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 W 12x B 3 has been reinvestigated by x-ray single crystal diffraction and revealed isotypism with the Mo 12x B 3 structure type (space group P6 3 /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 W 12x B 3 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 W 12x B 3 as well as of Rh in Mo 12x B 3 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: s 1 -W 12x Ir x B 2 with ReB 2 structure type (space group P6 3 /mmc; a = 0.2900, c = 0.7475 nm). The type of formation and crystal structure of diborides W 12x TM x B 2 (TM = Ru, Os, Ir) isotypic with ReB 2 were studied by x-ray powder diffraction and electron probe microanalysis in as cast state and after annealing at 1500°C. Accordingly, W 0.5 Os 0.5 B 2 (a = 0.29127(1), c = 0.7562(1) nm) forms directly from the melt, whereas W 0.4 Ru 0.6 B 2 (a = 0.29027(1), c = 0.74673(2) nm) and W 0.6 Ir 0.4 B 2 (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.

Structure of superhard tungsten tetraboride: A missing link between MB 2 and MB 12 higher borides

“…The structure of this material has been debated for more than half a century, but the need for a definitive solution has increased dramatically in recent years with the discoveries that WB 4.2 is both superhard and can serve as the parent phase for a large family of solid solutions that are even harder (8,9,11). Although the crystal structure reported here contains some elements postulated previously-sites that are only partly occupied by tungsten, for example (14,16,17,21)-the structure presents previously unidentified elements as well. The most important of these is the fact that the partially occupied tungsten sites that do not contain W atoms contain boron trimers, and these trimers are within the appropriate distance to couple with the boron layers, producing slightly distorted cuboctahedral boron cages.…”

“…Although this work confirmed the partial occupancy at one of the metal sites, the possibility of boron atoms filling vacancies in the structure was rejected, leaving voids in the structure. This model has been lent even more support by a recent single-crystal investigation by Zeiringer et al, who worked directly with the tungsten phase, referring to it analogously as W 1−x B 3 (P6 3 /mmc, a = 5.2012 Å and c = 6.3315 Å) (14), seemingly settling the matter.…”

“…Most importantly, however, this model provides insight into the rational causes of the extremely high hardness and solid solution hardening behavior observed in this boride. The history of previous attempts has already been explored in the recent work of Zeiringer et al (14), and therefore will be only briefly summarized here.…”

Crystal structures and constitution of the binary system iridium-boron

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GPa [2] (48 GPa (0.5 N) [3]) or 26.6 GPa (39.3 GPa (0.5 N)) [2] with rather different hardness values due to the anisotropic crystal structure (for details on the crystal structure of W 1−x B 3 , see [4]). Vicker's hardness measurements on the boron richest iridium boride IrB 1.35 revealed a load independent value of 18.2 GPa (at 9.81 N) but 49.8 GPa at 0.49 N [5] and at such low loads the hardness is therefore comparable with bulk samples of ReB 2 .

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In search of the elusive IrB2: Can mechanochemistry help?

“…Phases such as IrB 1. 35 and IrB 1.1 as well as the metal-rich phases IrB 0.9 and IrB 0.7 have been reported in a number of publications [17][18][19][20][21][22][23][24] and some of their properties have been investigated [12,[25][26][27]. Very high Vickers hardness of 49.8 GPa at 0.49N load was reported for IrB 1.35 [12].…”