Intervalence Charge Transfer State Research Articles

Defect compensation and intervalence charge transfer state-based Pr3+-doped niobate antithermal quenching phosphors.

Herein, a scheme of Sr2+/Ca2+ ion substitution was employed to simultaneously regulate the defect and intervalence charge transfer (IVCT) state of Sr2-xCaxNb2O7:Pr3+ phosphors, resulting in a dual-modulation strategy for enhancing phosphor thermal stability.

Modulating the antithermal-quenching of praseodymium-activated phosphor by the position of intervalence charge transfer state

So far, overcoming thermal quenching remains a huge challenge in the application of phosphor materials. The intervalence charge transfer (IVCT) state has an inestimable role in the compensation of antithermal-quenching effects and is widely used in the research of applications in optical temperature sensing, yet little study has been reported on its modulation of thermal quenching properties. In this contribution, the thermal quenching properties of praseodymium-activated solid-solution substituted GdNb1-xTaxO4 phosphors were investigated systematically for the inaugural time from the perspective of IVCT energy level positions. According to the empirical formula, the IVCT level positions in the phosphor increase sequentially from 32960.71 cm−1 to 33442.15 cm−1, the DFT calculates the extension of the substrate band gap from 3.6408 eV to 4.3066 eV, and the contributing energy level of the conduction band is determined. The variation in the position of the IVCT energy level explains the alteration of the luminescence intensity at room temperature for different composition phosphor 3P0 and 1D2 energy levels. More particularly, it was analyzed that in the variable temperature spectra, x = 0–0.75 exhibited different degrees of efficient antithermal-quenching properties at 303–523 K, while no antithermal-quenching was presented at x = 1. By analyzing the IVCT energy level position and other influencing factors such as band gap, the feasibility of the IVCT energy level position as a competing model for the compensation and quenching channels is established, and the internal mechanism of regulating thermal quenching property is revealed. Effective control of the phosphor resistance to thermal quenching performance was achieved, which directly determines whether enhanced or diminished thermal quenching can be obtained. This finding provides a new effective strategy for the tuning of thermal quenching luminescence of Pr3+-doped Nb/Ta system phosphors.

Highly sensitive dual-mode thermometry over a wide temperature range based on bandgap engineering in LiNb1-xTaxO3:Tb3+

Having proved that highly sensitive dual-mode optical thermometry could be implemented based on fluorescence intensity ratio (FIR) of the same green emission driven by varying excitations and fluorescence lifetime (FL) of Tb3+, the feasibility to manage thermometric performance over a wide temperature range via bandgap engineering is validated in LiNb1-xTaxO3:Tb3+. The vital thermally activated relaxation processes via an intervalence charge transfer state are altered by formulating crystal structure via partial replacement of Nb5+ with Ta5+, allowing adjustment of the sensitive temperature range which can be measured with the highest accuracy. Specifically, maximum relative sensitivities SR up to 1.43 and 2.36 %K−1 are respectively attained in FIR and FL modes of LiNb1·0Ta0·0O3:Tb3+. The upper limit of the sensitive temperature range is proved to be greatly extended from 260 to 440 K for LiNb0·2Ta0·8O3:Tb3+. This work may provide a perspective approach to design self-calibrated thermometers targeted at different temperature ranges for varying applications.

Defect elimination to enhance photoluminescence and optical transparency of Pr-doped ceramics for self-calibrated temperature feedback windows

