掺Gd3+,Y3+对PbWO_4低温热释光的影响

通过热释光方法研究了PbWO₄(PWO),PWO:Y³⁺,PWO:Gd³⁺多晶粉末及PWO,PWO:Y单晶的低温(<300K)热释光现象.多晶粉末中,掺杂Y³⁺或Gd³⁺都会大大降低甚至消除200K附近的热释光峰,同时产生新的热释光峰,分别位于125和150K(掺Y掺Gd).这表明掺三价离子除了起到电荷补偿作用以减少Pb³⁺,O^-浓度外,还可以产生新的陷阱能级.对于PWO:Y单晶,掺杂Y³⁺可以消除253K的热释光峰,即消除较深(～0.89eV)的陷阱,但PWO单晶中较浅的陷阱(～0.42eV)对应130K热释光峰仍然存在,对此进行讨论,它最可能源于氧空位缺陷.根据Pb³⁺,Gd³⁺,Y³⁺的电子库仑势不同,在PWO晶体中替代Pb²⁺后形成的电子陷阱深度有别(E_Pb>E_Gd>E_Y),从而解释了相应的热释光峰值温度的不同 关键词： 4')" href="#">PbWO₄ Y和Gd掺杂 热释光 陷阱     

Phase transition in the superconducting state in Gd-rich pseudoternary compounds (La1-xGdx)1.0Mo6Se8

A pronounced lambda-type specific heat anomaly in the superconducting state is reported for Gd-rich pseudoternary compounds (La_1-xGd_x)_1.0Mo₆Se₈. The anomaly is very similar to that previously reported for the compound Gd_1.2Mo₆Se₈. Both the temperature of the lambda anomaly (≈ 3.5 K) and the Curie-Weiss temperature were found to be nearly independent of x. In addition, evidence for the occurence of magnetic ordering at 0.8 K in Gd_1.2Mo₆Se₈ is presented. These observations support our earlier conjecture that the lambda anomaly at 3.5 K does not arise from magnetic ordering of the Gd³⁺ ions. The anomaly appears instead to be due to a different type of phase transition which is associated with the presence of the localized 4f electrons of the Gd³⁺ ions, but the exact nature of the transition remains to be established.     

Recombination luminescence of CaSO4:Tb3+ and CaSO4:Gd3+phosphors

A comparative study of the excitation of luminescence by VUV radiation as well as of thermally and photostimulated luminescence has been carried out for CaSO₄:Tb³⁺ and CaSO₄:Gd³⁺ phosphors, where Na⁺ or F⁻ ions are used for charge compensation. The distinction in hole processes for the phosphors with Na⁺ or F⁻ compensators is determined by the differing thermal stability of the holes localized at/near Tb³⁺Na⁺ and Gd³⁺Na⁺ (up to 100–160 K) or at/near Tb³⁺F⁻ V _Ca and Gd³⁺F⁻ V _Ca centers involving also a cation vacancy (up to 400–550 K). Tunnel luminescence in the pairs of localized electrons and holes nearby Tb³⁺ or Gd³⁺ has been detected. The mechanisms of electron-hole, hole-electron and tunnel recombination luminescence as well as a subsequent released energy transfer to RE³⁺ ions are considered.     

Growth, optical parameters, and spectroscopic properties of crystals of disordered scheelite-like molybdates NaLaxGd1 ? x(MoO4)2 (x = 0–1) activated by Tm3+ ions

Polarized optical absorption spectra in the range of 450–2000 nm have been recorded for the Czochralski-grown concentration series of single crystals of mixed scheelite-like molybdates NaLa _x Gd_{1 − x} (MoO₄)₂(x = 0–1) activated by Tm³⁺ ions. The intensity parameters Ω _t (t = 2, 4, 6) of the crystals grown were determined by the Judd-Ofelt method, and the values found were used to estimate the probabilities and emission branching ratios for a number of radiative transitions of Tm³⁺ ions. The main optical constants of an Na_0.909La_0.490Gd_0.498Tm_0.035Mo_2.020O_8.048 crystal were found and the temperature dependences of the refractive index for the ordinary and extraordinary waves were investigated in the temperature range of 293–373 K. The spectral dependences of the emission cross section for the expected ³ F ₄ → ³ H ₆ laser transition of Tm³⁺ ions in an Na_0.908La_0.301Gd_0.676Tm_0.048Mo_2.019O_8.043 crystal were measured in the spectral range of 1500–2000 nm. The gain cross section of the active medium on the ³ F ₄ → ³ H ₆ laser transition of Tm³⁺ ions in this crystal was calculated using the spectral dependences of the absorption and emission cross sections.     

