Searching for compounds with intrinsic low lattice thermal conductivity has been proven a successful strategy for achieving high thermoelectric performance. Herein, employing density functional theory calculations combined with electron and phonon Boltzmann transport theories, we report that Sr3AlSb3 and Ba3AlSb3 within the Zintl 3–1–3 compositional family exhibit record low thermal conductivities of 0.78 and 0.55 W/mK at room temperature, respectively. These low thermal conductivities are rooted in low-energy optical phonon modes with strong anharmonicity and the emergence of high-energy flat optical phonon modes with zero contribution to the lattice thermal conductivity. Heavier cationic atoms are found to soften low-lying optical phonon modes, which enhance phonon scattering and, therefore, favor a lower thermal conductivity. These combined characteristics lead to high and balanced figure of merit values around 2.3 for Zintl Ba3AlSb3 at both optimal p-type and n-type doping and high temperature. Our work highlights the important role of flat optical phonon modes on designing promising thermoelectric materials with intrinsic low thermal conductivity.

Colloidal quantum dots (QDs) of groups II−VI and III−V are key ingredients for next-generation light-emitting devices. Yet, many of them are heavy-element-containing or indirect bandgap, causing limited choice of environmental friendly efficient lightemitting materials. Herein, we resolve this issue by exploring potential derivatives of the parent semiconductors, thus expanding the material space. The key to success is the discovery of a principle for designing those materials, namely, cation stabilizing charged cluster network. Guided by this principle, three novel categories of cubic materials have been predicted, namely, porous binary compounds, I−II−VI ternary compounds, and I−II−III−V quaternary compounds. Using first-principles calculations, 65 realistic highly stable candidate materials have been theoretically screened. Their structural and compositional diversity enables a wide tunability of emitting wavelength from far-infrared to ultraviolet region. This work enriches the family of tetrahedral semiconductors and derivatives, which may be of interest for a broad field of optoelectronic applications.

Environmentally friendly colloidal quantum dots (QDs) of groups III–V are in high demand for next-generation high-performance light-emitting devices for display and lighting, yet many of them (e.g., GaP) suffer from inefficient band-edge emission due to the indirect bandgap nature of their parent materials. Herein, we theoretically demonstrate that efficient band-edge emission can be activated at a critical tensile strain γc enabled by the capping shell when forming a core/shell architecture. Before γc is reached, the emission edge is dominated by dense low-intensity exciton states with a vanishing oscillator strength and a long radiative lifetime. After γc is crossed, the emission edge is dominated by high-intensity bright exciton states with a large oscillator strength and a radiative lifetime that is shorter by a few orders of magnitude. This work provides a novel strategy for realizing efficient band-edge emission of indirect semiconductor QDs via shell engineering, which is potentially implemented employing the well-established colloidal QD synthesis technique.