Vacancy-Ordered Halide Perovskites for High Entropy Semiconductors

Due to their quasi-0D / molecular-aggregate type crystal structure, vacancy-ordered double perovskites (VODPs) with the chemical formula A₂BX₆, exhibit unusual material properties associated with both zero-dimensional and three-dimensional materials.^1–4 These include low thermal conductivity, high compressibility, and strong exciton binding despite relatively small semiconducting band gaps, making them potential candidates for a range of alternative applications, such as thermoelectrics, white-light emitters/phosphors, photocatalysts, non-linear optics and more.<br/> <br/>In this study, we report a combined experimental and computational investigation on the mixing behavior of cations in this system. Remarkably, we find ultra-low enthalpic costs to cation mixing, resulting in entropy dominance and ideal mixing behavior. This facilitates the room-temperature and low-temperature synthesis of high-entropy materials (high-entropy semiconductors) from these compounds, as demonstrated experimentally by Folgueras et al in <i>Nature</i>, 2023.⁵ We elucidate the underlying structural and electronic origins of this facile cation miscibility in these systems, and analyze the resulting optical, thermodynamic and structural changes upon cation mixing.<br/> <br/>Our work demonstrates that vacancy-ordered perovskites present an exciting new class of high-entropy semiconductors, synthesisable at much milder conditions than typical high-entropy materials. Moreover, we elucidate the origins of this behavior, allowing the extraction of general design rules for high-entropy semiconductors with tailored properties.<br/> <br/>1 S. R. Kavanagh, C. N. Savory, S. M. Liga, G. Konstantatos, A. Walsh and D. O. Scanlon, <i>J. Phys. Chem. Lett.</i>, 2022, <b>13</b>, 10965–10975.<br/>2 Y.-T. Huang, S. R. Kavanagh, D. O. Scanlon, A. Walsh and R. L. Z. Hoye, <i>Nanotechnology</i>, 2021, <b>32</b>, 132004.<br/>3 B. Cucco, C. Katan, J. Even, M. Kepenekian and G. Volonakis, <i>ACS Materials Lett.</i>, 2023, <b>5</b>, 52–59.<br/>4 B. Saparov, J.-P. Sun, W. Meng, Z. Xiao, H.-S. Duan, O. Gunawan, D. Shin, I. G. Hill, Y. Yan and D. B. Mitzi, <i>Chem. Mater.</i>, 2016, <b>28</b>, 2315–2322.<br/>5 M. C. Folgueras, Y. Jiang, J. Jin and P. Yang, <i>Nature</i>, 2023, 1–7.<br/>6 S. M. Liga‡ & S. R. Kavanagh‡, A. Walsh, D. O. Scanlon and G. Konstantatos. <i>J. Phys. Chem. C.</i>, 2023