On the nature of the valence transition in Eu(Pd1−xAux)2Si2

The intermediate valent compound series Eu(Pd_1−xAu_x)₂Si₂ has been studied by Mössbauer effect measurements on ¹⁵¹Eu(T = 4.2−300K) and ¹⁹⁷Au (4.2K) and by X-ray diffraction (10K, 300K). The temperature induced valence transition Eu²⁺ → Eu³⁺ for x < 0.175 is not of first order type, as suggested in a previous phase diagram [1]. The valence change of the Eu-ion is reflected also in the isomer shift of the ¹⁹⁷Au Mössbauer-resonance.     

Intermediate valence of europium in EuxLa1−xBe13

Mössbauer studies of the 21.6 keV transition of¹⁵¹Eu in cubic EuBe₁₃ and Eu_1/2La_1/2Be₁₃ in the temperature range 100 K to 600 K have been performed. The position of the absorption line in EuBe₁₃ moves from 0.60 mm/s at 100 K to 0.04 mm/s at 575 K. In Eu_1/2La_1/2B₁₃ the line moves from 0.50 mm/s at 100 K to –0.03 mm/s at 500 K. We conclude that Eu in Eu_xLa_1–xB₁₃ is in an intermediate valence state and we analyze the temperature dependence of the isomer shift in terms of an interconfigurational fluctuation model. The model contains temperature independent parameters E_exc, the interconfiguration excitation energy, T_f, the valence fluctuation temperature, and S₃, the isomer shift of pure Eu³⁺ in EuBe₁₃. Assuming S₃–S₂= 13 mm/s the analysis yields S₃=0.70 mm/s, T_f=400 K and E_exc=3000 K for EuBe₁₃, whereas T_f=460 K and E_exc=2750 K for Eu_1/2La_1/2B₁₃.     

High-temperature thermoelectric properties of Ca3Co4O9+δ with Eu substitution

We have prepared polycrystalline Ca_3−xEu_xCo₄O_9+δ (x=0, 0.15, 0.3 and 0.45) samples using a sol-gel process followed by SPS sintering and investigated the Eu substitution effects on their high-temperature thermoelectric properties. With the Eu substitution, both the electrical resistivity and thermopower increase monotonously. This could be attributed to the decrease of hole concentrations by substitution of trivalent Eu³⁺ for divalent Ca²⁺. The Eu substituted samples (x=0.15, x=0.3) have lower thermal conductivity than Ca₃Co₄O_9+δ due to their lower electronic and lattice thermal conductivity. The dimensionless figure of merit ZT reaches 0.3 at 1000 K for the sample of Ca_2.7Eu_0.3Co₄O_9+δ.     

Europium valencies in substituted EuCu2Si2 systems

Mössbauer studies of¹⁵¹Eu in EuCu_2?x Pt _x Si₂, EuCu₂Si_2?x Ge _x and Eu _x Gd_1?x Mn₂Si₂ (x≤0.25) at temperatures between 90 K and 425 K have been performed. All spectra are composed of at least two subspectra, corresponding to stable Eu²⁺ and intermediate valent Eu. The isomer shift of the stable divalent Eu (ordered magnetically up to 400 K in Eu_0.25Gd_0.75Mn₂Si₂) isS ₂=?7.5 mm/sec in comparison toS ₂=?10.5 mm/s for stable Eu²⁺ in EuMn₂Ge₂. This shows that the Eu²⁺ is under heavy internal pressure in these systems. From the temperature dependence of the hyperfine interaction parameters and line intensities conclusions are derived concerning interconfigurational excitation energies, their inhomogeneous width and recoil free fraction behaviour.     

Niobates et tantalates d'europium de type “Bronze de Tungstene Quadratique”—I: Proprietes electriques et magnetiques

The magnetic and electrical properties of some TTB type niobates and tantalates of Eu^II with general formula Eu_xNbO₃ (0.5? x? 0.6), Eu_0.5TaO₃ and EuBCM₅O₁₅ (M = Ta, Nb; B = Eu^II, Sr, Ba; C = Na, K) have been investigated. Suceptibility measurements showed a large proportion of Eu³⁺ ions among the Eu²⁺ ions, and this was confirmed by Mössbauer spectroscopy. Mössbauer spectra also showed the absence of any magnetic ordering of the europium ions down to 4.2K, and the absence of any charge hopping phenomena up to room temperature. Conductivity and Seebeck coefficient measurements revealed typical semiconductor behaviour from 77 to 800 K. The Seebeck coefficient vs temperature curves showed a hopping conductivity regime at low temperature and an extrinsic semiconductor conductivity at high temperature. A Goodenough type of derived schematic energy level model for the tungsten bronze type structure was found to be qualitatively in agreement with the experimental results.     

