Orthorhombic intermediate state in the zinc blende to rocksalt transformation path of SiC at high pressure

Pressure-Induced Phase Transitions in Bismutotantalite (BiTaO4): Insights from Single-Crystal Diffraction and Raman Spectroscopy

In situ high-pressure single-crystal X-ray diffraction and Raman spectroscopy analyses were performed on a natural bismutotantalite with an α-BiTaO4 structure. The results indicate that α-BiTaO4 transforms into an orthorhombic phase (HP γ-BiTaO4), likely through an intermediate orthorhombic phase (HP β-BiTaO4). The transition pressures are 11.0-11.8 GPa for α-BiTaO4 → HP β-BiTaO4 and 13.9-14.4 GPa for HP β-BiTaO4 → HP γ-BiTaO4 transition. The phase transitions are reversible. Although the structure of HP β-BiTaO4 was not successfully solved, the possible space group was determined to be Fmm2, with unit-cell parameters calculated at 12.2 GPa: a = 4.8158(18) Å, b = 33.8880(80) Å, c = 5.2910(5) Å. In contrast, the structure of HP γ-BiTaO4 was successfully solved and refined at 28.0 GPa, revealing a Pnma space group and unit-cell parameters of a = 9.8999(12) Å, b = 5.0435(16) Å, c = 10.8331(8) Å. The significant volume collapse of 6.2% through the phase transition and the increase in coordination numbers of Bi and Ta from 6 in α-BiTaO4 to 8/9 in HP γ-BiTaO4 indicate that the HP γ-BiTaO4 structure is considerably more compacted. Additionally, the equation of state for both α-BiTaO4 and HP γ-BiTaO4 was also studied.

Surface Reconstruction and Layer-Dependent Semiconductor-to-Metal Transition of Zinc-Blende CdSe

In this work, CdSe was taken as the representation to systematically investigate the (111) and (110) surface reconstructions, the electronic properties transition related to the layer size, and the corresponding physical mechanism through the density functional theory (DFT) calculation. For the (111) surface slab structure, the bulk truncated relaxation (BTR) surface and the honeycomb (HC) surface were carefully examined. The HC surface configuration, ignored by previous studies, is an energetically preferred surface compared to both the as-truncated and BTR configurations. Based on the HC surface, the band structure of the (111) surface shows a semiconductor character below four layers (4L). Surprisingly, the (111) CdSe turns metallic in the 4L system. In a higher-layer (>4L) system, the two side surfaces and internal regions show metallic and semiconductivity features, respectively. Such an abundant electronic properties transition should be attributed to the electron transfer under the intrinsic polarization perpendicular to the asymmetrical (111) plane. Different from the (111) surface, drastic structural reconstructions were not observed in the (110) surface and the band gap gradually decreased with the increasing number of layers until it approached the value in the bulk. Our results not only revealed the additional possible surface structure but also clarified the underlying mechanism of semiconductor-to-metal (even the edge metallic) transition related to the number of layers. All these findings could be extended to other II-VI group MX compounds for further development of electronic devices.

Strain-Dependent Effects on Confinement of Folded Acoustic and Optical Phonons in Short-Period (XC)m/(YC)n with X,Y (≡Si, Ge, Sn) Superlattices

Carbon-based novel low-dimensional XC/YC (with X, Y ≡ Si, Ge, and Sn) heterostructures have recently gained considerable scientific and technological interest in the design of electronic devices for energy transport use in extreme environments. Despite many efforts made to understand the structural, electronic, and vibrational properties of XC and XxY1-xC alloys, no measurements exist for identifying the phonon characteristics of superlattices (SLs) by employing either an infrared and/or Raman scattering spectroscopy. In this work, we report the results of a systematic study to investigate the lattice dynamics of the ideal (XC)m/(YC)n as well as graded (XC)10-∆/(X0.5Y0.5C)∆/(YC)10-∆/(X0.5Y0.5C)∆ SLs by meticulously including the interfacial layer thickness ∆ (≡1-3 monolayers). While the folded acoustic phonons (FAPs) are calculated using a Rytov model, the confined optical modes (COMs) and FAPs are described by adopting a modified linear-chain model. Although the simulations of low-energy dispersions for the FAPs indicated no significant changes by increasing ∆, the results revealed, however, considerable "downward" shifts of high frequency COMs and "upward" shifts for the low energy optical modes. In the framework of a bond polarizability model, the calculated results of Raman scattering spectra for graded SLs are presented as a function of ∆. Special attention is paid to those modes in the middle of the frequency region, which offer strong contributions for enhancing the Raman intensity profiles. These simulated changes are linked to the localization of atomic displacements constrained either by the XC/YC or YC/XC unabrupt interfaces. We strongly feel that this study will encourage spectroscopists to perform Raman scattering measurements to check our theoretical conjectures.

