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Stable compounds, crystal structures, and properties of polonium hydrides are investigated by DFT calculations.

High-pressure polymorphism and phase transitions have wide ranging consequences on the basic properties of ammonium nitrate. However, the phase diagram of ammonium nitrate at high pressure and high temperature is still under debate. This study systematically investigates the phase transitions and structural properties of ammonium nitrate at a pressure range of 5–60 GPa and temperature range of 250–400 K by ab initio molecular dynamics simulations. Two new phases are identified: one corresponds to the experimentally observed phase IV’ and the other is named AN-X. Simultaneously, the lattice strains play a significant role in the formation and stabilization of phase IV’, providing a reasonable explanation for experimental observation of phase IV-IV’ transition which only appears under nonhydrostatic pressure. In addition, 12 O atoms neighboring the NH (N atom in ammonium cation) atom are selected as reference system to clearly display the tanglesome rotation of ammonium cation.

We report on a first-principles study of the phase diagram, structures and properties of the Ru-H system in the H-rich regime over a wide range of pressures. The results show that RuH is thermodynamically stable and can coexist with RuH3 and RuH6 under pressure. RuH and RuH3 stoichiometries exhibit metallic character as a result of notable band structures, while RuH6 is a semiconductor. Strikingly, some hydrogen atoms pairwise couple into H2 units in the RuH6 compound. An estimation of superconducting transition temperature Tc is carried out by applying the Allen-Dynes modified McMillan equation for Fm3[combining macron]m (RuH), Pm3[combining macron]m (RuH3), and Pm3[combining macron]n (RuH3) structures and the resulting Tc reaches 0.41, 3.57 and 1.25 K at different pressures, respectively.

Single crystal Brillouin and Raman scattering measurements on NH3 in a diamond anvil cell have been performed under pressures up to 26?GPa at room temperature. The pressure dependencies of acoustic velocity, adiabatic elastic constants, and bulk moduli of ammonia from liquid to solid III and solid IV phase have been determined. All the nine elastic constants in orthorhombic structure phase IV were presented for the first time, each elastic constant grows monotonously with pressure and a crossover of the off-diagonal moduli C12 and C13 was observed at around 12?GPa because of their different pressure derivative values. We also performed ab initio simulations to calculate the bulk elastic moduli for orthorhombic ammonia, the calculated bulk moduli agree well with experimental results. In Raman spectra the very weak bending modes ?2 and ?4 for orthorhombic ammonia are both observed at room temperature, a transition point near 12?GPa is also found from the pressure evolution of the Raman bands. Copyright (c) 2011 John Wiley & Sons, Ltd.

The effects of high pressure on cyanuric chloride (C(3)N(3)Cl(3)), a remarkable crystal structure dominated by halogen bonds, have been studied by synchrotron X-ray diffraction and Raman spectroscopy in a diamond anvil cell. The results of high pressure experiments revealed that there was no obvious phase transition up to 30 GPa, indicating that halogen bonding is an effective noncovalent interaction to stabilize the crystal structure. Moreover, cyanuric chloride exhibited a high compressibility and a strong anisotropic compression, which can be explained by the layered crystal packing. Ab initio calculations were also performed to account for the high pressure Raman spectra and the high pressure behavior of halogen bonding.

We present a first-principles study on a phase of BC7 with Amm2 space-group symmetry (we call o-BC7), which is selected from more than a dozen of candidates via energetic, mechanical and dynamical stabilities within a wide pressure scale from 0 to at least 100 GPa. Our calculated results show that this structure is highly incompressible with large bulk and shear modulus of 388 GPa and 430 GPa, respectively, which is even higher than cubic BN. Further investigations revealed that the Vickers hardness (Hv ≈ 49.5 GPa) is higher than t-BC3 reported recently. And the superconducting critical temperature is calculated to be 38 K, which is comparable to that of MgB2.

The high-pressure phases of bromoform at zero temperature have been investigated by first-principles pseudopotential plane-wave calculations based on the density functional theory. A new high-pressure polar phase, ε, with space group CC has been found after a series of simulated annealing and geometry optimizations. Our calculated enthalpies showed that the transition from β phase to γ phase occurs at 1 GPa, then the γ phase transforms to the ε phase at 90 GPa. In addition, the Br···Br and C-H···Br interactions are the key factors for the polar aggregation in the ε phase. Further calculations show that the insulate-metal transition in ε phase due to band overlap happens at ~130 GPa.

