Crystal Field Sites Research Articles

Tunable afterglow Ca1 - xSrxAl2O4:Eu2+, Nd3+ phosphors with dependence of an excitation wavelength.

In the emerging field of high-capacity information encryption, multicolor, multitemporal, and multimodal luminescence inorganic materials are of great significance. However, conventional inorganic materials lack the flexibility to dynamically adjust the photon transition path, resulting in unicolor luminescence of the sample and reducing the reading and decoding levels. Herein, we elaborately designed the components for constructing dual-phase crystal fields for Eu2+ in phosphors based on a high temperature solid-state method. Specifically, SrAl2O4:Eu2+ crystal with a bright green afterglow and CaAl2O4:Eu2+ crystal with a blue afterglow were obtained in phosphors at the same time. As a result, a tunable afterglow behavior from blue to white was achieved due to the 4f65d1 → 4f7 transition of Eu2+ at different crystal field sites. Finally, the color tunable afterglow sample was used to explore the encryption and decryption processes of information, and the results showed that the prepared material has a good anti-counterfeiting performance, which is promising for the development of long persistent luminescent materials.

Spectral Broadening in Cr3+-Doped Sr0.92Mg0.91Al10.1O17 NIR Phosphor Realized by Multi-crystallographic Site Occupation

Near-infrared phosphor-converted light-emitting diodes (NIR pc-LEDs) play an important role in nondestructive testing, medical imaging, food analysis, and so on. Broadband NIR phosphors are needed in order to cover the characteristic wavelengths of various matters. In this paper, a NIR phosphor, Sr0.92Mg0.91Al10.1O17 (SMAO):Cr3+, with a full width at half maximum (FWHM) of 12–102 nm is achieved as a function of Cr3+ concentration. Two Cr3+ sites are observed by means of excitation spectra and fluorescence decay curves. The spectral broadening with the increase of Cr3+ concentration is attributed to the increase of occupation ratio on the weak crystal field site by Cr3+. In addition, the difference in thermal stability between 2E and 4T2 is employed to conduct the temperature measurement with a relative sensitivity of 0.43% K–1 at 270 K. A NIR pc-LED is fabricated using SMAO:0.03Cr3+ phosphor combined with a 395 nm LED chip with radiation flux of 2.2 mW@10 mA and 15 mW@100 mA.

Recent Advances in Chromium‐Doped Near‐Infrared Luminescent Materials: Fundamentals, Optimization Strategies, and Applications

AbstractDevelopment of chromium‐doped luminescent materials is pertinent to many emerging applications, ranging from agriculture, food industry to noninvasive health monitoring. The fundamental importance of chromium‐activated luminescent materials in the field of optics and biomedicine makes the rapid development of novel materials and relevant applications. Herein, the recent advances on the luminescence principle and photoluminescence (PL) optimization for Cr3+‐activated luminescent materials together with their potential applications are reviewed. The different types of most recently developed Cr3+‐doped luminescent materials and the design principles are systematically summarized. The associations between crystal structure and near‐infrared (NIR) PL properties, as well as performance‐evaluating parameters are introduced with the examples of known NIR emitting phosphors, which will be helpful to explore future NIR luminescent materials. Based on crystal field control, site engineering, and electron–phonon coupling, several efficient strategies for optimizing luminescence performances including bandwidth, thermal stability, and quantum efficiency of Cr3+‐doped NIR luminescent materials are proposed. Then, potential applications in the fields of food analysis, night vision, information encryption, and optical sensors are surveyed. Finally, the challenges of promising Cr3+‐doped luminescent materials are proposed.

Activators lattice migration strategy customized for tunable luminescence of Ce3+ doped β-Ca3(PO4)2

The luminescent properties of phosphors are determined by the electronic configuration of their activators and the crystal environment in which they are embedded. By manipulating the site-selective occupancy of activators, we can coordinately control the local crystal field and electron cloud distribution to master the movement of emission wavelength and obtain a color-tunable phosphor. The system of β-Ca3(PO4)2 is an excellent carrier for crystal-site engineering to modify luminescent performance. It has five distinct crystal sites and a strong photoluminescence response. However, due to low doping concentration and structural complexity, the specific occupation of Ce remains unknown. We discussed the specific position of Ce in this study and proposed a novel activators lattice migration strategy for controlling the relative strength of crystal field splitting (CFS) and the nephelauxetic effect (NE) to control luminescence properties. By combining X-ray Absorption Fine Structure (XAFS) with DFT calculation, we predicted the local environment of each crystal lattice and migrated Ce3+ from M(3) to a strengthened crystal field site with a lower coordination number, shorter bonding bond length, and greater polyhedral distortion, M(5), resulting in phosphors with tunable luminescence emission. Relevant parameters ΔD(Ce3+) were identified for dealing with interference caused by the electronegativity difference during a structural modification. Some potential applications emerged as a supplement and an intriguing QR-code and response program for epidemic prevention were demonstrated. This structure–activity relationship-based strategy provides new inspiration for designing luminescent materials with tunable emission and a broader range of applications.

