BC8 Phase Research Articles

Extreme Metastability of Diamond and its Transformation to the BC8 Post-Diamond Phase of Carbon.

Diamond possesses exceptional physical properties due to its remarkably strong carbon-carbon bonding, leading to significant resilience to structural transformations at very high pressures and temperatures. Despite several experimental attempts, synthesis and recovery of the theoretically predicted post-diamond BC8 phase remains elusive. Through quantum-accurate multimillion atom molecular dynamics (MD) simulations, we have uncovered the extreme metastability of diamond at very high pressures, significantly exceeding its range of thermodynamic stability. We predict the post-diamond BC8 phase to be experimentally accessible only within a narrow high pressure-temperature region of the carbon phase diagram. The diamond to BC8 transformation proceeds through premelting followed by BC8 nucleation and growth in the metastable carbon liquid. We propose a double-shock compression pathway for BC8 synthesis, which is currently being explored in experiments at the National Ignition Facility.

Origin of preferred orientation in an isotropic material: High pressure synthesis of bc8-Si

High pressure experiments and ab initio calculations are used to investigate unexpected crystallographic preferred orientation in the bc8 phase of silicon formed under non-hydrostatic conditions. Microstructural characterization in two orthogonal directions reveals that the preferred orientation is only visible when the sample is viewed perpendicular to the compression axis. Curiously, the elastic constants of bc8-Si are almost perfectly isotropic, making it counter-intuitive that preferred crystallographic orientation is observed. This conundrum is resolved by tracking the phase transformation pathway and computing the three-dimensional Young's modulus. We find the preferred orientation most likely originates from the highly anisotropic simple-hexagonal phase and is passed on to subsequent daughter phases via displacive phase transformations. Our investigation of preferred orientation in bc8-Si complements other high pressure studies where preferred orientation in silicon phases is often observed but not explained.

Double-Shock Compression Pathways from Diamond to BC8 Carbon.

Carbon is one of the most important elements for both industrial applications and fundamental research, including life, physics, chemistry, materials, and even planetary science. Although theoretical predictions on the transition from diamond to the BC8 (Ia3[over ¯]) carbon were made more than thirty years ago, after tremendous experimental efforts, direct evidence for the existence of BC8 carbon is still lacking. In this study, a machine learning potential was developed for high-pressure carbon fitted from first-principles calculations, which exhibited great capabilities in modeling the melting and Hugoniot line. Using the molecular dynamics based on this machine learning potential, we designed a thermodynamic pathway that is achievable for the double shock compression experiment to obtain the elusive BC8 carbon. Diamond was compressed up to 584GPa after the first shock at 20.5 km/s. Subsequently, in the second shock compression at 24.8 or 25.0 km/s, diamond was compressed to a supercooled liquid and then solidified to BC8 in around 1ns. Furthermore, the critical nucleus size and nucleation rate of BC8 were calculated, which are crucial for nano-second x-ray diffraction measurements to observe BC8 carbon during shock compressions. The key to obtaining BC8 carbon lies in the formation of liquid at a sufficient supercooling. Our work provides a feasible pathway by which the long-sought BC8 phase of carbon can be reached in experiments.

Large-scale CMOS-compatible process for silicon nanowires growth and BC8 phase formation

A novel, low temperature process for the formation of Si-BC8 phase is obtained while growing Silicon nanowires. The nanowires growth is performed in a CVD reactor under exposure of the substrate to microwaves, employing Sn nanospheres as catalyst and a flux of SiH4 as precursor, respectively. Microwaves allow for selective heating of the metal catalyst while keeping the substrate at low temperature. At the end of the process, silicon nanowires with the metal sphere on top are obtained, together with the (unexpected) transition of a portion of silicon substrate from the diamond to the Si-BC8 crystallographic phase. Silicon atoms in Si-BC8 phase are arranged in body-centered-cubic unit cells resulting into a different energy-wavevector diagram compared to the silicon diamond cubic phase. Indeed, Si-BC8 possesses a direct band gap as low as 30 meV at room temperature. These features may be employed in a large variety of applications, requiring CMOS-compatible manufacturing.Systematic structural analysis and a phenomenological model for Si-BC8 phase formation are discussed.

