Photoluminescence in the mixed valence compound Eu3S4

We have investigated the 5d-4f interband and the intra-4f photoluminescence of Eu₃S₄. An anomalous shift of the interband luminescence can be explained by temperature dependent valence fluctuations. The ferromagnetic ordering of Eu₃S₄ is also verified.     

Electron hopping and optic phonons in Eu3S4

Raman scattering on single crystals of Eu₃S₄ does not show the allowed q=o phonon modes in the cubic phase and exhibits no new modes in the distorted low temperature phase (T<186 K). Above the Curie temperature T_c=3.8 K the scattering is dominated by a spin-disorder induced one-phonon density of states allowing for the observation of the zone boundary phonon breathing mode of the S^2?ions. This mode does not show any anomaly near the charge order -disorder phase transition T_t=186 K. Temperature tunable spin fluctuations associated with the temperature activated Eu²⁺→Eu³⁺ electron hopping are detected in the scattering intensity, superimposed on the usual thermal spin disorder.     

Magnetic susceptibility and valence fluctuations in Sm3S4

The magnetic susceptibility of Sm₃S₄ single crystals has been measured between 1.8 K and 300 K. At low temperatures the susceptibility is composed of χ_Sm²⁺ + 2χ_Sm³⁺. Near and above 100 K the susceptibility of the Sm³⁺ ions is quenched due to the increasing valence fluctuation rate.     

Influence of pressure on the inhomogeneous mixed-valent state in Eu3S4

The 21.7 keV nuclear gamma resonance of ¹⁵¹Eu was used for studying the inhomogeneous mixed-valent compound Eu₃S₄ in the pressure range 0–15 kbar. At room temperature the activation energy for electron hopping between the crystallographically equivalent divalent and trivalent Eu sites was found to decrease with pressure by dE_a/dP=-1.8±0.3 meV kbar^-1. No change of the mean valence of Eu with pressure was observed.     

Influence of pressure on the inhomogeneous mixed-valent state in Eu3S4

The 21.7-keV nuclear gamma resonance of ¹⁵¹Eu was used for studying the inhomogeneous mixed-valent compound Eu₃S₄ in the presence range 0 to 15 kbar. At room temperature the activation energy for electron hopping between the crystallographically equivalent divalent and trivalent Eu sites was found to decrease with pressure by dE_a/dP = -1.8 ± 0.3 meV/kbar. No change of the mean valence of Eu with pressure was observed.     

Preparation, characterization and luminescence properties of porous Si3N4 ceramics with Eu2O3 as sintering additive

Porous Si₃N₄ ceramics with photoluminescence properties were prepared by pressureless sintering using α-Si₃N₄ powder as raw material and Eu₂O₃ as sintering additive. Chemical composition, phase formation, microstructure and photoluminescence properties of porous Si₃N₄ ceramics were studied by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence measurements (PL/PLE). The results show that single Eu₂O₃ additive promotes α→β transformation but not significant densification. A broad band emission center at 570 nm assigned to Eu²⁺ is observed, Eu³⁺ in Eu₂O₃ is (partially) converted to Eu²⁺ by reaction with Si₃N₄, which results in a lower β aspect ratio and β-content compared to the other Ln (Ln=lanthanide) oxide additives.     

Radiative properties of Eu in BaGa2S4

A photoluminescence study of the blue-green emitting BaGa₂S₄:Eu²⁺ phosphor is reported. Diffuse reflectance, excitation and emission spectra were examined with the aim to enlarge the fundamental knowledge about the emission of the Eu²⁺ rare earth ion in this lattice. The thermal dependence of the radiative properties and the influence of the Eu²⁺ concentration were investigated. The Stokes shift, the crystal field splitting and the activation energy of the thermal quenching were determined. By combining these results with data available in literature, we discussed the radiative properties of the BaAl₂S₄:Eu²⁺ blue phosphor in relation with those determined in this study for the isostructural BaGa₂S₄:Eu²⁺ phosphor.     

Photoluminescence of Eu in SrGa2S4

A photoluminescence (PL) study of the green-emitting SrGa₂S₄:Eu²⁺ phosphor is reported. Diffuse reflectance, excitation, and emission spectra were examined with the aim to enlarge the fundamental knowledge about the emission of the Eu²⁺ ion in this lattice. The thermal dependence of the radiative properties was investigated. In particular, the Stokes shift, the crystal field splitting and the activation energy of the thermal quenching were determined. By combining these results with the information presented in literature, we discussed the location of the Eu²⁺ levels relative to the valence and conduction bands of SrGa₂S₄.     

Preparation of Ca(1−x)EuxGa2S4 crystals and their photoluminescence, absorption and excitation spectra

The single crystal of CaGa₂S₄:Eu is expected as a useful laser material with a high quantum efficiency of light emission. However, as far as our knowledge is concerned, the systematic study of the mixed compounds of Ca_(1−x)Eu_xGa₂S₄ as a function of x has not been reported up to now. Here, we have first constructed the phase diagram of the CaGa₂S₄ and EuGa₂S₄ pseudo binary system, and show that it forms the solid solution. Then we have grown single crystals of these compounds. The maximum photoluminescence efficiency is achieved at x=0.25. From the three peak energies observed in the photoluminescence excitation (PLE) and absorption spectra, the 5d excited states are suggested to consist of three levels arising from the multiplets of Eu²⁺ ions.     

