【Frontiers in HaiChuan Technology】The Institute of Process Engineering, Chinese Academy of Sciences has developed metal-non-metal substitutional solid solutions

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Yang Jun, a researcher at the Institute of Process Engineering, Chinese Academy of Sciences, led his research team to phosphorylate pre-formed platinum (Pt) nanoparticles using tri-n-octylphosphine (TOP); phosphorus (P) atoms were able to replace some of the Pt atoms at the lattice sites, resulting in a Pt-P substitutional solid solution. This discovery not only helps to deepen the understanding of materials science but also holds the potential to enrich the content of basic textbooks on materials science and engineering. Recently, relevant research findings were published in Microscale. Studying substitutional solid solutions enables the design and optimization of the properties of metal materials and other alloys to meet various application requirements, which holds great significance for the fields of materials science and engineering. However, metal and non-metal elements typically form interstitial solid solutions, in which solute atoms (non-metal atoms) occupy the interstitial sites formed by the stacking of solvent or host atoms (metal atoms); it is less likely that they form substitutional solid solutions. However, for the Pt-P system, their formation of substitutional solid solutions does not seem to violate Hume-Rothery’s rule from the perspectives of crystal structure, atomic size, and electronegativity. The R&D team first prepared worm-like Pt nanoparticles in a mixture solvent of octadecene and oleamide, and then phosphorized them using TOP at high temperatures. Through observations using a transmission electron microscope, researchers found that the phosphating reaction can cause significant changes in the particle morphology; the particles after phosphating change from worm-like shapes to perfectly spherical substitutional solid solutions with a diameter of approximately 4.8 nanometers, and this solid solution can be confirmed through various characterization techniques. Theoretical calculations indicate that if the phosphided solid solution can maintain a stable face-centered cubic structure, the upper limit for P doping is approximately 10%, which is in good agreement with experimental observations and consistent with the limited solubility predicted by the difference in atomic sizes. The phenomenon of limited P-doping can be explained by the fundamental theories of materials science. Due to the difference in atomic sizes, the substitution of some Pt atoms with P atoms inevitably induces significant lattice distortion in the surrounding area, leading to an increase in the system’s internal energy. Although the substitution of Pt atoms with P atoms increases the number of possible states of the system, leading to an increase in entropy, at lower temperatures the increase in internal energy dominates, which may result in an increase in the system’s free energy. Therefore, there should be an upper limit to the substitution of P atoms; below this limit, the increase in internal energy caused by distortion is kept within certain bounds, allowing the crystal structure to remain stable.