Photoluminescence studies on Eu and Ce-doped Li2SrSiO4

The luminescence of Li₂SrSiO₄: 0.01Eu, xCe (x=0.0025, 0.005, 0.0075, and 0.01) is studied as a potential ultraviolet light-emitting diode (UV-LED) phosphor that is capable of converting the ultraviolet emission of a UV-LED into white light with good luminosity. There are broad blue and yellow emissions peaked at 413 and 575 nm, respectively. The two emissions come from d-f transitions of Ce³⁺ and Eu²⁺, respectively. The emission intensity of Li₂SrSiO₄: 0.01Eu, xCe reaches its maximum at x=0.0075. The energy transfer from Ce³⁺ to Eu²⁺ is demonstrated to be the type of electric dipole-dipole interaction with considerable spectral overlap and nonradiative transition is calculated to dominate. The Commission International de I’Eclairage (CIE) chromaticity coordinates of Ce³⁺/Eu²⁺ substituted compounds is also discussed.     

Enhanced electroluminescence of Eu by Tb in complexes Tb1−xEux(TTA)3Dipy

A series of rare earth ternary compounds of Tb_1−xEu_x(TTA)₃Dipy (HTTA=thenoyltrifluoroacetone, Dipy=2,2′-dipyridyl) have been synthesized, and the characteristics of the compounds have been performed by DTA-TG, IR, UV and fluorescence spectroscopy. Photoluminescence measurements indicated that the complexes of Eu(III) emit strong red luminescence under UV radiation. IR spectra suggest that complexes have been successfully synthesized, and TG curves indicate that the complexes are stable up to a temperature of about 220 °C. The Eu complex was blended with poly(N-vinylcarbazole) (PVK) and spin coated into films, and electroluminescence devices with the structure of Indium Tin Oxide (ITO)/PVK:Tb_1−xEu_x(TTA)₃Dipy/BCP(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)/aluminum quinoline (AlQ)/Al were fabricated, the luminescence of Eu³⁺ complexes enhances after doping with Tb³⁺. Therefore, it may be an effective method to improve the EL intensity of the lanthanide complex.     

Electron spin resonance and luminescence in Ga2−xEuxS3 single crystals

In present work electron spin resonance (ESR) and luminescence have been studied in Ga_2−xEu_xS₃ single crystals. The ESR and photoluminescence (PL) spectra of Ga_2−xEu_xS₃ were observed and the intensity of these spectra has increased linearly with the Eu concentrations in the samples. At the low temperature of 3.8 K in Ga_2−xEu_xS₃ single crystals, when the magnetic field has been applied as the perpendicular to the direction of [110], the hyperfine structure of ESR spectra has been observed. The exchange interaction of Eu²⁺ atoms in Ga_2−xEu_xS₃ single crystals have been determined as, g=4.2. The PL spectrum of pure Ga₂S₃ single crystals was recorded at 4.2 K and consisted of a narrow high-energy band at 2.53–2.38 eV and a low-energy band at 2.14–1.59 eV. Ga_2−xEu_xS₃ resulted in complete quenching of the impurity luminescence bands gave rise to a green europium band (2.38–2.14 eV) in the luminescence spectrum. Crystals of Ga_2−xEu_xS₃ emitted bright green electroluminescence (EL) spectrum when they excited with static and alternating electric fields at 77 K. We have also determined the field and frequency dependences of the EL.     

