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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.

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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 . 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-

In the present work we report on the synthesis, crystal structure, and physical properties (resistivity, magnetization, heat capacity) in combination with density functional theory (DFT) calculations of the electronic structure and phonon properties for the intermetallic compound LaPtSi. LaPtSi crystallizes in its own noncentrosymmetric structure type (space group I 4 1 md; a = 0.42502(1) nm and c = 1.4525(5) nm), which is an ordered ternary derivative of the centrosymmetric α-ThSi 2 -structure. The weakly correlated compound LaPtSi (Sommerfeld value γ = 6.5 mJ/molK 2 ) exhibits superconductivity below T c = 3.35 K and appears to be a fully gapped, weakly coupled s-wave BCS superconductor. The experimental observations are supported by DFT calculations which show that, despite a substantial spin-orbit splitting of the Fermi surfaces, a spin-singlet pairing is prevalent.