Compressibility and High-Pressure Structure of CaMg2Bi2 and YbMg2Bi2

Compounds with the formula AM2X2 in the CaAl2Si2 structure type have garnered increasing interest across various solid-state research domains, such as quantum topological and thermoelectric materials. Prior studies have identified high-pressure phase transitions in several compounds, including Mg3Sb2, Mg3Bi2, CaMn2Bi2, and SrAl2Si2. In this study, we investigate the structural behavior of CaMg2Bi2 and YbMg2Bi2 under varying pressure conditions. We synthesized crystals using the molten metal flux method and examined them through single-crystal synchrotron X-ray diffraction, employing diamond anvil cells to exert pressures up to 20 GPa. Our analysis reveals insights into the anisotropic compressibility of these materials, highlighting the more compressible and flexible octahedral A-Bi bonds as the primary contributors to this anisotropy. Moreover, we observed a phase transition in both CaMg2Bi2 and YbMg2Bi2 at pressures above 9.6 and 8.7 GPa, respectively. The newly identified high-pressure phase exhibits a distortion of the original CaAl2Si2 structure with space group C2/m. This high-pressure structure is distinct from that of related compounds (e.g., CaMn2Bi2, MgMg2Bi2), the latter exhibiting a square pyramidal coordination for the M site. These results illustrate how pressure can reorganize bonding and coordination in AM2X2 Zintl phases, providing insight into the structural mechanisms that govern phase stability. The anisotropy in compressibility may also influence directional phonon transport, with implications for tuning lattice thermal conductivity through pressure, strain, or chemical substitution.