Photoluminescence Properties of Double Perovskite Tantalates

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Article

Photoluminescence Properties of Double Perovskite Tantalates Activated with Mn , AELaTaO:Mn (AE = Ca, Sr, and Ba) 4+

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Yohei Takeda, Hideki Kato, Makoto Kobayashi, Shunsuke Nozawa, Hisayoshi Kobayashi, and Masato Kakihana J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06280 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 15, 2017

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The Journal of Physical Chemistry

Photoluminescence Properties of Double Perovskite Tantalates Activated with Mn4+, AE2LaTaO6:Mn4+ (AE = Ca, Sr, and Ba) Yohei Takeda,† Hideki Kato,*,† Makoto Kobayashi,† Shunsuke Nozawa,‡ Hisayoshi Kobayashi,§ and Masato Kakihana† †

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1

Katahira, Aoba-ku, Sendai 980-8577, Japan ‡

Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1

Oho, Tsukuba, Ibaraki, 305-0801, Japan §

Department of Chemistry and Materials Technology, Graduate School of Science and

Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Corresponding author: Hideki Kato ([email protected])

ACS Paragon Plus Environment

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Abstract

Tetravalent manganese is known as one of the candidate luminous centers to obtain red emission. There are still unclear factors in Mn4+-activated oxide phosphors to achieve intense emission. In this paper, we studied the photoluminescence properties of double perovskite-type tantalates AE2LaTaO6 (AE = Ca, Sr, and Ba) activated with Mn4+. All AE2LaTaO6:Mn exhibited Mn4+emission in deep red region under excitation by near ultraviolet-green light (300–570 nm) at room temperature. Co-substitution of Mg2+, Al3+, and Ti4+ compensates unbalanced charge caused by oxygen defects, resulting in the enhancement of Mn4+-emission. The present cosubstitution effect is different from usual co-substitution, such as the replacement of two Al3+ by Mn4+ and Mg2+, taking into consideration the charge balance between cations. Theoretical calculation of band structures based on density functional theory suggests the presence of two kinds of quenching schemes in AE2LaTaO6:Mn, photoionization and electron transfer from valence band to t2g orbitals of Mn.


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INTRODUCTION White light-emitting diodes (W-LEDs) are solid-state lightning devices which have been becoming popular recently in the fields of lighting and display due to long life time, high energy efficiency, environmental friendliness, and small size.1–5 Common W-LEDs achieve artificial white light emission by combination of LED chips and phosphors. Therefore, luminescence properties of phosphors used in W-LEDs directly affect characteristics of the phosphorconverted W-LEDs. Generally speaking, the commercial and popular W-LEDs are composed of a blue LED chip (InGaN) and a yellow phosphor Y3Al5O12:Ce3+.6–8 Although these two-color WLEDs exhibit high luminescence efficiency, color rendering index and color temperature are poor and high, respectively, due to the lack of green and red components. To solve these problems, various new phosphors have been studied in order to fabricate high performance phosphor-converted W-LEDs which are blended in blue, green and red components.9–12 The nitride phosphors activated with Eu2+ such as (Sr,Ca)AlSiN3:Eu2+ and M2Si5N8:Eu2+ (M = Ca, Sr, Ba) are energetically investigated as red-emitting phosphors for three-color W-LEDs.13–15 However, these Eu2+-activated nitride phosphors have two disadvantages. One is necessity of severe conditions for synthesis of nitrides, such as high temperature and high pressure. The other is high cost and maldistribution of europium. Besides such Eu2+-activated nitride phosphors, the red-emitting oxides phosphors activated with tetravalent manganese, which is one of the abundant elements, have received great interest recently.16–29 For Mn4+-activated oxide phosphors, aluminates such as CaAl4O7, Sr4Al14O25 and LaAlO3 are extensively investigated as host materials,18–20 whereas titanates are also reported as suitable hosts for Mn4+-activated phosphors, such as Mg2TiO4, Li2TiO3, Ln2MgTiO6 (Ln = La, Gd), Ln2ZnTiO6 and BaMg6Ti6O19.21–27 In addition, semiconducting materials based on group five