Contribution of Co3+ ions to the high-temperature magnetic and electrical properties of GdCoO3

The temperature dependence of the static magnetization of polycrystalline rare-earth cobaltite GdCoO₃ is measured in the temperature range 2?C800 K. The magnetic behaviors of GdCoO₃ and Gd³⁺ are found to be different at temperatures above room temperature, which is caused by the appearance of a contribution from Co³⁺ ions at high temperatures. The temperature dependence of the magnetic susceptibility of GdCoO₃ is determined by the magnetization of rare-earth gadolinium ions and the additional paramagnetic contribution induced by the thermally excited magnetic terms of Co³⁺ ions. The LDA + GTB method is used to calculate the electronic structure of GdCoO₃ in the temperature range 0?C300 K with allowance for strong electron correlations. The energy spectrum of GdCoO₃ is found to have intragap states that decrease the dielectric gap width with increasing temperature.     

Low temperature anomaly in Sc0.995Gd0.005H1.9

Gd³⁺ electron spin resonance linewidths and X-ray powder patterns have been determined from liquid nitrogen to room temperature on a hydrite of nominal composition Sc_0.995Gd_0.005H_1.9. These measurements reveal anomalous behavior below 140 K suggesting a phase transition.     

Phase transition in YbAl<Subscript>3</Subscript>C<Subscript>3</Subscript>

¹⁷⁰Yb M?ssbauer spectroscopy, temperature dependent X-ray, magnetisation and specific heat data are presented in the hexagonal intermetallic YbAl₃C₃, in order to shed light on the isostructural transition occurring near 80 K and to investigate the electronic state of the Yb ion above and below the transition. In the low temperature phase, we find that there occurs an atomic rearrangement in the hexagonal unit cell, leading to a strong symmetry lowering at the Yb site. We show that no magnetic ordering of the Yb³⁺ moments occurs down to 0.04 K, and we discuss this finding in terms of 4f-conduction electron hybridisation and geometric frustration.     

Thermal conductivity of YbInCu4

The electrical and thermal conductivities of an YbInCu₄ polycrystalline sample have been measured within the 4.2–300-K range. The behavior of the heat conductivity has been found to change sharply above and below T _v =70–75 K, the temperature corresponding to an isostructural phase transition from a state with an integral valence (T>T _v ) to a mixed-valence state (T<T _v ) of Yb ions. A preliminary qualitative analysis of the results is presented. Fiz. Tverd. Tela (St. Petersburg) 41, 1548–1551 (September 1999)     

Processes of the excitation energy migration and transfer in Ce-doped alkali gadolinium phosphates studied with time-resolved photoluminescence spectroscopy technique

Spectral-kinetic characteristics of Gd³⁺ and Ce³⁺ luminescence from a series of Ce³⁺-doped alkali gadolinium phosphates of MGdP₄O₁₂ type (M=Li, Na, Cs) have been studied within 4.2-300 K temperature range using time-resolved luminescence spectroscopy techniques. The processes of energy migration along the Gd³⁺ sub-lattice and energy transfer between the Gd³⁺ and Ce³⁺ ions have been investigated. Peculiarities of these processes have been compared for MGdP₄O₁₂ phosphate hosts with different alkali metal ions. A contribution of different levels from the ⁶P_j multiplet of the lowest Gd³⁺ excited state into the energy migration and transfer processes has been clarified. The phonon-assisted occupation of high-energy ⁶P_{5/2, 3/2} levels by Gd³⁺ in the excited ⁶P_j state has been revealed as a shift of Gd³⁺⁶P_j→⁸S_7/2 emission into the short-wavelength spectral range upon the temperature increase. The relaxation of excited Gd³⁺ via phonon-assisted population of Gd³⁺⁶P_5/2 level (next higher one to the lowest excited ⁶P_7/2) is supposed to be responsible for the rise in probability of energy migration within the Gd³⁺ sub-lattice initiating the Gd³⁺→Ce³⁺ energy transfer at T<150 K, whereas further intensification of Gd³⁺→Ce³⁺ energy transfer at T>150 K is explained by the increase in probability of Gd³⁺ relaxation into the highest ⁶P_3/2 level of the ⁶P_j multiplet. An efficient reversed Ce³⁺→Gd³⁺ energy transfer has been revealed for the studied phosphates at 4.2 K.     

