Pr3+ co-doped calcium aluminosilicate glass for tunable white lighting devices

Journal Pre-proof 2+,3+ 3+ Eu /Pr co-doped calcium aluminosilicate glass for tunable white lighting devices C.Y. Morassuti, L.H.C. Andrade, J.R. Silva, A.C. Bento, M.L. Baesso, F.B. Guimarães, J.H. Rohling, L.A.O. Nunes, G. Boulon, Y. Guyot, S.M. Lima PII:

S0925-8388(19)34565-7

DOI:

https://doi.org/10.1016/j.jallcom.2019.153319

Reference:

JALCOM 153319

To appear in:

Journal of Alloys and Compounds

Received Date: 27 August 2019 Revised Date:

5 December 2019

Accepted Date: 6 December 2019

Please cite this article as: C.Y. Morassuti, L.H.C. Andrade, J.R. Silva, A.C. Bento, M.L. Baesso, F.B. 2+,3+ 3+ Guimarães, J.H. Rohling, L.A.O. Nunes, G. Boulon, Y. Guyot, S.M. Lima, Eu /Pr co-doped calcium aluminosilicate glass for tunable white lighting devices, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153319. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Grupo de Espectroscopia Óptica e Fototérmica Universidade Estadual de Mato Grosso do Sul

Credit author statement

C.Y. Morassuti: Resources, Visualization. L.H.C. Andrade: Conceptualization, Resources, Writing – Review & Editing, Supervision. J.R. Silva: Resources, Visualization. A.C. Bento: Resources, Visualization. M.L. Baesso: Resources, Writing – Review & Editing, Supervision. F.B. Guimarães: Resources, Visualization. J.H. Rohling: Resources, Visualization. L.A.O. Nunes: Methodology, Writing – Review & Editing. G. Boulon: Writing – Review & Editing, Supervision. Y. Guyot: Methodology, Writing – Review & Editing. S.M. Lima: Conceptualization, Resources, Writing – Original Draft, Visualization, Supervision, Project administration, Funding acquisition

Cidade Universitária de Dourados - CP 351 - CEP 79804-970 - DOURADOS – MS – Brasil Tel. +55 67 3902-2555 / 2653 / 2656 - Fax. +55 67 3902-2652

Eu2+,3+/Pr3+ co-doped calcium aluminosilicate glass for tunable white lighting devices C.Y. Morassutia, L.H.C. Andradea, J.R. Silvaa, A.C. Bentob, M.L. Baessob, F.B. Guimarãesb, J.H. Rohlingb, L.A.O. Nunesc, G. Boulond, Y. Guyotd, and S.M. Limaa * a

Grupo de Espectroscopia Óptica e Fototérmica-GEOF, Centro de Estudos em Recursos

Naturais- CERNA, Universidade Estadual de Mato Grosso do Sul-UEMS, 351, Dourados, MS, Brazil. b

Grupo de Estudos de Fenômenos Fototérmicos, Departamento de Física, Universidade Estadual de Maringá, Av. Colombo 5790, 87020-900 Maringá, PR, Brazil.

c

d

Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, SP, Brazil.

Univ Lyon, Université Claude Bernard Lyon1, UMR5306 CNRS, Institut Lumière Matière (ILM), Villeurbanne 69622, France.

Graphical abstract

* Corresponding author:

Sandro Marcio Lima Universidade Estadual de Mato Grosso do Sul – UEMS Programa de Pós-Graduação em Recursos Naturais - PGRN Grupo de Espectroscopia Óptica e Fototérmica - GEOF CEP 79804-970 – C.P. 351 Dourados, MS, Brazil e-mail: [email protected] Phone: +55 67 3902 2555 Fax: +55 67 3902 2652

Abstract At the present time there is no doubt that almost every country is experiencing a revolutionary change in the used devices for artificial lighting whether for public, for domestic or for work place illumination. This movement is taking place towards the substitution of fluorescent lamps to LED-phosphor based devices under the justification of reduction in energy consumption and mitigation of the contamination of environment produced by the mercury of fluorescent lamps. In this aspect, despite of the great effort of worldwide scientists to obtain a white lighting source with the emitting spectrum similar to that of the sunlight, it is recognized that there is still a challenge to be faced of getting luminescent materials with broad band excitation in the UV-Vis regions and large emission in the visible in order to allow tunable white lighting generation. Motivated by this interest, in this paper, Eu2+,3+/Pr3+ co-doped calcium aluminosilicate glasses were synthesized and spectroscopically investigated aiming at the development of smart white lighting devices. A prototype was developed to study the sample with excitation at 445 nm light emission diode (LED). The results indicate high color rendering index (CRI Ra ~ 92-95) and tunable correlated color temperature (CCT from 5700 to 6600K), covering the whole spectral daylight range. By using a second 405nm LED together with the 445 nm one, the calculated Du’v’ values are suitable for indoor illumination. In this configuration, by adjusting the LEDs intensities it is possible to tune the CCT close to the white lighting region with CRI Ra nearly 95. These characteristics demonstrate that the Eu2+,3+/Pr3+ co-doped OH- calcium aluminosilicate glass is a strong candidate for smart white lighting with LEDs.

