The structure and luminescence properties of long afterglow phosphor Y3−xMnxAl5−xSixO12

A series of phosphors with the composition Y_3−xMn_xAl_5−xSi_xO₁₂ (x=0, 0.025, 0.050, 0.075, 0.150, 0.225, 0.300) were prepared with solid state reactions. The X-ray powder diffraction analysis of samples shows that the substitution of Mn²⁺ and Si⁴⁺ does not change the garnet structure of phosphors, but makes the interplanar distance decrease to a certain extent. The emission spectra show that Mn²⁺ in Y₃Al₅O₁₂ emits yellow-orange light in a broad band. With the increment of substitution content, the emission intensity of the phosphors increases firstly then decreases subsequently, and the emission peak moves to longer wavelength. Afterglow spectra and decay curves show that all the Mn²⁺ and Si⁴⁺ co-doped samples emit yellow-orange light with long afterglow after the irradiation of ultraviolet light. The longest afterglow time is 18 min. Thermoluminescence measurement shows that there exist two kinds of traps with different depth of energy level and their depth decreases with the increment of substitution content.     

Ionic conductivity in the single crystals of non-stoichiometric fluorite phases M1−xRxF2+x (M=Ca,Sr, Ba; R=Y,La-Lu)

The ionic conductivity has been measured of single crystals of rare earth fluoride solid solutions in Ca, Sr and Ba fluorides described by the formula M_1?xR_xF_2+x (M=Ca, Sr, Ba; R=Y, La-Lu). The measurements have been done using dc and ac within the temperature range 300–850 K. With the increase of RF₃ concentration up to x=0.05–0.15 (in different systems) conductivity steeply increases. With the further concentration growth the conductivity increases only slightly, reaching saturation value for the solid solution Ba_1?xR_xF_2+x. The dependence of solid solution conductivity on the chemical composition has been studied for isoconcentrates M_0.9R_0.1F_2.1. It has been established that in CaF_2- based series the crystals with R=Gd, Tb possess maximum conductivity and minimum activation energy. For SrF₂ and BaF₂-based series the highest conductivity was observed for crystals containing LaF₃. It has been established that for all the crystals, independent of the chemical composition and defect concentration, the conductivity logarithm at definite temperature linearly depends on the activation energy. Within the investigated class of substances the optimum composition for solid electrolytes is Sr_0.69La_0.31F_2.31.     

X-ray photoelectron spectroscopy characterization of the layered intercalated compound K2xMn1 − xPS3

Polycrystalline powders of the layered MnPS₃ compound have been intercalated with K⁺ ions by ion-exchange to yield the K_2xMn_1 − xPS₃ intercalate. X-ray photoelectron spectroscopy has been applied to learn about the electronic structure of this compound. In particular, we have studied the XPS spectra of the Mn 2p and 3p, P and S 2p, K 2p and 3p core levels and of the valence band region. The binding energies for various core levels of the elements present in this compound and their observed chemical shifts are analyzed. The data give evidence for the lack of non-equivalent atoms of K, Mn, P and S. Shake-up satellites are present at the Mn 2p and 3p core levels. The occurrence of such lines allows us to hypothesize that K_2xMn_1 − xPS₃ is a large-gap insulating Mn compound. Confirmation that only an ion transfer accompanies the intercalation process is given from both the strong observed similarity with the corresponding XPS spectra in MnPS₃ and the observed binding energy positions of the K 2p and 3p levels. As regards the valence band XPS spectrum, the observed analogies with the corresponding XPS spectra of the pure compound and of other K compounds have allowed us to single out two regions and their probable contributors.     