Pr-doped metal oxide polycrystalline transparent ceramics are highly desirable for photothermal window systems served in extreme environments, however, obtaining efficient photoluminescence together with high transparency in these ceramics is still posing serious challenges, which undoubtedly limits their applications. Here, Pr-doped Y₂Zr₂O₇ (YZO) transparent ceramics, as an illustrative example, are prepared by a solid-state reaction and vacuum sintering method. Owing to the elimination of defect clusters [ Pr_Y⁴⁺-O^2- - Pr_Y⁴⁺] and [Pr_Y⁴⁺ - e'] without introduction of impurities and additional defects, the fabricated YZO:Pr ceramics exhibit high transparency (74 %) and efficient photoluminescence (39-fold enhanced) after air annealing plus vacuum re-annealing treatment. Moreover, upon 295/450 nm excitation, the emission bands (blue, green, red, and dark red) from YZO:Pr ceramic present different temperature-dependent properties due to the thermal quenching channel generated by the intervalence charge transfer state (IVCT) between Pr³⁺ and Zr⁴⁺ ions. Furthermore, a self-calibrated temperature feedback window with same FIR model (I₆₁₃/I₅₀₃) under different excitation light sources (295 nm and 450 nm) is designed. The developed photothermal window operated in a wide temperature range (303 - 663 K) shows relatively high sensitivities (<em>S</em>_a and <em>S</em>_r reach 0.008 K^-1 at 663 K and 0.47 % K^-1 at 363 K, respectively), high repeatability (&gt; 98 %), and low temperature uncertainty (&lt; 3 K). This work presents a paradigm for achieving enhanced photoluminescence along with elevated transparency of lanthanides doped ceramics through vacuum re-annealing treatment engineering and demonstrates their promising potential for photothermal window systems.

Designing Color-tunable luminescent thermometers and information encryption based on Pr3+ or Sm3+ doped SrNb2O6: Tb3+ phosphors via energy transfer

Color-tunable luminescence intensity ratio thermometers and information encryption are firstly designed through different energy transfer mechanism. Herein, two types thermometric materials, SrNb2O6: Tb3+, Pr3+ and SrNb2O6: Tb3+, Sm3+ are prepared. The Tb3+ and Pr3+ typical emissions show luminescence mechanism between two intervalence charge transfer states. Whereas, Tb3+ and Sm3+ emissions are related to energy migration from intervalence charge transfer state to 4f level. Interestingly, (5D4→7F5)Tb3+/(1D2→3H4)Pr3+ or (5D4→7F5)Tb3+/(4G5/2→6H7/2)Sm3+ have different temperature sensing properties due to distinct energy transfer processes and provide visualization of temperature measurement. The former gains high sensitivity (Sa = 0.012 K−1) at high temperature region, and the latter reach as high as Sa = 0.028 K−1 at low temperature. Besides, using multi-mode color emission of Tb3+ or Pr3+ doped SrNb2O6 achieves information hiding. This work not only shows new strategy to develop high-efficiency luminescent thermometers but also develop flexible optical encryption technology.

La2Zr2O7:Pr3+ nanoparticles for luminescence thermometry based on a single parameter over a wide temperature range of 620 K

Luminescence thermometry based on the variation of the fluorescence intensity ratio (FIR) of rare-earth materials has become fascinating owing to their applicability in chemically and electromagnetically harsh environments. Relevant to their practical applicability, a wide temperature sensing range is urgently required for luminescent thermometric materials besides possessing a high relative sensitivity. Herein, we have demonstrated that pyrochlore La 2 Zr 2 O 7 :Pr 3+ (LZOP) nanoparticles (NPs) can serve as a promising optical temperature sensing material over a wide temperature range of 620 K. Specifically, we have taken advantage of the intervalence charge transfer state (IVCT) of Pr 3+ doping ion for luminescence temperature sensing and confirmed excellent optical thermometric performance from the LZOP NPs in a temperature sensing range of 85–705 K. The thermal sensing here could be measured by exploiting one thermometric parameter, i.e. the FIR between 1 D 2 → 3 H 4 and 3 P 0 → 3 H 4 transitions of Pr 3+ , and thus we only need to use one calibration formula for the whole temperature range. A maximum relative sensitivity of> 0.4%·K −1 from 165 to 205 K and a low temperature uncertainty of 1.21 K at 185 K is obtained. Based on the proposed configurational coordinate diagrams, A high-lying IVCT state was demonstrated to be the cause for slow thermal quenching at high temperatures, which broadened the working range of our thermometric sensing materials. This work provides a useful inspiration for exploring appropriate host materials with slow thermal-quenching channels to develop optical thermometric materials over a wide temperature sensing range. • Synthesized La 2 Zr 2 O 7 :0.1%Pr 3+ (LZOP) NPs by a low temperature molten salt method. • Demonstrated an excitation band at ~256 nm from the 4f-5d and IVCT transitions. • Confirmed their wide range thermal sensing potential using a single parameter.

