Fluorescence spectra and homogeneous line widths of Cr in glass nucleating ferroelectric Li2Ge7O15

We report optical studies of the new material – Cr³⁺-doped lithium–germanate glass, containing Li₂Ge₇O₁₅ (LGO) nanocrystalline particles. While only broadband ⁴T₂–⁴A₂ fluorescence from the low-field octahedral Cr³⁺ sites was observed from Cr³⁺ ions in the glass, in LGO nanocrystals high-field Cr³⁺ centers emit ²E–⁴A₂ (R–lines) fluorescence. The process of crystallization in the course of isothermal annealing of glasses was monitored spectroscopically and the nucleation of LGO crystallites was observed starting from the smallest clusters. Using the ²E–⁴A₂ fluorescence spectra it is possible to detect the ferroelectric phase transition in LGO:Cr³⁺ nanocrystals, whose critical temperature was found to be similar to that of the bulk crystals. Long-lived spectral holes were burned in the inhomogeneously broadened R-lines of Cr³⁺ in LGO nanocrystals at low temperatures. The linear temperature dependence of hole widths shows that the homogeneous broadening of ⁴A₂–²E transitions of Cr³⁺ in nanocrystals is due to interaction of Cr³⁺ electronic levels with the two-level systems (TLS) of the surrounding glass. The range of the Cr³⁺-TLS elastic dipole–dipole interaction is estimated.     

Electron paramagnetic resonance investigation of K3H(SO4)2 proton conductor doped with Cr ion

The proton arrangement around SO₄ units in K₃H(SO₄)₂ (KHS) was studied in detail by X-band CW EPR spectra of CrO₄³⁻ paramagnetic centre incorporated into KHS during the crystal growth process. The EPR data prove the theoretical model of coherent motion of protons and SO₄ units at the fast-proton conducting phase proposed by Ito and Kamimura.     

UV-laser-light-controlled photoluminescence of metal oxide nanoparticles in different gas atmospheres: BaTiO3, SrTiO3 and HfO2

The photoluminescence (PL) enhancement has been studied at room temperature using various specimen atmospheres (O₂ gas, CO₂ gas, CO₂–H₂ mixture gas, Ar–H₂ mixture gas and vacuum) under 325 nm laser light irradiation on various metal oxides. Of them, the results obtained for BaTiO₃ nanocrystals, SrTiO₃ ones and HfO₂ powder crystal are given in the present paper. Their PL were considerably increased in intensity by irradiation of 325 nm laser light in CO₂ gas and CO₂–H₂ mixture gas. The cause of the PL intensity enhancements is discussed in the light of the exciton theory, the defect chemistry and the photocatalytic theory. The results may be applied for the utilization of greenhouse gas (CO₂) and the optical sensor for CO₂ gas.     

Magnetic ordering and instability investigation of multiferroic Pb(Fe2/3W1/3)O3 ceramics by microwave dielectric spectroscopy

The microwave dielectric and magnetic properties of Pb(Fe_2/3W_1/3)O₃ multiferroic ceramics were investigated. A dielectric dispersion occurring in the frequency range 100 MHz-3 GHz and in a broad temperature range showed itself to be a powerful tool to detect magnetostrictive effects. The experimental results revealed the following remarkable features: the temperature dependence of f_R (characteristic frequency) and the dielectric strength Δε (characteristic of the dispersion) enabled us to identify not only the para-ferroelectric (T_C≈180 K) but also the para-antiferromagnetic (T_N≈340 K) phase transitions, while magnetic measurements revealed the para-antiferromagnetic ordering and a weak superexchange interaction (T_N2∼15 K). Additionally, both characterizations confirmed the existence of structural or magnetic instabilities around 250 K.     

Tm3+ 掺杂ZrF4 -BaF2 -LaF3 -AlF3 -NaF-PbF2玻璃激光制冷中荧光再吸收效应的理论分析

固体材料的激光制冷是近年来发展起来的一个新的研究领域. 掺Tm³⁺的ZrF₄-BaF₂-LaF₃-AlF₃-NaF-PbF₂(ZBLANP)玻璃材料是激光冷却的典型材料之一. 与另一种制冷掺杂离子Yb³⁺相比,Tm³⁺具有更好的制冷潜力. 目前制约材料制冷的一个主要机理就是荧光再吸收. 首先根据Tm³⁺:ZBLANP的光谱参数,利用半经典的随机行走模型得到了不同情况下的平均荧光再吸收次数,随后分析了荧光光子界面出射的全反射效应,并对所得结果进行了修正. 计算结果表明,荧光再吸收会导致量子效率降低0.5%-1%,出射荧光波长红移达到2-10 nm,激光制冷的效率和功率降低. 为了有利于荧光出射和净制冷的实现, 宜采用小体积细长棒的制冷元. 关键词： 激光制冷 稀土离子 荧光再吸收     

