An intrinsic criterion of defining ionic or covalent character of AB-type crystals based on the turning boundary radii calculated by an ab initio method

摘要

Based on the concept of the intrinsic characteristic atomic and ionic radii and an ab initio study, we present a new criterion of defining ionic or covalent character of AB-type crystals, showing that the ions in an ionic crystal do not necessarily contact each other in the classical meaning. The criterion states that an AB-type crystal is sorted as a primarily ionic in bonding nature if there exists a separation region which is the classically forbidden region for the electronic motion between a pair of adjacent cation A(+) and anion B- anion , and if there is no such a separation region for a pair of adjacent cation A(+) and anion B- cations in an AB-type crystal it is sorted as a primarily covalent one. This provides a fundamental understanding of the nature of ionic and covalent crystals. A comparison between the judging results of this criterion with that issued from the widely accepted Pauling's electronegativity criterion has been given, which demonstrates that in most cases both criteria give same judge results of the ionic or covalent character and the new criterion gives suitable judge results for a few cases when there are discrepancy between the two criterion judge rules.