Behavior of Polytype Structures in Poor-Stability Systems under Varying External Loading

An approach to investigating polytype transformations in close-packed crystals based on the generalized axial finite-dimensional Ising model is proposed. Within this approach, an analysis is made of the influence that model dimensions and long-range and many-body interactions exert on the stability of short-period and multi-layer structures and the sequence of polytype transformations. Using computer simulation techniques, the potential behavior of systems with polytype transformations is studied by varying the external shear stress and temperature under equilibrium and nonequilibrium conditions. The results obtained agree with the experimental data. This approach furnishes a means for predicting new multi-layer polytype structures and determining their symmetry.     

Microscopic Description of the Mechanism of Transition between the 2H and 4H Polytypes of Silicon Carbide

Physics of the Solid State - The mechanism of displacement of one close-packed SiC layer from one minimum position to another on the example of SiC polytype transition 2H → 4H has been...     

The most stable crystal structure and the formation processes of an order-disorder (OD) intermetallic phase in the Mg–Al–Gd ternary system

The most stable crystal structure for an 18R-type order-disorder (OD) intermetallic phase in the Mg–Al–Gd ternary system and its formation processes by annealing at 525?°C have been investigated by means of transmission electron microscopy and scanning transmission electron microscopy. The most energetically favourable polytype at 525?°C is found to be the structurally simplest one, a maximum degree of order polytype (monoclinic, 1M, space group: C2/m), described with a single stacking vector in stacking six-layer structural blocks. The formation of this simplest polytype occurs in the sequence of (i) enrichment of Gd and Al in four consecutive close-packed planes while keeping the hexagonal close-packed stacking of the AB-type, (ii) formation of Al₆Gd₈ clusters in the four consecutive atomic planes, introducing a stacking fault in the middle of the four consecutive atomic planes, (iii) thickening by the formation of Gd and Al-enriched four consecutive planes at a distance of two or three close-packed Mg atomic planes from the pre-existing Gd and Al-enriched four consecutive atomic planes so as to form six-layer and, sometimes seven-layer structural blocks, (iv) in-plane ordering of Al₆Gd₈ clusters in the four consecutive atomic planes and the stacking of structural blocks in the preferential stacking positions to form the OD structure, and (v) elimination of different structural blocks (other than six-layer ones) and the long-range ordering in the stacking of structural blocks.     

Effect of grain-boundary conditions on polytype transformations in close-packed crystals

Within the framework of a modified Ising model, the effect of the boundary conditions (“free ends” and periodic boundary conditions) on the polytype structure stability is studied in close-packed crystals. Using the ground state diagrams, feasible structures and transformation induced by an external field are identified. An analysis of the model system dimensions and long- and short-range many-body interaction on the shape of the ground state diagrams under different boundary conditions is made. The temperature effects on the polytype structure characteristics are discussed in detail. It is shown that both at the absolute zero and finite temperatures, the effects of the boundary conditions on certain properties could be predominant.__________This revised version was published online in May 2005 with a correction on affiliation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 61–66, November 2004.     

Special features of polytype transformations under changing external shear stress and temperature

Within the generalized axial finite-dimensional Ising model, possible model and experimental non-equilibrium polytype 3C-2H and 9R-3C transformations and twinning of the fcc-lattice of close-packed model crystals are considered. These transformations are realized in a great number of metals as well as cobalt, iron, and copper-base alloys and observed on variation of both the applied load and the temperature. The structure transformations simulated in the model may involve a number of multilayer metastable structures. Hysteresis phenomena found under variations of the external stress and temperature and their behavior in response to long-range interactions corresponding to atomic interactions in the fcc lattice in ten coordination spheres and in many-body interaction responsible for the stability of the 4H structure are considered. Factors accounting for the formation of metastable states and hysteresis effects are analyzed. It is shown that the crystal form may undergo a non-monotonic change as the applied load and temperature are varied. Siberian Physicotechnical Institute at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 80–86, September 1999.     

Polytypism of silicon carbide and Schottky barriers

The results obtained in our previous work [4] are revised taking into account the dependence of the electron affinity on the polytype of silicon carbide SiC. The dependence of the energy level of vacancies in a polytype of silicon carbide on the band gap is determined from the data on the Schottky barrier height and is explained in the framework of a simple two-band model.     