(Digital Presentation) La2Zr2O7:Pr3+ Nanoparticles for Luminescence Thermometry Based on a Single Parameter

Luminescence thermometry based on the variation of the fluorescence intensity ratio (FIR) of rare-earth materials has become fascinating owing to their applicability in chemically and electromagnetically harsh environments. Relevant to their practical applicability, a wide temperature sensing range is urgently required for luminescent thermometric materials besides possessing a high relative sensitivity. Herein, we have demonstrated that pyrochlore La2Zr2O7:Pr3+ (LZOP) nanoparticles (NPs) can serve as a promising optical temperature sensing material over a wide temperature range of 620 K. Specifically, we have taken advantage of the intervalence charge transfer state (IVCT) of Pr3+ doping ion for luminescence temperature sensing and confirmed excellent optical thermometric performance from the LZOP NPs in a temperature sensing range of 85-705 K. The thermal sensing here could be measured by exploiting one thermometric parameter, i.e. the FIR between 1D2 → 3H4 and 3P0 → 3H4 transitions of Pr3+, and thus we only need to use one calibration formula for the whole temperature range. A maximum relative sensitivity of > 0.4%·K−1 from 165 to 205 K and a low temperature uncertainty of 1.21 K at 185 K is obtained. Based on the proposed configurational coordinate diagrams, a high-lying IVCT state was demonstrated to be the cause for slow thermal quenching at high temperatures, which broadened the working range of our thermometric sensing materials. This work provides a useful inspiration for exploring appropriate host materials with slow thermal-quenching channels to develop optical thermometric materials over a wide temperature sensing range. Figure 1

Novel optical temperature sensors based on the emission of the Pr3+ ions doped Ca3(M,Ga)5O12 (M5+ = Nb, Ta) garnet phosphors

Pr3+-doped Ca3(Nb,Ga)5O12 (Pr:CNGG) and Ca3(Ta,Ga)5O12 (Pr:CTGG) phosphors with garnet structure have been systematically investigated as optical thermometric materials for potential applications in the field of sensing technology, for the first time to our knowledge. The ceramic samples of Pr:CNGG and Pr:CTGG with different concentrations of Pr3+ ions have been obtained by the solid-state reaction method. A comparative analysis to evidence the influence of the position of the inter-valence charge transfer (IVCT) states on the excited-state dynamics of Pr3+ ions in CNGG and CTGG materials is presented. The optical thermometric properties of the Pr:CNGG and Pr:CTGG samples were studied. The luminescence thermal quenching mechanisms of the 3P0 and 1D2 levels were investigated in detail based on the luminescence intensity ratio (LIR) of 1D2→3H4 / 3P0→3H4 (I606/I487) transitions over an extended temperature range (10–700 K). The maximum relative sensitivities were determined to be 1.64% K−1 at 10 K for 7 at% Pr:CTGG and 2.4% K−1 at 430 K for 1 at% Pr:CNGG, thus proving their potential as high-sensitivity temperature sensors in cryogenic and high-temperature domains, respectively.

Intervalence charge transfer and charge transport in the spinel ferrite ferromagnetic semiconductor Ru-doped CoFe2O4

