Silicon Clathrate Research Articles

Preparation of (321) Oriented Silicon Clathrate Film by a Sol-Gel Method

In this paper, Na+-doped silicon clathrate thin films were prepared on Si substrate by sol-gel method using zinc acetate and sodium carbonate as starting materials and the effects of the growth conditions on the structural of the films have been investigated by XRD, HRTEM, SAED analysis. The XRD results showed that there only existed a narrow and sharp peak around 2θ=32.94º which was assigned as the diffraction of Na8Si46(321). However, the peak at 2θ=32.94º disappeared when it was heated to above 640°C due to the decomposition of the silicon clathrates. Minute Zn2+ions in the gel was proved to promote the formation of Na-doped silicon clathrates. The HRTEM photos revealed that the thickness of the films was only about 15nm, and there existed no obvious interfaces between the film and the silicon substrate.

High Pressure Synthesis of Silicon Clathrates and Related Compounds

The use of high pressure and high temperature conditions is essential in the preparation of silicon clathrate compounds with electron excess compositions and related Si-rich binary compounds. The preparation conditions and crystal structures of Ba8Si46, Ba24Si100, BaSi6, LaSi5, LaSi10, NaxSi136 (x>24) and related Zintl silicides are discussed. These are the systems rich in superconductors.

Time-Resolved in Situ Synchrotron X-ray Diffraction Studies of Type 1 Silicon Clathrate Formation

Silicon clathrates are unusual open-framework solids formed by tetrahedrally bonded silicon that show remarkable electronic and thermal properties. The type I structure has a primitive cubic unit cell containing cages occupied by metal atoms to give compositions such as Na8Si46 and Na2Ba6Si46. Although their structure and properties are well described, there is little understanding of the formation mechanism. Na8Si46 is typically produced by metastable thermal decomposition under vacuum conditions from NaSi, itself an unusual structure containing Si44– polyanions. In this study, we used in situ synchrotron X-ray diffraction combined with rapid X-ray detection on samples taken through a controlled temperature ramp (25–500 °C at 8 °C/min) under vacuum conditions (10–4 bar) to study the clathrate formation reaction. We also carried out complementary in situ high-temperature solid-state 23Na NMR experiments using a sealed tube loaded under inert-gas-atmosphere conditions. We find no evidence for an intermediate amorphous phase during clathrate formation. Instead, we observe an unexpectedly high degree of structural coherency between the Na8Si46 clathrate and its NaSi precursor, evidenced by a smooth passage of several X-ray reflections from one structure into the other. The results indicate the possibility of an unusual, epitaxial-like, growth of the clathrate phase as Na atoms are removed from the NaSi precursor into the vacuum.

Semiclathrates of the Ge–P–Te System: Synthesis and Crystal Structures

Novel compounds [Ge(46-x) P(x) ]Te(y) (13.9≤x≤15.6, 5.92≤y≤7.75) with clathrate-like structures have been prepared and structurally characterized. They crystallize in the space group Fm ̅3 with the unit cell parameter changing from 20.544(2) to 20.698(2) Å (Z=8) on going from x=13.9 to x=15.6. Their crystal structure is composed of a covalently bonded Ge-P framework that hosts tellurium atoms in the guest positions and can be viewed as a peculiar variant of the type I clathrate superstructure. In contrast to the conventional type I clathrates, [Ge(46-x) P(x) ]Te(y) contain tricoordinated (3b) atoms and no vacancies in the framework positions. As a consequence of the transformation of the framework, the majority of the guest tellurium atoms form a single covalent bond with the host framework and thus the title compounds are the first representative of semiclathrates with covalent bonding. A comparison is made with silicon clathrates and the evolution of the crystal structure upon changing the tellurium content is discussed.

Chemical trends of the band gaps in semiconducting silicon clathrates

We have calculated the band structures of M8Si38Ga8 (M: Na, K, Rb, and Cs) using the density-functional theory with the generalized gradient corrected local density approximation. They are indirect semiconductors with the calculated gaps from 0.45 to 0.89 eV, the gaps becoming wider with the promotion to the heavier encapsulated guest alkali atoms. These are comparable to the calculated gap of 0.65 eV of crystalline Si with the diamond structure.

Chemical trends of the band gaps of idealized crystal of semiconducting silicon clathrates, M8Si38A8 (M = Na, K, Rb, Cs; A = Ga, Al, In), predicted by first-principle pseudopotential calculations

We have calculated the band structures of Si clathrate, M8Si38Ga8 (M=Na, K, Rb, and Cs), using the density-functional theory under the generalized gradient corrected local density approximation, where M is the encapsulated guest alkali atom. They are found to be indirect semiconductors with the calculated gaps (Eg) from 0.45 to 0.89eV, which should be compared to the calculated gap of 0.65eV of crystalline Si with the diamond structure. The gaps become wider with the promotion to the heavier guest alkali atoms and the reasons of gap widening are discussed using the calculated dependence of Eg on the cell-volume of guest-free silicon clathrate (Si46). Effect of the substitutional elements in the clathrate framework (Al and In in place of Ga) was also discussed.