The ab initio evolutionary algorithm is used to search for all thermodynamically stable Na-Te compounds at extreme pressure. In our calculations, several new structures are discovered at high pressure, namely, Imma Na2Te, Pmmm NaTe, Imma Na8Te2 and P4/mmm NaTe3. Like the known structures of Na2Te (Fm-3m, Pnma and P63/mmc), the Pmmm NaTe, Imma Na8Te2 and P4/mmm NaTe3 structures also show semiconductor properties with band-gap decreases when pressure increased. However, we find that the band-gap of Imma Na2Te structure increases with pressure. We presume that the result may be caused by the increasing of splitting between Te p states and Na s, Na p and Te d states. Furthermore, we think that the strong hybridization between Na p state and Te d state result in the band gap increasing with pressure.

The near-edge X-ray absorption fine structure (NEXAFS) spectra and crystal structures of the e - O 8 phase and the metallic ζ - O 2 phase have been investigated using full-potential linearized augmented plane-wave (FLAPW) and pseudopotential plane-wave methods. The change in NEXAFS spectra for the k -edge of the oxygen atom suggests a means for identifying the phase transition from e - O 8 to ζ - O 2 experimentally. The abrupt change of the bond length L (O1–O1) induced by pressure is believed to be responsible for the phase transition. There is no pressure-induced softening behavior in our calculated phonon dispersion curves near the phase transition pressure.

The high-pressure structures and superconductivity of iodine-doped hydrogen have been studied by ab initio calculations. Above 100 GPa, we discover a stable phase with Pnma symmetry in the H2I stoichiometry that consists of a monatomic iodine tube trapping hydrogen molecular units. Interestingly, H2 molecular units dissociate and form a novel atomic phase with R3̄m symmetry at 246 GPa. Further electron-phonon coupling calculations predict the critical temperature of superconductivity T(c) to be 3.8 K for the Pnma phase and 33 K for the R3̄m phase at 240 GPa. Significantly, the T(c) of the R3̄m phase is enhanced approximately 8 times that of the Pnma phase, which is mainly attributed to the reason that H2 molecules are broken exhibiting an atomic character in the R3̄m phase.

The structures of compressed halogen polyhydrides HnX (X = F, Cl and n1) and their evolution under pressure are studied using ab initio calculation based on density functional theory. HnF (n1) are metastable up to 300 GPa, whereas for HnCl (n1), four new stoichiometries (H2Cl, H3Cl, H5Cl, and H7Cl) are predicted to be stable at high pressures. Interestingly, triangular H3(+) species are unexpectedly found in stoichiometries H2F with [H3](+)[HF2](-), H3F with [H3](+)[F](-), H5F with [H3](+)[HF2](-)[H2]3, and H5Cl with [H3](+)[Cl](-)[H2] above 100 GPa. Importantly, formation processes of H3(+) species are clearly seen on the basis of comparing bond lengths, bond overlap populations, electron localization functions, and Bader charges as a functions of pressure. Further analysis reveals that the formation of H3(+) species is attributed to the pressure-induced charge transfer from hydrogen atoms to halogen atoms.

The structures, electron properties, and potential superconductivity of indium hydrides are systematically studied under high pressure by first-principles density functional calculations. Upon compression, two stable stoichiometries (InH3 and InH5) are predicted to be thermodynamically stable. Particularly, in the two compounds, all hydrogen atoms exist in the form of H2 or H3 units. The stable phases present metallic features with the overlap between the conduction and the valence bands. The Bader analysis indicates that charges transfer from In atoms to H atoms. Electron–phonon calculations show that the estimated transition temperatures (Tc) of InH3 and InH5 are 34.1–40.5 and 22.4–27.1 K at 200 and 150 GPa, respectively.