Luminescence properties of Cr:Y3Al5O12 nanocrystals

The Cr³⁺-doped Y₃Al₅O₁₂ (YAG) nanocrystalline powders were prepared by the Pechini method. The structural and phase evolution studies were performed using X-ray powder diffraction (XRD) method. The TEM images were taken in order to characterize the grain size and morphology of the Cr:YAG nanocrystals. Their excitation and luminescence spectra were investigated. It was found that luminescence properties of the Cr:YAG nanocrystals were strongly affected by size of the particles. With increasing the grain size the electron-phonon coupling decreases whereas the crystal-field strength increases.     

Self-compensation characteristics of Eu ions in BaTiO3

In order to clarify whether the mixed valence of Eu²⁺/Eu³⁺ exists in a self-compensation mode in Eu-doped BaTiO₃, the site occupation and valence state of Eu ions in barium titanate were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), electron spin resonance (ESR), Raman spectroscopy (RS), and dielectric measurements. The results indicate that Eu ions may enter both Ba- and Ti-sites as Eu³⁺, forming a self-compensation mode with the amphoteric behavior. Self-compensation characteristics of Eu ions in BaTiO₃ are reflected by an expansion in unit cell volume, evolution of the 830 cm^− 1 Raman band, strong diffusion of the dielectric peak, disappearance of the Eu²⁺ ESR broad signal, and a g = 2.004 signal independent of temperature.     

掺杂BaTiO3/SrTiO3多层膜与太赫兹波相互作用

掺杂半导体中的载流子吸收在THz波段非常明显,其相互作用研究是研制THz通信中的关键器件之一的基础。采用氟化氪（KrF）脉冲准分子激光烧蚀沉积（PLD）技术,制备了Ni掺杂BaTiO3/SrTiO3多层膜。基于辐射频率为3.09 THz、脉冲功率为10 mW量级的THz 量子级联激光器(QCL)光源研究了太赫兹波在Ni掺杂BaTiO3/SrTiO3多层膜中的传输,发现损耗主要是Ni颗粒的非共振吸收导致。     

掺杂BaTiO3/SrTiO3多层膜与太赫兹波相互作用

掺杂半导体中的载流子吸收在THz波段非常明显,其相互作用研究是研制THz通信中的关键器件之一的基础。采用氟化氪（KrF）脉冲准分子激光烧蚀沉积（PLD）技术,制备了Ni掺杂BaTiO3/SrTiO3多层膜。基于辐射频率为3.09 THz、脉冲功率为10 mW量级的THz 量子级联激光器(QCL)光源研究了太赫兹波在Ni掺杂BaTiO3/SrTiO3多层膜中的传输,发现损耗主要是Ni颗粒的非共振吸收导致。     

Luminescence studies on ZnO-P2O5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3

Zinc phosphate glasses doped with Gd₂O₃:Eu nanoparticles and Eu₂O₃ were prepared by conventional melt-quench method and characterized for their luminescence properties. Binary ZnO-P₂O₅ glass is characterized by an intrinsic defect centre emission around 324 nm. Strong energy transfer from these defect centres to Eu³⁺ ions has been observed when Eu₂O₃ is incorporated in ZnO-P₂O₅ glasses. Lack of energy transfer from these defect centres to Eu³⁺ in Gd₂O₃:Eu nanoparticles doped ZnO-P₂O₅ glass has been attributed to effective shielding of Eu³⁺ ions from the luminescence centre by Gd-O-P type of linkages, leading to an increased distance between the luminescent centre and Eu³⁺ ions. Both doped and undoped glasses have the same glass transition temperature, suggesting that the phosphate network is not significantly affected by the Gd₂O₃:Eu nanoparticles or Eu₂O₃ incorporation.     

Structure and phase transitions in chloroantimonate(V) crystals: [(C2H5)3NH]SbCl6 and [(C2H5)3NH]SbCl6·1/2[(C2H5)3NH]Cl

New triethylammonium salts: [(C₂H₅)₃NH]SbCl₆ (TCA) and [(C₂H₅)₃NH]SbCl₆·1/2[(C₂H₅)₃NH]Cl (TCAT) have been synthesized. The compounds crystallise in monoclinic symmetry: space groups P2₁/n and P2₁/c, for TCA at 293 K and TCAT at 100 K, respectively. The crystal structure of [(C₂H₅)₃NH]SbCl₆ consists of discrete ionic pairs—triethylammonium cations and hexachloroantimonate anions—linked via the bifurcated N-H?Cl hydrogen bonds. The crystal structure of [(C₂H₅)₃NH]SbCl₆·1/2[(C₂H₅)₃NH]Cl is composed of three symmetrically independent triethylammonium cations, chlorine anion and two symmetrically independent hexachloroantimonate anions. TCA undergoes a structural phase transition at 336 K (on heating) into the orthorhombic C222 space group, whereas TCAT reveals a structural phase transition at 332 K. The phase transitions are of the first order type. TCA shows a ferroelastic domain structure below 336 K. Differential scanning calorimetry, dilatometric, dielectric dispersion and Raman scattering measurements have been used to study the phase transition mechanisms in these triethylammonium salts.     

