Stable binary icosahedral quasicrystals (i-QCs) based on rare earth (RE) and cadmium are typically accessed by solution growth experiments, which operate in very narrow composition and temperature windows. Here, we present a procedure which allows study of peritectic reactions between approximant crystal (AC) phase and liquid yielding i-RECd and exemplify with i-GdCd and ternary variants where Cd is partially replaced by isovalent Zn (i-Gd(Cd,Zn)) or Mg (i-Gd(Cd,Mg)), or the 4f element Gd is replaced by nonmagnetic Y (i-(Gd,Y)Cd). The solubility limits for Zn and Mg substitution are about 10% and 20%, respectively, whereas Gd and Y show a complete solid solution behavior. We find that the peritectic decomposition temperature for i-GdCd is 390 degrees C, which is decreased when Gd is replaced by Y (i-YCd: 350 degrees C) and increased when Cd is replaced by Zn (i-Gd(Cd90Zn10): 440 degrees C), and especially by Mg (i-Gd(Cd80Mg20): 520 degrees C). Whereas substitution decisively alters the decomposition temperature (and hence stability) of the considered i-QCs, the decomposition temperature of the corresponding AC phases remains at around 700 degrees C. During the investigation of the pseudobinary phase diagrams Gd-(Cd95Zn5), Gd-(Cd90Zn10), and Gd-(Cd(80M)g(20)), faceted i-QCs grains with sizes up to 4 x 4 x 4 mm(3) could be isolated.

The intermetallic compound ZnSb is a (II-V) narrow gap semiconductor with interesting thermoelectric properties. Electrical resistivity, Hall coefficient, thermopower and thermal conductivity were measured up to 400 K on Ag-doped samples with concentrations 0.2, 0.5, 1, 2, and 3 at.%, which were consolidated to densities in excess of 99.5 % by spark plasma sintering. The work confirms a huge improvement of the thermoelectric Figure-of-merit, ZT, upon Ag doping. The optimum doping level is near 0.5 at.% Ag and results in ZT values around 1.05 at 390 K. The improvement stems from a largely decreased resistivity, which in turn relates to an increase of the hole charge carrier concentration by two orders of magnitude. It is argued that Ag can replace minute concentrations of Zn (on the order of 0.2 at.%) in the crystal structure which enhances the intrinsic impurity band of ZnSb. Excess Ag was found to segregate in grain boundaries. So the best performing material may be considered as a composite Zn similar to 0.998Ag similar to 0.002Sb/Ag-similar to 0.003.

Tsai-type quasicrystals and approximants are distinguished by a cluster unit made up of four concentric polyhedral shells that surround a tetrahedron at the center. Here we show that for Tsai-type 1/1 approximants in the RE-Au-Si systems (RE = Gd, Tb, Ho) the central tetrahedron of the Tsai clusters can be systematically replaced by a single RE atom. The modified cluster is herein termed a pseudo-Tsai cluster and represents, in contrast to the conventional Tsai cluster, a structural motif without internal symmetry breaking. For each system, single-phase samples of both pseudo-Tsai and Tsai-type 1/1 approximants were independently prepared as millimeter-sized, faceted, single crystals using the self-flux synthesis method. The full replacement of tetrahedral moieties by RE atoms in the pseudo-Tsai 1/1 approximants was ascertained by a combination of single-crystal and powder diffraction studies, as well as energy dispersive X-ray spectroscopy (EDX) analyses with a scanning electron microscope (SEM). Differential scanning calorimetry (DSC) studies revealed distinctly higher decomposition temperatures, by 5-35 K, for the pseudo-Tsai phases. Furthermore, the magnetic properties of pseudo-Tsai phases are profoundly and consistently different from the Tsai counterparts. The onset temperatures of magnetic ordering (T-mag) are lowered in the pseudo-Tsai phases by similar to 30% from 24 to 17 K, 11.5 to 8 K, and 5 to 3.5 K in the Gd-Au-Si, Tb-Au-Si, and Ho-Au-Si systems, respectively. In addition, the Tb-Au-Si and Ho-Au-Si systems exhibit some qualitative changes in their magnetic ordering, indicating decisive changes in the magnetic state/structure by a moment-bearing atom at the cluster center.

