Pressure-induced reconstructive phase transitions, polarization with metallicity, and enhanced hardness in antiperovskite MgCNi3.

In general, hydrostatic pressure can suppress electrical polarization, instead of creating and/or enhancing polarization like strain engineering. Here, a combination of first-principles calculations and CALYPSO crystal structures prediction is used to point out that hydrostatic pressure applied on antiperovskite MgCNi3 can stabilize polarization with metallicity, and thus a polar metal can exist under high pressure. Strikingly, the metallic polar phase of MgCNi3 exhibits an original linear-cubic coupling between polar and nonpolar modes, resulting in an asymmetrical double-well when the polarization is switched. Moreover, another novel phase of MgCNi3 under high pressure possesses an enhanced hardness stemming from a robust s-s electrons interaction of an unexpected C-C bond, rather than typical sp3 orbital hybridization. These discoveries open new routes to design superhard materials and polar metals.

Strain-induced structural phase transition, electric polarization and unusual electric properties in photovoltaic materials CsMI3 (M = Pb, Sn).

The structural phase transition, ferroelectric polarization, and electric properties have been investigated for photovoltaic films CsMI3 (M = Pb, Sn) epitaxially grown along (001) direction based on the density functional theory. The calculated results indicate that the phase diagrams of two epitaxial CsPbI3 and CsSnI3 films are almost identical, except critical transition strains varying slightly. The epitaxial tensile strains induce two ferroelectric phases Pmc21, and Pmn21, while the compressive strains drive two paraelectric phases P212121, P21212. The larger compressive strain enhances the ferroelectric instability in these two films, eventually rendering them another ferroelectric state Pc. Whether CsPbI3 or CsSnI3, the total polarization of Pmn21 phase comes from the main contribution of B-position cations (Pb or Sn), whereas, for Pmc21 phase, the main contributor is the I ion. Moreover, the epitaxial strain effects on antiferrodistortive vector, polarization and band gap of CsMI3 (M = Pb, Sn) are further discussed. Unusual electronic properties under epitaxial strains are also revealed and interpreted.

Hydrostatic pressure induced structural phase transition and mechanical properties of fluoroperovskite.

We perform the first-principles calculations combined with the particle swarm optimization algorithm to investigate the high-pressure phase diagrams of Na[Formula: see text]F3 ([Formula: see text] = Mn, Ni, Zn). Two reconstructive phase transitions are predicted from Pv-[Formula: see text] to pPv-[Formula: see text] at about 9 GPa and pPv-[Formula: see text] to ppPv-[Formula: see text] at around 26 GPa for NaZnF3. That is not the case for NaMnF3-a direct transition (reconstructive transition in nature but with the same Pnma space group) from Pv-[Formula: see text] to ppPv-[Formula: see text] phase around 12 GPa. Strikingly, our simulated results manifest that a disproportionation phase of NaZnF3 post-perovskite is uncovered along the way, which provides a successful explanation for the observed results in experiment. Additionally, the mechanical and thermal properties, especially the dynamical property, of the four NaZnF3 phases have also been studied. Here, we reveal the obvious softening of [Formula: see text]-wave velocity and bulk sound speed in pPv-[Formula: see text]-to-ppPv-[Formula: see text] transition, which may result in the discontinuity of seismic waves propagation through the Earth's interior.

Probing the Structural and Electronic Properties of Dirhenium Halide Clusters: A Density Functional Theory Study.

Dirhenium halide dianions received considerable attention in past decades due to the unusual metal-metal quadruple bond. The systematic structural evolution of dirhenium halide clusters has not been sufficiently studied and hence is not well-understood. In this work, we report an in-depth investigation on the structures and electronic properties of doubly charged dirhenium halide clusters Re2X82- (X = F, Cl, Br, I). Our computational efforts rely on the well-tested unbiased CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) method combined with density functional theory calculations. We find that all ground-state Re2X82- clusters have cube-like structures of D4h symmetry with two Re atoms encapsulated in halogen framework. The reasonable agreement between the simulated and experimental photoelectron spectrum of the Re2Cl82- cluster supports strongly the reliability of our computational strategy. The chemical bonding analysis reveals that the δ bond is the pivotal factor for the ground-state Re2X82- (X = F, Cl, Br, I) clusters to maintain D4h symmetric cube-like structures, and the enhanced stability of Re2Cl82- is mainly attributed to the chemical bonding of 5d orbital of Re atoms and 3p orbital of Cl atoms.

Structural Stability and Evolution of Medium-Sized Tantalum-Doped Boron Clusters: A Half-Sandwich-Structured TaB12- Cluster.