Inter-valence charge transfer (IVCT) is electron transfer between two metal $M$ sites differing in oxidation states through a bridging ligand: $M^{n+1} + M'^{m} \rightarrow M^{n} + M'^{m+1}$. It is considered that IVCT is related to the hopping probability of electron (or the electron mobility) in solids. Since controlling the conductivity of ferromagnetic semiconductors (FMSs) is a key subject for the development of spintronic device applications, the manipulation of the conductivity through IVCT may become a new approach of band engineering in FMSs. In Ru-doped cobalt ferrite CoFe$_2$O$_4$ (CFO) that shows ferrimagnetism and semiconducting transport properties, the reduction of the electric resistivity is attributed to both the carrier doping caused by the Ru substitution for Co and the increase of the carrier mobility due to hybridization between the wide Ru $4d$ and the Fe $3d$ orbitals. The latter is the so-called IVCT mechanism that is charge transfer between the mixed valence Fe$^{2+}$/Fe$^{3+}$ states facilitated by bridging Ru $4d$ orbital: Fe$^{2+}$ + Ru$^{4+}$ $\leftrightarrow$ Fe$^{3+}$ + Ru$^{3+}$. To elucidate the emergence of the IVCT state, we have conducted x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RPES) measurements on non-doped CFO and Co$_{0.5}$Ru$_{0.5}$Fe$_2$O$_4$ (CRFO) thin films. The observations of the XAS and RPES spectra indicate that the presence of the mixed valence Fe$^{2+}$/Fe$^{3+}$ state and the hybridization between the Fe $3d$ and Ru $4d$ states in the valence band. These results provide experimental evidence for the IVCT state in CRFO, demonstrating a novel mechanism that controls the electron mobility through hybridization between the $3d$ transition-metal cations with intervening $4d$ states.

Inner mechanism of Pr3+ luminescence thermometers based on the intervalence charge transfer state

Owing to their ultra-high temperature sensitivity, luminescence thermometers based on the intervalence charge transfer state of Pr3+ have drawn more and more attention. But the inner mechanism of Pr3+ luminescence thermometers, especially which parameters could affect the value of relative sensitivity, is still not fully revealed. In this paper, the luminescence thermometer properties of Ba1-xCaxTiO3:1%Pr3+ (x=0-0.7) with variation temperature have been systematically studied. The process of electrons in the 3P0 state passed through the intervalence charge-transfer state to the 1D2 state has been found to be sensitive to temperature variation. Through the intervalence charge transfer state engineering, we successfully realized the effective regulation of the intervalence charge transfer state energy position by replacing Ba2+ with Ca2+ and find that the effective energy gap between the 3P0 state and the intervalence charge transfer state is the key parameter that affects the relative sensitivity of luminescence thermometers. Moreover, the maximum relative sensitivity of Pr3+-doped BaTiO3, evaluated to be 2.26% K−1 at 413K under 450 nm excitation, is the one of the best performances among the Pr3+-based luminescence thermometers. This research could lead to the novel development of an efficient and high-performance optical thermometer.

Self-calibrated ratiometric thermometers and multi-mode anti-counterfeiting based on Ca2LaNbO6:Pr3+ optical material

Optical materials have been extensively used in thermometers and anti-countering for their non-contact, advantageous rapid, and high precision properties. Herein, a double-perovskite Ca2LaNbO6:Pr3+ color-tunable phosphor was synthesized. The emissions (blue, green, red1, and red2) from Ca2LaNbO6:Pr3+ phosphor have different temperature-dependent properties because of the thermal quenching channel generated by the intervalence charge transfer state between Pr3+ and Nb5+ ions. They can be used to realize four fluorescence intensity ratio models (Green/Blue, Green/Red2, Red1/Blue and Red1/Red2) to measure temperature with high absolute and relative sensitivities. Moreover, using the Ca2LaNbO6:Pr3+ phosphor and the carbon dots with green afterglow to create anti-counterfeiting patterns, the multi-mode anti-counterfeiting under different portable lamps (254 nm and 365 nm) was realized. Therefore, the prepared multifunctional phosphor indicated a potential application in both optical temperature sensing and anti-counterfeiting simultaneously.

