ABSTRACT
This work examines six structures (P4Ì 3m, P42 nm, R3m, P21/c, R3Ì m, 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 P4Ì 3m, P42 nm, and R3m reflect a variation of Pn3Ì m, previously reported as occurring on Be(CN)2 and Mg(CN)2 in X-ray diffraction studies, while the symmetries of P21/c, R3Ì m, and C2/m were selected from the P3Ì m1 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 (P4Ì 3m, P42 nm, and C2/m), Mg(CN)2 (P4Ì 3m, P42 nm, and C2/m), Ca(CN)2 (P21/c), Sr(CN)2 (P21/c and R3Ì m), and Ba(CN)2 (R3Ì m) 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 R3Ì m 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.