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

Abstract

In the search for MgB2-like phonon-mediated superconductors we have carried out a systematic density functional theory study of the Ca-B system, isoelectronic to Mg-B, at ambient and gigapascal pressures. A remarkable variety of candidate high-pressure ground states have been identified with an evolutionary crystal structure search, including a stable alkaline-earth monoboride oI8-CaB, a superconductor with an expected critical temperature (Tc) of 5.5 K. We have extended our previous study of CaB6 [Phys. Rev. Lett. 108, 102501 (2012)] to nearby stoichiometries of CaB6+x, finding that extra boron further stabilizes the proposed B24 units. Here an explanation is given for the transformation of cP7-CaB6 into the more complex oS56 and tI56 polymorphs at high pressure. The stability of the known metallic tP20 phase of CaB4 at ambient pressure is explained from a crystal structure and chemical bonding point of view. The tP20 structure is shown to destabilize at 19 GPa relative to a semiconducting MgB4-like structure due to chemical pressure from the metal ion. The hypothetical AlB2-type structure of CaB2, previously shown to have favorable superconducting features, is demonstrated here to be unstable at all pressures; two new metallic CaB2 polymorphs with unusual boron networks stabilize at elevated pressures above 8 GPa but are found to have very low critical temperatures (Tc ∼ 1 K). The stability of all structures has been rationalized through comparison with alkaline-earth analogs, emphasizing the importance of the size of the metal ion for the stability of borides. Our study illustrates the inverse correlation between the thermodynamic stability and superconducting properties and the necessity to carefully examine both in the design of new synthesizable superconducting materials.

Phase diagram and superconductivity of calcium borohyrides at extreme pressures

“…However, the pressure behavior of the binary systems (Ca-B, Ca-H, B-H), which form the edge of the ternary hull, was studied by ab-initio calculations in recent years, in search for possible superconductors. Superconducting structures were found in all three systems, with a maximum T c of 235 K in CaH 6 and 21 K in BH, both above 150 GPa, and 6 K in CaB at 30 GPa [39,64,65].…”

Synthesis of a predicted layered LiB via cold compression

“…The oI8 structure with zig-zagged B chains metastable in this case was predicted to be thermodynamically stable for the related CaB binary 50 . The instability of hP4-LiB 1D has been associated with a large DOS(E F ermi ) which can be lowered by change in stoichiometry 17 or the proposed distortion at high pressures (Figs.…”

“…Unexpectedly, our frozen phonon calculations 39,40 revealed that the family of Li 2n B 1D m phases across a large composition range becomes dynamically unstable under moderate pressures, a behavior not recognized in previous ab initio studies 17,[27][28][29][30]47,48 . Construction of most stable derived structures is a known challenge due to the possibility of multiple local minima with similar energies/enthalpies 43,49,50 . We carefully examined relevant stoichiometries with 1/2 ≤ x ≤ 4/7 under relevant (P, T ) conditions and identified several dynamically stable derivatives.…”

Electronic structure, stability, and oxidation of boron-magnesium clusters and cluster solids

“…31,32 Magnesium borides also find use as part of lightweight composite materials and in the thermoelectric properties of borides for systems like MgB 4 , MgB 7 , and other binary alloys. [33][34][35][36][37][38][39][40] The purpose of this work is to investigate the atomic and electronic structure, stability, and reactivity of B n Mg m − clusters containing up to 15 B and up to 3 Mg atoms. The ultimate objective is to identify clusters that could serve as a) Author to whom correspondence should be addressed.…”