A novel high-sensitive optical thermometer based on the multi-color emission in Pr3+ doped LiLaMgWO6 phosphors

Nowadays, optical thermometric materials with high sensing sensitivity in a wide temperature interval are urgently required due to the appealing prospect in the field of sensing technology. Herein, the double perovskite Pr3+: LiLaMgWO6 phosphors are successfully prepared via a high-temperature solid-state reaction method, and their concentration/temperature-dependent luminescence properties are discussed systematically. Upon UV excitation, the samples emit blue (3P0→3H4), green (3P0,1 → 3H5) and red emissions (1D2→3H4, 3P0→3F2) from Pr3+ ions. The different changing tendencies in relation to temperature are observed for the emissions from 3P0 and 1D2 multiplets, which are attributed to the effects of cross-over process to Pr3+-W6+ intervalence charge transfer (IVCT) state, cross relaxation process and multiphonon relaxation process. When the fluorescence intensity ratio (FIR) between 3P0→3F2 (654 nm) and 1D2→3H4 (616 nm) transitions is applied for temperature sensing, 0.5%Pr3+: LiLaMgWO6 displays a maximum absolute sensing sensitivity of 2421 × 10−4 K−1 at 573 K. Moreover, it can keep a comparatively high relative sensitivity (1%~3.25% K−1) and low temperature uncertainty (0.15–0.5 K) over a wide temperature range (~280 K), which are superior to those of many luminescence materials reported previously. The present study may provide useful design information to develop high-sensitive luminescent thermometers with a wide applicable temperature range.

Exploiting intervalence charge-transfer engineering to finely control (Ba, Sr)TiO3:Pr3+ luminescence thermometers

Most recently, it has been demonstrated that the use of 3P0–3H4/1D2-3H4 intensity ratio of Pr3+ transitions is potentially beneficial to luminescence thermometry. In this study, a study is conducted of (Ba1-xSrx)TiO3:0.5%:Pr3+ phosphors following Sr2+ concentrations range (x = 0–0.5) in terms of luminescence thermometry. Besides, an investigation is carried out into thermoluminescence properties, based on which a discussion is conducted about the potential mechanisms behind the experimental results. It is discovered from Pr3+-Ti4+ that the highly efficient thermometric characteristics contribute significantly to the intervalence charge transfer state. Furthermore, through intervalence charge-transfer engineering, the effective management of their thermometric properties is made achievable due to the Ba2+ substitution by Sr2+, which makes the range of the highest relative sensitivity controllable. In the meantime, the maximal relative temperature sensitivity of Pr3+-doped BaTiO3 reaches up to 2.77% K−1 at 371 K as measured. This is expected to shed light on the design of one optical thermometer calibrated itself, exhibiting great performance.

Mixed-Valence Lanthanide-Activated Phosphors: Invariance of the Intervalence Charge Transfer (IVCT) Absorption Onset across the Series

Recently, direct evidence was given for the existence of intervalence charge transfer (IVCT) states of Eu2+/Eu3+ pairs in Eu-doped fluorite hosts and their signature in absorption spectra was characterized [J. Phys. Chem. Lett. 2019, 10, 1851]. Here we show, by means of multiconfigurational ab initio calculations, that the IVCT absorptions of all 2+/3+ mixed valence lanthanides doped in CaF2 start basically at the same energy. This is rationalized with a simplified model of IVCT absorptions. Emissions above that energy are expected to be partly or totally quenched when 2+/3+ pairs are formed. On the basis of this finding and existing calculations, we report expected IVCT absorptions of mixed valence lanthanides doped in several fluoride, oxide, and sulfide hosts.

Pr3+ doped BaNb2O6 reddish orange emitting phosphor for solid state lighting and optical thermometry applications

A novel reddish-orange emitting BaNb2O6: Pr3+ phosphor was synthesized by conventional solid-state reaction method and its structural, morphological, optical properties and temperature dependent photoluminescence properties were investigated. The Le Bail profile matching of XRD patterns confirmed the orthorhombic phase presence and the corresponding cell parameters were calculated.Through a profound investigation of UV–visible diffuse reflectance spectra, a decrease in optical-band gap energy and an increase in Urbach tail width were noticed in doped sample as compared to undoped sample. Under the blue light excitation, the emission spectra exhibit peaks in reddish-orange region due to intra f-f transitions of Pr3+ ions. Strong covalence between Pr3+ and O2- was estimated by calculating nephelauxetic ratio, bonding parameter and covalency using the excitation spectra. The involvement of intervalence charge transfer (IVCT) state results in nonradiative relaxation increase, for which the emission peak intensities decreased with increase in temperature. The configurational coordinate model was used to demonstrate this phenomenon. The fluorescence intensity ratio technique was utilized for temperature sensing applications. The suitable CIE coordinates and significant temperature dependent PL makes the BaNb2O6: Pr3+ material to be a potential candidate for contributing red component in WLED and temperature sensing applications.