A broadband near‐infrared Sc1−x(PO3)3:XCr3+ phosphor with enhanced thermal stability and quantum yield by Yb3+ codoping

AbstractBroadband near‐infrared (NIR) phosphors converted light‐emitting diodes (pc‐LEDs) have attracted tremendous attention for their great potential in NIR spectroscopy applications. Herein, we report on the photoluminescence (PL) properties of Cr3+‐doped Sc(PO3)3, which exhibits a broadband NIR emission centered at 900 nm with a full width at half‐maximum (FWHM) of 161 nm upon excitation at 480 nm. In terms of the examination of spectroscopic parameters, we find that Cr3+ ions occupy a weak crystal field site (Dq/B = 1.94) in the Sc(PO3)3 host with a stronger electron–phonon coupling (Huang‐Rhys factor, S = 5.5), which results in a serious thermal quenching and a low internal quantum efficiency (IQE). Thermal stability and IQE of phosphors can be substantially enhanced by the introduction of Yb3+ ions. In the light of the analysis of excited‐state dynamics, we demonstrate that the enhancement mechanism is ascribed to the efficient Cr3+→Yb3+ energy transfer from the thermal sensitive Cr3+ centers to the thermal stable Yb3+ emitters. An NIR pc‐LED device has been finally fabricated by the combination of a blue‐emitting chip and a Cr3+/Yb3+ codoped Sc(PO3)3 phosphor whose potential application for broadband NIR pc‐LEDs is also discussed.

Sensitization Mechanism of Alumina Aerogels: Introducing Gd3+ as a Sensitizing Ion to Enhance the Luminescence Intensity of Tb3+

AbstractTo enhance the luminous intensity of phosphors containing Tb3+, Gd3+ was used as a sensitizing ion to recombine with Tb3+. Gd3+ and Tb3+ ions were loaded on the Al2O3 aerogel matrix with nano porous structure via a simple and facile sol‐gel method. After vacuum drying and calcination, Al2O3: Tb3+, Gd3+ composite aerogel materials were obtained. Al2O3 aerogel matrix has three‐dimensional nanostructure and good chemical stability, which can provide stable crystal field and emission site for the rare earth activated ions. The results indicated that the introduction of Gd3+ can effectively enhance the luminescence intensity of Al2O3: Tb3+ at 543 nm under 260 nm UV excitation. The optimal ratio of Gd3+ and Tb3+ was determined to be 1: 1. The mechanism of Gd3+ sensitization to Tb3+ can be considered as electric multipole resonance transmission. We expected that this work could provide guidance for the development of enhancing the luminous intensity of green phosphors for lamp.

Racah Parameter and Nephelauxetic Effect on Er<sup>3+</sup>/Nd<sup>3+</sup> Codoped Lithium Niobate Tellurite Glass

The modification of absorption characteristics in rare-earth doped tellurite glasses is important in photonics application. The Er3+/Nd3+ doped glasses of the form (69-x)TeO2-15Li2CO3-15Nb2O5-1Er2O3-(x)Nd2O3 with x= 0.4, 0.8 and 1.0 mol % are successfully made by using conventional melt-quenching technique. Physical properties such as polaron radius, interionic distance, field strength and average distance between acceptors and donors are determined. The absorption spectra reveal eleven prominent peaks centered at 6535.9 cm-1, 10245.9 cm-1, 11415.5 cm-1, 12437.8 cm-1, 13369.0 cm-1, 15267.2 cm-1, 15949.2 cm-1, 17127.1 cm-1, 19084.0 cm-1, 20449.9 cm-1 and 22271.7 cm-1 which corresponding to the transitions from ground state to various states of Nd3+ ions. The crystal field site increases from 8.120 to 8.169 cm-1 due to increase in contraction of Nd3+-ligand. The nephelauxetic function, increase 0.104 to 0.107. The increasing in value describes, the increasing in ionic bond from-0.0004 to-0.0011. The increment in exhibits the increase in the number of effective positive charge connected to neodymium atom and attracts the electrons from ligands as it increases from 1.0004 to 1.0011. These present glass shows the potential candidate of broadband amplifiers and lasers as it is comparable with other Nd3+ doped another host.