Pressure dependence of the absorption edge for Cd1-xMnxTe

The effect of pressure on the optical absorption edge of mixed crystals Cd_1-xMn_xTe with different manganese concentrations is reported. The observed absorption edge shifts to higher energy with increasing pressure at a rate of α=7?8×10^?3 eV/kbar and a second order coefficient of β=-4×10^?5 eV/kbar² for x<0.5, to lower energy with increasing pressure at a rate of α=-5.0 ×10^?3 eV/kbar for x?0.5. A phase transition occurs for all the samples studied. The absorption edge of the new phase is outside the wavenumber range of the instrument. The physical origins of different pressure coefficients are discussed in the light of the deformation potentials of energy band states and the hybridization of the Mn²⁺ 3d levels with the p-like states in the valence band.     

Charge order melting and magnetic transition in Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 system

The magnetic property of double doped manganite Nd_0.5(1+x)Ca_0.5(1−x)Mn_(1−x)Cr_xO₃ with a fixed ratio of Mn³⁺:Mn⁴⁺=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr³⁺ for Mn³⁺ ions causes the increase of magnetization and the rise of T_c. As the Cr³⁺ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn³⁺ and Cr³⁺ ions causes a magnetic upturn, namely, the second FM phase transition.     

Near infrared fluorescence and energy transfer in Ce/Nd Co-doped CaxSr1−xS

Novel near infrared (NIR) phosphors Ca_xSr_1−xS:Ce³⁺,Nd³⁺ were synthesized by a solid state reaction. The NIR emission was realized through an efficient absorption by the allowed 4f-5d transition of Ce³⁺ and efficient energy transfer to Nd³⁺ via well-matched energy levels. Ce³⁺ and Nd³⁺ content in CaS/SrS was optimized. It was found that CaS:Ce³⁺,Nd³⁺ gave much stronger NIR emission than that of SrS:Ce³⁺,Nd³⁺. Further studies on Ca_xSr_1−xS:Ce³⁺,Nd³⁺ indicated that both visible emission of Ce³⁺ and NIR emission of Nd³⁺ were observably affected by Ca/Sr ratio. The energy transfer efficiency, which can be estimated from fluorescence lifetime of Ce³⁺, increased from 52% to 74% for the Ca_xSr_1−xS:Ce³⁺,Nd³⁺ (x=0 to 1) series, accompanied with a shift of maximal emission wavelength of Ce³⁺ from 482 to 505 nm. The results showed that overlap between emission spectrum of Ce³⁺ and excitation spectrum of Nd³⁺ plays an important role in the energy transfer efficiency, and Ce³⁺ emitting in green or blue-greenish region sensitized the Nd³⁺ NIR fluorescence emission more efficiently than that in blue region.     

Effect of temperature and activator concentration on relaxation of excited states of Eu in K5Li2La1−xEuxF10 crystals

Single crystals of K₅Li₂La_1−xEu_xF₁₀ (where x=0.01,0.03,0.1,0.25,0.50,1) have been obtained from anhydrous fluorides by solid state reaction and then by the Bridgman crystal growth process. Optical properties of crystals were investigated at various temperatures between 5 and 300 K. Energy level scheme for Eu³⁺ has been constructed based on absorption and emission spectra recorded at 5 K. Emission from ⁵D₃, ⁵D₂, ⁵D₁ and ⁵D₀ levels has been studied in detail. It has been found that emission originating from the ⁵D₀ and ⁵D₁ levels is efficient and long lived for all Eu³⁺ concentrations. Estimated selfquenching rate of the ⁵D₁ luminescence is as small as about 10³ s⁻¹ in K₅Li₂EuF₁₀. Temperature dependence of this selfquenching is found to be consistent with a cross-relaxation process in which a donor ion makes the ⁵D₁-⁵D₀ transition and an acceptor ion makes the ⁷F₁-⁷F₃ transition. The ⁵D₂ luminescence is considerably strongly influenced by activator-activator interaction but it is still easily detectable. The ⁵D₃ luminescence is completely quenched in K₅Li₂EuF₁₀.     

