Pressure dependence of elastic constants of orthorhombic Li2Ge7O15 above and below the ferroelectric transition

The pressure derivatives of the elastic constants c_ij of orthorhombic Li₂Ge₇O₁₅ have been determined at 293 K by the method of pressure-induced shifts of resonance frequencies of thick plates at ca. 15 MHz in the range between 0 and 1500 bar. Approaching the transition at ca. 630 bar, all P_ij = dc_ij/dp (i, j = 1, 2, 3; p pressure) develop strongly negative values. At higher pressures a similar behaviour with reversed sign is observed. The pressure derivatives of the pure “shear resistances” c₄₄, c₅₅, and c₆₆ depend only slightly on pressure even in the vicinity of the transition. The main interactions driving the transition are of the totally symmetric type. The values dK^?1/dp (K volume compressibility) deviate strongly from the quasi-invariant value of ca. 5 observed in almost all stable crystals (dK^?1/dp = ? 1750 at 620 bar and 1380 at 700 bar). The anomalous piezoelastic behaviour reflects the anomalous thermoelastic behaviour: negative P_ij in the low pressure (high temperature) phase correspond to positive T_ij = d log c_ij/dT (T temperature) and vice versa.     

Fluorescence enhancement of oxide fluoride glass co-doped with TbF3 and SmF3

The fluorescence property of xTbF₃-BaF₂-AlF₃-GeO₂+ySmF₃ (x=0.01-40 mol%, y=0-5 wt%) glasses were investigated. The enhancement of Sm³⁺ fluorescence was recognized in the presence of Tb³⁺. Increasing Tb³⁺ content, the emission color changed from green to orange. When the intensity of fluorescence at 540 nm originated from Tb³⁺ is compared with that at 600 nm originated from Sm³⁺, the information about the concentration quenching of Tb³⁺ and Sm³⁺ was obtained. From these results, rare earth ions were dispersed identically in the glasses. After heating to 673 K or cooling to 77 K, the emission color of 20TbF₃-20BaF₂-10AlF₃-50GeO₂/mol%+0.05 wt% SmF₃ glass was reversibly changed from orange to green. In addition, while the emission from 10TbF₃-20BaF₂-10AlF₃-60GeO₂+0.01 wt% SmF₃ glass was green, its crystallized sample, prepared by annealing at 1073 K, exhibited an orange emission due to Sm³⁺ at room temperature.     

Theoretical investigation of optical spectra and covalent effect of Cr in Y2Ti2O7 and Y2Sn2O7

In this work, the complete matrix of optical spectral levels in trigonal symmetry of 3d² (3d⁸) ions are established on basis of strong field coupling mechanism by using two spin–orbit coupling parameters model. The contribution of the spin–orbit coupling of ligand to the optical spectra has been included in these formulas. As an application, the optical spectra of Cr⁴⁺ in Y₂Ti₂O₇ and Y₂Sn₂O₇ have been studied by the complete diagonalization (energy matrix) method. The covalent effect has been studied and the difficulty about Dq parameter in explanation of optical spectra of Cr-doped Y₂Ti₂O₇ and Y₂Sn₂O₇ is removed. The theoretical results are in good agreement with observed data.     

Nanocrystalline BaTiO3, doped with Eu and Cr: Fluorescence spectroscopy and spontaneous ordering effects

The results of optical studies of Cr³⁺ and Eu³⁺ ions doped nanocrystalline ferroelectric BaTiO₃ produced by the sol-gel process (particle size 20-40 nm) are reported. The Cr³⁺ impurity ions ²E-⁴A₂ (R-lines) fluorescence spectra in BaTiO₃ revealed significant differences from that reported in literature for the bulk material. At least three types of Cr³⁺ centers were found in the spectra. The temperature dependence of optical second harmonic generation in nanocrystalline BaTiO₃: Eu³⁺ shows a strong hysteresis in C_4v-O_h ferroelectric phase transition region, which was explained by the ordering effects in the system of electric dipole moments of dipole nanocrystals. The temperature dependencies of radiative lifetime of Eu³⁺⁵D₀ excited level reveal some hysteresis too. The possibility of influence of the ordering in the system of BaTiO₃:Eu³⁺ nanocrystals on the effective index of refraction of the medium and thus on the Eu³⁺⁵D₀ radiative lifetime, due to the local field effects is discussed.     

