Optimization of the Synthesis and Energy Transfer of Ca₂MgWO₆:Cr³⁺,Nd³⁺

Abstract

This work pertains to Cr³⁺ and Nd³⁺ co-activated Ca₂MgWO₆ phosphors synthesized by high temperature solid-state method using oxides and carbonates as raw materials. All luminescent samples according to Ca₂MgWO₆:Cr³⁺,Nd³⁺ include Cr³⁺ for the absorption of UV and visible radiation (230–800 nm) prior to energy transfer to Nd³⁺. As a result of the energy transfer between Cr³⁺ and Nd³⁺, we observe line emission originating from Nd³⁺ in the near infrared range additionally to the broad band near infrared emission from Cr³⁺ assigned to the spin-allowed ⁴T₂ → ⁴A₂ transition. The energy transfer from Cr³⁺ to Nd³⁺ is discussed via the variations of the lifetime data of Cr³⁺ and Nd³⁺. The strong absorption of Cr³⁺ in the ultraviolet range and the efficient energy transfer from Cr³⁺ to Nd³⁺ indicate that the herein presented material type can serve as a radiation converter for near infrared region light emitting diodes (NIR-LEDs) comprising an UV-A emitting (Al,Ga)N chip.

1. Introduction

Cr³⁺ is a widely applied activator ion in phosphors or solid state lasers emitting in the near infrared region (NIR) [1]. NIR radiation sources have a broad area of application in biomedical imaging, phototherapy, thermography, optical communication, and environment monitoring [2,3,4,5,6].

Incandescent and halogen lamps, silicon carbide heating elements, as well as GaAs or (Al,Ga)As LEDs are commonly used as NIR emitters. However, the application of these NIR sources is hampered by low efficiency, size, and (for NIR LEDs) their poor thermal stability, low electroluminescence efficiency, and sensitivity towards humidity [7]. To solve these problems, we can use LEDs based on a blue or UV-A emitting (Al,Ga)N semiconductor chip in combination with a suitable NIR emitting phosphor.

The rare earth ion Nd³⁺ offers suitable energy levels which, subsequent to an excitation process, lead to emission in the near infrared range and is therefore particularly well suited for application as a luminescent converter [8]. However, Nd³⁺ doped phosphors always exhibit narrow-band absorption features in the blue to ultraviolet region with a rather low absorption cross section due to the involved parity forbidden 4f–4f transitions. To overcome this drawback, the use of Cr³⁺ as a broadband sensitizer is widely discussed, since it exhibits up to three broad absorption bands in the near ultraviolet and visible spectral range due to its spin-allowed ⁴A₂/⁴T₂ and ⁴A₂/⁴T₁ transitions [9].

Ca₂MgWO₆ (CMW) is a low-cost material with high chemical and physical stability, which can be synthesized by a rather simple synthesis pathway and which can be doped with transition metal ions as soon as with rare earth metal ions [10]. In the past, several phosphors with CMW host have been reported, such as CMW:Eu³⁺ [11], CMW:Bi³⁺ [12], CMW:Bi³⁺,Sm³⁺ [13], CMW:Bi³⁺,Eu³⁺ [14], CMW:Er³⁺,Yb³⁺ [15], and CMW:Mn⁴⁺ [16]. Xu et al. studied CMW:Cr³⁺ and CMW:Cr³⁺,Yb³⁺. It turned out that Cr³⁺ in this host material can be very efficiently excited in the ultraviolet range [17,18].

Therefore, this work deals with the synthesis of this material by solid state reaction and the investigation of the energy transfer from Cr³⁺ to Nd³⁺ in a set of CMW:Cr³⁺,Nd³⁺ samples. After having confirmed the presence of single-phase materials, diffuse reflectance spectra and luminescence properties such as excitation spectra, emission spectra, and time-dependent spectroscopy were recorded.

2. Experimental Section

For the synthesis of the compounds according to

Table 1. Stoichiometric details of prepared samples.

