The lattice thermal conductivity, κL, is a fundamental physical property which characterizes the heat transport due to the solid’s lattice vibrations (phonons). This intrinsic property is of great importance for the performance of a wide variety of devices, such as thermal barrier coating, electronics, optoelectronic, and thermoelectrics. In thermoelectrics, i.e., materials that convert heat to electricity or vice versa, having low κL is critical for efficient thermal-to-electrical energy conversion. The heat transport by phonons which, in turn control the phonon group velocities and scattering rates, are related to the solid’s bond stiffness and anharmonicity. To study these effects, a combination of high-pressure synchrotron X-ray diffraction (HP-XRD) and high-temperature resonant ultrasound spectroscopy (HT-RUS) was used to quantify both local and average bondstiffness and anharmonic effects. High-pressure experiments were carried out to investigate the stability and bond stiffness of AM2X2 Zintl phases. For this purpose, single- and poly-crystals of several compounds, including Mg3Bi2, Mg3Sb2, CaMg2Bi2, and YbMg2Bi2, were synthesized. These samples were hydrostatically compressed inside diamond anvil cells for in-situ synchrotron radiation X-ray diffraction. As a result, the compressibility of the unit cell, lattice parameters, and individual bonds were obtained. It is shown that the octahedral A cation sites on all compositions are more compressible than the tetrahedral M cation sites. Furthermore, the influence of ionic radii on the compressibility was investigated. Additionally, reversible high-pressure phase transitions were discovered, and the high-pressure structures were solved, which are shown to be monoclinic, with space group C2/m. In the second part of this thesis, HT-RUS was used to investigate the average bond stiffness (e.g., as quantified by the elastic constants) as a function of temperature and composition in the (GeSe)1-x- (AgBiSe2)x (x=0, 0.1, 0.2, 0.3, 0.4) system. The crystal structure progressively transitions from an orthorhombic Pnma, to a rhombohedral R3m, to a cubic Fm3 ̄m arrangement, with marked consequences on the lattice thermal conductivity. With temperature, all the compositions eventually evolve to the rock-salt phase. From the HT-RUS experiments, it was determined that the cubic structure shows, on average, stiffer bonds compared to the other symmetries (rhombohedral and orthorhombic) present in this material system, which ultimately increases κL. In contrast, using scattering rate estimations, it is shown that an increase in alloying leads to a suppression of κL, mainly due to alloy scattering. From these observations, it is demonstrated how κL is suppressed by the combined influence of the softening of elastic constants and an increase in the scattering rate.
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