Luminescence and recombination processes in bulky Li6Gd x Y1 − x (BO3)3:Eu crystals

The luminescence and recombination processes in crystals of lithium borates Li₆Gd _x Y_{1 − x} (BO₃)₃:Eu have been investigated. The steady-state X-ray luminescence (XRL) spectra, the temperature dependences of the XRL intensity, and the thermally stimulated luminescence (TSL) spectra have been measured for the Li₆Gd(BO₃)₃, Li₆Eu(BO₃)₃, Li₆Y_0.5Gd_0.5(BO₃)₃:Eu, and Li₆Gd(BO₃)₃:Eu compounds in the temperature range of 90–500 K. It is established that the band at 312 nm, which is due to the ⁶ P _J → ⁸ S _7/2 transitions in Gd³⁺ ions and the group of lines at 580–700 nm, which are due to the ⁵ D ₀ → ⁷ F _J (J = 0 … 4) transitions in Eu³⁺ ions dominate in the XRL spectra. The XRL intensity in these bands increases several times with a change in temperature from 100 to 400 K for undoped crystals. The likely mechanisms of the observed temperature dependence of the XRL intensity and their relationship with the features of electronic-excitation energy transfer in these crystals are discussed. The spectra of all crystals under study exhibit a dominant composite TSL peak with a maximum at 110–160 K and a series of weaker peaks, the composition and structure of which depend on the crystal type. The nature of the shallow traps, which are responsible for the TSL at temperatures below room, and their relationship with the defects of lithium cation sublattice are discussed.     

Effects of Gd<Superscript>3+</Superscript> doping on structural and dielectric properties of PZT (Zr:Ti = 52:48) piezoceramics

The purpose of this research is to study the effect of doping Gd into Pb(Zr_0.52Ti_0.48)O₃ ceramics prepared by solid state reaction. X-ray diffraction patterns show that all PGZT samples are of tetragonal structure and the highest doping should be no more than 2 mole % Gd at which the unreacted oxides start to appear. The electron spin resonance (ESR) spectra of PGZT's indicate that Gd³⁺ can enter both A site of the perovskite structure instead of only A site as widely believed. The ESR peaks resonance shift towards low fields as the concentration is higher, which is due to the change in crystal field experienced by Gd³⁺ ions. At x =0.001, 0.005 and 0.01 dopings, two sets of powder ESR signals arising from Gd³⁺ (4f⁷, spin 7/2) ions at A site. The first set shows some fine structure having strong absorption peaks centered at 76.26 mT (g = 8.550). The second is a seven-peak spectrum centered at 206.01 mT (g = 3.165), which belongs to the Gd³⁺ ions at B sites. Furthermore, the overlapped ESR strong absorption peaks from 309.17 mT to 314.49 mT (g = 2.2818-2.1087) belong to Gd³⁺ of unreacted Gd₂O₃. The local environments of Gd³⁺ ions were verified from the calculated ESR spectra using appropriate spin Hamiltonian parameter, i.e. gyromagnetic tensor g, zero-field splitting D and hyperfine tensor A.     

Antiferromagnetic resonance and magnetic anisotropy in single crystals of the YFe3(BO3)4-GdFe3(BO3)4 system

The antiferromagnetic resonance in single crystals of the YFe₃(BO₃)₄-GdFe₃(BO₃)₄ system is studied in the frequency range 25–140 GHz and the temperature range 4.2–50.0 K. It is established that the YFe₃(BO₃)₄ crystal containing only the magnetic subsystem of Fe³⁺ ions is an antiferromagnet with an easy anisotropy plane. The temperature dependences of the gaps in the antiferromagnetic resonance spectra of GdFe₃(BO₃)₄ and Y _x Gd_{1 ? x} Fe₃(BO₃)₄ are used to calculate the contributions of the Fe³⁺ and Gd³⁺ subsystems to the magnetic anisotropy of these crystals. The contributions are found to be close in magnitude and have opposite signs. This leads to a relatively weak uniaxial anisotropy field in the crystals under investigation. Since the exchange interaction between the Gd³⁺ and Fe³⁺ ions magnetizes the magnetic subsystem of gadolinium, both subsystems start to contribute simultaneously at the Néel temperature of the iron subsystem.     

Luminescence and thermally stimulated recombination processes in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>:Ce<Superscript>3+</Superscript> crystals

The luminescence and thermally stimulated recombination processes in lithium borate crystals Li₆Gd(BO₃)₃ and Li₆Gd(BO₃)₃:Ce have been studied. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence), temperature dependences of the intensity of steady-state X-ray luminescence (XL), and thermally stimulated luminescence (TSL) spectra of these compounds have been investigated in the temperature range of 90–500 K. The intrinsic-luminescence 312-nm band, which is due to the ⁶ P _J → ⁸ S _7/2 transitions in Gd³⁺ matrix ions, dominates in the X-ray luminescence spectra of these crystals; in addition, there is a wide complex band at 400–420 nm, which is due to the d → f transitions in Ce³⁺ impurity ions. It is found that the steady-state XL intensity in these bands increases several times upon heating from 100 to 400 K. The possible mechanisms of the observed temperature dependence of the steady-state XL intensity and their correlation with the features of electronic-excitation energy transfer in these crystals are discussed. The main complex TSL peak at 110–160 K and a number of minor peaks, whose composition and structure depend on the crystal type, have been found in all crystals studied. The nature of the shallow traps that are responsible for TSL at temperatures below room temperature and their relation with defects in the lithium cation sublattice are discussed.     

