Structural principles and amorphouslike thermal conductivity of Na-doped Si clathrates

Predicted CsSi compound: a promising material for photovoltaic applications

Exploration of photovoltaic materials has received enormous interest for a wide range of both fundamental and applied research. Therefore, in this work, we identify a CsSi compound with a Zintl phase as a promising candidate for photovoltaic material by using a global structure prediction method. Electronic structure calculations indicate that this phase possesses a quasi-direct band gap of 1.45 eV, suggesting that its optical properties could be superior to those of diamond-Si for capturing sunlight from the visible to the ultraviolet range. In addition, a novel silicon allotrope is obtained by removing Cs atoms from this CsSi compound. The superconducting critical temperature (T_c) of this phase was estimated to be of 9 K in terms of a substantial density of states at the Fermi level. Our findings represent a new promising CsSi material for photovoltaic applications, as well as a potential precursor of a superconducting silicon allotrope.

Deviation from guest dominated glass like lattice dynamics in prototypical ternary Ba8Ni x Ge[Formula: see text] clathrates

Intermetallic guest filled clathrate cages have been identified as promising materials for thermoelectric applications. The structure, electronic structure, and phonon dynamics of type-I Ba filled Ni substituted Ge clathrates have been explored using density-functional theory. The formation energy of type-I Ba₈Ni _x Ge[Formula: see text] clathrates with y  vacancies [Formula: see text] in their framework is calculated as a function of concentration of Ni substitution x. Metal substitution i.e. with Ni destabilized the framework vacancies resulting in a framework devoid of vacancies beyond a particular concentration of Ni substituition (for [Formula: see text]). By tuning the concentration of Ni in the framework, n-type to p -type doping can be achieved while retaining the compositional homogeinity. Furthermore, Ni substitution in the framework lowered the thermal conductivity of these compounds. Results of molecular dynamics simulations showed that with the increase in temperature the guest, the substitutional host as well as the host atoms rattled to collectively lower the lattice thermal conductivity of these clathrates. This is found to be contradictory to the concept of guest dominated glass like phonon dynamics in these compounds.

First-Principles Analysis of Vibrational Properties of Type II SiGe Alloy Clathrates

We have mostly performed vibrational studies of Type-II silicon-germanium clathrate alloys, namely, Si_136-xGe_x (0 < x ≤ 128), using periodic density functional theory (DFT). Our computed lattice constant for various stoichiometric amount, namely, x, of Ge agrees to some extent with the observed X-ray diffraction (XRD) data, along with monotonically increasing dependence on x. According to our bandgap energy calculation via Vienna ab initio simulation package (VASP), Si₁₂₈Ge₈ has a “nearly-direct” bandgap of approximately 1.27 eV, which agrees well with the previously calculated result (~1.23 eV), which was obtained using the Cambridge sequential simulation total energy package (CASTEP). Most of our first-principles calculations focus on exploring the low-energy transverse acoustic (TA) phonons that contribute dominantly to the induction of negative thermal expansion (NTE) behavior. Moreover, our work has predicted that the Si₁₀₄Ge₃₂ framework exhibits NTE in the temperature range of 3–80 K, compared to the temperature regime (10–140 K) of NTE observed in such pure Si₁₃₆. It is posited that the increased number of Ge–Ge bonds may weaken the NTE effect substantially, as the composition, which is denoted as x, in Si_136-xGe_x is elevated from 32 (or 40) to 96 (or 104).

3D Printing Fabrication of Amorphous Thermoelectric Materials with Ultralow Thermal Conductivity

Thermoelectric materials are prepared by developing 3D printing technology. The 3D fabricated Bi0.5 Sb1.5 Te3 samples exhibit amorphous characteristics and thus show an ultralow thermal conductivity of 0.2 W m(-1) K(-1) . 3D printing fabrication readily generates bulk thermoelectric samples of any shape, which is not the case with traditional hot-pressing and spark plasma sintering methods.