Luminescence properties of LiPrxCe1−xP4O12

LiPr_1−xCe_xP₄O₁₂ (x=0, 0.002, 0.02; 0.1) powder samples were prepared using the melt solution technique. Luminescent parameters of LiPr_1−xCe_xP₄O₁₂ phosphors have been investigated under ultraviolet-vacuum ultraviolet (3-12 eV) synchrotron radiation and X-rays excitation at room and near liquid He temperatures. Excitation luminescence spectra of Ce³⁺ emission, luminescent spectra and decay curves from the lower excited state levels of the 4f¹5d¹ and 5d¹ electronic configuration of the Pr³⁺ and Ce³⁺, respectively, clearly indicate energy transfer from Pr³⁺ to Ce³⁺. Energy migration proceeds via the Pr-sublattice followed by nonradiation transfer from Pr³⁺ to Ce³⁺ ions.     

The structure and luminescence properties of a novel orange emitting phosphor Y3MnxAl5−2xSixO12

A series of phosphors with the composition Y₃Mn_xAl_5−2xSi_xO₁₂ (x=0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.6) was prepared through solid state reactions. X-ray powder diffraction analysis of samples shows that when co-doping content does not exceed 16% of Al³⁺, equimolar co-doping of Mn²⁺ and Si⁴⁺ does not change the garnet structure of phosphors, but makes the interplanar distance to decrease a certain extent. However, if the co-doping content exceeds 16%, new phases will form in the samples. The excitation and emission spectra of samples show that Mn²⁺ in Y₃Mn_xAl_5−2xSi_xO₁₂ emits broadband orange light (peak wavelength varies from 586 to 593 nm). With an increment in co-doping content, the emission intensity of the phosphors increases when the value of x is lower than 0.1 while it decreases when it is higher than 0.1 and the emission peak moves to a longer wavelength.     

Fundamental optical properties of Cd1-xMnxSe single crystals

The absorption near the fundamental edge of Cd_1-xMn_xSe was measured in the composition range 0<x<0.3 at room and liquid nitrogen temperature with the electric field of the radiation parallel and perpendicular to the hexagonal axis. An exponential dependence of the absorption coefficient versus photon energy was found, and a linear dependence of energy gap E₀ on composition was obtained. The room temperature reflectivity measurements in the energy range 2.5–5.2eV, for two polarization of light were performed, and a linear dependence of the interband transitions energies vs. alloy composition was found.     

The influence of easy direction of magnetisation on 59Co NMR spectrum in R2(Co1−xMnx)17 compounds

The spin echo NMR spectra of ⁵⁹Co in R₂(Co_1-xMn_x)₁₇, (R = Y, Gd) measured at 4.2 K are reported. The large shift of resonance lines is observed, that is explained as caused by reorientation of easy axis of magnetisation from easy plane to easy direction (c axis). It is suggested to explain quantitatively the spectra, that only two of four Co sites (9d and 18f) in R₂Co₁₇ structure play a dominant role in determining of anisotropy energy and the Co atoms at the 6c sites (“dumb-bell” atoms) give no direct contribution to the anisotropy energy of the compound. The corresponding changes of local anisotropy energy and the orbital part of cobalt magnetic moment characteristic for each of cobalt structural sites are calculated and discussed.     

Conductivity enhancement in fluorite-structured Ba1−xLaxF2+x solid solutions

The ionic conductivity of single crystals of the fluorite-structured solid solutions Ba_1?xLa_xF_2+x(10^?3 <×<0.45) has been studied as a function of temperature and composition in the range 300–900 K. Three regions can be discerned in the concentration dependence of the ionic conductivity: a dilute concentration region (x<10^?3), where classic relations between solute content and ionic conductivity hold; an intermediate concentration region (10^?3<x?5×10^?2), where large changes occur in the conductivity activation enthalpy and the magnitude of the conductivity; and a concentrated solid solution region (x?5×10^?2) characterized by enhanced ionic motion. In the dilute region the migration enthalpy for interstitial fluoride ions is determined to be 0.714 eV, while a value of 0.39 eV is found for the (La_BaF_i)^X association enthalpy. The defect chemistry in the intermediate concentration region is shown to be controlled by a superlinear increase of the concentration of mobile defects, while in the concentrated solid solution region a composition-independent amount of ≈1 mole% of interstitial fluoride ions with enhanced mobility, carry the current.     