CaxZn1–xO alloys are potential candidates to achieve wide band-gap, which might significantly promote the band gap engineering and heterojunction design. We performed a crystal structure search for CaO-ZnO system under pressure, using an ab initio evolutionary algorithm implemented in the USPEX code. Four stable ordered CaxZn1–xO structures are found in the pressure range of 8.7–60 GPa. We further constructed the pressure vs. composition phase diagram of CaO-ZnO alloys based on the detailed enthalpy calculations. With the increase in Ca concentration, the CaO-ZnO alloy first undergoes a hexagonal to monoclinic transition and then transforms back to a hexagonal phase. At Above 9 GPa, there is no cubic structure in the alloys, in contrast to the insostructural components (B1-B1). The band gap of the CaxZn1–xO alloy shows an almost linear increase as a function of the Ca concentration. We also investigated the variation regularity of the band gap under pressure.

The crystal structures of Rh2B and RhB2 at ambient pressure were explored by using the evolutionary methodology. A monoclinic P21/m structure of Rh2B was predicted and donated as Rh2B-I, which is energetically much superior to the previously experimentally proposed Pnma structure. At the pressure of about 39 GPa, the P21/m phase of Rh2B transforms to the C2/m phases. For RhB2, a new monoclinic P21/m phase was predicted, named as RhB2-II, it has the same structure type with Rh2B. Rh2B-I and RhB2-II are both mechanically and dynamically stable. They are potential low compressible materials. The analysis of electronic density of states and chemical bonding indicates that the formation of strong and directional covalent B-B and Rh-B bonds in these compounds contribute greatly to their stabilities and high incompressibility.

Tungsten-nitrogen (W-N) compounds are studied via a combination of first-principles calculations and variable-composition evolutionary structure searches. New candidate ground states and high-pressure phases at 3 : 2, 1 : 1, and 5 : 6 compositions are uncovered and established for possible synthesis. We found that the structures in 4/5-fold N coordination (i.e., NbO-WN and W5N6) are more favoured for the W-N system at low-pressures compared with the conventional 6-fold phases (rs-WN and δ-WN). We attribute the low N coordination feature of W-N ground states to the enhanced W 5d-N 2p orbital hybridization and strong covalent W-N bonding, which involves the full-filling of W-N bonding and antibonding states and can remarkably improve the mechanical strength and hardness. These findings not only clarify the phase diagram of the W-N system, but also shed light on the correlations of hardness with microscopic crystal and electronic structures.

The metallization, superconductivity, and high-pressure structures of AlH3(H2) are explored at 25-300 GPa by ab initio first principle calculations.

The crystal structures and properties of hafnium hydride under pressure are explored using the first-principles calculations based on density function theory. The material undergoes pressure-induced structural phase transition I4/mmm→Cmma→P21/m at 180 and 250 GPa, respectively and all of these structures are metallic. The superconducting critical temperature Tc values of I4/mmm, Cmma and P21/m are 47–193 mK, 5.99–8.16 K and 10.62–12.8 K at 1 atm, 180 and 260 GPa, respectively. Furthermore, the bonding nature of HfH2 is investigated with the help of the electron localization function, the difference charge density and Bader charge analyses, which show that HfH2 is classified as a ionic crystal with the charges transferring from Hf atom to H.

First-principle evolutionary calculation was performed to search for all probable stable lithium tellurium compounds. In addition to the well-known structures of Fm-3m Li2Te and Pnma Li2Te, several novel structures, including those of P4/nmm Li2Te, Imma Li8Te2, and C2/m Li9Te2, were determined under high pressure. The transformation sequence of Li2Te induced by pressure was presented as follows. The phase transition occurred at 7.5 GPa while transforming from Fm-3m phase to Pnma structure, then transformed to P4/nmm phase at 14 GPa. P4/nmm Li2Te can remain stable at least up to 140 GPa. Li8Te2 and Li9Te2 were stable at 8–120 GPa and 80–120 GPa, respectively. Interestingly, Li8Te2 and Li9Te2 were predicted to be metallic under high pressure, Li2Te would metalize on compression. P4/nmm Li2Te is likely a super ionic conductor due to the special characteristic. Metallic P4/nmm Li2Te may be a candidate mixed conductor material under extreme pressure. Charge transfer was studied using Bader charge analysis. Charge transferred from Li to Te, and the relative debilitated ionicity between Li and Te atoms existed at high pressure.

