Context: Li2VMgO6, Li2NbMgO6, and Li2TaMgO6 double perovskite compounds were energetically the most stable in the FM phase. The lattice constants were 7.63 Å, 7.94 Å, and 7.95 Å, and the Curie temperatures were 910.451 K, 930.739 K, and 1258.821 K, respectively. The wide bandgap semiconductor characters were provided in the GGA-PBE methods as 2.139 eV, 4.209 eV, and 5.007 eV, respectively. This wide band gap semiconductor state in the majority carriers and the metallic state in the minority states made these double perovskites true half-metallic ferromagnetics. The bulk modulus obtained in the ground state calculations and the values obtained from thermodynamic calculations were relatively close. Debye temperatures in the initial state conditions were 747 K, 685.13 K, and 587.77 K, respectively. The total magnetic moment values were calculated as 3.00 µB/f.u. The most significant contribution to this value came from oxygen atoms. Methods: The theoretical calculations of Li2VMgO6, Li2NbMgO6, and Li2TaMgO6 double perovskite alloys were performed using the WIEN2k program developed by Blaha et al. The electronic calculations were made with GGA-PBE, GGA + mBJ, and GGA + U approximations in the space number 225 and the Fm-3 m symmetry group. The thermodynamic calculations were performed using Gibbs2. In thermodynamic calculations, temperature increases were determined as 100 K and temperature values were increased from 0 to 1200 K. [ABSTRACT FROM AUTHOR]