Spin-glass-like behaviour of the mixed spinel systems MnxZn1-xCr2O4 and Mn0.75Mg0.25(Cr1-yVy)2O4

NMR and susceptibility measurements have been made on a randomly mixed insulating ferrimagnet and antiferromagnet, Mn_xZn_1-xCr₂O₄. The thermoremanence and the induced unidirectional anisotropy were observed for concentrations lower than x = 0.80, after field cooling. The compound Mn_0.75Mg_0.25Cr₂O₄ shows similar behaviour. When the latter is doped with V³⁺ at the B sites, its magnetic anisotropy increases strongly, but the change in the unidirectional anisotropy is smooth.     

Concentration dependence of the Ce3+ and Tb3+ luminescence of Ce1-xTbxMgAl11O19

The quenching of the Ce³⁺ emission and the increase of the Tb³⁺ emission with increasing x of Ce_1-xTb_xMgAl₁₁O₁₉ for x ? 0.35 is ascribed to energy transfer from Ce³⁺ to Tb³⁺, which is restricted to nearest rare earth neighbours. This transfer is almost complete at x ? 0.35. The decrease of the Tb³⁺ emission at higher Tb³⁺ concentrations is not due to Tb³⁺ concentrational quenching, but due to the limited solubility of Tb³⁺ in the CeMgAl₁₁O₁₉ phase.     

X-ray photoelectron study of Sn1−xMnxTe semimagnetic semiconductors

X-ray photoelectron (XPS) studies of core-levels in Sn_1−xMn_xTe (x < 0.1) semimagnetic semiconductors have been performed. The spectra were acquired under UHV conditions from the clean (as-cleaved or in-situ scraped) crystal surface. The single-phase NaCl structure of the alloys studied was verified by X-ray diffraction (XRD). The structure of Sn 3d and Te 3d core-levels in SnMnTe was found fully consistent with that of SnTe. Remarkable qualitative similarity of the Mn 2p spectrum of Sn_1−xMn_xTe (x = 0.09) with the case of zinc-blende MnTe [R.J. Iwanowski, M.H. Heinonen, E. Janik, Chem. Phys. Lett. 387 (2004) 110] has been shown: (1) the same binding energies (BEs) of the main contributions to the Mn 2p_3/2 line, related to Mn²⁺ state of the bulk MnTe bond; (2) occurrence of low BE component in the Mn 2p spectrum, indicative of clean-surface species containing reduced-valence Mn ions (i.e. Mn^q+, where 0 < q < 2); (3) strong satellites of the 2p_3/2 (Mn²⁺ related) parent lines. In SnMnTe, the highest intensity ratio of the satellite to main peak (ever reported for Mn 2p photoelectron spectrum) was revealed; this was interpreted in terms of the so-called charge-transfer model.     

Diode laser action in Pb1-xMnxS

Diode laser action has been observed in Pb_1-xMn_xS alloy system for molar fraction of MnS x = 0.014 at the emission energy of 320 meV at 15 K. Combined current and magnetic field tuning has been examined in the range of 2.4–9.0 A and 0–14 kG, respectively and a quasi continuous tuning in the energy range from 320.5 meV to 322.0 meV was achieved. This is a first report about laser action in semiconductors containing paramagnetic ions.     