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elements such as La2LiTaO6 and Ba2LaNbO6 also exhibit photoluminescence with Mn4+activation.28,29 We have recently discovered that Mn-activated double perovskite-type titanates, La2MgTiO6 and La2ZnTiO6, show strong deep-red emission at room temperature in spite of that SrTiO3 of the typical perovskite titanate does not show Mn4+-emission at room temperature due to significant temperature quenching.27 The theoretical study suggested that the less probability of the electron transfer from the valence band to the empty t2g orbitals of Mn in the excited state is an important factor to obtain meaningful emission at room temperature. In SrTiO3:Mn, t2g orbitals of Mn are embedded in a valence band while those in La2MgTiO6:Mn locate above a valence band. Band potentials of perovskite-type tantalates vary with kinds of A-site cation,30 therefore it is expected that investigation into a series of double perovskite-type compound gives more meaningful information about relationship between photoluminescence properties and band structures. In this study, we examined photoluminescence properties of Mn4+-activated double perovskite-type tantalates AE2LaTaO6 (AE = Ca, Sr, and Ba). The relationship between thermal quenching properties and band structures is also discussed. EXPERIMENTAL SECTION Synthesis. Mn-substituted samples, AE2LaTa(1–x)MnxO6, were synthesized by a polymerizable complex (PC) method employing anhydrous citric acid (Wako Pure Chemical; 98%) and propylene glycol (Kanto Chemical; 99%) as chelating and esterification reagents, respectively, in the similar manner described elsewhere.31 CaCO3 (Kanto Chemical; 99.99%), SrCO3 (Kanto Chemical; 99.9%), BaCO3 (Kanto Chemical; 99.99%), La(NO3)2·6H2O (Wako Pure Chemical; 99.9%), Ta(OC2H5)5 (Kojundo Chemical; 99.999%), and Mn(NO3)2·6H2O (Wako Pure Chemical;


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98.0%) were used as raw materials. The amorphous precursors obtained by the PC method were fired at 1473 K for 2 h in air. Additional annealing was performed for some Ba2LaTaO6 samples at 1473 K for 5 h under air, O2, or H2(4%)/Ar atmosphere to investigate the influences of annealing under different atmospheres. Samples co-substituted with Mn and either Mg, Al, or Ti (AE2LaTa(1–x–y)MnxMyO6 (M = Mg, Al, and Ti)) were also synthesized by the PC method employing Mg(NO3)2·6H2O (Kanto Chemical; 99.9%), Al(NO3)3·9H2O (Kanto Chemical; 98.0%), or Ti[O(CH2)3CH3]4 (Kanto Chemical; 97%) as a source of co-substituent in the same manner for the Mn-substituted samples. Characterization. The crystal phases of samples obtained were confirmed by X-ray diffraction analysis (XRD, Bruker AXS; D2 Phaser). Photoluminescence (PL) and corresponding excitation (PLE) spectra were taken using fluorescence spectrometers (Hitachi; F-4500 and Jasco; FP6500). Reflectance spectra of Mn-activated samples were taken by a fluorescence spectrometer equipping an integration sphere whereas those of non-activated samples were recorded by an absorption spectrometer equipping an integration sphere (Shimadzu; UV-3100). PL spectra were also taken at low temperature (77–300 K) using a cryostat (Janis; VPF-475). Measurements of X-ray absorption near-edge structure (XANES) at Mn-K edge were carried out in a fluorescence mode at BL9A of the Photon Factory in High Energy Accelerator Research Organization (Tsukuba, Japan) under approval of the Photon Factory Program Advisory Committee (2014S2006). Band Structure Calculation. The band structures were calculated by the plane wave based density functional theory (DFT) using CASTEP program.32,33 The Perdew-Burke-Ernzerhof (PBE) functional was used together with the ultrasoft-core potentials.34–36 The cutoff energies were set to 300 eV. The electron configurations of the atoms were O: 2s22p4, Ca: 3s23p64s2, Mn:


