Structure and dynamics of the pure and mixed fluorites <Emphasis Type="Italic">Me</Emphasis>F<Subscript>2</Subscript> (<Emphasis Type="Italic">Me</Emphasis> = Ca,Sr, Ba,and Pb)

The structure and dynamics of the crystal lattice of MeF₂ fluorites (Me = Ca, Sr, Ba, and Pb) under external hydrostatic compression (0–3.5 GPa) are calculated within the shell model in the pair potential approximation. The first-order structural phase transition from the cubic to the orthorhombic phase in these crystals under pressure is investigated. The effect of chemical pressure on the BaF₂ crystal is analyzed by the simulation of mixed crystals, namely, Ba_1?xCa_xF₂ and Ba_1?xSr_xF₂. It is demonstrated that the supercell method, as applied to the simulation of mixed crystals, results in a lower lattice energy per formula unit as compared to the lattice energy obtained by the virtual-crystal method.     

Pressure and temperature dependence of the emission in BaF2:Eu and SrF2:Eu

The 4f⁶5d¹(e)→4f⁷ emission (normal) of Eu²⁺ in SrF₂ and normal and anomalous emissions of Eu²⁺ in BaF₂ are studied as a function of pressure and temperature. Although in BaF₂ both emissions (anomalous and normal) shift with pressure in the same spectral direction (a red shift), the anomalous emission converts to the normal emission when the pressure increases to 33 kbar. Considering the dependence of BaF₂:Eu²⁺ luminescence on the pressure and temperature we have found that spectral transformation takes place due to a smooth pressure-induced level crossing at approximately the same pressure as the phase transition.     

Site-selective laser spectroscopy of BaF2:Eu3+

The selective laser excitation of the flourescence of Eu³⁺ ions is used to investigate the defect sites in BaF₂:Eu³⁺ for Eu³⁺ concentrations ranging from 0.03 to 2.43 mol%. We identified the fluorescence lines arising from the (Eu³⁺, F^-_i) dipole of C_3v symmetry and established its energy level diagram. The compensating ion is an interstitial F^-_i ion located in the next-nearest-neighbour site, with respect to the Eu³⁺ ion. This (Eu³⁺, F^-_i) dipole dominates in BaF₂. Fluorescence lines assignable to next-nearest-neighbour pairs of (Eu³⁺, F^-_i) dipoles have been found above 0.1 mol% dopant concentration.     

Phase transitions under pressure and exciton-phonon interactions in GaSxSe1−x crystals

This paper presents the results of investigating the optical properties of GaS_xSe_1−x crystals near the fundamental absorption edge at hydrostatic pressure up to 1.2 GPa and T = 300 K.The crystals with x=0; 0.2; 0.25 are shown to be subjected to phase transformations in the investigated pressure range. The values of P_cr (phase transition pressure) and baric coefficients of band gap widths of the investigated crystals in low- and high-pressure phases have been estimated.     

Structural transition of BaF2 nanocrystals under high pressure

The structural transition of BaF₂ nanocrystals is studied by in situ high pressure synchrotron radiation X-ray diffraction measurements up to about 21.2 GPa at ambient temperature. Two phase transformations were observed at 5.8 and 14.4 GPa, and the two high pressure phases are identified as orthorhombic (Pnma) phase and hexagonal (P6₃/mmc) phase by Rietveld refinement. Upon decompression, the α-PbCl₂-type metastable phase is retained when the pressure is released. Two phase transformations of the BaF₂ nanocrystals are higher than that in bulk BaF₂. It is proposed that the size effects are found to influence the BaF₂ nanocrystals high-pressure behaviors and the surface energy plays a significant role in the structural stability.     

Charge states of the activator and size effects in BaF2: Eu nanophosphors

ABSTRACT

According to the spectra of stationary X-ray excited luminescence (XEL) of BaF₂: Eu nanophosphors at 80 and 294 K, it was revealed that the thermal annealing of fine-grained nanoparticles (d?=?35?nm) in the range of 400–1000°C, which is accompanied by an increase of their sizes in the range of 58–120?nm, does not result in effective changes of the charge state of Eu^{3 +} → Eu^{2 +} activator, in contrast to CaF₂: Eu nanoparticles. The maximum light output of X-ray excited luminescence of BaF₂: Eu nanophosphors in the 590?nm emission band of Eu³⁺ ion was observed at an annealing temperature of 600°C with the average size of nanoparticles 67?nm. The subsequent growth of annealing temperatures, especially in the range of 800–1000°C, causes decrease in the light output of X-ray excited luminescence due to the increase of defect concentration in the lattice as a result of sharp increase of nanoparticle sizes and their agglomeration. In BaF₂: Eu nanoparticles of 58?nm size, according to the thermostimulated luminescence (TSL) spectrum, transformation of Eu³⁺ → Eu²⁺ under the influence of long-time X-ray irradiation was revealed for the peak of 151?K. Thus, X-ray excited luminescence spectra of BaF₂: Eu nanophosphors are formed predominantly due to the emission of Eu³⁺ ions, while emission of Eu²⁺ ions is observed in the TSL spectra.     

