Novel highly luminescent europium dinitrosalicylates

A series of lanthanide dinitrosalicylates M₃Ln(3,5-NO₂-Sal)₃ · nH₂O (Ln = Eu, Gd; M = Li, Na, K, Cs) was synthesized. It was found that the luminescence efficiency of some M₃Eu(3,5-NO₂-Sal)₃ · nH₂O compounds was near to the high efficiency of europium dibenzoylmethanate with 1,10-phenanthroline, Eu(DBM)₃ · Phen. The luminescence excitation spectra, electron-vibrational luminescence spectra, and vibrational IR spectra were investigated. The energy of the lowest excited triplet state of the ligand was obtained from phosphorescence spectra of M₃Gd(3,5-NO₂-Sal)₃ · nH₂O, M(3,5-NO₂-HSal) · nH₂O, and M₂(3,5-NO₂-Sal) · nH₂O. The details of the structure of compounds were discussed. The influence of different M-cations on the Eu³⁺ luminescence efficiency and on the processes of excitation energy transfer to a Eu³⁺ ion was analyzed. The presence of large alkali metal cations in lanthanide dinitrosalicylates and an increase in the temperature weaken the network of hydrogen bonds and, to some extent, the “ligand–metal” bonds. This is a cause of a long-wavelength shift of the intraligand charge transfer (ILCT) band in Eu³⁺ excitation spectra arising at inclusion of Cs⁺ instead of Li⁺ cations in the crystal lattice and at the heating of compounds. A change of the energies of ligand electronic states at substitution of Li⁺ and Na⁺ for Cs⁺ can give a tenfold enhancement of the Eu³⁺ luminescence efficiency at 300 K.     

Lattice disorder in alumina single crystals produced by ion bombardment

The effect of 40 keV Kr ion bombardment of Al₂O₃ single crystals has been investigated using a Rutherford backscattering-channelling (RBC) technique. Curves of lattice disorder were found to be sigmoidal; the disorder increasing slowly up to a fluence of 1 × 10¹⁵ ions cm^–2 and then accelerating to a saturation level at 1 × 10¹⁶ ions cm^–2. These doses are about 100 times higher than comparable values found for elemental and III–V semiconductor compounds. The number of displaced atoms per incident ion estimated for an ion dose of 3 ×10¹⁴ ions cm^–2 was found to be less than that calculated using Sigmund's equation. This difference is discussed in terms of defect recombination and re-ordering during bombardment. Measurements taken from the RBC random spectra before and after each bombardment have indicated that the stoichiometry of the alumina crystals did not alter even at the highest bombardment fluences used.     

Influence of calcination temperature on luminescent properties of Sr3Al2O6:Eu, Dy phosphors prepared by sol–gel-combustion processing

The detailed preparation process of Eu²⁺ and Dy³⁺ ion co-doped Sr₃Al₂O₆ phosphor powders with red long afterglow by sol–gel-combustion method in the reducing atmosphere is reported. X-ray diffraction, scanning electron microscopy and photoluminescence spectroscopy are used to investigate the effects of synthesis temperature on the crystal characteristics, morphology and luminescent properties of the as-synthesized Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors. The results reveal that Sr₃Al₂O₆ crystallizes completely when the combustion ash is sintered at 1200 °C. The excitation and the emission spectra indicate that the excitation broad-band lies chiefly in visible range and the phosphor powders emit strong light at 618 nm under the excitation of 472 nm. The light intensity and the light-lasting time of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors are increased when increasing the calcination temperatures from 1050 to 1200 °C. The afterglow of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors sintered at 1200 °C lasts for over 600 s when the excited source is cut off. The red emission mechanism is discussed according to the effect of nephelauxetic and crystal field on the 4f⁶5d¹ → 4f⁷ transition of the Eu²⁺ ions.     

