  Based on crystal chemistry rationale and calculations within the density
functional theory DFT, the U:B system is complemented with additional binary
compounds UB3, U2B6, and UB6 possessing two-dimensional 2D and 3D boron
substructures. Observations are supported quantitatively with the trends of
cohesive energies, charge transfers onto the boron sub-lattice and geometry
optimized structures. The results point out to a structure crossover from
hexagonal (layer B network) to 3D boron network at compositions above UB3 found
to be connected with a threshold amount of charge onto boron which is ~0.46.
From the energy-volume of states EOS considering spin degenerate and
spin-polarized configurations, hexagonal UB3, and cubic UB6 were found in a
stable ferromagnetic ground state with 1.47 and 2.40Bohr Magnetons spin-only
moments. The volume variations of magnetization show respectively a smooth and
abrupt evolution for UB3 and UB6.
