Thermoelectric properties of porous silicon

We have studied the thermoelectric properties of porous silicon, a nanostructured, yet single-crystalline form of silicon. Using electrochemical etching, liquid-phase doping, and high-temperature passivation, we show that porous Si can be fabricated such that it has thermoelectric properties superior to bulk Si, for both n- and p-type doping. Hall measurements reveal that the charge carrier mobility is reduced compared to the bulk material which presently limits the increase in thermoelectric efficiency.     

Electronic structure of low-pressure and high-pressure phases of silicon disulfide

Self-consistent density functional theory calculations of band spectra, densities of states as well as the spatial distribution of valence electron charge density were carried out for the low-pressure α-phase and the high-pressure β-phase of SiS₂. Group-theoretical analysis performed for both phases enabled the symmetry of wave functions in a number of high-symmetry points of the Brillouin zone as well as the structure of valence band representations to be found. Based on the calculations of the band structure, the orthorhombic α-phase of SiS₂ was determined to be an indirect-gap semiconductor with the band gap E _gi = 2.44 eV (T ₁ → X ₈ transition), while the β-phase was shown to be direct gap with E _gd = 2.95 eV (Г ₃ → Г ₂ transition). The calculated energy distribution of the total density of states in the valence band of α-SiS₂ qualitatively and quantitatively correlates with the main experimental features of the X-ray photoelectron spectrum.     

Metadynamics simulations of the high-pressure phases of silicon employing a high-dimensional neural network potential

We study in a systematic way the complex sequence of the high-pressure phases of silicon obtained upon compression by combining an accurate high-dimensional neural network representation of the density-functional theory potential-energy surface with the metadynamics scheme. Starting from the thermodynamically stable diamond structure at ambient conditions we are able to identify all structural phase transitions up to the highest-pressure fcc phase at about 100 GPa. The results are in excellent agreement with experiment. The method developed promises to be of great value in the study of inorganic solids, including those having metallic phases.     

Electrical properties of the high-pressure phases of gallium and indium tellurides

A study has been made of the resistance ρ, the thermopower S, and magnetoresistance MR of Ga₂Te₃ and α-In₂Te₃ single crystals at pressures P up to 25 GPa. It is found that the resistance ρ and |S| sharply decrease at ∼0–5 and 1.5–3 GPa, respectively. The semiconductor-metal phase transitions in the temperature range from 77 to 300 K are established from the sign reversal of the temperature coefficient of ρ to occur at P>4.4 and >1.9 GPa. The values S ≈+(10–20)μ V/K for the metallic phases with a Bi₂Te₃-type structure agree with those for liquid In₂Te₃ and Ga₂Te₃. Negative MR is revealed in In₂Te₃ at P≈1.9 GPa. No MR is observed in Ga₂Te₃ up to 25 GPa. The variation of the electronic structure of In₂Te₃ and Ga₂Te₃ under pressure is discussed. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 6, 2000, pp. 1004–1008. Original Russian Text Copyright ? 2000 by Shchennikov, Savchenko, Popova.     

Predicted novel high-pressure phases of lithium

Under high pressure, "simple" lithium (Li) exhibits complex structural behavior, and even experiences an unusual metal-to-semiconductor transition, leading to topics of interest in the structural polymorphs of dense Li. We here report two unexpected orthorhombic high-pressure structures Aba2-40 (40 atoms/cell, stable at 60-80 GPa) and Cmca-56 (56 atoms/cell, stable at 185-269 GPa), by using a newly developed particle swarm optimization technique on crystal structure prediction. The Aba2-40 having complex 4- and 8-atom layers stacked along the b axis is a semiconductor with a pronounced band gap >0.8 eV at 70 GPa originating from the core expulsion and localization of valence electrons in the voids of a crystal. We predict that a local trigonal planar structural motif adopted by Cmca-56 exists in a wide pressure range of 85-434 GPa, favorable for the weak metallicity.     