In situ pressure-induced solid-state amorphization in Sm2(MoO4)3, Eu2(MoO4)3 and Gd2(MoO4)3 crystals: chemical decomposition scenario

The effect of pressure on the phase transformations in Sm₂(MoO₄)₃, Gd₂(MoO₄)₃ and Eu₂(MoO₄)₃ crystals has been studied in situ using synchrotron radiation. All three isostructural compounds undergo a structural phase transition at 2.2-2.8 GPa to a new phase, which is interpreted as a possible precursor of amorphization. Amorphization in these crystals occurs irreversibly over a wide pressure range, and its mechanism, interpreted as a chemical decomposition, is found to be weakly affected by the degree of hydrostaticity.     

Li and Nb codoping in dielectric properties and phase transitions of KTaO3: Reentrant dipolar glass and long-range dipole ordering

Abstract

The highly polarizable perovskite-type oxide, KTaO₃ doped simultaneously with Li⁺ and Nb⁵⁺ (K_1?xLi_xTa_1?yNb_yO₃, KLTN), reveals unexpected properties and ordering effects. Studies of the dielectric permittivity ?'(T, f) (10—300K, 100Hz-1 MHz) for x = 0.0014 and y = 0.024 show collective dipolar ordering effects with a transition from paraelectric into a mixed phase (coexisting dipole-glass-like and long-range ordered ferroelectric phases) taking place near 39 K. At 15 K another phase transition into a reentrant dipolar glass-like state is observed. Such a sequence of transitions and the existence of a reentrant glass state are unknown for electrical dipolar systems.     

Thermoluminescence spectroscopy of Eu and Mn doped BaMgAl10O17

Thermoluminescence (TL) studies of Eu²⁺ and Mn²⁺ doped BaMgAl₁₀O₁₇ (BAM) are reported and discussed. The TL spectra that are measured after irradiation with ultraviolet (120-) show a series of TL peaks between 100 and . The TL spectra are similar for BAM with the two dopants, which suggest that the shallow traps are typical for the BAM host lattice. Using the Hoogstraaten analysis trap depths between 0.1 and are determined. A model is proposed based on thermally activated recombination in local TL centres (not via the conduction band). Further support for this model is obtained from the observation that the TL signal is strongest for excitation around the band edge of BAM . Upon heating the samples in air all low temperature TL peaks decrease in intensity. In addition a new peak appears in the TL spectrum, which is connected with a deeper trap and also a partial oxidation of Eu²⁺ to Eu³⁺ is observed. The luminescence efficiency is lower and the UV induced degradation is faster after annealing in air. These results indicate that the shallow traps are related to oxygen vacancies. The shallow traps do not have a negative influence on performance (efficiency and degradation) of BAM as a lighting phosphor. The luminescence efficiency and stability are strongly influenced by the formation of Eu³⁺ and a deeper trap during annealing in air. Subsequent annealing in a reducing atmosphere restores the original properties.     

High-temperature B2 ordering in Fe50Co50 alloy

High-temperature (above 1200 °C) B2 ordering was experimentally found in the Fe₅₀Co₅₀ alloy with the help of the electron diffraction. Using the X-ray photoelectron spectroscopy method the “ordering-phase separation” transition was shown to lead to changes both in the 3d-electron spectra of the valence band (3d-valence electron localization on Fe atoms increases at phase separation and it decreases at ordering). The investigation of multiplet splitting of the 3s-electron spectra of core-level electrons reveals that magnetic moments on Fe and Co atoms in the Fe₅₀Co₅₀ alloy are larger, when there is the tendency toward ordering, and they are smaller in the case of the tendency toward phase separation. A number of examples are presented, which are indicative of the fact that chemical ordering can influence magnetic ordering by the formation of the respective microstructures and magnetic ordering has no influence on chemical ordering. It can be concluded that the phase transition “ordering-phase separation” takes place both at 730 °C and at the temperature somewhat higher that 1200 °C.     