VUV spectroscopy of wide bandgap materials

In this paper we will present VUV spectroscopy experiments performed at the Superlumi station of Hasylab, DESY, Hamburg, on samples of BaF₂ crystals activated with Ce and BaF₂, (Ba,La)F₂ crystals activated with Er. The results of these experiments include time resolved luminescence and luminescence excitation spectra obtained under wavelength selective VUV and UV excitation by pulsed synchrotron radiation.We will reveal the information provided by the VUV/UV excitation spectra of the Ce³⁺ 5d → 4f as well as Er³⁺ 4fⁿ⁻¹5d → 4fⁿ and 4fⁿ → 4fⁿ emissions on energy transfer mechanisms from the fluoride host to the rare earth ion. We will demonstrate that the fast energy transfer channels involve bound excitons while the generation of free electrons and holes leads to slower processes dependant on hole and/or electron trapping.We will demonstrate that differences between the excitation spectra of the 5d → 4f emission in Ce and 4f¹⁰5d → 4f¹¹ emission in Er activated BaF₂ are generated by the coupling of the 4f → 5d transition to the 4f¹⁰ core of the Er³⁺ ion. We will also identify the additional band, absent for Ce, which is due to the exchange split high spin (HS) state of the 4f¹⁰5d configuration responsible for the slow decay of the excited Er³⁺ ions in BaF₂ and (Ba,La)F₂.Finally we will provide evidence and explain why the dominant VUV 4f¹⁰5d → 4f¹¹ Er³⁺ emission in BaF₂ is spin-forbidden and slow while in the mixed (Ba,La)F₂ crystals it is spin-allowed and fast.     

Characteristic features of ion-photon emission from yttrium-iron garnets

The main parameters of the ion-photon emission (IPE) arising from the bombardment of yttrium-iron garnets (YIG) samples with Ar⁺-ions were studied. The essential difference in the spectral composition, quantum yield of emissions, and spatial distribution of radiation registered from YIG to that from samples of pure metal iron was determined. Such difference in parameters of IPE is due to contribution of new mechanisms of formation of excited particles leaving the sample during ion bombardment YIG compared with emission from a metal. It was suggested the excited atoms are formed during the decay of Me_mO_n complexes ejected from solids.     

Preparation of SrAl2O4: Eu, Dy nanometer phosphors by detonation method

SrAl₂O₄: Eu²⁺, Dy³⁺ nanometer phosphors were synthesized by detonation method. The particle morphology and optical properties of detonation soot that was heated at different temperatures (600–1100 °C) had been studied systematically by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Results indicated SrAl₂O₄: Eu²⁺, Dy³⁺ nanometer powders in monoclinic system (a = 8.442, b = 8.822, c = 5.160, β = 93.415) can be synthesized by detonation method, when detonation soot was heated at 600–800 °C. The particle size of SrAl₂O₄: Eu²⁺, Dy³⁺ is 35 ± 15 nm. Compared with the solid-state reaction and sol-gel method, synthesis temperature of the detonation method is lower about 500 and 200 °C respectively. After being excited under UN lights, detonation soot and that heated at 600–1100 °C can emit a green light.     

Spectroscopic characteristics of Olgite (Ba,Sr)(Na,Sr)2Na[PO4]2 crystals doped with Sm ions

Sm³⁺ ions doped Olgite crystals (Ba,Sr)(Na,Sr)₂Na[PO₄]₂ were prepared by high temperature solid-state reaction. Sm²⁺ ions were obtained by X-ray irradiation reduction. The samples were investigated by X-ray diffraction, SEM, photoluminescence and decay curves measurements. In (Ba,Sr)(Na,Sr)₂Na[PO₄]₂, the influence of different mole ratios of Sr to Ba atoms on the crystal structure, reducing efficiencies of Sm³⁺ to Sm²⁺ and luminescence properties of Sm²⁺ ions were discussed. It is found that the conversion of Sm³⁺ → Sm²⁺ after X-ray irradiation is efficient in this phosphate. The emission of Sm²⁺ in this host after excitation into the 4f⁵ 5d¹ levels shows ⁵D₀ → ⁷F_J (J = 0, 1, 2) emission together with a broad emission band. The characteristic of Sm²⁺ ions luminescence was discussed.     

Characterisation of thermo-mechanical properties of MgO–Al2O3–SiO2 glass ceramic with different heat treatment temperatures

The effects of heat treatment temperature on crystallisation behaviour, precipitated phases and thermo-mechanical properties of some MgO–Al₂O₃–SiO₂ (MAS) glass-ceramics were investigated. Crystallisation behaviour of MgO–Al₂O₃–SiO₂ glasses in the presence of TiO₂ as a nucleation agent was studied. The crystalline phases present in the heat treated samples were identified by X-ray diffraction (XRD). It was observed from XRD studies that magnesium aluminium titanate initially precipitated and when the heat treatment temperature was increased to 1140 °C, depending on the thermal history, either magnesium silicate, aluminium titanate and quartz or magnesium aluminium titanate, magnesium aluminate and quartz were precipitated. SEM observation revealed that the heat treatment led to phase separation of droplet-shaped crystals before the needle-shaped crystals formed at 1140 °C. The effect of annealing temperature on the density and mechanical properties of these glass-ceramic were characterised by nanoindentation and the results revealed a significant increase in hardness of the fully crystallised system.     

