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In this work, we discover anomalously low lattice thermal conductivity (<0.25 W/mK at 300 °C) in the Hg-containing quaternary diamond-like semiconductors within the Cu2IIBIVTe4 (IIB: Zn, Cd, Hg) (IV: Si, Ge, Sn) set of compositions. Using high-temperature X-ray diffraction, resonant ultrasound spectroscopy, and transport properties, we uncover the critical role of the antisite defects HgCu and CuHg on phonon transport within the Hg-containing systems. Despite the differences in chemistry between Hg and Cu, the high concentration of these antisite defects emerges from the energetic proximity of the kesterite and stannite cation motifs. Our phonon calculations reveal that heavier group IIB elements not only introduce low-lying optical modes, but the subsequent antisite defects also possess unusually strong point defect phonon scattering power. The scattering strength stems from the fundamentally different vibrational modes supported by the constituent elements (e.g., Hg and Cu). Despite the significant impact on the thermal properties, antisite defects do not negatively impact the mobility (>50 cm2/(Vs) at 300 °C) in Hg-containing systems, leading to predicted zT > 1.5 in Cu2HgGeTe4 and Cu2HgSnTe4 under optimized doping. In addition to introducing a potentially new p-type thermoelectric material, this work provides (1) a strategy to use the proximity of phase transitions to increase point defect phonon scattering, and (2) a means to quantify the power of a given point defect through inexpensive phonon calculations.

GeTe-Sb2Te3 alloys have been widely studied for use in rewritable media, and in recent years, they have emerged as excellent thermoelectric materials, with reports of zT&gt;2 for Ge-rich compositions. GeTe-Sb2Te3 alloys exhibit a solid-state phase transition from a layered structure with rhombohedral symmetry to a cubic rocksalt structure, which plays an important role in their thermoelectric behavior. Here, we investigate the impact of the phase transition on the thermal expansion and elastic moduli of (GeTe)17Sb2Te3 using high-temperature X-ray diffraction and resonant ultrasound spectroscopy. The high-temperature elastic moduli of GeTe, Sb2Te3, and Bi2Te3 were also measured for comparison. While it is typical for materials to soften with increasing temperature due to thermal expansion, our study reveals anomalous hardening of the elastic moduli in (GeTe)17Sb2Te3 at temperatures below the phase transition, followed by further hardening at the transition temperature. In contrast, the elastic moduli of GeTe, Sb2Te3, and Bi2Te3 soften with increasing temperature. We attribute the anomalous hardening of (GeTe)17Sb2Te3 to the gradual vacancy diffusion accompanying the transition from a layered to a cubic structure. The stiffening elastic moduli lead to increasing speed of sound, which impacts the lattice thermal conductivity by flattening the temperature dependence.

A study of 63 metal-tantalate-oxides was conducted in search of heavy scintillator materials operating at ambient temperature. Tantalates are known to have slow scintillation decay times, however, the high atomic number of tantalum (73) provides good stopping power for gamma rays. The samples were synthesized by solid state reactions. The physical, optical, and scintillation properties of these materials were evaluated by X-ray diffraction, X-ray excited luminescence and pulsed X-ray luminescence. Out of the 63 synthesized tantalates examined, only 12 had luminosity values greater than 1,000 ph/MeV at room temperature. From these, three compounds ScTaO 4 , YTa 3 O 9 , and Zn 3 Ta 2 O 8 have greater than 40% of their emission in the first µs. The brightest and fastest compound of those tested was Zn 3 Ta 2 O 8 with an estimated luminosity of 26,000 ph/MeV and a main decay time of 600 ns from its crystalline powder. Further attention is given to Zn 3 Ta 2 O 8 and Mg 4 Ta 2 O 9 scintillation properties, demonstrating their potential for scintillation applications.