Keywords: White lighting, LEDs, calcium aluminosilicate, praseodymium, europium.

1. Introduction Recently, many efforts have been done in order to develop materials for white light emission diodes (WLEDs) devices due to long lifetime and low energy consumption when applied in displays and for artificial lighting [1-4]. These advantages transformed the WLEDs as almost the unique option to replace the traditional incandescent or fluorescent lamps. Basically, there are two ways to produce white light using LEDs and phosphors materials: the first one is combining emissions from different near violet and/or violet LEDs to excite a phosphor, that emits a broad visible light ranging from blue to red region; the second way is combining a blue LED emission, which is partially absorbed and transmitted by a phosphor layer, with a broad green-red emission band from the phosphors [3, 5, 6]. In this second system, the transmitted blue LED emission is also used to add with the phosphors emission, so that the complete blue-green-red emissions create a visible emission with high color-rendering index (CRI). The yellow emitting Ce3+:YAG (Yttrium Aluminum Garnet) is widely used as a phosphor coupled in a blue LED as mentioned above. In this structure, Ce3+ presents a broad emission in the visible region (centered at 18083 cm-1 = 553 nm with band-width of 2995 cm1

) and an efficient absorption of blue light (from 420 to 480 nm) [6]. The correlated color

temperature (CCT) obtained with this material with blue LED excitation is very high (CCT ~ 6283K) [2]. However, a drawback of the Ce3+:YAG crystal is its low CRI value, due to its weak red emission (CRI ~70). In order to overcome this problem, many red-emitting components (Eu3+, Sm3+ and Pr3+) have been added as dopant or co-dopants in different structures [7]. Among them, Pr3+ is a promising candidate due to its absorption bands around 450 nm and the red emissions (1D2 → 3H4 and 3P0 → 3H6 transitions) that starts from different energy levels [8, 9]. According to the used concentration, these emissions can be tuned and combined with green-yellow emitting phosphors for white light generation [10]. Recently

Sr2SiO4 crystal structure was co-doped with Eu2+ and Pr3+ and its luminescent characteristics were investigated for color tuning possibility in white light generation [11]. The results indicated that the material exhibits a yellow-to-orange emission tunable with the excitation in different Eu2+ sites. There was observed an energy transfer from Eu2+ to Pr3+ that is dependent to the Eu2+ sites, suggesting that the material can be used for tuning white LED. Previous works reported that the (low- and high-) silica content in calcium aluminosilicate glasses (LSCAS and CAS, respectively) are promising materials for different applications due to its good chemical stability, they allow incorporation of high doping concentration of lanthanide ions, they present high transmission from the ultraviolet up to the infrared (up to 5.5 µm) and by supporting high mechanical loads [12]. When these materials are prepared with lanthanide ions, they exhibit optical properties that qualify them to be applied for different optical applications. It was demonstrated that Ce3+-doped LSCAS glass has potential for white lighting purposes when it is excited by commercially available UV LEDs [13, 14]. Besides, the coexistence of Eu2+ and Eu3+ oxidation states in CAS glass has indicated that it can be tailored for tunable white lighting by combining adjustment of the matrix composition (changing the silica concentration), melting environment and pumping wavelength [15-17]. More recently, Pr3+-doped CAS glass was investigated and an intense and broad orange-red emission with quantum efficiency of 0.37 were measured, indicating that the material is promising as red phosphor for white light generation [18]. In this work, Eu2+,3+/Pr3+ co-doped calcium aluminosilicate glasses were optically investigated to be applied as phosphors for white light production. Different Eu2O3 and Pr6O11 concentrations were studied and a prototype was developed in two configurations: the first, a single 405 nm UV-LED was used as excitation source; and secondly, a combined 405 and 445 nm LEDs were used to tune better CCT and CRI indexes to produce white lighting. The spectroscopic characterization show that the studied system exhibits high CRI value (~ 95)

and tunable CCT values (5100-5700K), which are rare parameters found in glasses for white lighting application.