Influence of BaO on spectral studies of Cr2O3 doped titanium-boro-tellurite glasses

Abstract Barium-titanium-boro-tellurite glasses containing the transition metal (TM) Cr3+ ions with different BaO mol% were prepared using the melt quenching method. From the XRD, SEM and EDS studies, it was confirmed that the prepared glasses were impurity-free and their atomic arrangement is non-crystalline in nature. The UV–visible spectra exhibited a dominant high absorption band corresponding to the enrgy of ~15974 cm−1 and three weak low-intensity bands around 23641 cm−1, 14881 cm−1 and 14306 cm−1. The identified bands are assigned to 4A2g→4T2g (F), 4A2g→4T1g (F), 4A2g→2T1g (G) and 4A2g→2Eg (G) transitions respectively. The optical band gap values are increasing while the refractive index values are decreasing with increasing BaO content. The crystal-field parameter Dq values and Racah parameter (B) values were found to change non-linearly with the barium oxide content which suggests a change in the environment around Cr3+ ions. From the value of Dq/B, it can be concluded that the Cr3+ ions are localized in the weak crystal field sites. The EPR spectra which were recorded at X-band frequencies exhibited four resonance signals at g 1 ≈ 1.97, g 2 ≈ 2.38, g 3 ≈ 4.25 and g 4 ≈ 5.23. The theoretical g-values were evaluated from the Macfarlane's perturbation formulae. The FTIR and Raman spectra show broad peaks which are the characteristic features of the prepared glass component vibrating modes.

Preparation and Spectroscopic Studies of Cr3+-doped Aluminium Calcium Sodium Borate Glasses

Abstract Aluminium calcium sodium borate glasses doped with different concentrations of chromium(III) oxide have been prepared by melt-quenching technique with chemical compositions of 20Al2O3:20CaO:20Na2O:(40-x)B2O3:xCr2O3 where (x = 0.00, 0.01, 0.02, 0.03, 0.04 and 0.05 mol%). All Cr3+-doped glass samples show clear and transparent with green color center. The densities of the Cr3+-doped samples are around 2.48 to 2.49 g/cm3 while the refractive indexes are around 1.54 to 1.55. The optical absorption spectra show the absorption band at around 604 nm corresponding to the 4A2g→4T2g transition of Cr3+ ions. The excitation spectra consists of two broad bands at around 423 and 567 nm related to the transitions from the ground state 4A2g to the excited states 4T1g and 4T2g, respectively. The emission spectra, excited with 567 nm excitation wavelength, show the band at 690 nm related to the 2Eg→4A2g transition. The emitted light determined by the CIE 1931 chromaticity diagram is shown in red region. The calculated value of Dq/B = 2.28 indicates that the Cr3+ ions in the glass samples are center in intermediate crystal field site which lies between strong and weak crystal field.

CO2‐induced chemo‐mechanical alteration in reservoir rocks assessed via batch reaction experiments and scratch testing

AbstractThe injection of carbon dioxide (CO2) into geological formations results in a chemical re‐equilibration between the mineral assemblage and the pore fluid, with ensuing mineral dissolution and re‐precipitation. Hence, target rock formations may exhibit changes of mechanical and petrophysical properties due to CO2 exposure. We conducted batch reaction experiments with Entrada Sandstone and Summerville Siltstone exposed to de‐ionized water and synthetic brine under reservoir pressure (9–10 MPa) and temperature (80°C) for up to four weeks. Samples originate from the Crystal Geyser field site, where a naturally occurring CO2 seepage alters portions of these geologic formations. We conducted micro‐scratch tests on rock samples without alteration, altered under laboratory conditions, and naturally altered over geologic time. Scratch toughness and hardness decrease as a function of exposure time and water salinity up to 52% in the case of Entrada and 87% in the case of Summerville after CO2‐induced alteration in the laboratory. Imaging of altered cores with SEM‐EDS and X‐ray microCT methods show dissolution of carbonate and silica cements and matrix accompanied by minor dissolution of Fe‐oxides, clays, and other silicates. Parallel experiments using powdered samples confirm that dissolution of carbonate and silica are the primary reactions. The batch reaction experiments in the autoclave utilize a high fluid to rock volume ratio and represent an end member of possible alteration associated with CO2 storage systems. These types of tests serve as a pre‐screening tool to identify the susceptibility of rock facies to CO2‐related chemical‐mechanical alteration during long‐term CO2 storage. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.