Non-stoichiometry in Ca2Al2SiO7 enabling mixed-valent europium toward ratiometric temperature sensing

Eu2+/Eu3+ mixed-valence couple co-doped material holds great potential for ratiometric temperature sensing owing to its different electronic configurations and electron-lattice interaction. Here, the correlation of nonstoichiometry in chemical composition, phase structures and luminescence propertis of Ca2Al2Si1−xO7:Eu is discussed, and controlled Eu2+/Eu3+ valence and tunable emission appear with decreasing Si content. It is found that the 2Ca2+ + Si4+ ↔ Eu2+ + Eu3+ + Al3+ cosubstitution accounts for the structural stability and charge balance mechanism. Benefiting from the diverse thermal dependent emission behaviors of Eu2+ and Eu3+, Ca2Al2Si1−xO7:Eu thermometer exhibits excellent temperature sensing performances with the maximum absolute and relative sensitivity being 0.024 K−1 (at 303 K) and 2.46% K−1 (at 443 K) and good signal discriminability. We propose that the emission quenching of Eu2+ is ascribed to 5d electrons depopulation through Eu2+/Eu3+ intervalence charge transfer state, while the quenching of Eu3+ comes from multi phonon relaxation. Our work demonstrates the potential of Ca2Al2Si1−xO7:Eu for noncontact optical thermometry, and also highlights mixed valence europium containing com pounds toward temperature sensing.

A ratiometric optical thermometer with multi-color emission and high sensitivity based on double perovskite LaMg0.402Nb0.598O3: Pr3+ thermochromic phosphors

Currently, non-contact fluorescence intensity ratio (FIR)-based luminescent thermometry has been extensively attracted great attention for its promising applications in electromagnetic field, micro-temperature field and thermally harsh environments. In this work, the double-perovskite LaMg0.402Nb0.598O3: Pr3+ (LMNO: Pr3+) thermometric phosphor is firstly designed and successfully synthesized via a high-temperature solid-state method. Under 450 nm excitation, the as-prepared samples simultaneously exhibit blue emission (3P0 → 3H4), green emission (3P1 → 3H4) and red emission (1D2 → 3H4, 3P0 → 3F2) of Pr3+. They present different dependence on the temperature due to the intervalence charge transfer state (IVCT). Accordingly, the four FIR models between 3P1 → 3H4 and 3P0 → 3H4 (G/B), 3P1 → 3H4 and 3P0 → 3F2 (G/R2), 1D2 → 3H4 and 3P0 → 3H4 (R1/B) and 1D2 → 3H4 and 3P0 → 3F2 (R1/R2) are used as temperature detecting signal in the range of 298–523 K, and the maximum absolute and relative sensitivity of LaMg0.402Nb0.598O3: 1.2% Pr3+ sample reached 0.0597 K−1 at 523 K and 0.7250% K−1 at 473 K, respectively. Excellent temperature sensing features are also demonstrated in the LaMg0.402Nb0.598O3: 0.3% Pr3+ and LaMg0.402Nb0.598O3: 2.0% Pr3+ samples. Except for high sensitivity for temperature sensing, the designed Pr3+-doped double-perovskite materials also realize the self-calibration by simultaneous monitoring of four models of FIR. Moreover, after five cycles, the relative luminescence intensity of LaMg0.402Nb0.598O3: 1.2% Pr3+ sample remains stable. These results indicate that LaMg0.402Nb0.598O3: Pr3+ phosphors have great promising application as self-calibrated optical temperature sensors.