Cr4+在Al2O3多晶体中的光谱性能研究

采用传统无压烧结工艺制备Cr:Al₂O₃透明多晶陶瓷.测定了其退火前后的吸收光谱和荧光光谱，发现在Al₂O₃六配位的八面体结构中，Cr⁴⁺的荧光发射也处在1100—1600nm波段的红外区间，荧光发射峰位于1223nm附近，类似Cr⁴⁺在四面体中的发光行为.同时由于氧化铝晶格常数较小，晶体场强较强，使Cr⁴⁺:Al₂O_{3< 关键词： 4+')" href="#">Cr⁴⁺ 2}O₃透明陶瓷')" href="#">Cr:Al₂O₃透明陶瓷 光谱性质 八面体     

Ce3+掺杂SiO2-Al2O3-Gd2O3玻璃的闪烁性能

通常, Ce离子掺杂的低密度玻璃有较高的发光效率, 而高密度的Ce离子掺杂玻璃其发光效率很低. 为了解释这一现象, 采用高温熔融法获得了SiO₂-Al₂O₃-Gd₂O₃三元系统的玻璃形成区, 并在还原气氛下制备了Ce³⁺掺杂SiO₂-Al₂O₃-Gd₂O₃以及SiO₂-Al₂O₃-Gd₂O₃-Ln₂O₃ (Ln=Y, La, Lu)闪烁玻璃, 研究了其光谱和闪烁性能. 测试结果显示: 随着Gd₂O₃含量增加, 玻璃紫外截止波长发生红移, 荧光强度降低, 衰减时间缩短; 加入Lu₂O₃, La₂O₃, Y₂O₃后, 紫外截止波长发生红移, 荧光强度降低, 衰减时间变短; 当Gd₂O₃超过10% mol时, X射线荧光积分光产额从相当于锗酸铋 晶体的61%降低到13%. 荧光强度降低、衰减时间缩短的原因是随着玻璃的紫外截止波长红移玻璃的能带宽度变窄, 使得Ce³⁺离子的d电子轨道开始接近玻璃的导带, Ce³⁺离子受辐射后跃迁到d电子轨道的电子会通过导带与玻璃中的空穴复合, 产生电荷迁移猝灭效应.     

Crystal field interactions in DyFe2Si2 and DyFe2Ge2

Two sets of crystal field (CF) parameters have been proposed for DyFe₂Si₂, none of which could provide a simultaneous explanation of the available experimental data, particularly at low temperatures (below 100 K). The set derived from magnetic studies could not even explain the thermal variation of the magnetic specific heat reported in the same work. Although the set of CF parameters, obtained from a fit to the Mossbauer spectra, could provide a fairly good explanation of the thermal variation of the magnetic susceptibilities along the c-axis, it could not explain the observed thermal variation of other reported experimental findings. In the present work, an appraisal of the CF parameters proposed earlier has been done and a set of CF parameters has been derived, which provide a simultaneous explanation of all the available experimental data. The effect of substitution of Ge for Si on the magnetic properties and the magnetic specific heat of DyFe₂Si₂ has been studied in the framework of one electron crystal field model. The inelastic neutron scattering studies and EPR measurements are required to check the predicted Stark energies and the paramagnetic resonance g-values.     

Vibrational and Fe-Mössbauer spectra of LaFeGe2O7 and NdFeGe2O7

The infrared, Raman and ⁵⁷Fe-Mössbauer spectra of LaFeGe₂O₇ and NdFeGe₂O₇ were recorded and analysed on the basis of their structural characteristics. Some comparisons with the stoichiometrically related materials containing the heavier lanthanides are made, showing that it is possible to differentiate spectroscopically both groups of materials. The Mössbauer parameters clearly reflect the small structural differences in the FeO₅-polyhedra present in these compounds.     