Effect of Mg and Ti doping on the luminescence of Y2O3:Eu

The Y₂O₃:Eu³⁺,Mg²⁺,Ti^IV materials (x_Eu: 0.02, x_Mg: 0.08, x_Ti: 0.04) were prepared by solid state reaction. The purity and crystal structure of the material was studied with the X-ray powder diffraction. Luminescence properties were studied in the UV-VUV range with the aid of synchrotron radiation. The emission of Y₂O₃:Eu³⁺,Mg²⁺,Ti^IV had a maximum at 612 nm (λ_exc: 250 nm) due to the ⁵D₀→⁷F₂ transition of Eu³⁺. The excitation spectra (λ_em: 612 nm) showed a broad band at 233 nm, due to the charge transfer transition between O²⁻ and Eu³⁺, and at 297 nm due to the Ti→Eu³⁺ energy transfer. Only very weak persistent luminescence was discovered. In the room and 10 K temperature excitation spectra, the line at 208 nm is due to the formation of a free exciton (FE) and a broad band at 199 nm was related to the valence-to-conduction band absorption of the Y₂O₃ host lattice. The absorption edge was ca. 205 nm giving 6.1 eV as the energy gap of Y₂O₃.     

Synthesis and photoluminescence characteristics of (Sr, Ca)3B2O6:Eu for application in white light-emitting diodes

A series of yellow-green (Sr, Ca)₃B₂O₆:Eu phosphors have been synthesized using precursors prepared via a facile sol-gel route. The solid-solution phases crystallized to materials with the formula of Sr_3−x−yCa_xEu_yB₂O₆ with varied Ca²⁺ and Eu²⁺ contents. The emission peak centered at 540 nm under near-UV excitation exhibited a broad-band distribution in the range of 450-650 nm. The dependences of the luminescence intensity on the contents of Ca²⁺ substitution and Eu²⁺ dopant were also investigated. The composition in the host lattice sensitively affected the chromaticity index. Sr_1.21Ca_1.7Eu_0.09B₂O₆ (SCB:0.09Eu) was shown to possess the highest intensity and broadest emission band. Calcining temperature was shown to greatly influence the luminescent properties of SCB:0.09Eu. It is concluded that SCB:0.09Eu can be used as an efficient yellow-green phosphor for white light-emitting diodes (white LEDs) applications.     

Synthesis and Eu concentration-dependent photoluminescence of Gd2−xEuxO3 nanowires

The nanowire growth behavior and photoluminescence characteristics of red-emitting oxide phosphor Gd_2−xEu_xO₃ have been investigated in the function of activator (Eu³⁺) concentrations (x=0.08, 0.12, 0.16, 0.20, and 0.24). Nanowires of Gd_2−xEu_xO₃ phosphor were prepared by the dehydration of corresponding hydroxides Gd_1−x/2Eu_x/2(OH)₃ obtained by the hydrothermal reaction. Highly uniform nanowires of 20-30 nm in diameter can grow up to several tens of micrometers in length. A number of defects on the surface of Gd_1.92Eu_0.08O₃ nanowires, which are induced during structural transformation from hexagonal hydroxide to cubic oxide, strongly decrease the luminescence efficiency in comparison with that of the bulk phosphor. In contrast, the photoemission intensity of nanowires is significantly improved with increasing Eu³⁺ content (x) of Gd_2−xEu_xO₃ solid solution. The highest relative emission intensity of nanowires is observed when the x value is close to x=0.20. This content is much higher than the optimal concentration of Eu³⁺ (x=0.08-0.10) for the bulk Gd₂O₃:Eu powder.     

Exchange and hyperfine fields acting on dilute Eu In SmCo5, Sm2(Co1-xFex)17 and Sm2CoxMy( rm M = Fe, Cu, Zn, Zr)

Mössbauer source experiments of dilute ¹⁵³Eu and ⁵⁷Fe in SmCo₅ and ¹⁵³Eu in Sm₂(Co_1-xFe_x)₁₇ and Sm₂ Co_x M_y at 4.1 K were performed. After the ${}^{153}\text{Sm}\text{→β?}{}^{153}$ decay the Eu ion is trivalent and exhibits extremely large hyperfine interactions due to strong exchange fields. Since the exchange interactions are comparable to the Eu³⁺ spin-orbit coupling, we calculated the expectation values of the Eu³⁺ spin, magnetic hyperfine field and electric field gradient as a function of exchange field and second order crystalline field, by diagonalization of the full Hamiltonian of spin orbit, exchange and crystalline field. For SmCo₅ and Sm₂(Co_1-xFe_x)₁₇ the exchange and crystalline fields are known and thus allow us to analyze our experimental results and obtain the polarized conduction electron contributions to the magnetic hyperfine field. The contribution due to magnetic neighbour polarization of conduction electrons is found to be linear in exchange field. The experimental results together with the theoretical analysis yield the Eu electric field gradient 4f Sternheimer shielding factor R_Q to be 0.26±04. It is shown that Mössbauer studies of two probes (¹⁵⁵Gd and Eu³⁺) in magnetic systems yield directly the second order crystalline field, the exchange field and the various contributions to the hyperfine field acting on the Eu nucleus. From the experimentally measured magnetic hyperfine fields alone, approximate values for the exchange fields in the mixed systems Sm₂Co_xM_y were determined.     

