Pressure induced phase transitions of bulk CsGeCl3 and ultrafast laser pulse induced excited-state properties of CsGeCl3 quantum dots.

First-principles calculations are carried out to investigate the structural, electronic, and optical properties of CsGeCl3. The results indicate that CsGeCl3 undergoes three structural phase transitions from Cm or R3m to Pm3̄m at 8.5 GPa, from Pm3̄m to ppPv-Pnma at 9.4 GPa, and from ppPv-Pnma to I4mm at 64 GPa, respectively. Meanwhile, the relation between the band gap and pressure implies that the band gap value of ppPv-Pnma is 1.56 eV at 40 GPa, making it a potential photovoltaic material. Based on pressure-induced stable structures, the CsGeCl3 quantum dots (QDs) have been fabricated to investigate the excited-state properties by tuning ultrafast laser pulses based on time-dependent density functional theory (TDDFT). The excited-state properties show that CsGeCl3 QDs have a wider absorption range compared with their bulk materials and their optical responses can be regulated by changing the laser intensity and wavelength. Our results further reveal that the R3m-QDs exhibit excellent optical performance and have potential applications in optoelectronic devices.

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.

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.

Prediction of a novel magnetoelectric switching mechanism in multiferroics.

We report a first-principles study of the recently predicted Pmc21 phase of the multiferroic BiFeO3 material, revealing a novel magnetoelectric effect that makes it possible to control magnetism with an electric field. The effect can be viewed as a two-step process: Switching the polarization first results in the change of the sense of the rotation of the oxygen octahedra, which in turn induces the switching of the secondary magnetic order parameter. The first step is governed by an original trilinear-coupling energy between polarization, octahedral tilting, and an antiferroelectric distortion. The second step is controlled by another trilinear coupling, this one involving the predominant and secondary magnetic orders as well as the oxygen octahedral tilting. In contrast with other trilinear-coupling effects in the literature, the present ones occur in a simple ABO3 perovskite and involve a large polarization.

Highly-efficient heterojunction solar cells based on 2D Janus transition-metal nitride halide (TNH) monolayers with ultrahigh carrier mobility.

Symmetry breaking has a crucial effect on electronic band structure and subsequently affects the light-absorption coefficient of monolayers. We systematically report a family of two-dimensional (2D) Janus transition-metal nitride halides (TNHs, T = Ti, Zr, Hf, Fe, Pd, Pt, Os, and Re; H = Cl and F) with breaking of both in-plane and out-of-plane structural symmetry. The dynamical, thermal and mechanical stabilities are calculated to check the stability of the Janus TNHs. The electric properties of ten TNHs are studied via the HSE06+SOC method and the band gaps range from 0.93 eV (PdNCl) to 4.74 eV (HfNCl). Desirable light adsorption coefficients of up to 105 cm-1 are obtained for the Janus TNHs with no central symmetry. The Janus OsNCl monolayer shows excellent electrical transport properties and ultrahigh carrier mobility (104 cm2 V-1 s-1). Heterojunctions formed by stacking two Janus TNH monolayers are further investigated for solar cell applications. Eight of the heterojunctions have type-II band alignments. Surprisingly, the OsNCl/FeNCl heterojunction has a power conversion efficiency (PCE) of 23.45%, which is a larger value compared to the PCE of GeSe/SnSe heterostructures (21.47%). The optical properties and the built-in electric field of the OsNCl/FeNCl heterojunction are investigated. These results indicate that the stable Janus TNH monolayers have potential applications in photoelectric devices, and the vertical heterojunctions can be used in solar cells.

Strain-Induced Structural Phase Transitions in Epitaxial (001) BiCoO3 Films: A First-Principles Study.

We have simulated BiCoO3 films epitaxially grown along (001) direction with density functional theory computations. Leading candidates for the lowest-energy phases have been identified. The tensile strains induce magnetic phase transition in the ground state (P4mm symmetry) from a C-type antiferromagnetic order to a G-type order for the in-plane lattice parameter above 3.922 Å. The G-type antiferromagnetic order will be maintained with larger tensile strains; however, a continuous structural phase transition will occur, combining the ferroelectric and antiferrodistortive modes. In particular, the larger tensile strain allows an isostructural transition, the so-called Cowley's ''Type Zero'' phase transitions, from Cc-(I) to Cc-(II), with a slight volume collapse. The orientation of ferroelectric polarization changes from the out-of-plane direction in the P4mm to the in-plane direction in the Pmc21 state under epitaxial tensile strain; meanwhile, the magnetic ordering temperature TN can be strikingly affected by the variation of misfit strain.

Laser-Driven Insulator-Metal Phase Transitions in CsPbI3 Quantum Dots and Influence of Doped Metal Nanowires.

