A clear effect of charge compensation through Na+ co-doping on the luminescence spectra and decay kinetics of Nd3+-doped CaAl4O7

doi:10.1016/j.jssc.2016.03.031

Journal of Solid State Chemistry

Volume 238, June 2016, Pages 259-266

https://doi.org/10.1016/j.jssc.2016.03.031 Get rights and content

Abstract

We present a detailed analysis of luminescence behavior of singly Nd³⁺ doped and Nd³⁺, Na⁺ co-doped calcium aluminates powders: Ca_1−xNd_xAl₄O₇ and Ca_1−2xNd_xNa_xAl₄O₇ (x=0.001–0.1). Relatively intense Nd³⁺ luminescence in IR region corresponding to typical ⁴F_3/2→⁴I_J (J=9/2–13/2) transitions with maximum located at about 1079 nm was obtained in all samples on direct excitation into f–f levels. The effect of dopant concentration and charge compensation by co-doping with Na⁺ ions on morphology and optical properties were studied. The results show that both, the Nd³⁺ concentration and the alkali metal co-doping affected the optical properties but had no influence on the powders morphology. The studies of luminescence spectra (298 and 77 K) in a function of dopant concentration showed an increasing distortion of the local symmetry of Nd³⁺with raising activator content due to certain defects created in the crystal lattice. On the other hand Na⁺ addition led to significant narrowing of absorption and luminescence bands and also a reduction of the number of their components, showing smaller disturbance of Nd³⁺ ions local symmetries. Consequently, charge compensated by Na⁺ co-doping materials showed significantly enhanced Nd³⁺ luminescence. The decrease of emission intensity and luminescence lifetimes with increase of activator concentration was attributed mainly to phonon-assisted cross-relaxation processes between Nd³⁺ ions. Analysis with Inokuti–Hirayama model indicated dipole–dipole mechanism of ion-ion interaction. Na⁺ addition led to much smaller concentration quenching due to smaller clustering of dopant ions in CaAl₄O₇ lattice.

Graphical abstract

Introduction

Calcium aluminates have been of considerable interest for possible applications as efficient phosphors, materials of construction, catalyst support, components in bioceramics as well as model systems for the study of crystallization phenomena in amorphous precursors [1], [2], [3], [4], [5]. Various crystalline phases: CaAl₂O₄, CaAl₄O₇, Ca₃Al₂O_6, CaAl₁₂O₁₉, Ca₁₂Al₁₄O₃₃ exist in the mixed oxide CaO-Al₂O₃ system. All these compounds were used as host materials for rare earth doped phosphors because they offer a high optical transparency from UV to NIR spectral range as well excellent mechanical and thermal properties combined with chemical inertness [2], [3], [4], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Among them CaAl₂O₄ with spinel structure co-doped with Eu²⁺and Nd³⁺ (Dy³⁺) is well known commercial phosphor showing the longest persistent luminescence attractive for applications in optical storage media and field emission dispalys [9].

CaAl₄O₇ has monoclinic crystal structure with a C2/c space group. Ca²⁺ion occupies single crystallographic site of C₂ (pseudo-C_2v) symmetry surrounded by five oxygen ions, whereas Al³⁺ions are distributed over two independent tetrahedral sites, whose surroundings are significantly distorted giving C₁ local symmetries [16], [17]. The interesting unique feature of CaAl₄O₇ structure is the existence of so-called “triclusters” consisting of an oxygen atom surrounded by three Al tetrahedra. Studies on luminescence properties of Eu³⁺ doped and Eu³⁺, Na⁺ co-doped CaAl₄O₇ proved that lanthanide ion substitute Ca²⁺ion in the crystal lattice [18]. It is known that when trivalent lanthanide ion is incorporated in place of divalent Ca ion vacancies, interstitials, impurities and other trapping defects are often formed in order to attain charge balance. In materials, where a certain defects occur with some regularity in the crystal lattice, only a small number of optical centers may be so perturbed that their optical transition frequencies are sufficiently different from those of majority and as a consequence a distinct sites can be identified. In the case of random distribution of defects, where perturbative effects of defect decreases with distance rather small perturbations are most probable and statistical distribution of rare earth–defects distance produces inhomogeneously broadened lineshapes [19]. Both above effects can lead to the reduction in the luminescence intensity. On the other hand, it is well known that addition of alkali metal ions as charge compensators may decrease the perturbative effect of the defects and consequently increase the luminescent efficiency of phosphors [20], [21], [22].

