Study On Group Electronegativity And Optical Properties Of Borate Crystals

On the basis of their abundant structures and excellent optical properties, borate crystals are widely used as laser materials, luminescence host materials and non-linear optical materials especially, and therefore they have been the hotspot of research in science. In borate crystals, the boron atoms are hybridized forming sp2 structure or sp3 structure, then the hybridized boron atoms bond with oxygen atoms to form trigonal BO3 group or tetrahedral BO4 group. These groups either exist alone in crystals or connect each other via common oxygen atoms to form rigid boron-oxygen polymeric anion groups, which are called fundamental building blocks (FBBs). The FBBs can polymerize into complex anion frameworks of borate crystals further.The concept of electronic polarizability describes the power of electron distortion in external electric field, which is the origin in nonlinear optical (NLO) properties of crystals and has been an important parameter in designing optical-electric devises. It is indicated that electronic polarizability is related closely to element electronegativity of atoms composing of materials in early research. The concept of electronegativity (EN) describes "the power of an atom in a molecule to attract electrons to itself". For more than 70 years, EN has been a basic atomic parameter widely used in the fields of chemistry, physics and materials science and so on. According to Sanderson's idea of EN equalization, we have proposed a method to calculate the EN values of anion groups of borate crystals with gradual geometric mean. Considering the cubic root of EN of anion groups as the approximate radii of groups and the effect of deviation between B-O actual bond length and B-O ideal bond length on the radii of groups, the empirical quantitative relationship between the EN and the electronic polarizability of anion groups of borate crystals have been obtained with our proposed covalent electronegativity model early. Based on the empirical relationship and the structures of anion groups of borate crystals, the electronic polarizabilities of anion groups of borate crystals can be forecasted effectively, which can be used to estimate the molecule electronic polarizabilities of borate crystals with the sum rule of electronic polarizability. And we have forecasted the refractive indices of borate crystals with the Lorenz-Lorentz equation further.Finally, the empirical relationship between the maximal second NLO coefficients and the refractive indices of borate crystals have been established, which can be used to estimate the maximal second NLO coefficient from borate crystals'refractive indices. And our proposed relationship between group EN and group electronic polarizability can be used to forecast the refractive indices of borate crystals. So, we can estimate the maximal second NLO coefficients of borate crystals from their EN of anion groups, which can be used to search new second NLO borate crystals.