ABSTRACT
This chapter focuses on several groups of chemical elements, all of which have one common feature: one of their d electron shells is unfilled. The number of d electrons in such a shell is increasing by one when moving across the periodic table rows from left to right until the d shell becomes completed with 10 electrons. The elements can easily form positively charged ions with several electrons in their unfilled open d-electron shells and varying oxidation states. Being placed into suitable crystals and/or glasses, these ions can be a reason for noticeable changes of the optical properties of these host materials. An additional interest represents a non-dimensional C/B ratio, since it allows the reduction of the number of electrostatic parameters and is used for a simplified representation of the energy levels splitting of d-electrons in cubic crystal fields by the Tanabe-Sugano diagrams.