Until now, only scant studies have been performed on luminescence properties of Nd³⁺-doped CaAl₄O₇ having grossite monoclinic structure. In the early 70s Lindop et. al. reported optical properties of single crystal of 1%-doped CaAl₄O₇. They found stimulated emission cross section for Nd³⁺ to be comparable with the value for YAG [23], [24].

In this work, we report on the studies on singly (Nd) and doubly (Nd, Na) doped CaAl₄O₇ powder materials. The effects of activator concentration and charge compensation by means of Na⁺ ions on morphology, luminescence and absorption spectra as well as decay kinetics of the powders are presented and discussed.

Section snippets

Experimental

Powder materials of Nd³⁺-doped and Nd³⁺–Na⁺ co-doped calcium aluminates: CaAl₄O₇:Nd(x%) and CaAl₄O₇:Nd(x%), Na(x%) (x=0.001–0.1) were prepared via modified Pechini citrate process, according to the procedure described in [18]. The metal nitrates: Al(NO₃)₃·9H₂O, Ca(NO₃)₂·4H₂O, Nd(NO₃)₃·6H₂O, NaNO₃, citric acid (CA) and ethylene glycol (EG) were used as starting materials. First, the required amount of CA was dissolved in EG, followed by the addition of stoichiometric amounts of Al(NO₃)₃·9H₂O,

Structural and morphological characterization

X-ray powder diffraction technique was used to check the crystallographic purity of all studied powders. The obtained XRD patterns exhibit a very similar profiles and match well the grossite monoclinic phase of calcium aluminate with the C2/c space group (ICSD file 14270) [16], [17]. The measured diffractograms for all singly Nd³⁺ doped and Nd³⁺, Na⁺ co-doped CaAl₄O₇ are presented in Fig. 1. In the grossite structure, Ca²⁺ion occupies one coordinate site of C₂ (pseudo-C_2v) symmetry surrounded

Conclusions

Grossite monoclinic phases of Nd³⁺ doped and Nd³⁺, Na⁺ co-doped CaAl₄O₇ were successfully prepared by modified Pechini method at 950 °C. Under 585 nm excitation both types of powders exhibited relatively strong luminescence in IR region corresponding to the ⁴F_3/2→⁴I_J (J=9/2–13/2) transition of Nd³⁺ with the dominant component around 1079 nm. Detailed analysis of the absorption and emission spectra of CaAl₄O₇:Nd³⁺ has shown an increasing distortion of the local symmetry of Nd³⁺ with raising dopant

Acknowledgment

This work was supported by POIG.01.01.02-02-006/09 project co-funded by European Regional Development Fund within the Innovative Economy Program. Priority I, Activity 1.1. Sub-activity 1.1.2, which is gratefully acknowledged.

References (30)

S. Jonas et al.
Ceram. Int.
(1999)
T. Aitasalo et al.
Opt. Mater.
(2004)
D. Jia
J. Lumin.
(2006)
M. Puchalska et al.
J. Lumin.
(2013)
A. Suresh Kumar et al.
Spectrochim. Acta A: Mol. Biomol. Spectrsc.
(2015)
J. McKittrick et al.
Displays
(1999)
Xiaorui Gao et al.
J. Solid State Chem.
(2008)
Buhao Zhang et al.
J. Solid State Chem.
(2014)
M. Puchalska et al.
Opt. Mater.
(2010)
A.B. Gawande et al.
Opt. Mater.
(2014)

M. Puchalska et al.

J. Lumin.

(2012)

A. Khanna et al.

J. Solid State Chem.

(2015)

M. Sobczyk

J. Quant. Spectrosc. Radiat. Transf.

(2013)

Renping Cao et al.

J. Solid State Chem.

(2014)

M. Sobczyk

Opt. Mater.