Binary silicon boride SiB3 has been reported to occur in two forms, as disordered and nonstoichiometric alpha-SiB3-x, which relates to the alpha-rhombohedral phase of boron, and as strictly ordered and stoichiometric beta-SiB3. Similar to other boron-rich icosahedral solids, these SiB3 phases represent potentially interesting refractory materials. However, their thermal stability, formation conditions, and thermodynamic relation are poorly understood. Here, we map the formation conditions of alpha-SiB3-x and beta-SiB3 and analyze their relative thermodynamic stabilities. alpha-SiB3-x is metastable (with respect to beta-SiB3 and Si), and its formation is kinetically driven. Pure polycrystalline bulk samples may be obtained within hours when heating stoichiometric mixtures of elemental silicon and boron at temperatures 1200-1300 degrees C. At the same time, alpha-SiB3-x decomposes into SiB6 and Si, and optimum time-temperature synthesis conditions represent a trade-off between rates of formation and decomposition. The formation of stable beta-SiB3 was observed after prolonged treatment (days to weeks) of elemental mixtures with ratios Si/B = 1:11:4 at temperatures 1175-1200 degrees C. The application of high pressures greatly improves the kinetics of SiB3 formation and allows decoupling of SiB3 formation from decomposition. Quantitative formation of beta-SiB3 was seen at 1100 degrees C for samples pressurized to 5.5-8 GPa. beta-SiB3 decomposes peritectoidally at temperatures between 1250 and 1300 degrees C. The highly ordered nature of beta-SiB3 is reflected in its Raman spectrum, which features narrow and distinct lines. In contrast, the Raman spectrum of alpha-SiB3-x is characterized by broad bands, which show a clear relation to the vibrational modes of isostructural, ordered B6P. The detailed composition and structural properties of disordered alpha-SiB3-x were ascertained by a combination of single-crystal X-ray diffraction and Si-29 magic angle spinning NMR experiments. Notably, the compositions of polycrystalline bulk samples (obtained at T <= 1200 degrees C) and single crystal samples (obtained from Si-rich molten Si-B mixtures at T > 1400 degrees C) are different, SiB2.93(7) and SiB2.64(2), respectively. The incorporation of Si in the polar position of B-12 icosahedra results in highly strained cluster units. This disorder feature was accounted for in the refined crystal structure model by splitting the polar position into three sites. The electron-precise composition of alpha-SiB3-x is SiB2.5 and corresponds to the incorporation of, on average, two Si atoms in each B-12 icosahedron. Accordingly, alpha-SiB3-x constitutes a mixture of B10Si2 and B11Si clusters. The structural and phase stability of alpha-SiB3-x were explored using a first-principles cluster expansion. The most stable composition at 0 K is SiB2.5, which however is unstable with respect to the decomposition beta-SiB3 + Si. Modeling of the configurational and vibrational entropies suggests that alpha-SiB3-x only becomes more stable than beta-SiB3 at temperatures above its decomposition into SiB6 and Si. Hence, we conclude that alpha-SiB3-x is metastable at all temperatures. Density functional theory electronic structure calculations yield band gaps of similar size for electron-precise alpha-SiB2.5 and beta-SiB3, whereas alpha-SiB3 represents a p-type conductor.

In today’s society, where energy conservation and green energy are buzz words, new scientific discoveries in green energy harvesting is key. This work focuses on materials capable of recycling low value thermal energy. Low value thermal energy, waste heat, is for free, and can be transformed into valuable electricity via thermoelectric technology. A thermoelectric device cleanly converts heat into electricity through the Seebeck effect. Thermoelectric devices can play an important role in satisfying the future global need for efficient energy management, however, the primary barrier of improving thermoelectric devices is the materials themselves.

The aim of this thesis is to identify new compositions and structures for thermoelectric materials. In particular, the concept of “electron poor framework semiconductors” is explored. Electron Poor Framework Semiconductors (EPFS) are materials at the border between metals and non-metals, which often show intricate and unique structures with complex bonding schemes. Generally, constituting elements should be from group 12(II) (Zn, Cd), 13(III) (B, Al, Ga, In), 14(IV) (Si, Ge, Sn, Pb), 15(V) (Sb, Bi), and 16(VI) (Te), i.e. elements which have a similar electronegativity (between 1.5-2.0). All EPFS materials have in common highly complex crystal structures, which are thought to be a consequence of their electron-poor bonding patterns. EPFS materials have an intrinsically very low – glass like - lattice thermal conductivity. The focus of this thesis is on combinations of group 12(II) (Zn) with 16 (V) (As, Sb), and 13(III) (B) with 14(IV) (Si).

ZnSb possesses a simple structure with 8 formula units in an orthorhombic unit cell, it is considered a stoichiometric compound without noticeable structural disorder. In this thesis ZnSb is used as a model system to establish more broadly structure–property correlations in Sb based EPFS materials.

ZnSb was established to possess an impurity band that determines electrical transport properties up to 300–400 K. Doping of ZnSb with Ag seems to enhance the impurity band by increasing the number of acceptor states and improving charge carrier density by two orders of magnitude. ZT values of Ag doped ZnSb are found to exceed 1 at 350 K. The origin of the low thermal conductivity of ZnSb was traced back to a multitude of localized low energy optic modes, acting as Einstein-like rattling modes.

ZnAs was accessed through high pressure synthesis. The compound is isostructural to ZnSb and possess an indirect band gap of 0.9 eV, which is larger than that for ZnSb (0.5 eV). The larger band gap is attributed to the higher polarity of Zn-As bonds. The electrical resistivity of ZnAs is higher and the Seebeck coefficient is lower compared to ZnSb. However, ZnAs and ZnSb exhibit similarly low lattice thermal conductivity, although As is considerably lighter than Sb. This was explained by their similar bonding properties.

Lastly, the longstanding mystery of SiB₃ phases was resolved. The formation of metastable and disordered α-SiB_3-x is fast and thus kinetically driven, whereas formation of stable β-SiB₃ is slow and not quantitative unless high pressure conditions are applied. This thesis work established reproducible synthesis routes for both materials. The fast kinetics can be exploited for simultaneous synthesis and sintering of α -SiB_3-x specimens in a SPS device. It is suggested that α -SiB_3-x represents a promising refractory thermoelectric material.