First-principles calculations have been performed on NaAlH 4 using the generalized gradient approximation pseudopotential method. The predicted β-NaAlH 4 (α-LiAlH 4 -type) structure is energetically more favorable than α-NaAlH 4 for pressures over 15.9 GPa, which is apparently correlated with the experimental transition pressure 14 GPa. This transition is identified as first-order in nature with volume contractions of 1.8%. There is no pressure-induced softening behavior from our calculated phonon dispersion curves near the phase transition pressure. Based on the Mulliken population analysis, the β-NaAlH 4 structure is expected to be the most promising candidate for hydrogen storage.

The high pressure crystal structure of BI3 with space group P21/c consisting of B2I6 dimers is determined using an ab initio evolutionary algorithm for crystal structure prediction in conjunction with density functional theory.

The high pressure structures, metallization, and superconductivity of recently synthesized H2-containing compounds (H2S)2H2 are elucidated by ab initio calculations. The ordered crystal structure with P1 symmetry is determined, supported by the good agreement between theoretical and experimental X-ray diffraction data, equation of states, and Raman spectra. The Cccm structure is favorable with partial hydrogen bond symmetrization above 37 GPa. Upon further compression, H2 molecules disappear and two intriguing metallic structures with R3m and Im-3m symmetries are reconstructive above 111 and 180 GPa, respectively. The predicted metallization pressure is 111 GPa, which is approximately one-third of the currently suggested metallization pressure of bulk molecular hydrogen. Application of the Allen-Dynes-modified McMillan equation for the Im-3m structure yields high Tc values of 191 K to 204 K at 200 GPa, which is among the highest values reported for H2-rich van der Waals compounds and MH3 type hydride thus far.

The MgxZn(1-x)O alloy system may provide an optically tunable family of wide band gap materials that can be used in various UV luminescences, absorption, lighting, and display applications. A systematic investigation of the MgO-ZnO system using ab initio evolutionary simulations shows that MgxZn(1-x)O alloys exist in ordered ground-state structures at pressures above about 6.5 GPa. Detailed enthalpy calculations for the most stable structures allowed us to construct the pressure-composition phase diagram. In the entire composition, no phase transition from wurzite to rock-salt takes place with increasing Mg content. We also found two different slops occur at near x = 0.75 of Eg-x curves for different pressures, and the band gaps of high pressure ground-state MgxZn(1-x)O alloys at the Mg concentration of x0.75 increase more rapidly than x0.75.

High-pressure structures of MH2 (M = V, Nb) are explored through ab initio evolutionary methodology. As the same main group metal hydrides, VH2 and NbH2 adopt the same tetragonal structure with space group Fm-3m at low pressures. However, at high pressures VH2 and NbH2 possess Pnma and P6₃mc phases differently. The two phase transitions are both the first order phase transition identified by volume collapses. Our calculations suggest that two high-pressure structures have both dynamical and mechanical stability up to 100 GPa. Pnma VH2 and P6₃mc NbH2 are metallic phases demonstrated by the band structure and density of states. However, their superconducting temperatures are only several Kelvins.

The structures and properties of rhenium nitrides are studied with density function based first principle method. New candidate ground states or high-pressure phases at Re:N ratios of 3:2, 1:3, and 1:4 are identified via a series of evolutionary structure searches. We find that the 3D polyhedral stacking with strong covalent N-N and Re-N bonding could stabilize Re nitrides to form nitrogen rich phases, meanwhile, remarkably improve the mechanical performance than that of sub-nitrides, as Re3N, Re2N, and Re3N2. By evaluating the trends of the crystal configuration, electronic structure, elastic properties, and hardness as a function of the N concentration, we proves that the N content is the key factor affecting the metallicity and hardness of Re nitrides.

Pressure-induced phase transitions of BaH₂ have been studied by ab initio calculations. Our results show that BaH₂ transforms from the cotunnite structure to the InNi₂-type structure at about 2.3 GPa, which is in agreement with experimental results. The InNi₂ phase is predicted to be an insulator and transforms to a metallic phase with an AlB₂-type structure at about 34 GPa. Under higher pressure, a post-AlB₂ phase with the YbZn₂-type structure (space group Imma, 4 f.u./cell) is predicted, which is both dynamically and mechanically stable. Analysis of the enthalpies for both AlB₂ and YbZn₂ phases further supports the existence of this new phase. The [AlB₂ → YbZn₂] structural phase transition is identified as a second-order nature, driven by the softening of the transverse acoustic phonon mode at the L point (0.5, 0.0, 0.5).