Effect of Eu contents on the structure and properties of BaLa2ZnO5:Eu phosphors

A series of Eu³⁺ doped BaLa₂ZnO₅ was prepared by a Pechini-type sol-gel method. The effect of Eu³⁺ concentration on the structure and the luminescent properties of the samples BaLa_2−xEu_xZnO₅ were investigated by powder X-ray diffraction (XRD) and photoluminescence (PL) spectra. The sample adopts the tetragonal or orthorhombic structure as the Eu³⁺ concentration reaches below 1.2 or over 1.6, respectively. The sample is a mixture of tetragonal and orthorhombic phases as the contents of dopants lie between 1.2 and 1.6. The phosphor offers the strongest red light as content of Eu³⁺ is 0.4. In addition, the crystallization processes of the complex precursors were examined by simultaneous thermogravimetry and differential thermal analysis (TG-DTA), and also the optimal heating temperature was investigated.     

压致硼酸盐晶态物质的非晶化现象研究

 本文用X射线衍射、Eu²⁺的发射谱与激发谱、拉曼谱以及扫描电镜（SEM）颗粒形貌，研究了常压下合成的具有正交结构的SrB₂O₄:Eu²⁺晶体在3.0~7.0 GPa压力下的晶态非晶化现象。分析结果表明，压力导致晶粒细化和晶态非晶化。晶粒尺寸由常压下的微米量级细化为几十个纳米量级，随压力的变化为：2 μm（0.1 MPa），49.4 nm（3.0 GPa），29.7 nm（5.0 GPa），25.1 nm（7.0 GPa）。晶态与非晶态体积比随压力增加而减小，分别为：70/30（3.0 GPa），63/37（5.0 GPa），57/43（7.0 GPa）。在压力下Eu²⁺是处在晶态与非晶态两种不同的低对称的环境中。纳米级晶粒是以亚晶粒形式存在于微米级大晶粒中的，压致非晶态可能组成了纳米亚晶粒的界面区。     

Pressure dependence of the Raman spectra of indium sulphide

Raman spectra of InS single crystals have been studied at different hydrostatic pressures up to 1.2 GPa. Mode-Grüneisen parameters have been obtained for Raman-active normal modes. It is shown that the variations observed in Raman spectra with growing pressure can be interpreted from the standpoint of the structural phase transition D¹²_2h → D¹⁷_4h in InS as the hydrostatic pressure continues to increase. The transition pressure has been evaluated at (7 ± 1) GPa.     

Photoluminescence and Raman spectroscopy studies of Eu3+-doped rutile TiO2 nanocrystals at high pressures

Nanocrystal samples (particle size about 90 nm) of Eu³⁺-doped rutile titanium dioxide (TiO₂) nanocrystals (rutile Eu³⁺/TiO₂ nanocrystals) were synthesized by the sol–gel method with hydrothermal treatment. The pressure effect on photoluminescence (PL) and Raman spectra of the rutile Eu³⁺/TiO₂ nanocrystals was investigated with a diamond anvil cell under hydrostatic pressure condition. Raman spectra of the samples at high pressures indicated that the critical pressure for the transition from the rutile phase to a new baddeleyite-type phase was between 10 and 14.2 GPa. The position of Raman bands shifted to high wavenumbers and the PL intensity of ⁵D ₀→⁷F ₂ transition of Eu³⁺ decreased down to zero with the increase of pressure before the phase transition occurred. After releasing the pressure, the rutile phase was not recovered and a α-PbO₂-type phase was observed at ambient pressure.     

Structure of the intermediate high-pressure phases of ternary lead tellurides

In chambers with diamond anvils, the structure of high-pressure phases of ternary lead tellurides Pb_1?x Sn_xTe (x = 0.29) and Pb_1?x Mn_xTe (x = 0.05) and nonstoichiometric crystals Pb_0.55Te_0.45, Pb_0.45Te_0.55 is analyzed by the synchrotron radiation diffraction method at pressures of P up to 14 GPa. The orthorhombic structure of the intermediate high-pressure phase (space group Pnma) is determined for all the samples above 6 GPa. Models of the phase transition in PbTe from the initial rock salt structure to the orthorhombic phase, which constitutes a distorted variant of NaCl, as well as the properties of this phase, are discussed.     

