Unraveling the Structure and Bonding Evolution of the Newly Discovered Iron Oxide FeO2

Recently reported synthesis of FeO2 at high pressure has stimulated great interest in exploring this new iron oxide and elucidating its properties. Here, we present a systematic computational study of crystal structure, chemical bonding, and sound velocity of FeO2 in a wide range of pressure. Our results establish thermodynamic stability of the experimentally observed pyrite phase (P-phase) of FeO2 at pressures above 74 GPa and unveil two metastable FeO2 phases in Pbcn and P4(2)/mnm symmetry at lower pressures. Simulated x-ray diffraction (XRD) spectra of Pbcn and P4(2)/mnm FeO2 match well with measured XRD data of the decompression products of P-phase FeO2, providing compelling evidence for the presence of these metastable phases. Energetic calculations reveal unusually soft O-O bonds in P-phase FeO2 stemming from a low-frequency libration mode of FeO6 octahedra, rendering the O-O bond length highly sensitive to computational and physical environments. Calculated sound-velocity profiles of P-phase FeO2 are markedly different from those of the Pbcn and P4(2)/mnm phases, underscoring their distinct seismic signatures. Our findings offer insights for understanding the rich structural, bonding, and elastic behaviors of this newly discovered iron oxide

Crystal structure prediction using the Minima Hopping method

A structure prediction method is presented based on the Minima Hopping method. Optimized moves on the configurational enthalpy surface are performed to escape local minima using variable cell shape molecular dynamics by aligning the initial atomic and cell velocities to low curvature directions of the current minimum. The method is applied to both silicon crystals and binary Lennard-Jones mixtures and the results are compared to previous investigations. It is shown that a high success rate is achieved and a reliable prediction of unknown ground state structures is possible.Comment: 9 pages, 6 figures, novel approach in structure prediction, submitted to the Journal of Chemical Physic

Exploring the high-pressure materials genome

A thorough in situ characterization of materials at extreme conditions is challenging, and computational tools such as crystal structural search methods in combination with ab initio calculations are widely used to guide experiments by predicting the composition, structure, and properties of high-pressure compounds. However, such techniques are usually computationally expensive and not suitable for large-scale combinatorial exploration. On the other hand, data-driven computational approaches using large materials databases are useful for the analysis of energetics and stability of hundreds of thousands of compounds, but their utility for materials discovery is largely limited to idealized conditions of zero temperature and pressure. Here, we present a novel framework combining the two computational approaches, using a simple linear approximation to the enthalpy of a compound in conjunction with ambient-conditions data currently available in high-throughput databases of calculated materials properties. We demonstrate its utility by explaining the occurrence of phases in nature that are not ground states at ambient conditions and estimating the pressures at which such ambient-metastable phases become thermodynamically accessible, as well as guiding the exploration of ambient-immiscible binary systems via sophisticated structural search methods to discover new stable high-pressure phases.Comment: 14 pages, 6 figure

Low-energy structures of zinc borohydride Zn(BH4_4)2_2

We present a systematic study of the low-energy structures of zinc borohydride, a crystalline material proposed for the hydrogen storage purpose. In addition to the previously proposed structures, many new low-energy structures of zinc borohydride are found by utilizing the minima-hopping method. We identify a new dynamically stable structure which belongs to the $I4_122$ space group as the most stable phase of zinc borohydride at low temperatures. A low transition barrier between $I4_122$ and $P1$, the two lowest-lying phases of zinc borohydride is predicted, implying that a coexistence of low-lying phases of zinc borohydride is possible at ambient conditions. An analysis based on the simulated X-ray diffraction pattern reveals that the $I4_122$ structure exhibits the same major features as the experimentally synthesized zinc borohydride samples.Comment: Version accepted by Phys. Rev. B. Manuscript has 8 pages, 5 figures, 2 tables (with 6 pages, 5 figures, 2 tables in supplemental material

Low-density silicon allotropes for photovoltaic applications

Silicon materials play a key role in many technologically relevant fields, ranging from the electronic to the photovoltaic industry. A systematic search for silicon allotropes was performed by employing a modified ab initio minima hopping crystal structure prediction method. The algorithm was optimized to specifically investigate the hitherto barely explored low-density regime of the silicon phase diagram by imitating the guest-host concept of clathrate compounds. In total 44 metastable phases are presented, of which 11 exhibit direct or quasi-direct band-gaps in the range of $\approx$1.0-1.8 eV, close to the optimal Shockley-Queisser limit of $\approx$1.4 eV, with a stronger overlap of the absorption spectra with the solar spectrum compared to conventional diamond silicon. Due to the structural resemblance to known clathrate compounds it is expected that the predicted phases can be synthesized

Emergence of hidden phases of methylammonium lead-iodide (CH3_3NH3_3PbI3_3) 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, 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

Prediction of a novel monoclinic carbon allotrope

A novel allotrope of carbon with $P2/m$ symmetry was identified during an \emph{ab-initio} minima-hopping structural search which we call $M10$-carbon. This structure is predicted to be more stable than graphite at pressures above 14.4 GPa and consists purely of $sp^3$ 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