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3d54s2, Sr: 4s24p65s2, Ba: 5s25p66s2, La: 5s25p65d16s2, and Ta: 5d36s2. Super cells of Ca32La16Ta15MnO96, Sr16La8Ta7MnO48, and Ba32La16Ta15MnO96, which were constructed from crystal structure data of AE2LaTaO6,37–39 were employed for models of AE2LaTaO6:Mn4+. From the experimental finding, the local electronic structure for the substituted Mn atom is known to be a 4+ cation, and the Mn ion is in the quintet state. However, the super cells have even number of electrons, and then the calculations were carried out with the super cells possessing –1 charge and the quintet state for the three systems. Geometry optimization was carried out with respect to all atomic coordinates, and the lattice parameters were fixed to the original crystalline structure. The lattice parameters of super cells are listed in Table S1. RESULTS AND DISCUSSION PL Properties of Mn-Activated Ba2LaTaO6 (BLT:Mn). In BLT:Mn (x = 0.1–1%), XRD patterns indicated that all samples were obtained in pure BLT phase with a B-site ordered double perovskite structure (Figure S1). No changes in diffraction peak intensity indicate that amounts of Mn substitution do not affect crystallinity of the BLT phase. The shifts in diffraction peak position were negligible. In six-fold coordination environment, ionic radius of Mn4+ (67 pm) is much smaller than that of La3+ (117 pm) but close to that of Ta5+ (78 pm).40 Therefore, negligible shifts indicate that Mn4+ ions are substituted for Ta5+ sites rather than La3+ sites. BLT:Mn exhibited sharp deep red luminescence at 681 nm attributed to 2Eg→4A2g transition of Mn4+ when excited at 300–570 nm (Figure 1). Strong excitation bands in 300–440 nm and in 460–570 nm are attributed to spin-allow 4A2g→4T1g and 4A2g→4T2g transition, respectively, while a weak excitation band owing to spin-forbidden 4A2g→2T2g transition around 430 nm is not obvious and


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is embedded in the tail of the 4A2g→4T1g band.41 Thus, it has been revealed that BLT is available as a host material for Mn4+-activated phosphors. No changes in emission wavelengths and the

Relative intensity

2

(a)

1

0 300 2 Relative intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400

(b)

500

600

0.5% 0.3% 1% 0.2%

1

0.1%

0 550 0.2 Relative intensity

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600

650

700

750

(c)

0.1

0 550

600

650 700 Wavelength / nm

750

Figure 1. (a) PLE spectrum of BLT:Mn(0.5%) monitored at 681 nm and PL spectra of BLT:Mn(0.1–1%) excited at (b) 334–350 nm and (c) 240 nm. shape of emission bands were observed even when the concentration of Mn4+ was varied from 0.1 to 1%. The emission intensity increased with Mn concentration up to 0.5%, however, further increase in Mn concentration caused concentration quenching. When BLT:Mn was excited at shorter wavelengths than 300 nm, BLT:Mn exhibited broad orange emission around 570–700 nm in addition to sharp deep red emission as shown in Figure 1c. The H2-reduction treatment enhanced the broad orange emission while the sharp deep red emission was suppressed as shown


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in Figure 2. This alteration in PL behavior seemed to be due to change in the status of Mn. XANES spectra shown in Figure 2 indicate that as-prepared BLT:Mn(1%) has an absorption edge at a slightly lower energy side than MnO2 whereas the reduced sample shows the same onset energy as MnO. Thus, a small part of Mn is incorporated as Mn2+ in as-prepared BLT:Mn in spite of that the majority is present as Mn4+, and most of the Mn4+ ions are reduced to Mn2+ by the H2-reduction treatment. These results suggest that the broad orange emission is attributed to Mn2+ ions in octahedral coordination sites.42–44 In a magnified PLE spectrum of reduced BLT:Mn (inset of Figure 2a), typical excitation bands attributed to d-d transition of Mn2+ were seen although their intensities were very weak. The orange emission was observed under excitation at 418 nm in addition to the Mn4+-emission while excitation at 400 nm corresponding to a valley in the PLE spectrum did not give the orange emission (Figure S2). Moreover, non-doped BLT reduced by H2 did not show the orange emission. From these results, we concluded that the orange emission is originated from Mn2+ ions incorporated into the BLT lattice but not from the defects. The highly symmetric environment may strictly forbid such non-allowed d-d transition. Mn ions are likely to be substituted for Ta5+ with 4+ of oxidation state rather than 2+ in consideration of oxidation number of the site, however Mn2+ ions are actually present in asprepared BLT:Mn. Oxygen defects would allow the substitution of Mn2+ according to the formula Ba2LaTa0.99MnII0.01zMnIV0.01(1–z)O6–(0.005+0.01z) (an example for the sample with 1% of Mn substitution). In other words, Mn2+ is owing the responsibility to compensate the decrease in total negative charge caused by the oxygen defects in BLT:Mn.


