Spectroscopic characteristics of praseodymium-doped cubic double sodium-yttrium fluoride crystals Na0.4Y0.6F2.2:Pr3+. Intensities of optical transitions and luminescence kinetics

Using the Bridgman-Stockbarger technique, we have grown a series of cubic crystals Na_0.4Y_0.6F_2.2:Pr³⁺ (NYF:Pr³⁺) with a content of praseodymium in the range of 0.04–9 at %. We have determined the composition of crystals, evaluated their optical quality, and found the incorporation coefficient of Pr³⁺ ions into the Na_0.4Y_0.6F_2.2 matrix (K _Pr ～ 0.9). We have examined optical spectra of NaYF:Pr³⁺ crystals at room and low (7 K) temperatures in the range of 200–2500 nm. The low-temperature absorption spectra of NYF:Pr³⁺ crystals have been shown to consist of broad weakly structured bands. Based on the analysis of low-temperature absorption spectra, the structure of the Stark splitting of praseodymium levels has been represented in terms of a model of “quasi-centers,” which are characterized by an inhomogeneous broadening of Stark components. From experimental absorption cross-section spectra at T = 300 K, we have calculated oscillator strengths for transitions from the ground state ³ H ₄ to excited multiplets ³ H ₅, ³ H ₆, ³ F _j (j = 2, 3, 4), ¹ G ₄, ¹ D ₂, and (³ P _j ,¹ I ₆) (j = 0, 1, 2). Using the Judd-Ofelt method, we have determined intensity parameters Ω _t and found that Ω₂ = 0, Ω₄ = 4.4 × 10^?20, and Ω₆ = 2.28 × 10^?20 cm². With these values, we have calculated the probabilities of radiative transitions, the branching coefficients, and the lifetimes of the radiative levels ¹ D ₂ and ³ P ₀. The probabilities of multiphonon nonradiative transitions in NYF:Pr³⁺ crystals have been estimated. Using the method of kinetic spectroscopy with selective excitation, we have investigated the luminescence decay kinetics of praseodymium from the ³ P ₀ and ¹ D ₂ levels upon their selective resonant excitation by nanosecond laser pulses. The inference has been made that Na_0.4Y_0.6F_2.2:Pr³⁺ crystals are processable; admit doping by praseodymium in high concentrations; and, with respect to all their radiative characteristics, can be potentially considered as active media for converters of optical radiation and solid-state continuously tunable lasers in the visible range.     

掺杂稀土离子晶体中的电磁感应光透明现象

对在掺杂稀土离子晶体中实现电磁感应光透明进行了实验研究。首先,以Er3＋∶YAG晶体为样品,用旋转波近似下的密度矩阵方程理论计算了探测场的吸收特性随Er3＋离子浓度的变化规律,结果表明：在探测场失谐Δp=0时,形成了一个对于探测光透明的窗口,从而在理论上论证了在掺杂稀土离子晶体中实现电磁感应光透明效应的可行性。设计了一个以Pr3＋∶Y2SiO5晶体为样品的实验激发方案,吸收光谱显示,当温度为6 K时其在共振吸收峰处可形成一个完全透明的窗口,实现了在掺杂离子晶体中的电磁感应光透明。实验还分析了工作温度、耦合场失谐对探测光透过率的影响,结果显示：当样品温度上升到15 K时,透明窗口消失;耦合场的失谐量越大,透射率越小。     

Scintillation properties of PbWO4 crystals doped with the rare-earth ions

In PbWO₄(PWO) crystals grown by Czochralski method the influence of atmosphere of the growth (O₂, air) and doping with the rare-earth ions of different types (A³⁺=Lu³⁺, Gd³⁺,Tb³⁺,Eu³⁺ as well as doubly doped A³⁺–Li⁺) on light yield and luminescence decay were analyzed. PWO scintillator with the ultra-fast (τ=0.5 ns) main component of luminescence decay (87% of total light yield) was obtained using the O₂-growth atmosphere and doping by Eu₂O₃ at a concentration of 5000 ppm. It is concluded that the decrease of decay constant of the main scintillation component is the result of the resonant energy transfer between the centers of “blue” PWO luminescence (λ_max=420 nm) and the 4f–4f-transitions of Eu³⁺ ions in this spectral region.     

Spectroscopic investigations of wide-band fluoride crystals doped with ions of some rare-earth elements under X-ray excitation

Results of studying the properties of luminescence spectra of wide-band fluoride CaF₂, Ca_0.9Y_0.1F_2.1, Na_0.4Y_0.6F_2.2, Na_0.4Yb_0.6F_2.2, and BaYb₂F₈ crystals (pure and doped with Ce³⁺ and Yb³⁺ ions) under synchrotron radiation excitation with energies ranging from 3 to 60 keV are presented. Luminescence spectra are obtained in the region 200–600 nm. Transmission spectra are studied, and the effect of intense short-wavelength radiation on the optical properties of these crystals is studied.     

