Fengjuan Pan

Site Occupancies, Luminescence, and Thermometric Properties of LiY9(SiO4)6O2:Ce3+ Phosphors

In this work, we report the tunable emission properties of Ce3+ in an apatite-type LiY9(SiO4)6O2 compound via adjusting the doping concentration or temperature. The occupancies of Ce3+ ions at two different sites (Wyckoff 6h and 4f sites) in LiY9(SiO4)6O2 have been determined by Rietveld refinements. Two kinds of Ce3+ f-d transitions have been studied in detail and then assigned to certain sites. The effects of temperature and doping concentration on Ce3+ luminescence properties have been systematically investigated. It is found that the Ce3+ ions prefer occupying Wyckoff 6h sites and the energy transfer between Ce3+ at two sites becomes more efficient with an increase in doping concentration. In addition, the charge-transfer vibronic exciton (CTVE) induced by the existence of free oxygen ion plays an important role in the thermal quenching of Ce3+ at 6h sites. Because of the tunable emissions from cyan to blue with increasing temperature, the phosphors LiY9(SiO4)6O2:Ce3+ are endowed with possible thermometric applications.

VUV-vis photoluminescence, X-ray radioluminescence and energy transfer dynamics of Ce3+ and Pr3+ doped LiCaBO3

A series of LiCaBO3:Ce3+/Pr3+ phosphors were prepared by a high-temperature solid state reaction method. A structure refinement for the LiCaBO3 compound was performed based on powder X-ray diffraction (XRD) data. The energies of the crystal field split excited 5d states of Ce3+ and Pr3+ in LiCaBO3 were determined from synchrotron radiation VUV-UV excitation spectra. Furthermore, the influence of the doping concentration and temperature on the emission properties of Ce3+ was investigated, and the vacuum referred binding energy (VRBE) scheme for all lanthanide 4f and 5d states in LiCaBO3 was constructed to estimate the thermal activation energy of Ce3+ luminescence quenching. The Pr3+ to Ce3+ energy transfer (ET) and its influence on luminescence decays of Pr3+ and Ce3+ were studied in detail. Finally, the X-ray excited luminescence spectra were measured to evaluate the possible X-ray detection applications.

Blue-emitting phosphor Ba4OCl6:Eu2+ with good thermal stability and a tiny chromaticity shift for white LEDs

An intense blue-emitting phosphor, Ba4OCl6:Eu2+, was developed by a solid state reaction at low synthesis temperature. The photoluminescence excitation and emission spectra, concentration effect, thermal-dependent luminescence quenching properties, chromaticity shift, activity energy and luminous efficiency of radiation are investigated. These results show that Eu2+ exhibits a broadband excitation extending from 250 to 400 nm and emits an intense blue light at 450 nm with a tiny colour chromaticity shift in the temperature range of 350–490 K. The external quantum efficiency of Ba3.97OCl6:Eu0.032+ is about 26.6%. It demonstrates that Eu2+ doped Ba4OCl6 can be excited by an n-UV LED chip and is an effective blue-emitting phosphor potentially useful in white LEDs.

Double substitution induced tunable luminescent properties of Ca3−xYxSc2−xMgxSi3O12:Ce3+ phosphors for white LEDs

Y3+–Mg2+ pairs were incorporated into the Ca3Sc2Si3O12 for Ca2+–Sc3+ sites to investigate their structural and optical properties systematically, their potential applications in white light-emitting diodes, as well as the coexistent shrinkage effects of the first sphere and the second sphere based on the Rietveld refinement analysis of X-ray diffraction data. Due to the substitution of Y3+–Mg2+, an obvious red shift of the emission occurs. CIE chromaticity coordinates, quantum efficiency as well as thermal stability suggest that this double substitution is an efficient way to tune the luminescence properties of phosphors. The white LED devices were also successfully fabricated which demonstrates that such a kind of double substitution will be a promising way to broaden the applications of phosphors in advanced lighting in the future.

Defect Structure, Phase Separation, and Electrical Properties of Nonstoichiometric Tetragonal Tungsten Bronze Ba0.5-xTaO3-x

New insight into the defect chemistry of the tetragonal tungsten bronze (TTB) Ba(0.5-x)TaO(3-x) is established here, which is shown to adapt to a continuous and extensive range of both cationic and anionic defect stoichiometries. The highly nonstoichiometric TTB Ba(0.5-x)TaO(3-x) (x = 0.25-0.325) compositions are stabilized via the interpolation of Ba(2+) cations and (TaO)(3+) groups into pentagonal tunnels, forming distinct Ba chains and alternate Ta-O rows in the pentagonal tunnels along the c axis. The slightly nonstoichiometric Ba(0.5-x)TaO(3-x) (x = 0-0.1) compositions incorporate framework oxygen and tunnel cation deficiencies in the TTB structure. These two mechanisms result in phase separation within the 0.1< x < 0.25 nonstoichiometric range, resulting in two closely related (TaO)(3+)-containing and (TaO)(3+)-free TTB phases. The highly nonstoichiometric (TaO)(3+)-containing phase exhibits Ba(2+) cationic migration. The incorporation of (TaO)(3+) units into the pentagonal tunnel and the local relaxation of the octahedral framework around the (TaO)(3+) units are revealed by diffraction data analysis and are shown to affect the transport and polarization properties of these compositions.

