Structural, Electronic, and Thermal Properties of CdSnAs2.

Structural, electrical, and thermal properties of CdSnAs2, with analyses from temperature-dependent transport properties over a large temperature range, are reported. Phase-pure microcrystalline powders were synthesized that were subsequently densified to a high-density homogeneous polycrystalline specimen for this study. Temperature-dependent transport indicates n-type semiconducting behavior with a very high and nearly temperature independent mobility over the entire measured temperature range, attributed to the very small electron effective mass of this material. The Debye model was successfully applied to model the thermal conductivity and specific heat. This work contributes to the fundamental understanding of this material, providing further insight and allowing for investigations into altering this and related physical properties of these materials for technological applications.

Synthesis and Thermal Properties of Solid-State Structural Isomers: Ordered Intergrowths of SnSe and MoSe2.

A family of structural isomers [(SnSe)1.05]m(MoSe2)n were prepared using the modulated elemental reactant method by varying the layer sequence and layer thicknesses in the precursor. By varying the sequence of Sn-Se and Mo-Se layer pairs deposited and annealing the precursors to self-assemble the targeted compound, all six possible isomers [(SnSe)1.05]4(MoSe2)4, [(SnSe)1.05]3(MoSe2)3[(SnSe)1.05]1(MoSe2)1, [(SnSe)1.05]3(MoSe2)2[(SnSe)1.05]1(MoSe2)2, [(SnSe)1.05]2(MoSe2)3[(SnSe)1.05]2(MoSe2)1, [(SnSe)1.05]2(MoSe2)1[(SnSe)1.05]1(MoSe2)2[(SnSe)1.05]1(MoSe2)1, and [(SnSe)1.05]2(MoSe2)2[(SnSe)1.05]1(MoSe2)1[(SnSe)1.05]1(MoSe2)1 were prepared. The structures were characterized by X-ray diffraction and electron microscopy which showed that all of the compounds have very similar c-axis lattice parameters and in-plane constituent lattice parameters yet distinct isomeric structures. These studies confirm that the structure, order, and thickness of the constituent layers match that of the precursors. The cross-plane thermal conductivity is found to be very low (∼0.08 Wm(-1) K(-1)) and independent of the number of SnSe-MoSe2 interfaces within uncertainty. The poor thermal transport in these layered isomers is attributed to a large cross-plane thermal resistance created by SnSe-MoSe2 and MoSe2-MoSe2 turbostratically disordered van der Waals interfaces, the density of which has less variation among the different compounds than the SnSe-MoSe2 interface density alone.

Preparation, formation, and structure of [(SnSe)1.04]m(MoSe2)n intergrowth compounds (0 < m and n < 32) from designed precursors.

A detailed synthetic approach, using the method of modulated elemental reactants, is described for the preparation of MX-TX(2) (M = metal, X = chalcogen, and T = transition metal) solid-state intergrowths with m and n values significantly larger than previously reported. As a specific example, we demonstrate the ability to synthesize more than 500 distinct intergrowth compounds in a single ternary system, Sn-Mo-Se. A simple method for determination of the chemical composition of the constituent layers in the precursor and product is described for cases in which both structural components contain one or more common elements. X-ray reflectivity, laboratory and synchrotron X-ray diffraction, scanning transmission electron microscopy, and high-resolution transmission electron microscopy imaging, and electron microprobe analysis provide conclusive evidence of the formation of layered intergrowths with well-defined structure and composition. The ability to access a large range of monochalcogenide thicknesses allows a size-dependent structural transition in the SnSe component to be controlled and tracked and indicates that intergrowth materials such as those described here comprise novel material systems in which size-dependent phenomena can be precisely controlled.

Precursor Routes to Complex Ternary Intermetallics: Single-Crystal and Microcrystalline Preparation of Clathrate-I Na8Al8Si38 from NaSi + NaAlSi.

Single crystals of the ternary clathrate-I Na8Al8Si38 were synthesized by kinetically controlled thermal decomposition (KCTD), and microcrystalline Na8Al8Si38 was synthesized by spark plasma sintering (SPS) using a NaSi + NaAlSi mixture as the precursor. Na8AlxSi46-x compositions with x ≤ 8 were also synthesized by SPS from precursor mixtures of different ratios. The crystal structure of Na8Al8Si38 was investigated using both Rietveld and single-crystal refinements. Temperature-dependent transport and UV/vis measurements were employed in the characterization of Na8Al8Si38, with diffuse-reflectance measurement indicating an indirect optical gap of 0.64 eV. Our results indicate that, when more than one precursor is used, both SPS and KCTD are effective methods for the synthesis of multinary inorganic phases that are not easily accessible by traditional solid-state synthesis or crystal growth techniques.

Raman spectroscopy insights into the size-induced structural transformation in SnSe nanolayers.

Raman spectroscopy is used to probe the structural changes in [SnSe]m[MoSe2]n ferecrystal thin films as a function of m, the number of bilayers of SnSe. In spite of the interleaved structure in the intergrowths, Raman spectra can be described as a superposition of spectra from the individual components, indicating that the interaction at the interface between the components is relatively weak. Analysis of room-temperature Raman spectra indicate that the MoSe2 layers separating the SnSe layers are nanocrystalline in all of the samples studied, with little change as the number of Se-Mo-Se trilayers (n) or SnSe bilayers (m) increases, reflecting the rotational disorder between adjacent trilayers. A thickness-dependent, continuous transition occurs in the SnSe layer as m is increased, from a pseudotetragonal structure when the layers are thin to a bulk-like orthorhombic SnSe structure when the SnSe layer thickness is increased. Polarization analysis of the Raman scattering from these materials allows the symmetry evolution of the SnSe layers through this transition to be determined.

