Charge transfer from Eu to trivalent lanthanide co-dopants: Systematic behavior across the series.

Electron transfer processes between lanthanide activators are crucial for the functional behavior and performance of luminescent materials. Here, a multiconfigurational ab initio study reveals how direct metal-to-metal charge transfer (MMCT) between the Eu luminescence activator and a Ln co-dopant (Ln = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) systematically dictates the luminescence and optical properties of CaF. The combination of the structures and energies of the electronic manifolds, the vibrational force constants, and the structural properties of the donor and acceptor in the host determines the predictions of five different behaviors of CaF:Eu, Ln co-doped materials after MMCT absorption: formation of stable traps, MMCT emission, emission quenching, Ln emission, and Eu emission.

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

Efficient broadband infrared (IR) light-emitting diodes (LEDs) are needed for emerging applications that exploit near-IR spectroscopy, ranging from hand-held electronics to medicine. Here we report broadband IR luminescence, cooperatively originating from Eu and Tb dopants in CaS. This peculiar emission overlaps with the red Eu emission, ranges up to 1200 nm (full-width-at-half-maximum of 195 nm) and is efficiently excited with visible light.

Fine-Tuning the Cr R-Line by Controlling Pauli Antisymmetry Strength.

Fine-tuning of the Cr zero-phonon R-line fluorescence (E-A) has recently been achieved by varying the composition of Cr-doped solid solution garnets. Chemical substitutions of Y with Lu and Tb produce R-line shifts and multiplets that cannot be associated with direct or indirect changes in the Cr-O covalency alone and raise important questions on the nature of the interactions between the CrO active center and its environment, hence on the impact of local disorder. In this Letter we show that Pauli antisymmetry interactions with next-nearest neighbor ions, which are commonly omitted from theoretical analyses, induce a nephelauxetic effect on the CrO active center that shifts the R-line.

Direct Evidence of Intervalence Charge-Transfer States of Eu-Doped Luminescent Materials.

Direct evidence is given for the existence of intervalence charge transfer (IVCT) states of Eu/Eu pairs in Eu-doped CaF, SrF, and BaF. They are detected in diffuse reflectance spectra. In doped materials, IVCT states, in which an electron transfer occurs between two metal sites differing only in oxidation state, are rather difficult to observe because the absorption bands are extremely broad and flat, their intensity is low, and no emission follows the IVCT absorptions.

Color Control of Pr Luminescence by Electron-Hole Recombination Energy Transfer in CaTiO and CaZrO.

Controlling luminescence in phosphors able to produce several emissions from different stable excited states determines their use in optical devices. We investigate the color control mechanism that quenches the greenish-blue emission in favor of the red one in the archetype phosphor CaTiO:Pr. State-of-the-art ab initio calculations indicate that direct host-to-dopant energy transfer (released by electron-hole recombination following the interband excitation and structural reorganization) selectively populates the D red luminescent state of Pr and bypasses the P greenish-blue emitter.

X-ray Excitation Triggers Ytterbium Anomalous Emission in CaF:Yb but Not in SrF:Yb.

Materials that luminesce after excitation with ionizing radiation are extensively applied in physics, medicine, security, and industry. Lanthanide dopants are known to trigger crystal scintillation through their fast d-f emissions; the same is true for other important applications as lasers or phosphors for lighting. However, this ability can be seriously compromised by unwanted anomalous emissions often found with the most common lanthanide activators.

New Insights in 4f(12)5d(1) Excited States of Tm(2+) through Excited State Excitation Spectroscopy.

Optical excitation of ions or molecules typically leads to an expansion of the equilibrium bond lengths in the excited electronic state. However, for 4f(n-1)5d(1) excited states in lanthanide ions both expansion and contraction relative to the 4f(n) ground state have been reported, depending on the crystal field and nature of the 5d state. To probe the equilibrium distance offset between different 4f(n-1)5d(1) excited states, we report excited state excitation (ESE) spectra for Tm(2+) doped in CsCaBr3 and CsCaCl3 using two-color excited state excitation spectroscopy.

Metal-to-metal charge transfer between dopant and host ions: Photoconductivity of Yb-doped CaF2 and SrF2 crystals.