2. Experimental Calcium aluminosilicate (CAS) glasses, in wt.%, were prepared using high-purity oxides of (28-x-y) Al2O3, 34CaO, 34SiO2, 4MgO, and with x = 0 and 0.5% of Eu2O3 and y = 0, 0.2, 0.5, 1.0, 2.0, and 4.0 wt.% of Pr6O11. The mixtures were melted for two hours, under vacuum condition, at 1550°C to minimize OH- molecules from the glass structure. The glass tempering was performed inside the vacuum chamber, followed by thermal annealing to remove internal stress. For the spectroscopic characterization, the samples were cut and optically polished to be approximately 2.0 mm thick. For the prototype test, a piece of the sample was crushed using an agate mortar and pestle to obtain a grain size of tens of micrometer. It is possible to guarantee that this particle size maintains the glass sample characteristics. This procedure was useful to increase the scattering and consequently the phosphor emission intensity, as usually done with phosphor tests [19]. The excitation and emission spectra were obtained at front face configuration using a SPEX Fluorolog spectrofluorometer (0.22m, Spex/1680) equipped with a Xe-lamp as the excitation source and a photomultiplier (Hamamatsu/R928) for detection. The excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode as the reference detector. After the spectroscopic measurements, the 0.5 wt.% of Eu2O3/1.0 wt.% of Pr6O11 codoped CAS sample was crushed in agate mortar and pestle to obtain a grain size of approximately 30 µm. An experimental setup was mounted to record the spectroscopic data for the combined LEDs + phosphor material. Three pallets (layers) of powder grain of sample with different thickness: h1, h2, and h3 (1.00, 1.25 and 1.50 mm), corresponding to a weight of

0.200, 0.250, and 0.300g respectively were prepared. Each pallet was at first placed over the head of a single 405 nm commercial LED and then over two coupled LEDs emitting at 405 and 445 nm. The violet and blue LEDs excitation intensities were controlled by a current control with the 0 – 25V/5A VCC (Cidepe EQ030J) power source.

3. Results and discussion 3.1. Spectroscopic investigation The photoluminescence excitation (PLE) spectra for the 0.5 wt.% Eu2O3:CAS and 0.5 wt.% Pr6O11:CAS single doped glasses and for the 0.5 wt.% Eu2O3/ 0.5 wt.% Pr6O11:CAS codoped glass are plotted in Figure 1a. These spectra were obtained by measuring the emission at 600nm, so that all transitions from the studied systems can be noted in the excitation spectra. For the single Pr doped CAS glass, the transitions from 3H4 ground state to the up levels 3P0, 3P1 (1I6) and 3P2 can be seen around 480, 469 and 442 nm, respectively [18]. A small absorption band around 320nm is assigned to the glass defects and at 260 nm the glass charge transfer band (band gap edge) is observed. The CAS glass doped with 0.5 wt.% Eu2O3 spectrum exhibits two superposed (overlapped) absorption bands with a maximum centered at 323 (eg) and 275 nm (t2g) attributed to the parity-allowed electric dipole transition, 4f7 (8S7/2) → 4f65d of Eu2+ ions [16]. The excitation region between 350 and 460 nm is attributed to Eu2+ absorption bands, as previously identified [16]. An interesting aspect to be noted is that by exciting the Eu2+ in the border (in the 350-460 nm region) the luminescence is more efficient than exciting at the maximum absorption position. This occur because the upper 5d levels of Eu2+ are located close to the conduction band of the glass. In this case, some photoionization process appends and quenches the emission. This photoionization process is particularly efficient because the excited electron in Eu2+ are like delocalized electron. Finally, for the co-doped glass the PLE spectrum corresponds practically to the sum