Radiation resistance diagnostics of wide-gap optical materials

Novel approach in the detection of radiation damage created by ion beams in optical materials was demonstrated. Protons of the energy of 100 keV and fluence of 10¹⁷ cm⁻² create sufficient amount of crystal lattice defects in the thin surface layer for testing of optical materials needed for future fusion reactors. These structural defects can be detected and analysed using the spectra of cathodoluminescence excited in the irradiated layer by an electron beam with adjustable energy. The method was verified by the enhanced intensity of F-type luminescence that reflects the creation of radiation-induced oxygen vacancies in MgO and Al₂O₃ crystals. Low radiation resistance of nominally pure (Lu_1-xGd_x)₂SiO₅ crystals was demonstrated by almost total suppression of intrinsic luminescence after the same irradiation.     

MgAl2O4-γ-Al2O3 solid solution interaction: mathematical framework and phase separation of α-Al2O3 at high temperature

Although existence of MgAl₂O₄-γ-Al₂O₃ solid solution has been reported in the past, the detailed interactions have not been explored completely. For the first time, we report here a mathematical framework for the detailed solid solution interactions of γ-Al₂O₃ in MgAl₂O₄ (spinel). To investigate the solid solubility of γ-Al₂O₃ in MgAl₂O₄, Mg-Al spinel (MgO-nAl₂O₃; n = 1, 1.5, 3, 4.5 and an arbitrary high value 30) precursors have been heat treated at 1000°C. Presence of only non-stoichiometric MgAl₂O₄ phase up to n = 4.5 at 1000°C indicates that alumina (as γ-Al₂O₃) present beyond stoichiometry gets completely accommodated in MgAl₂O₄ in the form of solid solution. γ → α alumina phase transformation and its subsequent separation from MgAl₂O₄ has been observed in the Mg-Al spinel powders (n > 1) when the 1000°C heat treated materials are calcined at 1200°C. In the mathematical framework, unit cell of MgAl₂O₄ (Mg₈Al₁₆O₃₂) has been considered for the solid solution interactions (substitution of Mg²⁺ ions by Al³⁺ ions) with γ-Al₂O₃. It is suggested that combination of unit cells of MgAl₂O₄ takes part in the interactions when n > 5 (MgO-nAl₂O₃).     

Kinetic analysis of spinel formation at the interface of CaO-Al2O3 fibre-reinforced Al 7075 composites

A kinetic analysis was performed on the spinel (MgAl₂O₄) formation reaction at the interfacial region of the calcium aluminate (CaO-Al₂O₃) fibre-reinforced aluminum 7075 alloy matrix composites (CA/Al 7075) processed at 1200°K. The counterdiffusion model of Mg²⁺ and Al³⁺ fluxes was employed for the spinel formation reaction. From the parabolic relation between the thickness of the spinel layer and time the reaction rate constant was determined to be a function of the average diffusion coefficient of Al³⁺ ions. By applying the parabolic equation to the experimental data of the CA/Al 7075 the average diffusion coefficient of Al³⁺ ions in the MgAl₂O₄ spinel at 1200°K was calculated and this value appeared to be in small deviation from the extrapolation of previous diffusion coefficient data.     

Low temperature mullite crystallization in Al- and Si-alkoxide derived homogeneous gels

A homogeneous mullite gel (3:2 Al₂O₃:SiO₂ molar ratio) was made from tetraethoxysilane (TEOS) and aluminum isopropoxide (Al(OCH(CH₃)₂)₃) at room temperature within a relatively quick three days. Mullite was the only crystalline phase to form during calcination; metastable phases like aluminosilicate spinel did not appear. When heated at 20 °C/min, crystalline mullite (65 mol% Al₂O₃) started forming at 575 °C and reached 27 wt.% by 900 °C. Major crystallization occurred at ~ 1000 °C with a concurrent increase of Al₂O₃ concentration (68–69 mol%) in the mullite phase. The alumina content decreased towards stoichiometric (3Al₂O₃·2SiO₂) mullite at even higher temperatures. When the gel remained in the amorphous state, low temperature preheating significantly improved crystallization at higher temperatures. After preheating at 425 °C for 24 h, 78 wt.% of the final product was crystallized when it was subsequently annealed at 750 °C for 5 min. Only 20 wt.% crystallized without preheating.     

