121 research outputs found
Accelerated materials design approaches based on structural classification: Application to low enthalpy high pressure phases of SH<sub>3</sub> and SeH<sub>3</sub>
We propose a methodology that efficiently ass-eses major characteristics in the energy landscape for agiven space of configurations (crystal structures) underpressure. In this work we study SH3 and SeH3, both of fun-damental interest due to their superconducting properties.Starting from the crystal fingerprint, which defines config-urational distances between crystalline structures, we in-troduce an optimal one dimensional metric space that isused to both classify and characterize the structures. Fur-thermore, this is correlated to the electronic structure. Ouranalysis highlights the uniqueness of the Im-3m phase of H3S and H33Se for superconductivity. This approach isan useful tool for future material design applications
Prediction of a novel monoclinic carbon allotrope
A novel allotrope of carbon with symmetry was identified during an
\emph{ab-initio} minima-hopping structural search which we call -carbon.
This structure is predicted to be more stable than graphite at pressures above
14.4 GPa and consists purely of bonds. It has a high bulk modulus and is
almost as hard as diamond. A comparison of the simulated X-ray diffraction
pattern shows a good agreement with experimental results from cold compressed
graphite.Comment: 3 pages, 3 figure
Emergence of hidden phases of methylammonium lead-iodide (CHNHPbI) upon compression
We perform a thorough structural search with the minima hopping method (MHM)
to explore low-energy structures of methylammonium lead iodide. By combining
the MHM with a forcefield, we efficiently screen vast portions of the
configurational space with large simulation cells containing up to 96 atoms.
Our search reveals two structures of methylammonium iodide perovskite (MAPI)
that are substantially lower in energy than the well-studied experimentally
observed low-temperature orthorhombic phase according to density
functional calculations. Both structures have not yet been reported in the
literature for MAPI, but our results show that they could emerge as
thermodynamically stable phases via compression at low temperatures. In terms
of the electronic properties, the two phases exhibit larger band gaps than the
standard perovskite-type structures. Hence, pressure induced phase selection at
technologically achievable pressures (i.e., via thin-film strain) is a route
towards the synthesis of several MAPI polymorph with variable band gaps
Elemental Phosphorus: structural and superconducting phase diagram under pressure
Pressure-induced superconductivity and structural phase transitions in
phosphorous (P) are studied by resistivity measurements under pressures up to
170 GPa and fully crystal structure and superconductivity
calculations up to 350 GPa. Two distinct superconducting transition temperature
(T) vs. pressure () trends at low pressure have been reported more
than 30 years ago, and for the first time we are able to reproduce them and
devise a consistent explanation founded on thermodynamically metastable phases
of black-phosphorous. Our experimental and theoretical results form a single,
consistent picture which not only provides a clear understanding of elemental P
under pressure but also sheds light on the long-standing and unsolved
superconductivity trend. Moreover, at higher pressures we predict a
similar scenario of multiple metastable structures which coexist beyond their
thermodynamical stability range. Metastable phases of P experimentally
accessible at pressures above 240 GPa should exhibit T's as high as 15 K,
i.e. three times larger than the predicted value for the ground-state crystal
structure. We observe that all the metastable structures systematically exhibit
larger transition temperatures than the ground-state ones, indicating that the
exploration of metastable phases represents a promising route to design
materials with improved superconducting properties.Comment: 14 pages, 4 figure
Enhancing the superconducting transition temperature of BaSi2 by structural tuning
We present a joint experimental and theoretical study of the superconducting
phase of the layered binary silicide BaSi2. Compared with the layered AlB2
structure of graphite or diboride-like superconductors, in the hexagonal
structure of binary silicides the sp3 arrangement of silicon atoms leads to
corrugated sheets. Through a high-pressure synthesis procedure we are able to
modify the buckling of these sheets, obtaining the enhancement of the
superconducting transition temperature from 4 K to 8.7 K when the silicon
planes flatten out. By performing ab-initio calculations based on density
functional theory we explain how the electronic and phononic properties of the
system are strongly affected by changes in the buckling. This mechanism is
likely present in other intercalated layered superconductors, opening the way
to the tuning of superconductivity through the control of internal structural
parameters.Comment: Submitte
Emergence of superconductivity in doped H<sub>2</sub>O ice at high pressure
We investigate the possibility of achieving high-temperature superconductivity in hydrides under pressure by inducing metallization of otherwise insulating phases through doping, a path previously used to render standard semiconductors superconducting at ambient pressure. Following this idea, we study H2O, one of the most abundant and well-studied substances, we identify nitrogen as the most likely and promising substitution/dopant. We show that for realistic levels of doping of a few percent, the phase X of ice becomes superconducting with a critical temperature of about 60 K at 150 GPa. In view of the vast number of hydrides that are strongly covalent bonded, but that remain insulating up to rather large pressures, our results open a series of new possibilities in the quest for novel high-temperature superconductors
Primeras etapas en el crecimiento de pelÃculas delgadas de teloruro de cadmio (CdTe) por sublimación en espacio cerrado
Emergence of hidden phases of methylammonium lead iodide (CH3NH3PbI3) upon compression
We perform a thorough structural search with the minima hopping method (MHM) to explore low-energy structures of methylammonium lead iodide. By combining the MHM with a forcefield, we efficiently screen vast portions of the configurational space with large simulation cells containing up to 96 atoms. Our search reveals two structures of methylammonium iodide perovskite (MAPI) that are substantially lower in energy than the well-studied experimentally observed low-temperature Pnma orthorhombic phase according to density functional calculations. Both structures have not yet been reported in the literature for MAPI, but our results show that they could emerge as thermodynamically stable phases via compression at low temperatures. In terms of the electronic properties, the two phases exhibit larger band gaps than the standard perovskite-type structures. Hence, the pressure-induced phase selection at technologically achievable pressures (i.e., via thin-film strain) is a viable route towards the synthesis of several MAPI polymorph with variable band gaps
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