of Eu2+,3+ and Pr3+ emissions. A broad excitation region ranging between 250 and 500 nm indicates that the co-doped system is useful for white light applications, since blue and/or ultraviolet LEDs can be combined to excite the glass system. The 445 nm excitation wavelength was chosen for the photoluminescence measurements in the Eu/Pr-codoped CAS system. Five samples were studied all of them with fixed Eu2O3 (at 0.5 wt%) concentration and varying the praseodymium amount as xPr6O11, with x = 0.2, 0.5, 1.0, 2.0, and 4.0. The used europium concentration was that presented the maximum of Eu2+ emission intensity [16]. Figure 1b shows the photoluminescence (PL) spectra from 450 to 800 nm for all studied glasses. The spectra present a broad emission band from Eu2+ which overlaps with those from the Pr3+ ions. By increasing the Pr3+ concentration, the emission peaks from the 3P0 → 3Hj transitions (j = 4, 5 and 6) at 495, 537 and 646 nm, and from 1D2 → 3H4 and 3P0 → 3F2 transitions, around 604 and 713 nm, respectively, are highlighted. It was difficult to identify the contribution from Eu3+ emission in the spectra due to its overlap with the more intense Eu2+ and Pr3+ emissions. By deconvoluting the PL spectra of Figure 1b using the three colors matching functions established by the Commission Internationale de l’Éclairage (CIE) in 1931 [20], the (x, y) coordinates were calculated for each studied sample and are plotted in Figure 2. This procedure was also used to determine the coordinates for the previously studied Eu2O3 [16] and Pr6O11 [18] single doped CAS glasses (also shown in Fig. 2). As noted, the (x, y) values obtained for the 0.5 wt.% of Eu2O3 single doped CAS glass show that the sample show a PL that is between the green and yellow region, indicating that this system has a poor red contribution to the spectrum. This behavior was also observed for Eu2+/CN-:KCl/KCN crystal [21]. On the other hand, the (x, y) coordinates for the Pr3+ single doped CAS glasses reported at the literature [18] have a strong dependence on concentration, changing from the red region (0.61, 0.36) for lower concentration (0.2 wt.% Pr6O11) to the orange (0.47, 0.41) for higher

concentration (2.0 wt.% Pr6O11). For the latter sample, the correlated color temperature (CCT) was calculated as 2369K with a distance from the color coordinates to the Planckian locus, Du’v’, value of (-0,009). This low value for Du’v’ indicates how close the material emission is to the ideal source. Since CCT parameter specify the color appearance of the emitted light in relation to the light color from an ideal black-body reference source at a particular temperature, the low CCT value for this sample indicates that it can be added to materials that require orange emission. In this work we took advantage of this observation and investigated the emissions from the Eu/Pr co-doped CAS system. In Figure 2, it can be noted that the (x, y) coordinates for the Eu/Pr co-doped CAS glasses are in the same region (0.43, 0.50) between the position for the Eu single doped CAS glass and the Pr doped one. The position is remaining in the yellow region, indicating that an excitation at 445nm with a blue LED can provide the blue component for white light (0.33, 0.33) coordinates (shown in the dashed line in Fig. 2). 3.2. Prototype development with 445 nm LED In order to perform white light generation test, powder grain of the 0.5 wt.% Eu2O3/1.0 wt.% Pr6O11 co-doped CAS glass was deposited on the top of a commercial blue LED at 445 nm. The setup was described on the experimental section. Figure 3 shows the luminescence spectra recorded by varying the LED current from 20 to 400 mA three different sample thicknesses (powder layers of grain sample), designated as h1, h2 and h3. It can be observed in all data that by increasing the applied current from 20 to 400 mA, an increase in the PL intensity is observed. The spectra obtained for the optical path h2 (plotted in part (b)) are those ones that exhibit higher PL intensity for both LED and Eu/Pr co-doped CAS glass contributions. The color rendering index (CRI Ra) and CCT colorimetric parameters were calculated for each spectrum (different current) and for the three thicknesses investigated, and the results are presented in Table 1. All spectra in Figure 3 show very high values for CRI Ra

(ranging between 85.1 and 95.0), and it can be noted that their values are tunable, ranging from 85.1 – 92.7 for the h1, 91.2 – 95.0 for h2, and 93.9 – 90.0 for h3 when varying the current. These CRI values are considered very appropriate for indoor illumination as described by ANSI (American National Standards Institute). The values are higher than those reported by Ce3+:YAG phosphor [2], which exhibit poor red light component. According to the used thickness layers and set current, the CCT values can be tuned from 4304K to 8365K, crossing the temperature region for white light generation (~6500K). In Figure 4 it is observed the chromaticity analysis for the emission spectra showed in Figure 3 for the 0.5 wt.% Eu2O3/1.0 wt.% Pr6O11 co-doped CAS glass. It is possible to observe that by increasing the layer thickness, the color appearance changes in terms of the (x, y) coordinates from blue/violet to yellow/orange region. The h2 layer present coordinates in the CIE 1931 diagram around the standard CIE coordinates to white light of (0.33, 0.32) for 20 mA and (0.33, 0.34) for 400 mA, achieving the white light (0.33, 0.33) at 155 mA. The CCT values showed in Table 1 to this layer can be tunable from 5706K to 6610K covering the typical daylight range. 3.3. Prototype development with 405 and 445 nm LEDs In order to improve the contribution of the Eu2+ emission, a violet LED at λexc = 405 nm was coupled to excite the 0.5 wt.% Eu2O3/1.0 wt.% Pr6O11 co-doped CAS glass simultaneously. The h2 powder layer thickness was chosen and the PL obtained results are shown in Figure 5(a – g). In Figure 5a, the electric current for the 445 nm LED was fixed at 20 mA, and the 405 nm LED current was varied from 20 to 400 mA. In the other parts (b, c, d, e, f and g) of Figure 5, the same current range was used for the 405 nm LED, while the current for the 445 nm LED was always fixed (at 65, 110, 155, 200, 300 and 400 mA, respectively). Due to layer thickness and the allowed electrical dipole transitions of Eu2+ that presents high absorption coefficient in calcium aluminosilicate glasses [15, 17], no

