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We study the structures and magnetic phase stability of rare earth diboride compounds, RB2, using density functional simulations within the local density approximation. At zero pressure the hexagonal (P6/mmm) structure is energetically stable and at high pressure these materials prefer to keep the same structure. The five different elastic constants, bulk modulus, shear modulus, and hardness of all the hexagonal compounds have been calculated. The pressure dependence of the volumes is determined. The calculated large bulk modulus and high hardness reveal that they are incompressible and hard materials. The structural parameters and magnetic phase stability for RB2 (R = Tb, Dy, Ho, Er, Tm, and Yb) compare quite well with experimental results. For the others rare earth materials, our results are predictions.

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a b s t r a c tWe studied from first-principles the recently discovered BiS 2 -layered superconductor La 0.5 Th 0.5 OBiS 2 and compared with LaO 0.5 F 0.5 BiS 2 . Firstly, we have performed a global geometry optimization in order to predict an accurate ground state. In contract to the parent semiconductor LaOBiS 2 and according to other recent works, the band structure of both materials presents superconducting behavior, four bands of Bi6p orbitals that cross a Fermi-level. Additionally, the systematic study of the electronic properties as a function of pressure shows the same behavior in both materials "Enhancement of superconductivity T C ", where the Fermi-level is shifted upward and the Fermi surface presents a similar dispersion. These similarities between tetravalent cation electron-doped via Th þ4 substitution for La þ3 (La 0.5 Th 0.5 OBiS 2 ), and monovalent anion electron-doped via F À substitution for O 2À (LaO 0.5 F 0.5 BiS 2 ) may predict the same electronic properties.