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Relative intensity

(a)

0.004

0.8 0 350

0.4

400 450 λ / nm

λEm: 618 nm

reduced

500

λEx: 270 nm

as-prep.

0 300 (b)

400

500 600 Wavelength / nm (c)

0.8 as-prep. 0.4 reduced

Normalized absorbance

λEx: 350 nm λEm: 681 nm

Relative intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


700

reduced

as-prep. MnO MnO2

0 300 400 500 600 700 Wavelength / nm

6520

6540 6560 6580 Energy / eV

6600

Figure 2. PLE and PL of as-prepared and reduced BLT:Mn(1%) excited at (a) 270 nm and (b) 350 nm. Inset of (a) represents a magnified PLE spectrum of the reduced sample. (c) XANES spectra of BLT:Mn(1%) with reference samples of MnO and MnO2 (dashed lines). Effects of Co-substitution. In the cases of Eu2+-activated phosphors, co-existing Eu3+ ions weaken intensity of Eu2+-emission because Eu3+ ions work as a quencher for luminescence of Eu2+.45 It is thought that co-existing Mn2+ ions also decrease the efficiency of Mn4+-emission. Therefore, co-substitution was examined for BLT:Mn to enhance Mn4+-emission by elimination of co-existing Mn2+. As mentioned above, the formation of Mn2+ is due to oxygen defects in BLT:Mn. The formation of Mn2+ will be suppressed if co-dopants compensate the decreased negative charge. An Al3+ ion possessing lower oxidation number than Ta5+ was chosen as a codopant for the charge compensation. The Mn4+-emission was enhanced by 1.8 times with cosubstitution of Al3+ (BLT:Mn-Al) while the Mn2+-emission became negligible as shown in Figure 3a,b. No changes are observed in XRD patterns between BLT:Mn and BLT:Mn-Al as shown in


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Figure S3. This suggests that the enhancement of the Mn4+-emission is caused by changes in the status of Mn rather than the improvement of crystallinity. Two absorption bands coinciding with strong excitation bands for the Mn4+-emission were obviously strengthened by the co-substitution of Al3+ in spite of the same total concentration of Mn (Figure 3c). Thus, the co-substitution of Al3+ obviously increased the population of Mn4+ by elimination of Mn2+. In BLT:Mn-Al, the formation of Mn2+ is suppressed because co-substituted Al3+ ions take over the responsibility of the charge compensation from Mn ions. According to our hypothesis, other low valence ions also function as efficient co-dopants. Co-substitution with Mg2+ or Ti4+ indeed enhanced the Mn4+emission intensity (Figure S4). The co-substitution of Mg2+ and Ti4+ also showed the similar phenomena to that of Al3+ in absorption spectra and XRD patterns, that is, co-substitution made

Relative intensity

(a) Mn(0.2%)-Al(0.2%)

2

Mn(0.2%) 1

0 300

0.04

0.02

0 550

500 600 Wavelength / nm 100 (c) PLE

Reflectance (%)

Relative intensity

(b)

400

600 650 700 750 Wavelength / nm

700

50

0 300


Intensity / arb. units

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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600

Figure 3. (a) PL and PLE spectra of BLT:Mn(0.2%) and BLT:Mn(0.2%)-Al(0.2%) taken with excitation at 340 nm and monitoring at 681 nm. (b) PL spectra taken with excitation at 270 nm. (c) Reflectance spectra with a representative PLE spectrum (dashed line) of BLT:Mn.


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the absorption by Mn4+ intense and gave no changes in crystallinity as shown in Figure S5. On the other hand, additional annealing for BLT:Mn in O2 atmosphere was not effective to eliminate co-existing Mn2+ ions. The annealing treatment in O2 resulted in suppression of the Mn4+emission and enhancement of the Mn2+-emission (Figure S6). Annealing in air also showed similar effects on luminescence properties. These results suggest that the high temperature treatment facilitates the formation of oxygen defects in BLT. Thus, the co-substitution of lowvalence ions is effective to improve the Mn4+-emission from the BLT:Mn phosphors. There are many reports on enhancement of the Mn4+-emission by co-substitution with a concept of charge compensation