Insight into the structural, elastic and electronic properties of a new orthorhombic 6O-SiC polytype

Different polytypes of SiC are described and predicted in literature. Here, we report the first occurrence of an orthorhombic 6O-SiC polytype as rock-forming mineral in the nickel laterite mine of Tiebaghi (New Caledonia). This new class of SiC crystallizes in the space group Cmc2₁ with 12 atoms per unit cell [a = 3.0778(6) Å, b = 5.335(2) Å, c = 15.1219(6) Å, α = 90°, β = 90°, γ = 120°]. The density of 6O-SiC is about 3.22 g/cm³ and the calculated indirect bandgap at room temperature of 3.56 eV is identical to 6H-SiC. Our results suggest that 6O-SiC is the intermediate state in the wurtzite to rocksalt transformation of 6H-SiC.

Temperature-induced phase transitions in the rock-salt type SiC: a first-principles study

The phase transitions in the rock-salt type SiC (B1-SiC) under decompression are studied in the framework of first-principles molecular dynamics simulations up to room temperature. The transformation pathways were determined based on an analysis of the symmetry and phonon spectra of high-symmetry transient structures identified in the simulations. The plausible pathways of the transformation ofB1-SiC into the 3C-, 2H-, 4H-, 12R-SiC polytypes were suggested. The transformation paths were found to depend on both the availability of soft phonon modes in an unreconstructed phase and the initial conditions of the simulation. It is shown that an increase in cell volume at decompression leads to the condensation of a certain phonon mode. As a result, an intermediate state forms due to the atomic displacements and to subsequent strains related to this mode. All the decompressed structures were compressed back under pressure of 120-250 GPa depending on the type of the decompressed phase and simulation temperature that was in the range of 300-1200 K. The suggested scheme of structural identification can be used to determine the transition paths for the structural transformations of other similar structures under pressure.

Hysteresis and bonding reconstruction in the pressure-induced B3-B1 phase transition of 3C-SiC

The determination of kinetic factors affecting phase metastability is crucial for the design of materials out of the ambient conditions. At a given temperature, the kinetic barrier associated with the reconstruction of the bonding network of a pressure-induced phase transition can be only overcome at pressures where the available vibrational energy of the system is equal or higher than the corresponding activation energy. Our work demonstrates that these pressures provide boundaries to hysteresis cycles that can be evaluated following a three-step computational strategy: (i) total energy electronic structure calculations, (ii) determination of vibrational contributions by means of a simple Debye model, and (iii) description of the energetic profile along the transition path in the framework of the martensitic approximation. In the 3C-SiC polytype, our results reveal that the high pressure rock-salt (B1) structure can not be quenched on release of pressure unless temperature is close to 0 K. The B1 phase transforms back to the low-pressure zinc blende (B3) polymorph at 300 K if pressure is below 30 GPa, in very good agreement with experimental observations. These results are supported by a full characterization of the B3-B1 energetic transition profile in terms of the chemical changes of the bonding network topologically analysed with the electron localization function.

Computer simulations of 3C-SiC under hydrostatic and non-hydrostatic stresses

Uniaxial [001] stress induces a semiconductor–metal transition in 3C-SiC.

Phase transformation of cadmium sulfide under high temperature and high pressure conditions

Cadmium sulfide (CdS) is one of the most significant wide band gap semiconductors, and knowledge of the phase transformation of CdS under high temperature and pressure is especially important for its applications. The pressure-temperature phase diagram and the phase transformation pathways of CdS have been investigated by using density functional theory combined with quasiharmonic approximation. Our results indicated that under ambient conditions, wz-CdS is a stable phase, while under high temperature and pressure, rs-CdS becomes the stable phase. It is also found that zb-CdS is an intermediate phase in transforming from rs-CdS to wz-CdS. Therefore, although there are no zb-CdS phase regions in the CdS pressure-temperature phase diagram, zb-CdS can be found in some prepared experiments.

