Pt-B System Revisited: Pt2B, a New Structure Type of Binary Borides. Ternary WAI(12)-Type Derivative Borides

摘要

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 degrees 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, beta = 104.401(4)degrees, 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 P6(3)/mmc, a = 0.279377(2) nm, c = 1.04895(1) nm, R-F = 0.075, R-I = 0.090]. The structure of the new "lT-Pt3B" is still unknown. The formation of previously reported Pt similar to 4B has not been confirmed from binary samples. Exploration of the Pt-rich section of the Pt-Cu-B system at 600 degrees C revealed a new ternary compound, Pt12CuB6-y [X-ray single-crystal data: space group Im (3) over bar, 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 similar to 7 atom % Cu for Pt2B but to be insignificant for "lT-Pt3B. The architecture of the new Pt2B structure combines puckered layers of boron-filled and empty [Pt-6] octahedra (anti-CaCl2-type fragment) alternating along the x axis with a double layer of boron-semifilled [Pt-6] trigonal prisms interbedded with a layer of empty tetrahedra and tetragonal pyramids (B-deficient alpha-TlI fragment). Assuming boron vacancies ordering (space group R3), the Pt12CuB6-y structure exhibits serpentine-like columns of edge-connected boron-filled [Pt-6] trigonal prisms running infinitely along the z axis and embedding the icosahedrally coordinated Cu atom. Pt2B, (Pt1-yCuy)(2)B (y = 0.045), and Pt12CuB6-y (y = 3) behave metallically, as revealed by temperature-dependent electrical resistivity measurements.