Metal Deficiency Tailored by the 18-Electron Rule Stabilizes Metal-Based Inorganic Compounds

The 18-electron (18-e–) rule is typically restricted to predicting the stability of transition-metal-based complexes. Herein, we report the use of the 18-e– to predict the stability in a family of intermetallics MNiSn (M = V, Cr, Fe, and Co) crystallizing in the Co1.75Ge structure type. Site deficiencies at the M site obtained from single-crystal X-ray diffraction are understood as attaining a stable noble gas electronic configuration. The density functional theory -based structure calculation confirms that the deficient structure is more stable than the ideal occupation available at the crystal lattice. MnNiSn, which crystallizes in the half-Heusler crystal structure, depicts the role of covalent radii of the constituent elements in determining the crystal structure. Using X-ray absorption spectroscopy and X-ray photoelectron spectroscopy, the local structure of the above-mentioned compounds was also elucidated, supporting the role of deficiency tuned valence fluctuation to attain a 18-e– configuration that eventually leads to the formation of stable compounds.