contribution at 405 nm due to the LED is observed in the spectra. This is an important advantage in this configuration once the ultraviolet-A LED emission may increase the risk of eye damage [22]. As can be observed in the insets, by increasing the applied current in the 405 nm LED, the (x, y) coordinates change towards yellow region. When the 445 nm LED current is increased, in each inset in the figures it is observed a tendency to centralize the variation induced by the 405 nm LED excitation around the white region. Figure 6a shows the PL spectra for the 0.5 wt.% Eu2O3/1.0 wt.% Pr6O11 co-doped CAS glass at five combinations of the excitation power of both violet and blue LEDs. These spectra represent the better (high) CRI and acceptable Du’v’ values that are possible to tune a high CCT range. The respective colorimetric parameters are presented in Table 2. It can be observed that the (x, y) coordinates (showed in Figure 6b) are close to the white light region (0.33, 0.33) and the CRI Ra values remain between 92 and 95 to all configuration. The CCT values can be tunable from 5140 – 5702K with Du’v’ values less than 0.02, which are very appropriate for phosphor application as defined by the Japanese Standard Association (JIS C8152-2). These results indicate that the co-doped studied system is a promising material to be used as phosphor to develop WLEDs with high colorimetric parameters.

4. Conclusions A broad emission band of Eu2+ combined with the Pr3+ emissions were investigated in the Eu/Pr-codoped calcium aluminosilicate glasses. The sample was used as phosphor in a prototype with UV-blue LEDs to investigate their colorimetric parameters. The obtained CCT values cover the daylight range presenting Du’v’ less than 0.02, which is acceptable by the Japanese Standard Association. Adjusting the thickness of the phosphor layer, the emission can be tuned in a high CCT range (from 4304 to 8365K) with CRI Ra values higher than 90. By using a 445 and 405 nm commercial LEDs at different applied currents, the colorimetric

parameters do not loss quality: CRI Ra values ranging between 92 and 95, with tunable CCT values from 5141 to 5702K with |Du’v’| values ≤ to 0.01. These results show that the Eu2+,3+/Pr3+co-doped CAS glasses are promising materials for tunable white light devices using cheap commercial blue and violet LEDs.

Acknowledgments The authors thank Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Apoio ao Desenvolvimento do Ensino, Ciência e Tecnologia do Estado de Mato Grosso do Sul (FUNDECT - 59/300.031/2015 and 23/200.735/2014) for financial support.

Table 1. Colorimetric parameter data obtained for 0.5 wt.% Eu2O3/1.0 wt.% Pr6O11 co-doped CAS glass at different thicknesses (h1, h2 and h3) and LED current (LED intensity).

h1 = 1.00 mm

h2 = 1.25 mm

h3 = 1.50 mm

I445 (mA)

CRI Ra

CCT (K)

CRI Ra

CCT (K)

CRI Ra

CCT (K)

20

85.1

8365

91.7

6616

93.9

4528

65

88.9

7496

92.2

6074

94.5

4394

110

89.4

7486

93.2

5943

92.4

4316

155

90.1

7390

94.0

5869

92.4

4323

200

91.0

7199

94.1

5802

91.7

4283

300

91.9

6978

94.7

5733

91.7

4295

400

92.7

6928

95.0

5706

90.9

4304

Table 2. Colorimetric parameter data for 0.5 wt.% Eu2O3/1.0 wt.% Pr6O11 co-doped CAS glass at different 405 and 445 nm LED electric current with h2 = 1.25 mm thickness.