Stress- and Temperature-Dependent Wideband Fluorescence of a Phosphor Composite for Sensing Applications

As both stress and temperature can alter the lattice of a solid phosphor in a similar way, the dopant rare-earth ion may be used as a probe for either parameter using the same sensing mechanism. Here, a wideband phosphor, YAG:Ce is used as a micro stress sensor at very low magnitudes of compressive stress. The sensing resolution can be smaller than 0.06 N/mm2 in the range of 0 ~ 1.2 N/mm2 with a sensitivity of 11 cm−1/MPa. The sensitivity is about a hundred times larger than the coefficient reported under hydrostatic pressure. The barycenter energy of the emission band of a phosphor can be used to indicate stress and/or temperature change, producing a precision beyond the spectroscopic resolution limit. The sensing functions obtained from the barycenter shift are superior to those from peak shifts. The stress/temperature dependent barycenter shift of the observed fluorescence bands of YAG:Ce were similar in magnitude. Mechanisms involving the nephelauxetic effect, crystal field effect and site symmetry of dopant ions are explored as explanations of the experimental phenomena.

Luminescence and other spectroscopic properties of purple and green Cr-clinochlore

For the first time ever, the luminescence spectra of Cr3+ centers in two chlorite crystals are presented. Chromium ions occupy the strong crystal-field site M4 in the brucite sheet and the intermediate crystal-field site in the inner octahedral sheet for purple and green chlorite, respectively. We discuss the influence of an effective positive charge on the Cr3+ ion and an effective negative charge of ligands on the differences in the values of the Dq and B parameters. It is concluded that the presence of Fe2+ ions and other point defects, as well as concentration quenching, causes the very short luminescence lifetimes of chromium ions.

Growth and spectroscopic characteristics of Cr3+:KSc(WO4)2 Crystal

The Cr3+:KSc(WO4)2 crystal was grown by top seed solution method. The absorption and luminescence spectra were investigated. The absorption cross section σa is 1.16×10−19cm−2 at 469nm for the 4A2→4T1 transition and 4.98×10−20cm−2 at 684nm for the 4A2→4T2 transition, respectively. The emission cross section σe of 4T2→4A2 transition is 4.86×10−20cm2 at 916nm with FWHM of 200nm and fluorescence lifetime is 19.83μs. Based on the absorption and emission spectra, the crystal field strength Dq, the Racah parameters B and C, the effective phonon energy ħω and the Huang-Rhys factor S were calculated. Cr3+ ions in KSc(WO4)2 crystal occupy the weak crystal field sites and have stronger coupling to crystal lattice.

Spontaneous and stimulated emission spectroscopy of a Nd(3+)-doped phosphate glass under wavelength selective pumping.

The influence of the host matrix on the spectroscopic and laser properties of Nd(3+) in a K-Ba-Al phosphate glass has been investigated as a function of rare-earth concentration. Site-selective time resolved laser spectroscopy and stimulated emission experiments under selective wavelength laser pumping show the existence of a very complex crystal field site distribution of Nd(3+) ions in this glass. The peak of the broad stimulated (4)F(3/2)→(4)I(11/2) emission shifts in a non monotonous way up to 3 nm as a function of the excitation wavelength. This behavior can be explained by the relatively moderate inter-site energy transfer among Nd(3+) ions found in this system and measured by using fluorescence line narrowing spectroscopy. The best slope efficiency obtained for the laser emission was 40%.

Growth, Structure, and Optical Properties of the Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 Crystal

The Cr3+-doped K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal was grown from a flux of K2Mo3O10 by the top seeded solution growth method. The K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal crystallizes in the rhombohedral system in space group R3c̅. The K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal has a highly disordered structure. Investigation of the spectral properties of the Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal shows that it exhibits broad absorption and emission bands, which are caused by the disordered structure of the K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal, except for the broad emission of Cr3+ ions. On the basis of the absorption and emission spectra, the crystal field strength Dq/B and Racah parameter C have been calculated. The Dq/B value of 2.18 implies that Cr3+ ion occupies the low crystal field site in the Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal. These results suggest that the Cr3+:K0.6(Mg0.3Sc0.7)2(MoO4)3 crystal is a potential tunable laser crystal material.

