Recently, Professor Zhang Kan from the College of Materials Science and Engineering, Jilin University, together with collaborators, published an article in Advanced Materials entitled Core electron count as a versatile and accurate new descriptor for sorting mechanical properties of diverse transition metal compounds. This work reports a novel discovery regarding the critical role of core electron count (CEC) in governing the bonding behavior of transition metal-light element (TM-LE) compounds, revealing an unprecedented vital influence of CEC on material bonding characteristics. A brand-new composite descriptor combining CEC with valence electron concentration (VEC) is proposed, which precisely establishes the correlation between the VEC–CEC pair and the evolutionary trends of mechanical properties for transition metal nitrides (TMN) and transition metal diborides (TMB₂).
Mechanical properties are among the most fundamental and essential material characteristics, largely determining the application scope and service lifetime of materials. With the rapid advancement of industrial technologies, materials for core equipment face diversified service scenarios, imposing broader and more refined requirements on mechanical performance. Accordingly, identifying key factors (descriptors) that can accurately and systematically describe the evolution of material mechanical properties is of great significance to enable high-throughput screening, classification and prediction of mechanical behaviors.
TM-LE compounds have attracted extensive attention owing to their excellent comprehensive performance including chemical inertness, thermal stability and oxidation resistance. Meanwhile, most of these materials exhibit hard or even superhard characteristics, rendering them widely applicable. Nevertheless, transition metals cover a broad range of elements, and TM-LE systems formed with light elements feature multicomponent and polymorphic structural features. Therefore, exploring reliable descriptors for mechanical properties of TM-LE systems is critical for targeted optimization and rational design of their mechanical performance. Previous studies have proven that VEC can capture the overall mechanical property trends of structurally distinct TMB₂, TMN and TMC, demonstrating universal applicability across different crystal structures. However, VEC alone suffers from prominent limitations: for a given TM-LE system, compounds with identical VEC display widely scattered mechanical property values, and extensive overlapping of mechanical performance occurs among TM-LE compounds with similar VEC values. It is thus urgent to develop a secondary descriptor complementary to VEC to achieve refined, precise characterization of mechanical properties of TM-LE compounds, providing effective criteria for designing materials with balanced high hardness and high ductility.
Building on existing research concerning VEC-dependent mechanical behaviors, the team put forward the concept of correlating full electronic structures with mechanical properties of TM-LE compounds and constructed the novel composite VEC–CEC descriptor. This descriptor successfully reproduces the mechanical property trends of TMN and TMB₂ with disparate crystal structures and bonding types, while resolving the data dispersion issue observed for samples sharing identical VEC in both systems, achieving high prediction accuracies of 95.06% for TMN and 92.59% for TMB₂, respectively. As VEC rises, TMN and TMB₂ generally undergo a transition from brittle to ductile behavior. For TMN or TMB₂ samples with fixed VEC, increasing the CEC of solute atoms leads to pronounced electron delocalization in all crystallographic directions. This phenomenon originates from the strong screening effect of core electrons on the Coulomb interaction between valence electrons and positively charged atomic nuclei. Such enhanced electron delocalization further strengthens d–d orbital interactions between valence electrons of metal atoms and improves material ductility. The VEC–CEC descriptor holds great promise for designing and predicting mechanical properties of a much broader range of material systems, opening a new avenue for targeted development of materials with customized mechanical performance.
Figure caption: The novel composite descriptor VEC–CEC for mechanical property evaluation of TMN and TMB₂.
Zhang Rui and Gu Xinlei, doctoral candidates from the College of Materials Science and Engineering, Jilin University, are co-first authors of this paper. The corresponding authors are Professor Zhang Kan (College of Materials Science and Engineering) and Dr. Liu Chang (School of Physics), Jilin University. Professor Chen Changfeng from the University of Nevada, Las Vegas, USA, also participated in this research. This work was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Jilin Province, and the "Golden Seed" Program of Jilin University.
