Crystal Structure of W1−x B3 and Phase Equilibria in the Boron-Rich Part of the Systems Mo-Rh-B and W-{Ru,Os,Rh,Ir,Ni,Pd,Pt}-B

Abstract

The crystal structure of W1−x B3 has been reinvestigated by x-ray single crystal diffraction and revealed isotypism with the Mo1−x B3 structure type (space group P63/mmc; a = 0.52012(1), c = 0.63315(3) nm; R F = 0.040). As a characteristic feature of the structure, planar hexagonal metal layers (1/3 of atoms removed from ordered positions) sandwich planar boron honeycomb layers. One of the two W-sites shows a random defect of about 73%. Strong metal boron and boron-boron bonds are responsible for high mechanical stability. Although W1−x B3 at about 80 at.% B is the metal boride richest in boron, it contains no directly linked three-dimensional boron framework. The solubility of Rh, Ir, Ni, Pd and Pt in W1−x B3 as well as of Rh in Mo1−x B3 has been investigated in as cast state and after annealing. Furthermore, phase equilibria in the boron rich part of the corresponding isothermal sections W-TM-B (TM = Rh, Ir at 1100 °C, TM = Ni, Pd at 900 °C and TM = Pt at 800 °C) and Mo-Rh-B (at 1100 °C) have been established. A ternary compound only forms in the system W-Ir-B: τ1-W1−x Ir x B2 with ReB2 structure type (space group P63/mmc; a = 0.2900, c = 0.7475 nm). The type of formation and crystal structure of diborides W1−x TM x B2 (TM = Ru, Os, Ir) isotypic with ReB2 were studied by x-ray powder diffraction and electron probe microanalysis in as cast state and after annealing at 1500 °C. Accordingly, W0.5Os0.5B2 (a = 0.29127(1), c = 0.7562(1) nm) forms directly from the melt, whereas W0.4Ru0.6B2 (a = 0.29027(1), c = 0.74673(2) nm) and W0.6Ir0.4B2 (a = 0.29263(1), c = 0.75404(8) nm) are incongruently melting. Annealing at 1500 °C leads in case of the iridium compound to an almost single-phase product but the same procedure does not increase the amount of the ruthenium diboride.