The crystal structure and bonding characteristics of intermetallic compounds critically influence their thermal and elastic properties. Polymorphic phase transitions – where crystal structures transform without altering atomic composition – offer a unique window to directly probe the relationship between atomic arrangement and thermal properties. Intermetallic Zintl compounds provide an intriguing case study, as they exhibit both ionic and covalent bonding frameworks within a single crystal lattice. Within the AMX Zintl family (where A = alkali-metal or alkaline earth metal, M = transition metal, X = non-metal), a series of closely related crystal structures feature a covalent sublattice that transitions progressively from a two-dimensional (2D), graphene-like configuration to a fully interconnected three-dimensional (3D) network. By examining these crystallographic transitions, we uncovered concrete correlations between the dimensionality of covalent bonding and the resulting thermal properties. We first explored the changes in thermal transport properties in the compound YbCuBi, as its crystal structure transitions from a flat 2D covalent sublattice (Space group – P63/mmc) to a buckled quasi-2D covalent network (Space group – P63mc) with periodic interlayer interactions. Using a combination of resonance ultrasound spectroscopy, inelastic neutron scattering, and first-principles calculations, we studied the impacts of this crystallographic transition on acoustic and optical phonons. Thermal conductivity measurements elucidated how changes in phonon energies and elastic behavior impact the thermal transport characteristics. Building on this, we also investigated a quasi-2D to 3D covalent phase transition in the CaAgSb1-xBix solid solution. Isoelectronic substitution of Sb by Bi systematically alters the elastic properties, while the crystallographic transition induces a ‘step-like’ change. Lattice parameters from x-ray diffraction reveal the underlying mechanism of the phase transition, while resonance ultrasound spectroscopy elucidates its impact on elastic properties. We also explore the limitations of Weidemann-Franz approximation for heat transport by charge carriers, highlighting the boundaries in our current understanding of thermal transport by quasiparticles. In the process, we discover promising thermoelectric properties in CaAgSb, attributed to its low thermal conductivity and high electronic mobility. According to the single parabolic band model, reducing carrier concentration could potentially enhance the thermoelectric performance of CaAgSb. Therefore, we conduct a ‘phase-boundary mapping’ study, combining first-principles density functional theory calculations and experiments to elucidate the behavior of carrier-generating defects under different growth conditions. We identify Ag vacancies as the defects with the lowest formation energy under all growth conditions, limiting Fermi level tuning to a narrow window, and suggesting that other routes may be needed to optimize the thermoelectric efficiency of CaAgSb.
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