First-principles study of the optical properties of BeO in its ambient and high-pressure phases

Optical properties such as the dynamic dielectric function, reflectance, and energy-loss function of beryllium oxide (BeO) in its ambient and high-pressure phases are reported for a wide energy range of 0-50 eV. The calculations of optical properties employ first-principles methods based on all-electron density functional theory together with sum over states and finite-field methods. Our results show subtle differences in the calculated optical properties of the wurtzite, zincblende, rocksalt and CsCl phases of BeO, which may be attributed to the higher symmetry and packing density of these phases. For the wurtzite phase, the calculated band gap of 10.4 eV corresponds well with the experimental value of 10.6 eV and the calculated (average) index of refraction of 1.70 shows excellent agreement with the experimental value of 1.72.     

First principles study on the structural and optical properties of the high-pressure ZnO phases

A new high-pressure tetragonal phase (B10) of ZnO is investigated with an ab initio calculation based on density functional theory and is compared with the cubic B1 (rocksalt structure) and B2 (CsCl structure) phases at high pressure. It is found that the B10 phase has a more covalent nature than the B2 phase. The B1, B2, and B10 phases are semiconductors and their band gap energies are determined to be 3.73, 3.15, and 3.27 eV, respectively. The B10 phase has a similar optical response to the B2 phase, but not the B1 phase. The similarity of dielectric function between B10 and B2 phases are the result of the similar profiles of electronic density of state.     

New metastable phases of silicon

It has been known since 1963 that a metastable phase (III,BC-8) results from the decompression of silicon from its high pressure metallic phases. However recent theoretical studies suggest the possibility of several metastable phases with similar total energies. Upon rapid release of pressure from the metallic state, two new, metastable, phases of Si have been discovered, with tetragonal structures. X-ray diffraction data on these new phases are presented.     

Superconductivities of high-pressure phases in the metal-hydrogen systems

Abstract

The superconducting properties of the d-metal hydrides rank among the most interesting and less studied of their characteristics. Correct and complete data have only been obtained for one superconducting hydride, the palladium hydride'. The long studies on phases forming in the d-metal-hydrogen system in the well- mastered pressure range up to tens of atmospheres have not exhibited any new superconductors, and the scope of systems which could be of interest from this viewpoint has been mainly exhausted'.     

Thermoelectric properties of hydrogen ion-irradiated silicon crystals under ultrahigh pressures of up to 20 GPa

The thermopower S of p-Si samples containing a thin hydrogenated layer was studied at high pressures P of up to 20 GPa. Near the phase transition to a white-tin lattice (P ≤ 10 GPa), the thermopower of samples with a hydrogenated layer was found to decrease as compared to that in the starting p-Si samples. However, the values of S of high-pressure metallic phases with a β-Sn structure and orthorhombic (above 12 GPa) and simple hexagonal (above 16 GPa) structures are approximately the same for different sample groups. The variation in crystal structure induced by pressure treatment was studied by ultrasoft x-ray spectroscopy. The observed decrease in S induced by the pressure treatment is tentatively assigned to the formation of an amorphous phase in addition to the Si-III metastable phase.     

Stabilization of high-pressure phases in nanocrystalline metals and alloys

It is shown experimentally that the formation of submicrocrystalline and nanocrystalline states significantly affects the stability of the high pressure hcp phase in iron-based alloys. It is established that the manganese segregation from the bulk of ɛ-phase grain to the grain boundary under high hydrostatic pressure can influence the stability of this phase in nanocrystalline alloys.     

Thermoelectric properties of tunnel junctions

The thermoelectric (Seebeck) coefficient α and thermoelectric quality factor (figure of merit) ZT are estimated for a tunnel junction in metals. It is shown that α can be of the order of hundreds of μV/K while ZT can approach values ∼0.1–1. The maxima of α(h) and ZT(h) correspond to a certain width h of the tunnel junction; such h is about a few nanometers. The results we obtained can find applications in the constructions of novel thermoelectric generators.