High temperature sensitivity phosphor based on an old material: Red emitting H3BO3 flux assisted CaTiO3: Pr3+

High temperature sensitivity phosphor based on an old material red emitting CaTiO3: Pr3+ phosphor prepared via H3BO3 flux assisted solid-state reaction is reported. The typical sample possesses the orthorhombic perovskite phase and no impurity is found. The optical bandgap value is ~3.59 eV. There exists a single red emission with ideal color purity. The average long-afterglow decay time is 87.7s, the average trap depth is 0.76 eV. The maximum absolute sensitivity at 473 K is equal to ~0.0258 K-1 and the maximum relative sensitivity is estimated to be 7.69% K−1 at 298 K with the resolution of 0.26–0.65 K, indicating the suitability of the material for temperature sensitivity application. A possible mechanism for the quenching of 3Pj emission at room temperature and the thermal quenching process of 1D2 level through intervalence charge transfer state are proposed from the configurational coordinate diagram.

Color tuning and white light emission in a single-phased La2Ti2O7:Pr3+, Tb3+ phosphor

Color-tunable and white-emitting Pr3+ and Tb3+ co-doped La2Ti2O7 phosphors were prepared via a molten salt method. The as-synthesized samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflection spectra (DRS), photoluminescence spectra (PL), fluorescence decay curves and temperature-dependent photoluminescence spectra in detail. The La2Ti2O7:Pr3+, Tb3+ phosphors are uniform square-like nanobelt and the band gap of La2Ti2O7 host is calculated to be 3.83 eV by DRS. Under 296 nm wavelength UV excitation, the La2Ti2O7:Pr3+ compounds show strong red emission with peaking at 612 nm. Four typical emission lines of Tb3+ ions are observed from the region of 480–650 nm in La2Ti2O7:Tb3+ phosphors, and the strongest green emission peak is located at 545 nm. What's more, the emission color of La2Ti2O7:0.001Pr3+, 0.010 Tb3+ can be tuned from white (0.3274, 0.3379) to pink (0.4777, 0.2890) upon the excitation from 290 to 348 nm and the emission color of La2Ti2O7:0.001Pr3+, xTb3+ can be also tuned from white to green by changing the concentration of Tb3+. The La2Ti2O7:Pr3+, Tb3+ phosphors have the poor temperature dependence, resulting from the value of ΔR (the displacement between parabolas of the intervalence charge transfer states and ground states) for Tb3+ and Pr3+ both are larger. The results indicate that La2Ti2O7:Pr3+, Tb3+ phosphors could be a promising tunable and white light emission phosphor.

3P0 → 1D2 non-radiative relaxation control via IVCT state in Pr3+-doped Na2Ln2Ti3O10 (Ln=La, Gd) micro-crystals with triple-layered perovskite structure

A comparative study on luminescence properties and their nature in relation to different substitutions at the rare-earth site of Pr3+-doped layered perovskite Na2Ln2Ti3O10 (Ln = La, Gd) micro-crystals has been reported. At room temperature, the sample for Ln = La exhibits intense greenish-blue (3P0 → 3H4) and red (1D2 → 3H4) emissions under bandgap excitation, whereas for Ln = Gd the red emission can be only observed and all 3P0-related emissions were completely quenched. It turns out that 3P0→ 1D2 nonradiative relaxation in Na2Ln2Ti3O10 (Ln = La, Gd) critically depends on the energy location of Pr3+-Ti4+ intervalence charge transfer state and thus on the location of Pr3+ ground state 3H4 with respect to the top of the valence band. Temperature-dependent photoluminescence spectra in the 4.5–300 K range reveals a significant increase of Pr3+ luminescence, which is ascribed to an efficient thermally-activated energy transfer process from host to Pr3+ ions. Lower energy of self-trapped exciton state relative to the bottom of conduction band for Ln = Gd is responsible for thermoluminescence observed at higher temperatures than that for Ln = La sample. In the case of Ruddlesden-Popper type layered perovskite oxide compounds our results show a possibility of controlling the 3P0 → 1D2 nonradiative relaxation of Pr3+ ions and the energy distance of the relatively shallow traps in relation to the bottom of conduction band, giving a way for specific band-gap engineering in these materials.