Initio Structure Search Research Articles

Efficient modeling of organic adsorbates on oxygen-intercalated graphene on Ir(111)

Organic charge transfer complexes (CTCs) can be grown as thin films on intercalated graphene (Gr). Deciphering their precise film morphologies requires global ab initio structure search, where configurational sampling is computationally intractable unless we reconsider the model for the complex substrate. In this study, we employ charged freestanding Gr to approximate an intercalated Gr/O/Ir(111) substrate, without altering the adsoption properties of deposited molecules. We compare different methods of charging Gr and select the most appropriate substitute model for Gr/O/Ir(111) that maintains the adsorption properties of fluorinated tetracyanoquinodimethane (${\mathrm{F}}_{4}\mathrm{TCNQ}$) and tetrathiafulvalene (TTF), prototypical electron acceptor/donor molecules in CTCs. Next, we apply our model in the Bayesian optimization structure search method and density-functional theory to identify the stable structures of ${\mathrm{F}}_{4}\mathrm{TCNQ}$ and TTF on supported Gr. We find that both molecules physisorb to Gr in various configurations. The narrow range of adsorption energies indicates that the molecules may diffuse easily on the surface and molecule-molecule interactions likely have a central role in film formation. Our study shows that complex intercalated substrates may be approximated with charged freestanding Gr, which can facilitate exhaustive structure search of CTCs.

Properties of Alkaline-Earth-Metal Polynitrogen Ternary Materials at High Pressure.

We report the formation of cubic and tetragonal BaSrN3 at 100 GPa using an ab initio structure search method. Pressure ramping to 0 GPa reveals a reaction pressure threshold of 4.92 and 7.23 GPa for the cubic and tetragonal BaSrN3, respectively. The cubic phase is stabilized by coulombic interaction between the ions. Meanwhile, tetragonal BaSrN3 is stabilized through an expansion of the d-orbital in Ba and Sr atoms that is compensated by delocalization of π-electrons in N through reduction of π overlap. Elastic properties analysis suggests that both phases are mechanically stable. The structures also have high melting points as predicted using an empirical model, and all imaginary modes vanishes at about 2000 K. These results have significant implication for the design of cleaner and environmentally friendly high energy density materials.

New tetrahedral polymorphs of the group-14 elements

In this short communication we present the main results of an ab initio structural search for new polymorphs of the group-14 elements C, Si, and Ge, with a focus on low-density and energetically favorable sp 3-bonded tetrahedral polymorphs which could constitute candidate phases to be synthesized in the laboratory. We propose new orthorhombic and tetragonal polymorphs with enthalpies that are lower or comparable to those of other polymorphs of the group-14 elements that have either been experimentally synthesized or recently proposed as the structure of unknown phases obtained in experiments. The new phases have interesting structural and electronics properties. These include an orthorhombic phase that in Si is found to have a quasi-direct band gap at the zone center of about the right size for optimal photovoltaic applications, and a tetragonal phase with a chiral topology that displays an exotic pentagonal pattern and whose vibrational properties match those of an unknown phase of Si.

Disorder-order phase transition at high pressure in ammonium fluoride

Solid ${\mathrm{NH}}_{4}\mathrm{F}$ displays intriguing parallels with ice despite its apparently ionic character. Here we investigate its phase diagram in low-temperature and high-pressure regions using Raman spectroscopy, x-ray diffraction, and ab initio structure search calculations. We focus on the high-pressure cubic phase which resembles that found in ice under pressure and is also the ambient pressure phase of other ammonium halides. We detect a disorder-order transition above 10 GPa, recalling those found both in other ammonium halides and in ice. The transition reveals itself in the pressure dependence of several Raman modes as well as through the progressive splitting of lattice and bending modes of the cubic phase at low temperatures. An in-depth analysis of the Raman modes and their evolution is made.

ChemInform Abstract: ZnSb Polymorphs with Improved Thermoelectric Properties.

AbstractNovel ZnSb polymorphs with improved thermoelectric (TE) properties are discovered by a systematic ab initio structural search based on DFT calculations revealing the glass‐like energy landscape and polymorphism of the system.

ZnSb Polymorphs with Improved Thermoelectric Properties

ZnxSb1–x systems are one of the most promising materials for thermoelectric applications due to their low cost, low toxicity, and high thermoelectric figure of merit. We report the discovery and characterization of novel ZnSb polymorphs with improved thermoelectric properties from a systematic ab initio structural search. The putative ground state at low temperature is a structure with Pbca symmetry. Corresponding electron and phonon transport properties are computed, where the lattice thermal conductivity was accurately obtained through third-order interatomic force constants. At 500 K, both the reduced figure of merit S2/L as well as the lattice thermal conductivity are superior to any previously reported numbers on ZnSb polymorphs, rendering the Pbca phase a promising candidate for an exceptionally good thermoelectric material.

Prediction of a stable half-metal ferromagnetic BaCl solid

The modification of Ba in BaCl compounds from alkaline-metal to transition- and half-metal behavior is explored. High-pressure structural changes in BaCl are predicted using an ab initio structure search method. Dynamically stable bcc and $R\ensuremath{-}3m$ forms of BaCl are predicted at 15 and 10 GPa, respectively. The BaCl forms are more stable than elemental Ba plus $\mathrm{BaC}{\mathrm{l}}_{2}$ above \ensuremath{\sim}10 GPa. Ba in stable BaCl adopts transition-metal properties via an $s\ensuremath{-}d$ transition. At ambient pressure the fcc structure is ferromagnetic, and the bcc structure is half metallic and ferromagnetic. The transition-metal electronic structure found is sufficient to support superconductivity, with ${T}_{c}$ as high as 3.4 K near ambient pressure.

Investigation of new phases in the Ba-Si phase diagram under high pressure using ab initio structural search.

Barium silicides are versatile materials that have attracted attention for a variety of applications in electronics and optoelectronics. Using an unbiased structural search based on a particle-swarm optimization algorithm combined with density functional theory calculations, we investigate systematically the ground-state phase stability and the structural diversity of Ba-Si binaries under high pressure. The phase diagram turns out to be quite intricate, with several compositions stabilizing/destabilizing as a function of pressure. In particular, we identify novel phases of BaSi, BaSi2, BaSi3, and BaSi5 that might be synthesizable experimentally over a wide range of pressures. Our results not only clarify and complete the previously known structural phase diagram, but also provide new insights for understanding the Ba-Si binary system.

Sodium–gold binaries: novel structures for ionic compounds from an ab initio structural search

Intermetallic compounds made of alkali metals and gold have intriguing electronic and structural properties that have not been extensively explored. We perform a systematic study of the phase diagram of one binary system belonging to this family, namely NaxAu1−x, using the ab initio minima hopping structural prediction method. We discover that the most stable composition is NaAu2, in agreement with available experimental data. We also confirm the crystal structures of NaAu2 and Na2Au, that were fully characterized in experiments, and identify a candidate ground-state structure for the experimental stoichiometry NaAu. Moreover, we obtain three other stoichiometries, namely Na3Au2, Na3Au and Na5Au, that could be thermodynamically stable. We do not find any evidence for the existence of the experimentally proposed composition NaAu5. Finally, we perform phonon calculations to check the dynamical stability of all reported phases and we simulate x-ray diffraction spectra for comparison with future experimental data.