Comparative study of the luminescence of Al2O3:Ti and Al2O3 crystals under VUV synchrotron radiation excitation

The comparative study of the luminescent properties of Al₂O₃:Ti crystal in comparison with those for undoped Al₂O₃ crystal counterpart is performed under synchrotron radiation excitation with an energy of 3.7–25 eV. Apart from the main emission band peaked at 725 nm related to the ²E → ²T₂ radiative transitions of Ti³⁺ ions, the luminescence of excitons localized around Ti ions in the band peaked at 290 nm and the luminescence of F⁺–Ti and F–Ti centers in the bands peaked at 325 and 434 nm are also found in the emission spectra of Al₂O₃:Ti crystal. We show also that the luminescence of Ti³⁺ ions in Al₂O₃:Ti crystal can be effectively excited by the luminescence of excitons localized around Ti dopant as well as by the luminescence of F–Ti centers.     

Microstructure, mechanical, and in vitro properties of mica glass-ceramics with varying fluorine content

The design and development of glass ceramic materials provide us the unique opportunity to study the microstructure development with changes in either base glass composition or heat treatment conditions as well as to understand processing-microstructure-property (mechanical/biological) relationship. In the present work, it is demonstrated how various crystal morphology can develop when F⁻ content in base glass (K₂O–B₂O₃–Al₂O₃–SiO₂–MgO–F) is varied in the range of 1.08–3.85% and when all are heat treated at varying temperatures of 1000–1120°C. For some selected heat treatment temperature, the heat treatment time is also varied over 4–24 h. It was established that with increase in fluoride content in the glass composition, the crystal volume fraction of the glass-ceramic decreases. Using 1.08% fluoride, more than 80% crystal volume fraction could be achieved in the K₂O–B₂O₃–Al₂O₃–SiO₂–MgO–F system. It was observed that with lower fluoride content glass-ceramic, if heated at 1040°C for 12 h, an oriented microstructure with ‘envelop like’ crystals can develop. For glass ceramics with higher fluorine content (2.83% or 3.85%), hexagonal-shaped crystals are formed. Importantly, high hardness of around 8 GPa has been measured in glass ceramics with maximum amount of crystals. The three-point flexural strength and elastic modulus of the glass-ceramic (heat treated at 1040°C for 24 h) was 80 MPa and 69 GPa of the sample containing 3.85% fluorine, whereas, similar properties obtained for the sample containing 1.08% F⁻ was 94 MPa and 57 GPa, respectively. Further, in vitro dissolution study of the all three glass-ceramic composition in artificial saliva (AS) revealed that leached fluoride ion concentration was 0.44 ppm, when the samples were immersed in AS for 8 weeks. This was much lower than the WHO recommended safety limits of 1.5 ppm. Among all the investigated glass-ceramic samples, the glass ceramic with 3.85% F⁻ content in base glass (heat treated at 1040°C for 12 h), exhibits the adherence of Ca–P layer, which consists of spherical particles of 2–3 μm. Other ions, such as Mg⁺² and K⁺¹ ion concentrations in the solution were found to be 8 and 315 ppm after 8 weeks of leaching, respectively. The leaching of all metal ions is recorded to decrease with time, probably due to time-dependent kinetic modification of sample surface. Summarizing, the present study illustrates that it is possible to obtain a good combination of crystallization, mechanical and in vitro dissolution properties with the careful selection of base glass composition and heat treatment conditions.     

Chemical effects in Zr- and Co-implanted sapphire

Sapphire samples were implanted with Zr and Co ions to a fluence of 5 × 10¹⁷ ions/cm² in MEVVA implanter operated at 65 kV. XPS and RBS spectra were studied in the course of annealing within the temperature range of 200–1400 °C. It is concluded that Zr diffuses to the surface and forms ZrO₂ precipitates, whereas Co migrates to the bulk and forms CoAl₂O₄ spinel which gives the crystal a bluish color. These changes are consistent with thermodynamical properties of the implanted ions.     

Effect of SHI irradiation on the morphology of SnO2 thin film prepared by reactive thermal evaporation

SnO₂ thin films prepared by reactive thermal evaporation on glass substrates were subjected to 120 MeV Ag⁹⁺ ion irradiation. The surface topography progression using the swift heavy ion irradiation was studied. It shows creation of unique surface morphologies and regular structures on the surface of the SnO₂ thin film at particular fluences. Field Emission Scanning electron microscopy (FE-SEM) and Atomic force microscopy (AFM) are used for investigating the effect of Ag ions at different fluences on the surface of SnO₂. The morphological changes suggest that ion assisted/induced diffusion process play a significant role in the evolution of nanostructures on SnO₂ surface. The roughness increases from 9.4 to 14.9 with fluence upto 1 × 10¹² ions/cm² and beyond this fluence, the roughness decreases. Ion-beam induced recrystallization at lower fluences and amorphization or disordering of crystals at higher fluences are understood based on the thermal spike model.     