Growth and spectral properties of Cr3+/RbAl(MoO4)2 crystal

The Cr3+/RbAl(MoO4)2 crystal was grown by top seeding solution growth method, and its thermal and spectral properties were investigated. Based on the absorption and photoluminescence spectra, the crystal field strength Dq, the Racah parameters B and C, the effective phonon energy ħω and the Huang–Rhys factor S were calculated. The results of spectroscopic characterisation show that the Cr3+ ions occupy medium crystal field sites in Cr3+/RbAl(MoO4)2 and Cr3+/RbAl(MoO4)2 crystals may be a potential candidate for tunable laser applications.

Luminescence Properties and Local Structures of Eu3 + in EuBaB9O16 Crystal

The red phosphor was prepared by the conventional solid-state reaction method. The site-selective excitation and emission spectra and the luminescence decay curves have been investigated by using a pulsed, tunable, narrowband dye laser in the region in crystals at 10 and 300 K. Non site-selective emission spectra under UV excitation were also performed for comparison. The study focused on the crystallographic distribution and local symmetry of the sites in . The excitation spectra corresponding to the transition show that there are at least three different inequivalent crystallographic site distributions in lattices. From the analysis by site-selective spectroscopy, the rare-earth ions substitute for the crystal-field sites with the hexagonal or symmetry in the lattices. The influence of local structures of the sites relates with the excellent thermal stability of the luminescence of .

Influence of MgO codoping on the optical properties of Er3+-doped near-stoichiometric LiNbO3

Four near-stoichiometric lithium niobate (NSLN) crystals codoped with Er3+ (1 mol%) and MgO (0, 0.5, 1.0, and 2.0 mol%) were grown from K2O-based flux in air using top seeded solution growth technique. The [Li]/[Nb] ratio, estimated from the blueshift of ultraviolet absorption edge, is 97.2% in NSLN:Er. MgO; codoping can increase the segregation coefficient of Er3+ in NSLN:Er:MgO crystal. The photorefractive damage threshold is enhanced by three orders of magnitude for NSLN:Er codoped with 1 mol% MgO, it coincides with the peak shift of OH− absorption spectrum from 3481 to 3535 cm−1. Judd–Ofelt theory based on absorption spectra is used to analyze the influence of MgO concentration on the Judd–Ofelt intensity parameter, transition strength, fluorescence branching ratio, and stimulated emission cross section. From the time-resolved emission spectra and the comparison among emission spectra, two Er3+ crystal-field sites are ascertained in NSLN:Er codoped with 2 mol% MgO, this coincides with the bimodal structure in X-ray photoelectron spectrometry spectra. The upconversion processes under pulse excitation is proposed based on the pump energy dependence and decay kinetics. The distribution of Er3+-clustered sites in NSLN:Er:MgO series is discussed based on the nonexponential decay curves monitored at 550 nm under two-photon excitation.

Broadband laser tunability of Nd^3+ ions in 08CaSiO_3-02Ca_3(PO_4)_2 eutectic glass

Site-selective spectroscopy and stimulated emission experiments performed in the (4)F(3/2)-->(4)I(11/2) laser transition of Nd(3+)-doped 0.8CaSiO(3-) 0.2Ca(3)(PO(4))(2) eutectic glass are presented. The spectral features of the excitation spectra and those of spontaneous and stimulated emissions reveal the existence of a very complex crystal field site distribution for Nd(3+) ions. As a consequence, the stimulated emission of Nd(3+) in this glass shows a tunability of about 10 nm as a function of excitation wavelength.

Spectroscopic characteristic and energy levels of Cr 3+ in Cr 3+:KAl(MoO 4) 2 crystal

This paper reports polarized spectral properties and energy levels of Cr 3+ in KAl(MoO 4) 2 crystal. The absorption and emission cross sections are estimated as 3.72×10 –20 cm 2 at 669 nm and 2.74×10 –20 cm –2 at 823 nm for σ -polarization, respectively. The energy levels of Cr 3+ ion in KAl(MoO 4) 2 crystal were calculated based on the Tanabe–Sugano theory. It is suggested that Cr 3+ ions occupy at an intermediate crystal field site in Cr 3+:KAl(MoO 4) 2.