Dopant-to-host electron transfer is calculated using ab initio wavefunction-based embedded cluster methods for Yb/Ca pairs in CaF2 and Yb/Sr pairs in SrF2 crystals to investigate the mechanism of photoconductivity. The results show that, in these crystals, dopant-to-host electron transfer is a two-photon process mediated by the 4f(N-1)5d excited states of Y b(2+): these are reached by the first photon excitation; then, they absorb the second photon, which provokes the Y b(2+) + Ca(2+) (Sr(2+)) → Y b(3+) + Ca(+) (Sr(+)) electron phototransfer. This mechanism applies to all the observed Y b(2+) 4f-5d absorption bands with the exception of the first one: Electron transfer cannot occur at the first band wavelengths in CaF2:Y b(2+) because the Y b(3+)-Ca(+) states are not reached by the two-photon absorption.

Configuration coordinate energy level diagrams of intervalence and metal-to-metal charge transfer states of dopant pairs in solids.

Configuration coordinate diagrams, which are normally used in a qualitative manner for the energy levels of active centers in phosphors, are quantitatively obtained here for intervalence charge transfer (IVCT) states of mixed valence pairs and metal-to-metal charge transfer (MMCT) states of heteronuclear pairs, in solid hosts. The procedure relies on vibrational frequencies and excitation energies of single-ion active centers, and on differences between ion-ligand distances of the donor and the acceptor, which are attainable empirically or through ab initio calculations. The configuration coordinate diagrams of the Yb(2+)/Yb(3+) mixed-valence pair in Yb-doped YAG and the Ce(3+)/Yb(3+) heteronuclear pair in Ce,Yb-codoped YAG, are obtained and described.

Intervalence charge transfer luminescence: interplay between anomalous and 5d - 4f emissions in Yb-doped fluorite-type crystals.

In this paper, we report the existence of intervalence charge transfer (IVCT) luminescence in Yb-doped fluorite-type crystals associated with Yb(2+)-Yb(3+) mixed valence pairs. By means of embedded cluster, wave function theory ab initio calculations, we show that the widely studied, very broad band, anomalous emission of Yb(2+)-doped CaF2 and SrF2, usually associated with impurity-trapped excitons, is, rather, an IVCT luminescence associated with Yb(2+)-Yb(3+) mixed valence pairs. The IVCT luminescence is very efficiently excited by a two-photon upconversion mechanism where each photon provokes the same strong 4f(14)-1A1g→ 4f(13)((2)F7/2)5deg-1T1u absorption in the Yb(2+) part of the pair: the first one, from the pair ground state; the second one, from an excited state of the pair whose Yb(3+) moiety is in the higher 4f(13)((2)F5/2) multiplet.

Intervalence charge transfer luminescence: the anomalous luminescence of cerium-doped Cs₂LiLuCl₆ elpasolite.

The existence of intervalence charge transfer (IVCT) luminescence is reported. It is shown that the so called anomalous luminescence of Ce-doped elpasolite Cs2LiLuCl6, which is characterized mainly by a very large Stokes shift and a very large band width, corresponds to an IVCT emission that takes place in Ce(3+)-Ce(4+) pairs, from the 5de(g) orbital of Ce(3+) to 4f orbitals of Ce(4+). Its Stokes shift is the sum of the large reorganization energies of the Ce(4+) and Ce(3+) centers formed after the fixed-nuclei electron transfer and it is equal to the energy of the IVCT absorption commonly found in mixed-valence compounds, which is predicted to exist in this material and to be slightly larger than 10,000 cm(-1).

Blue absorption and red emission of Bi2+ in solids: strongly spin-orbit coupled 6p levels in low symmetry fields.

Wave function embedded cluster ab initio calculations on a (BiO8)(14-) cluster under the effects of a high symmetry Oh confinement potential are used to study the energies of the (2)P1/2, (2)P3/2(1), and (2)P3/2(2) spin-orbit coupling levels of the 6s(2)6p configuration of Bi(2+) in Oh, D4h, D2h, D4, D2d, D2, S4, C4v, C4, C3v, C2v, C2, Cs, and C1 fields, together with the (2)P1/2→(2)P3/2(1) and (2)P1/2→(2)P3/2(2) absorption oscillator strengths and the (2)P3/2(1) radiative lifetime. These levels are responsible for the blue absorptions and the red-orange emissions produced when Bi(2+) is doped in borates, phosphates, sulphates, and other hosts. It is found that the splitting of (2)P3/2 is mainly due to the tetragonal D4h and orthorhombic D2h components of the actual field.