First-principles calculations of carbon clathrates: Comparison to silicon and germanium clathrates

We employ state-of-the-art first-principles calculations based on density-functional theory and density-functional perturbation theory to investigate relevant physical properties and phase diagram of the free guest type-I $(X\text{\ensuremath{-}}46)$ and type-II $(X\text{\ensuremath{-}}34)$ carbon clathrates. Their properties and those of silicon and germanium diamonds, and clathrates have been computed and compared within the same approach. We briefly present and discuss their structural, cohesive, and electronic properties. In particular, we present different results about electronic properties of carbon clathrates. From the symmetry analysis of electronic states around the band gap, we deduce their optical properties, and we forecast the effects of hypothetical-doped elements on their electronic band gap. We then report first-principles calculations of vibrational, thermodynamical, and elastic properties. Whereas vibrational properties of Si and Ge systems can be linked through their atomic weight ratio, we show that the vibrational properties of carbon structures differ strongly. Raman and infrared spectra of all clathrates are also calculated and compared. The effects of pressure and temperature on thermodynamical properties (heat capacity, entropy, thermal expansion, etc.) within static and quasiharmonic approximations are investigated. It is shown that thermodynamical properties of carbon clathrates and diamond present a similar evolution up to high pressures (100 GPa) and over a large range of temperatures ([0, 1500] K). Then we deduce the equilibrium phase diagram $(P,T)$ of C-2/C-34/C-46. We conclude the paper with a presentation of elastic properties computed from acoustic slopes.

ChemInform Abstract: High-Pressure Synthesis of a New Silicon Clathrate Superconductor, Ba8Si46.

A new silicon clathrate compound with a composition of Ba8Si46 was prepared under high-pressure and high-temperature conditions. The compound was isomorphous with Na8Si46 and became a superconductor with a transition temperature of 8.0 K. Barium atoms occupy all of the Si20 and Si24 cages of the clathrate structure. This is the first clathrate superconductor obtained as a bulk phase.

Framework Contraction in Na-Stuffed Si(cF136)

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

Silicon clathrates and carbon analogs: high pressure synthesis, structure, and superconductivity

Compounds with cage-like structures of elemental silicon and carbon are comparatively reviewed. Barium containing silicon clathrate compounds isomorphous with type I gas hydrates were prepared using high pressure and high temperature (HPHT) conditions, and found to become superconductors. The application of HPHT conditions to Zintl binary silicides have produced a number of silicon-rich cage-like structures including new clathrate structures; most of them are superconductors. Carbon analogs of silicon clathrates can be prepared by 3D polymerization of C(60) under HPHT conditions, which are new allotropes of carbon with expanded framework structures. The crystal chemistry and characteristic properties of some related compounds are also reviewed.

Raman spectroscopy of type-I and type-VIII silicon clathrate alloys Sr 8Al xGa 16− xSi 30

Raman spectra of type-I and type-VIII silicon clathrates have been measured at ambient temperature and pressure for Sr 8Al x Ga 16− x Si 30 alloys prepared by changing the Al/Ga ratio. Although the broadening of Raman bands appeared, due to the disorder of the host framework by replacing the Si sites by Al and Ga atoms, the phase transition from type-I to type-VIII ( x = from 7 to 8) was confirmed from the following changes; in type-VIII phase, (1) Sr guest vibrations shift toward higher frequencies by the decrease in the size of host cage and (2) the appearance of sharp and strong peak of Sr vibration at about 75 cm −1. All Sr vibrational frequencies are consistently compared with the Einstein temperatures estimated from the atomic displacement parameters (ADPs) in X-ray diffraction (XRD) data.

ChemInform Abstract: Controlled Thermal Decomposition of NaSi to Derive Silicon Clathrate Compounds.

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A study of the vibrational and thermoelectric properties of silicon type I and II clathrates

First principles calculations are employed to investigate both type I and II silicon clathrates. The phonon and infrared spectra of both types of clathrate are obtained. We find a localized vibrational mode of Si46 whose frequency is 375.0 cm−1, where the vibration in the mode localizes in the hexagonal chain. The heat capacity of both clathrates is the same as that of the diamond phase Si (d-Si). When the temperature is lower than 100 K, the Debye temperatures of the clathrates are higher than that of d-Si; however, the Debye temperatures of both clathrates at high temperature (>100 K) are lower than that of the d-Si. The mean free paths (λ) and thermal conductivities (κ) of the clathrates are larger than those of d-Si at low temperature. The Seebeck coefficients (S) of the clathrates are higher than that of d-Si in the temperature interval 300–1000 K; however, both clathrates exhibit a lower value of σ/τ when compared to the d-Si.

