A new series of Sr-based phosphates, Sr9−xMnxEu(PO4)7, were synthesized using the high-temperature solid-state method in air. It was found that these compounds have the same structure as ...strontiowhitlockite, which is a β-Ca3(PO4)2 (or β-TCP) structure. The concentration of Mn2+ ions required to form a pure strontiowhitlockite phase was determined. An unusual partial reduction of Eu3+ to Eu2+ in air was observed and confirmed by photoluminescence (PL) and electron spin resonance (ESR) spectra measurements. The PL spectra recorded under 370 nm excitation showed transitions of both 4f5d–4f Eu2+ and 4f–4f Eu3+. The total integral intensity of the PL spectra, monitored at 395 nm, decreased with increasing Mn2+ concentration due to quenching effect of Eu3+ by the Mn2+ levels. The temperature dependence of Eu2+ photoluminescence in a Sr9−xMnxEu(PO4)7 host was investigated. The conditions for the reduction of Eu3+ to Eu2+ in air were discussed.
In this study, a new series of phosphors, Ca9−xZnxGd0.9(PO4)7:0.1Eu3+ (x = 0.00–1.00, step dx 0.05), was synthesized, consisting of centro- and non-centrosymmetric phases with β-Ca3(PO4)2-type ...structure. Crystal structures with space groups R3c (0.00 ≤ x < 0.35) and R3¯c (x > 0.8) were determined using X-ray powder diffraction and the method of optical second harmonic generation. In the region 0.35 ≤ x ≤ 0.75, phases R3c and R3¯c were present simultaneously. Refinement of the Ca8ZnGd(PO4)7 crystal structure with the Rietveld method showed that 71% of Gd3+ ions are in M3 sites and 29% are in M1 sites. A luminescent spectroscopy study of Ca9−xZnxGd0.9(PO4)7:0.1Eu3+ indicated the energy transfer from the crystalline host to the Gd3+ and Eu3+ luminescent centers. The maximum Eu3+ luminescence intensity corresponds to the composition with x = 1.
TmMgB5O10 spontaneous crystals were synthesized via the flux-growth technique from a K2Mo3O10-based solvent. The crystal structure of the compound was solved and refined within the space group P21/n. ...The first principles calculations of the electronic structure reveal that TmMg-pentaborate with an ideal not defected crystal structure is an insulator with an indirect energy band gap of approximately 6.37 eV. Differential scanning calorimetry measurements and powder X-ray diffraction studies of heat-treated solids show that TmMgB5O10 is an incongruent melting compound. A characteristic band of the Tm3+ cation corresponding to the 3H6 → 1D2 transition is observed in the photoluminescence excitation spectra of TmMg-borate. The as-obtained crystals exhibit intense blue emission with the emission peaks centered at 455, 479, 667, and 753 nm. The most intensive band corresponds to the 1D2 → 3F4 transition. TmMgB5O10 solids demonstrate the thermal stability of photoluminescence.
A new series of Sr-based phosphates, Srsub.9−xMnxEu(POsub.4)sub.7, were synthesized using the high-temperature solid-state method in air. It was found that these compounds have the same structure as ...strontiowhitlockite, which is a β-Casub.3(POsub.4)sub.2 (or β-TCP) structure. The concentration of Mnsup.2+ ions required to form a pure strontiowhitlockite phase was determined. An unusual partial reduction of Eusup.3+ to Eusup.2+ in air was observed and confirmed by photoluminescence (PL) and electron spin resonance (ESR) spectra measurements. The PL spectra recorded under 370 nm excitation showed transitions of both 4f5d–4f Eusup.2+ and 4f–4f Eusup.3+. The total integral intensity of the PL spectra, monitored at 395 nm, decreased with increasing Mnsup.2+ concentration due to quenching effect of Eusup.3+ by the Mnsup.2+ levels. The temperature dependence of Eusup.2+ photoluminescence in a Srsub.9−xMnxEu(POsub.4)sub.7 host was investigated. The conditions for the reduction of Eusup.3+ to Eusup.2+ in air were discussed.
•Phosphors with whitlockite-type structure Sr8M2+Sm1-х(PO4)7:xEu3+, M = Mg2+, Zn2+ were synthesized by three type of routes.•The influence of the synthetic route on the luminescent properties was ...revealed.•Solid-state reaction method leads to better crystallinity, and to improvement of the luminescent proprieties.