Effect of glycine substitution on the ferroelectric phase of betaine arsenate [(CH3)3NCH2COO·H3AsO4]

The present work reports an experimental investigation on the influence of glycine (NH₂CH₂COOH) substitution in the polar properties and the critical dynamics of the molecular ferroelectric betaine arsenate, (CH₃)₃NCH₂COO·H₃AsO₄. The dielectric dispersion (20 Hz<ν<3 MHz) and the thermally induced displacement currents are investigated in detail over the extended Curie region of the system (130 K<T<100 K). The results obtained for a single crystal with nominal glycine content of 20% are analyzed, compared with those obtained for pure betaine arsenate and discussed within the scope of a phenomenological Landau model previously used to describe a system with competing ferroelectric and structural instabilities.     

Study of molybdenum coordination state and crystallization behavior in MoO3-La2O3-B2O3 glasses by Raman spectroscopy

The ternary MoO₃-La₂O₃-B₂O₃ glasses containing a large amount of MoO₃ (10-50 mol%) are prepared, and their structure and crystallization behavior are examined from the Raman scattering spectrum measurements and X-ray diffraction analyses. It is found that the glass transition and crystallization temperatures and the thermal stability against crystallization decrease with increasing MoO₃ content. It is suggested that the main coordination state of Mo⁶⁺ ions in the glasses is isolated (MoO₄)²⁻ tetrahedral units giving strong Raman bands at 830-860 and 930 cm⁻¹. It is found that the crystalline phases in the crystallized glasses are mainly LaMoBO₆ and LaB₃O₆, and the main crystallization mechanism in MoO₃-La₂O₃-B₂O₃ glasses is surface crystallization. LaMoBO₆ crystals are found to give strong Raman bands at 810-830 and ∼910 cm⁻¹.     

Hydrothermal preparation and photoluminescent properties of MgAl2O4: Eu spinel nanocrystals

Nanoplates of the MgAl₂O₄ spinel doped with Eu³⁺ ions were prepared by a microwave assisted hydrothermal method. Structural properties of the precursor calcined at 700 and 1000 ^oC powders were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). According to the obtained XRD patterns the formation of single-phase spinels after calcination was confirmed. The average spinel particle size was determined to be 11 nm after calcination at 700 °C and it increased up to 14 nm after calcination at 1000 °C. The photoluminescent properties of prepared powders with different Eu³⁺ ion concentrations (0-5% mol) were investigated using excitation and emission spectroscopy at room and low temperatures (77 K).     

EPR and optical measurements of weakly doped LiNbO3: Er

Optical spectroscopy measurements of the congruent LiNbO₃ (LN) single crystals, weakly doped with Er (0.1–0.3 wt%) and Er (0.3 wt%) and Yb (0.5 wt%), have been carried out. The shape of the optical absorption and additional absorption bands registered after γ-irradiation suggests the presence of Er³⁺ ion pairs. EPR investigations were performed for LN single crystal doped with Er (0.1 wt%). Unusual behaviour of the temperature dependence of the intensity and linewidth of the main EPR line—corresponding to the fine transition of ^evenEr³⁺—ions, is reported. The main EPR line appears to be a superposition of several paramagnetic centres originating from isolated ^evenEr³⁺ ions and ^evenEr³⁺–^evenEr³⁺ pairs of ions. In low temperature region (below 20 K), the main EPR signal is dominated by signals arising from ^evenEr³⁺–^evenEr³⁺ pairs of ions. The inverse intensity of the EPR line at low temperature region fulfils the Curie–Weiss law and enabled to determine the Curie–Weiss constant Θ=1.5±0.5 K. The positive sign of Θ suggests that the ferromagnetic interactions arise in the system of ^evenEr³⁺–^evenEr³⁺ ion pairs in LN. Our results suggest that the distribution of Er ions in congruent LN is not homogeneous and Er impurity ions can form clusters in host lattice even in the case of weak erbium doping.     

Phase compositions and ferroelectric properties of Pb(Zr0.52,Ti0.48)O3 thin films with different highly orientations prepared by a sol-gel route

It is shown that different highly oriented Pb(Zr_0.52,Ti_0.48)O₃ films can be obtained on Pt/Ti/SiO₂/Si substrate using a sol-gel technique. The effects of pyrolysis temperature on the orientation, phase composition and ferroelectric properties of the films are investigated. It is found the ferroelectric hysteresis loops of (1 1 1)-oriented film, (1 1 1) and (1 0 0) mix-oriented film can both be saturated when the external electric field is large enough, whereas the hysteresis loop of (1 0 0)-oriented film is difficult to saturate. The analysis of X-ray diffraction indicated the possibility of different phase composition in different oriented films under large film residual stress. Higher remnant polarization (53 μC/cm²) for (1 0 0)-oriented film can be attributed to its more tetragonal phase composition, which results in that the in-plane domain switching can continuously occur with external electric field increasing.