Cooperative nonradiative cross-relaxation in crystals of La(1−x)CexF3 solid solutions

The phenomenon of concentration quenching of the luminescence from donor ions in crystals of La_1?x Ce_xF₃ solid solutions was studied. A cooperative nonradiative energy transfer from a single excited neodymium ion to a cooperative acceptor representing a couple of cerium ions, as well as from a single erbium ion to three cerium atoms (also forming a cooperative acceptor center), was observed.     

Luminescence and scintillation properties of Lu0.8Gd1.2SiO5:Ce and Lu1.8Gd0.2SiO5:Ce single crystals: A comparative study

The luminescence and scintillation properties of Lu_0.8Gd_1.2SiO₅:Ce and Lu_1.8Gd_0.2SiO₅:Ce single crystals are compared. At 662 keV γ-rays, light yield (LY) value of 29,800 ph/MeV obtained for Lu_1.8Gd_0.2SiO₅:Ce is much higher than that of 20,200 ph/MeV obtained for Lu_0.8Gd_1.2SiO₅:Ce. The energy resolution of 6.0% obtained for Lu_0.8Gd_1.2SiO₅:Ce is better than that of 7.7% for Lu_1.8Gd_0.2SiO₅:Ce due to its better intrinsic resolution and proportionality of LY. The LY and energy resolution for α-rays, as well as a LY ratio under excitation with α- and γ-rays (α/γ ratio) are also determined. The intrinsic LY and light loss coefficient under excitation with α- and γ-rays are evaluated. The photofraction at 662 keV γ-rays is also determined and discussed.     

Two-step photoconductivity in LiY<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>Lu<Subscript>1 – <Emphasis Type="Italic">x</Emphasis> </Subscript>F<Subscript>4</Subscript>:Ce,Yb crystals

Photoconductivity of LiY_xLu_1–xF₄:Ce,Yb (x = 0–1) crystals is measured under one- and two-step excitation. It is established that the photoconductivity is due to intra-center transitions from excited states of Ce³⁺ ions. The position of the ground 4 f-state of Ce³⁺ ion relative to the bottom of the conduction band is determined. The choice of pumping conditions to obtain the lasing on the 5d–4f transitions of trivalent cerium in these active media is substantiated.     

The nature of activation centers in Y2SiO5:Pr3+, Gd2SiO5:Pr3+, and Lu2SiO5:Pr3+ crystals

A complex study of the energy spectra and relaxation channels for the excitation energy of activation centers in Y₂SiO₅:Pr³⁺, Lu₂SiO₅:Pr³⁺, and Gd₂SiO₅:Pr³⁺ was performed. An analysis of the low-temperature optical spectra showed that the energy parameters and the character of field splitting of the ¹ D ₂ and ³ H ₄ activator ion terms were substantially different in crystals of different crystallographic types. The pseudosymmetry effect was observed in splitting of the ¹ D ₂ and ³ H ₄ terms of Pr³⁺ ions situated in nonequivalent crystal lattice cation sites of Y₂SiO₅ and Lu₂SiO₅. Activator ions nonuniformly populated nonequivalent cation sites of the Y₂SiO₅ crystal lattice. At high activator ion concentrations (>1 at %), luminescence decay in Y₂SiO₅ could not be described by a simple exponential time dependence. The complex luminescence decay law was caused by activator ion excitation energy migration and capture by acceptors. The role of energy acceptors was played by activator ion dimers.     

Multiplication of electronic excitations in nanophosphors Lu2O3:Eu and Lu2O3:Tb

Luminescence properties of Lu₂O₃:Eu³⁺ and Lu₂O₃:Tb³⁺ nanocrystalline powders with the particle size varying from 46 to 6 nm were studied under excitation by synchrotron radiation in the photon energy range (up to ∼22.5 eV) covering the region where the processes of multiplication of electronic excitation occur. It was found that the excitation spectra of Tb³⁺ emission from all Lu₂O₃:Tb³⁺ nanopowders have similar behavior, whereas the shape of the excitation spectra of Eu³⁺ emission from Lu₂O₃:Eu³⁺ nanopowders strongly depends on the particle size. The difference in the behavior of Lu₂O₃:Eu³⁺ and Lu₂O₃:Tb³⁺ nanophosphor systems was explained by different mechanisms of the energy transfer from the host to Eu³⁺ or Tb³⁺ ions (either the hole or electron recombination mechanism, respectively), which are differently influenced by losses of electronic excitations near the particle surface.     