Transition-metal (TM)-doped boron clusters have received considerable attention in recent years, in part, because of their remarkable size-dependent structural and electronic properties. However, the structures of medium-sized boron clusters doped with TM atoms are still not well-known because of the much increased complexity of the potential surface as well as the rapid increase in the number of low-energy isomers, which are the challenges in cluster structural searches. Here, by means of an unbiased structure search, we systematically investigated the structural evolution of medium-sized tantalum-doped boron clusters, TaBn0/- (n = 10-20). The results revealed that TaBn0/- (n = 10-15) clusters adopt half-sandwich molecular geometries, with the notable exception of TaB10-, while for n = 16-18 and 19-20, the lowest-energy clusters are characterized by drum-type geometries and tubular molecules with two B atoms on the top, respectively. Good agreement between the calculated and experimental photoelectron spectra strongly support the validity of our global minimum structures. Molecular orbital and adaptive natural density partitioning analyses indicate that the enhanced stability of half-sandwich TaB12- is due to the strong interaction of the Ta atom (5d orbitals) with surrounding B atoms (2p orbitals) and σ B-B bonds in the B12 moiety.

Prediction of hypervalent molecules: investigation on MnC (M = Li, Na, K, Rb and Cs; n = 1-8) clusters.

New hypervalent molecules have emerged from a systematic exploration of the structure and bonding of MnC (M = Li, Na, K, Rb and Cs; n = 1-8) clusters via an unbiased CALYPSO structure investigation combined with density functional theory. The global minimum structures are obtained at the B3LYP/6-311+G* and CCSD(T)/6-311+G* levels of theory. The observed growth behavior clearly indicates that the ground state of MnC (M = Li, Na, K, Rb and Cs; n = 1-8) is transformed from a planar to a three-dimensional (3D) structure at n = 4. A maximum of six alkali atoms can be bound atomically to a carbon atom. The determination of the averaged binding energies Eb(n), fragmentation energies ΔE(n) and HOMO-LUMO energy gaps unambiguously supports the stability of M6C. This indicated conclusively that 6 is a magic Li-coordination number for C. The nature of bonding is further investigated by an insightful analysis of the highest occupied molecular orbital (HOMO) and the topology of chemical bonds for the most stable clusters. In the final step, electron localization functions (ELF) and density of states (DOS) are determined in order to consolidate the acquired information on the studied electronic structures.

Exploration of the Structural, Electronic and Tunable Magnetic Properties of Cu4M (M = Sc-Ni) Clusters.

The structural, electronic and magnetic properties of Cu4M (M = Sc-Ni) clusters have been studied by using density functional theory, together with an unbiased CALYPSO structure searching method. Geometry optimizations indicate that M atoms in the ground state Cu4M clusters favor the most highly coordinated position. The geometry of Cu4M clusters is similar to that of the Cu5 cluster. The infrared spectra, Raman spectra and photoelectron spectra are predicted and can be used to identify the ground state in the future. The relative stability and chemical activity are investigated by means of the averaged binding energy, dissociation energy and energy level gap. It is found that the dopant atoms except for Cr and Mn can enhance the stability of the host cluster. The chemical activity of all Cu4M clusters is lower than that of Cu5 cluster whose energy level gap is in agreement with available experimental finding. The magnetism calculations show that the total magnetic moment of Cu4M cluster mainly come from M atom and vary from 1 to 5 µB by substituting a Cu atom in Cu5 cluster with different transition-metal atoms.

Evolution of the Structural and Electronic Properties of Medium-Sized Sodium Clusters: A Honeycomb-Like Na20 Cluster.

Sodium is one of the best examples of a free-electron-like metal and of a certain technological interest. However, an unambiguous determination of the structural evolution of sodium clusters is challenging. Here, we performed an unbiased structure search among neutral and anionic sodium clusters in the medium size range of 10-25 atoms, using the Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) method. Geometries are determined by CALYPSO structure searches, followed by reoptimization of a large number of candidate structures. For most cluster sizes the simulated photoelectron spectra of the lowest-energy structures are in excellent agreement with the experimental data, indicating that the current ground-state structures are the true minima. The equilibrium geometries show that, for both neutral and anionic species, the structural evolution from bilayer structures to layered outsides with interior atoms occurs at n = 16. A novel unprecedented honeycomb-like structure of Na20 cluster with C3 symmetry is uncovered, which is more stable than the prior suggested structure based on pentagonal structural motifs.

Large polarization and dielectric response in epitaxial SrZrO3 films.