(2013)

Cited by (13)

Low-temperature optical thermometer based on the luminescence of the double perovskite Ba<inf>2</inf>MgWO<inf>6</inf>: Nd3+
2023, Journal of Luminescence
Here, we report Nd³⁺ - doped Ba₂MgWO₆ (BMW) as a potential remote luminescent thermometer, working over a wide operating temperature range up to 673 K. The crystal structure, morphology and luminescence of samples were characterized. The lifetime of Nd³⁺ ⁴F_3/2 decreases from 0.55 ms to 0.46 ms when the content of Nd³⁺ increases caused by the cross-relaxation. The temperature sensing ability of Ba₂MgWO₆ was evaluated for the first time by applying the thermally coupled Nd³⁺ levels as well as based on the emissions ratio of Nd³⁺ ions and the host. An impressive maximum of relative sensitivity (S_r = 4.46%K⁻¹ at 198 K) with very small uncertainty ( $δ T = 0.005 K$ ) working in 77–248 K was achieved for the sample BMW: 4%Nd³⁺. The results promote the BMW: Nd³⁺ double perovskite as a promising optical thermometer that can operate in a wide operating temperature, especially in the low – temperature range.
Influence of temperature on near-infrared luminescence, energy transfer mechanism and the temperature sensing ability of La<inf>2</inf>MgTiO<inf>6</inf>: Nd3+ double perovskites
2021, Sensors and Actuators, A: Physical
Citation Excerpt :
It is over two orders of magnitude shorter than the decay time of Nd3+ and decreases slightly with increasing Nd3+ content. Generally, at the low temperature, the decays are slower and close to single – exponential than those obtained at room temperature similarly as was found in the case of CaAl4O7:Nd3+ [34]. However, an opposite phenomenon at which the 300 K decay time (red circles) is longer than the one at 77 K (black squares) (see Fig. 7b) was observed in this work and also had been reported in some earlier publications [22,24,26].
La₂MgTiO₆: Nd³⁺ samples synthesized by the coprecipitation method crystallized in an orthorhombic phase with space-group Pbnm (62). The SEM images of the sample showed heterogeneous morphology and the average crystallite size was 0.7 μm. Luminescent spectra, typical for Nd³⁺ ions at low symmetry site (C_s), were observed in all samples excited at 266 nm and 808 nm. The highest emission intensity was obtained for the sample doped with 3% Nd³⁺. The emission lifetimes at 77 K and 300 K shorten with increasing Nd³⁺ concentration due to cross-relaxation processes. Noticeably, the lengthening of the decay times at 300 K, as compared to 77 K, resulted from the thermalization of the higher Stark component of the ⁴F_3/2 level. In addition, the experimental data were analysed using the Inokuti – Hirayama model; the energy transfer between Nd³⁺ ions was predominantly regulated by the dipole–dipole interaction. The critical transfer distance $R_{0}$ , critical concentration $C_{0}$ , energy transfer parameter $C_{d a}$ , and energy transfer probability $W_{d a}$ were found to be $4.9 Å, 2 \times 10^{21}$ ions∙cm⁻³, $5.67 \times 10^{- 41}$ cm⁶∙s^-1, and 4004 s^-1, respectively. Furthermore, the temperature–dependent luminescence exhibited good thermal stability with temperature quenching at 450 K. The temperature sensing ability of the sample doped with 5 % Nd³⁺, based on the emission of two thermally coupled ⁴F_5/2 and ⁴F_3/2 levels, was investigated. The highest value of the relative sensitivity was 0.81 and 0.83 % K^-1 at 248 K and 275 K, respectively.
Luminescence properties of orange emitting CaAl<inf>4</inf>O<inf>7</inf>:Sm3+ phosphor for solid state lighting applications
2020, Solid State Sciences
Sm³⁺-doped CaAl₄O₇ phosphor has been synthesized using the sol-gel method with a varying doping concentration of Sm³⁺ ions. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images were used to study the structural and morphological properties of the material. The UV-VIS-NIR spectrum was also recorded to analyze the optical transitions of CaAl₄O₇ phosphor doped with Sm³⁺ ions. The photoluminescence (PL) spectra of the phosphors were also recorded to examine the luminescence properties. The PL spectra exhibit intense orange emission at 602 nm that was obtained for the prepared samples using the 403 nm excitation. The optimized concentration of Sm³⁺ ions in CaAl₄O₇ was found to be 0.03 mol, beyond which concentration quenching was observed. The CIE color chromaticity coordinates of all the synthesized phosphors lie in the orange region of the chromaticity diagram. All of these results indicate the candidature of CaAl₄O₇ phosphor doped with Sm³⁺ ions for lighting and display applications.
Phase transition and optical properties of Ba<inf>1-x</inf>Zn<inf>x</inf>Al<inf>2</inf>O<inf>4</inf>:0.1% Eu3+ prepared via citrate sol-gel method
2019, Journal of Luminescence
Citation Excerpt :
It is well known that if a trivalent lanthanide ion is incorporated in the site of divalent cation, the vacancies, interstitials, molecular impurities and other trapping defects are often formed in order to restore charge balance. In the field of luminescence, the charge balance process might affect different emission and afterglow mechanisms [6]. Eu3+ is a well-known trivalent lanthanide dopant that has an emission spectrum that strongly depends on the symmetry of the site of occupation.