Large splittings of the 4f shell of Ce3+ in garnets.

Ab initio embedded cluster calculations on Ce(3+)-doped Y3Al5O12, Lu3Al5O12, Gd3Al5O12, Y3Ga5O12, Lu3Ga5O12, and Gd3Ga5O12, which do not make use of any adjustable parameter, support recent assignments of the seventh 4f level of Ce(3+) in garnets [Przybylińska et al., Appl. Phys.

Ab initio theoretical study on the 4f(2) and 4f5d electronic manifolds of cubic defects in CaF2:Pr3+.

Wave function-based embedded cluster ab initio calculations have been carried out in order to study the 4f(2) and 4f5d energy levels of the cubic substitutional defect of Pr-doped CaF2. The 4f(2) → 4f5d absorption spectrum and 4f5d → 4f(2) emission spectrum have been calculated. The 4f(2 1)S0 level is found to be immersed in the 4f5d(eg) manifold, so that no quantum cutting from (1)S0 can occur and only strong 4f5d(eg) → 4f(2) emission is predicted, which supports previous assumptions made in order to explain results in CaF2:Pr(3+).

Host effects on the optically active 4f and 5d levels of Ce3+ in garnets.

When Ce(3+) is doped in a garnet, it substitutes for some cations. The effect of the substitution on the optically active 4f and 5d levels of Ce(3+) can be analyzed in terms of an undistorted substitution followed by a structural relaxation, but, whereas the contribution of the undistorted substitution can be predicted/calculated using the crystallographic structure of the pure garnet, which is at hand, that of the structural relaxation demands the detailed local structure around the Ce(3+) impurity, which is hard to know. Hence the importance of knowing the role of the undistorted substitution.

Effective Hamiltonian parameters for ab initio energy-level calculations of SrCl2:Yb2+ and CsCaBr3:Yb2+.

Calculated energy levels from recent ab initio studies of the electronic structure of SrCl2:Yb(2+) and CsCaBr3:Yb(2+) are fitted with a semi-empirical 'crystal-field' Hamiltonian, which acts within the model space 4f(14) + 4f(13)5d + 4f(13)6s. Parameters are obtained for the minima of the potential energy curves for each energy level and also for a range of anion-cation separations. The parameters are compared with published parameters fitted to experimental data and to atomic calculations.

Radial correlation effects on interconfigurational excitations at the end of the lanthanide series: a restricted active space second order perturbation study of Yb2+ and SrCl2:Yb2+.

At the end of the lanthanide series, 4f → 5d and other interconfigurational transitions, in which one electron is excited from a tight 4f orbital to a much more diffuse one, occur with a break of many f-f pairs, which make the electron correlation effects dominant. For instance, the large energy gap of 25 000 cm(-1) (∼29 500 cm(-1) without spin-orbit coupling) above the 4f(14) ground state of the SrCl2:Yb(2+) material is mostly due to electron correlation. In effect, a minimal multiconfigurational restricted active space (RASSCF) calculation that includes only the 4f(14) ground and 4f(13)5d and 4f(13)6s open-shell excited configurations gives a very small gap (5400 cm(-1)), whereas the correlation corrections to the 4f(14) → 4f(13)5d(eg) transition energies at the second order perturbation theory (RASPT2) level are very large: 35 599 ± 439 cm(-1), in average, for all excited states.

Yb2+-doped SrCl2: electronic structure of impurity states and impurity-trapped excitons.

First-principles electronic structure calculations of the excited states of Yb(2+)-doped SrCl(2) crystals up to 65,000 cm(-1) reveal the existence of unexpected excited states with double-well potential energy surfaces and dual electronic structure lying above and very close in energy to the 4f(13)5d manifold, with which they interact strongly through spin-orbit coupling. The double-well energy curves result from avoided crossings between Yb-trapped exciton states (more stable at short Yb-Cl distances) and 4f(13)6s impurity states (more stable at long Yb-Cl distances); the former are found to be preionization states in which the impurity holds the excited electron in close lying empty interstitials located outside the YbCl(8) moiety. Spin-orbit coupling between the double-well states and the lower lying 4f(13)5d impurity states spreads the dual electronic structure character to lower energies and, hence, the instability of the divalent oxidation state is also spread.