A series of new Sr-based phosphors Sr8M2+Sm1-х(PO4)7:xEu3+, M = Mg2+, Zn2+ with β-Ca3(PO4)2-type structure was synthesized and studied. The Sr8MgEu(PO4)7 was obtained by three types of synthetic routs: high temperature solid-state, hydrothermal and sol-gel methods. The influence of synthesis routes and annealing conditions on luminescent properties was studied and analyzed taking into account the data on average crystallite size and grain size determination. It was shown that the synthesis method has a defining role on the phosphor’s properties. The solid-state route gives the best powders crystallinity and the highest intensity of emission. The samples were characterized by strong emission in visible region at 615 nm originating from 5D0 → 7F2 transition of Eu3+ and 600 nm originating from the 5G5/2 → 4H7/2 transition of Sm3+. Also, it was revealed that Zn-substituted samples exhibit higher intensity than Mg-containing ones due to larger particle size and lower scattering of light and higher light absorption. Photoluminescence decay curves were measured and the dependence of decay times on activator concentrations was revealed. The CIE chromaticity coordinates were close to red standard. The prototypes of LED devices were characterized by strong emission with warm correlated color temperature. The investigated compounds can be used as a potential commercial phosphor for solid-state lighting application.
Ca8CdEu(PO4)7 polycrystalline material with a β-Ca3(PO4)2-type (β-TCP) structure was obtained using a solid state technique. The structure was refined by Rietveld method using powder synchrotron ...X-ray diffraction data. The analysis of applying approaches confirmed that the Ca8CdEu(PO4)7 structure is similar to R3¯с structures of Ca8MEu(PO4)7 (M = Mg, Zn). The photoluminescence properties were studied under UV excitation and compared with previously investigated Ca8MEu(PO4)7 (M = Mg, Zn, Ca). The substitution of Ca2+ by M2+ influences on the efficiency of energy transfer between the host and the luminescent ion. The influence of second coordination-sphere interactions on the luminescent properties of Ca8MEu(PO4)7 was revealed. Based on the difference on the PL spectra of Eu3+ in Ca8MEu(PO4)7 with R3с or R3¯с structures the site-selective method for determination of space group in β-TCP-type compounds was proposed. It is thus shown that the Eu3+ was used as an optical probe to determine the site symmetry of studied hosts.
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•Ca8CdEu(PO4)7 red phosphor with a β-TCP structure was synthesized by a solid state technique.•The structure and luminescent properties were systematically studied.•The site-selective method for determination of space group in β-TCP-type compounds was proposed.•The properties of Ca8CdEu(PO4)7 were compare with Ca8MEu(PO4)7 similar compounds.
A series of novel phosphates with the general formulas Ca9Gd0.9−xTm0.1Smx(PO4)7 and Ca9Gd0.9−yTmySm0.1(PO4)7 were synthesized by solid-state method. As-obtained phosphates were characterized by ...powder X-ray diffraction and second harmonic generation analyses, dielectric measurements, luminescence spectroscopy. All samples were single phase and characterized by the whitlockite-type structure with space group R3c. An influence of admixture concentration of REE3+ ions in the initial host on dielectric properties was studied in details. Synthesized phosphates are characterized by intensive luminescence. The emission in the orange region of the visible spectrum is observed for Ca9Gd0.9Sm0.1(PO4)7 with a maximum intensity band at 602 nm. The line in blue region at 455 nm, which corresponds to 1D2 → 3F4 Tm3+ transition, is registered for Ca9Gd0.9Tm0.1(PO4)7. Emission in the white region of CIE coordinates was registered for Tm-Sm co-doped compounds.
The effects of a uniform external magnetic field on the thermomagnetic convection of magnetic fluid in a cylindrical enclosure are numerically studied. It was found that uniform magnetic field can be ...cause the convection in the non-uniformly heated magnetic fluid in a cylindrical enclosure even in the case of zero gravity. There are two threshold values of fluid magnetization and temperature gradient. Above the first threshold the convective flow is realized as circular flow, above the second one there are two cells in the enclosure. The angle between a magnetic field strength and a temperature gradient is found to be significant factor influencing structure of a convective flow and heat transfer control.
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