First-Principles Calculations with Six Structures of Alkaline Earth Metal Cyanide A(CN) (A = Be, Mg, Ca, Sr, and Ba): Structural, Electrical, and Phonon Properties.

This work examines six structures (4̅3, 4 , 3, 2/, 3̅, and 2/) of alkaline earth metal cyanide A(CN) (A = Be, Mg, Ca, Sr, and Ba) using first-principles calculations. The symmetries of 4̅3, 4 , and 3 reflect a variation of 3̅, previously reported as occurring on Be(CN) and Mg(CN) in X-ray diffraction studies, while the symmetries of 2/, 3̅, and 2/ were selected from the 3̅1 symmetry found using Mg(OH) as the initial structures, with -OH being replaced by -CN. The band structure, density of states, and phonon properties of all A(CN) structures were then investigated using density functional theory (DFT), with a generalized gradient approximation (GGA) applied for the exchange and correlation energy values. The simulation results for the phonon spectra indicate that the stable structures are Be(CN) (4̅3, 4 , and 2/), Mg(CN) (4̅3, 4 , and 2/), Ca(CN) (2/), Sr(CN) (2/ and 3̅), and Ba(CN) (3̅) at 0 GPa. For the effects of high pressure, Ca(CN) and Sr(CN) were thus found to be stable as 2/ at pressures above 10 and 3 GPa, respectively, while Ca(CN) is as stable as 3̅ above 15 GPa. In the calculated band structures, all of the compounds with the 2/ structure demonstrated good conductivity, while the other structures have a band gap range of 2.83-6.33 eV.