An environment-dependent interatomic potential for silicon carbide: calculation of bulk properties, high-pressure phases, point and extended defects, and amorphous structures

An interatomic potential has been developed to describe interactions in silicon, carbon and silicon carbide, based on the environment-dependent interatomic potential (EDIP) (Bazant et al 1997 Phys. Rev. B 56 8542). The functional form of the original EDIP has been generalized and two sets of parameters have been proposed. Tests with these two potentials have been performed for many properties of SiC, including bulk properties, high-pressure phases, point and extended defects, and amorphous structures. One parameter set allows us to keep the original EDIP formulation for silicon, and is shown to be well suited for modelling irradiation-induced effects in silicon carbide, with a very good description of point defects and of the disordered phase. The other set, including a new parametrization for silicon, has been shown to be efficient for modelling point and extended defects, as well as high-pressure phases.

New transformation mechanism for a zinc-blende to rocksalt phase transformation in MgS

The stability of the zinc-blende structured MgS is studied using a constant pressure ab initio molecular dynamics technique. A phase transition into a rocksalt structure is observed through the simulation. The zinc-blende to rocksalt phase transformation proceeds via two rhombohedral intermediate phases within R3m (No:160) and [Formula: see text] (No:166) symmetries and does not involve any bond breaking. This mechanism is different from the previously observed mechanism in molecular dynamics simulations.

CRYSTAL: a computational tool for the ab initio study of the electronic properties of crystals

CRYSTAL [1] computes the electronic structure and properties of periodic systems (crystals, surfaces, polymers) within Hartree-Fock [2], Density Functional and various hybrid approximations.

CRYSTAL was developed during nearly 30 years (since 1976) [3] by researchers of the Theoretical Chemistry Group in Torino (Italy), and the Computational Materials Science group in CLRC (Daresbury, UK), with important contributions from visiting researchers, as documented by the main authors list and the bibliography.

The basic features of the program CRYSTAL are presented, with two examples of application in the field of crystallography [4, 5].

Putting the squeeze on NaCl: modelling and simulation of the pressure driven B1-B2 phase transition

We used a geometric approach to derive models for the transition B1 (NaCl) to B2 (CsCl) type structure. This enabled us to construct several dynamical transition states, corresponding to different mechanistic pathways. From this, transition trajectories were obtained, which served as starting points for path sampling molecular dynamics simulations. We point out the difference of this approach from approaches based on energy calculations, and demonstrate that a preferred mechanism can be clearly discriminated.

Ab initio molecular dynamics simulation of a pressure induced zinc blende to rocksalt phase transition in SiC

The high-pressure induced phase transformation from the zinc blende to rocksalt structure in SiC has been studied by the ab initio molecular dynamics method. The simulations showed that SiC passes through a tetragonal intermediate state before transforming to a monoclinic phase at 160 GPa. The mechanism for this phase transformation agrees well with recent ab initio MD simulations, in which the applied pressure was as high as ∼600 GPa, but in the present study the transformation occurs at much lower pressure.

Thickness-Dependent Phase Transition of AlxGa1-xN Thin Films on Strained GaN

We report our investigation of phase transition of AlxGa1-xN thin films on GaN made by employing first-principles calculations. A critical thickness of two AlGaN molecular layers is determined for the wurtzite-to-zinc blende structural transition under compressive strains, which is associated with the second-nearest-neighbor interaction of electron bonds. Higher AlN mole fractions are found to favor the phase transition because of strong push toward covalency of the Al-N bonds under strains. Electronic structure results show that, after the phase transition, the spontaneous and piezoelectric polarizations of the AlGaN films are significantly reduced.

Multimillion Atom Simulations of Dynamics of Oxidation of an Aluminum Nanoparticle and Nanoindentation on Ceramics