Undoped and Ba_1-xZn_xAl₂O₄:0.1% Eu³⁺ series (0 ≤ x ≤ 1) were successfully prepared using the citrate sol-gel method. X-ray diffraction (XRD) results revealed that depending on the Zn moles, the prepared samples could be indexed to hexagonal BaAl₂O₄ and cubic ZnAl₂O₄ structures. Scanning electron microscopy (SEM) shows that varying Zn moles changes the morphology of the prepared sample. Transmission electron microscopy (TEM) results confirmed the nano-crystallite shape and size of the structures seen in SEM, and the multiphase nature predicted by XRD. The photoluminescence (PL) results showed distinct emission peaks, which were either attributed to the host intrinsic intra-band gap defects or to characteristic transitions within Eu³⁺ ions. The International Commission on Illumination (CIE) colour coordinates showed that the emission colour can be tuned when varying the Zn mole.
Luminescences of Bi3+ and Bi2+ ions in Bi-doped CaAl<inf>4</inf>O<inf>7</inf> phosphor powders obtained via modified Pechini citrate process
2019, Journal of Alloys and Compounds
Citation Excerpt :
Various crystalline compounds existing in the CaO–Al2O3 systems, CaAl2O4, CaAl4O7, Ca3Al2O6, CaAl12O19, Ca12Al14O33, have been employed as host materials for transition metal and rare-earth doped phosphors [47,48]. In particular, CaAl4O7 (grossite) is a very promising host for optoelectronic applications because of its high mechanical strength, high chemical and temperature stability as well as a large range of transmittance in UV–Vis–NIR part of spectrum [46,49,50]. In the present research the CaAl4O7 was activated with Bi to learn on its luminescent properties.
In the present work, bismuth-doped CaAl₄O₇ phosphor powders were obtained via modified Pechini citrate process and their photoluminescence, radioluminescence and thermally stimulated luminescence properties were studied down to 20 K. At room temperature excitation in UV led to broad-band ³P_0,1 → ¹S₀ Bi³⁺ luminescence with three maxima located in blue-green part of spectrum as well as Bi²⁺ emission around 760 nm assigned to the ²P_3/2 → ²P_1/2 transition. At 150 K an additional Bi³⁺ emission band peaking at 370 nm appeared and its intensity increased significantly upon further cooling. The presence of a few luminescent centers of Bi³⁺ in CaAl₄O₇ was assigned to the presence of defects disturbing the local symmetry of activator ions. The radioluminescence spectra showed only two bands, an intense one at 760 nm clearly related to Bi²⁺ and a very weak component at 555 nm associated with one of the Bi³⁺ sites. These two emissions were also observed in the thermoluminescence of the material following its exposure to X-rays. The Bi³⁺ luminescence is suitably positioned to contribute the cyan color to white-LEDs. Its quantum yield is satisfactory, close to 40%. The Bi²⁺ 720–850 nm emission is attractive for bioimaging as it appears in the I^st biological window.
Relationship between the optical properties and the structure of a new complex: Nd(III)-isothiocyanate-2,2′-bipyridine
2018, Journal of Molecular Structure
Citation Excerpt :
The experimental data of the decay time of the 4F3/2 state of the Nd3+ ion for the title compound were fitted using the biexponential function (see Fig. 7). These values are small and amount to 0.65 and 0.16 μs The definition of ‘small’ respects the values of 193 μs [49] or 151 μs [50] for the Nd3+ ion in the nearest inorganic neighbour. Also, the values are small in comparison with typical decay times for Nd complexes with organic ligands: sulfonylamidophospate with NdO bonds (3–5 μs) [11] or phosphonic analogue of EDTA with NdO,N bonds (18–31 μs) [30].
A new compound, [Nd(NCS)₃ (2,2′-bipyridine)₂(H₂O)] (C₂₈H₂₄N₈NdO₂S₃), was obtained and investigated for its photophysical properties. Coordination systems of lanthanide-organic compounds are promising materials with applications as efficient luminophores. Organic ligands with chromophore groups (for example double bonds or aromatic rings) can intensify luminescence efficiency (antenna effect). The title compound crystallizes in a triclinic system with the space group P-1 and the following unit cell parameters: a = 9.698 (3), b = 13.000 (4), c = 13.219 (4) Å, α = 110.36 (3), β = 94.12 (2) γ = 97.53 (2)°. The coordination number of the Nd³⁺ ion is 8. The molecules of 2,2′-bipyridine coordinate to the Nd³⁺ ion via N atoms. Thus, the neodymium complex, based on this ligand, might have different optical properties from complexes with coordinating Nd–ligand bonds via O atoms bonds only. Additionally, we used another ligand – isothiocyanate ion – inorganic ion, which also coordinates via N atoms. In the absorption and emission spectra of the title compounds at low temperatures, the splitting of the 4f–4f transitions indicates a single low symmetry of the Nd³⁺ ion in the title compound. This fact confirms our X-ray structural results. The lower nephelauxetic parameter (β), positive bonding parameter (b^1/2), and Sinha parameter (δ) indicate a somewhat covalent character of the bond between the metal and the ligand but the NdN,O coordination bonds of [Nd(NCS)₃ (2,2′-bipyridine)₂(H₂O)] indicate a more covalent character than NdO-only bonds but less covalent than NdN-only bonds. The experimental data of the decay time of the ⁴F_3/2 state of the Nd³⁺ ion for the title compound were fitted using the biexponential function. These values are small and amount to 0.65 and 0.16 μs

