It is about 20 years ago that I started to collect data from literature on the 4f-5d transitions energies of the lanthanides in inorganic compounds. This curiosity driven research was inspired by my desire to understand why and how the emission from Ce³⁺, applied as luminescence activator in scintillation crystals, changes with type of compound. It was also inspired by observations and reports on constant energy differences between 4f-5d transition energies when that of Ce, Pr, and Nd in the same compound are compared. Today, data has been collected on 2000+ compounds with information on all divalent and all trivalent lanthanides extracted from 7000+ manuscripts. Information that comprises not only 4f-5d transitions but also VBàLn charge transfer band energies, host exciton energies, luminescence decay and quenching data. The data base is current being augmented with data on the 6s²-elements and transition metal elements.

Figure 1. A typical VRBE diagram, here for LaAlO₃, illustrating the location of Ln²⁺ 4f-levels (in red) and Ln³⁺ 4f-levels (in blue). Open data symbols are the VRBE in the lowest energy Ln³⁺ level.

A brief overview on the methods and models derived from this curiosity driven research is presented. Particularly, attention is focussed on the chemical shift model that enables to construct so-called vacuum referred binding energy (VRBE) diagrams as illustrated in Figure 1. It is a unique method that provides information on the VRBE at the top of the valence band and the bottom of the CB in a routine fashion. The electronic structure of inorganic compounds can be compared with each other with reference to a common energy, i.e., the energy of an electron at rest in vacuum. Figure 2 shows a stacked VRBE diagram. It turns out that when comparing different compounds with each other, the VRBE in the lanthanide 4fⁿ ground state levels show much less variation than the VRBE at the conduction band bottom or valence band top. This illustrates that in order to understand (electronic) structure property relationships we should understand what controls the VRBE at the CB-bottom and VB-top rather than what controls the VRBE in the 4fⁿ ground state.

Figure 2. A stacked diagram of VRBE schemes of various oxide compounds illustrating that the lanthanide 4f ground state levels of Eu²⁺ and Ce³⁺ are fairly compounds invariant but that host band energies change widely.

Problem with such new insight is that rarely VRBE information is provided. Photo-electron spectroscopy techniques and ab-initio or first principles calculations commonly relate energies with respect to the Fermi energy but not to the vacuum level. Electrochemistry does make use of a common energy reference. This is the standard hydrogen potential, and since that potential is equivalent to -4.44 eV on the VRBE-scale, one may, although rarely done so, translate redox potentials into VRBE energies. This means that data on redox potentials, as is abundantly available in the studies on materials for photo-catalytic splitting of water, can be used to derive VRBE energies. In Li-ion battery research, the common reference is the Li^0/+ redox potential, and again the data on materials for battery research can be translated into VRBE data. Above demonstrates that the different disciplines; luminescence materials, materials for catalysis, and materials for batteries are “speaking their own languages”. However, as a matter of fact, many properties relate to one and the same, i.e., the binding energy of electrons with respect to the vacuum level. I will demonstrate how the different disciplines connect and how insight from luminescence studies may be of use to the other disciplines and vice versa.