Formation of metastable bc8 phase from crystalline Si0.5Ge0.5 by high-pressure torsion

Si0.5Ge0.5 crystals were subjected to severe plastic deformation using high-pressure torsion (HPT). A metastable body-centered-cubic phase having a bc8 structure was formed by HPT processing with anvil rotations. High-resolution transmission electron microscopy analysis revealed that nanograins having diamond-cubic (dc) and bc8 phases were present in the HPT-processed sample. The bc8 phase was reverse transformed back to the dc phase after annealing. The resistivity of the crystalline Si0.5Ge0.5 (~9 × 10−3 Ω·cm) increased by roughly a factor of 6 after compression, and remained on the same order of magnitude but decreased slightly after 10 anvil rotations despite the grain refinement. These results indicate that the electrical behavior of bc8-Si0.5Ge0.5 is similar to that of bc8-Si.

Colloidal Synthesis of an Exotic Phase of Silicon: The BC8 Structure

Creating allotropes and polymorphs of nanoparticles (NPs) has gained tremendous momentum in recent times. Group 14 (C, Si, Ge) has a number of allotropes; some with significant applications. Here we report the synthesis of Si NPs crystallizing in the BC8 structure via a colloidal route for the first time. The BC8 structure is a metastable structure of Si that can be accessed from the β-Sn form through the release of high pressure. These Si BC8 structured NPs were synthesized via reduction of SiI4 with n-butyllithium, capped with octanol and precipitated from solution. The transmission electron microscopy lattice fringes as well as the selected area electron diffraction pattern of the precipitate are consistent with the BC8 structure. The LeBail whole profile fitting of powder X-ray diffraction data also confirms the structure as the BC8 phase. The Raman spectrum provides further evidence to support the BC8 structure. With proper tuning of the band gap these NPs could be potential candidates for solar cells.

Extending the Density Functional Tight Binding Method to Carbon Under Extreme Conditions

We report herein on simulations of carbon under pressures up to 2000 GPa and 30 000 K using the density functional tight binding method (DFTB) with a parameter set we have specifically designed for these conditions. The DFTB method can provide a high throughput simulation capability compared to Kohn–Sham density functional theory while retaining most of its accuracy. We fit the DFTB repulsive energy to measured and computed diamond isothermal compression data and show that this yields accurate compression curves for diamond, graphite, and the BC8 phase, as well as material properties for all three phases. We then show that our new repulsive energy yields predictions of the Hugoniot of diamond shock compressed to the conducting liquid that are within the range of different experimental measurements. Our results provide a straightforward method by which DFTB can be extended to studies of covalently bonded materials under extremely high pressures and temperatures such as the interiors of planets and other la...

The bonding configuration in a partially relaxed pseudomorphic epilayer of SiGe: evidenceof the BC-8 phase of silicon

The bonding configuration of a silicon host lattice in a carbon-induced partially relaxedpseudomorphic epilayer of SiGe is studied using x-ray photoelectron spectroscopy,high-resolution XRD and Raman spectroscopic techniques. In-plane and out-of-planestrains are estimated using high-resolution x-ray diffraction. Careful analysis ofthe x-ray photoelectron spectra (XPS) suggests that the bonding environmentof the silicon host lattice is modified, and is attributed to the presence ofgraphite-like-sp2 bonding inaddition to the tetrahedral-sp3 bonding. These results are further supported by our Raman studies. Our Ramanstudies indicate the presence of the BC-8 phase; a high-pressure phase ofsilicon. The modified configuration of the silicon host lattice at high pressureis responsible for the observed changes in the XPS and Raman spectra. Theseresults are also compared with the carbon incorporated silicon epilayers(Si1−xCx) having negligible strain relaxation. We attribute these effects to the strain-induced effectsand not to the compositional effects of Ge.