such

as

Sr4Al14O25:Mn-Mg,

La2LiTaO6:Mn-Mg,

Y3Al5O12:Mn-Mg

and

CaAl12O19:Mn-Mg.19,28,46,47 The co-substitution in these phosphors adopts general charge compensation strategy between cations and is done with the replacement of ions with lower and higher oxidation numbers; replacement of two Al3+ with Mg2+ and Mn4+ in aluminates whereas replacement of Ta5+ with Mn4+ accompanying with replacement of Li+ by Mg2+ in La2LiTaO6. The present co-substitution strategy is absolutely different from the reported ones. In cosubstituted BLT, both dopants, Mn4+ and either Mg2+, Al3+, or Ti4+, take lower oxidation numbers than Ta5+. It should be noticed that the substitution of these ions for La3+ sites can be reasonably excluded by large differences in ionic radii, La3+: 117 pm, Mn4+: 67 pm, Mg2+: 86 pm, Al3+: 67.5 pm, and Ti4+: 74.5 pm.40 The concept of charge compensation for oxygen defects by low-valence ions is new strategy to stabilize Mn4+ at Ta5+ sites.


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3

Mn(x%)-Al(x%) Relative PL intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Relative PL intensity


2

1 0

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Mn(0.4%)-Al(y%)

3

2 1

0 0 0.5 1 Mn concentration (at.%)

0

1 2 3 4 5 Al concentration (at.%)

Figure 4. Dependence of PL intensity on Mn concentration in BLT:Mn(x%)-Al(x%) and Al concentration in BLT:Mn(0.4%)-Al(y%). To obtain maximum emission intensity, optimization of concentration of dopants was carried out for BLT:Mn-Al, As first, amounts of Mn were varied to find optimum Mn concentration, where the amounts of Al were set at the same as those of Mn. The strongest Mn4+emission was obtained with 0.4% of Mn-activation in BLT:Mn-Al (Figures 4 and S7). Then, the amounts of Al were varied for the samples with 0.4% of Mn concentration. All samples were obtained as pure phase of BLT even with large amounts of Al such as 5% (Figure S8). Diffraction peaks slightly shifted to higher angle side with increases in the amount of Al, indicating substitution of Al3+ for Ta5+. A small amount of Al co-substitution (0.1%) was effective to obtain intense Mn4+-emission, however the emission intensity was not sensitive to concentration of Al3+ (Figures 4 and S7). The internal quantum yields were determined at room temperature to be 39.7% with 74.4% of absorption at 350 nm for BLT:Mn(0.5%) and 49.2% with 73.1% of absorption at 345 nm for BLT:Mn(0.4%)-Al(0.4%), respectively. Thus, cosubstitution of Al improved not only the apparent emission intensity but also the internal quantum yield. This proves that the co-existing Mn2+ works as a quencher of Mn4+-emission as Eu3+ does in the Eu2+-activated phosphors.


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Thermal quenching property is an important characteristic for phosphors. PL properties of BLT:Mn(0.4%)-Al(0.4%) were investigated at 25–200 ºC with 25 ºC of increment (Figure 5). Rising temperature did not affect the peak wavelength and the shape of emission spectra. However, emission intensity significantly decreased with rising temperature; 56% at 75 ºC and 29% at 100 ºC. The significant thermal quenching behavior is not favored for phosphors in WLEDs but available for optical thermometry sensors.48

1

1

25 ºC

0.8

Relative integral intensity

Relative intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50 ºC

0.6 75 ºC

0.4 100 ºC

0.2

175 ºC 200 ºC

0 600

125 ºC 150 ºC

0.8 0.6 0.4 0.2 0


750

0 50 100 150 200 Temperature / ºC

Figure 5. PL spectra and integral intensity of BLT:Mn(0.4%)-Al(0.4%) at 25–200 ºC under excitation at 350 nm. Comparison of PL Properties Between Mn-Activated AE2LaTaO6. As described above, BLT is available as a host for Mn4+-activated phosphor. Other double perovskite tantalates, Ca2LaTaO6 (CLT) and Sr2LaTaO6 (SLT), are also candidates for host materials due to the similarity in crystal structure. All of AE2LaTaO6 are crystallized in the B-site ordered double perovskite structure, however, CLT has slightly different feature from SLT and BLT in terms of kinds of B-site cations.37–39 B-sites are occupied by La and Ta in SLT and BLT while Ca and Ta occupy the B-sites in CLT and A-sites are shared by Ca and La. Each Mn-activated sample was obtained as a pure phase (Figure S9) and exhibited Mn4+-emission in deep red region at room