Optical decoherence and persistent spectral hole burning in Tm:LiNbO3

We report studies of decoherence and spectral hole burning for the 794 nm optical transition of thulium-doped lithium niobate. In addition to transient spectral holes due to the ³H₄ and ³F₄ excited states of Tm³⁺, persistent spectral holes with lifetimes of up to minutes were observed when a magnetic field of a few hundred Gauss was applied. The observed anti-hole structure identified the hole burning mechanism as population storage in the ¹⁶⁹Tm nuclear hyperfine levels. In addition, the magnetic field was effective in suppressing spectral diffusion, increasing the phase memory lifetime from 11 μs at zero field to 23 μs in a field of 320 Gauss applied along the crystal’s c-axis. Coupling between Tm³⁺ and the ⁷Li and ⁹³Nb spins in the host lattice was also observed and a quadrupole shift of 22 kHz was measured for ⁷Li at 1.7 K. A Stark shift of 18 kHz cm/V was measured for the optical transition with the electric field applied parallel to the c-axis.     

Optical decoherence and persistent spectral hole burning in Er:LiNbO3

Developing new resonant optical materials for spatial-spectral holography and quantum information applications requires detailed knowledge of the decoherence and population relaxation dynamics for the quantum states involved in the optical transitions, motivating the need for fundamental material studies. We report recent progress in studying these properties in erbium-doped lithium niobate at liquid helium temperatures. The influence of temperature, applied magnetic fields, measurement timescale, and dopant concentration were probed using photon echo spectroscopy and time-resolved spectral hole burning on the 1532 nm transition of Er³⁺:LiNbO₃. Effects of spectral diffusion due to interactions between Er³⁺ ions and between the Er³⁺ ion and ⁷Li and ⁹³Nb nuclear spins in the host lattice were observed. In addition, long-lived persistent spectral storage of seconds to minutes was observed due to non-equilibrium population redistribution among superhyperfine states.     

Eu photoluminescence enhancement due to thermal energy transfer in Eu2O3-doped SiO2-B2O3-PbO2 glasses system

In this work, Eu³⁺-doped lead borosilicate glasses (SiO₂-B₂O₃-PbO₂) synthesized by fusion method had their optical properties investigated as a function of temperature. Atomic Force Microscopy images obtained for a glass matrix annealed at 350 and 500 °C show a precipitated crystalline phase with sizes 11 and 21 nm, respectively. Besides, as the temperature increases from 350 to 300 K a strong Eu³⁺ photoluminescence (PL) enhancement takes place. This anomalous feature is attributed to the thermally activated carrier transfer process from nanocrystals and charged intrinsic defects states to Eu³⁺ energy levels. In addition, the PL peaks in this temperature range were assigned to the Eu³⁺ transitions ⁵D₀→⁷F₂, at 612 nm, ⁵D₀→⁷F₁, at 595 nm, and ⁵D₀→⁷F₀, at 585 nm. It was also observed that the ⁵D₀→⁷F₃ and ⁵D₀→⁷F₄ PL bands at 655 and 700 nm, respectively, show a continuous decrease in intensity as the temperature increases.     

Luminescence enhancement in (Nd+Ce) doped SiO2 : Al2O3 sol–gel glasses

An enhancement in NIR luminescence from Nd³⁺-doped Ce³⁺ co-doped SiO₂+Al₂O₃ sol–gel glasses has been observed. The lasing transition (⁴F_3/2→⁴I_11/2) at 1072 nm from the dual rare-earth Nd³⁺+Ce³⁺-doped glasses has shown an emission strength of about five times that of the single rare-earth ion Nd³⁺-doped glass. From the measurement of lifetimes of the transition at 1072 nm, the transfer rate (W_tr), critical distance (R₀) and energy transfer efficiency (η) of the neodymium glasses have been calculated.     