Structures of the cubic and rhombohedral high-pressure modifications of silicon as packing of the rod-like substructures determined by the algebraic geometry

Tessellations by generating clusters are proposed for the high-pressure phases BC8 and R8 of silicon. The structures of both high-pressure phases are represented by the parallel packings of rods. The latter are stacks of distorted icosahedra, joined by a common triangular face and flattened out along the threefold symmetry axis. Along the rod axis there is an alternation of empty and double-centered icosahedra. The empty icosahedra are additionally distorted in the cubic BC8 phase by antiparallel rotation about the rod axis, while the double-centered icosahedra are distorted by that rotation in the rhombohedral R8 phase. A possible mechanism of the reversible BC8 <--> R8 transformation is proposed as the rotation about the rod axis of the common triangular face of the neighboring icosahedra, thus transforming between distorted and undistorted icosahedra. The graphs of the generating clusters for both the BC8 and R8 structures are determined by two subconfigurations of the same construction of the finite projective geometry.

Evidence for nanoindentation-induced phase transformations in germanium

Nanoindentation experiments were performed using Berkovich and cube-corner indenters to investigate whether nanoindentation-induced phase transformations, such as those observed in silicon, also occur in germanium. Although the indentation load-displacement curves for germanium do not show the unloading pop-out or elbow phenomena observed in silicon, clear evidence for phase transformations was obtained by scanning electron microscopy (SEM) and micro-Raman spectroscopy. SEM showed that there is extruded material around the contact periphery of cube-corner hardness impressions that is metalliclike in its flow characteristics, just as in silicon. Micro-Raman spectroscopy revealed more direct evidence by identifying amorphous and what may be the crystalline BC8 (Ge-IV) phase. The fact that these phenomena are observed primarily and reproducibly only for the cube-corner indenter suggests that the contact geometry significantly affects the transformation behavior. Results are discussed in terms of possible deformation mechanisms and how they may be influenced by the indenter geometry.

Contact resistance and phase transformations during nanoindentation of silicon

Abstract An electrically conductive VC tip has been used in conjunction with ex-situ transmission electron microscopy (TEM) and micro-Raman spectroscopy to examine the behaviour of silicon during nanoindentation testing. During loading, the contact resistance is low because of the presence of the metallic β-Sn phase of silicon. During unloading, the semimetallic BC8 phase and the rhombohedral R8 phase are formed, giving rise to an increase in the contact resistance. In large indentations there is a pronounced discontinuity in both the contact resistance-depth and load-depth curves during unloading. This signifies the formation of a phase with lower electrical resistance. TEM analysis suggests that this phase is amorphous silicon formed in the centre of large indentations. The position of the discontinuity on the load-depth curve is found to depend on the unloading rate, as would be expected for the generation of an amorphous phase. Micro-Raman analysis is largely in agreement with the TEM results, but the amorphous silicon is seen most clearly when there are cracks present, suggesting that the amorphous phase may be subsurface or generated during the rapid propagation of cracks.

Contact resistance and phase transformations during nanoindentation of silicon

Abstract An electrically conductive VC tip has been used in conjunction with ex-situ transmission electron microscopy (TEM) and micro-Raman spectroscopy to examine the behaviour of silicon during nanoindentation testing. During loading, the contact resistance is low because of the presence of the metallic β-Sn phase of silicon. During unloading, the semimetallic BC8 phase and the rhombohedral R8 phase are formed, giving rise to an increase in the contact resistance. In large indentations there is a pronounced discontinuity in both the contact resistance-depth and load-depth curves during unloading. This signifies the formation of a phase with lower electrical resistance. TEM analysis suggests that this phase is amorphous silicon formed in the centre of large indentations. The position of the discontinuity on the load-depth curve is found to depend on the unloading rate, as would be expected for the generation of an amorphous phase. Micro-Raman analysis is largely in agreement with the TEM results, but the amorphous silicon is seen most clearly when there are cracks present, suggesting that the amorphous phase may be subsurface or generated during the rapid propagation of cracks.