View all citing articles on Scopus

View full text

Article preview

Journal of Solid State Chemistry

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Structural and morphological characterization

Conclusions

Acknowledgment

References (30)

Ceram. Int.

Opt. Mater.

J. Lumin.

J. Lumin.

Spectrochim. Acta A: Mol. Biomol. Spectrsc.

Displays

J. Solid State Chem.

J. Solid State Chem.

Opt. Mater.

Opt. Mater.

J. Lumin.

J. Solid State Chem.

J. Quant. Spectrosc. Radiat. Transf.

J. Solid State Chem.

Opt. Mater.

Cited by (13)

Low-temperature optical thermometer based on the luminescence of the double perovskite Ba<inf>2</inf>MgWO<inf>6</inf>: Nd<sup>3+</sup>

Influence of temperature on near-infrared luminescence, energy transfer mechanism and the temperature sensing ability of La<inf>2</inf>MgTiO<inf>6</inf>: Nd<sup>3+</sup> double perovskites

Luminescence properties of orange emitting CaAl<inf>4</inf>O<inf>7</inf>:Sm<sup>3+</sup> phosphor for solid state lighting applications

Phase transition and optical properties of Ba<inf>1-x</inf>Zn<inf>x</inf>Al<inf>2</inf>O<inf>4</inf>:0.1% Eu<sup>3+</sup> prepared via citrate sol-gel method

Luminescences of Bi<sup>3+</sup> and Bi<sup>2+</sup> ions in Bi-doped CaAl<inf>4</inf>O<inf>7</inf> phosphor powders obtained via modified Pechini citrate process

Relationship between the optical properties and the structure of a new complex: Nd(III)-isothiocyanate-2,2′-bipyridine