Pt–B System Revisited: Pt2B, a New Structure Type of Binary Borides. Ternary WAl12-Type Derivative Borides

Abstract

On the basis of a detailed study applying X-ray single-crystal and powder diffraction, differential scanning calorimetry, and scanning electron microscopy analysis, it was possible to resolve existing uncertainties in the Pt-rich section (≥65 atom % Pt) of the binary Pt–B phase diagram above 600 °C. The formation of a unique structure has been observed for Pt₂B [X-ray single-crystal data: space group C2/m, a = 1.62717(11) nm, b = 0.32788(2) nm, c = 0.44200(3) nm, β = 104.401(4)°, R_F2 = 0.030]. Within the homogeneity range of “Pt₃B”, X-ray powder diffraction phase analysis prompted two structural modifications as a function of temperature. The crystal structure of “hT-Pt₃B” complies with the hitherto reported structure of anti-MoS₂ [space group P6₃/mmc, a = 0.279377(2) nm, c = 1.04895(1) nm, R_F = 0.075, R_I = 0.090]. The structure of the new “T-Pt₃B” is still unknown. The formation of previously reported Pt_～4B has not been confirmed from binary samples. Exploration of the Pt-rich section of the Pt–Cu–B system at 600 °C revealed a new ternary compound, Pt₁₂CuB_6–y [X-ray single-crystal data: space group Im3?, a = 0.75790(2) nm, y = 3, R_F2 = 0.0129], which exhibits the filled WAl₁₂-type structure accommodating boron in the interstitial trigonal-prismatic site 12e. The isotypic platinum–aluminum–boride was synthesized and studied. The solubility of copper in binary platinum borides has been found to attain ～7 atom % Cu for Pt₂B but to be insignificant for “T-Pt₃B”. The architecture of the new Pt₂B structure combines puckered layers of boron-filled and empty [Pt₆] octahedra (anti-CaCl₂-type fragment) alternating along the x axis with a double layer of boron-semifilled [Pt₆] trigonal prisms interbedded with a layer of empty tetrahedra and tetragonal pyramids (B-deficient α-TI fragment). Assuming boron vacancies ordering (space group R3), the Pt₁₂CuB_6–y structure exhibits serpentine-like columns of edge-connected boron-filled [Pt₆] trigonal prisms running infinitely along the z axis and embedding the icosahedrally coordinated Cu atom. Pt₂B, (Pt_1–yCu_y)₂B (y = 0.045), and Pt₁₂CuB_6–y (y = 3) behave metallically, as revealed by temperature-dependent electrical resistivity measurements.