Tetrahedral structures with icosahedral order and their relation to quasicrystals

The possible existence of quasicrystals in tetrahedral phases is considered. It is shown that one of the well-known crystalline silicon phases (the BC8 phase or silicon III) is characterized by the icosahedral local order with three-quarters of the interatomic bonds being directed along the fivefold axes of an icosahedron. This crystal is considered as an approximant of an icosahedral quasicrystal. Higher order approximants and other tetrahedral structures related to quasicrystals are also constructed. It is shown that in these structures, the formation of the intrinsic phason disorder with the preservation of the energetically favorable coordination number four is possible. The ab initio quantum-mechanical calculations for carbon and silicon show that, although all the considered phases are metastable, their energies only slightly differ from the energies of the corresponding stable phases.

Silicon and carbon structures with icosahedral order, phason jumps, and disorder

After an introductory survey, a recent development in the theory of quasicrystal-related four-coordinated structures is presented. Two crystalline approximants, 1/0 (known as the BC8 phase) and 1/1, are studied in detail. A novel method for construction of four-coordinated structures from the Frank-Kasper dense-packed systems is suggested and several new structures are constructed. The energies of those structures and topological phason disorder are studied ab initio.

Icosahedral order and disorder in tetrahedral semiconductors: Three-dimensional and six-dimensional views

Different approaches to tetrahedrally coordinated quasicrystals are discussed. It is shown that the BC8 phase, which is observed in silicon and germanium, has a hidden icosahedral symmetry of tetrahedral interatomic bonds. In this cubic crystal with the Ia3̄ space group, 75% of all bonds are directed almost exactly along fivefold icosahedral axes. This structure (16 atoms per unit cell) is the 1/0 approximant of a hypothetical icosahedral quasicrystal with a body-centered six-dimensional lattice. The 1/1 approximant is also constructed (64 atoms per unit cell with Ia3̄ space symmetry). A specific type of topological disorder, related with partial phason flips, is shown to be possible in both the approximants. The structure of the approximants and the atomic flips are analyzed both in physical and in perpendicular space and their energies are studied ab initio within the density functional theory. The considered phason disorder provides a new insight into crystalline-to-amorphous transformations.

Size-dependent Phase Transformations During Point Loading of Silicon

Using a unique combination of in situ electrical and acoustical measurements and ex situ transmission electron microscopy, the phase transformations of silicon during point loading were shown to exhibit a strong dependence on the size of the deformed volume. For nanometer-size volumes of silicon, the final phase was the body centered cubic structure BC8, but for larger volumes it was amorphous. The size dependence was explained by considering how shear stress fields vary with contact size and how interfacial effects between the silicon substrate and the BC8 phase determine its stability. For both small and large contacts the presence of a nonmetallic phase (assumed to be the Rhombohedral structure R8) was observed.

Quasicrystal-related phases in tetrahedral semiconductors: Structure, disorder, andab initiocalculations

We show that special structural changes (phason atomic jumps) can transform the BC8 and R8 phases, known to occur in Si and Ge, into a phase with tetrahedral coordination. These structural changes are analogous to the BC8-R8 transformation. The phase has the body-centered-tetragonal symmetry (the space group ${C}_{4h}^{6}\ensuremath{-}{I4}_{1}/a)$ with 16 atoms per unit cell or 8 atoms per primitive cell and will be referred to as BT8. All the atoms occupy equivalent general positions $x,y,z$ and there are three different tetrahedral bonds. It is shown that all three phases, BT8, BC8, and R8 may be considered as a result of phason-ordering transitions from a disordered phase with $\mathrm{Ia}3\ifmmode\bar\else\textasciimacron\fi{}d$ symmetry. An infinite number of other structures with tetrahedral bonding (ordered and disordered) can be constructed this way. The phason jumps are typical of these phases because they are related to hypothetical icosahedral quasicrystals. In particular, the BC8 and the recently suggested BC32 structure may be considered as $1/0$ and $1/1$ cubic approximants of the same quasicrystal. We present a detailed ab initio study of BT8 and BC32 phases in silicon within density-functional theory in the local-density approximation. Our results show that the energetics and diffraction patterns of the BT8 phase are very similar to those of the BC8 and R8 phases. We conclude that the available diffraction data do not exclude the existence of the BT8 phase. In contrast, the energy of the BC32 structure is significantly larger (for positive pressure), and the existence of this phase is therefore questionable. The pressure dependence of the bond lengths, angles, and the lattice parameters of the BT8 phase is investigated. Our estimation of the energy barrier for phason defect formation in the BC8 phase is about 0.12 eV per jumping atom and the energy of the defect is very small (less then 0.02 eV per jumping atom), therefore this type of defect could be present in real samples. Finally, we show that the computed elastic constants of the BC8 phase are almost isotropic as expected for the approximants of icosahedral quasicrystals.