Sample Number	Atom-% Cr3+	Atom-% Nd3+	Atom-% Li+	Atom-% Na+	Formula
1	0	0	0	0	Ca2MgWO6
2	1	0	2	0	Ca2Mg0.97Cr0.01Li0.02WO6
3	1	0.5	2	1	Ca1.97Nd0.01Na0.02Mg0.97Cr0.01Li0.02WO6
4	1	1	2	2	Ca1.94Nd0.02Na0.04Mg0.97Cr0.01Li0.02WO6
5	1	1.5	2	3	Ca1.91Nd0.03Na0.06Mg0.97Cr0.01Li0.02WO6
6	1	2	2	4	Ca1.88Nd0.04Na0.08Mg0.97Cr0.01Li0.02WO6
7	1	4	2	8	Ca1.76Nd0.08Na0.16Mg0.97Cr0.01Li0.02WO6
8	1	6	2	12	Ca1.64Nd0.16Na0.24Mg0.97Cr0.01Li0.02WO6
9	1	8	2	16	Ca1.52Nd0.16Na0.32Mg0.97Cr0.01Li0.02WO6
10	1	0	0	0	Ca2Mg0.99Cr0.01WO6
11	1	0	1	0	Ca2Mg0.98Cr0.01Li0.01WO6
12	0	1	0	0	Ca1.98Nd0.02MgWO6
13	0	1	0	1	Ca1.96Nd0.02Na0.02MgWO6
14	0	1	0	2	Ca1.94Nd0.02Na0.04MgWO6
15	1	1	0	0	Ca1.98Nd0.02Mg0.99Cr0.01WO6
16	1	1	0.5	0.5	Ca1.97Nd0.02Na0.01Mg0.985Cr0.01Li0.005WO6
17	1	1	1	1	Ca1.96Nd0.02Na0.02Mg0.98Cr0.01Li0.01WO6
18	1	1	1.5	1.5	Ca1.95Nd0.02Na0.03Mg0.975Cr0.01Li0.015WO6

, we used the high temperature solid state synthesis. Stoichiometric amounts of the educts CaCO₃, MgO, WO₃, Na₂CO₃, Li₂CO₃, Cr₂O₃, and Nd₂O₃ were used, and these were ground in agate mortar with the addition of acetone. The mixture was then dried and transferred to a corundum crucible. The samples were first heated for 2 h at 600 °C and then for 5 h at 1300 °C. Between the heating steps and after the final sintering, the sinter bodies were ground again to obtain a fine beige powder.

The samples were identified by X-ray powder diffraction (XRD) (PANalytical X’Pert Pro, Malvern Panalytical B.V., Almelo, The Netherlands) with Cu K_α radiation (λ = 0.15406 nm) operating at 40 kV and 40 mA. XRD patterns were collected from 10° ≤ 2Θ ≤ 80°.

Diffuse reflectance spectra were recorded on a FLS920 spectrometer (Edinburgh Instruments Ltd., Kirkton Campus, UK) equipped with a 450 W Xe arc lamp, a cooled (−20 °C) single-photon counting photomultiplier R2658P (Hamamatsu Photonics K.K., Hamamatsu City, Japan), and a Spectralon integration sphere. BaSO₄ (99.998%, Merck KGaA, Munich, Germany) was used as a reflectance standard.

The photoluminescence excitation (PLE) and emission (PL) spectra at room temperature were measured on a FLS920 spectrometer (Edinburgh Instruments Ltd., Kirkton Campus, UK) equipped with a 450 W Xe arc lamp, mirror optics for powder samples, and a cooled (−20 °C) single-photon counting photomultiplier R2658P (Hamamatsu Photonics K.K., Hamamatsu City, Japan). The decay curves were recorded on the same instrument (FLS920), and a µF900 flash lamp was used as the excitation source.