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temperature as shown in Figure 6. The co-substitution of Al3+ was also effective for CLT:Mn and SLT:Mn as well as BLT:Mn. This emphasizes the usefulness of the charge compensation between oxygen defects and low valence cations in AE2LaTaO6:Mn. The emission intensity widely varied with the host materials and the order of emission intensity was SLT < CLT < BLT. The emission peak wavelengths also varied with the host materials; 696, 700 and 681 nm in CLT:Mn, SLT:Mn and BLT:Mn, respectively. There is a relationship between emission 3 PLE

Relative intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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PL BLT:Mn-Al BLT:Mn

2

CLT:Mn-Al 1 SLT:Mn-Al SLT:Mn

CLT:Mn

0 300

400


700

800

Figure 6. PL and PLE spectra of AE2LaTaO6:Mn(0.4%)Al(0.4%) and AE2LaTaO6:Mn(0.5%). Excitation wavelength: 330 nm for CLT:Mn and CLT:Mn-Al, 322 nm for SLT:Mn and SLT:MnAl, and 340 nm for BLT:Mn and BLT:Mn-Al. Monitored wavelength: 695 nm for CLT:Mn and CLT:Mn-Al, 700 nm for SLT:Mn and SLT:Mn-Al, and 681 nm for BLT:Mn and BLT:Mn-Al. wavelengths and the Mn-O bond lengths analyzed from optimized structures by DFT calculation (Table S2), that is, the emission peak wavelength becomes shorter when an average Mn-O bond length is longer. PL measurements at low temperature were conducted to obtain further information about Mn-activated AE2LaTaO6 (Figure 7). In both CLT:Mn and BLT:Mn, as temperature was raised, the emission at shorter wavelength than ca. 670 nm was increased while


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2

λex: 320 nm

0.6

CLT:Mn

1 300 K

Relative intensity

Relative intensity

80 K

λex: 320 nm 80 K

SLT:Mn

0.4

0.2 300 K

0

0


BLT:Mn 80 K

3 2

650 700 750 Wavelength / nm

750

300 K

1

Normalized integral intensity

λex: 350 nm

4

Relative intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

BLT:Mn CLT:Mn

0.5

SLT:Mn

0

0 650 700 750 Wavelength / nm

100 150 200 250 300 Temperature / K

Figure 7. PL spectra of AE2LaTaO6:Mn(0.5%) in the range of 80–300 K and temperature dependence of integral intensity. Temperature decreased from 300 K to 80 K with a step of 20 K. that at longer wavelengths was decreased. SLT:Mn also showed the similar phenomenon except for the threshold wavelength (675 nm). The emission bands enhanced and suppressed with increasing temperature are attributed to anti-Stokes and Stokes bands, respectively.49 Integral emission intensities are analyzed to judge actual changes in emission intensities (Figure 7). SLT:Mn and CLT:Mn suffered remarkable thermal quenching and SLT:Mn showed more significant quenching than CLT:Mn. On the other hand, BLT:Mn survived thermal quenching without obvious decrease in integrated intensity in range of 80–300 K. These differences in thermal quenching properties result in the order of emission intensity at room temperature (SLT < CLT < BLT). The band gaps of CLT, SLT and BLT were determined from the absorption onsets in UV-vis spectra of non-doped samples to be 5.17, 4.98, and 4.77 eV, respectively


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(Figure S10). Our previous report suggests that oxide materials possessing wider band gaps are favorable as hosts for Mn4+-activated phosphors because energy barrier for electron transfer from the valence band to t2g orbitals of Mn is an important factor to obtain intense Mn4+-emission.27 However, the order of PL intensity (SLT < CLT < BLT) is inconsistent with the order of band gaps (BLT < SLT < CLT). Therefore, band structure was investigated by the DFT method to clarify what factors affect the PL intensity of Mn-activated AE2LaTaO6. 30

CLT:Mn

O2p

Ta5d

Density of state

20 Mn3d t2g(α)

10

eg(α)

0 -10

t2g(β)

-20 -30 -2

0

20

Density of state

2

4

O2p

SLT:Mn

Ta5d

Mn3d t2g(α)

10

eg(α)

0 -10

t2g(β)

-20 -2 30

0

BLT:Mn

20

Density of state

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4 Ta5d

O2p Mn3d t2g(α)

10

2

eg(α)

0 -10 t2g(β)

-20 -30 -2

0 Energy / eV

2

4

Figure 8. PDOS of AE2LaTaO6:Mn.