Photoluminescence in MgxSr1−xAl2O4:Eu, Ndand electron-vibrational interaction in the Eu 5d states

Green phosphor compositions Mg_xSr_1−xAl₂O₄:Eu, Nd (with x=0.05-0.25) were prepared by solid state reaction method. The effect of Mg substitution on photoluminescence characteristics was investigated. The photoluminescence show intense green emission for MgSrAl₂O₄:Eu²⁺, Nd³⁺ with long persistence. This green emission corresponds to transitions from 4f⁶5d¹ to 4f⁷ of Eu²⁺ ion. Comparative analysis of the excitation and emission spectra were used to evaluate the crystal field splitting of the 5d states of Eu²⁺ and the parameters of electron-vibrational interaction, such as Huang-Rhys factor, effective phonon energy, and zero-phonon line position.     

Luminescent properties of KGd1−x(WO4)2:Eux and KGd1−x(WO4)2−y(MoO4)y:Eux phosphors in UV-VUV regions

KGd_1−x(WO₄)_2−y(MoO₄)_y:Eu³⁺_x(0.1?x?0.75, y=0 and 0.2) phosphors are synthesized through traditional solid-state reaction and their luminescent properties in ultraviolet (UV) and vacuum ultraviolet (VUV) regions are investigated. Under 147 nm excitation, these phosphors show characteristic red emission with good color purity. In order to improve their emission intensity, the MoO₄²⁻ (20 wt%) is introduced into the anion of KGd_1−x(WO₄):Eu³⁺_x. The Mo⁶⁺ and Eu³⁺ co-doped KGd(WO₄)₂ phosphors show higher emission intensity in comparison with the singly Eu³⁺-doped KGd(WO₄)₂ in VUV region. The chromaticity coordination of KGd_0.45(WO₄):Eu³⁺_0.55 is (x=0.669, y=0.331), while that of KGd_0.45(WO₄)_1.8(MoO₄)_0.2:Eu³⁺_0.55 is (x=0.666, y=0.334) in VUV region.     

Ca1−xMo1−ySiyO4:Eux: A novel red phosphor for white light emitting diodes

Single phase of Ca_1−xMo_1−ySi_yO₄:Eu_x³⁺ (0.18?x?0.26, 0?y?0.04) was synthesized by solid-state method. The photoluminescence investigation indicated that Ca_1−xMoO₄:Eu_x³⁺ (0.18?x?0.26) could be effectively excited by 393 and 464 nm, and it exhibited an intense red emission at 615 nm. The introduction of Si⁴⁺ ions did not change the position of the peaks but strongly enhanced the emission intensity of Eu³⁺ under 393 and 464 nm excitations and showed very good color purity. The emission intensity of optimal Ca_0.8Mo_0.98Si_0.02O₄:Eu_0.2³⁺ sample (excited by 393 nm) was about 5.5 times higher than that of the phosphor Y₂O₂S:0.05Eu³⁺. So this phosphor could be nicely suitable for the application of the UV LED chips.     