Low temperature transport properties and heat capacity of single-crystal Na8Si46

The low temperature thermal conductivity, resistivity, and Seebeck coefficient of single-crystal Na(8)Si(46) are investigated revealing the intrinsic low temperature transport properties of this material. Metallic conduction is observed, with a higher residual resistance ratio than any other known type I clathrate. Heat capacity together with thermal conductivity provide insight into the Na disorder inside the polyhedra formed by the Si framework. Single-crystal structural refinement and thermal property analyses reveal anisotropic disorder for Na inside the tetrakaidecahedra due to a reduction in the symmetry inside these polyhedra, unlike that for Na inside the dodecahedra.

Off-center rattling and anisotropic expansion of type-I clathrates studied by Raman scattering

Dynamical motions of the guest ions in type-I clathrates Sr8Ga16Si(30-x)Gex and Ba8Ga16Si(30-x)Gex have been studied by Raman scattering spectroscopy, to clarify the role of guest vibration modes in these systems with unusual thermal transport behaviors. An anomalous decrease of the guest energies with decreasing temperature is observed for both systems. The Ge-doping expands the cage surrounding the 6d site anisotropically for Sr8Ga16Si(30-x)Gex, but isotropically for Ba8Ga16Si(30-x)Gex. Especially for Sr8Ga16Si(30-x)Gex, off-center rattling arises simultaneously with the anisotropic expansion, and it is confirmed that these anomalies play a crucial role to suppress lattice thermal conductivity in these systems.

Anharmonic motions of Kr in the clathrate hydrate

The anomalous glass-like thermal conductivity of crystalline clathrates has been suggested to be the result of the scattering of thermal phonons of the framework by 'rattling' motions of the guests in the clathrate cages. Using the site-specific (83)Kr nuclear resonant inelastic scattering spectroscopy in combination with conventional incoherent inelastic neutron scattering and molecular-dynamics simulations, we provide unambiguous evidence and characterization of the effects on these guest-host interactions in a structure-II Kr clathrate hydrate. The resonant scattering of phonons led to unprecedented large anharmonic motions of the guest atoms. The anharmonic interaction underlies the anomalous thermal transport in this system. Clathrates are prototypical models for a class of crystalline framework materials with glass-like thermal conductivity. The explanation of the unusual molecular dynamics has a wide implication for the understanding of the thermal properties of disordered solids and structural glasses.

Exceptional ideal strength of carbon clathrates

We study by means of ab initio calculations the ideal tensile and shear strengths of the C-46 clathrate phase. While its bulk modulus and elastic constants are smaller than in diamond, its strength is found to be in all directions larger than the critical stresses associated with the diamond [111] planes of easy slip. This can be related to the frustration by the clathrate cage structure of the diamond to graphite instability under nonhydrostatic stress conditions [corrected] The criteria for designing strong materials are discussed.

First principles calculations of electronic structures and metal mobility of NaxSi46 and NaxSi34 clathrates

Energetics, geometry, electronic band structures, and charge transfer for Na(x)Si(46) and Na(x)Si(34) clathrates with different degrees of cavity filling by sodium, and the mobility of the Na atom inside the different cavities are studied using first principles density functional calculations within the generalized gradient approximation. The stabilization of the clathrate lattice and the cell volume variation upon the inclusion of Na (which appears to move easily in the larger cavities of Na(x)Si(34), thus justifying the experimental observations) are discussed in connection with the onset of the repulsion between Na and Si for distances shorter than approximately 3.4 A. For all degrees of filling of the different cavities examined we find that the electron population of the s orbitals in the partially ionized Na atoms increases with a decrease in the size of the cavity, and that the Na states contribute significantly to the density of states at the Fermi level and thus influence the properties of these compounds.