The crystal structure and superconducting properties of monatomic bromine under high pressure have been studied by first-principles calculations. We have found the following phase transition sequence with increasing pressure: from body-centered orthorhombic (bco, phase II) to body-centered tetragonal structure (bct, phase III) at 126 GPa, then to face-centered cubic structure (fcc, phase IV) at 157 GPa, which is stable at least up to 300 GPa. The calculated superconducting critical temperature T(c) = 1.46 K at 100 GPa is consistent with the experimental value of 1.5 K. In addition, our results of T(c) decrease with increasing pressure in all the monatomic phases of bromine, similar to monatomic iodine. Further calculations show that the decrease of λ with pressure in phase IV is mainly attributed to the weakening of the 'soft' vibrational mode caused by pressure.

Ab initio calculations are performed to probe the hydrogen bonding, structural, and superconducting behaviors of HBr and HCl under high pressure. The calculated results show that the hydrogen bond symmetrization (Cmc2(1)--Cmcm transition) of HBr and HCl occurs at 25 and 40 GPa, respectively, which can be attributed to the symmetry stretching A(1) mode softening. After hydrogen bond symmetrization, a pressure-induced soft transverse acoustic phonon mode of Cmcm phase is identified and a unique metallic phase with monoclinic structure of P2(1)/m (4 molecules/cell) for both compounds is revealed by ab initio phonon calculations. This phase preserves the symmetric hydrogen bond and is stable in the pressure range from 134 to 196 GPa for HBr and above 233 GPa for HCl, while HBr is predicted to decompose into Br(2)+H(2) above 196 GPa. Perturbative linear-response calculations predict that the phase P2(1)/m is a superconductor with T(c) of 27-34 K for HBr at 160 GPa and 9-14 K for HCl at 280 GPa.

In situ Raman scattering and synchrotron X-ray diffraction have been used to investigate the effects of high pressure on the structural stability of guanidinium nitrate (C(NH{sub 2}){sub 3}{sup +} {center_dot} NO{sub 3}{sup -}, GN), a representative two-dimensional supramolecular architecture of hydrogen-bonded rosette network. This study has confirmed a structural phase transition observed by Raman scattering and X-ray diffraction at {approx}1 GPa and identified it as a space group change from C2 to P2{sub 1}. The high-pressure phase remained stable up to 22 GPa. We discussed the pressure-induced changes in N-H stretching vibration in Raman spectra and proposed that this phase transition is due to the rearrangements of the hydrogen-bonding networks.

Single-crystal samples of the 1:1 adduct between cyanuric acid and melamine (CA.M), an outstanding case of noncovalent synthesis, have been studied by Raman spectroscopy and synchrotron X-ray diffraction in a diamond anvil cell up to pressures of 15 GPa. The abrupt changes in Raman spectra around 4.4 GPa have provided convincing evidence for pressure-induced structural phase transition. This phase transition was confirmed by angle dispersive X-ray diffraction (ADXRD) experiments to be a space group change from C2/m to its subgroup P2(1)/m. On release of pressure, the observed transition was irreversible, and the new high-pressure phase was fully preserved at ambient conditions. We propose that this phase transition was due to supramolecular rearrangements brought about by changes in the hydrogen bonding networks.

The effects of high pressure on the structural stability of the melamine-boric acid adduct (C3N6H(6).2H3BO3, M.2B), a three-dimensional hydrogen-bonded supramolecular architecture, were studied by in situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy. M.2B exhibited a high compressibility and a strong anisotropic compression, which can be explained by the layerlike crystal packing. Furthermore, evolution of XRD patterns and Raman spectra indicated that the M.2B crystal undergoes a reversible pressure-induced amorphization (PIA) at 18 GPa. The mechanism for the PIA was attributed to the competition between close packing and long-range order. Ab initio calculations were also performed to account for the behavior of hydrogen bonding under high pressure.