Investigation of the GdMn2O5 multiferroic by the μSR method

The GdMn₂O₅ multiferroic (a ceramic sample and a sample consisting of a large array of randomly oriented single crystals with linear dimensions 2–3 mm) has been studied by the μSR method within the temperature range 10–300 K. Three anomalies in the temperature behavior of the parameters of the muon polarization relaxation function, namely, close to the phase transition driven by the onset of long-range magnetic order in the manganese ion subsystem (T _N1 = 40–41 K), near the lock-in transition initiated by an abrupt change of the wave vector of magnetic order (T _L = 35 K), and close to the Gd³⁺ ion ordering temperature (T _N2 = 15 K), have been found. An analysis of the time spectra of muon spin precession in the internal magnetic field of the samples has revealed two positions of preferable muon localization sites in samples, which differ in precession frequencies and the character of their behavior with temperature. The lower-frequency precession driven by Mn⁴⁺ ions, ferromagnetic Mn4⁺-Mn⁴⁺ + muonium complexes, and Gd³⁺ions is observed throughout the temperature region T < T _N1 and is practically independent of temperature. At temperatures T < T _L = 35 K, a higher-frequency precession associated with Mn³⁺ ions appears also. It is characterized by a temperature dependence ～(T/T _N1)^β with the index β = 0.39, which is typical of Heisenberg-type 3D magnets. For T < T _N1, a deficiency of the rest total asymmetry is observed. This phenomenon can probably be assigned to formation of muonium, which suggests that charge transfer processes play an important role in formation of long-range magnetic order.     

Optical spectra and luminescence dynamics of the Dy-doped Gd2SiO5 single crystal

Spectroscopic properties of the Dy³⁺:Gd₂SiO₅ (GSO) single crystal were investigated. The polarized absorption and unpolarized emission spectra were measured at temperature ranging from 10 K to 300 K. Experimental oscillator strengths were determined from room temperature polarized absorption spectra and phenomenological intensity parameters Ω _t were calculated by using the standard Judd–Ofelt theory. Low-temperature measurements were used to determine the energy level structure of two nonequivalent Dy³⁺ sites in the GSO crystalline host. Analysis of spectra and decay curves of the ⁴F_9/2 emission revealed that Dy³⁺ ions entering nine-coordinated sites with C _3v symmetry and Dy³⁺ ions entering the seven-coordinated sites with C _s symmetry form two distinct, well-isolated subsystems weakly coupled by the spectral energy migration process. In addition to dissimilar crystal field splitting of multiplets, the two subsystems differ significantly in the efficiency of excitation energy transfer between dysprosium ions, thereby showing dissimilar self-quenching of the ⁴F_9/2 emission. Besides, only one of the two Dy³⁺ subsystems is coupled to Gd³⁺ ions by nonradiative Gd³⁺–Dy³⁺ energy transfer process. Laser potential related to the ⁴F_9/2→⁶H_13/2 yellow luminescence of dysprosium ions was assessed based on evaluation of the emission cross-section values. It was concluded that the Dy:Gd₂SiO₅ (Dy:GSO) is a promising material for the visible laser operation.     

Molecular dynamics study of ion transport in CaF2(10 mol % LaF3)

Abstract

We present the results of a molecular dynamics study of CaF₂(10 mol % LaF₃). No diffusion of fluorine ions is observed in pure CaF₂ below the transition temperature (T_C ≈ 1460K) to the fast-ion phase. However, doping with La³⁺ ions promotes fluorine ion diffusion in this temperature range, and tends to suppress it above T_C. Various ion migration mechanisms are discussed.     