Mössbauer study of cobalt ferrite nanocrystals substituted with rare-earth Y ions

Mössbauer spectroscopy is used to characterize the crystallite size and structure of CoFe_2−xY_xO₄ (x=0, 0.1, 0.3, 0.5) ferrite nanocrystallites synthesized by the sol-gel auto-combustion method. The effect of the substitution of Fe³⁺ ions by Y³⁺ ions on the structure of cobalt ferrite nanocrystallites is investigated. The Mössbauer spectra showed two sets of six-line hyperfine patterns for all the samples, indicating the presence of Fe in both A and B-sites. On increasing the concentration of doped Y, the hyperfine field strength and the isomer shift first increase and then decrease, whereas the quadrupole splitting continuously increases. The superparamagnetism was observed for all the samples and the change of ratio of the superparamagnetism component reflects the size of crystal grain.     

EPR study of clusters of rare-earth ions in mixed fluoride crystals

The EPR spectra of the (BaF₂)_{1 ? x} (CeF₃) _x system are studied for the concentrations x = 0, 0.001, 0.002, 0.005, 0.01, and 0.02. The appearance of new tetragonal centers is detected beginning from x = 0.002, the intensity of these centers being maximal at x = 0.01. The (CaF₂)_{1 ? x ? y} (CeF₃) _x (YF₃) _y double solutions with x = 0.001 and y from 0 to 0.02 are also studied. In addition to the ordinary tetragonal center, beginning from y = 0.001, a new tetragonal center appears with the same structure as in the previously studied mixed crystals based on BaF₂—namely, the Ce³⁺-□-R³⁺ chain elongated along the fourfold axis substitutes the Ca²⁺-Ca²⁺-Ca²⁺ and Ba²⁺-Ba²⁺-Ba²⁺ chains in regular CaF₂ and BaF₂ crystals (□ is the cation vacancy, and R³⁺ is the Ce³⁺, La³⁺, or Y³⁺ trivalent ion).     

High-temperature VUV spectroscopy of KYF4 crystals doped with Nd3+, Er3+ and Tm3+ ions

Thermal quenching of 5d-4f luminescence from Nd³⁺, Er³⁺ and Tm³⁺ ions doped into KYF₄ crystals has been investigated in the temperature range up to ∼750 K where this luminescence is completely quenched. The obtained temperatures of thermal quenching (T_q) are ∼270, 495, 450 K for Nd³⁺, Er³⁺, Tm³⁺, respectively. At high temperatures, thermal quenching of 5d-4f luminescence from Nd³⁺ and Er³⁺ is accompanied by the appearance of 4f-4f luminescence from the lower-energy 4f levels. It has been shown that the dominating mechanism of thermal quenching for Nd³⁺ and Er³⁺ ions is thermally stimulated non-radiative transitions (intersystem crossing) from the 5d states to lower-energy 4f levels, namely ²G(2)_9/2 and ²F(2)_7/2, respectively, whereas for the Tm³⁺ ion, thermally stimulated ionization of 5d electrons to the conduction band states is responsible for thermal quenching of 5d-4f luminescence. The energy gap between the lowest Tm³⁺ 5d level and the bottom of the KYF₄ conduction band has been estimated to be 0.66 eV.     

Ultraviolet-visible spectroscopic characterization of lanthanum beryllate crystals doped with Er,Nd, or Pr ions

Spectroscopic characterization of lanthanum beryllate La₂Be₂O₅ (BLO) single crystals doped with trivalent ions of Eu, Nd or Pr, was carried out in the ultraviolet-visible spectral range using synchrotron radiation spectroscopy in combination with conventional optical absorption and luminescence spectroscopy techniques. On the basis of the obtained data, the energy level diagram for these trivalent impurity ions in BLO host lattice was developed; the optical and electronic properties of the crystals were determined; the possibility of the 4f-4f, 4f-5d and charge transfer transitions was analyzed; spectroscopic properties of the lattice defects formed during the introduction of trivalent impurity ions in the BLO host lattice, were investigated. We found that the lattice defects are responsible for a wide-band photoluminescence (PL) in the energy region of 400–600 nm. The most efficient excitation of the defect photoluminescence in the energy gap of BLO occurs in broad PL excitation-bands at 270 and 240 nm. The PL intensity of defects depends on the type of impurity ion and increases in the sequence: Pr-Nd-Er.     