The dependence of superconducting transition temperature ${T}_{c}$ on hydrostatic and nonhydrostatic pressures for monatomic phases of iodine has been studied by first-principles pseudopotential plane-wave method. It is shown that the ${T}_{c}$ of both phases II and III under hydrostatic pressures are in agreement with the experimental data while the ${T}_{c}$ of phase IV under hydrostatic pressures decreases with increasing pressure, contrary to the experimental results. In order to explore the origin of difference between experimental and theoretical results, we have studied the effect of nonhydrostatic pressure on the superconductivity of monatomic iodine, and found that the symmetry of phase IV changes from face-centered cubic to face-centered orthorhombic (fco) under anisotropic stresses. Further calculations show that the ${T}_{c}$ of this fco structure increases with increasing pressure, in good agreement with the experimental results, which is mainly attributed to the nonhydrostatic pressure-induced enhancement of the electronic density of states at the Fermi level and electron-phonon coupling matrix element $⟨{I}^{2}⟩$.

Crystal structures of bromine under high pressure have been studied by employing plane-wave pseudopotential method with the generalized gradient approximation. It is found that the band overlap in the molecular $Cmca$ phase, which causes the pressure-induced insulator-to-metal transition, occurs at about $55\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. Geometry optimization shows that the bromine changes to a face-centered orthorhombic (fco) phase with equal interatomic distances ${d}_{1}={d}_{2}={d}_{3}$ at about $75\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, but this fco structure is mechanically unstable with shear elastic stiffness coefficient ${C}_{66}l0$. For understanding the structure of this phase, we have modeled an incommensurate structure by a rational approximation with modulation vector $k=(0.25,0,0)$ according to the previous research results in solid iodine. Our results show that the enthalpy of this modulated phase is lower than that of the fco solid, and the elastic stiffness coefficients $({C}_{ij})$ satisfy the Born stability criteria, indicating that the modulated structure is more thermodynamically stable and mechanically stable. In addition, through comparing the x-ray diffraction patterns of our structure with the experimental one, we conclude that the structure of bromine phase V is close to our modulated structure. It is clearly illustrated that the phase transition from $Cmca$ phase to the incommensurate phase is associated with the instability of the shear elastic stiffness coefficient ${C}_{44}$ which is related to the softening of the long-wavelength part of the transverse branch near the center of the first Brillouin zone. With the increasing of pressure, the modulated phase transforms into the monatomic phase II with body-centered orthorhombic structure at about $100\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, which is in agreement with the experimental result.

Two high density structures β- and γ-LiAlH4 are detected in LiAlH4, a promising hydrogen storage compound, upon compression in diamond anvil cells, investigated with synchrotron X-ray diffraction and first-principle calculations. The joint of the experimental and theoretical results has confirmed the sequence of the pressure-induced structural phase transitions from α-LiAlH4 (space group P21/c) to β-LiAlH4 (P21/c-6C symmetry), and then to γ-LiAlH4 (space group Pnc2), which are not reported in previous literatures. At the α to β transition point for LiAlH4, the estimated difference in cell volume is about 20%, while the transformation from β to γ phase is with a volume drop smaller than 1%. The α to β phase transition is accompanied by the local structure change from a AlH4 tetrahedron into a AlH6 octahedron, which contributes to a large volume collapse.

According to the fact that methane has not been metalized and may be dissociated under high pressure, we present a structural characterization of compressed Mg-methane compounds obtained using first-principles molecular dynamics within density-functional theory. In this article, we propose two structures with space groups orthorhombic Pban and tetragonal P4/nmm, to be stable in the pressure ranges of 0–75 GPa and 75–120 GPa, respectively. The calculated results indicate that the orthorhombic Pban phase is a semiconductor and the tetragonal P4/nmm phase is a superconductor with Tc of 84–121 K in the pressure range of 75–120 GPa.

The structural, electronic, and dynamical properties and intermolecular interactions of CH4H2 are investigated based on first-principles calculations. Enthalpy calculations indicate that the P structure is the most stable phase below 15.6 GPa. On compression, the P212121 phase possesses the lowest enthalpy, then the P21/C becomes energetically favorable above 98.2 GPa. The pressure-induced hardening behavior of H2-bonds in the CH4H2 system reproduces the experimental data well. In addition, the CH4 molecules remain as tetrahedra due to the weak interactions of molecular H2 and CH4. At higher pressure, the orientation of H2 molecules in CH4H2 is also disordered and this resembles the findings for pure H2. It shows that comparing to pure H2, the addition of CH4 does not have much limitations on the rotation of the H2 molecules.