High energy photoelectron spectroscopy of correlated electron systems and recoil effects in photoelectron emission

The bulk sensitivity of the high energy photoelectron spectroscopy (PES) in the soft and hard X-ray regions is gradually recognized to be essential for studying the electronic structures of strongly correlated electron systems with different surface and bulk electronic structures. With increasing hν, the kinetic energy (E_K) of photoelectrons is increased with realizing longer inelastic mean free path. Studies of three-dimensional genuine bulk band dispersions and bulk Fermiology are only possible by the soft X-ray angle resolved PES (ARPES). Examples are given for Nd_{2 - x}Ce_xCuO₄ and CeRu₂X₂(X: Si, Ge). The temperature dependence of the Yb 4f spectra provides useful information on valence fluctuation and Kondo resonance. The applicability of the single impurity Anderson model (SIAM) and its limit is discussed in the examples of Yb_{1 - x}Lu_xAl₃ and YbAl₃. The importance of the bulk sensitivity is also demonstrated by the soft and hard X-ray PES for SmOs₄Sb₁₂. When E_K becomes very large, the nucleus (ion) recoil effects become observable on the core photoelectron emission from light elements. The example is shown for Yb_7/8Lu_1/8B₁₂ and the influence of the recoil effects for the future high energy ARPES is briefly discussed.     

The charge-density-wave transition in Lu5Ir4Si10 under uniaxial pressure

Abstract

An uniaxial pressure cell for low temperature use is described in detail. Then we present data of the electrical resistance of single crystals of Lu₅Ir₄Si₁₀, which is known to show a charge-density-wave transition around 83 K and to become superconducting near 3.8 K, both phenomena being anticorrelated under pressure. Since the CDW in Lu₅Ir₄Si₁₀ is a quasi one-dimensional phenomenon because of a chain-like structure, it responds to uniaxial pressure in a specific way.     

Influence of unstable valence of cerium ions on the crystal field in ReNi compounds

To examine the effect of hybridization of 4f electrons with conduction electrons on the crystal field potential using neutron spectroscopy, we studied the effects of the crystal electric field (CEF) in intermetallic compounds of the type ReNi, in which chemical substitution is followed by a transition of the cerium ions from an intermediate valence state to the Kondo state. Measurements were performed both on cerium ions in the compounds Ce_1−x La_xNi (x=0.5, 0.8), where they have a whole-number population of the 4f shell, and on the paramagnetic impurity ion Nd in the series of compounds Re_1−x Nd_xNi (Re=Ce, La, Y), in which the cerium ions are found either in an intermediate valence state or in the Kondo state. From the neutron inelastic magnetic scattering spectra on Nd ions, we have reconstructed the crystal field parameters in ReNi compounds and calculated the CEF level diagram of Ce ions in these compounds as functions of the interion distances Re-Ni. The results of our calculations are in good agreement with the experimentally determined splitting diagram of the ground-state multiplet of the Ce ions. We have determined that as the degree of hybridization with the conduction electrons grows the CEF potential varies considerably and the effective splitting of the 4f shell of the cerium ions increases. The estimated energy scale of the splitting of the ground-state multiplet of the Ce³⁺ ions in the ReNi CEF (Δ_CEF∼15 meV) turns out to be commensurate with the Kondo temperature (T _K ;140 K for CeNi. Analysis indicates that the CEF potential has a substantial effect on the formation of the valence-unstable ground state of the f shell in this compound. Zh. éksp. Teor. Fiz. 113, 1731–1747 (May 1998)     

Symmetry Classification of the One–Dimensional Second Order Equation of a Hydrodynamic Type

Abstract

The paper contains a symmetry classification of the one–dimensional second order equation of a hydrodynamical type L(Lu)+λLu=F (u), where L ≡ ? _t+u? _x. Some classes of exact solutions of this equation are given.     

Pressure induced anisotropic magnetovolume phenomena in Lu2Fe17 Intermetallics

Magnetisation and magnetic susceptibility of a Lu₂Fe₁₇ single crystal have been studied under hydrostatic pressure up to 1.2 GPa at temperatures down to 5 K using a SQUID magnetometer. The ferromagnetic phase of Lu₂Fe₁₇ is suppressed rapidly above a critical pressure P _C = 0.4 GPa in the whole temperature range below the critical temperature T _C . A magnetic phase diagram of Lu₂Fe₁₇ has been constructed using results of the magnetic susceptibility measurements under pressure. A pressure induced incommensurate antiferromagnetic phase exhibits metamagnetic transitions with the increasing critical magnetic field H _C under pressure. Taking into account recent neutron diffraction data, the pressure induced anisotropic changes of the lattice parameters of the Lu₂Fe₁₇ are discussed.     

