The devised structures were subjected to geometry relaxations of the atomic positions and the lattice constants down to the ground state characterized by minimum energy. The iterative computations were performed using the DFT-based plane-wave Vienna Ab initio Simulation Package (VASP) . The projector augmented wave (PAW) method was used for the atomic potentials . The exchange X and correlation C effects (XC) were treated within a generalized gradient approximation (GGA) scheme . Test calculations with a hybrid functional HSE06 did not result in significant changes of the GGA results. The relaxation of the atoms to the ground state geometry was performed using a conjugate-gradient algorithm . The Blöchl tetrahedron method with corrections according to the Methfessel and Paxton scheme was applied for geometry optimization and energy calculations, respectively. A special k-point sampling was used to approximate the reciprocal space Brillouin-zone (BZ) integrals. For better reliability, the optimization of the structural parameters was carried out along with successive self-consistent cycles with increasing k-mesh until the forces on the atoms were below 0.02 eV/Å and the stress components below 0.003 eV/Å 3 .