Electronegativity And Superhard Materials Design

The concept of electronegativity(EN) was proposed by Pauling in 1932,which describes "the power of an atom in a molecule to attract electrons to itself".For more than 70 years,EN has been a basic parameter widely used in the fields of chemistry and materials science and so on.EN of bonding atoms is closely related to the structure and property of materials,which provides an important guide in the study of structure-property relationship and the prediction of novel materials.In our previous work,based on the electrostatic potential of elements in covalent crystals, an EN model of elements in covalent crystals was proposed,by which the EN values of 58 elements in the periodic table with different bonding electrons and coordination numbers were determined.From the viewpoint of EN,a microscopic hardness model was established,by which the hardness of covalent or polar covalent materials can be satisfactorily predicted solely in terms of EN of constituent atoms and crystal structure data.Recently,a new promising approach to obtain novel superhard materials is to combine transition metals(TMs) possessing high bulk modulus with small,covalent bond-forming atoms such as boron,carbon,nitrogen,or oxygen.However,it is still under debate whether TMs combining with light elements can form superhard materials.Additionally,there are some difficulties in measuring the hardness of the new materials.Thus,the purpose of this work is to propose an effective method to identify the hardness of transition metal(TM) compounds and to further yield hints for the exploration of novel superhard materials.We hypothesize that the light elements obeying the octet rule in stable TM compounds, by which the number of TM bonding electrons is determined.Then the EN values of TMs are obtained according to our method of calculating the covalent EN of atoms.Finally the hardness model based on the viewpoint of EN is extended to TM compounds.Particularly,the TM compounds with light elements linking to both TM and light elements are separated into two sub-systems according to the different coordinated atoms of light elements,by which the hardness of such a catalogue of compounds is successfully predicted.The calculated hardness values of TM borides,carbides,nitrides and oxides agree well with available experimental data.The enhanced effect of d valence electrons on hardness is well reflected by the bond EN as the bonding environment of constituent atoms has been fully considered by their EN values.The negative effect of metallicity on hardness has been reflected qualitatively by our method dealing with TM bonding electrons.It is found that the strong covalent bonding between light elements is much favorable for the formation of superhard TM materials.Thus,the current research provides us a useful guide in the study of novel superhard materials.