碳化硅多型体连生现象的X射线研究

本工作用劳埃法对分布于三十二炉的四百多个碳化硅单晶进行了类型分布、连生规律和结构完整性等的分析。大量的观察表明,碳化硅片状晶体中的共轴平行连生是一种普遍的现象,二种以上类型的连生占晶体总数的三分之二。文中列出了各种连生类型的统计。系统地报导了SiC晶体连生状况的X射线法实验结果。对各炉晶体所作的类型分布的分析表明,不同的制备条件下影响较大的是颜色和结构完整度。类型分布的规律性并不明显,邻近的晶体类型可以完全相异。用实验数据分析了类型、颜色和蚀坑间的关系。对碳化硅多型体产生的机理作了讨论。     

Investigation on the formation of lonsdaleite from graphite

Structural stability and the possible pathways to experimental formation of lonsdaleite—a hexagonal 2H polytype of diamond—have been studied in the framework of the density functional theory (DFT). It is established that the structural transformation of orthorhombic Cmmm graphite to 2H polytype of diamond must take place at a pressure of 61 GPa, while the formation of lonsdaleite from hexagonal P6/mmm graphite must take place at 56 GPa. The minimum potential barrier height separating the 2H polytype state from graphite is only 0.003 eV/atom smaller than that for the cubic diamond. The high potential barrier is indicative of the possibility of stable existence of the hexagonal diamond under normal conditions. In this work, we have also analyzed the X-ray diffraction and electron-microscopic data available for nanodiamonds found in meteorite impact craters in search for the presence of hexagonal diamond. Results of this analysis showed that pure 3C and 2H polytypes are not contained in the carbon materials of impact origin, the structure of nanocrystals found representing diamonds with randomly packed layers. The term “lonsdaleite,” used to denote carbon materials found in meteorite impact craters and diamond crystals with 2H polytype structure, is rather ambiguous, since no pure hexagonal diamond has been identified in carbon phases found at meteorite fall sites.     

On the calculation of the linear and quadratic dielectric susceptibilities of hexagonal silicon carbide

The electron and lattice contributions to the linear and quadratic dielectric susceptibilities of the hexagonal polytype of silicon carbide 2H-SiC are calculated within the Harrison bond-orbital model. The results obtained are in satisfactory agreement with the calculations performed by other authors and are close in order of magnitude to the corresponding values for the 6H-SiC polytype.     

Determination of size and concentration of critical clusters of silicon carbide in the vapour phase in homogeneous nucleation process

The concentrations of clusters of various size in the atmosphere during silicon carbide crystal growth have been calculated on the basis of fundamental ideas of homogeneous nucleation theory, taking into account the specific parameters of silicon carbide. It has been shown that the cluster concentration are sufficiently high to conclude that this is the dominant influence during the initial stages of crystal growth. In this way the assumption of the polymer theory of polytypism, namely that the polytype properties of silicon carbide can be determined from the composition of the gas phase, containing sufficiently large clusters with various polytype structures, has been confirmed.     

Asymmetric split-vacancy defects in SiC polytypes: a combined theoretical and electron spin resonance study

Transition metal defects were studied in different polytypes of silicon carbide (SiC) by ab initio supercell calculations. We found asymmetric split-vacancy (ASV) complexes for these defects that preferentially form at only one site in hexagonal polytypes, and they may not be detectable at all in cubic polytype. Electron spin resonance study demonstrates the existence of ASV complex in niobium doped 4H polytype of SiC.     

Specific features of the formation of the ferroelectric phase in polytypes of TlGaSe2 crystals

The polytypism of layered crystals of thallium gallium diselenide TlGaSe₂ has been found to substantially affect the temperature of phase transformations and the mechanism of formation of the polar state in these ferroelectrics. In particular, it is shown that the phase transition observed in the C-TlGaSe₂ polytype is an improper ferroelectric phase transition occurring at a temperature T _c ≈ 108 K, whereas the phase transition observed in the 2C-TlGaSe₂ polytype is a proper ferroelectric phase transition occurring at a higher temperature T _c ≈ 111 K. It is concluded that the elucidation of the polytype of a particular sample is a necessary stage of investigation of the TlGaSe₂ crystals.     

