The ability to predict materials with desired thermal conductivity from a large material database can significantly improve the efficiency of experimental work. Lattice thermal conductivity is controlled by the velocity and relaxation time of phonons (lattice vibrations). Phonon scattering is closely related to the anharmonic lattice vibrations of a material, while phonon velocity depends on density and, bond stiffness. In this research, the relationship between structure, bonding, and thermal properties is discussed in two classes of layered materials, AM2X2 intermetallic compounds and GeTe - Sb2Te3 alloys. First, we study the origin of the anomalously low lattice thermal conductivity of MgMg2Sb2 compare to other isostructural AMg2Pn2 compounds (A = Mg, Ca, Yb, and Pn = Sb and Bi ). By employing high-temperature X-ray diffraction (XRD) and resonant ultrasound spectroscopy (RUS) techniques, we have shown that the low lattice thermal conductivity is due to previously-unrecognized soft shear modes and highly anharmonic acoustic phonons in layered MgMg2Sb2. Combined with the phonon calculations from our collaborators, we attribute the anomalous thermal behavior of MgMg2Sb2 to the instability of the vibrational modes that originated from the weak bonding of the Mg, which is too small for the octahedral site. Second, we investigate the phase stability of the AMg2Pn2 system with mixed occupancy of Mg, Ca, Sr, or Ba on the cation (A) site. We show that the small ionic radius of Mg2+ leads to limited solubility when alloyed with larger cations such as Sr or Ba. Third, by performing in-situ high-pressure synchrotron X-ray diffraction, we showed that a few AM2X2 compounds can exhibit phase transitions at high-pressure, most of which are previously unrecognized. In addition, we observed that the compressibility of MgMg2Sb2 and MgMg2Bi2 is near-isotropic, whereas other isostructural AM2X2 compounds show clear signs of anisotropy between the in-plane and out-of-plane compressibility as is typical of layered compounds. We have analyzed the compressibility, transition pressure/temperature, anisotropy, as well as the type of phase transition to develop a deeper understanding of the stability and bond strength of different AM2X2 compounds. Lastly, we observed and explained the lattice stiffening and flattened lattice thermal conductivity curve with increasing temperature in GeTe - Sb2Te3 alloys. Unlike most compounds that soften with increasing temperature, the elastic moduli of (GeTe)17 - Sb2Te3 stiffen with increasing temperature before the phase transition. We investigate GeTe, Sb2Te3, and (GeTe)17 - Sb2Te3 from room temperature up to the phase transitions with high-temperature XRD, high-temperature RUS, and transport property measurements. We attribute the stiffening behavior to the gradual diffusion of layered vacancies to random vacancies on the cation site, which profoundly impact the elastic properties and the transport properties of the material.
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