First-principles calculations are performed to investigate the ferroelectric and dielectric properties of (001) epitaxial SrZrO3 thin films under misfit strain. A rich phase diagram is predicted. By condensing the polar instability, the ferroelectric Pmc21 and Ima2 phases can coexist under tensile strain (about 3.7%-5.2%/5.7%). Combining in-plane ferroelectric (FExy) and out-of-plane in-phase antiferrodistortive (IAFDz) modes, another new Pmc21 state (P > 56 µC cm(-2)) occurs with increase in the tensile strain. The paraelectric I4/mcm and ferroelectric P4mm phases emerge around -3.2%/-3.7% and -6.4%/-7.4% compressive strain, respectively. The former exhibits an intense out-of-plane dielectric response, while the latter possesses a rather large polarization (∼ 110 µC cm(-2)). The large polarization and dielectric response are discussed in relationship to strain-driven structural distortion.

Understanding the structural transformation, stability of medium-sized neutral and charged silicon clusters.

The structural and electronic properties for the global minimum structures of medium-sized neutral, anionic and cationic Sin(µ) (n = 20-30, µ = 0, -1 and +1) clusters have been studied using an unbiased CALYPSO structure searching method in conjunction with first-principles calculations. A large number of low-lying isomers are optimized at the B3PW91/6-311 + G* level of theory. Harmonic vibrational analysis has been performed to assure that the optimized geometries are stable. The growth behaviors clearly indicate that a structural transition from the prolate to spherical-like geometries occurs at n = 26 for neutral silicon clusters, n = 27 for anions and n = 25 for cations. These results are in good agreement with the available experimental and theoretical predicted findings. In addition, no significant structural differences are observed between the neutral and cation charged silicon clusters with n = 20-24, both of them favor prolate structures. The HOMO-LUMO gaps and vertical ionization potential patterns indicate that Si22 is the most chemical stable cluster, and its dynamical stability is deeply discussed by the vibrational spectra calculations.

Theoretical studies of the dependence of EPR parameters on local structure for the tetragonal Er(3+) centres in YVO4 and ScVO4.

The dependences of the EPR parameters on the local distortion parameters Δθ and ΔR as well as the crystal-field parameters have been studied by diagonalizing the 364×364 complete energy matrices for a tetragonal Er(3+) centre in the YVO4 and ScVO4 crystals. The results show that the local distortion angle Δθ and the fourth-order crystal-field parameter A4 are most sensitive to the EPR g-factors g// and g⊥, whereas the local distortion length ΔR and the second-order parameter A2 are less sensitive to the g-factors. Furthermore, we found that the abnormal EPR g-factors for the Er(3+) ion in the ScVO4 may be ascribed to the stronger nephelauxetic effect and covalent bonding effect, as a result of an expanded local distortion for the Er(3+) centre in the ScVO4 crystal. Simultaneously, the contributions of the J-J mixing effects from the terms of excited states to the EPR parameters have been evaluated quantitatively.

Chemisorption-induced two- to three-dimensions structural transformations in gold pentamer (CO)(n)Au5(-) (n =0-5).

Understanding the geometry structures of gold clusters, especially with adsorbates, is essential for designing highly active gold nanocatalysts. Here, CO chemisorption onto the Au5(-) cluster is investigated using the density functional calculations. It is found that chemisorption of CO molecules can induce previously unreported two- to three-dimensions (3D) structural changes. Even a single CO chemisorption induces a major structural change to explain the huge blue-shift in photoelectron spectroscopy (PES). The apex site in the parent Au5(-) cluster is not always the most preferred site for the chemisorption, and two bridged adsorption CO molecules are observed in the lowest-energy (CO)3Au5(-) cluster. A clear splitting is observed in the first PES of (CO)4Au5 (-), and calculated planar and 3D geometries are likely coexisting in the cluster beam. The fifth CO adsorption leads to the structural transformation of Au5 skeleton to create more apex sites to accommodate five CO molecules. The structural properties, together with the vertical electron detachment energy (VDE) and binding energies calculations indicate that the chemisorption-saturated number is 5.

Probing the structural and electronic properties of bimetallic chromium-gold clusters CrmAun(m+n≤6): comparison with pure chromium and gold clusters.

