Resolving the pressure induced ‘self-insertion’ in skutterudite CoSb3

CoSb₃ belongs to the skutterudite family of compounds and serves as a crucial platform for the exploration of thermoelectric materials, however, its importance is equally high for studies of strong correlations at high pressures. Under compression it undergoes a ‘self-insertion’ isostructural transition resulting in a peculiar redistribution of large Sb atoms between different crystallographic sites. We conducted a comprehensive investigation of the structural phase stability of CoSb₃ up to 70 GPa using single crystal samples characterized employing conventional single crystal X-ray diffraction and X-ray scattering focused on measuring Bragg peak at high resolution (including elements of Bragg Coherent Diffraction Imaging). We explore the compression behavior of CoSb₃ in three different pressure transmitting media (PTM) and address several important, but previously unexplored topics: the influence of various PTMs and nonhydrostatic stresses on the strongly correlated system of CoSb₃, including the ‘self-insertion’ crossover, the phase stability of CoSb₃, the compound’s polymorphism, its crystal chemistry, and its peculiar evolution under pressure at ambient temperature. Among other important observations, we track the population of Sb atoms within the dodecahedral sites of CoSb₃ on compression, during the process of ‘self-insertion’, and on decompression. We detect that ‘self-insertion’ may not only reduce the solid’s compressibility, but also make it negative. Finally, but not least, we report that the ‘self-insertion’ crossover is an important step preceding a previously unknown phase transformation from cubic $\mathrm{Im}\bar{3}$ CoSb₃ into trigonal $R\bar{3}$ occurring above 40 GPa, and discuss the distinctive behavior of CoSb₃ phases and their structural frameworks.