RESUMEN
Novel temperature-sensitive luminescent materials, Ba2Gd2Ge4O13 doped with Bi3+, Bi3+/Sm3+ or Bi3+/Eu3+ ions, have been prepared using a conventional solid-state reaction technique. The XRPD study has verified that all the synthesized germanates crystallize in the monoclinic system (space group C2/c, Z = 4). The crystal lattice of the compounds consists of zigzag chains of edge-sharing Me2O7 (Me = Bi3+, Eu3+ or Sm3+) dimers, [Ge4O13] units, and ten-coordinated Ba atoms. Under UV excitation the powders exhibit the luminescence corresponding to the 3P1 â 1S0 (310-550 nm) transition in Bi3+ and 4G5/2 â 6HJ (550-730 nm) in Sm3+ or 5D0 â 7FJ (570-720 nm) transitions in Eu3+ ions. Heating of Ba2Gd1.94Bi0.01Sm0.05Ge4O13 and Ba2Gd1.89Bi0.01Eu0.1Ge4O13 phosphors leads to an irregular decrease in the intensity of the main emission lines. It has been found that the fluorescence intensity ratio between a wide band in the 310-530 nm region and peaks at longer wavelengths may be successfully used as a temperature-dependent characteristic. Absolute/relative sensitivity values for Ba2Gd1.94Bi0.01Sm0.05Ge4O13 and Ba2Gd1.89Bi0.01Eu0.1Ge4O13 germanates reach 0.19% K-1/0.80% K-1 and 2.21% K-1/0.58% K-1, respectively. The above parameters indicate that Ba2Gd2Ge4O13:Bi3+/Sm3+ or Bi3+/Eu3+ samples can be used as potential luminescent materials for non-contact temperature measurement.
RESUMEN
A new series of Ba2RE2Ge4O13 (RE = Pr, Nd, Gd, Dy) germanates and Ba2Gd2-xEuxGe4O13 (x = 0.1-0.8) solid solutions have been synthesized using the solid-state reaction technique and characterized by X-ray powder diffraction. All compounds crystallize in the monoclinic system, space group C2/c, Z = 4. The crystal lattice consists of RE2O12 dimers, zigzag C2-symmetric [Ge4O13]10- tetramers, and ten-coordinated Ba atoms located in voids between polyhedra. The density-functional theory (DFT) calculations performed on a rich set of Ba2RE2Ge4O13 compounds have confirmed the high thermodynamic stability of monoclinic modification. Under ultraviolet (UV) light excitation Ba2Gd2-xEuxGe4O13 phosphors exhibit an orange-red emission corresponding to the characteristic f-f transitions in Eu3+ ions. The highest intensity of lines at 580 nm (5D0â7F0), 582-602 nm (5D0â7F1), 602-640 nm (5D0â7F2), 648-660 nm (5D0â7F3), and 680-715 nm (5D0â7F4) is observed for the samples with x = 0.4-0.6. The possibility of their application has been assessed by studying their color characteristics, quantum efficiency, and thermal stability. The obtained data indicate that Ba2Gd2-xEuxGe4O13 solids can be considered as promising materials for UV-excited phosphor-converted light-emitting diodes (LEDs) if an aluminum nitride substrate (λex = 255 nm) is used as a semiconductor chip.
RESUMEN
The rechargeable Na-ion batteries attract much attention as an alternative to the widely used but expensive Li-ion batteries. The search for materials with high sodium diffusion is important for the development of solid state electrolytes. We present the results of experimental and ab initio studies of the Na-ion diffusion mechanism in Na9Sc(MoO4)6. The ion conductivity reaches the value of 3.6 × 10-2 S cm-1 at T â¼ 850 K. The 23Na and 45Sc NMR data reveal the coexistence of three different types of Na-ion motion in the temperature range from 300 to 750 K. They are activated at different temperatures and are characterized by substantially different dynamics parameters. These features are confirmed by ab initio calculations of activation barriers for sodium diffusion along various paths.