Pressure effects on the superconducting transition of ytterbium doped Ce0.6Yb0.4FeAsO0.9F0.1

The Effect of pressure on the superconducting transition temperature of Yb doped Ce_0.6Yb_0.4FeAsO_0.9F_0.1 has been investigated for the first time using resistivity and magnetization studies. Increase in chemical pressure by substitution of smaller Yb³⁺ ions in place of Ce³⁺ ions results in a significant enhancement of T_c from 38 K (Yb free) to 47 K (40% Yb). Enhancement in T_c with external pressure has been observed for this compound up to a maximum value of T_c = 48.7 K at 1 GPa, beyond which T_c starts decreasing monotonously. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoluminescence and photoconductivity of indium selenide single crystals doped with rare-earth elements

The paper discusses the results of the investigation of photolominescence spectra and photoconductivity spectral distribution of Nd-; Yb- and Er- doped InSe single crystals at 77 and 293 K. Rare-earth additions into InSe contributes to the appearance of new bands in the wavelength region 1.02÷1.06 mkm. The photoconductivity spectra obtained for the mentioned crystals at 77 and 293 K are given.     

Ferroelectric properties of strontium barium niobate crystals doped with rare-earth metals

The influence of doping of the Sr_xBa_1?x Nb₂O₆ (x=0.61) crystals (SBN-0.61) by rare-earth element (RE) dopants has been investigated. Taking into account the data available in the literature, it is demonstrated that the introduction of all rare-earth metals is accompanied by a substantial decrease in the temperature of the phase transition T _p and an increase in its smearing. The shift in the T _p temperature for certain rare-earth metals ranges up to 20 K per atomic percent of dopant in the crystal. It is experimentally established that, in the SBN-0.61: Yb, SBN-0.61: Ce, SBN-0.61: Tm, SBN-0.61: La, and SBN-0.61: (Ce + La) crystals, a decrease in T _p brings about a considerable increase in the permittivity and the piezoelectric and electrooptical coefficients. The conclusion is drawn that the doping by rare-earth metals provides a means of optimizing the properties of strontium barium niobates. The pulse switching of the SBN-0.61 and SBN-0.61: RE crystals is studied for the first time. It is found that the switching is characterized by a number of features, the most important of which is a decrease in the switched charge after the application of external fields. This effect associated with the specific features of the switching in the relaxor ferroelectric is assumed to be responsible for the instability of the parameters for strontium barium niobate material.     

Luminescence properties of solid solutions of borates doped with rare-earth ions

The structural and luminescence properties of Lu _x Y_{1 ? x} BO₃ solid solutions doped with Ce³⁺ or Eu⁺³ have been investigated. It has been found that the solid solutions crystallize in the vaterite phase with a lutetium concentration x < 0.5. For a higher lutetium concentration x, the solid solutions contain an additional calcite phase with a content less than 5 wt %. The luminescence spectra are characterized by intensive impurity emission under excitation with the synchrotron radiation in the X-ray and ultraviolet spectral ranges. It has been shown that, as the lutetium concentration x in the Lu _x Y_{1 ? x} BO₃: Ce³⁺ solid solutions increases, the emission intensity smoothly decreases, which is associated with a gradual shift of the Ce³⁺ 5d(1) level toward the bottom of the conduction band, as well as with a decrease in the band gap. It has been established that, in the Lu _x Y_{1 ? x} BO₃: Eu³⁺ solid solutions with intermediate concentrations x, the efficiency of energy transfer to luminescence centers increases. This effect is explained by the limited spatial separation of electrons and holes in the solid solutions. It has been demonstrated that the calcite phase adversely affects the luminescence properties of the solid solutions.     

Spectroscopy of dielectric nanocrystals doped by rare-earth and transition-metal ions

This paper reviews studies on the optical properties of dielectric nanocrystals doped by rare-earth and transition-metal ions. Manifestations of small nanocrystal size in the optical properties of impurity ions are considered. Two types of phenomena are discussed: (i) those associated with modification of the vibrational spectrum of small particles and (ii) those due to the interaction of impurity ions with the medium in which they are embedded.     

ENDOR and transferred hyperfine interaction of impurity rare-earth ions with nearest diamagnetic ions in crystals

The tetragonal Er³⁺ ion associated with the interstitial F^? ion along the [100] axis in CaF₂ is studied using ENDOR. The parameters of the transferred hyperfine interaction and of the nuclear Zeeman interaction of the surrounding fluorine ions are determined. Anomalously large values of the pseudo-nuclear Zeeman effect on the F^? nuclei are found. The theoretical analysis of these parameters is carried out in a frame of operator techniques in the theory of transferred hyperfine interactions. A number of useful relations for practical calculations of the values of the local field at ligand nuclei are reported.     

Optical characteristics of alkali-halide crystals doped with nickel ions

Scientific-Research Institute of Mechanics and Physics at the Saratov State University, 112-A, Bolshaya Kozach'ya Str., Saratov, 410071, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 3, pp. 235–238, May–June, 1995.