Bimetallic chromium-gold CrmAun(m+n≤6) clusters are systematically investigated using the density functional theory at PW91P86 level with LanL2TZ basis set to understand the evolution of various structural, electronic, magnetic, and energetic properties as a function of size (m+n) and composition (m/n) of the system. Theoretical results show a logical evolution of the properties depending on the size and the composition of the system. Cr m clusters clearly prefer 3D structures while Au n clusters favor planar configurations. The geometry of the bimetallic Cr m Au n clusters mainly depends on their composition, i.e., clusters enriched in Cr atoms prefer 3D structures while increasing Au contents promotes planar configurations. The stability is maximized when the composition of binary Cr m Au n clusters is nearly balanced. Meanwhile, the number of hetero Cr-Au bonds and charge transfer from Cr to Au are maximized for clusters with m≈n. The most probable dissociation channels of the Cr m Au n clusters are calculated and analyzed. Natural population analysis reveals that Au atoms tend to be negatively charged while Cr atoms tend to be positively charged. Combined with the trend that Au atoms favor the surface/edges/vertices and Cr atoms tend to be inside, the outer part of the cluster tends to be negatively charged, and the inner part tends to be positively charged.

Probing the structural and electronic properties of small aluminum dideuteride clusters.

Adsorption of deuterium on the neutral and anionic Aln(λ) (n=1-9, 13; λ=0, -1) clusters has been investigated systematically using density functional theory. The comparisons between the Franck-Condon factor simulated spectra and the measured photoelectron spectroscopy (PES) of Cui and co-workers help to search for the ground-state structures. The results showed that D2 molecule tends to be dissociated on aluminum clusters and forms the radial AlD bond with one aluminum atom. By studying the evolution of the binding energies, second difference energies and HOMO-LUMO gaps as a function of cluster size, we found Al2D2, Al6D2 and Al7D2(̄) clusters have the stronger relative stability and enhanced chemical stability. Also, considering the larger adsorption energies of these three clusters, we surmised that Al2, Al6 and Al7(̄) may be the better candidates for dissociative adsorption of D2 molecule among the clusters we studied. Furthermore, the natural population analysis (NPA) and difference electron density were performed and discussed to probe into the localization of the charges and reliable charge-transfer information in AlnD2 and AlnD2(̄) clusters.

Probing the geometries, relative stabilities, and electronic properties of neutral and anionic Ag(n)S(m) (n + m ≤ 7) clusters.

The geometry structures, relative stabilities, and electronic properties of neutral and anionic Ag(n)S(m) (n + m ≤ 7) clusters have been investigated systematically by means of density function theory (DFT). The results of geometry optimization show that the most stable configurations of binary Ag(n)S(m)°/⁻ clusters have an early appearance of 3D structure at n = 3, m = 1, differing from those of pure silver and sulfur clusters. Moreover, the ground-state structures prefer low spin multiplicity (singlet or doublet) except for S2, Ag2S3, Ag2S4, Ag4S3, and Ag2S5. The calculated electron detachment energies (both vertical and adiabatic) are in good agreement with experimental data. This further lends considerable credence for the lowest-energy structures and the chosen computational method. By calculating the binding energies, fragmentation energies, second-order difference of energies and HOMO-LUMO energy gaps of neutral and anionic Ag n S m clusters, the relative stability and electronic property as a function of cluster size are discussed in detail. Further, in order to understand the nature of the bond in doped clusters and pure clusters, we have performed the contour maps of their HOMOs and analyzed their composition.

Structural and relative stabilities, electronic properties, and hardness of iron tetraborides from first prinicples.

First-principles calculations were carried out to investigate the structure, phase stability, electronic property, and roles of metallicity in the hardness for recently synthesized FeB4 with various different structures. Our calculation indicates that the orthorhombic phase with Pnnm symmetry is the most energetically stable one. The other four new dynamically stable phases belong to space groups monoclinic C2/m, orthorhombic Pmmn, trigonal R3̅m, and hexagonal P63/mmc. Their mechanical and thermodynamic stabilities are verified by calculating elastic constants, formation enthalpies, and phonon dispersions. We found that all phases are stabilized further under pressure. Above the pressure of about 50 GPa, the formation enthalpy of Pmmn is almost equal to that of P63/mmc phase. The analysis on density of states not only demonstrates that formation of strong covalent bonding in these compounds contributes greatly to their stabilities but also that they all exhibit metallic behavior which does not relate to the approach used. By considering metallic contributions, the estimated Vickers hardness values based on the semiempirical model show that the OsB4-structured FeB4, with a hardness of 48.1 GPa, well exceeding the limitation of superhardness (40 GPa), is more hard than the most stable phase. The others are predicted to be potential hard materials. Moreover, the atomic configuration and strong B-B covalent bonds are found to play important roles in the hardness of materials.

Phase stability, mechanical properties, hardness, and possible reactive routing of chromium triboride from first-principle investigations.