Electronic spectra of Yb2+-doped SrCl2.

The absorption and emission spectra of Yb(2+)-doped SrCl(2) have been calculated on the basis of ab initio quantum chemical calculations which consider recently found, unexpected excited states with double-well energy curves and complex electronic structure, resulting from avoided crossings between Yb-trapped excitons and Yb impurity states, which influence prominent spectral features. The root mean square deviation and largest absolute error of the calculated energy levels are 394 and -826 cm(-1), respectively. The YbCl(8) moiety breathing mode vibrational frequencies and bond lengths of the lowest states are consistent with observed vibrational progressions and energy shifts induced by uniaxial compression.

Energy gaps in the 4f(13)5d(1) manifold and multiple spontaneous emissions in Yb(2+)-doped CsCaBr(3).

Multiple spontaneous 4f(13)5d(1) --> 4f(14) emissions are predicted in Yb(2+)-doped CsCaBr(3) crystals by ab initio quantum chemical calculations. Four emission bands are found at 23,900, 26,600, 34,600, and 43,900 cm(-1) that should be experimentally observable at low temperatures. The first, third, and fourth bands are slow, electric dipole forbidden emissions that can be described as spin-forbidden.

Spin-forbidden and spin-enabled 4f(14)-->4f(13)5d(1) transitions of Yb(2+)-doped CsCaBr3.

The lowest part of the 4f-->5d absorption spectrum of Yb(2+)-doped CsCaBr(3) crystals has been calculated using methods of quantum chemistry and it is presented here. A first, low-intensity band is found on the low energy side of the spectrum, followed by several strong absorption bands, in agreement with experimental observations in trivalent and divalent lanthanide ions of the second half of the lanthanide series, doped in crystals. Based on Hund's rule, these transitions are usually interpreted as "spin-forbidden" and "spin-allowed" transitions, but this interpretation has been recently questioned in the literature.

Improved embedding ab initio model potentials for embedded cluster calculations.

An improvement in the method of production of embedding ab initio model potentials (AIMP) for embedded cluster calculations in ionic solids is proposed and applied to the oxides CeAlO(3), CeO(2), and UO(2). The improvement affects the calculation of one of the AIMP components, the Pauli repulsion operator, which prevents the cluster electrons from collapsing onto the occupied orbitals of the host in embedded cluster calculations and, so, their over occupancy. The linear constants involved in such operator are proposed to be obtained in embedded cluster calculations in the perfect host, with the requirement that local structures calculated with working embedded clusters of relatively small size agree with those calculated with reference embedded clusters of much larger size.

Embedding Fragment ab Initio Model Potentials in CASSCF/CASPT2 Calculations of Doped Solids: Implementation and Applications.

In this article, we present a fragment model potential approach for the description of the crystalline environment as an extension of the use of embedding ab initio model potentials (AIMPs). The biggest limitation of the embedding AIMP method is the spherical nature of its model potentials. This poses problems as soon as the method is applied to crystals containing strongly covalently bonded structures with highly nonspherical electron densities.

Detailed interpretation of the 5f-6d absorption spectrum of U3+ in Cs2NaYCl6 and high pressure effects based on an ab initio simulation.

The 5f-->6d(t(2g)) absorption spectrum of U(3+)-doped Cs(2)NaYCl(6) is simulated with a quantum chemical ab initio embedded-cluster approach applied to U(3+) substitutional defects of O(h) local symmetry. The first-principles results help to provide a detailed interpretation of the very rich experimental absorption spectrum of this material between 14 000 and 25 000 cm(-1). Also, the effects of high pressures up to 26 kbars on the absorption spectrum are predicted, the most relevant feature being a redshift of around 21 cm(-1)/kbar, which is the fingerprint of a bond length shortening upon 5f-->6d(t(2g)) excitation.

Geometry and electronic structure of impurity-trapped excitons in Cs2GeF6:U4+ crystals. The 5f17s1 manifold.

Excitons trapped at impurity centers in highly ionic crystals were first described by McClure and Pedrini [Phys. Rev. B 32, 8465 (1985)] as excited states consisting of a bound electron-hole pair with the hole localized on the impurity and the electron on nearby lattice sites, and a very short impurity-ligand bond length.