Vacuum Referred Binding Energies of the Lanthanides in Transition Metal Oxide Compounds

Abstract

The electronic level schemes for divalent and trivalent lanthanide ions in rare earth (La, Gd, Y, Lu, Sc) vanadate, niobate, tantalate, and in alkaline earth (Ba, Sr, Ca, Mg) titanate, molybdate, and tungstate compounds are presented. Use is made of data from luminescence excitation and absorption spectra of lanthanide (mostly Eu 3+ , Pr 3+ , and Tb 3+ ) impurities in those compounds. By means of the chemical shift model, binding energies, relative to the vacuum energy, of electrons in the impurity levels and the host bands are obtained. It reveals clear trends in conduction band and valence band energy with changing size of the rare earth or the alkaline earth ion. The bottom of the conduction band is dominated by 3d, 4d, or 5d orbitals, and it is found that the binding energy at the conduction band bottom tends to decrease with higher orbital number. The vanadates, titanates, molybdates, niobates, tungstates, and tantalates are widely used in many areas of applied physics and materials science. TiO 2 is famous for its photocatalytic activity used to produce hydrogen in electrochemical cells upon absorption of sunlight. Many other titanates and also vanadates are actively being studied for such applications.1 It is then crucial that the electron binding energy E V at the top of the valence band and E C at the bottom of the conduction band lie closely below and above the redox potentials for hydrogen and oxygen production. Lanthanides in transition metal (TM) based compounds can produce very bright luminescence. For bright luminescence to occur, the location of the emitting impurity level relative to E C is important. A location too close to E C will result in poor quantum efficiency or even a total absence of emission because of thermal or autoionization of the excited electron to the conduction band. Whether a lanthanide or a transition metal impurity can trap an electron from the conduction band or a hole from the valence band is also controlled by the impurity level locations in the band gap. 2That same location tells us the preferred impurity valence state.3 The above examples demonstrate that knowledge on the electronic structure, i.e., the absolute binding energy of the electrons in impurity states, host band states, or molecules, is crucial for our understanding of the performance of materials.There is an abundance of data on lanthanide spectroscopy covering many thousands of different compounds. Data that can provide information on the location of lanthanide impurity levels relative to the valence and conduction bands of the host compound. Once the location of the 4f n ground state energy of one particular lanthanide impurity is known, say Eu 2+ , then it can be predicted for all other divalent lanthanides. This good predictability is caused by the atomic like and well-shielded nature of the inner lanthanide 4f-orbitals.4 Host referred binding energy (HRBE) schemes as for GdVO 4 in Fig. 1 where all energies are referred to the top of the valence band, see the righthand energy scale, can be made routinel...

Charge carrier trapping processes in lanthanide doped LaPO4, GdPO4, YPO4, and LuPO4

“…One may now compare the binding energy at the valence band top or conduction band bottom in various compounds with respect to a same reference energy [16][17][18][19] . This model shows that the VRBE in the 4f n ground state of trivalent and divalent lanthanides is almost independent on the type of compound [20][21][22][23] . 3 Consequently, lanthanide related hole trap depth can be tuned by altering the VRBE at the top of the valence band and electron trap depth through tailoring the VRBE at the bottom of the conduction band 7 .…”

Weak thermal quenching of the luminescence in the Ca3Sc2Si3O12:Ce3+garnet phosphor

“…The way in which we have built the VRBE diagrams for the different hosts is based on published methods. 11,[35][36][37][38][39][40][41][42][43] The model, called the chemical shift model, provides electronic structure with absolute binding energies relative to the energies of the electron at rest, in vacuum.…”

Energy level modeling of lanthanide materials: review and uncertainty analysis

“…The period of the main vibrational mode at the defect site is taken for t 0 which is around 0.5-1 Â 10 À13 s. 126,129 DE T can be obtained from TQ measurements using two methods: one can fit eqn (17) to the experimental quenching profile, or one can take T 0.5 , i.e. The lifetime of the excited state can be obtained from time-resolved luminescence spectroscopy.…”