First-Principles Calculations with Six Structures of Alkaline Earth Metal Cyanide A(CN)(2) (A = Be, Mg, Ca, Sr, and Ba): Structural, Electrical, and Phonon Properties

[Image: see text] This work examines six structures (P4̅3m, P4(2)nm, R3m, P2(1)/c, R3̅m, and C2/m) of alkaline earth metal cyanide A(CN)(2) (A = Be, Mg, Ca, Sr, and Ba) using first-principles calculations. The symmetries of P4̅3m, P4(2)nm, and R3m reflect a variation of Pn3̅m, previously reported as occurring on Be(CN)(2) and Mg(CN)(2) in X-ray diffraction studies, while the symmetries of P2(1)/c, R3̅m, and C2/m were selected from the P3̅m1 symmetry found using Mg(OH)(2) as the initial structures, with −OH being replaced by −CN. The band structure, density of states, and phonon properties of all A(CN)(2) 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)(2) (P4̅3m, P4(2)nm, and C2/m), Mg(CN)(2) (P4̅3m, P4(2)nm, and C2/m), Ca(CN)(2) (P2(1)/c), Sr(CN)(2) (P2(1)/c and R3̅m), and Ba(CN)(2) (R3̅m) at 0 GPa. For the effects of high pressure, Ca(CN)(2) and Sr(CN)(2) were thus found to be stable as C2/m at pressures above 10 and 3 GPa, respectively, while Ca(CN)(2) is as stable as R3̅m above 15 GPa. In the calculated band structures, all of the compounds with the C2/m structure demonstrated good conductivity, while the other structures have a band gap range of 2.83–6.33 eV.