RESUMEN
The apatite-like NaLa9(GeO4)6O2:Nd3+,Ho3+ phosphor is prepared using the solid-state method. Rietveld refinement of high-resolution time-of-flight neutron powder diffraction measurements indicate that this compound crystallizes in the hexagonal system with space group P63/m, Z = 1 and unit cell parameters a = 9.88903(6) Å, c = 7.25602(5) Å, V = 614.521(7) Å3 at room temperature. The 4f sites are statistically occupied by La, Nd and Na, while 6h sites are occupied by La and Nd. Luminescence in the near- and middle-IR range caused by the transitions in neodymium and holmium ions is excited under 808 nm laser diode radiation. The highest emission intensity in NaLa9-x-yNdxHoy(GeO4)6O2 is attained at trace amounts of holmium, and it decreases sharply when y increases to 0.01. The IR phosphors have a good thermal stability and exhibit a very weak upconversion emission in the red spectral range upon 808 nm excitation. A scheme of excitation and emission pathways involving ground/excited state absorption, energy transfer, cross-relaxation, nonradiative multiphonon relaxation processes in Nd3+ and Ho3+ ions has been proposed. The data analysis indicates that Nd3+ ions serve as sensitizers for Ho3+ ions in these compounds, stimulating intense 2.1 µm and 2.7 µm emissions. These apatite-related germanate phosphors are promising materials for near- and middle-infrared solid-state lighting applications.
RESUMEN
Simultaneous emission lines around 1.05µm, 1.3µm, 1.8µm, 2.1µm and 2.7µm have been observed in Li7La3-xNdxHf2O12:Ho3+ (x=0.00-0.15) under 808nm laser diode excitation. Near-infrared luminescence due to holmium ions with residual concentration in the Li7La3Hf2O12 host has been studied. The intensity of 2.1 and 2.7µm lines associated with 5I7â5I8 and 5I6â5I7 transitions in Ho3+ depends on the neodymium codopant concentration. This result indicates that Nd3+ ions can be potentially used as sensitizers for Ho3+ ions to stimulate the intense near-infrared emission in this system. Possible energy transfer mechanisms between lanthanide ions have been briefly discussed.
RESUMEN
Two new solid MO(OH)2 (M = Zr, Hf) oxyhydroxides have been synthesised by an ion-exchange reaction from Li2MO3 (M = Zr, Hf) precursors obtained by a citrate combustion technique. The crystal structure of the oxyhydroxides has been solved by direct methods and refined using Rietveld full profile fitting based on X-ray powder diffraction data. Both oxyhydroxides crystallize in a P2(1)/c monoclinic unit cell and have a structure resembling that of the related salts. Detailed characterisation of the fine-structure features and chemical bonding in precursors and oxyhydroxide powders has been performed using vibrational spectroscopy, nuclear magnetic resonance spectroscopy, scanning electron microscopy, pair distribution function analysis and quantum-chemical modelling.
RESUMEN
Crystal structures of TiO(OH)(2) and Li(2)TiO(3) have been studied in detail and refined using X-ray powder diffraction data. Both compounds possess a high concentration of defects in the structure. The crystal structure of the Li(2)TiO(3) salt obtained at 700 degrees C reveals stacking faults of LiTi(2) metal layers, which leads to the appearance of short-range order in three possible space groups: C2/c, C2/m, P3(1)12. The possibility to stabilise this imperfect state increases the mobility of the Li(+) ions in the structure and allows the complete exchange of lithium by hydrogen in acid water solutions with formation of TiO(OH)(2). The crystal structure of TiO(OH)(2) belongs to the layered double hydroxide structure type with the 3R(1) sequence of oxygen layers and can be described as a stacking of charge-neutral metal oxyhydroxide slabs [(OH)(2)OTi(2)O(OH)(2)]. TiO(OH)(2) is the first layered double hydroxide structure formed by a cation with oxidation state +4 only.