3. Results and Discussion

Ca₂MgWO₆ crystalized in a monoclinic structure with space group P2₁/n and was characterized by B-Site cation ordering and a^-a^-c⁺-type BO₆ octahedral tilting. The lattice parameters were a = 5.4199 Å, b = 5.5479 Å, and c = 7.7147 Å [19]. Calcium atoms were coordinated by twelve oxygen atoms, where four oxygen atoms were significantly shortened. The average distance for the Mg–O octahedra was 2.069 Å and for the W–O octahedra was 1.922 Å, which were in good agreement with the ionic radii according to Shannon [r(Mg²⁺) = 0.86 Å, r(W⁶⁺) = 0.74 Å, r(O²⁻) = 1.28 Å] [20].

Because of the strong ligand-field stabilization energy of Cr³⁺ ions ([Ar]3d³ configuration) in sixfold coordination and the similar ionic radii between Cr³⁺ (0.755 Å) and Mg²⁺ (0.86 Å), chromium preferred to occupy octahedral magnesium sites. Another reason for occupying the magnesium sites instead of the tungsten sites was the smaller difference in ion charge. Due to the similar ionic radii, Nd³⁺ (1.410 Å) occupied Ca²⁺ (1.480 Å) lattice sites.

A charge balance was thus necessary, which was achieved by replacing Mg²⁺ with Li⁺ and Ca²⁺ with Na⁺ in appropriate concentration. To avoid the formation of color centers, twice the amount of charge compensation was used, which is described further on.

The XRD pattern as shown in Figure 1 proves that no changes in the crystal structure could be observed upon substituting with Cr³⁺ and Nd³⁺. All the diffraction peaks were well indexed to those of the monoclinic Ca₂MgWO₆ phase (ICDD (International Centre for Diffraction Data): 04-012-4403).

Moreover, Figure 2 depicts a series of reflection spectra of Ca₂MgWO₆ in order to underline the necessity of the application of a double amount of charge compensation.

As displayed in Figure 2a Ca₂MgWO₆ was doped with Cr³⁺ and different amounts of Li⁺ to reveal the effect of charge compensation. For all three samples, the three expected spin-allowed [Ar]3d³–[Ar]3d³ transitions of Cr³⁺ having the electron configuration [Ar]3d³ were observed. These were in the UV range with its center at 345 nm (⁴A₂ → ⁴T₁(⁴P)), in the green spectral range with its center at 475 nm (⁴A₂ → ⁴T₁(⁴F)), and in the deep red spectral range with its center at 694 nm (⁴A₂ → ⁴T₂(⁴F)) [17]. The sample 10 in which no charge compensation was present showed a slightly worse reflectivity, whereas the other two samples (2 and 11) had identical reflection spectra.

Figure 2b shows the reflection spectra of the Ca₂MgWO₆:Nd³⁺ samples, where the charge was balanced with different amounts of Na⁺. Again, only a slight improvement of the reflectivity could be observed. For all three samples (12–14), the typical 4f–4f-transitions of Nd³⁺ were observed. These were located at 530 nm (⁴I_9/2 → ⁴G_7/2), 590 nm (⁴I_9/2 → ⁴G_5/2), 685 nm (⁴I_9/2 → ⁴F_9/2), and 750 nm (⁴I_9/2 → ⁴F_7/2).

Figure 2c shows the reflection spectra of Ca₂MgWO₆ samples co-doped with Cr³⁺ and Nd³⁺, where the charge was balanced with different amounts of Li⁺/Na⁺ (samples 4 and 15–18). In contrast to Ca₂MgWO₆:Cr³⁺ and Ca₂MgWO₆:Nd³⁺, another very broad absorption band was recorded, which only occurred if Cr³⁺ and Nd³⁺ were present at the same time. Therefore, we assumed the presence of a charge transfer transition between Cr³⁺ and Nd³⁺. This charge transfer was significantly suppressed by the addition of Li⁺/Na⁺ due to the better ion distribution, and thus this additional absorption band disappeared gradually until the amount of charge compensation was doubled. Then, only the typical spin-allowed transitions of Cr³⁺ and the spin-forbidden transitions of Nd³⁺ were visible.