EPR of trivalent iron centers in a BaF2: Fe crystal

An orthorhombic paramagnetic Fe³⁺ ion center (concentration of iron ions is 0.1 at. %) was found using EPR in BaF₂: Fe crystals irradiated by x rays. The EPR spectra recorded in the Q range at a temperature T = 77 K exhibit both the fine structure typical of a center with effective spin S = 5/2 and a superhyperfine structure (SHFS) indicating the SHFS interaction of the electronic moment of the center with the nuclear magnetic moments of its six ligands (F^? ions). An analysis of the SHFS reveals that this center forms through the replacement of a Ba²⁺ cation by a Fe²⁺ cation, which transforms into a Fe³⁺ (⁶ A _1g ) cation under x-ray irradiation and shifts into a neutral position along the C₂ axis of the cubic coordination shell of the replaced host cation.     

Luminescent properties of stabled hexagonal phase Sr1−xBaxAl2O4:Eu (x=0.37-0.70)

Stabled hexagonal phase Sr_1−xBa_xAl₂O₄:Eu²⁺ (x=0.37-0.70) was prepared by solid-state method. Result revealed that the structure behavior of the SrAl₂O₄:Eu²⁺ calcined at 1350 °C in a reducing atmosphere for 5 h strongly depended on the Ba²⁺ concentration. With increasing Ba²⁺ concentration, a characteristic hexagonal phase can be observed. When 37-70% of the strontium is replaced by barium, the structure of the prepared sample is pure hexagonal. Photoluminescence and excitation spectra of the samples with different x and doped with 2% Eu²⁺ were investigated. Changes in the emission spectra were observed in the two different phases. The green emission at 505 nm from Eu²⁺ was found to be quite strong in the hexagonal phase. The intensity and peak position of the green luminescence from Eu²⁺ changed with increasing content of Ba²⁺. The strongest green emission was obtained from Sr_0.61Ba_0.37Al₂O₄:Eu²⁺. The decay characteristics of Sr_1−xBa_xAl₂O₄:Eu²⁺ (x=0.37-0.70) showed that the life times also varied with the value of x. Furthermore, the emission colors and decay times varying with x could be ascribed to the variation of crystal lattice.     

Tetragonally distorted Cr2+ ions in BaF2 and SrCl2 studied by ESEEM spectroscopy

Electron spin echoes on tetragonally distorted non-Kramers Cr²⁺ ions in chromium doped BaF₂ and SrCl₂ single crystals have been observed. The superhyperfine interaction (shf) with the nearest halide ions has been studied using the electron spin echo envelope modulation (ESEEM) signal. For Cr²⁺ in BaF₂ the nearest coordination shell consists of four equivalent fluorine ions in a plane perpendicular to the defect tetragonal axis, the Cr²⁺ ion being off-centre displaced towards the fluorine plane. The Cr²⁺ shf tensor for BaF₂ has been determined. For SrCl₂:Cr ESEEM signals associated to the shf interaction with nearest chloride ions were observed but the measurements did not allow us to give a definitive defect model in this case.     

Thermoluminescence and opitical absorption of BaF2 crystals

Abstract

Nominally pure and Dy-doped BaF₂ crystals were investigated concerning their optical absorption (OA) and thermoluminescence (TL) properties. Peaks at 120—150 and 200°C were observed for a heating rate of 1.7°C/s. The TL response for γ-rays and the TL emission spectra were obtained for these peaks. Except for the purest crystal, all BaF₂ crystals produced OA bands before irradiation typical of Ce³⁺ ions. After irradiation, Dy doped crystals showed bands due to Dy²⁺ ions. A nominally pure sample gave bands related to Ce²⁺ ions and photochromic centers of Ce³⁺ ions. and photochromic centres of Ce³⁺ ions. The correlation between some OA bands and TL peaks is discussed.     

