Fig. 1. (a) A convex hull diagram for the Ca–CaCl₂ system at selected pressures. At a given pressure, the compounds located on the convex hull are thermodynamically stable. ΔH denotes the enthalpy of formation per atom. (b) Pressure–composition phase diagram of the Ca–CaCl₂ system.

Fig. 2. Crystal structures of stable Ca–Cl compounds. (a) P-62m-CaCl₂ at 60 GPa along the (001) and (110) directions. (b) Fm-3m-CaCl at 30 GPa. (c) Pm-3m-CaCl at 50 GPa. (d) P-1-Ca₅Cl₆ at 50 GPa. (e) R-3-Ca₃Cl₄ at 80 GPa. The large blue and small green spheres represent Ca and Cl atoms, respectively.

Fig. 3. Electronic structures of Fm-3m-CaCl at 30 GPa and Pm-3m-CaCl at 50 GPa. (a) Band structure and projected density of states (PDOS) of Fm-3m-CaCl at 30 GPa. (b) Band structure and PDOS of Pm-3m-CaCl at 50 GPa. (c) ELF of Fm-3m-CaCl at 30 GPa. The ELF = 0.8 isosurface is shown. (d) ELF of Pm-3m-CaCl at 50 GPa (showing the ELF = 0.6 isosurface). The large blue and small green spheres represent Ca and Cl atoms, respectively.

Fig. 4. Ferromagnetism of Pm-3m-CaCl under high pressure. (a) Spin density at 50 GPa. The 0.0035 e/bohr³ isosurface is shown. The blue and green spheres represent Ca and Cl atoms, respectively. (b) Magnetic moment per formula unit as a function of pressure. The increasing delocalization of the d electrons of calcium leads to magnetic collapse at 110 GPa.

Fig. 5. Powder XRD patterns and equation of state. (a) Pm-3m-CaCl and P-62m-CaCl₂ synthesized at 50(3) GPa. The X-ray wavelength is 0.6199 Å. Experimental data (black points) are compared with simulated patterns with DFT (blue and green lines) and full-profile refinements using the Rietveld method (red line) with differences. Some peaks marked by asterisks may belong to impurities. The positions of reflections of Pm-3m-CaCl and P-62m-CaCl₂ are indicated by blue and green tick marks, respectively. (b) Powder XRD patterns of Pm-3m-CaCl and P-62m-CaCl₂ at different pressures. Some peaks marked by asterisks may belong to impurities. (c) Equation of state of CaCl synthesized in DAC in comparison with that of Ca.⁵⁰ Experimental parameters (symbols) are compared with DFT predictions (lines). The error bars correspond to the experimental pressure uncertainty due to pressure gradients and pressure measurements.³⁹