Difference in the outermost layer between TaB2(0001) and HfB2(0001)

Atomic and electronic structures of TaB2(0001) and HfB2(0001) surfaces are investigated with use of the first-principles pseudopotential calculations. Our calculated surface formation energies indicate that the graphitic-boron-terminated TaB2(0001) surface is energetically more favorable, whereas the HfB2(0001) surface prefers termination with a close-packed Hf layer. These Endings are consistent with experimental facts. We have also found the difference in the surface relaxation between them. The first interlayer spacing of the B-terminated TaB2 surface is expanded by 0.39%. On the other hand, the outermost Hf-terminated HfB2 surface is contracted by 4.8%. [S0163-1829(99)05147-4].