Pt-B system re-visited: Pt2B, a new structure type of binary borides. Ternary WAl12-type derivative borides

Abstract

Based on a detailed study applying X-ray single crystal and powder diffraction, DSC and SEM analysis it was possible to resolve existing uncertainties in the Pt-rich section (65 at.% Pt) of the binary Pt-B phase diagram above 600C. The formation of a unique structure has been observed for Pt2B (X-ray single crystal data: SG C2/m, a=1.62717(11) nm, b=0.32788(2) nm, c=0.44200(3) nm, =104.401(4)º, RF2=0.030). Within the homogeneity range of "Pt3B", X-ray powder diffraction phase analysis prompted two structure modifications as a function of temperature. The crystal structure of "hT-Pt3B" complies with the hitherto reported structure of anti-MoS2 (SG P63/mmc, a=0.279377(2) nm, c=1.04895(1) nm; RF=0.075, RI=0.090). The structure of the new "ℓT-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 Pt-Cu-B system at 600C revealed a new ternary compound Pt12CuB6-y (X-ray single crystal data: SG Im , a=0.75790(2), y=3; RF2=0.0129) which exhibits the filled WAl12-type structure accommodating boron in the interstitial trigonal-prismatic site 12e. The isotypic platinum-aluminium-boride was synthesized and studied. Solubility of copper in binary platinum borides has been found to attain about 7 at.% 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 [Pt6] octahedra (anti-CaCl2-type fragment) alternating along the x axis with a doubled layer of boron-semifilled [Pt6] trigonal prisms interbedded with a layer of empty tetrahedra and tetragonal pyramids (B-deficient α-TℓI fragment). Assuming boron vacancies ordering (SG R3), the Pt12CuB6-y structure exhibits the serpentine-like columns of edge-connected boron filled [Pt6] trigonal prisms running infinitively along z axis and embedding the icosahedrally coordinated Cu atom. Pt2B, (Pt1-yCuy)2B (y=0.045) and Pt12CuB6-y (y=3) behave metallic, as revealed from temperature dependent electrical resistivity measurements.