Crystal structures and constitution of the binary system iridium-boron

Abstract

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 . Quite high hardness values were also recorded for IrB 1.1 thin films (on SiO 2 substrate) revealing an intrinsic film hardness of 43(±5) GPa [6].Investigations on the constitution of the binary system Ir-B revealed the existence of three compounds Ir 3 B 2 , IrB and IrB 2 [7−9]. Some of the early reports were mainly concerned with the metal-rich eutectic (1046°C at 21.4 at.% B [10]), with the optimization of synthesis techniques [8] and with the stability of IrB 1.1 against various acids and bases [11]. Melting point (T m = 1190 ± 20°C), microhardness 1652 ± 80 kgf/mm 2 , Seebeck coefficient (20−800°C, S V , min = −9 μV/K at 350°C) and electrical resistance (20−800°C) for IrB 1.1 were reported by Samsonov et al. [8]. X-ray powder and single crystal (Weissenberg) photographs served to evaluate the crystal structure of IrB 1.1 , which was reported to be isotypic with the α-ThSi 2 structure type (space group I4 1 /amd; a = 0.2810, c =1.0263 nm) exhibiting a severe defect at the boron sites (8e sites randomly occupied by ~50% of B atoms) [12]. For the compound richest in boron, 'IrB 2 ' , monoclinic symmetry (space group C2/m) was established from X-ray single crystal multi-film Weissenberg photographs yielding a crystal structure described as a stacking of puckered boron layers (A) and puckered double layers of metal atoms (B) in the simple sequence ABAB in c-direc-

High-Pressure Synthesis of β-Ir4B5and Determination of the Compressibility of Various Iridium Borides

“…Furthermore, the investigations show that β-Ir 4 B 5 possesses a phase width and could therefore be labeled as β-Ir 4 B 5± x , as Zeiringer et al also suggested for other iridium borides. 10 However, in the following, we use the idealized formula β-Ir 4 B 5 . Figure 3 shows the structure, which can be described as stacking of alternating puckered boron layers (B, B′) and puckered iridium layers (A, A′) in the stacking sequence ABA′B′ in the c direction of the crystal structure.…”

“…Iridium–iridium distances in the framework range from 2.7332(5) to 2.9489(4) Å, which is similar to the iridium–iridium distances in other borides and metallic iridium. 10,26,28,61,62 Whereas the boron atoms B2 at the edge of the ribbons are coordinated by four iridium and two boron atoms, the boron atoms B1 have four close boron neighbors and four iridium atoms, leading to an 8-fold coordination (4 Ir + 4 B). The bond lengths of Ir–B within the iridium cage range from 2.128(8) to 2.249(11) Å and within the trigonal prism from 2.108(19) to 2.249(11) Å, which is slightly shorter than the sum (2.25 Å) of the covalent radii of the B atom ( r = 0.84 Å) and the Ir atom ( r = 1.41 Å).…”

“…9 Most borides can be synthesized at ambient pressure, which makes these materials interesting for industrial applications, as this synthesis route is relatively inexpensive and easily accessible. 1,3,4,8−10 …”

“…10,28 These phases were synthesized via high temperature syntheses by Aronsson et al 25,26 and by Rogl et al 28 More recently, Zeiringer et al reinvestigated the chemical compositions and structures of these iridium borides and suggested a slightly different labeling for a more suitable specification of the phase width. 10 In 2016, Xie et al successfully used a mechanochemical approach to synthesize two new iridium borides, namely, IrB 2 and IrB. 29 Rau et al and Latini et al investigated the hardness of Ir 4 B 5+ x (formerly IrB 1.35 ) and Ir 5 B 4+ x (formerly IrB 1.1 ) and discovered that very thin films of both phases show a high hardness of 49.8 at a load of 0.49 N for Ir 4 B 5+ x and 43 GPa at 0.49 for Ir 5 B 4+ x .…”

“…Numerous predictions of novel iridium borides with varying Ir:B ratio, which possibly could be synthesized under high-pressure/high-temperature conditions and are expected to show interesting mechanical properties, have been made. 10,12−14,31−34 …”