We report for the first time the synthesis of well-defined bismuth/ phosphorus nanoparticles (Bi/P NPs) based on silyl-halide elimination by reaction of BiX 3 (X = I, Cl) and P(SiMe 3) 3 in the presence of different types of stabilizing ligands, such as cinchonidine, 4-(3-phenylpropyl)pyridine, and polyvinylpyrroli-done. This synthetic approach led to spherical, small, and monodisperse NPs [mean diameter ca. 2.0−2.5 nm determined by transmission electron microscopy (TEM)]. Wide-ranging characterization of these NPs, including TEM, powder X-ray diffraction, and small-angle X-ray scattering, and several spectroscopic techniques, such as X-ray fluorescence, IR, and solid-state NMR, proved the formation of a core/shell structure constituted by a crystalline Bi(0) core and amorphous P-containing shells, representing a unique case of phase segregation for metal/phosphorus materials reported in the literature. The assessed reactivity of the as-prepared Bi/P NPs evidenced their potential application in the synthesis of phosphine (PH 3) in a safer way than the conventional approach using P 4 (white phosphorus) and in a sharp contrast with the reported reactivity of amorphous red phosphorus. ■ INTRODUCTION Bulk metal phosphides (M x P y) are nowadays receiving intense attention in the field of materials due to their wide scope of properties and applications in metallurgy and semiconductors for electronics or optics, among others. 1−9 The structure and composition of these M x P y binary species have been accurately analyzed for almost all combinations with metals, except with the heavy elements of group 15 (As, Sb, Bi). 10 Among them, bismuth phosphide is particularly appealing because it is isoelectronic with PbS, a well-known intrinsic semiconductor used for an extensive range of applications, but exhibiting strong environmental constraints. 11−17 Concerning its preparation, two synthetic approaches have been reported to obtain Bi x P y. The first methodology relied on the dissolution of phosphorus in molten bismuth, which however did not lead to the desired compound but to the segregation of the elements into their pure form, leading to the synthesis of black and violet allotropes of P and Bi(0). 18,19 Based on the successful syntheses of other metal phosphides, M x P y , under milder conditions via the formation of Me 3 SiX (X = Cl, Br, I) as concomitant products, we pursued this strategy for the synthesis of Bi x P y. 20 Notably, the generation of the strong Si−X bond acts as a driving force for the reaction. Following this strategy, Allen and co-workers reported the formation of an insoluble black material at room temperature from the reaction of BiCl 3 with P(SiMe 3) 3. 21 A range of analyses were carried out to characterize this material, including scanning electron microscopy, energy-dispersive X-ray spectrometry, X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS), thermogravimetric analysis and differential scanning calorimetry (TGA and DSC), as well as conductivity measurements. These analyses prompted the authors to propose the formation of amorphous hybrid "BiP" materials. However, the characterization of the as-prepared materials did not provide unequivocal evidence for the formation of the expected product, whose precise structure remains unknown. From another standpoint, molecular compounds featuring discrete Bi/P bonds are rather rare. Their synthesis also relies on substitution reactions between reagents containing P−Si (or P−Li) and Bi−X bonds. More importantly for the present study, the stability of the Bi/P bond appears to vary in these molecular species. Indeed, Coles and co-workers reported in 2016 that the compound "Bi(NON R) (PPh 2)" (NON R = [O(SiMe 2 NR) 2 ], where R = tBu, 2,6-iPr 2 C 6 H 3) was unstable and readily evolved to form a P−P bond (Ph 2 P−PPh 2) and a Bi−Bi bond, whereas the related Bi(NON R)(PCy 2) was a stable compound at room temperature. 22 Between molecules and bulk material domains, nanoparticles (NPs) may provide a field of interesting and distinctive properties. In this context, the case of "BiP" is particularly appealing for two reasons. First, a fundamental question can be asked: are Bi/P bonds strong enough in nanoalloys versus Bi− Bi and P−P bonds in the elemental materials? In other words,