Phonon spectra of stoichiometric rare-earth compound of EuGa2S4

Investigations of EuGa₂S₄ have been done on the photoluminescence (PL) related to the transition between 4f⁶5d and 4f⁷ configuration of the Eu²⁺ ion and its excitation (PLE) spectra, Raman scattering and infrared absorption. The energies of phonons coupled to the ground and the excited states of the transition are analyzed to be 34 and 19 meV from the shapes of the PL and PLE bands, respectively. The former corresponds to the energy of the Raman line showing the highest intensity. The latter is close to the value obtained from analysis of the temperature dependence of the half width of the PL band. These correspondences indicate that the relevant emission of EuGa₂S₄ surely has phonon-terminated character.     

Influence of [S2O3]/[Sm] on Sm2S3 thin films deposited by liquid phase deposition method on self-assembled monolayers

Sm₂S₃ thin films were prepared on Si (1 0 0) substrates using SmCl₃ and Na₂S₂O₃ as precursors by liquid phase deposition method on self-assembled monolayers. The influence of the molar concentration ratio of [S₂O₃²⁻]/[Sm³⁺] on the phase compositions, surface morphologies and optical properties of the as-deposited films were investigated. The as-deposited Sm₂S₃ thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), ultraviolet-visible (UV-vis) and photoluminescence spectrum (PL). Results show that it is important to control the [S₂O₃²⁻]/[Sm³⁺] during the deposition process and monophase Sm₂S₃ thin films with orientation growth along (0 1 1) direction can be achieved when [S₂O₃²⁻]/[Sm³⁺] = 2.0, pH 3.0, with citric acid as a template agent. The as-deposited thin films exhibit a dense and crystalline surface morphology. Good transmittance in the visible spectrum and excellent absorbency of ultraviolet light of the thin films are observed, and the band gap of the thin films first decrease and then increase with the increase of the [S₂O₃²⁻]/[Sm³⁺]. The as-deposited thin films also exhibit red photoluminescence properties under visible light excitation. With the increase of the [S₂O₃²⁻]/[Sm³⁺] in the deposition solution, the PL properties of Sm₂S₃ thin films are obviously improved.     

Raman studies of (Fe,V)3S4 solid solution

Raman scattering of Fe_xV_3-xS₄ with 0?x?2 has been studied. The observed spectra of V₃S₄, one of the end members of this solid solution, have been assigned to the Raman active lattice modes based on the C_2h³ space group symmetry. On the basis of the results obtained for V₃S₄, the nature of the metal-metal interactions and the site distributions of Fe atoms in (Fe, V)₃S₄ have been discussed.     

Magnetic susceptibility and valency state in Sm3S4

The lack of pronounced temperature dependence in the magnetic susceptibility of Sm₃S₄ rules out a magnetic ground state for Sm ions. Comparison with semiconducting and metallic phases of SmS suggest the presence of one Sm²⁺ ion for two non-integral valency Sm ions.     

Luminescent characterization of CaAl2S4:Eu powder

The photoluminescent (PL) emission and excitation behaviour of green-emitting CaAl₂S₄:Eu²⁺ powder phosphor is reported in detail. CaAl₂S₄:Eu²⁺ emission provides good CIE colour coordinates (x=0.141; y=0.721) for the green component in display applications. Powder with a dopant concentration of 8.5 mol% shows the highest luminescence efficiency. Temperature dependence of the radiative properties, such as luminescence intensity and decay time, was investigated. In particular, the Stokes shift, the mean phonon energy, the redshift, the energy of the f→d and d→f transition and the crystal field splitting of the CaAl₂S₄:Eu²⁺ emission were determined. The thermal quenching of the emission was examined.     

Photoluminescence of chromium doped CdIn2S4

Well-resolved photoluminescence spectra of CdIn₂S₄: Cr³⁺ recorded at 77K and 2K have been analysed. The R-lines from the doublet ²E-⁴A₂ transition and their associated vibronic spectra have been assigned. These results yield a revised value for the crystal field parameter of B = 618 cm^-1     

X-ray absorption spectroscopic study of mixed valence systems EuCu2Si2, YbCu2Si2 and Sm4Bi3

X-ray absorption spectroscopy technique is employed to determine the valence of the rare earth ions in EuCu₂Si₂, YbCu₂Si₂ and Sm₄Bi₃. In each case, two absorption peaks corresponding to two different valence states of respective rare earth ions have been observed. Low temperature (77 K) study of EuCu₂Si₂ indicates distinct change in the relative intensities of the absorption peaks compared to those registered at room temperature (300 K). It is inferred from the change in the relative intensities that the population of Eu²⁺ in EuCu₂Si₂ decreases at liquid nitrogen temperature compared to Eu³⁺. Conclusions drawn from these results agree well with those reported by others using different experimental techniques. In Sm₄Bi₃, Sm²⁺ and Sm³⁺ are found to occur in the ratio of 3:1.     