Therefore, the reflection spectra of samples shown in Figure 2d (Ca₂MgWO₆:1%Cr,y%Nd) were synthesized with just this required amount of Li⁺/Na⁺ (Samples 2–9). Additionally, the undoped compound (Sample 1) was plotted here, which was used to determine the band gap of Ca₂MgWO₆, which was derived to be 4.0 eV. As described above, one could observe the transitions of Cr³⁺ as well as the transitions of Nd³⁺. As the proportion of Nd³⁺ increased, the absorption lines of Nd³⁺ gained more intensity.

Figure 2. Diffuse reflection spectra of Ca₂MgWO₆ with (a) 1% Cr³⁺ and different amounts of Li⁺; (b) 1% Nd³⁺ and different amounts of Na⁺; (c) 1% Cr³⁺, 1% Nd³⁺ and different amounts of Li⁺ and Na⁺; and (d) 1% Cr³⁺, (0–8)% Nd³⁺ and double amount of Li⁺ and Na^+.

Figure 2. Diffuse reflection spectra of Ca₂MgWO₆ with (a) 1% Cr³⁺ and different amounts of Li⁺; (b) 1% Nd³⁺ and different amounts of Na⁺; (c) 1% Cr³⁺, 1% Nd³⁺ and different amounts of Li⁺ and Na⁺; and (d) 1% Cr³⁺, (0–8)% Nd³⁺ and double amount of Li⁺ and Na^+.

Figure 3 shows the excitation spectra of Ca₂MgWO₆ doped with 1% Cr³⁺ and a variable content of Nd³⁺ between 0 and 8 atom-%. The emission monochromator in Figure 3a was fixed to the emission of the Cr³⁺ emission band at 825 nm, while in Figure 3b, it was set to the line emission of Nd³⁺ located at 879 nm.

In the excitation spectrum monitoring the Cr³⁺ emission, all three expected 3d–3d excitation bands were seen in the UV (maximum at 330 nm), the green (maximum at 490 nm), and the deep red (maximum at 687 nm) spectral range. They could be clearly assigned to the spin-allowed crystal-field transitions ⁴A₂ → ⁴T₁(⁴P), ⁴A₂ → ⁴T₁(⁴F), and ⁴A₂ → ⁴T₂(⁴F) of Cr³⁺. However, the high intensity of the former transition in this compound was not typical, which showed a comparison to other Cr³⁺ doped compounds. Mostly, the two transitions in the visible spectral region exhibited much higher intensity [21,22,23,24]. The decrease in excitation intensity occurred with increasing content of Nd³⁺, as expected for efficient energy transfer from Cr³⁺ to Nd³⁺. The excitation lines visible at about 580 nm, which were caused by the presence of Nd³⁺, point to the presence of back transfer from Nd³⁺ to Cr³⁺, because there were no Nd³⁺ emission lines in the emission range set here.

The excitation spectrum of Nd³⁺ emission also showed the three typical broad excitation bands of Cr³⁺ in the UV, the green, and the deep red spectral range, which also confirmed efficient energy transfer from Cr³⁺ to Nd³⁺. In addition, excitation lines were found at 530 nm, 590 nm, 750 nm, and 800 nm, which could be assigned to the intraconfigurational 4f–4f transitions ⁴I_9/2 → ⁴G_7/2, ⁴I_9/2 → ⁴G_5/2, ⁴I_9/2 → ⁴F_7/2, and ⁴I_9/2 → ⁴F_5/2 of Nd³⁺.

