Mixed-Valence Lanthanide-Activated Phosphors: Invariance of the Intervalence Charge Transfer (IVCT) Absorption Onset across the Series

Abstract

Recently, direct evidence was given for the existence of intervalence charge transfer (IVCT) states of Eu 2+ /Eu 3+ pairs in Eu-doped fluorite hosts and their signature in absorption spectra was characterized [J. Phys. Chem. Lett. 10(2019)1851]. Here we show, by means of multiconfigurational ab initio calculations, that the IVCT absorptions of all 2+/3+ mixed valence lanthanides doped in CaF 2 start basically at the same energy. This is rationalized with a simplified model of IVCT absorptions. Emissions above that energy are expected to be partly or totally quenched when 2+/3+ pairs are formed. On the basis of this finding and existing calculations, we report expected IVCT absorptions of mixed valence lanthanides doped in several fluoride, oxide, and sulfide hosts.

Broadband infrared LEDs based on europium-to-terbium charge transfer luminescence

“…The reason is that, when Tb 3+ is extrinsically compensated, the formation of Eu 3+ will also be favored 29 , generating undesired Eu 2+ –Eu 3+ (–Na + ) centers in addition to the intended Eu 2+ –Tb 3+ (–Na + ) centers. The former centers are responsible for the luminescence efficiency drop by IVCT quenching 28 , 43 .…”

“…As an example, the ground state of a Eu 2+ –Tb 3+ pair is denoted as 4 f 7 ( 8 S 7/2 )–4 f 8 (1 A 1 g ). Diabatic potential energy curves have proven their use by successfully explaining qualitative trends of CT processes upon chemical substitutions in host compounds 22 , 23 , 28 , 43 .…”

Charge transfer from Eu2+ to trivalent lanthanide co-dopants: Systematic behavior across the series

“…8 and 28, using the results of independent embedded cluster calculations from Ref. 30. They are aimed at describing spectroscopically relevant forward and backward electron transfers between Eu Ca and Ln Ca substitutional defects with non-local charge compensations.…”

“…The electronic structures of the electron donor and acceptor octahedral embedded clusters (LnF 8 ) 6− and (LnF 8 ) 5− were obtained with the suite of programs MOLCAS, 31 using D 2h symmetry, in two step spin-orbit coupling (SOC) SA-RASSCF/MS-RASPT2/RASSI-SO DKH calculations. 30 First, the spin-orbitfree many-electron relativistic second-order Douglas-Kroll-Hess (DKH) Hamiltonian 32,33 was used to perform all-electron calculations with the same type of basis set used in the calculations of CaF 2 :Yb 2+ , Yb 3+ in Ref. 9: Gaussian atomic natural orbital relativistic basis sets ANO-RCC for the lanthanides 34 and fluorine, 35 with respective contractions (25s22p15d11f 4g2h)/[9s8p5d4f 3g2h] (quadruple zeta with polarization quality) and (14s9p4d)/[5s4p3d] (quadruple zeta with polarization without f -function quality); plus the occupied 3s and 3p orbitals of the embedding Ca 2+ ions, contracted as (20s15p)/[1s1p] and located at the 12 neighbor sites ( 1 2 , 1 2 , 0), acting as cluster-host orthogonalization functions; 36 and 5 s-type Gaussian type functions located at the six ( 1 2 , 0, 0) interstitial sites surrounding the LnF 8 cube in the fluorite structure, whose orbital exponents were optimized to give a minimal impurity-trapped exciton energy in CaF 2 :Yb 2+ .…”

“…[21][22][23][24][25][26][27] In addition, it has been shown that diabatic combinations of such calculations on isolated ions in different oxidation states can successfully describe the optical spectra and other relevant features of IVCT and MMCT states. [8][9][10][11]15,[28][29][30] Here, this calculation method is systematically applied to study MMCT states of the co-doped materials CaF 2 :Eu 2+ , Ln 3+ (Ln 3+ = Ce 3+ , Pr 3+ , Nd 3+ , Pm 3+ , Sm 3+ , Eu 3+ , Gd 3+ , Tb 3+ , Dy 3+ , Ho 3+ , Er 3+ , Tm 3+ , and Yb 3+ ). After consideration of the effect of local vibrations and geometry changes upon electron transfer, systematic variations are found that predict distinct functional behaviors of co-doped materials.…”