Controlled thermal decomposition of NaSi to derive silicon clathrate compounds

Formation conditions of two types of sodium containing silicon clathrate compounds were determined by the controlled thermal decomposition of sodium monosilicide NaSi under vacuum. The decomposition began at 360 °C. Much higher decomposition temperatures and the presence of sodium metal vapor were favorable for the formation of type I clathrate compound Na 8Si 46. Type II clathrate compound Na x Si 136 was obtained as a single phase at a decomposition temperature <440 °C under the condition without sodium metal vapor. The type I clathrate compound was decomposed to crystalline Si above 520 °C. The type II clathrate compound was thermally more stable, and retained at least up to 550 °C in vacuum.

Phonon density of states, anharmonicity, electron-phonon coupling, and possible multigap superconductivity in the clathrate superconductorsBa8Si46andBa24Si100: Factors behind large difference inTc

We report a detailed study of specific heat, electrical resistivity, and thermal expansion in combination with inelastic neutron and inelastic x-ray scattering to investigate the origin of superconductivity in the two silicon clathrate superconductors ${\text{Ba}}_{8}{\text{Si}}_{46}$ and ${\text{Ba}}_{24}{\text{Si}}_{100}$. Both compounds have a similar structure based on encaged barium atoms in oversized silicon cages. However, the transition temperatures are rather different: 8 and 1.5 K, respectively. By extracting the superconducting properties, phonon density of states, electron-phonon coupling function, and phonon anharmonicity from these measurements, we discuss the important factors governing ${T}_{c}$ and explain the difference between the two compounds.

Publisher's Note: Charge transfer in silicon clathrates studied by Compton scattering [Phys. Rev. B76, 233104 (2007)

Publisher's Note: Charge transfer in silicon clathrates studied by Compton scattering [Phys. Rev. B<b>76</b>, 233104 (2007)

Charge transfer in silicon clathrates studied by Compton scattering

High-resolution Compton profiles of the clathrates ${\mathrm{Na}}_{8}{\mathrm{Si}}_{46}$, ${\mathrm{K}}_{7.6}{\mathrm{Si}}_{46}$, and ${\mathrm{Si}}_{136}$ were measured to study the charge transfer of the alkali-metal guest atoms to the clathrates silicon host lattice. The experiment was accomplished utilizing the Compton spectrometer at beamline ID15B of the European Synchrotron Radiation Facility with an incident energy of $56.7\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ and a momentum space resolution of $0.16\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.}$ The valence electron contribution to the total Compton profile of the sodium and potassium guest atoms was extracted and is compared with calculations employing density-functional theory using Becke exchange and Perdew-Wang correlation. Good overall agreement between experiment and theory is found and proves the quality of the underlying wave functions used for the theoretical predictions. On this basis, the charge transfer in these systems is estimated by a Mulliken population analysis and discussed for the different guest atoms.

Hydrogen Encapsulation in a Silicon Clathrate Type I Structure: Na5.5(H2)2.15Si46: Synthesis and Characterization

A hydrogen-encapsulated inorganic clathrate, which is stable at ambient temperature and pressure, has been prepared in high yield. Na5.5(H2)2.15Si46 is a sodium-deficient, hydrogen-encapsulated, type I silicon clathrate. It was prepared by the reaction between NaSi and NH4Br under dynamic vacuum at 300 degrees C. The Rietveld refinement of the powder X-ray diffraction data is consistent with the clathrate type I structure. The type I clathrate structure has two types of cages where the guest species, in this case Na and H2, can reside: a large cage composed of 24 Si, in which the guest resides in the 6d crystallographic position, and a smaller one composed of 20 Si, in which the guest occupies the 2a position. Solid-state 23Na, 1H, and 29Si MAS NMR confirmed the presence of both sodium and hydrogen in the clathrate cages. 23Na NMR shows that sodium completely fills the small cage and is deficient in the larger cage. The 1H NMR spectrum shows a pattern consistent with mobile hydrogen in the large cage. 29Si NMR spectrum is consistent with phase pure type I clathrate framework. Elemental analysis is consistent with the stoichiometry Na5.5(H2.15)2Si46. The sodium occupancy was also examined using spherical aberration (Cs) corrected scanning transmission electron microscopy (STEM). The high-angle annular dark-field (HAADF) STEM experimental and simulated images indicated that the Na occupancy of the large cage, 6d sites, is less than 2/3, consistent with the NMR and elemental analysis.