We have developed a first-principles-based hierarchical simulation framework, which seamlessly integrates (1) a quantum mechanical description based on the density functional theory (DFT), (2) multilevel molecular dynamics (MD) simulations based on a reactive force field (ReaxFF) that describes chemical reactions and polarization, a nonreactive force field that employs dynamic atomic charges, and an effective force field (EFF), and (3) an atomistically informed continuum model to reach macroscopic length scales. For scalable hierarchical simulations, we have developed parallel linear-scaling algorithms for (1) DFT calculation based on a divide-and-conquer algorithm on adaptive multigrids, (2) chemically reactive MD based on a fast ReaxFF (F-ReaxFF) algorithm, and (3) EFF-MD based on a space-time multiresolution MD (MRMD) algorithm. On 1920 Intel Itanium2 processors, we have demonstrated 1.4 million atom (0.12 trillion grid points) DFT, 0.56 billion atom F-ReaxFF, and 18.9 billion atom MRMD calculations, with parallel efficiency as high as 0.953. Through the use of these algorithms, multimillion atom MD simulations have been performed to study the oxidation of an aluminum nanoparticle. Structural and dynamic correlations in the oxide region are calculated as well as the evolution of charges, surface oxide thickness, diffusivities of atoms, and local stresses. In the microcanonical ensemble, the oxidizing reaction becomes explosive in both molecular and atomic oxygen environments, due to the enormous energy release associated with Al-O bonding. In the canonical ensemble, an amorphous oxide layer of a thickness of approximately 40 angstroms is formed after 466 ps, in good agreement with experiments. Simulations have been performed to study nanoindentation on crystalline, amorphous, and nanocrystalline silicon nitride and silicon carbide. Simulation on nanocrystalline silicon carbide reveals unusual deformation mechanisms in brittle nanophase materials, due to coexistence of brittle grains and soft amorphous-like grain boundary phases. Simulations predict a crossover from intergranular continuous deformation to intragrain discrete deformation at a critical indentation depth.

Phase Equilibria and Transition Mechanisms in High-Pressure AgCl by Ab Initio Methods

The theoretical study of pressure-driven phase transformations by means of ab initio quantum mechanical methods, in the frame of the extended Landau approach, is considered. A specific application to AgCl is presented: the system shows, on increasing pressure, four polymorphs with rock salt- (Fmm), KOH- (P2(1)/m), TlI- (Cmcm), and CsCl- (Pmm) type structures. The method of constant-pressure enthalpy minimization was used for all phases, by fully relaxing the corresponding crystal structures. Periodic ab initio energy calculations were performed by the CRYSTAL03 code, employing a DFT-GGA-PBE functional with a localized basis set of Gaussian-type functions. The three phase transitions were predicted to occur at 3.5, 6.0, and 17.7 GPa, respectively, against pressures of 6.6, 10.8, and 17 GPa from literature experimental results. The rock salt- to KOH-type and KOH- to TlI-type displacive transformations show a weak first-order character. The TlI- to CsCl-type reconstructive transition is sharply first-order, and its kinetic mechanism was studied in detail on the basis of a P2(1)/m pathway, similar to that previously found for the rock salt- to CsCl-type transformation of NaCl. An activation enthalpy of 0.011 eV was found at the equilibrium pressure of 17.7 GPa.

Universal transition state for high-pressure zinc blende to rocksalt phase transitions

First-principles density functional calculations show that the high-pressure transitions of different semiconductors from zinc blende to rocksalt go through a transition state, which is universal in the sense that its position along the path and the corresponding geometry is independent of the chemical components of the semiconductor. This is explained using a Landau-like model expansion of the free energy in cosine functions of atomic position.

Pentagonal dodecahedranes: polyfunctionalization and MS fragmentation

With the use of four- to eight-fold functionalized dodecahedranes (1-3), opportunities to arrive at highly strained dodecahedranes with two to four pairs of vicinal, eclipsed bromine substituents through front-side substitution and addition reactions have been explored. In standard processes, the interception of beta-OCH(3) radical/cationic intermediates was not problematic (9-12, 37, 50). The interception of beta-CO(2)R radicals was possible for Cl(*) (18) but not for Br(*) (17). The interception of beta-chloro radicals was possible for Cl(*) (27) but not for Br(*) (26), and the interception of beta-Br cations ("bromonium ions") with Br(-) was modest (45) to highly inefficient (24, 26). Two X-ray structural analyses (dimethoxy dibromide 9 and tetramethoxy tetrabromide 53) indicated the structural consequences of the molecular strain introduced by the two (four) vicinal CH(3)O/Br pairs. A systematic analysis of the MS spectra confirms that, in virtually all cases studied, the elimination of the substituents occurs without significant carbon-cage disruption, leading ultimately to multiply unsaturated dodecahedral ions for dodecahedrahexa(C(20)H(8))enes, -hepta(C(20)H(6))enes, and -octa(C(20)H(4))enes.