Electronic and Structural Properties of MPt_xB_6-2x (M = Y, Yb): Structural Disorder in an Octahedral Boron Framework

Two new ternary platinum borides, YPt_xB_6-2x and YbPt_xB_6-2x, were obtained by argon-arc melting of the elements followed by annealing at 780 °C (750 °C). The structures of these compounds combine the fragments of CaB₆- and AuCu₃-type structures [space group Pm3̅m; x = 1.15, a = 4.0550(4) Å and x = 1.34, a = 4.0449(2) Å for YPt_xB_6-2x and YbPt_xB_6-2x, respectively; single-crystal X-ray diffraction]. Two possible variants of B/Pt ordering (space group P4/mmm) were created via a group-subgroup approach targeting the derived stoichiometry. The architecture of the type-I YPt_xB_6-2x structure model (a' = a, b' = b, c' = c) combines the 4.8² boron nets alternating with the layers of Y and Pt; the type-II YPt_xB_6-2x structure model (a' = 2a, b' = 2b, c' = c) exhibits columns of linked [B₂₄] truncated cubes filled with Y running along the c axis. The striking features of both structural models are [B₄Pt₂] octahedra. The structural similarities with hitherto reported structures (YB₂C₂, M₂Ni₂₁B₂₀, MNi₂₁B₂₀, and ErNiB₄) were drawn supporting the verity of these models. A chemical bonding analysis for type-I and type-II YPt_xB_6-2x based on electron localization function distribution revealed a two-center interaction forming the 4.8² boron nets for type-I YPt_xB_6-2x and a covalent bonding within [B₄Pt₂] octahedra as well as a two-center interaction for B-B intraoctahedral bonds for type-II YPt_xB_6-2x. Analysis of Bader charges revealed the cationic character of the yttrium atoms. The interactions for nondistorted areas of the structures agree well with the bonding picture calculated for constituent building structures, YB₆ and YPt₃. Electronic structure calculations predict YPt_xB_6-2x to be a metal with the density of states of around N(E_F) = 1 states eV^-1 f.u.^-1. The exploration of the Y-Pt-B system in the relevant concentration range elucidated the homogeneity field of YPt_xB_6-2x (0.90 ≤ x ≤ 1.40) and revealed the existence of three more ternary phases at 780 °C: YPt₂B (space group P6₂22), YPt₃B (space group P4mm), and YPt₅B₂ (space group C2/m).