EPR observation of phase transitions in calcium cadmium acetate hexahydrate: Using Mn2+ and Cu2+ ions as probe

Results of temperature dependence of EPR spectra of Mn²⁺ and Cu²⁺ ions doped calcium cadmium acetate hexahydrate (CaCd(CH₃COO)₄·6H₂O) have been reported. The investigation has been carried out in the temperature range between room temperature (～ 300 K) and liquid nitrogen temperature. A I-order phase transition at 146 ± 0.5 K has been confirmed. In addition a new II-order phase transition at 128 ± 1 K has been detected for the first time. There is evidence of large amplitude hindered rotations of CH₃ groups which become frozen at ～ 128 K. The incorporation of Cu²⁺ and Mn²⁺ probes at Ca²⁺ and Cd²⁺ sites respectively provide evidence that the phase transitions are caused by the molecular rearrangements of the common coordinating acetate groups between Ca²⁺ and Cd²⁺ sites. In contradiction to the previous reports of a change of symmetry from tetragonal to orthorhombic below 140 K, the symmetry of the host is concluded to remain tetragonal in all the three observed phases between room temperature and liquid nitrogen temperature.     

Intensities of hypersensitive transitions in garnet crystals doped with Er<Superscript>3+</Superscript> ions

We examine the oscillator strengths and the intensity parameters Ω _t (t = 2, 4, 6) of yttrium-aluminum, scandium-containing, and gallium garnet crystals doped with Er³⁺ ions. A comparative analysis of the oscillator strengths and the intensity parameters Ω _t (t = 2, 4, 6) of garnets with different contents of Al³⁺ and Sc³⁺ ions (Gd_2.4Er_0.5Sc_1.8Al_3.3O₁₂, Gd_2.4Er_0.5Sc_1.9Al_3.2O₁₂, Gd_2.4Er_0.5Sc_2.0Al_3.1O₁₂) is performed, as a result of which the oscillator strengths and the intensity parameters Ω _t (t = 2, 4, 6) of these crystals are shown to have close values. We find that Ca₃(NbGa)₅O₁₂ crystals doped with Er³⁺ ions are characterized by highest values of the oscillator strengths for hypersensitive transitions and of the intensity parameter Ω₂ of Er³⁺ ions compared to the values of these quantities in the examined garnet crystals, which is determined by the fact that the symmetry of the local environment of Er³⁺ ions in these crystals is C ₁, C ₂, or C _2ν. We reveal that, as the concentration of Er³⁺ ions in these crystals increases from 1 to 39 at %, both the oscillator strength of the hypersensitive transition ⁴ I _15/2 → ² H _11/2 of Er³⁺ ions and their intensity parameter Ω₂ tend to decrease, which can be related to an increase in the relative fraction of Er³⁺ ions with higher symmetry of the local environment.     

Paramagnetic centers in two phases of manganese-doped lanthanum gallate

An EPR study of two phases of manganese-doped lanthanum gallate (with a first-order structural transition occurring at 430 K) has revealed Gd³⁺, Fe³⁺, and Mn⁴⁺ centers at room temperature and 438 K. The parameters of spin Hamiltonians are determined for the Gd³⁺, Fe³⁺, and Mn⁴⁺ rhombohedral centers in the high-temperature phase (with no other centers found here) and for the monoclinic center Gd³⁺ in the low-temperature phase. Both in the orthorhombic and in the rhombohedral phase, crystallographic twins (or ferroelastic domains) are observed.     

Comment on magnetism and superconductivity in rutheno cuprates: RuSr2GdCu2O8 and RuSr2Gd1.5Ce0.5Cu2O10

Both RuSr₂GdCu₂O_8-δ (Ru-1212) and RuSr₂Gd_1.5Ce_0.5Cu₂O_10-δ (Ru-1222) exhibits magnetism and superconductivity, as seen by magnetization vs. temperature behavior measured in 5 Oe field. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization data show branching at around 140 K and 100 K with a cusp at 135 K and 80 K and a diamagnetic transition around 20 K and 30 K in the ZFC part, for Ru-1212 and Ru-1222, respectively. The isothermal magnetization possesses a non-linear contribution due to a ferromagnetic component at low temperatures below 50 K for both samples. The resistance vs. temperature behavior of the samples in applied fields of 0, 3 and 7 T confirmed superconductivity, with a different type of broadening of the superconductivity transition under magnetic fields for Ru-1212 from that known for conventional high-T _c superconductors. The magnetoresistance (MR) is negative above the Ru magnetic ordering temperature at 135 K. Below the Ru magnetic ordering temperature, MR displays a positive peak at low fields and becomes negative at higher fields for Ru-1212. For Ru-1222, MR remains negative both above and below the ordering temperature. A maximum of 2% is observed for the negative MR value at the Ru magnetic ordering temperature. An electron diffraction pattern obtained for the Ru-1212 sample shows two types of superstructure: one has a weak spot at the centre of the a–b rectangle, and the other only along the b direction. Interestingly, Ru-1222 shows only clean a–b and a–c planes, without any superstructures.