Changes of the local magnetic properties of the optically excited Nd3+ ions and their manifestation in the near IR spectra of the Nd0.5Gd0.5Fe3(BO3)4 crystal

Polarized absorption spectra of f–f transitions ⁴I_9/2 → ⁴F_3/2 and (²H_9/2 + ⁴F_5/2) in the Nd³⁺ ion in the Nd_0.5Gd_0.5Fe₃(BO₃)₄ single crystal were studied as a function of temperature in the range of 2–40 K and as a function of magnetic field in the range of 0–65 kOe at 2 K. It was found out that the selection rules for f–f electron transitions substantially changed in the magnetically ordered state of the crystal, and they strongly depended on the orientation of the Fe and Nd ions magnetic moments relative to the light polarization. The splitting of the ground and excited states of the Nd³⁺ ion in the exchange field of the Fe sublattice were determined. It was revealed that the value of the exchange splitting (the exchange interaction) in the excited states did not correlate with the theoretical Landé factors. The Landé factors of the excited states were experimentally found. In general, the local magnetic properties in the vicinity of the excited ion depend substantially on its electron state. In particular: (1) in one of the excited states a weak ferromagnetic moment appears, (2) the changes of type of the local magnetic anisotropy take place in some excited states, and (3) in some excited states the energetically favorable orientation of the Nd³⁺ ion magnetic moment is opposite to that in the ground state. In some excited states the nonequivalent Nd³⁺ centers were found out.     

Nature of intrinsic and impure luminescence of MAlP2O7 crystals

The results of the photoluminescence (PL) investigation of pure and chromium-doped MAlP₂O₇ (M = Na, K, Cs) compounds are presented. The spectra of the intrinsic luminescence of MAlP₂O₇ crystals consist of a separated UV band at a peak position near 330 nm and a complex wide band which covers the region of visible light up to 750 nm at excitation by VUV synchrotron radiation. The “red” band in 600–1000 nm diapason appears in the PL spectra of crystals doped with chromium ions. The effect of the temperature on the structure, the peak positions and intensities of the luminescence bands was studied. An assumption about the nature of the intrinsic PL was made. The “red” luminescence was considered as a result of the ⁴Т₂ → ⁴А₂ radiation transitions in the impurity Cr³⁺ ions located in the intermediate crystal field.     

Rapid densification and reaction sequences in self-reinforced Y-α-SiAlON ceramics with barium aluminosilicate as an additive

Y-α-SiAlON (Y_1/3Si₁₀Al₂ON₁₅) ceramics with 5 wt.%BaAl₂Si₂O₈ (BAS) as an additive were synthesized by spark plasma sintering (SPS). The kinetic of densification, phase transformation sequences and grain growth during sintering process were investigated. Full densification could be achieved by 1600 °C without holding and using a heating rate of 100 °C min⁻¹, but the transformation from α-Si₃N₄ to α-SiAlON is not completed simultaneously with the densification process. The equilibrium phase assemblage could be reached after SPS at 1800 °C for 5 min and the resultant material possesses self-reinforced microstructure with high hardness of 19.2 GPa and fracture toughness of 6.8 MPa m^1/2. The complete crystallization of BAS is beneficial to the high temperature mechanical properties. The obtained could maintain the room strength up to 1300 °C.     

Study on structural phase transitions and relaxation processes of Cs2Co(SO4)2·6H2O and Cs2Zn(SO4)2·6H2O crystals

The structural phase transitions and relaxation processes of Cs₂Co(SO₄)₂·6H₂O and Cs₂Zn(SO₄)₂·6H₂O single crystals were investigated, with the phase transitions of both crystals being determined from NMR data. The spin–lattice relaxation time, T₁, of the ¹³³Cs nucleus in two crystals undergoes a significant change near the phase transition temperature, T_C, and these changes coincide with the changes in the splitting of the ¹³³Cs resonance lines. The variations in the temperature dependence for the splitting of the ¹³³Cs resonance lines and T₁ near T_C are related to changes in the symmetry of surrounding Cs⁺. In addition, the ¹³³Cs T₁ of Cs₂Co(SO₄)₂·6H₂O, which contains paramagnetic ions, was found to be shorter than that of Cs₂Zn(SO₄)₂·6H₂O. This relaxation time is inversely proportional to the square of the magnetic moment of the paramagnetic ions. The differences between the ¹³³Cs T₁ of these compounds are probably due to the differences between the electronic structures of their metal ions.