Optical absorption edge of a new GaSe polytype

The optical energy gap of the δ modification of GaSe is reported. The energy shift of the exciton in this polytype is explained in terms of interlayer separation by comparison with the ? structure under high pressure.     

Phase transformations in PbI2 crystals with temperature

A systematic study of phase transformation with temperature has been undertaken in single crystals of lead iodide grown in gel. The crystals of the polytype 2H, which is known to be the common modification of PbI₂ at room temperature, have been finally found to transform into polytype 12R after heating at 150°C. During the intermediate period of heating the crystals show features of disorder, viz. streaking or arcing or both, on their X-ray diffraction photographs. The higher polytypes 12H and 16H do not show any change even after prolonged heating at 150°C. If silver iodide is added in a minute quantity during crystallization, the process of phase transformation is appreciably altered. The results have been discussed.     

Pseudospin splitting of the energy spectrum of planar polytype graphene-based superlattices

The energy spectrum of planar polytype graphene-based superlattices has been investigated. It is shown that their energy spectrum undergoes pseudospin splitting due to the asymmetry of quantum wells forming the superlattice potential profile.     

Packing spheres tightly: influence of mechanical stability on close-packed sphere structures

Many experiments and simulations of packings of monodisperse hard spheres report a dominance of the face-centered cubic structure in the hexagonally close-packed limit, even though it has no significant energetic or entropic gain over other close-packed configurations. Combining simulations and experiments, we demonstrate that a simple mechanical instability which occurs during the packing process may play an important role in selecting the face-centered cubic structure over other close-packed alternatives. Our argument is supported by detailed quantitative analyses of key configurations in sphere packings and highlights the importance of the packing dynamics. The proposed mechanism is elementary and should therefore play a role in a wide range of sphere systems.     

Density of states of superlattices with multiple layers per period

Density-of-states (DOS) investigations of polytype superlattices (SLs), whose period consists of more than two different layers, are presented. Local DOS (LDOS) is computed as a function of both the electron energy and the space coordinate, which illustrates — in a direct way — the spatial localization, over the SL period, of states forming the energy minibands. A few examples of AlGaAs-based polytype SLs with a coupled-well, step-well and δ-doped basis are analysed, indicating that various LDOS distributions within SL minibands can be arranged. By changing the basis geometry, the electronic DOS can either be selectively confined to a particular SL layer or extend over the whole SL period, which provides a means of modifying the spatial overlap of states from different SL minibands and thus, tuning the respective interband transitions. In particular, polytype SLs with similar miniband structures may exhibit essentially different LDOS features and, consequently, the different transport and optical characteristics. Therefore, it is emphasized that — for a particular device application of a complex-basis SL — not only the electronic level structure should be appropriately designed, but also the corresponding space-charge distributions should be carefully considered.     

Description of the geometry of crystals with a hexagonal close-packed structure based on pair interaction potentials

The pair force interaction potential that allows one to describe a deviation from spherical symmetry, which is typical for hexagonal close-packed structures, is constructed using the ??spherically symmetric?? Mie potential that depends only on the interatomic distance. The parameters of the considered potential, which ensure the stability of hexagonal close-packed lattices, are obtained for a wide range of metals, namely, beryllium, gadolinium, hafnium, holmium, dysprosium, yttrium, cobalt, lutetium, magnesium, osmium, rhenium, ruthenium, scandium, thallium, terbium, technetium, titanium, thulium, cerium, zirconium, and erbium. It is shown that for this pair interaction potential the hexagonal close-packed structure is energetically more favorable than the face-centered cubic structure. The proposed potential can be used to perform computational experiments and analytical investigations.     

On the calculation of spontaneous polarization of the 2H-SiC polytype

An expression for the dipole moment of the tetrahedral bond is derived for the hexagonal 2H-SiC polytype within the Harrison bond-orbital model. The polarization calculated from this expression is in satisfactory agreement with the results of earlier pseudopotential calculations.     

用高分辨电子显微术直接鉴定ZnS多型体中堆垛序列和缺陷

用倾斜束照明的电子显微术在分辨率为3.12?的水平上摄照了点阵象,直接观察了ZnS多型体的堆垛次序,这方法被用于揭示ZnS堆垛的无序和层错。 关键词：