The first-principles calculations are employed to provide a fundamental understanding of the structural features and relative stability, mechanical and electronic properties, and possible reactive route for chromium triboride. The predicted new phase of CrB3 belongs to the rhombohedral phase with R-3m symmetry and it transforms into a hexagonal phase with P-6m2 symmetry at 64 GPa. The mechanical and thermodynamic stabilities of CrB3 are verified by the calculated elastic constants and formation enthalpies. Also, the full phonon dispersion calculations confirm the dynamic stability of predicted CrB3. Considering the role of metallic contributions, the calculated hardness values from our semiempirical method for rhombohedral and hexagonal phases are 23.8 GPa and 22.1 GPa, respectively. In addition, the large shear moduli, Young's moduli, low Poisson's ratios, and small B∕G ratios indicate that they are potential hard materials. Relative enthalpy calculations with respect to possible constituents are also investigated to assess the prospects for phase formation and an attempt at high-pressure synthesis is suggested to obtain chromium triboride.

Probing the structural, bonding, and magnetic properties of cobalt coordination complexes: co-benzene, co-pyridine, and co-pyrimidine.

Neutral and anionic Co1,2(benzene)1,2, Co1,2(pyridine)1,2, and Co1,2(pyrimidine)1,2 complexes have been investigated within the framework of an all-electron gradient-corrected density functional theory. The ground-state structures for each size clusters were identified based on the geometry optimization. Meanwhile, their electron affinities and vertical detachment energies were predicted and compared with the experimental values. By analyzing the pattern of highest occupied molecular orbitals (HOMOs), we found that the bond formation of these Co-organic complexes mainly arises from the 3d/4s electrons of the cobalt atoms and the π-cloud of the organic molecules. More importantly, we presented an approach to map and analyze the Co-organic interactions from another perspective. The scatter plots of the reduced density gradient (RDG) versus ρ allow us to identify the different types of interactions, and the maps of the gradient isosurfaces show a rich visualization of chemical bond and steric effects. Their magnetic properties were studied by determining the spin magnetic moments and visualizing the spin density distributions. Finally, the natural population analysis (NPA) charge was calculated to achieve a deep insight into the distribution of electron density and the reliable charge-transfer information.

Formation and properties of iron-based magnetic superhalogens: a theoretical study.

In order to explore new magnetic superhalogens, we have systematically investigated the structures, electrophilic properties, stabilities, magnetic properties, and fragmentation channels of neutral and anionic Fe(m)F(n) (m = 1, 2; n = 1-7) clusters using density functional theory. Our results show that a maximum of six F atoms can be bound atomically to one Fe atom, and the Fe-Fe bonding is not preferred in Fe2F(n)(0/-) clusters. The computed electron affinities (EAs) indicate that FeF(n) with n ≥ 3 are superhalogens, while Fe2F(n) can be classified as superhalogens for n ≥ 5. To further understand their superhalogen characteristic, the natural population analysis charge distribution and the HOMOs of anionic clusters were also analyzed. When the extra negative charge and the content of HOMO are mainly located on F atoms, the clusters could be classified as superhalogens with EAs substantially surpass that of Cl. By calculating the binding energies per atom and the HOMO-LUMO gaps, FeF3, FeF4(-), Fe2F4, Fe2F5(-), and Fe2F7(-) clusters were found to have higher stabilities, corresponding to the Fe atoms that are attained at their favorite +2 and +3 oxidation states. Furthermore, we also predicted the most preferred fragmentation channel and product for all the ground state clusters. Even more striking is the fact that both neutral and anionic Fe(m)F(n) (m = 1, 2; n = 1-7) clusters carry large magnetic moments which mainly come from 3d orbital of iron atom.

Structures, electrophilic properties, and hydrogen bonds of cytidine, uridine, and their radical anions: Microhydration effects.

Structures, electrophilic properties, and hydrogen bonds of the neutral and anionic monohydrated nucleoside, (cytidine)H2O, and (uridine)H2O have been systematically investigated using density functional theory. Various water-binding sites were predicted by explicitly considering the optimized monohydrated structures. Meanwhile, predictions of electron affinities and vertical detachment energies were also carried out to investigate their electrophilic properties. By examining the singly occupied molecular orbital and natural population analysis, we found the excess negative charge is localized on the cytidine and uridine moiety in anionic monohydrates. This may be the reason why the strength of hydrogen bonding undergoes an obvious change upon the extra electron attachment. Based on the electron density (ρ) and reduced density gradient (RDG), we present an approach to map and analyze the weak interaction (especially hydrogen bond) in monohydrated cytidine and uridine. The scatter plots of RDG versus ρ allow us to identify the different type interactions. Meanwhile, the maps of the gradient isosurfaces show a rich visualization of hydrogen bond, van der Waals interaction, and steric effect.