Energy level structure of4f5dstates and the Stokes shift inLaPO4:Pr3+: A theoretical study (original) (raw)
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Journal of Physics: Condensed Matter, 2013
The 4f-5d excitation and emission spectra of Ce 3+ and Pr 3+ ions in Ca 9 Lu(PO 4 ) 7 as recently reported (2012 J. Phys.: Condens. Matter 24 385502) were further analyzed and simulated by employing the effective Hamiltonian model for the 4f N and 4f N−1 5d electronic configurations of impurity lanthanide ions and the exchange charge model of crystal-field theory. The multi-site effect on the 4f-5d transition spectra was explicitly discussed from the points of view of the local structure and site occupation ratios of lanthanide ions in Ca 9 Lu(PO 4 ) 7 . An excellent agreement between the predicted and measured spectra confirms the validity of the performed calculations. Based on these energy level and intensity calculation results, the radiative lifetimes of the 5d-4f emissions of Ce 3+ and Pr 3+ ions have been modeled to show nearly independent temperature trends. Comparison with the measured lifetimes suggests the nonradiative relaxation process in this host is probably related to the intrinsic defect states. In addition to the studies of the 4f-5d transitions, a general theoretical scheme to calculate the lowest 4f-6s transition energy of the Ce 3+ ion was proposed for the first time on the basis of the ligand polarization model. The predicted 6s energy position of the Ce 3+ ion in Ca 9 Lu(PO 4 ) 7 is solid evidence corroborating our previous spectroscopic assignment.
Physical Review B, 2001
The excitation and emission properties of Pr 3ϩ and Ce 3ϩ doped M SO 4 (M 2ϩ ϭCa 2ϩ , Sr 2ϩ , and Ba 2ϩ) were investigated at 293 and 10 K. The lowest 5d level in CaSO 4 :Pr 3ϩ is located below the 4 f 2 ͓ 1 S 0 ͔ level resulting in allowed 4 f 5d→4 f 2 transitions. In BaSO 4 and SrSO 4 the lowest 5d level appears above the 1 S 0 level and 4 f 2 ͓ 1 S 0 ͔→4 f 2 emission is observed. However, also 4 f 5d→4 f 2 emission occurs, suggesting the presence of two different Pr centers. The unusual temperature dependence of the emission, points, however, to a thermal excitation process from the 1 S 0 state to the 4 f 5d states involving only one site. The internal quantum efficiency of the 3 P 0 and 1 D 2 emission in BaSO 4 is estimated to be 0.1 and 1 %, respectively. It will be demonstrated that under host lattice excitation part of the energy is transferred to 4 f 2 states with lower energy than the 1 S 0 or the 4 f 5d states. The possibility for quantum splitting to occur in Pr 3ϩ doped oxides and fluorides will be discussed. Trends observed in the interaction between the crystal field and the 5d electron of Ce 3ϩ , will be applied to predict the energy of the 4 f 5d states of Pr 3ϩ .
Density functional theory calculations of structural, electronic and optical properties of LaPO4:Eu
Optical Materials, 2014
Monoclinic LaPO 4 and LaPO 4 :Eu have been prepared by the hydrothermal method. The phase composition, UV-Vis absorption spectrum, excitation and emission spectra of as-obtained products were measured. Theoretical calculations of the structural, electronic and optical properties of LaPO 4 and LaPO 4 :Eu were also carried out. The results indicated that the lattice parameters, energy gap and optical properties were in good agreement with the experimental results. The impurity energy levels induced by the 4f states of Eu expanded the absorption edge and decreased the band gap. The charge transfer energy of O(2p)-Eu(4f) calculated was about 4.41 eV, which was close to the value achieved in excitation spectrum (4.5 eV).
Journal of Physics and Chemistry of Solids, 1995
LiY, _,Pr,F, (x < 0.1) powder samples and in LiY,_,Pr,F, (x GO.02) single crystals have been investigated. The concentration quenching of the Pr'+ emission is observed to be stronger in (La, _xPr,),O,. This is ascribed to a more efficient cross-relaxation via superexchange interaction between Pr" ions. This interaction is shown to be effective over longer distances in La,O,:Pr than in LiYF,: Pr (viz. IO and 5 A, respectively). The shift of the 4f5d configuration to lower energies with increasing Pr3+ concentration, which is only observed for (La, _xPr,),Ol, gives support for this conclusion. Moreover, the vibronic spectra and the vibronic transition probabilities of Pr 3+ in LaOF and La,O, are reported and compared.
Electronic Structure and Properties of CePO4
2010
Using a combination of density functional theory (DFT) calculations and experiments, we determine the structural and electronic properties of cerium orthophosphate (CePO 4), a promising proton-conducting electrolyte for fuel cell applications. To better account for strongly-localized Ce 4f electrons, we use a DFT + U approach, where the exchange-correlation functional is augmented with an adjustable effective Hubbard-like parameter U. We find that the calculated structural properties are in good agreement with x-ray diffraction measurements, largely independent of the value of U used. However, the electronic structure is much more sensitive to U, and values of U = 2.5-3 eV for Ce 4f states provide excellent agreement between the calculated density of states and measured photoemission spectra near the valence-band edge, validating the efficacy of a DFT + U-based approach for this system. With a judicious choice of U determined from photoemission experiments, this work provides a natural starting point for future studies of charge transport and charged defect formation and migration in this important class of compounds.
Optical and structural characterisation of pure and Pr 3+ doped LaPO 4 and CePO 4 nanocrystals
Journal of Alloys and Compounds, 2011
The rod- and stick-shaped nanocrystalline cerium and lanthanum orthophosphates, pure and Pr3+ doped, have been synthesised by the hydrothermal method in acidic (pH 1) or alkaline (pH 11) environment. Subsequent calcination of as-obtained powders at different temperatures, 500 and 900 °C, led to the formation of hexagonal and monoclinic phases, respectively. The hexagonal phase at room temperature has been identified as hydrated orthophosphate with zeolite water inside the channels of the structure. The monoclinic phase is free of water.The average grain size is about 12–30 nm depending on the calcination temperature and preparation conditions. The structural, morphological and optical (vibrational and luminescent) properties have been characterised and their changes due to the temperature and different acidity in the starting materials have been studied.► The nanoparticles (12–30 nm) of hydrated (hexagonal) and water-free (monoclinic) phases of lanthanum and cerium orthophosphates La(Ce)PO4 were obtained by the hydrothermal method. ► Differences between hexagonal and monoclinic nanocrystals are very well reflected in the vibrational spectra and a slight influence of the environment conditions (pH) of the crystallisation is found. ► The structure differences and environment conditions of the crystallisation (pH) play role in the luminescent properties.