Structural study of Pr1−xCaxMnO3 and Y1−xCaxMnO3 perovskites

The structural properties of Prx_1?xCa_xMnO₃ and Y_1?xCa_xMnO₃ perovskite systems were studied by X-ray and magnetic susceptibility measurements. The differences in the structural properties of both systems are attributed to the differences of tolerance factors, i.e. of ionic radii of the ions present and the results are interpreted as being due to simultaneous action of steric effect, orbit-lattice interactions of Mn³⁺ ions and Coulomb interactions among 3d electrons. Their individual contributions depending on the compositions are analyzed. Besides the usual cooperative Jahn-Teller effect existing in PrMnO₃ a new type of transformation was found which is connected, under condition of Mn³⁺ ? Mn⁴⁺ valency migration, with the formation of local Jahn-Teller distortions. It is characterized by the equality of v_hop and v_phon at a critical temperature T_crit; it has been found that T_crit depends only very weakly on the composition. A correpsonding O'→O change in the yttrium system is, however, rather continuous which is explained by a strong influence of the cooperative buckling effect. An analysis of the magnetic data points to the presence of double exchange in the praseodymium system at high temperatures, while at low temperatures this mechanism applies only in the region, where cooperative Jahn-Teller effect exists, i.e. at x0.4.     

Crystal structure and photoluminescence of (Y1−xCex)2Si3O3N4

Oxonitridosilicate phosphors with compositions of (Y_1−xCe_x)₂Si₃O₃N₄ (x=0−0.2) have been synthesized by solid state reaction method. The structures and photoluminescence properties have been investigated. Ce³⁺ ions have substituted for Y³⁺ ions in the lattice. The emission and excitation spectra of these phosphors show the characteristic photoluminescence spectra of Ce³⁺ ions. Based on the analyses of the diffuse reflection spectra and the PL spectra, a systematic energy diagram of Ce³⁺ ion in the forbidden band of sample with x=0.02 is given. The best doping Ce content in these phosphors is ∼2 mol%. The quenching temperature is ∼405 K for the 2 mol% Ce content sample. The luminescence decay properties were investigated. The primary studies indicate that these phosphors are potential candidates for application in three-phosphor-converted white LEDs.     

Optical,magneto-optical and mössbauer spectroscopy on Co3+ substituted cobalt ferrite Co2+Fe2−xCo3+xO4(0 ⩽ x ⩽ 2)

The magneto-optical spectra of Co_1+xFe_2?xO₄ show with increasing Co³⁺ content an increasing intensity of the 4A₂ ? 4T₁(F) and 4A₂ ? 4T₁(P) transition of Co²⁺ at 0.8 and 2.0 eV. A decrease in the Co²⁺-Fe³⁺ charge transfer transitions on octahedral sites is found. In the optical spectra a strong increase in optical absorption is found with dominant transitions at 0.8, 1.6 and 2.6 eV due to Co³⁺ crystal field transitions on octahedral sites and a Co²⁺-Co³⁺ charge transfer. Conversion Electron Mössbauer Spectroscopy has been used to determine the cation distribution in the surface layer of the samples. The results indicate a shift of Co²⁺ from octahedral to tetrahedral sites when Co³⁺ is substituted in CoFe₂O₄. This results in enhanced optical absorption, enhanced magneto-optical effects and a lower Curie temperature.     

Laser time-resolved selective excitation of Nd3+ ions in KNdP4O12 crystals

The emission spectra of Nd³⁺ ions in KNd_xRE_1?xP₄O₁₂ (RE = Y, La and Pr) and KNd_xCr_1?xP₄O₁₂ crystals were investigated. Under selective excitation into ²G$\text{7}\text{2}$ + ⁴G$\text{5}\text{2}$ multiples at 1.6 K the fluorescence of Nd³⁺ ions in non-equivalent crystal sites was observed. The excitation spectrum of the ${}^4\text{F}\text{3}\text{2}$ fluorescence had a complex satellite structure. Time resolved measurements showed the dependence of the fluorescence decay on the excitation wavelength. Selective excitation into the satellite lines at the wings of the main transition led to strongly non-exponential decay. The low temperature results indicated that there is no spectral energy transfer between ions in different types of sites.