I405(mA)

I445(mA)

CCT

Du’v’

x

y

CRI Ra

20

135

5702

- 0.008

0.328

0.323

93

65

150

5473

- 0.002

0.333

0.338

95

110

170

5337

0.002

0.336

0.349

95

155

170

5233

0.006

0.340

0.359

94

200

160

5141

0.010

0.343

0.370

92

6 λem = 600 nm

(a)

PLE ( arb. units )

5

CAS glass doped with: 0.5wt.% Pr6O11 + 0.5wt.% Eu2O3 0.5wt.% Eu2O3

2+

Eu

0.5wt.% Pr6O11 2+

Eu

4

3+

Pr

3 2 glass charge transfer band

1

glass defects

0 250

16

300 350 400 Wavelength ( nm )

(b)

450 500 550

0.5 wt%Eu2O3/Xwt.%Pr6O11 co-doped CAS glass: 3

P 0→ 3H4 3

PL ( arb. units )

3+

Eu

12

P 0 →3 H 5

X= 3

0.2 0.5 1.0 2.0 4.0

P 0 →3 H 6

1

D 2 →3 H 4

3

P0→3F2

8 3

P 0 →3 F 3 3

4 λexc = 445 nm

0

500

600 Wavelength ( nm )

Figure 1. Morassuti et al.

P 0 →3 F 3 3

700

P 0 →3 F 4

800

Figure 2. Morassuti et al.

1.5 h1 = 1.00 mm

1.0

Current ( mA ) 20 65 110 155 200 300 400

λex445 nm

high current

0.5

PL ( arb. units )

1.5

(a)

h2 = 1.25 mm

1.0

0.5

1.5

(b)

h3 = 1.50 mm

1.0

0.5

(c)

0.0 400

500

600

Wavelength ( nm ) Figure 3. Morassuti et al.

700

800

520

0.8

Current(mA) 20 400

540

560

0.6 500

580

y

h3

0.4

600

h2 h1

620 (0.33, 0.33)

0.2 480

0.0 0.0

460

0.2

0.4 x

Figure 4. Morassuti et al.

0.6

0.8

Figure 5. Morassuti et al.

(a) 520

0.8

540

560

0.6 500

y

580 600

0.4

620

0.2 480

0.0 0.0

460

0.2

0.4 x

Figure 6. Morassuti et al.

0.6

0.8 (b)

Figure Captions Figure 1. Photoluminescence excitation (PLE) spectra (a) for Eu2+,3+:CAS, Pr3+:CAS and Eu2+,3+/Pr3+:CAS glasses, observing the emission at 600 nm. Photoluminescence (PL) spectra (b) for different ions concentration in the co-doped system, performed with excitation at 445 nm.

Figure 2. Chromaticity analysis in the CIE-1931 diagram for the Eu2+,3+:CAS [16], Pr3+:CAS [18] and Eu2+,3+/Pr3+:CAS glasses.

Figure 3. Photoluminescence (PL) spectra for the 0.5 wt.% Eu2O3/1.0 wt.% Pr6O11 co-doped CAS glass obtained with excitation at 445 nm. The electric current changed from 20 to 400 mA and the used powder thicknesses of the glass (small pieces of sample) were (a) h1 = 1.00 mm, (b) h2 = 1.25 mm and (c) h3 = 1.50 mm.

Figure 4. Chromaticity analysis in CIE-1931 diagram by calculating the (x, y) coordinates from the PL spectra of different sample powder thicknesses (small pieces of sample) and electric currents plotted in Figure 3.

Figure 5. Photoluminescence spectra of 0.5 wt.% Eu2O3/1.0 wt.% Pr6O11 co-doped CAS glass obtained with different 405 and 445 nm electric current. The insets show the (x, y) coordinates for each spectrum in the partial CIE-1931 chromaticity diagram.

Figure 6. (a) Photoluminescence spectra of 0.5 wt.% Eu2O3/1.0 wt.% Pr6O11 co-doped CAS glass with electric currents applied in both 405 and 445 nm LEDs to achieve maximum CRI Ra values. (b) CIE-1931 diagram showing the coordinates for the PL spectra.

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Grupo de Espectroscopia Óptica e Fototérmica Universidade Estadual de Mato Grosso do Sul

Highlights

Eu2+,3+/Pr3+ co-doped CAS glasses are promising materials for tunable white light devices. Correlated color temperature values cover the daylight range. Color rendering index Ra values ranging between 92 and 95.

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Declaration of interests x The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: