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Y10Ta4O25:Eu3+ presents peculiar spectra and excellent red luminescence properties due to its special fluorite-related structure. The microstructure was characterized by the site-selective luminescence of Eu3+ activators.

Tungsten bronze-type ceramics have been widely investigated as one of the best ferroelectric frameworks. RE-doped Ba2NaNb5O15 phosphors have been reported with unsatisfactory luminescence efficiencies. It is attractive to develop a ferroelectric showing multifunctionality with electric and luminescence performances. In this work, the cation substitutions of (Eu3+ → Ba2+) and (Ti4+ → Nb5+) are realized in tungsten bronze-type Ba2NaNb5O15 to develop optoelectronic ceramics (luminescence + dielectric). The introduction of framework-disturbances is regarded to be an effective strategy on modification of the luminescence and electronic properties. The ceramics were prepared using the high-temperature solid-state reaction. The cation disorder due to Eu3+/Ti4+ co-doping can significantly enhance the luminescence efficiency and thermal stability. Meanwhile, Eu3+/Ti4+ co-doping has a strong influence on the microwave dielectric performances, that is, the dielectric constant (er) values and quality factor (Q × f) were significantly improved. The temperature coefficient of resonant frequency (τf) is optimized. The improvement of optoelectronic effects shows a significant dependence on cation disorder of the ferroelectric framework.

In this work, a series of mixed pyrophosphates (Mg1-xMnx)2P2O7 (x = 0-1.0) were prepared for the first time using a solid-state reaction method, which exhibits two kinds of structural variants, that is, α-(low temperature) and β-(high temperature) phases. The detailed phase-formations were determined via structural Rietveld refinements and the luminescence transitions of 4T1 → 6A1 in Mn2+. The phase-formation of (Mg1-xMnx)2P2O7 (x = 0-1.0) shows a strict dependence on Mn2+ doping contents in the lattices. (Mg1-xMnx)2P2O7 has an α-Mg2P2O7 phase when the Mn2+-doping concentration is lower than x ≤ 0.1. With an increase of Mn2+ substitution above 20 mol% (x = 0.2-1.0), (Mg1-xMnx)2P2O7 presents only the β-phase even at room temperature, while (Mg1-xMnx)2P2O7 (x = 0.15) shows a mixed formation of α- and β-Mg2P2O7 phases. The crystallographic surrounding of the Mn2+ activators in different structures had a strict influence on the spectral profile, luminescence efficiency, and color centers. Interestingly, in this series of phosphors, (Mg1-xMnx)2P2O7 (x = 0.15) with the mixed phases of α- and β-type has overwhelming luminescence abilities such as the best luminescence intensity and high thermal stability. The pyrophosphates were confirmed to qualify for red-emitting LED lamps.

Aluminosilicate with the cordierite structure (Mg2Al4Si5O18) represents a big family of technologically important compounds with typical α- and β-phases. Crystallographic sites for transition metal and rare earth ions in the cordierite structure have always been a controversial topic. In this work, the (Mg1−xMnx)2Al4Si5O18 (x = 0–1.0) solid solution is firstly confirmed via Rietveld structural refinements and luminescence dynamics. The well-crystallized ceramics were prepared by high-temperature solid-state reaction, and then characterized using structural, morphological, luminescence, decays, and thermal quenching measurements. (Mg1−xMnx)2Al4Si5O18 (x = 0–1.0) undergoes structural changes from the orthorhombic β-phase (x = 0–0.2), α-phase (x = 0.3–0.9) and Mn-cordierite (x = 0.95–1.0) (isostructural to β-phase). It is technically meaningful that high-temperature α-phase cordierite Mg2Al4Si5O18 can be stabilized via Mn2+-doping (30–90 mol%). The crystallographic site-occupation of Mn2+ activators is clearly identified. Interestingly, (Mg1−xMnx)2Al4Si5O18 (x = 0.01–1.0) presents tunable colors produced by two distinct luminescence centers, that is, green Mn2+(A) and red Mn2+(B) bands centered at about 530 and 650 nm, respectively. The most efficient excitation wavelengths and decay times of Mn2+(A) are distinct from those of Mn2+(B). The Mn2+(A) green centers are only observed in β-phase cordierite (x = 0.01–0.2) with a dominant emission intensity compared with Mn2+(B), while the Mn2+(B) red centers are observed in all samples (x = 0.01–1.0). The Mn2+(A) center fills in the hexagonal channels, while Mn2+(B) is related to the substitution of Mn2+ for the octahedral Mg2+ site in the cordierite lattices. The luminescence mechanism of Mn2+ in (Mg1−xMnx)2Al4Si5O18 was proposed. The results can be used for basic and application research studies in a wide family of rare earth or transition-metal-doped cordierite materials.

There is a close correlation between the photoluminescence and photocatalysis of a semiconductor because the two processes involve the same electronic transition. Eu3+ is one of the most popular rare-earth ions; it presents efficient luminescence and improved photocatalysis. Usually, the luminescence from the higher states (5D1,2) of Eu3+ can be hardly observed due to cross-relaxation processes. This work demonstrates a particularly developed transition from the 5D1 level of Eu3+ and its significant contribution to the improved photocatalysis of (Bi3Li)O4Cl2via prolonging the decay time of the excited state. Single-phase (Bi3−3xEu3xLi)O4Cl2 (x = 0, 0.01, 0.03, 0.05) was synthesized by the sol–gel method in combination with a solid-state reaction. This is a bismuth layer structure with [(Bi/Li)O2]2+ layers formed by alternative Bi3+ and Li+ stripes. The intrinsic luminescence of (Bi3Li)O4Cl2 (λem = 500 nm) with a decay time of 0.26 μs was detected even at 300 K. Unusually, the phosphor shows a prominent transition from 5D1 in (Bi3−3xEu3xLi)O4Cl2. Moreover, in addition to 5D0 → 7F3, 5D0 → 7FJ (J = 0, 1, 2, 4) is characterized by intense transitions with comparable intensities. The intrinsic emission of (Bi3Li)O4Cl2 has a decay time which involves the band transition of an electron from the valence band to the conduction band. In Eu3+-doped (Bi3Li)O4Cl2, there are midgap states formed by the 5D1 level with a longer lifetime of about 60 μs. The D1 levels of Eu3+ significantly contribute to the separation of light-induced charges by prolonging the decay time of the excited states. This work demonstrates a simple strategy to develop optical materials with simultaneous luminescence and has improved the photocatalysis in bismuth oxychlorides featuring strong polarization and rigid phonons.

A Mn4+ doped Na2WO2F4 phosphor was synthesized through a two-step wet chemical method. The relationship between crystal structure and luminescence properties is discussed and unusual strong intense zero phonon lines (ZPLs) have been found in a distorted octahedral environment. The power dependent luminescence spectra exhibit the existence of down conversion luminescence intensity saturation under a high pumping power limit. The fluorescence intensity ratios of anti-Stokes bands to the ZPL and Stokes bands reveal an obvious temperature dependent relationship based on thermal de-population from the low states to the upper states of an intrinsic Mn4+ 2Eg → 4A2g transition. The temperature dependent emission intensity of Mn4+ is investigated by changing the excitation power, and an optical temperature sensitivity as high as 0.00658 K−1 is achieved at 193 K with the intensity ratio of anti-Stokes bands to the ZPL under 488 nm excitation by a Xenon lamp. This work presents a new method to realize optical thermometry at low temperature by controlling the intensity ratio of the anti-Stokes bands to the ZPL.

Cation disorder in the phosphor lattice could be one of the effective approaches to modify the luminescence efficiency. In this work, cation substitutions of (Mo6+ → V5+) and (Na+ → Mg2+) were conducted in the self-activated NaMg2V3O10. All the samples of Na1+xMg2−xV3−xMoxO10 (x = 0, 0.01, 0.05, 0.1, 0.2, 0.3) were prepared via solid-state reaction. The morphological properties were measured via SEM and EDS analyses. Structural Rietveld refinement was performed to investigate the microstructure in the lattices. The cation substitution brings about structural disorder in the phosphor, which exerts great modifications in the luminescence properties. NaMg2V3O10 presents an intrinsic indirect transition with a band gap of 3.22 eV. The incorporation of Mo6+ and Na+ in the lattices moves the optical absorption to a longer wavelength bringing about a narrower band gap. The luminescence intensity, thermal stability and corresponding lifetime were modified by the cation disorder in the self-activated phosphor.

Mn4+-activated cubic phase Ba5AlF13 red phosphors were prepared by the two-step coprecipitation method. The structural and optical features were characterized on the basis of X-ray diffraction (XRD), transmission electron microscopy (TEM), emission and excitation spectra, and luminescence decay curves. The Ba5AlF13:Mn4+ phosphors can be efficiently excited by near-UV to blue light and exhibit bright red emission at around 627 nm, which is assigned to the 2Eg → 4A2g transition of the 3d3 electrons in [MnF6] octahedra. Temperature dependent emission spectra and decay curves from 10 to 550 K were measured to deeply understand the luminescence mechanism of Mn4+ in the Ba5AlF13 lattice. Notably, this novel red phosphor shows excellent anti-thermal quenching behaviour (∼700% of emission intensity at 300 K relative to 10 K).

A novel red emitting Cs2WO2F4:Mn4+ phosphor was successfully synthesized by a two-step wet chemical method. The crystal structure, morphology, and elemental composition were confirmed by powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS), respectively. The luminescence properties were investigated from emission, excitation and luminescence decay curves in the temperature region of 10-500 K. The application of non-contact optical thermometry of Cs2WO2F4:Mn4+ based on the fluorescence intensity ratio (FIR) of the two coupled anti-Stokes and Stokes sidebands is discussed. The as-prepared Cs2WO2F4:Mn4+ phosphor shows a bright narrow red emission at 632 nm under excitation by a blue lamp at 470 nm and it also presents a broad and yellow-white intrinsic tungstate emission (∼520 nm) under UV excitation. The mechanism of energy transfer from [WO2F4]2- (the sensitizer) to Mn4+ (the activator) is discussed.

Three isomorphic lanthanide metal–organic frameworks (Ln-MOFs) have been constructed from lanthanide formate skeletons and 3,5-bis(4′-carboxy-phenyl)-1,2,4-triazole (H2bct) connectors. Further, by adjusting the co-doping ratio of different Ln3+ ions into the framework, two doped Ln-MOFs are synthesized and show tunable luminescence emission including white-light emission.

Polycrystalline Cs5V3O10 micro-particles were synthesized by a solid-state reaction. The vanadate shows intrinsic self-activated luminescence of a single broad band with a peak at 520 nm, extending from about 400 nm to 720 nm. This asymmetric band is composed of two bands due to the electronic transitions from the 3T1 and 3T2 excited states to the 1A1 ground state in [VO4]3− centers. The same emission band was obtained for micro- and nanoparticles. It is suggested that the broadening of the emission band arises from single [VO4]3− molecules. The emission and decay curve profiles indicate that the emission is not due to different kinds of [VO4]3− centers but only one kind of [VO4]3− center. When the temperature is increased from 10 K to 450 K, the emission intensity increases below 150 K and decreases above 150 K, and an unusual blue shift is observed. The observed temperature dependence can be understood by the relaxation processes of the emitting 3T1 and 3T2 states including the thermal feeding by the lower-energy 3T1 state to the higher-energy 3T2 state.

The red-emitting Eu3+-doped rare earth antimonates R3SbO7 (R = La, Gd, Y) were prepared by high-temperature solid-state reaction. The crystal-phase formations were verified by X-ray powder diffraction (XRD) and structural refinements. The luminescence properties such as photoluminescence (PL) excitation and emission spectra, fluorescence decay curves, absolute luminescence quantum efficiency (QE), CIE color coordinates and the dependence of luminescence intensity on doping level were investigated. The luminescence QE, CIE color coordinates, and the spectrum characteristics of Eu3+ ions have a strong dependence on both R (La, Gd, Y) cations and Eu3+ doping levels. The 5D0 → 7F4 emission peak at 710 nm was the dominant transition in Eu3+-doped La3SbO7, while 5D0 → 7F0 at 580 nm was the strongest transition in Eu3+-doped Y3SbO7. The luminescence properties were discussed on the basis of the crystal structure. Different Eu3+ luminescence centers, such as isolated centers, pair broadening, and cluster centers, were discussed based on the dependence of the lifetime values on Eu3+ concentration. The luminescence QE of La3SbO7:0.4Eu3+ can reach 63.8% under excitation of UV light at room temperature; thus, it can have potential application as a red-emitting phosphor for solid-state lighting.

CaB2O4 powders and ceramics were prepared by the conventional solid-state reaction. In vitro hydroxyapatite (HA) mineralization was investigated by soaking the samples in simulated body fluid (SBF) for various time periods. X-ray diffraction and structural refinements, scanning electron microscopy and X-ray energy-dispersive spectra measurements were applied to investigate apatite formation before and after immersion in SBF. HA can easily form flower-like nanostructures with nano-needles even when soaked in SBF for several hours. The in vitro bioactivity of CaB2O4 was attributed to easy formation of B–OH groups in the CaB2O4 structure when soaked in SBF solutions. In the process of mineralization, the luminescence evolution of Eu3+ ions, a well-known structural probe, was detected by photoluminescence spectra and photoluminescence decay curves. This suggested that the process of mineralization can be monitored by the luminescence intensity of Eu3+ ions in the mineralization products. The current study will open up a new and simple in vivo avenue for in situ monitoring of hydroxyapatite conversion using a fiber luminescence spectrometer.

Correction for ‘Synthesis, structure and optical performance of red-emitting phosphor Ba5AlF13:Mn4+’ by Lin Qin et al., RSC Adv., 2017, 7, 49473–49479.

Eu2+- and Eu3+-doped MgZn2(PO4)2 were prepared using the high temperature solid-state reaction method. The excitation spectra in the UV and VUV region, the emission spectra, and the luminescence decay curves of the Eu2+ and Eu3+ ions were investigated. The crystallographic distributions of the Eu2+ and Eu3+ sites in MgZn2(PO4)2 lattices are different. MgZn2(PO4)2:Eu2+ presents a bluish green luminescence with two distinct emission centers at 450 and 525 nm. MgZn2(PO4)2:Eu3+ presents an reddish orange color with luminescence transitions from the 5D0 level of the Eu3+ ions. There exist two Eu2+-crystallographic sites in MgZn2(PO4)2:Eu2+, which occupy both Mg2+ and Zn2+ sites. However, there is only one Eu3+-site in the MgZn2(PO4)2:Eu3+ lattices, which was confirmed by site-selective excitation and emission spectroscopy, and the luminescence decay in the 5D0 → 7F0 region of the Eu3+ ions under a pulsed, tunable, narrowband dye laser. The Eu3+ ion was suggested to occupy the octahedral Mg2+-site in the MgZn2(PO4)2 lattices. The spectral characteristics, the temperature-dependent luminescence intensity and activation energies of thermal stabilities and the microstructures were discussed on the basis of the crystal structure and the luminescence results.

Mn2+-doped phosphates Na2CaMg1−xMnx(PO4)2 (x = 0.05–1.0) were prepared by conventional solid-state reaction. X-ray powder diffraction (XRD), the emission and excitation spectra, and decay measurements were employed to characterize the synthesized phosphors. The XRD patterns show that Na2CaMg1−xMnx(PO4)2 (x = 0.05–0.3) forms the single low temperature monoclinic phase α-Na2CaMg(PO4)2 with the crystal group of P21/c (No.14). The heavily Mn2+-doped Na2CaMg1−xMnx(PO4)2 (x = 0.6–1.0) crystallizes in the single high-temperature trigonal phase of β-Na2CaMg(PO4)2 with space-groupPm1 (No. 164). The members with x = 0.3–0.5 form a series of solid solutions containing two distinct phases, α- and β-Na2CaMg(PO4)2. The dependence of luminescence spectra on the Mn2+-doping concentration in Na2CaMg1−xMnx(PO4)2 (x = 0.05–1.0) was investigated. The great red-shift of Mn2+ emission with increasing Mn2+-concentration in Na2CaMg(PO4)2 were observed. The results are discussed in relation with the detailed crystal structure and the spectral analyses. The CIE coordinates and the luminescence decay (lifetimes) of Mn2+ ions were discussed in order to further investigate the potential applications.

Eu3+ doped red-emitting phosphors of double phosphates Ca9R(PO4)7 (R = Al, Lu) were synthesized by a general high temperature solid-state reaction. The phosphors were characterized by X-ray powder diffraction (XRD). The detailed luminescence properties, e.g., the emission spectra under the excitation of UV light, the photoluminescence excitation spectra and decay lifetimes were reported. The phosphors can be efficiently excited by near UV light to realize an intense red luminescence (613 nm) corresponding to the electric dipole transition 5D0 → 7F2 of Eu3+ ions. The luminescence properties and the potential applications were analyzed. These phosphors were investigated by the site-selective emission spectra and the fluorescence decay curves in the 5D0 → 7F0 region using a pulsed, tunable, narrowband dye laser. It is suggested that Eu3+ ions have three different crystallographic sites doped in Ca9Al(PO4)7, and five sites in Ca9Lu(PO4)7 host. The site assignments of Eu3+ ions in Ca9R(PO4)7 (R = Al, Lu) were discussed on the base of both optical spectroscopy results and structural analysis.