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Figure 8 depicts projected density of states (PDOS) near the band gap of AE2LaTaO6:Mn. We should notify that energies of excitation and emission cannot be discussed from differences in energy levels of Mn orbitals observed in calculated band structures although those events are caused by electron transition between Mn 3d orbitals. Because those energies should be discussed with differences in total energy between ground and excited states.50 However, these band structures calculated for the models in the 4A2g state reflect the relationship between Mn orbitals and conduction/valence bands of hosts.27 In all AE2LaTaO6:Mn, valence and conduction bands are composed of O 2p and Ta 5d orbitals, respectively. SLT:Mn shows a remarkable difference among AE2LaTaO6:Mn in terms of the location of empty eg(a) orbitals of Mn. The eg(a) orbitals are located below the conduction bands in CLT:Mn and BLT:Mn like as the previous La2MgTiO6:Mn,27 in contrast, those in SLT:Mn are embedded in its conduction band. It would be due to the large splitting between t2g(a) and eg(a) caused by the remarkably short MnO bond lengths in SLT:Mn (Table S2). On the other hand, differences are seen in the location of t2g(a) in AE2LaTaO6:Mn. The t2g(a) orbitals are almost embedded in the valence band in SLT:Mn while those in CLT:Mn and BLT:Mn are located above the valence band. BLT:Mn has a larger energy gap between t2g(a) and the valence band than CLT:Mn. Figure 9 shows electron density contour maps of top four occupied orbitals including the highest occupied molecular orbital (HOMO). The orbitals #579 are HOMO in CLT:Mn and BLT:Mn. Mn 3d orbitals are seen in the top three occupied orbitals from HOMO to the third highest occupied orbital (expressed as HOMO–2) in CLT:Mn and BLT:Mn. Only O 2p orbitals contribute to the fourth highest occupied orbital (HOMO–3), indicating that HOMO–3 corresponds to the top of valence band. Larger contributions of O 2p to orbitals from HOMO to HOMO–2 in CLT:Mn indicate stronger


17


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hybridization between Mn 3d and O 2p. Such stronger interaction between Mn 3d and O 2p may cause the smaller energy gap between t2g(a) and the valence band in CLT:Mn.

CLT:Mn #578 (HOMO-1)

#579 (HOMO)

Ca La Mn 3d + O 2p #577 (HOMO-2)

Mn 3d + O 2p

Ta

#576 (HOMO-3)

Mn O

Mn 3d + O 2p

O 2p

BLT:Mn #579 (HOMO)

#578 (HOMO-1)

Ba Mn 3d (+ O 2p) #577 (HOMO-2)

Mn 3d (+ O 2p) #576 (HOMO-3)

La Ta Mn O

Mn 3d (+ O 2p) O 2p

Figure 9. Electron density counter maps of CLT:Mn and BLT:Mn. Figure 10 illustrates the proposed mechanism based on the results of DFT calculation. In the excited states (4T1g and 2Eg), Mn4+ has an empty t2g(a) orbital (i.e. two of t2g(a) orbitals are occupied) and either one eg(a) or t2g(b) orbital is occupied in the state of 4T1g and 2Eg, respectively. The eg(a) orbitals are embedded in the conduction band in SLT:Mn. Transfer of an excited electron in eg(a) to the conduction band, which is called as photoionization and is one of quenching paths, easily occurs with embedded eg(a). In addition, no energy gap between t2g(a)