The intermetallic compound ZnSb is an interesting thermoelectric material largely due to its low lattice thermal conductivity. The origin of the low thermal conductivity has so far been speculative. Using multitemperature single crystal x-ray diffraction (9-400 K) and powder x-ray diffraction (300-725 K) measurements, we characterized the volume expansion and the evolution of structural properties with temperature and identified an increasingly anharmonic behavior of the Zn atoms. From a combination of Raman spectroscopy and first principles calculations of phonons, we consolidate the presence of low-energy optic modes with wave numbers below 60 cm(-1). Heat capacity measurements between 2 and 400 K can be well described by a Debye-Einstein model containing one Debye and two Einstein contributions with temperatures Theta(D) = 195 K, Theta(E1) = 78 K, and Theta(E2) = 277K as well as a significant contribution due to anharmonicity above 150 K. The presence of a multitude of weakly dispersed low-energy optical modes (which couple with the acoustic, heat carrying phonons) combined with anharmonic thermal behavior provides an effective mechanism for low lattice thermal conductivity. The peculiar vibrational properties of ZnSb are attributed to its chemical bonding properties, which are characterized by multicenter bonded structural entities. We argue that the proposed mechanism to explain the low lattice thermal conductivity of ZnSb might also control the thermoelectric properties of other electron poor semiconductors, such as Zn4Sb3, CdSb, Cd4Sb3, Cd13-xInyZn10, and Zn5Sb4In2-delta.

ZnAs was synthesized at 6 GPa and 1273 K utilizing multianvil highpressure techniques and structurally characterized by single-crystal and powder X-ray 7 diffraction (space group Pbca (No. 61), a = 5.6768(2) angstrom, b = 7.2796(2) angstrom, c = 7.5593(2) angstrom, Z = 8). The compound is isostructural to ZnSb (CdSb type) and displays multicenter bonded rhomboid rings Zn2As2, which are connected to each other by classical two-center, two-electron bonds. At ambient pressure ZnAs is metastable with respect to Zn3As2 and ZnAs2. When heating at a rate of 10 K/min decomposition takes place at similar to 700 K. Diffuse reflectance measurements reveal a band gap of 0.9 eV. Electrical resistivity, thermopower, and thermal conductivity were measured in the temperature range of 2-400 K and compared to thermoelectric ZnSb. The room temperature values of the resistivity and thermopower are similar to 1 Omega cm and +27 mu V/K, respectively. These values are considerably higher and lower, respectively, compared to Zn Sb. Above 150 K the thermal conductivity attains low values, around 2 W/m.K, which is similar to that of ZnSb. The heat capacity of ZnAs was measured between 2 and 300 K and partitioned into a Debye and two Einstein contributions with temperatures of theta(D) = 234 K, theta(E1) = 95 K, and theta(E2) = 353 K. Heat capacity and thermal conductivity of ZnSb and ZnAs show very similar features, which possibly relates to their common electron-poor bonding properties.

Yb-Cd (Tsai-type) quasicrystals constitute the largest icosahedral quasicrystal family where Yb can be replaced by other rare earth elements (RE) and Cd by pairs of p- and d-block elements. YbCd6 is a prototype 1/1 Tsai-type approximant phase which has a similar local structure to the Yb-Cd quasicrystal. In this study, the syntheses of Yb15.78Au65.22Ge19.00, Gd14.34Au67.16Ge18.5 and Gd14.19Au69.87Si15.94 Tsai-type 1/1 quasicrystal approximants are optimized using the self-flux technique. The crystal structures of the compounds are refined by collecting single crystal x-ray diffraction data. The structural refinements indicated that the compounds are essentially isostructural with some differences at their cluster centers. The basic polyhedral cluster unit in all the three compounds can be described by concentric shells of icosahedra symmetry and of disordered tetrahedra and/or a rare earth atom at the cluster center. Furthermore, the thermoelectric properties of the compounds are probed and their dimensionless figures of merit are calculated at different temperatures. A significant difference is observed in their thermoelectric properties, which could arise due to the slight difference in their crystal structure and chemical composition, as we move from Ge to Si and/or Gd to Yb. Therefore, this study shows the systematic effect of the chemical substitution of structurally similar materials on their thermoelectric properties.

The intermetallic compound ZnSb is an electron poor (II-V) semiconductor with interesting thermoelectric properties. Electrical resistivity, thermopower and thermal conductivity were measured on single crystalline and various polycrystalline specimens. The work establishes the presence of impurity band conduction as an intrinsic phenomenon of ZnSb. The impurity band governs electrical transport properties at temperatures up to 300-400 K after which ZnSb becomes an intrinsic conductor. Furthermore this work establishes an inherently low lattice thermal conductivity of ZnSb, which is comparable to the state-of-the- art thermoelectric material PbTe. It is argued that the impurity band relates to the presence of Zn defects and the low thermal conductivity to the electron-poor bonding properties of ZnSb.