Raman microspectroscopy study of processing-induced phase transformations and residual stress in silicon

Raman spectroscopy was used for analysis of phase transformations and residual stress in machined silicon. Wear debris from dicing of silicon was scanned with a Raman spectrometer. Recorded spectra manifest the presence of amorphous Si, hexagonal phase (Si-IV), bc8 phase (Si-III) and pristine Si-I under residual stress. On surfaces of diced wafers as well as lapped silicon wafers, the r8 phase (Si-XII) was detected in addition to the above phases. The composition of phases in diced cross sections of silicon wafers differs dramatically between high and low speed cuts. The quantification of these phases was attempted by curve fitting each spectrum with corresponding peaks of each phase. Subsequently, relative intensity maps of specific phases were generated. Thus, Raman spectroscopy studies of machined surfaces demonstrated metallization of Si under a variety of machining conditions including lapping, grinding, scratching, dicing and slicing. All metastable phases of silicon disappear after etching and polishing of respective wafers. No evidence of phase transformations was found on a quartz-damaged silicon wafer surface. Residual stress having a characteristic distribution was observed in this case.

Icosahedral order and disorder in semiconductors

Hidden icosahedral symmetry of tetrahedral interatomic bonds is found in the BC8-Ia3 phase of silicon and germanium (75% of all bonds are directed almost exactly along fivefold icosahedral axes). It is shown that this phase may be considered as a crystalline approximant of a hypothetical icosahedral quasicrystal with a body-centred six-dimensional lattice and that all the atomic positions in BC8 crystals may be obtained as projections from that six-dimensional lattice. The diffraction pattern of the BC8 phase has hierarchical organisation typical of both approximants and quasicrystals. A specific type of topological disorder, related with phason flips, is suggested. The experimentally observed BC8-R8 phase transition can be considered as resulting from coherent phason flips of this type. Dual relationships with approximants of conventional quasicrystals are used to construct a higher order approximant without dangling bonds. A new scenario for crystalline-to-amorphous transformation is discussed.

Phase transformations of silicon caused by contact loading

Combining hardness indentation tests and micro-Raman spectroscopy it is shown that metallic Si-II is produced near the interface of a diamond indenter and silicon to a depth of about 0.5 μm, where the highest stresses (hydrostatic and deviatoric) exist. At fast unloading rates Si-II transforms to the amorphous state, whereas a mixture of the r8 high pressure polymorph Si-XII and the bc8 phase Si-III forms upon a slow load release. The region of Si-III+Si-XII is surrounded by the wurtzite structured Si-IV, where the stresses during the indentation had not been high enough to cause the transition to the metallic state. Thus, because of shear deformation a direct transformation to Si-IV takes place. Outside the phase-transformed regions the classical aspects of indentation-induced deformation by dislocation glide, twinning and crack formation are observed. Annealing of the high pressure phases leads to the formation of Si-IV at moderate temperatures and to the reversal to the original diamond structure (Si-I) at temperatures above 500 °C. Using the laser beam of the Raman spectrometer to anneal the samples the phase transitions could be monitored directly. The formation of silicon polymorphs other than amorphous or metallic structures during hardness indentation is, to the best of our knowledge, reported here for the first time. The results compare well with the polymorphism in Si that is known from diamond anvil cell experiments.