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orbitals and the valence band facilitates non-radiative recombination via electron transfer from the valence band to the empty t2g(a) orbital.27 A hole is formed in the valence band when an electron in the valence band moves to an empty t2g(a) orbital. Then, non-radiative recombination between the electron in the t2g(b) and the hole occurs. Thus, the significant thermal quenching and poor emission ability of SLT:Mn are caused by the photoionization and the electron transfer from the valence band to the empty t2g(a) orbital. CLT:Mn has narrower energy gap between t2g(a) and valence band than BLT:Mn due to the stronger interaction between Mn 3d and O 2p. Therefore, raising temperature promotes electron transfer from the valence band to the empty t2g(a) orbital in CLT:Mn. In contrast, the larger energy barrier in BLT:Mn prevents from such electron transfer in the range of 80–300 K, resulting in the strong emission at room temperature. CLT:Mn4+

SLT:Mn4+

e–

CB eg(α)

photoionization

t2g(β)

t2g(α) VB

BLT:Mn4+

strong emission

e–

e– e–

transfer

Figure 10. Mechanism in AE2LaTaO6:Mn. CONCLUSIONS We have found that double perovskite-type AE2LaTaO6:Mn exhibits deep red-emission attributed to Mn4+ ions and orange-emission attributed to Mn2+ ions. Co-substitution of low-valence ions such as Mg2+, Al3+, and Ti4+ for Ta5+ improves the internal quantum yield of the Mn4+-emission. It is due to elimination of co-existed Mn2+ by the compensation of unbalanced charge caused by oxygen defects. The oxidation numbers of both Mn4+ and co-substituents are lower than Ta5+.


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The present co-substitution is different from usual co-substitution of higher and lower valence cations, such as the replacement of two Al3+ by Mn4+ and Mg2+. Two different quenching paths are proposed for AE2LaTaO6:Mn in spite of members in the same family of double perovskitetype tantalates. One is the photoionization, which is caused by the eg(a) orbitals embedded into the conduction band. The other is the electron transfer from the valence band to an empty t2g(a) orbital. In SLT:Mn, both of the photoionization and the electron transfer owe the significant thermal quenching at low temperature (80–300 K) and poor emission at room temperature. The thermal quenching in CLT:Mn is promoted via the electron transfer from the valence band to an empty t2g(a) by rising temperature in the range of 80–300 K due to the small energy barrier. In contrast, BLT:Mn is tough against thermal quenching in such low temperature range because of the large energy barrier from the valence band to t2g(a) orbitals and no overlaps between the conduction band and the eg(a) orbitals. Thus, this work indicates importance of large energy gaps between the valence band and t2g(a) orbitals and between the conduction band and eg(a) orbitals in Mn4+-activated oxide phosphors. ASSOCIATED CONTENT Supporting Information. Lattice parameters of the calculation models, Mn-O bond lengths, and details of XRD patterns, luminescence spectra, and reflectance spectra of non-doped hosts (PDF) are available free of charge via Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected].


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was partly supported by KAKENHI Grant Numbers (JP24107004, JP16H02391, and JP15H00890). REFERENCES (1) Krames, M. R.; Shchekin, O. B.; M.-Mach, R.; Mueller, G. O.; Zhou, L.; Harbers, G.; Craford, M. G. Status and Future of High-Power Light-Emitting Diodes for Solid-State Lighting. J. Display Technol. 2007, 3, 160–175. (2) Smet, P. F.; Parmentier, A. B.; Poelman, D. Selecting Conversion Phosphors for White Light-Emitting Diodes. J. Electrochem. Soc. 2011, 158, R37–R54. (3) Schubert, E. F.; Kim, J. K. Solid-State Light Sources Getting Smart. Science 2005, 308, 1274–1278. (4) Chen, L.; Lin, C.-C.; Yeh, C.-W.; Liu, R.-S. Light Converting Inorganic Phosphors for White Light-Emitting Diodes. Materials 2010, 3, 2172–2195. (5) Xia, Z.; Xu, Z.; Chen, M.; Liu, Q. Recent Developments in the New Inorganic Solid-State LED Phosphors. Dalton Trans. 2016, 45, 11214–11232.


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TOC Graphic Thermal quenching

BLT:Mn4+

1

strong emission

Relative intensity

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CLT:Mn4+

BLT:Mn4+

SLT:Mn4+ CB eg(α) photoionization t2g(β)

e–

CLT:Mn4+ SLT:Mn4+

0

100 200 300 Temperature / K

e–

e–

t2g(α) VB

e– transfer


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Photoluminescence Properties of Double Perovskite Tantalates

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