Features of the local structural disorder in Li2O-CaF2-P2O5 glass-ceramics with Cr2O3 as nucleating agent

Li₂O-CaF₂-P₂O₅ glasses mixed with different concentrations of Cr₂O₃ (ranging from 0 to 1.0 mol%) were crystallized. The samples were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy (EDS), differential thermal analysis and conventional spectroscopic techniques. The X-ray diffraction and scanning electron microscopic studies reveal the presence of lithium phosphate, calcium phosphate and chromium phosphate (complexes of Cr³⁺, Cr⁵⁺ and Cr⁶⁺ ions) crystal phases. The study on DTA suggests that the crystallization is predominantly due to the surface crystallization when the concentration of nucleating agent Cr₂O₃ is around 0.8 mol%. The IR and Raman spectral studies of these samples indicate that the sample crystallized with 0.8 mol% Cr₂O₃ is more compact and possesses high rigidity due to the presence of chromium ions largely in tetrahedral positions.     

Electron energy loss spectra from polycrystalline Cr and Cr2O3 before and after surface reduction by Ar bombardment

Electron energy loss spectra (ELS) have been obtained from polycrystalline Cr and Cr₂O₃ before and after surface reduction by 2 keV Ar⁺ bombardment. The primary electron energy used in the ELS measurements was systematically varied from 100 to 1150 eV in order to distinguish surface versus bulk loss processes. Two predominant loss features in the ELS spectra obtained from Cr metal at 9.0 and 23.0 eV are assigned to the surface and bulk plasmon excitations, respectively, and a number of other features arising from single electron transitions from both the bulk and surface Cr 3d bands to higher-lying states in the conduction band are also present. The ELS spectra obtained from Cr₂O₃ exhibit features that originate from both interband transitions and charge-transfer transitions between the Cr and O ions as well as the bulk plasmon at 24.4 eV. The ELS feature at 4.0 eV arises from a charge-transfer transition between the oxygen and chromium ions in the two surface layers beneath the chemisorbed oxygen layer, and the ELS feature at 9.8 eV arises from a similar transition involving the chemisorbed oxygen atoms. The intensity of the ELS peak at 9.8 eV decreases after Ar⁺ sputtering due to the removal of chemisorbed oxygen atoms. Sputtering also increases the number of Cr²⁺ states on the surface, which in turn increases the intensity of the 4.0 eV feature. Furthermore, the ELS spectra obtained from the sputtered Cr₂O₃ surface exhibit features characteristic of both Cr⁰ and Cr₂O₃, indicating that Ar⁺ sputtering reduces Cr₂O₃. The fact that neither the surface- nor the bulk-plasmon features of Cr⁰ can be observed in the ELS spectra obtained from sputtered Cr₂O₃ while the loss features due to Cr⁰ interband transitions are clearly present indicates that Cr⁰ atoms form small clusters lacking a bulk metallic nature during Ar⁺ bombardment of Cr₂O₃.     

Fluorescence features of Sm ions in Na2SO4-MO-P2O5 glass system—Influence of modifier oxide

Na₂SO₄-P₂O₅:Sm³⁺ glasses mixed with three different alkaline earth modifier oxides viz., MgO, CaO and BaO were prepared. Optical absorption and photoluminescence spectra of these glasses have been recorded at room temperature; the Judd-Ofelt theory was successfully applied to characterize these spectra. From this theory, various radiative properties like transition probability A, branching ratio β_r, the radiative lifetime τ_r, for various emission levels in the spectra of these glasses have been determined and reported. An attempt has also been made to throw some light on the relationship between the structural modifications and luminescence efficiencies in the change of scenario for modifying oxides in the glass network.     