Syntheses of optically efficient (La1−x−yCexTby)F3 nanocrystals via a hydrothermal method

Optically efficient cerium and terbium doped lanthanide fluoride (La_1−x−yCe_xTb_y)F₃ nanocrystals with different doping concentrations have been synthesized by a hydrothermal route in the presence of ethylenediamine tetraacetic acid disodium salt (EDTA). The results showed that the formation of nanocrystals with different morphologies depends on terbium ion Tb³⁺ doping concentration, but independent of cerium ion Ce³⁺ doping concentration. With increase in Tb³⁺ doping concentration, the morphologies of nanocrystals evolved from a spherical shape to a plated-like one. In addition, both the photoluminescence quantum yield (PL QY) and the fluorescence lifetime of nanocrystals increased with the increase in Ce³⁺ doping concentration in cerium and terbium co-doped system. The PL QY reached up to 55%, and the lifetime up to 7.3 ms. Transmission electron microscopy (TEM), X-ray diffraction (XRD), selected area electron diffraction (SAED), X-ray fluorescence (XRF), energy dispersive spectroscopy (EDS), ultraviolet-visible (UV-vis) absorption, photoluminescence (PL) and infrared (IR) spectroscopies were employed to characterize the properties of nanocrystals. The growth mechanism of nanocrystals with different morphologies and optical properties of nanocrystals with different doping concentrations were investigated.     

Absence of pressure-induced valence change in CeAs

The pressure-volume relationship for cerium monoarsenide has been determined up to a pressure of 32 GPa using energy dispersive X-ray diffraction. Contrary to expectations based on the behaviour of CeP, cerium arsenide does not show a pressure-induced valence change in this pressure range. Instead we find a structural transition from the NaCl-type to the CsCl-type structure, which exhibits a large hysteresis. Contrary to other results our value of the bulk modulus (B₀ = 69 ± 1 GPa) agrees well with the value expected from an Anderson-Nafe plot for CeAs with the Ce ion in the trivalent state.     

Luminescence spectroscopy of excitons and antisite defects in Lu3Al5O12 single crystals and single-crystal films

The nature of the intrinsic luminescence of the lutetium aluminum garnet Lu₃Al₅O₁₂ (LuAG) has been analyzed on the basis of time-resolved spectral kinetic investigations upon excitation of two model objects, LuAG single crystals and single-crystal films, by pulsed X-ray and synchrotron radiations. Due to the differences in the mechanisms and methods of crystallization, these objects are characterized by significantly different concentrations of Lu_Al antisite defects. The energy structure of luminescence centers in LuAG single crystals (self-trapped excitons (STEs), excitons localized near antisite defects, and Lu_Al antisite defects) has been established. For single-crystal LuAG films, grown by liquid-phase epitaxy from a Pb-containing flux, the energy parameters of the following luminescence centers have been determined: STEs in regular (unperturbed by the presence of antisite defects) sites of the garnet lattice and excitons localized near Pb²⁺ ions. The structure of the luminescence centers, related to the background emission of impurity Pb²⁺ ions, has also been established in the UV and visible ranges. It is suggested that, in contrast to the two-halide hole self-trapping, a self-trapped state similar to STEs in simple oxides (Al₂O₃, Y₂O₃) is formed in LuAG; this state is formed by self-trapped holes in the form of singly charged O^? ions and electrons localized at excited levels of Lu³⁺ cations.     

EPR study of Ce ions in lutetium silicate scintillators Lu2Si2O7 and Lu2SiO5

Two Ce³⁺-doped scintillator crystals, LSO (Lu₂SiO₅:Ce) and LPS (Lu₂Si₂O₇:Ce), are studied by EPR spectroscopy. The analysis indicates that Ce³⁺ substitutes for Lu³⁺ ion in a C₂-symmetry site for LPS and in two C₁-symmetry sites for LSO, with a preference for the largest one, with 6+1 oxygen neighbors. Angular dependence of the EPR spectrum shows that the electronic ground state of Ce³⁺ is different in these two matrices. It is mainly composed of |M_J|=5/2 state in LPS and |M_J|=3/2 state in LSO. The temperature dependence of the linewidth shows a noticeably long spin lattice relaxation time, especially in LPS, which is the result of a stronger crystal field in LPS than in LSO.     

Energy transfer and sensitised luminescence in single crystals of CaF2:(Ce+Mn) system

The measurement of absorption, luminescence, and relative intensity of cerium and manganese emission during fluorescence, on single crystals of CaF₂:(Ce+Mn), have been reported in this paper. The occurence of the absorption bands have been explained in the following manner: (i) 250 mu band due to the transfer transition from F^? to²D, the excited state of Ce³⁺, (ii) 305 mμ band due to the characteristic transition of Ce³⁺ and (iii) 335 mμ band due to the perturbed level of the lattice. The emission bands have been explained like this, (i) 320 and 340 mμ bands due to the transitions of²D to²F_5/2,7/2 levels of Ce³⁺ (ii) 380 and 440 mμ bands corresponding to the perturbed levels and (iii) 520 mμ band due to transfer of energy from cerium centres to manganese centres. The results of the present investigation indicate that energy transfer in this system occures not only from cerium to manganese centres but also from cerium to cerium and cerium to perturbed levels of the lattice. It has also been observed that energy transfer process is temperature dependent in this case. An energy level scheme is proposed to explain the transfer mechanism.     

