The Synthesis, Structure, and Luminescent Properties of TmMgB5O10 Crystals.

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.

New Series of Red-Light Phosphor Ca9-xZnxGd0.9(PO4)7:0.1Eu3+ (x = 0-1).

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 ß-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.

Co-Doping Effect of Mn2+ and Eu3+ on Luminescence in Strontiowhitlockite Phosphors.

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 ß-Ca3(PO4)2 (or ß-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.

K5Eu1-xTbx(MoO4)4 Phosphors for Solid-State Lighting Applications: Aperiodic Structures and the Tb3+ → Eu3+ Energy Transfer.

This paper describes the influence of sintering conditions and Eu3+/Tb3+ content on the structure and luminescent properties of K5Eu1-xTbx(MoO4)4 (KETMO). KETMO samples were synthesized under two different heating and cooling conditions. A K5Tb(MoO4)4 (KTMO) colorless transparent single crystal was grown by the Czochralski technique. A continuous range of solid solutions with a trigonal palmierite-type structure (α-phase, space group R3̅m) were presented only for the high-temperature (HT or α-) KETMO (0 ≤ x ≤ 1) prepared at 1123 K followed by quenching to liquid nitrogen temperature. The reversibility of the ß â†” α phase transition for KTMO was revealed by a differential scanning calorimetry (DSC) study. The low-temperature (LT)LT-K5Eu0.6Tb0.4(MoO4)4 structure was refined in the C2/m space group. Additional extra reflections besides the reflections of the basic palmierite-type R-subcell were present in synchrotron X-ray diffraction (XRD) patterns of LT-KTMO. LT-KTMO was refined as an incommensurately modulated structure with (3 + 1)D superspace group C2/m(0ß0)00 and the modulation vector q = 0.684b*. The luminescent properties of KETMO prepared at different conditions were studied and related to their structures. The luminescence spectra of KTMO samples were represented by a group of narrow lines ascribed to 5D4 → 7FJ (J = 3-6) Tb3+ transitions with the most intense emission line at 547 nm. The KTMO single crystal demonstrated the highest luminescence intensity, which was ∼20 times higher than that of LT-KTMO. The quantum yield λex = 481 nm for the KTMO single crystal was measured as 50%. The intensity of the 5D4 → 7F5 Tb3+ transition increased with the increase of x from 0.2 to 1 for LT and HT-KETMO. Emission spectra of KETMO samples with x = 0.2-0.9 at λex = 377 nm exhibited an intense red emission at ∼615 nm due to the 5D0 → 7F2 Eu3+ transition, thus indicating an efficient energy transfer from Tb3+ to Eu3+.

The role of anionic heterovalent [PO4]3- → [GeO4]4- substitution on the luminescence properties of inorganic phosphors with the ß-Ca3(PO4)2-type structure: new data based on accurate crystal structure refinement.

A series of solid-solution phosphate germanates Ca8+0.5xZnEu(PO4)7-x(GeO4)x (x = 0, 0.2, 0, 4, 0.6, 0.8, 1) with the ß-Ca3(PO4)2-type structure were synthesized by solid-state reactions. The limit of existence of a single-phase solid solution was determined by X-ray diffraction patterns and it was found at x = 0.8. The heterovalent tetrahedral [PO4]3- → [GeO4]4- substitution requires a charge compensation according to the scheme: [PO4]3- + ½ □ → [GeO4]4- + ½ Ca2+. The additional amount of Ca2+ ions in the crystal structure was detected at the M4 site during Rietveld refinement. It was shown that in ß-Ca3(PO4)2-type compounds, charge balancing is not provided by the randomly distributed oxygen vacancies but only by the partial occupancy of the M4 site. The presence of Ca2+ at the M4 site leads to a polar structure with the space group R3c which was confirmed by an SHG test for all single-phase samples. It was shown that the Ge4+ ions preferably occupy the T3 site in the structure, which is connected through common oxygen with the cationic M1-M5 sites. The analysis of the similarity of the previously reported Ca9La(GeO4)0.75(PO4)6 compound reveals an unexpectedly high value. The same structural similarity evaluation of the studied compound Ca8.1EuZn(PO4)6.8(GeO4)0.2 in the present work with the initial model gives a very small value, which indicates a good match between the initial and under-consideration structures. The luminescence properties of Eu3+ were investigated from the point of view of crystal structures and anionic substitutions. The integral intensity increased linearly with the [PO4]3- → [GeO4]4- substitution. It can be concluded that the anionic substitution on Ge4+ can improve the luminescence characteristics. The present study includes new data on the anionic substitution based on accurate crystal structure refinement.