All-inorganic CsPbI3 perovskite quantum dots (QDs) have received extensive attention in developing optoelectronic devices due to their outstanding properties. Here, using time-dependent density functional theory (TDDFT), the optical properties of the three distinct phases (α, γ, and δ) of the CsPbI3 QDs are investigated. Surprisingly, the δ phase structured QDs exhibit stronger optical absorption properties than the α and γ phase QDs when exposed to equivalent laser irradiation. Considering the quantum size effect, size regulation is also performed on the three structures, the results reveal a significant improvement in optical properties as the size increases in the direction of laser irradiation. More interestingly, Ag-hybrid QDs show better optical gain and maintain a laser-driven metallic state. Our results demonstrate the great potential of size adjustment and metal nanowire coupling in improving the optoelectronic properties of QDs and developing efficient photovoltaic devices.

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.

Theoretical study on local defect structure of (FeO4)(5-) clusters in YGG and LGG crystals.

The optical spectrum and the local defect structure of tetrahedral (FeO4)(5-) clusters are investigated in yttrium gallium garnet (YGG) and lutetium gallium garnet (LGG) crystals by means of the 252x252 complete energy matrices for d(5) configuration ions in tetragonal ligand field. The results show that the local defect structures around tetrahedral Fe3+ centers display an expansion effect. Simultaneously, the (FeO4(5-) clusters in the two different crystals have very similar local structures, which are close to those in YIG garnet crystal. Finally the relative curves of the zero-field splitting energies DeltaE1 and DeltaE2 in the ground state (6)A(1) varying with the structure parameter theta are plotted.

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.

Tetragonal distortion of structural defects in Cr3+ doped in several perovskites calculated from the EPR and optical data: a complete energy matrix study.

This paper describes a novel application of a ligand field model in the study of the local molecular structure of the (CrF 6) (3-) coordination complex. Based on the ligand field model, the complete energy matrix which contains the electron-electron repulsion interaction, the ligand field interaction, the spin-orbit coupling interaction, and the Zeeman interaction, has been constructed for a d (3) configuration ion in a tetragonal ligand field. In order to study the relation between the EPR, the optical spectra, and the local lattice structures around the centers with tetragonal symmetry in AMF 3 codoped with Cr (3+) and Li (+) ions, a three-layer-ligand model is proposed. By diagonalizing the complete energy matrix and employing the three-layer-ligand model, the variational ranges of the local structural parameters around the Cr (3+) ions are determined, respectively. The results show that the local lattice structures around the Cr (3+) ions in AMF 3 exhibit a compressed distortion, and the magnitude of distorted parameter Delta R 1 of the Cr (3+)-V M center is different from that of the Cr (3+)-Li (+) center in AMF 3. The compressed distortion is ascribed to the fact that the radius of the Cr (3+) ion is smaller than those of M (2+) (M = Cd, Mg, Zn). Moreover, a linear correlation between the difference in the magnitude of distortion parameterDelta R for two different defect centers and the difference in the corresponding values of the zero-field-splitting parameter Delta D are found first.

Theoretical study of EPR spectra for Mn2+ ion in [Mg(H2O)6]SnCl6 single crystal.

The relationship between the impurity structures and the electron paramagnetic resonance (EPR) parameters D, (a-F) have been studied by diagonalizing the complete energy matrices for Mn2+ ion in [Mg(H2O)6]SnCl6 single crystal in a trigonal ligand field within a weak-field-representation. It is shown that the local lattice structure around Mn2+ ion in [Mg(H2O)6]SnCl6 exhibits an elongation distortion which is different at 290 K and 77 K. The local structure parameters R=2.223+/-0.027A, theta=52.966+/-0.004 degrees and R=2.205+/-0.030A, theta=53.155+/-0.047 degrees for Mn2+ ion in [Mg(H2O)6]SnCl6 are determined at different temperatures 290 K and 77 K, respectively, and EPR parameters D and (a-F) can also get a satisfactory explanation simultaneously.

Creating multiferroics with large tunable electrical polarization from paraelectric rare-earth orthoferrites.

The quest for materials possessing both a magnetic ordering temperature above room temperature and a large electrical polarization is an important research direction in order to design novel spintronic and memory devices. Up to now, BiFeO3 and related systems are the only known compounds simultaneously possessing such characteristics. Here, first-principles calculations predict that another family of materials, namely epitaxial films made of rare-earth orthoferrites (RFeO3), can also exhibit such desired features. As a matter of fact, applying a large enough strain to these compounds, which are nominally paraelectric and have a high magnetic transition temperature, is predicted to render them ferroelectric, and thus multiferroic. At high compressive strain, the resulting ferroelectric phase of RFeO3 systems having large rare-earth ions is even a tetragonal state characterized by a giant polarization and axial ratio. For large tensile strain, two striking inhomogenous ferroelectric phases--including one never observed before in any perovskite--are further predicted as having significant polarization. A multiphase boundary also occurs, which may lead to optimization of properties or unusual features. Finally, many quantities, including electrical polarization and magnetic ordering temperature, are tunable by varying the epitaxial strain and/or chemical pressure.

