesium oxide to TPa pressures and eV temperatures

By applying auxiliary-field quantum Monte Carlo, we calculate the equation of state (EOS) and B1-B2 phase transition of magnesium oxide (MgO) up to 1 TPa. The results agree with available experimental data at low pressures and are used to benchmark the performance of various exchange-correlation functionals in density functional theory calculations. We determine PBEsol is an optimal choice for the exchange-correlation functional and perform extensive phonon and quantum molecular-dynamics calculations to obtain the thermal EOS. Our results provide a preliminary reference for the EOS and B1-B2 phase boundary of MgO from zero up to 10,500 K.

cond-mat.mtrl-sci↗