超声场对钛基PbO2电极电化学氧化Cr3+过程电流效率的影响

本文研究了超声场对Ti／SnO2 Sb2O3／PbO2电极电化学氧化Cr3 过程的影响,探讨了在不同的操作电流密度、反应温度、硫酸浓度及Cr3 浓度下,超声场对硫酸介质中Cr3 电化学氧化生成Cr2O72-过程中的电流效率的影响,并用扫描电镜对有无超声场时不同反应时间下的电极形貌进行了分析。实验结果表明:在相同的反应条件下,有超声场作用时Cr3 电化学氧化过程的电流效率明显高于无超声场时的电流效率。扫描电镜测定发现,有超声场作用时不同反应时间下电极的形貌有明显的变化。     

Optical character of Er/Yb co-doped P2O5-CaO-Na2O-Al2O3-AgO phosphate glass

The up-conversion (UC) and near infrared (NIR) luminescence of Er³⁺/Yb³⁺ co-doped phosphate glass are investigated. In the UC emission range, the 523 nm, 546 nm green emissions and the 659 nm red emission are observed. With the increasing pump power, the intensity ratios of I₅₂₃/I₆₅₉, I₅₄₆/I₆₅₉ and I₅₂₃/I₅₄₆ increase gradually. The phenomenon is reasonably interpreted by theoretical analysis based on steady state rate equations. The emission cross section of the infrared emission at 1546 nm is larger (about 6.7 × 10^− 21 cm²), which is suitable for making fiber amplifier.     

Luminescence characteristics of Cu- and Eu-doped Li2B4O7

Photoluminescence, radioluminescence as well as optical absorption techniques were used to characterize Cu- and Eu-doped lithium tetraborate. The effect of host modification (single crystal and glassy state) and a limited range of dopant content were investigated. Time-resolved measurements were helpful to identify the emitting species.     

Emission features of Ho ion in Nb2O5, Ta2O5 and La2O3 mixed Li2O-ZrO2-SiO2 glasses

Li₂O-ZrO₂-SiO₂: Ho³⁺ glasses mixed with three interesting d-block elemental oxides, viz., Nb₂O₅, Ta₂O₅ and La₂O₃, were prepared. Optical absorption and photoluminescence spectra of these glasses have been recorded at room temperature. The luminescence spectra of Nb₂O₅ and Ta₂O₅ mixed Li₂O-ZrO₂-SiO₂ glasses (free of Ho³⁺ ions) have also exhibited broad emission band in the blue region. This band is attributed to radiative recombination of self-trapped excitons (STEs) localized on substitutionally positioned octahedral Ta⁵⁺ and Nb⁵⁺ ions in the glass network. The Judd-Ofelt theory was successfully applied to characterize Ho³⁺ spectra of all the three glasses. From this theory various radiative properties, like transition probability A, branching ratio β_r and the radiative lifetime τ_r, for ⁵S₂ emission levels in the spectra of these glasses have been evaluated. The radiative lifetime for ⁵S₂ level of Ho³⁺ ions has also been measured and quantum efficiencies were estimated. Among the three glasses studied the La₂O₃ mixed glass exhibited the highest quantum efficiency. The reasons for such higher value have been discussed based on the relationship between the structural modifications taking place around the Ho³⁺ ions.     

Synthesis and characterization of thermoluminescent Li2B4O7 nanophosphor

Lithium borate (Li₂B₄O₇) is a low Z_eff, tissue equivalent material that is commonly used for medical dosimetry using the thermoluminescence (TL) technique. Nanocrystals of lithium borate were synthesized by the combustion method for the first time in the laboratory. TL characteristics of the synthesized material were studied and compared with those of commercially available microcrystalline Li₂B₄O₇. The optimum pre-irradiation annealing condition was found to be 300 °C for 10 min and that of post-irradiation annealing was 300 °C for 30 min. The synthesized Li₂B₄O₇ nanophosphor has very poor sensitivity for low doses of gamma up to 10¹ Gy whereas from 10¹ to 4.5×10² Gy this phosphor exhibits a linear response and then from 4.5×10² to 10³ Gy it shows supralinearity. Thermoluminescence properties of Li₂B₄O₇ nanophosphor doped with Cu has also been investigated in this paper. It shows low fading and a linear response over a wide range of gamma radiation from 1×10² to 5×10³ Gy. Therefore the synthesized lithium borate nanophosphor doped with Cu may be used for high dose measurements of gamma radiations.