Localization of Oxygen Interstitials in CeSrGa3O7+δ Melilite

The solubility of Ce in the La(1-x)Ce(x)SrGa3O(7+δ) and La(1.54-x)Ce(x)Sr0.46Ga3O(7.27+δ) melilites was investigated, along with the thermal redox stability in air of these melilites and the conductivity variation associated with oxidization of Ce(3+) into Ce(4+). Under CO reducing atmosphere, the La in LaSrGa3O7 may be completely substituted by Ce to form the La(1-x)Ce(x)SrGa3O(7+δ) solid solution, which is stable in air to ∼600 °C when x ≥ 0.6. On the other side, the La(1.54-x)Ce(x)Sr0.46Ga3O(7.27+δ) compositions displayed much lower Ce solubility (x ≤ 0.1), irrespective of the synthesis atmosphere. In the as-made La(1-x)CexSrGa3O(7+δ), the conductivity increased with the cerium content, due to the enhanced electronic conduction arising from the 4f electrons in Ce(3+) cations. At 600 °C, CeSrGa3O(7+δ) showed a conductivity of ∼10(-4) S/cm in air, nearly 4 orders of magnitude higher than that of LaSrGa3O7. The oxidation of Ce(3+) into Ce(4+) in CeSrGa3O(7+δ) slightly reduced the conductivity, and the oxygen excess did not result in apparent increase of oxide ion conduction in CeSrGa3O(7+δ). The Ce doping in air also reduced the interstitial oxide ion conductivity of La1.54Sr0.46Ga3O7.27. Neutron powder diffraction study on CeSrGa3O7.39 composition revealed that the extra oxygen is incorporated in the four-linked GaO4 polyhedral environment, leading to distorted GaO5 trigonal bipyramid. The stabilization and low mobility of interstitial oxygen atoms in CeSrGa3O(7+δ), in contrast with those in La(1+x)Sr(1-x)Ga3O(7+0.5x), may be correlated with the cationic size contraction from the oxidation of Ce(3+) to Ce(4+). These results provide a new comprehensive understanding of the accommodation and conduction mechanism of the oxygen interstitials in the melilite structure.

Stabilization and tunable microwave dielectric properties of the rutile polymorph in α-PbO2-type GaTaO4-based ceramics

The effects of Ti substitution on the crystal chemistry and microwave dielectric properties of Ga1−xTa1−xTi2xO4 were investigated. GaTaO4 adopts a 1:1-ordered monoclinic α-PbO2 type structure. At 1300 °C the Ga1−xTa1−xTi2xO4 system formed a monoclinic α-PbO2 type solid solution when x ≤ 0.05, and further Ti substitution in Ga1−xTa1−xTi2xO4 induced a phase transformation from α-PbO2 to rutile, leading to a rutile solid solution when x ≥ 0.15. Within the intermediate composition range 0.075 ≤ x ≤ 0.1, a mixture of α-PbO2 and rutile phases formed, which gradually transformed into a single rutile phase upon being fired at 1350–1400 °C. At 1400 °C the compositions where x = 0–0.025 remained in monoclinic α-PbO2 type phases, while the x = 0.05 composition transformed into a rutile polymorph, implying that Ti substitution is favorable for stabilizing the high temperature rutile polymorph of GaTaO4 down to room temperature. The monoclinic Ga1−xTa1−xTi2xO4 (x = 0, 0.05) ceramics exhibited a low permittivity (er) values of ∼16–20, high Qf values ∼45 000–68 000 GHz and large negative temperature coefficients of the resonance frequency, τf of −56 ppm °C−1 to −47 ppm °C−1. Ti substitution in Ga1−xTa1−xTi2xO4 increased er to ∼40 and τf to ∼53 ppm °C−1 in the range x = 0–0.4, while the Qf values exhibited a tendency to decrease with Ti substitution. The rutile solid solution showed, for the first time, a tunable τf from a negative value to a positive value and optimum microwave dielectric properties were achieved for rutile Ga0.75Ta0.75Ti0.5O4: er ∼ 37, Qf ∼ 30 000 GHz and τf ∼ 4.4 ppm °C−1. The factors controlling the dielectric loss and τf in Ga1−xTa1−xTi2xO4 are discussed in terms of the polymorphism, defects, charge disorder and polarizability associated with the Ti substitution.

Nonstoichiometric Control of Tunnel-Filling Order, Thermal Expansion, and Dielectric Relaxation in Tetragonal Tungsten Bronzes Ba0.5–xTaO3–x

Ordering of interpolated Ba(2+) chains and alternate Ta-O rows (TaO)(3+) in the pentagonal tunnels of tetragonal tungsten bronzes (TTB) is controlled by the nonstoichiometry in the highly nonstoichiometric Ba0.5-xTaO3-x system. In Ba0.22TaO2.72, the filling of Ba(2+) and (TaO)(3+) groups is partially ordered along the ab-plane of the simple TTB structure, resulting in a √2-type TTB superstructure (Pbmm), while in Ba0.175TaO2.675, the pentagonal tunnel filling is completely ordered along the b-axis of the simple TTB structure, leading to a triple TTB superstructure (P21212). Both superstructures show completely empty square tunnels favoring Ba(2+) conduction and feature unusual accommodation of Ta(5+) cations in the small triangular tunnels. In contrast with stoichiometric Ba6GaTa9O30, which shows linear thermal expansion of the cell parameters and monotonic decrease of permittivity with temperature within 100-800 K, these TTB superstructures and slightly nonstoichiometric simple TTB Ba0.4TaO2.9 display abnormally broad and frequency-dependent extrinsic dielectric relaxations in 10(3)-10(5) Hz above room temperature, a linear deviation of the c-axis thermal expansion around 600 K, and high dielectric permittivity ∼60-95 at 1 MHz at room temperature.