Studies of EPR spectra and defect structure for Er³âº ions in BaF2 and SrF2 crystals.

The local lattice structure and electron paramagnetic resonance (EPR) spectra have been studied systematically by diagonalizing 364 × 364 complete energy matrices for a f(11) ion in a trigonal ligand-field. By simulating the calculated Stark levels and EPR parameters to the experimental results, the shift parameters are determined for Er(3+) ions in BaF(2) and SrF(2). The results show that the trigonal center is attributed to an interstitial F(-) ion located at the [111] axis of the cube, and the nearest ligand close to the charge compensator has a displacement towards central ion by 0.042Å for L center in BaF(2):Er(3+) and 0.026Å for J center in SrF(2):Er(3+), respectively. Moreover, the relationships between g-factors and shift parameter ΔZ as well as orbit reduction factor k' are discussed.

A DFT study on equilibrium geometries, stabilities, and electronic properties of small bimetallic Na-doped Au(n) (n = 1-9) clusters: comparison with pure gold clusters.

A systematic study on the geometric structures, relative stabilities, and electronic properties of small bimetallic Au(n)Na (n = 1-9) clusters has been performed by means of first-principle density functional theory calculations at the PW91PW91 level. The results show that the optimized ground-state isomers adopt planar structures up to n = 5, and the Na-capped geometries are dominant growth patterns for n = 6-9. Dramatic odd-even alternative behaviors are obtained in the second-order difference of energies, fragmentation energies, highest occupied-lowest unoccupied molecular orbital energy gaps, and chemical hardness for both Au(n)Na and Au(n+1) clusters. It is found that Au(5)Na and Au(6) have the most enhanced stability. Here, the size evolutions of the theoretical ionization potentials are in agreement with available experimental data, suggesting a good prediction of the lowest energy structures in the present study. In addition, the charge transfer has been analyzed on the basis of natural population analysis.

Density functional study on size-dependent structures, stabilities, electronic and magnetic properties of Au(n)M (M = Al and Si, n = 1-9) clusters: comparison with pure gold clusters.

The density functional theory (DFT) method has been employed to systematically investigate the geometrical structures, relative stabilities, and electronic and magnetic properties of Au(n)M (M = Al and Si, n = 1-9) clusters for clarifying the effect of Al(Si) modulation on the gold nanostructures. Of all the clusters studied, the most stable configurations adopt a three-dimensional structure for Au(n)Al at n = 4-8 and Au(n)Si at n = 3-9, while for pure gold systems, no three-dimensional lowest energy structures are obtained. Through a careful analysis of the fragmentation energy, second-order difference of energy, HOMO-LUMO energy gap, and magnetic moment as a function of cluster size, an odd-even alternative phenomenon has been observed. The results show that the clusters with even-number valence electrons have a higher relative stability, but lower magnetic moments. Furthermore, Al(Si) doping is found to enhance the stabilities of gold frameworks. In addition, the charge analysis has been given to understand the different effects of individual doped atom on electronic properties and compared further.

EPR theoretical study of local molecular structure and thermal expansion coefficient for octahedral Mn2+ centers in zinc fluosilicate.

A theoretical method for studying the inter-relation between electronic and molecular structure has been proposed by diagonalizing the complete energy matrices for a d(5) configuration ion in a trigonal ligand field and considering the second-order and fourth-order EPR parameters D and (a - F) simultaneously. As for ZnSiF(6).6H(2)O:Mn(2+) and ZnSiF(6).6D(2)O:Mn(2+) complex molecules, the local lattice distortion and local thermal expansion coefficient for the octahedral Mn(2+) centers in zinc fluosilicate have been investigated, respectively. The calculations indicate that the local lattice structure around an octahedral Mn(2+) center has an expansion distortion, whether the Mn(2+) ion is doped in ZnSiF(6).6H(2)O or ZnSiF(6).6D(2)O. Moreover, the total tendency of the local lattice expansion distortion will be more and more obvious with the temperature rising, apart from some slight variations at T = 60 K for the ZnSiF(6).6H(2)O. By simulating the two low-symmetry EPR parameters D and (a - F) simultaneously, the local lattice structure parameters R and theta have been determined to vary from 2.204 Angstroms to 2.256 Angstroms and from 53.417 degrees to 52.710 degrees, respectively, in the temperature range 19-297 K for ZnSiF(6).6H(2)O:Mn(2+) and to vary from 2.215 Angstroms to 2.255 Angstroms and from 53.346 degrees to 52.714 degrees, respectively, in the temperature range 50-300 K for ZnSiF(6).6D(2)O:Mn(2+). Subsequently the dependence of local thermal expansion coefficients on the temperature is studied and the corresponding theoretical values of the local thermal expansion coefficients are reported firstly. Some characteristics of local thermal expansion coefficients of Mn(2+) in ZnSiF(6).6H(2)O:Mn(2+) and ZnSiF(6).6D(2)O:Mn(2+) systems are also analyzed.