High-pressure structures of MH2 (M = V, Nb) are explored through ab initio evolutionary methodology. As the same main group metal hydrides, VH2 and NbH2 adopt the same tetragonal structure with space group Fm-3m at low pressures. However, at high pressures VH2 and NbH2 possess Pnma and P63mc phases differently. The two phase transitions are both the first order phase transition identified by volume collapses. Our calculations suggest that two high-pressure structures have both dynamical and mechanical stability up to 100 GPa. Pnma VH2 and P63mc NbH2 are metallic phases demonstrated by the band structure and density of states. However, their superconducting temperatures are only several Kelvins.

The structural stability and electrical properties of AlB2-type MnB2 were studied based on high pressure angle-dispersive x-ray diffraction, in situ electrical resistivity measured in a diamond anvil cell (DAC) and first-principles calculations under high pressure. The x-ray diffraction results show that the structure of AlB2-type MnB2 remains stable up to 42.6 GPa. From the equation of state of MnB2, we obtained a bulk modulus value of 169.9±3.7 GPa with a fixed pressure derivative of 4, which indicates that AlB2-type MnB2 is a hard and incompressible material. The electrical resistance undergoes a transition at about 19.3 GPa, which can be explained by a transition of manganese 3d electrons from localization to delocalization under high pressure.

The phase stability, mechanical properties and metallic properties of tantalum nitrides are extensively studied by means of first principles calculations. The relationship between nitrogen concentration and physical properties of tantalum nitrides has been systematically investigated. With the nitrogen concentration increasing, it is found that the feature of covalent bonding enhances and the directionality of the covalent bonding and hardness of tantalum nitrides reduce. While these make the ductility of tantalum nitrides improve with the nitrogen concentration increasing. The intensity of metallic properties of tantalum nitrides can be effectively adjusted by controlling the nitrogen concentration and pressure. When the tantalum: nitrogen ratio reaches Ta:N = 1:3, remarkable nitrogen–nitrogen bonds are found in TaN3. The hardness of TaN3 abnormally increases with reference to that of the preceding composition Ta3N5-II. The potential synthesis routes of tantalum nitrides are suggested.

The structural stability, mechanical properties, and dynamical properties of T carbon-like structures were extensively studied by first-principles calculations using density functional theory. A novel modulated T carbon-like carbon allotrope (T-II carbon) is predicted by means of first principles calculations. This structure has 8 atoms in the unit cell, possesses the Pnm space group, and can be derived by stacking up two T carbons together. T-II carbon is a semiconductor with band gap 0.88 eV and has a higher hardness (27 GPa) than that of T carbon (5.6 GPa). The calculations of ideal strength and the electron localization function indicate that T-II carbon has better ability to resist shear strain than T carbon.

A systematical study of the orientational behavior of C60 molecules in single wall carbon nanotubes (SWCNTs) with different chirality and diameter has been performed by using a model of an infinite long nanotube filled with two C60 (denoted as C60-1 and C60-2) molecules. We studied the preferred orientation of the C60-1 molecule when the neighboring C60-2 molecule was fixed at the pentagon, double-bond, and hexagon orientations respectively. Our results showed that the C60-1 molecule prefers the pentagon (hexagon) orientation when the tube diameter is smaller (larger) than 1.31 nm (1.36 nm). For the tube diameter in between, the preferred molecular orientation of C60-1 changes from pentagon to hexagon with the increasing tube diameter when the neighboring C60-2 molecule is fixed at the pentagon or double-bond orientation. A novel vertex orientation for the C60-1 molecule has been found when the C60-2 molecule is fixed at the hexagon orientation.

Magnesium borohydride (Mg(BH4)2) is one of the potential hydrogen storage materials. Recently, two experiments [Y. Filinchuk, B. Richter, T. R. Jensen, V. Dmitriev, D. Chernyshov, and H. Hagemann, Angew. Chem., Int. Ed. 50, 11162 (2011); L. George, V. Drozd, and S. K. Saxena, J. Phys. Chem. C 113, 486 (2009)] found that α-Mg(BH4)2 can irreversibly be transformed to an ultra dense δ-Mg(BH4)2 under high pressure. Its volumetric hydrogen content at ambient pressure (147 g/cm(3)) exceeds twice of DOE's (U.S. Department of Energy) target (70 g/cm(3)) and that of α-Mg(BH4)2 (117 g/cm(3)) by 20%. In this study, the experimentally proposed P4(2)nm structure of δ-phase has been found to be dynamically unstable. A new Fddd structure has been reported as a good candidate of δ-phase instead. Its enthalpy from 0 to 12 GPa is much lower than P4(2)nm structure and the simulated X-ray diffraction spectrum is in satisfied agreement with previous experiments. In addition, the previously proposed P-3m1 structure, which is denser than Fddd, is found to be a candidate of ε-phase due to the agreement of Raman shifts.

