Stability and superconductivity of Ca-B phases at ambient and high pressure

In the search for MgB2-like phonon-mediated superconductors we have carried\nout a systematic density functional theory study of the Ca-B system,\nisoelectronic to Mg-B, at ambient and gigapascal pressures. A remarkable\nvariety of candidate high-pressure ground states have been identified with an\nevolutionary crystal structure search, including a stable alkaline-earth\nmonoboride oI8-CaB, a superconductor with an expected critical temperature (Tc)\nof 5.5 K. We have extended our previous study of CaB6 [Phys. Rev. Lett. 108,\n102501 (2012)] to nearby stoichiometries of CaB[6+x], finding that extra boron\nfurther stabilizes the proposed B24 units. Here an explanation is given for the\ntransformation of cP7-CaB6 into the more complex oS56 and tI56 polymorphs at\nhigh pressure. The stability of the known metallic tP20 phase of CaB4 at\nambient pressure is explained from a crystal structure and chemical bonding\npoint of view. The tP20 structure is shown to destabilize at 19 GPa relative to\na semiconducting MgB4-like structure due to chemical pressure from the metal\nion. The hypothetical AlB2-type structure of CaB2, previously shown to have\nfavorable superconducting features, is demonstrated here to be unstable at all\npressures; two new metallic CaB2 polymorphs with unusual boron networks\nstabilize at elevated pressures above 8 GPa but are found to have very low\ncritical temperatures (Tc ~1 K). The stability of all structures has been\nrationalized through comparison with alkaline-earth analogs, emphasizing the\nimportance of the size of the metal ion for the stability of borides. Our study\nillustrates the inverse correlation between the thermodynamic stability and\nsuperconducting properties and the necessity to carefully examine both in the\ndesign of new synthesizable superconducting materials.\n