Experimental confirmation of the stability and chemical bonding analysis of the high-pressure phases Ca-I, II, and III at pressures up to 52 GPa

Synchrotron x-ray powder diffraction on Ca was performed at pressures up to 52.6(3) GPa using a diamond-anvil cell at ambient temperature. The sequence of phase transitions from fcc (Ca-I) to bcc (Ca-II) at 19.8 GPa and bcc to simple cubic (sc Ca-III) at 33 GPa was confirmed. A sample of the Ca-III phase was remelted at a pressure above 40 GPa and still remained in the sc structure. The quality of the obtained powder diffraction patterns doubtlessly confirms the stability of the sc phase within the framework of the experiment at pressures above 33 GPa. The bulk modulus of the high-pressure phases was calculated based on fitting of third-order Birch-Murnaghan equations of state with K=17.4(5) GPa, K(')=3.22(4) for fcc Ca-I, K=51.9(3.4) GPa, K(')=4.2(6) for bcc Ca-II, and K=165.2(4.7) GPa, K(')=1.0(3) for sc Ca-III, respectively. In terms of electron localizability indicator (ELI-D), the chemical bonding in the calcium modifications may be described as multicenter bonding. With increasing pressure, the number of centers per bond changes from 6 (and 4) in Ca-I via 4+2 for Ca-II to 8 for Ca-III.