Figure 4 depicts the emission spectra of the compounds Ca₂MgWO₆:1%Cr,y%Nd, whereby y was between 0 and 8. In the absence of Nd³⁺ (sample 2), solely the broadband emission of Cr³⁺ was visible (⁴T₂ → ⁴A₂), which was located between 700 and 1000 nm and culminated at 825 nm. After adding the Nd³⁺, the emission intensity of the Cr³⁺ transition decreased, and new emission lines could be measured in the spectral range between 870 and 930 nm. Up to a Nd content of 1.5% (Sample 5), the Nd³⁺ emission intensity increased due to the gradually promoted energy transfer, while at even higher concentrations, it declined again due to concentration quenching.

Figure 5 shows the decay curves of the compounds Ca₂MgWO₆:1%Cr, y%Nd, with excitation at λ_Ex = 365 nm in each case.

In Figure 5a, the Cr³⁺ emission subsequent to UV-A excitation was observed at 825 nm. If only Cr³⁺ ions were present (Sample 2), a monoexponential decay curve was observed, which could be fitted with a decay time of τ = 28 µs. This fit very well with a spin-allowed transition originating from the ⁴T₂ state. Upon co-doping with Nd³⁺ ions, biexponential decay curves were just obtained, with the first component decaying faster and the second component decaying much more slowly than observed for the sample without Nd³⁺. From this, it can be concluded that the energy was firstly transferred to the Nd³⁺ ions, and, secondly, back transfer to the Cr³⁺ ions occurred. The more Nd³⁺ ions were present in the compound (Samples 8 and 9), the more the observed decay curve could be approached by a monoexponential function. This points to the conclusion that the back transfer was no longer the rate-determining step.

In Figure 5b, the emission was observed at 879 nm. If solely Cr³⁺ ions were present Sample 2), a monoexponential decay behavior was present, which corresponded to the previously described decay behavior of Cr³⁺. By co-doping with Nd³⁺ ions, biexponential decay curves with significantly delayed decay behavior were initially obtained. With a further increase of the Nd³⁺ concentration (Sample 6), there was a monoexponential decay curve yielding a decay time of 120 µs. A further increase led again to a biexponential decay curve but with a shorter decay time compared to the previous samples.

Figure 5. Decay curves of Ca₂MgWO₆ doped with 1% Cr³⁺ and (0–8)% Nd³⁺ and double amount of Li⁺ and Na⁺; (a) emission monochromator fixed at emission line of Cr³⁺ (825 nm), (b) emission monochromator fixed at emission line of Nd³⁺ (879 nm).

Figure 5. Decay curves of Ca₂MgWO₆ doped with 1% Cr³⁺ and (0–8)% Nd³⁺ and double amount of Li⁺ and Na⁺; (a) emission monochromator fixed at emission line of Cr³⁺ (825 nm), (b) emission monochromator fixed at emission line of Nd³⁺ (879 nm).

4. Conclusions

In summary, a set of single phase samples of Ca₂MgWO₆:Cr³⁺,Nd³⁺ were prepared by a conventional solid state reaction due to sintering at 1300 °C. Additionally, the synthesis procedure of this material was optimized by adding the double molar amount of Li⁺ and Na⁺ for charge compensation. The presence of highly pure samples and the lack of the formation of color centers were confirmed by reflection spectra.

Photoluminescence properties were also studied in detail. The excitation spectra of the Cr³⁺ activator extended from 250 to 750 nm, while the emission spectra of the Cr³⁺ spread out from 700 to 1000 nm. Efficient energy transfer occurred from Cr³⁺ to Nd³⁺, which was proven by the decline of luminescence intensity and the decay time of Cr³⁺ by ascending the concentration of the co-dopant Nd³⁺. Since the activator Cr³⁺ exhibited broadband absorption between 250 and 750 nm, especially between 250 and 430 nm, and Nd³⁺ emit in the NIR range, the Ca₂MgWO₆:Cr³⁺,Nd³⁺ phosphors are promising materials to be employed as luminescent converters in ultraviolet (Al,Ga)N or near UV (In,Ga)N emitting LEDs.