Superconductivity in doped sp3 semiconductors: the case of the clathrates

We present a joint experimental and theoretical study of the superconductivity in doped silicon clathrates. The critical temperature in Ba(8)@Si-46 is shown to strongly decrease with applied pressure. These results are corroborated by ab initio calculations using MacMillan's formulation of the BCS theory with the electron-phonon coupling constant lambda calculated from perturbative density functional theory. Further, the study of I(8)@Si-46 and of gedanken pure silicon diamond and clathrate phases doped within a rigid-band approach show that the superconductivity is an intrinsic property of the sp(3) silicon network. As a consequence, carbon clathrates are predicted to yield large critical temperatures with an effective electron-phonon interaction much larger than in C60.

High-pressure Raman study of Ba doped silicon clathrate

Vibrational properties of Ba doped Si clathrate are investigated at high pressures up to 20 GPa by Raman spectroscopy. Vibrations related with Ba encaged in the Si cages are observed below 100 cm(-1) by low-frequency Raman measurements of Ba8Si46 and Ba6.6Si46 clathrates. The high-pressure Raman spectra obtained for both compression and decompression processes reveal a new phase transition at 7 GPa, and the reversibility of the phase transition at 15 GPa. We investigate the mechanisms of these phase transitions, the interaction between the guest and host frame, and the volume dependence of the vibrational modes.

Structural stability and phase transitions in K8Si46 clathrate under high pressure

The structural stability of type-I K8Si46 clathrate has been investigated at high pressure by synchrotron x-ray diffraction. In contrast to that observed in the Na-doped structure-II analogue [A. San-Miguel, Phys. Rev. Lett. 83, 5290 (1999)]], no phase separation into the beta-Sn Si structure was identified at 11 GPa. Instead, K8Si46 is found to undergo a transition to an isostructural positional disordered phase at around 15 GPa. Ab initio phonon band structure calculations reveal a novel phenomenon of phonon instabilities of K atoms in the large cavities is responsible for this transition. Above 32 GPa, the new structure transforms into an amorphous phase.

Bcc and Fcc transition metals and alloys: a central role for the Jahn-Teller effect in explaining their ideal and distorted structures

Transition metal elements, alloys, and intermetallic compounds often adopt the body centered cubic (bcc) and face centered cubic (fcc) structures. By comparing quantitative density functional with qualitative tight-binding calculations, we analyze the electronic factors which make the bcc and fcc structures energetically favorable. To do so, we develop a tight-binding function, DeltaE(star), a function that measures the energetic effects of transferring electrons within wave vector stars. This function allows one to connect distortions in solids to the Jahn-Teller effect in molecules and to provide an orbital perspective on structure determining deformations in alloys. We illustrate its use by considering first a two-dimensional square net. We then turn to three-dimensional fcc and bcc structures, and distortions of these. Using DeltaE(star), we rationalize the differences in energy of these structures. We are able to deduce which orbitals are responsible for instabilities in seven to nine valence electron per atom (e(-)/a) bcc systems and five and six e(-)/a fcc structures. Finally we demonstrate that these results account for the bcc and fcc type structures found in both the elements and binary intermetallic compounds of group 4 through 9 transition metal atoms. The outline of a theory of metal structure deformations based on loss of point group operation rather than translational symmetry is presented.

Tailoring band gap and hardness by intercalation: an ab initio Study of I(8)@Si-46 and related doped clathrates

We present an ab initio study of the structural and electronic properties of the recently synthesized I(8)@Si-46 clathrate which is shown to be a degenerate p-type doped system. The intercalation significantly opens the band gap to a 1.75 eV value within the density functional theory. We study further the intercalation by other neighboring elements. A quasiparticle study reveals that such systems can display a band gap in the "green-light" energy range. Finally, we show that the bulk modulus can be increased to values equivalent to the one of the diamond phase.

Theoretical study of the lattice thermal conductivity in Ge framework semiconductors

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 with that of diamond-phase Ge, the clathrate conductivity is reduced by approximately 1 order of magnitude due to the open framework itself. The addition of an encapsulated (rattling) Sr guest atom produces a further order of magnitude reduction in the conductivity, making it comparable to that of amorphous Ge. Our results are consistent with experiments, and have impact on the search for improved thermoelectric materials.