Chemical Bonding Trends in Y ₂ <i>M</i> ₃ Si ₅ ( <i>M</i> = Mn–Cu, Tc–Pd, Re–Pt): A Study within the Broad <i>R</i> ₂ <i>M</i> ₃ <i>X</i> ₅ Intermetallic Family

A comparative analysis of the chemical bonding in the Y₂<i>M</i>₃Si₅ (<i>M</i> = Mn-Cu, Tc-Pd, Re-Pt) intermetallic compounds is presented, aiming at elucidating the chemical factors governing their crystallization into tetragonal (<i>tP</i>40-Sc₂Fe₃Si₅), monoclinic (<i>mS</i>40-Lu₂Co₃Si₅), or orthorhombic (<i>oI</i>40-U₂Co₃Si₅) structures. This study provides the first comprehensive bonding investigation of Y₂<i>M</i>₃Si₅ compounds with <i>M</i> = Fe, Co, and Ni, each adopting one of the three structure types. Employing projected crystal orbital Hamilton population curves (pCOHP), integrated pCOHP (IpCOHP), and integrated crystal orbital bond index (ICOBI) analyses, the bonding scenario is revealed to be primarily dominated by polar covalent <i>M</i>-Si interactions, followed by Y-Si, with Si-Si bonds playing a secondary role. This highlights a bonding picture more complex than that predicted by the Zintl concept. Extending the analysis to all transition metals and prototypes, regardless of their thermodynamic stability, allows for a systematic comparison of bonding in both stable and metastable configurations. The covalency distribution within the unit cell, quantified as IpCOHP%, exhibits periodic trends across the transition metal series, both along periods and down groups. The maximization of <i>M</i>-Si IpCOHP% emerges as the key chemical factor in stabilizing one structure type over another, aligning with experimental observations.