Luminescent properties of HTP AgGd1−xW2O8:Eux and AgGd1−x(W1−yMoy)2O8:Eux phosphor for white LED

Intense red phosphors, AgGd_1−xEu_x(W_1−yMo_y)₂O₈ (x=0.0-1.0, y=0.0-1.0), have been synthesized through traditional solid-state reaction and characterized by X-ray diffraction (XRD) and photoluminescence (PL). XRD results reveal that AgGd_1−xEu_xW₂O₈ synthesized at 1000 °C has a tetragonal crystal structure, which is named as high temperature phase (HTP) AgGdW₂O₈. All phosphors compositions with Eu³⁺ show red and green emission on excitation either in the charge-transfer or Eu³⁺ levels. Analysis of the emission spectra with different Eu³⁺ concentrations reveal that the optimum dopant concentration for Eu³⁺ is x=0.6 in the HTP AgGd_1−xEu_xW₂O₈ (x=0.0-1.0). Studies on the AgGd_0.4Eu_0.6(W_1−yMo_y)₂O₈ (y=0.0-1.0) and AgGd_1−xEu_x(W_0.7Mo_0.3)₂O₈ (x=0.0-1.0) show that the emission intensity is maximum for compositions with y=0.3 and x=0.5, respectively, and a decrease in emission intensity is observed for higher y or x values. The Mo⁶⁺ and Eu³⁺ co-doped AgGd(WO₄)₂ phosphors show higher emission intensity in comparison with the singly Eu³⁺-doped AgGd(WO₄)₂ in UV region. The intense emission of the tungstate/molybdate phosphors under 394 and 465 nm excitations, respectively, suggests that these materials are promising candidates as red-emitting phosphors for near-UV/blue GaN-based white LED for white light generation.     

Europium valency in EuCu2−x Fe x Si2, EuCu2Si2−x Ge x,EuCu2−x Si2+x and EuNi2−x Si2+x

Mössbauer spectroscopy, X-ray photoemission spectroscopy and magnetization studies of EuCu_2?x Fe _x Si₂, EuCu₂Si_2?x Ge _x , EuCu_2?x Si_2+x , and EuNi_2?x Si_2+x have been performed. In EuFe₂Si₂ and EuNi₂Si₂ the Eu ion is trivalent, in EuCu₂Si₂ it is of intermediate valency. In all systems withx=0, many inequivalent Eu sites of intermediate valency are formed. In EuCu_2?x Fe _x Si₂, the average valency of Eu moves nonmonotonically toward Eu³⁺. In EuCu₂Si_2?x Ge _x , EuCu_2?x Si_2+x and EuNi_2?x Si_2+x , the Eu average valency moves quickly towards Eu²⁺. In all three systems, whenx=1, all Eu ions are already stable divalent. The systems EuCuSi₃ and EuNiSi₃ order magnetically at 40 K and 34 K, and exhibit hyperfine fields of 323 kOe and 458 kOe, respectively. XPS and Mössbauer isomer shift determinations of the average Eu valence as a function of temperature in EuCu_1.5Fe_0.5Si₂ are in good agreement. The experimental observations concerning the Eu valencies can not be explained solely in terms of the local volume available to the Eu ion; the nature of chemical environment plays a dominant role.     

Luminescent spectral characteristics of EuxLa1 - xTa7O19 polycrystals

Eu_xLa_{1 - x}Ta₇O₁₉ geptatantalates have been synthesized (x = 0.005-10). The luminescence and excitation spectra of these geptatantalates have been investigated at 77 and 300 K. It is supposed that the Eu³⁺ luminescence spectrum for all x may be interpreted within one type optical center with D_2d symmetry. The energy level diagram of the luminescence center has been worked out. It has been found that there is concentration dependence quenching of an europium luminescence. The reasons for this are discussed.     

Electric field gradient in cubic intermetallic europium compounds with unstable europium valence

Spin echo NMR measurements in EuPd₂ and EuPt₂ are reported. The europium hyperfine fields were determined and found to be equal to B_hf = + (2.08 ± 0.04) T in EuPd₂ and B_hf = -(3.33 ± 0.02) T in EuPt₂. The Eu¹⁵³ resonance lines show well resolved quadrupolar structures with a line separation of 0.7 MHz in EuPd₂ and 1.4 MHz in EuPt₂. Analysis of the NMR results obtained in the isoelectronic pseudobinary series Eu_xCa_1?xPd₂, Eu_xBa_1?xPd₂, Eu_xCa_1?xPt₂ and Eu_0.95Ba_0.05Pt₂ suggests that the observed quadrupolar interaction is partially related to a not completely divalent character of Eu in these compounds. This is in keeping with thermodynamic arguments given earlier by Miedema.     

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.