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

This work examines six structures (P4̅3m, P42nm, R3m, P21/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, P42nm, 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 P21/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, P42nm, and C2/m), Mg(CN)2 (P4̅3m, P42nm, and C2/m), Ca(CN)2 (P21/c), Sr(CN)2 (P21/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.