A systematic study of structure and dynamics behaviors of encapsulated C60H18 molecules in single wall carbon nanotube (SWCNT) has been performed by energetic analysis and molecular dynamics simulation. The results show that the C60H18 molecule inside SWCNT exhibit a diameter dependent orientation behavior in the range of 1.41–1.57 nm, in which the preferable tilted and parallel orientation have been found in nanotubes with diameter smaller and larger than 1.54 nm, respectively. It is found that the preferable orientations of C60H18 molecules are weakly affected by the C60H18–C60H18 intermolecular interaction, but unaffected by the tube chirality. The translation movement of C60H18 molecules along the tube axis direction in the nanotubes is restricted with diameter smaller than 1.45 nm, while spontaneous movement has been observed upon increasing the tube diameter. For the molecular dynamics simulation, various stacking arrangements of C60H18 molecules have been presented with the tube diameter (dt) in the range from 1.57 to 2.71 nm.

The previously proposed theoretical and experimental structures, bond characterization, and compressibility of Mg(BH4)2 in a pressure range from 0 to 10 GPa are studied by ab initio density-functional calculations. It is found that the ambient pressure phases of meta-stable I41/amd and unstable P-3m1 proposed recently are extra stable and cannot decompose under high pressure. Enthalpy calculation indicates that the ground state of F222 structure proposed by Zhou et al. [2009 Phys. Rev. B 79 212102] will transfer to I41/amd at 0.7 GPa, and then to a P-3m1 structure at 6.3 GPa. The experimental P6122 structure (α-phase) transfers to I41/amd at 1.2 GPa. Furthermore, both I41/amd and P-3m1 can exist as high volumetric hydrogen density phases at low pressure. Their theoretical volumetric hydrogen densities reach 146.351 g H2/L and 134.028 g H2/L at ambient pressure, respectively. The calculated phonon dispersion curve shows that the I41/amd phase is dynamically stable in a pressure range from 0 to 4 GPa and the P-3m1 phase is stable at pressures higher than 1 GPa. So the I41/amd phase may be synthesized under high pressure and retained to ambient pressure. Energy band structures show that they are both always ionic crystalline and insulating with a band-gap of about 5 eV in this pressure range. In addition, they each have an anisotropic compressibility. The c axis of these structures is easy to compress. Especially, the c axis and volume of P-3m1 phase are extraordinarily compressible, showing that compression along the c axis can increase the volumetric hydrogen content for both I41/amd and P−3m1 structures.

In 2009, a super-hard MnB2 with ReB2-type structure was predicted as being in the ground state. However, it has not been synthesized successfully in about two years either by high temperature and high pressure (HTHP) method or by the arc-melting method. To obtain the accurate synthesis conditions, the P–T phase boundary between AlB2-type and ReB2-type MnB2 has been studied by first-principles lattice dynamics calculations within quasi-harmonic approximation (QHA). Our results show that the ReB2-type MnB2 can be synthesized only below 1020 K at ambient pressure. Pressure effect makes their transition temperature decrease. If the pressure is higher than 38 GPa, only AlB2-type MnB2 can be obtained. The synthesis temperatures of previous experiments (either HTHP or arc-melting method) are all above 1020 K, so that only AlB2-type MnB2 can be synthesized. Therefore, it is essential to control the temperature accurately to synthesize the ReB2-type MnB2. On the other hand, the pressure should be controlled to be as low as possible. Further analyses show that the thermodynamic stability of MnB2 at high temperature mostly depends on the vibration frequency of Mn atoms. The stronger interactions between Mn and B in the ReB2-type MnB2 induce the vibration frequencies of Mn atoms shift to higher and increase the Gibbs free energy, causing the thermodynamics instability of ReB2-type MnB2 at high temperature. Therefore, there is no ReB2-type MnB2 synthesized at the temperature higher than 1020 K.