(TPP)NO3的合成、表征与分子识别NO

在氯仿与无水乙醇的混合溶剂(体积比为1:1)中,四苯基卟啉(TPP)与Ce(NO₃)·6H₂O混合反应后,得产物Ce(TPP)NO₃. 通过紫外-可见光谱、红外光谱、荧光光谱、质谱、核磁共振氢谱的分析与表征,四苯基卟啉与铈原子以四齿方式进行配位,在同一个铈原子上还有一个硝酸根配位. 向Ce(TPP)NO₃的二氯甲烷溶液中通入NO气体,NO可以配位在同一个铈原子上,得到新的配合物Ce(TPP)(NO)NO₃,向此溶液中通入N₂,金属卟啉配合物可以恢复为配合物Ce(TPP)NO₃.     

Magnetic HFI of cerium in an iron garnet investigated by angular correlations

The magnetic HFI of cerium ion in a lanthanum-doped yttrium iron garnet is investigated by the gamma-gamma angular correlation technique. It is found that cerium, formed after radioactive decay of La¹⁴⁰, reaches the ground state of Ce³⁺ in a time short compared with the nuclear state life-time of 4.9 nsec. Quasi-static as well as time-dependent interactions are detected, yielding an internal field ofH _eff=(21.5± 3) kOe at the nuclei of cerium. The molecular field theory is applied in the analysis of data, and values for the exchange field, the spinlattice relaxation time and the magnectic moment of cerium ion in an iron garnet at room temperature are deduced. These values are:H _exch=(147±20)kOe, τ _c =(4.5±0.6)×10^?13 sec and 〈μ_z〉=(0.017±0.002) Bohr magnetons.     

Single-crystalline films of Ce-doped YAG and LuAG phosphors: advantages over bulk crystals analogues

Luminescent properties of phosphors based on single-crystalline films (SCF) of Y₃Al₅O₁₂:Ce (YAG:Ce) and Lu₃Al₅O₁₂:Ce (LuAG:Ce) garnet have been analyzed in comparison with single-crystal (SC) analogues. It has been shown that the main peculiarity of luminescent properties of YAG:Ce and LuAG:Ce SCF as compared to SC is determined by the extremely low concentration of Y_Al³⁺ and Lu_Al³⁺ antisite defects (AD) in SCF. The advantages of phosphors based on YAG:Ce and LuAG:Ce SCF are caused by the absence in these SCF the additional channels for dissipation of excitation energy connected with AD and vacancy-type defects.     

Vacuum ultraviolet spectroscopy of pure and Ce3+-doped Na0.4Lu0.6F2.2

The spectroscopic properties of wide-band fluoride Na_0.4Lu_0.6F_2.2 crystals activated by Ce³⁺ were investigated. The absorption edge for the matrix was found to be at about 9.5 eV. In the 4- to 8-eV region of the absorption spectrum of Na_0.4Lu_0.6F_2.2: Ce³⁺, all 4f-5d transitions of the Ce³⁺ ion are observed. In Na_0.4Lu_0.6F_2.2: Ce³⁺ crystals, ultraviolet/visible emission, reflection and time-resolved vacuum ultraviolet/ultraviolet excitation spectra were recorded at liquid helium and room temperatures.     

Theoretical investigations on the high light yield of the LuI3:Ce scintillator

The extremely high scintillation efficiency of lutetium iodide doped by cerium is explained as a result of at least three factors controlling the energy transfer from the host matrix to activator. We propose and theoretically validate the possibility of a new channel of energy transfer to excitons and directly to cerium, namely the Auger process when Lu 4f hole relaxes to the valence band hole with simultaneous creation of additional exciton or excitation of cerium. This process should be efficient in LuI₃, and inefficient in LuCl₃. To justify this channel, we perform calculations of density of states using a periodic plane-wave density functional approach. The second factor is the increase of the efficiency of valence hole capture by cerium in the row LuCl₃-LuBr₃-LuI₃. The third one is the increase of the efficiency of energy transfer from self-trapped excitons to cerium ions in the same row. The latter two factors are verified by cluster ab initio calculations. We estimate either the relaxation of these excitations and barriers for the diffusion of self-trapped holes (STH) and self-trapped exciton (STE). The performed estimations theoretically justify the high LuI₃:Ce³⁺ scintillator yield.