Characterization of Hydrogen-Encapsulated Type-I Silicon Clathrate

This study describes a new stable inorganic clathrate compound encapsulating hydrogen in the Si cage structure. In this work, the characterization of the new type-I Si clathrate, Na6(H2)2Si46, is illustrated. Na6(H2)2Si46 was prepared from NaSi and NH4X (X = Cl and Br) under dynamic vacuum at 300 oC. Rietveld refinement of powder X-ray diffraction data was consistent with the clathrate type-I structure. Solid-state 1 H, 29 Si and 23 Na MAS NMR confirmed the presence of both hydrogen and sodium in the clatharte cages. 23 Na NMR indicated that the guest sites are not fully occupied by Na. In order to validate the structure and investigate the Na content, the synthesized particles were examined using spherical aberration (Cs) corrected VG HB501 scanning transmission electron microscope (STEM) operated at 100 kV. Fig. 2 shows a high-angle annular dark-field (HAADF) image taken along [100] with the convergence semi-angle of 20 mrad and collection semi-angle of 70-210 mrad. Image calculation was made with Kirkland’s code [3] with the thickness of 21 nm and defocus value of 22.5 nm. The calculated image inserted in Fig. 2 is in good agreement with the experimental one. The ring pattern and the center of the ring correspond to the Si24 cage and the 6d sites, respectively. When observed along [100], the 6d sites are aligned in an atomic column which is apart from the nearest columns by 0.23 nm so that the Na occupancy in the 6d sites can be determined by comparing the experimental intensity profiles with calculated ones. Fig. 3 shows an averaged experimental image using 2dx software [4] and calculated images with different Na occupancies in the 6d sites as well as intensity profiles along the line X-Y. The peak intensity at the

Electronic and vibrational properties of framework-substituted type-II silicon clathrates

Framework substitution has not been widely reported among the type-II group-IV clathrates. We have performed a theoretical study of the ${\mathrm{Cs}}_{8}{\mathrm{Ga}}_{8}{\mathrm{Si}}_{128}$ and ${\mathrm{Rb}}_{8}{\mathrm{Ga}}_{8}{\mathrm{Si}}_{128}$ clathrates, using the local-density approximation (LDA) to density-functional theory. In ${\mathrm{Rb}}_{8}{\mathrm{Ga}}_{8}{\mathrm{Si}}_{128}$ and ${\mathrm{Cs}}_{8}{\mathrm{Ga}}_{8}{\mathrm{Si}}_{128}$, eight Si atoms are substituted by Ga at the $8a$ crystallographic sites of the framework. We predict that both ${\mathrm{Rb}}_{8}{\mathrm{Ga}}_{8}{\mathrm{Si}}_{128}$ and ${\mathrm{Cs}}_{8}{\mathrm{Ga}}_{8}{\mathrm{Si}}_{128}$ are semiconductors with LDA indirect band gaps in the range of $0.73--0.77\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Similar substitution with Ge (instead of Ga) rendered the materials metallic. We have compared the electronic and vibrational properties of these Ga-substituted materials with those of the partially filled ${\mathrm{Rb}}_{8}{\mathrm{Si}}_{136}$ and completely filled ${\mathrm{Na}}_{16}{\mathrm{Rb}}_{8}{\mathrm{Si}}_{136}$ clathrates. The phonon-dispersion curves show low-frequency guest ``rattling'' modes for each material. We have used these results to estimate the temperature-dependent values of the isotropic mean-square displacement amplitude $({U}_{\mathrm{iso}})$ for the various guest atoms in ${\mathrm{Na}}_{16}{\mathrm{Rb}}_{8}{\mathrm{Si}}_{136}$, ${\mathrm{Rb}}_{8}{\mathrm{Ga}}_{8}{\mathrm{Si}}_{128}$, and ${\mathrm{Cs}}_{8}{\mathrm{Ga}}_{8}{\mathrm{Si}}_{128}$. Our estimates for Na and Rb in ${\mathrm{Na}}_{16}{\mathrm{Rb}}_{8}{\mathrm{Si}}_{136}$ are in good agreement with experiment. We predict that the Rb guests will have low-frequency vibrational modes and higher values of ${U}_{\mathrm{iso}}$ than the Cs guests. The large values of ${U}_{\mathrm{iso}}$, the localized low-frequency modes, and the semiconducting properties of ${\mathrm{Rb}}_{8}{\mathrm{Ga}}_{8}{\mathrm{Si}}_{128}$ indicate that this type of framework-substituted type-II clathrate may be potentially useful for thermoelectric applications.