Influence of co-doping different rare earth ions on CaGa2S4: Eu, RE(RE=Ln) phosphors

Photoluminescent phosphors CaGa₂S₄: Eu²⁺, RE³⁺ (RE³⁺ including all rare earth ions except for Sc³⁺, Pm³⁺, Eu³⁺ and Lu³⁺) were prepared by sintering at high temperature in a reductive atmosphere, and their luminescent properties were studied intensively. The influences of co-doping rare earth ions on their luminescent properties were also investigated. No remarkable differences were found from excitation spectra of co-doped phosphors CaGa₂S₄: Eu²⁺, RE³⁺ in contrast with that of phosphor CaGa₂S₄: Eu²⁺, but there were a few differences in emission spectra of Ce³⁺, Pr³⁺ or Ho³⁺ co-doped phosphors. Phosphors CaGa₂S₄: Eu²⁺, RE³⁺ (RE=Ce, Pr, Gd, Tb, Ho and Y) had persistent afterglow, and very short afterglow was shown for Nd³⁺ or Er³⁺ co-doped phosphors, but no long afterglow appeared when auxiliary activator was La³⁺, Sm³⁺, Dy³⁺, Tm³⁺ or Yb³⁺. Among the phosphors with long-lasting phosphorescence, in our experiments, CaGa₂S₄: Eu²⁺, Ho³⁺ had the longest and the highest brightness long yellow afterglow. Thermo-luminescence of all co-doped phosphors was measured to find the answer of different influences from different rare earth auxiliary activators.     

Energy transfer in Y3Al5O12:Ce, Pr and CaMoO4:Sm, Eu phosphors

Non-radiative energy transfers (ET) from Ce³⁺ to Pr³⁺ in Y₃Al₅O₁₂:Ce³⁺, Pr³⁺ and from Sm³⁺ to Eu³⁺ in CaMoO₄:Sm³⁺, Eu³⁺ are studied based on photoluminescence spectroscopy and fluorescence decay patterns. The result indicates an electric dipole-dipole interaction that governs ET in the LED phosphors. For Ce³⁺ concentration of 0.01 in YAG:Ce³⁺, Pr³⁺, the rate constant and critical distance are evaluated to be 4.5×10⁻³⁶ cm⁶ s⁻¹ and 0.81 nm, respectively. An increase in the red emission line of Pr³⁺ relative to the yellow emission band of Ce³⁺, on increasing Ce³⁺ concentration is observed. This behavior is attributed to the increase of spectral overlap integrals between Ce³⁺ emission and Pr³⁺ excitation due to the fact that the yellow band shifts to the red spectral side with increasing Ce³⁺ concentration. In CaMoO₄:Sm³⁺, Eu³⁺, Sm³⁺-Eu³⁺ transfer occurs from ⁴G_5/2 of Sm³⁺ to ⁵D₀ of Eu³⁺. The rate constant of 8.5×10⁻⁴⁰ cm⁶ s⁻¹ and the critical transfer distance of 0.89 nm are evaluated.     

Optical gain due to the Eu transition in the alloy of Ca1-xEuxGa2S4

Room temperature photoluminescence quantum efficiency of the alloy of Ca_1−xEu_xGa₂S₄ was measured as a function of x, and was found to be nearly unity under excitation at peak wavelength of excitation spectrum (510 nm) in the x range of 0.01≤x≤0.2. At larger x values, it tends to decrease, but still as high as 30% for stoichiometric compound EuGa₂S₄. Taking these backgrounds into account, pump-probe experiments were done with Ca_1−xEu_xGa₂S₄ for searching optical gain at x=0.2. The optical gain of nearly 30 cm⁻¹ was confirmed to exist, though the pumping induced transient absorption which seems to limit the higher gain was found.     

ESR spectra of Eu centers in defect Ga2S3 single crystals

In this paper, the author presents the results of measurements of the low-temperature and angular dependences of the ESR spectra of Eu²⁺ centers in defect Ga₂S₃ single crystals in the temperature range 8–29 K and for 0–180° orientations of the static magnetic field. The electron structure of impurity ¹⁵¹Eu atoms in Ga₂S₃:Eu single crystals has been studied by using the ESR method at different doping proportions of Eu atoms. Ga₂S₃ single crystals were grown from the melt using the Bridgman method. The Eu concentration was determined by atomic absorption analysis and X–ray fluorescence analysis (XRFA). By investigation on the ESR spectra, the author has first determined the values of charge states for Eu, which have turned out to be a Eu²⁺(4f⁷) ion with spin S=7/2, g=4.18±0.02 and concentration of the states of Eu N=6.3×10¹⁴ cm⁻³.