Telluride misfit layer compounds: [(PbTe)1.17]m(TiTe2)n.

Telluride misfit layer compounds are reported for the first time. These compounds were synthesized using a novel approach of structurally designing a precursor that would form the desired product upon low-temperature annealing, which allows the synthesis of kinetically stable products that do not appear on the equilibrium phase diagram. Four new compounds of the [(PbTe)(1.17)]m(TiTe2)n family are reported, and their structures were examined by a variety of X-ray diffraction techniques.

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).

Preparation and crystal growth of Na24Si136.

The synthesis and single crystal growth of clathrate-II Na(24)Si(136) is performed in one step applying the spark plasma treatment to the precursor Na(4)Si(4). The reported results demonstrate a new route to intermetallic compounds facilitated by the electric field and current. SPS is revealed to offer significant opportunities as a novel preparatory method for synthesis and crystal growth of solid state materials.

Zintl Phases as Reactive Precursors for Synthesis of Novel Silicon and Germanium-Based Materials.

Recent experimental and theoretical work has demonstrated significant potential to tune the properties of silicon and germanium by adjusting the mesostructure, nanostructure, and/or crystalline structure of these group 14 elements. Despite the promise to achieve enhanced functionality with these already technologically important elements, a significant challenge lies in the identification of effective synthetic approaches that can access metastable silicon and germanium-based extended solids with a particular crystal structure or specific nano/meso-structured features. In this context, the class of intermetallic compounds known as Zintl phases has provided a platform for discovery of novel silicon and germanium-based materials. This review highlights some of the ways in which silicon and germanium-based Zintl phases have been utilized as precursors in innovative approaches to synthesize new crystalline modifications, nanoparticles, nanosheets, and mesostructured and nanoporous extended solids with properties that can be very different from the ground states of the elements.

Synthesis and crystal structure of Na1-xGe3+z: a novel zeolite-like framework phase in the Na-Ge system.

A novel crystalline binary phase is reported in the Na-Ge system, with an entirely new, zeolite-like crystal structure solved and refined by the combination of synchrotron X-ray and neutron powder diffraction techniques.

Confined lattice dynamics of single and quadruple SnSe bilayers in [(SnSe)(1.04)](m)[MoSe2](n) ferecrystals.

The Sn specific densities of phonon states in the SnSe subunits of [(SnSe)1.04]m[MoSe2]n ferecrystals with (m,n) = (1,1), (4,1) and in bulk SnSe were derived from nuclear inelastic scattering by the (119)Sn Mössbauer resonance. Using different measurement configurations, phonons with polarization parallel and perpendicular to the ferecrystal plane were specifically probed. Vibrational properties and phonon spectral weight are found to strongly depend on the phonon polarization and layer count m. A highly peculiar feature of these ferecrystal densities of phonon states is the emergence of rather sharp high energy vibrational modes polarized perpendicular to the ferecrystal plane, which contrasts with usual findings in thin layered structures and nanostructured materials in general, and a depletion of modes with a gap appearing between acoustic and high energy modes. The spectral weight of these phonons depends on the overall SnSe content, m, but cannot be unambiguously attributed to SnSe-MoSe2 interfaces. Considering the low energy part of lattice dynamics, ferecrystals exhibit rather low average phonon group velocities as compared to the speed of sound in the long wavelength limit. For the (1,1) ferecrystal, this effect is most pronounced for vibrations polarized in the ferecrystal plane. Thus, an experimental microscopic origin for the vibrational and bonding anisotropy in subunits of ferecrystals is provided.

Binary Alkali-Metal Silicon Clathrates by Spark Plasma Sintering: Preparation and Characterization.

The binary intermetallic clathrates K8-xSi46 (x = 0.4; 1.2), Rb6.2Si46, Rb11.5Si136 and Cs7.8Si136 were prepared from M4Si4 (M = K, Rb, Cs) precursors by spark-plasma route (SPS) and structurally characterized by Rietveld refinement of PXRD data. The clathrate-II phase Rb11.5Si136 was synthesized for the first time. Partial crystallographic site occupancy of the alkali metals, particularly for the smaller Si20 dodecahedra, was found in all compounds. SPS preparation of Na24Si136 with different SPS current polarities and tooling were performed in order to investigate the role of the electric field on clathrate formation. The electrical and thermal transport properties of K7.6Si46 and K6.8Si46 in the temperature range 4-700 K were investigated. Our findings demonstrate that SPS is a novel tool for the synthesis of intermetallic clathrate phases that are not easily accessible by conventional synthesis techniques.

Influence of guest loading on thermal properties of NaxSi136 clathrates.

Thermal properties of a series of type II clathrates of the formula NaxSi136 with 0 < x < 24 and Na guests occupying the Si cages have been investigated over the temperature range from 2 to 300 K. Heat capacity and thermal conductivity results show that the structure is remarkably responsive to the loading of Na guests. The response is phononic: the host lattice expands in a non-monotonic way, and first stiffens, then relaxes at low loading into the larger Si28 cages (x < 9), then stiffens again as the Na concentration increases further. The response is also electronic, through changes in electronic properties as additional Na is loaded into the smaller Si20 cages at high loading (x > 9). In total, the influence of the guest loading illustrates the complexities of structure-property relations in a guest-host system.