Colloidal synthesis of BaF2 nanoparticles and their application as fillers in polymer nanocomposites

Nanoparticles of pure and Eu-doped BaF₂ have been prepared through sol-gel colloidal synthesis. In addition, BaF₂-filled PMMA polymer nanocomposites were fabricated and dielectric properties were measured. The as-synthesized pure and Eu-doped BaF₂ nanoparticles were analyzed by both X-ray diffraction and transmission electron microscopy and consisted of crystalline BaF₂ particles with an average diameter of 13.6 nm with a standard deviation of about ±2.4 nm. The photoluminescence properties of the pure and Eu-doped (2%, 4% and 8%) nanoparticles showed characteristic emission of Eu³⁺ (⁵D₀→⁷F _J (J=1–4) transitions). We also measured significantly enhanced dielectric breakdown strength of up to 30% for BaF₂ nanocomposites over the unfilled PMMA polymer. This study thus offers some promise of sol-gel synthesis of nanocomposite dielectrics with great potential for use as electrical insulation materials in cryogenic high-voltage applications.     

Thermoelectric properties of high-pressure silicon phases

The thermo emf in Czochralski-grown silicon single crystals (Cz-Si) was experimentally studied in a range of pressures up to 20 GPa. The pressure dependences revealed phase transitions in the metallic phase of silicon, which passed from tetragonal to orthorhombic and then to hexagonal lattice. The high-pressure silicon phases, as well as the metallic high-pressure phases in A_NB_8?N semiconductors, possess conductivity of the hole type. As the pressure decreases, the emf behavior reveals transitions to the metastable phases Si-XII and Si-III. Preliminary thermobaric treatment of the samples at a pressure of up to 1.5 GPa and a temperature of T=50–650°C influences the thermoelectric properties of Cz-Si at high pressures.     

Multiphonon radiationless phenomena in Mn2+-Doped Ca1-x Sr x F2 and BaF2 Fluorites

This work investigates the surprising disappearance of the Mn²⁺ photoluminescence on passing from CaF₂:Mn²⁺ to SrF₂:Mn²⁺ or BaF₂:Mn²⁺ using pressure spectroscopy. We show that the loss of luminescence in these systems is associated with non-radiative thermally activated processes whose activation energy and pre-exponential rates strongly depend on the crystal volume irrespective of the chemical composition. A salient feature is the PL enhancement observed in the cotunnite high-pressure phase along the series. This enhancement is explained in terms of the large volume reduction at the phase transition, as well as by the presence of low-symmetry crystal fields attained at the cation sites leading to an increase of the radiative transition rate by the electric-dipole mechanism.     

Photo-and pressure-induced transformations in the linear orthorhombic polymer of C<Subscript>60</Subscript>

Stability of the linear orthorhombic polymer of C₆₀ under pressure and laser irradiation is studied by Raman scattering and X-ray diffraction measurements. The Raman spectrum at ambient pressure remains unchanged, in the time scale of the experiment, up to an intensity of 3200 W/cm² of the 514.5 nm line of an Ar⁺ laser, but irreversible changes are observed at higher intensities. The Raman spectra recorded at increased pressure show similar irreversible changes even at the laser intensity as low as 470 W/cm². The X-ray diffraction and Raman measurements of the pressure-treated samples, performed after pressure release, show that the nonirradiated material does not exhibit any changes in the crystal structure and phonon spectra. This behavior indicates a pressure-enhanced photo-induced transformation to a new polymeric phase characterized by a Raman spectrum that differs from those of the other known polymeric phases of C₆₀. The Raman spectra of the phototransformed linear orthorhombic polymer of C₆₀ were measured at a pressure of up to 29 GPa. The pressure dependence of the Raman mode frequencies show singularities near 4 GPa and 15 GPa, respectively, related to a reversible phase transition and an irreversible transformation to a metastable disordered phase. The diffuse Raman spectrum of the disordered phase does not exhibit substantial changes with an increase in pressure up to 29 GPa. The high-pressure phase transforms to a mixture of pristine and dimerized C₆₀, after pressure release and exposure to ambient conditions for 30 h. The text was submitted by the authors in English.     

Spectral-kinetic characteristics of crystals and nanoceramics based on BaF<Subscript>2</Subscript> and BaF<Subscript>2</Subscript>: Ce

Optical characteristics of BaF₂ and BaF₂: Ce single crystals and nanoceramic materials prepared from these single crystals by uniaxial hot pressing have been studied. It has been shown that the introduction of Ce₃₊ ions into BaF₂ hardly affects the ultrafast (∼0.9 ns) luminescence component. The integrated luminescence intensity of the BaF₂: Ce nanoceramics is higher than that of the corresponding single crystal and considerably higher that the intensity of the undoped BaF₂ crystal. It has been demonstrated that the slow (several hundred nanoseconds) component of the luminescence decay of BaF₂: Ce is due to the energy transfer from excitons to Ce³⁺ ions.