The lattice thermal conductivity of Ge clathrates is investigated by evaluating the linear response theory heat current correlation functions using molecular dynamics. Clathrate crystals with and without guest atoms in their fullerane cages are studied. In comparison that diamond-phase Ge, clathrate reduced approximately 1 order magnitude due to open framework itself. addition an encapsulated (rattling) Sr atom produces a further reduction conductivity, making it comparable amorphous Ge. Our...

By treating a small cluster embedded in an effective medium (described here by the coherent-potential approximation) we can reproduce "exact" numerical frequency-distribution spectra of vibrating-linear-chain alloy ${A}_{c}{B}_{1\ensuremath{-}c}$ with mass disorder. The theory is especially applicable to concentrations $0.05\ensuremath{\lesssim}c\ensuremath{\lesssim}0.95$ throughout regime, where other theories are quantitatively unreliable. Unlike purely calculations, present method be...

Systematic crystal structure refinements from powder X-ray diffraction data as well density functional theory calculations demonstrate that the silicon clathrate II Si(cF136) exhibits a lattice contraction Na is introduced solely into Si(28) cages. When Si(20) cages, in addition, begin to be filled with Na, contrasting expansion results. The nonmonotonic structural response filling an indication of markedly dissimilar guest-framework interactions for Na@Si(20) and Na@Si(28).

Using an ab initio local density approximation (LDA) method, we have studied the structural properties and electronic band structures of several tin-based clathrate materials. We find that pure type-I $({\mathrm{Sn}}_{46})$ type-II $({\mathrm{Sn}}_{136})$ frameworks are metastable, expanded-volume phases Sn they semiconductors. further Cs encapsulated binary is metallic if full stoichiometry $({\mathrm{Cs}}_{8}{\mathrm{Sn}}_{46})$ assumed. However, our calculations suggest vacancy formation...

Raman-scattering spectra of the type II clathrates Cs8Na16Si136, Cs8Na16Ge136, and Si136 were studied employing different laser wavelengths. Most Raman-active vibrational modes these compounds identified. Polarization measurements used to identify symmetric modes. The lowest frequency optic “rattle” mode corresponding vibrations Cs atoms inside hexakaidecahedra is identified for both Si Ge clathrate compounds. We compare experimental data directly with theoretical calculations. These...

The conduction-electron-spin-resonance (CESR) spectra of small metallic particles are expected to exhibit very different behavior from those bulk metals and depend on particle size. In particular, Kawabata has made a comprehensive theoretical study the size dependence CESR line shape predicts that, for diameters, small-particle $g$ shift contains contribution which adds is proportional square diameter. this paper we develop formalism enables us reexamine as function size, compare predictions...

The vibrational frequency-dsitribution spectra of the random one-dimensional ternary alloy ${A}_{c}{B}_{1\ensuremath{-}c}C$ are accurately predicted for first time by a statistical theory small clusters embedded in coherent-potential-approximation effective medium. can be applied practically to real three-dimensional alloys.

Framework substitution has not been widely reported among the type-II group-IV clathrates. We have performed a theoretical study of ${\mathrm{Cs}}_{8}{\mathrm{Ga}}_{8}{\mathrm{Si}}_{128}$ and ${\mathrm{Rb}}_{8}{\mathrm{Ga}}_{8}{\mathrm{Si}}_{128}$ clathrates, using local-density approximation (LDA) to density-functional theory. In ${\mathrm{Cs}}_{8}{\mathrm{Ga}}_{8}{\mathrm{Si}}_{128}$, eight Si atoms are substituted by Ga at $8a$ crystallographic sites framework. predict that both...

Abstract Clathrate materials have been the subject of intense interest and research for thermoelectric application. Nevertheless, from very large number conceivable clathrate structures, only a small fraction them examined. Since thermal conductivity clathrates is inherently due to their unit cell size open-framework structure, current on focused finding ones with power factor. Here we predict an extraordinarily factor type-VIII Si 46 . We show existence density closely packed elongated...

A new type of total-energy functional for semiconductors is proposed, using a form based on an approximate quantum-mecahnical analysis. Two types terms are included: pair giving matrix description atom's local environment, and cluster describing the ring topology. The scheme reproduces both covalent metallic behavior semiconductor materials in appropriate limits. parametrized applied to Si.

A simple theory of deep levels due to vacancies paired with nearest-neighbor $s{p}^{3}$-bonded substitutional impurities in zinc-blende hosts is presented, and the major chemical trends such 12 different semiconductors are predicted. The results these calculations show that when an impurity complexed a vacancy, its band gap may be significantly altered comparison those isolated impurity. Complexing vacancy can drive shallow, level deeper into gap, or cause move across become resonant either...

The dynamical two-point correlation functions of the high-temperature Heisenberg paramagnet as described by nonlinear integral equations generated moment expansions developed Reiter are investigated. In particular, for magnetic dipole and quadrupole obtained solved in a lowest-order self-consistent approximation. These directly measurable electromagnetic, neutron-scattering, acoustic experiments dense insulators. From solution functions, theoretical values spin-diffusion coefficient...

Although clathrate materials are known for their small thermal conductivity, they have not shown a large thermoelectric power factor so far. We present the band structures of type VIII Si, Ge, and Sn clathrates as well alkali alkaline-earth intercalated Si clathrates. Our calculations revealed that this group has potentially factors due to existence number carrier pockets near edges. In particular, we calculated charge transport properties Si46-VIII both n-type p-type materials. The...

