Daniel B. Moore

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.

Misfit layer compounds and ferecrystals: Model systems for thermoelectric nanocomposites

A basic summary of thermoelectric principles is presented in a historical context, following the evolution of the field from initial discovery to modern day high-zT materials. A specific focus is placed on nanocomposite materials as a means to solve the challenges presented by the contradictory material requirements necessary for efficient thermal energy harvest. Misfit layer compounds are highlighted as an example of a highly ordered anisotropic nanocomposite system. Their layered structure provides the opportunity to use multiple constituents for improved thermoelectric performance, through both enhanced phonon scattering at interfaces and through electronic interactions between the constituents. Recently, a class of metastable, turbostratically-disordered misfit layer compounds has been synthesized using a kinetically controlled approach with low reaction temperatures. The kinetically stabilized structures can be prepared with a variety of constituent ratios and layering schemes, providing an avenue to systematically understand structure-function relationships not possible in the thermodynamic compounds. We summarize the work that has been done to date on these materials. The observed turbostratic disorder has been shown to result in extremely low cross plane thermal conductivity and in plane thermal conductivities that are also very small, suggesting the structural motif could be attractive as thermoelectric materials if the power factor could be improved. The first 10 compounds in the [(PbSe)1+δ]m(TiSe2)n family (m, n ≤ 3) are reported as a case study. As n increases, the magnitude of the Seebeck coefficient is significantly increased without a simultaneous decrease in the in-plane electrical conductivity, resulting in an improved thermoelectric power factor.

Synthesis and characterization of turbostratically disordered (BiSe)1.15TiSe2

The synthesis and characterization of turbostratically disordered (BiSe)1.15TiSe2 is reported. Specular and in-plane x-ray diffraction studies indicate an alternating structure containing two planes of a distorted rock salt structured BiSe and a Se–Ti–Se trilayer of TiSe2 with independent lattices. The title compound was found to be turbostratically (rotationally) disordered about the c-axis, and the BiSe layer displays an orthorhombic in-plane structure with a = 4.562(2) A and b = 4.242(1) A. Temperature dependent electrical resistivity reveals that the disordered compound is metallic, but with less temperature dependence than may be expected for a 3D crystal, which is attributed to the lack of coherent vibrations due to the turbostratic disorder. The room temperature resistivity was found to be ρ = 5.0 × 10−6 Ωm with a carrier concentration of n = 5 × 1021 cm−3. Comparing the carrier concentration to (PbSe)1.16TiSe2 suggests that the bismuth is trivalent and donates an electron to the conduction band of the TiSe2 constituent.

Carrier dilution in TiSe2 based intergrowth compounds for enhanced thermoelectric performance

Synthesis and electrical properties of kinetically stabilized (PbSe)1+δ(TiSe2)n thin-film intergrowths are reported for 1 ≤ n ≤ 18. A linear increase in the c-lattice parameter of the intergrowth is observed as n is increased and the slope is consistent with the inclusion of an additional TiSe2 structural unit as n is incremented by 1 and the observed intercept is consistent with the expected thickness of a PbSe bilayer. The charge donated to the TiSe2 constituent from the PbSe is diluted across more layers as n is increased, leading to a systematic increase in the Seebeck coefficient. The room temperature resistivity values of the reported compounds are all on the order of 10−5 Ω m and depend on defect densities that affect the mobility, making the magnitude of the resistivity less sensitive to n. The temperature dependence is metallic for large n, with a slight upturn at low temperatures due to localization of carriers for small n values. The power factor increases with n, including the highest reported for chalcogenide misfit layered and related compounds, showing that nanostructuring and modulation doping are an effective means of tuning the power factor of thermoelectric intergrowth materials. Since these compounds have very low thermal conductivity due to structural anisotropy and misregistration between intergrowth constituents, this suggests that varying their nanoarchitecture is a promising approach to obtain high values of zT.

Kinetically Controlled Formation and Decomposition of Metastable [(BiSe)1+δ]m[TiSe2]m Compounds

Preparing homologous series of compounds allows chemists to rapidly discover new compounds with predictable structure and properties. Synthesizing compounds within such a series involves navigating a free energy landscape defined by the interactions within and between constituent atoms. Historically, synthesis approaches are typically limited to forming only the most thermodynamically stable compound under the reaction conditions. Presented here is the synthesis, via self-assembly of designed precursors, of isocompositional incommensurate layered compounds [(BiSe)1+δ] m[TiSe2] m with m = 1, 2, and 3. The structure of the BiSe bilayer in the m = 1 compound is not that of the binary compound, and this is the first example of compounds where a BiSe layer thicker than a bilayer in heterostructures has been prepared. Specular and in-plane X-ray diffraction combined with high-resolution electron microscopy data was used to follow the formation of the compounds during low-temperature annealing and the subsequent decomposition of the m = 2 and 3 compounds into [(BiSe)1+δ]1[TiSe2]1 at elevated temperatures. These results show that the structure of the precursor can be used to control reaction kinetics, enabling the synthesis of kinetically stable compounds that are not accessible via traditional techniques. The data collected as a function of temperature and time enabled us to schematically construct the topology of the free energy landscape about the local free energy minima for each of the products.

Detection of nanoscale embedded layers using laboratory specular X-ray diffraction

Unusual specular X-ray diffraction patterns have been observed from certain thin film intergrowths of metal monochalcogenide (MX) and transition metal dichalcogenide (TX2) structures. These patterns exhibit selective “splitting” or broadening of selected (00l) diffraction peaks, while other (00l) reflections remain relatively unaffected [Atkins et al., Chem. Mater. 24, 4594 (2012)]. Using a simplified optical model in the kinematic approximation, we illustrate that these peculiar and somewhat counterintuitive diffraction features can be understood in terms of additional layers of one of the intergrowth components, MX or TX2, interleaved between otherwise “ideal” regions of MX-TX2 intergrowth. The interpretation is in agreement with scanning transmission electron microscope imaging, which reveals the presence of such stacking “defects” in films prepared from non-ideal precursors. In principle, the effect can be employed as a simple, non-destructive laboratory probe to detect and characterize ultrathin laye...

Synthesis of four new members of the (PbSe) 1.16 (TiSe 2 ) n (n = 1, 2, 3, and 4) Family of ferecrystals

A new family of turbostratically disordered misfit layered compounds (ferecrystals), (PbSe)1.16(TiSe2)n, is described with n values of 1, 2, 3 and 4. These compounds were synthesized using the modulated elemental reactant method (MER). The high-resolution transmission electron micrographs of these compounds clearly show rotational disorder in the constituent layers of these compounds. θ – 2θ x-ray diffraction scans contain many 00l superlattice diffraction peaks. The c axis lattice parameters determined from this data show a systematic increase as n, the number of TiSe2 layer, is increased.