Clathrates can form a type of guest-host solid structures that, unlike most crystalline solids, have very low thermal conductivity. It is generally thought that the guest atoms caged inside host framework act as ``rattlers'' and induce lattice dynamics disorders responsible for small We performed systematic study dynamical properties type-VIII clathrates with alkali alkaline-earth guests, i.e., ${X}_{8}\mathrm{S}{\mathrm{i}}_{46}$ ($X=\mathrm{Na}$, K, Rb, Cs, Ca, Sr, Ba). The energy...

The material design of type-VIII clathrate Si46 is presented based on first principles. structural, electronic, elastic, vibrational, and thermodynamic properties this hypothetical are presented. Our results predict that an indirect semiconductor with a bandgap 1.24 eV. band structure revealed interestingly large number electron pockets near both conduction valance edges. Such density states the edges, which higher than best thermoelectric materials discovered so far, can result in power...

We have extended the Kobayashi, Sankey, and Dow [Phys. Rev. B 25, 6367 (1982)] theory of deep levels in Hg1−xCdxTe to include (vacancy, impurity) nearest-neighbor pairs. In qualitative agreement with results obtained by these workers for isolated point defects, we find that composition dependences (dE/dx) defect energy associated such complexes depend on site occupied impurity atom. Furthermore, some produced a complex are very different than dE/dx corresponding defects. thus suggest this...

The contribution of alloy scattering to the electronic rate at Brillouin-zone center in 18 III-V ternary semiconducting alloys is calculated Born approximation using semi-empirical tight-binding parameters Vogl, Hjalmarson, and Dow. Three types disorder for a type ${A}_{x}{B}_{1\ensuremath{-}x}\mathrm{C}$ are included these calculations. These (1) ordinary diagonal or energies associated with constituents $A$ $B$, (2) off-diagonal transfer between $C$ $B$ $C$, (3) redistribution which takes...

We have performed density functional calculations of the vibrational modes three tin-based, cagelike clathrate materials: pure type-I $({\mathrm{Sn}}_{46})$ and type-II $({\mathrm{Sn}}_{136})$ frameworks, donor/acceptor compensated compound ${\mathrm{Cs}}_{8}{\mathrm{Ga}}_{8}{\mathrm{Sn}}_{38}.$ also theoretically identified infrared- Raman-active in these materials computed their Raman spectra. In addition, we report measurements spectrum...

Abstract We have studied the vibrational properties of some compounds based on Type II silicon, germanium, and tin‐based clathrate materials using LDA electronic structure methods. The lattices in these framework open cages which can contain weakly bound guest impurities, guests produce local (“rattling”) modes. Such modes may scatter extended, heat carrying acoustic framework, potentially reducing thermal conductivity. present results for spectra Na 16 Cs 8 Si 136 , Ge 24 Sn . discuss...

A general first-principles method for the calculation of dynamical spin correlation functions in a system randomly distributed, strongly interacting spins with multipolar interactions is presented. The valid high-temperature limit and applicable at all concentrations $c$. As numerical example, case dipolar forces treated magnetic-resonance frequency line shape calculated variety values trend change $c$ obtained agrees phenomenological theories experiment. theoretical linewidth has correct...

We studied the structural and electronic properties of some Ba Sr guest-containing type-I semiconductor clathrate alloys ${\text{Ba}}_{8}{\text{Ga}}_{16}{\text{Si}}_{x}{\text{Ge}}_{30\ensuremath{-}x}$ ${\text{Sr}}_{8}{\text{Ga}}_{16}{\text{Si}}_{x}{\text{Ge}}_{30\ensuremath{-}x}$ for three values Si composition $x$ $(x=0,5,15)$. Our calculations are based on generalized gradient approximation to density functional theory. Starting with stable structures semiconductors...

The neutral vacancy (V) and the four vacancy-hydrogen complexes ({V,${\mathrm{H}}_{\mathit{n}}$}, n=1,...,4) in silicon are studied ${\mathrm{H}}_{\mathit{n}}$${\mathrm{Si}}_{63}$ periodic supercells with ab initio pseudo-atomic-orbital molecular-dynamics method developed by Sankey co-workers. Results presented for (i) equilibrium configurations of V {V,${\mathrm{H}}_{\mathit{n}}$} complexes, (ii) diffusion properties {V,${\mathrm{H}}_{1}$}, (iii) vibrational modes...

The embedded-cluster method is modified to treat electronic spectra in ternary alloys and applied the calculation of density states for one-dimensional, one-state-peratom, alloy ${A}_{x}{B}_{1\ensuremath{-}x}C$ nearest-neighbor tight-binding approximation. As a test this method, some representative cases "persistence" "amalgamation" regimes are presented compared with "exact" same obtained 10 000-atom random chains by use negative-eigenvalue theorem. For cluster containing eight unit cells...

We have performed density-functional calculations of the vibrational and thermal properties some type-I semiconductor clathrate ``alloys'' ${\text{Ba}}_{8}{\text{Ga}}_{16}{\text{Si}}_{x}{\text{Ge}}_{30\ensuremath{-}x}$ ${\text{Sr}}_{8}{\text{Ga}}_{16}{\text{Si}}_{x}{\text{Ge}}_{30\ensuremath{-}x}$ as a function Si composition $x$. find that guest-atoms Ba Sr produce localized modes lying below $80\text{ }{\text{cm}}^{\ensuremath{-}1}$, which tend to reduce acoustic bandwidth host material....