Photoluminescence Properties of Layered Perovskite-Type Strontium Scandium Oxyfluoride Activated With Mn4+

Introduction

White light emitting diodes (W-LEDs) based on blue-LEDs are widely spreading to various fields as highly efficient solid lightings (Lin et al., 2016; Adachi, 2018; Wang et al., 2018). Artificial white light is basically obtained by combination of blue and yellow light emitted from a blue-LED chip and a yellow-emitting phosphor Y₃Al₅O₁₂:Ce, respectively. Such white light is inevitably cool white with high color temperature due to poor emission strength of Y₃Al₅O₁₂:Ce in red region. Efficient red-emitting phosphors are added to achieve artificial warm white light by tuning color temperature. Nitride phosphors activated with Eu²⁺ such as (Sr,Ca)AlSiN₃:Eu²⁺ and M₂Si₅N₈:Eu²⁺ (M = Ca, Sr, and Ba) are extensively studied and commercially used as the red-emitting phosphors (Li et al., 2006, 2009; Uheda et al., 2006; Watanabe and Kijima, 2009; Tsai et al., 2015; Wang et al., 2018). However, requirements of high temperature and high pressure in synthesis of nitrides are drawbacks of nitride phosphors rising the costs. Therefore, development of alternative yellow—to red-emitting phosphors activated with Eu²⁺, which can be synthesized milder conditions in comparison with nitrides, is also conducted for oxides, phosphates, and oxyhalides (Toda et al., 2006; Daicho et al., 2012, 2018; Kim et al., 2013; Sato et al., 2014; Wen et al., 2016). Besides, phosphors activated with Mn⁴⁺ have been recently paid attention due to capability of red emission using wide variety of host materials (Srivastava and Beers, 1996; Seki et al., 2013; Ye et al., 2013; Sasaki et al., 2014; Wang et al., 2014; Takeda et al., 2015, 2017; Zhou et al., 2016; Cai et al., 2017; Wu et al., 2017; Xi et al., 2017; Zhang et al., 2017; Adachi, 2018; Jansen et al., 2018). Octahedral 6-fold coordination sites are preferred for substitution of Mn⁴⁺ ions. Fluorides and aluminates are paid much attention as hosts of Mn⁴⁺-activated phosphors from the viewpoints of their insulating nature and octahedral sites. Besides, titanates having semiconducting nature are also available for hosts of Mn⁴⁺-activated phosphors (Srivastava and Beers, 1996; Seki et al., 2013; Ye et al., 2013; Sasaki et al., 2014; Takeda et al., 2015; Zhang et al., 2017). We have recently reported that double perovskite-type titanates La₂MTiO₆ (M: Mg and Zn) are available as host materials of Mn⁴⁺-activated phosphors although a representative perovskite-type titanate SrTiO₃ doped with Mn⁴⁺ could not show any luminescence at room temperature due to significant thermal quenching at low temperature, ~100 K (Takeda et al., 2015). Low temperature photoluminescence measurements and theoretical band structure calculations have revealed the importance of relative position of Mn 3d orbitals to valence and conduction bands of host materials in order to avoid electron transfer from the valence band to empty t_2g orbital of Mn and photoionization. The knowledge obtained from the previous researches encourages us to expand the research target for Mn⁴⁺-activated phosphors to Sr₂TiO₄ possessing a K₂MgF₄ type layered perovskite structure. Both SrTiO₃ and Sr₂TiO₄ are members in a perovskite family composed of the same constituent elements. SrTiO₃ of the representative perovskite-type compound is composed of TiO₆ octahedra sharing corners infinitely, building three-dimensional bulky structure, while Sr₂TiO₄ has layers of the two-dimensional perovskite slab with a single TiO₆ thickness separated by SrO layers as depicted in Figure 1. The decreases in structural dimension cause widening band gaps (Reyes-Lillo et al., 2016), which is thought to be a positive factor to suppress the electron transfer and/or the photoionization. Therefore, it is expected that comparison photoluminescence properties between SrTiO₃:Mn and Sr₂TiO₄:Mn gives important information to understand the relationship between structural dimension and photoluminescence properties with Mn⁴⁺-activation.

FIGURE 1

Figure 1. Crystal structures of SrTiO₃ and Sr₂TiO₄.

In this research, we investigated photoluminescence properties of Mn⁴⁺-activated layered perovskite compounds. The differences in photoluminescence properties especially thermal quenching properties between SrTiO₃:Mn and Sr₂TiO₄:Mn are discussed from features in crystal structures. In addition, we also investigated into photoluminescence properties of Sr₂ScO₃F:Mn possessing the K₂MgF₄ type structure as well as Sr₂TiO₄:Mn.

Experiments

Sample Preparation

All powder samples were synthesized by a solid state reaction method using SrCO₃ (Kanto, 99.9%), SrF₂ (Wako, 99.5%), rutile type TiO₂ (Kojundo Chemical, 99.9%), Sc₂O₃ (Shin-Etsu Chemical, 99.99%), and Mn(NO₃)₂ 6H₂O (Wako, 98.0%) as raw materials. The stoichiometric mixtures of the raw materials were calcined at 1473 K for 5 h in air using alumina crucibles. Where, concentration of Mn substitution was fixed at 0.2 atom% to Ti or Sc. Non-doped samples were also synthesized by the same manner.

Characterization of Samples

Crystal phases of obtained samples were confirmed by powder X-ray diffraction (XRD) technique (Bruker, D2 Phaser). Photoluminescence measurements were performed using fluorescence spectrometers (Hitachi; F-4500 and Jasco; FP-6500). Photoluminescence spectra were also taken at low temperature (80–300 K with a step of 20 K) using a cryostat (Janis; VPF-475) under vacuum. Diffuse reflectance spectra of non-doped samples were taken by an absorption spectrometer equipping an integration sphere (Shimadzu; UV-3100). The band gaps of the non-doped samples with indirect transition were determined from (αhν)^1/2–hν plot, where α, h, and ν represent Kubelka-Munk function, Planck constant, and frequency, respectively.

Band Structure Calculation

The band structures were calculated by the plane wave based density functional theory (DFT) using CASTEP program (Payne et al., 1992; Milman et al., 2000). The Perdew-Burke-Ernzerhof (PBE) functional was used together with the ultrasoft-core potentials (Vanderbilt, 1991; Perdew et al., 1996, 1997). The cutoff energies were set to 300 eV. The electron configurations of the atoms were O: 2s²2p⁴, F: 2s²2p⁵, Sc: 3s²3p⁶3d¹4s², Ti: 3s²3p⁶3d²4s², Mn: 3d⁵4s², and Sr: 4s²4p⁶5s². Super cells of Sr₁₆Ti₇MnO₃₂ and Sr₁₆Sc₇MnO₂₅F₇ were employed for models of Sr₂TiO₄:Mn and Sr₂ScO₃F:Mn, respectively. Where, one F atom was also replaced with an O atom accompanied by the substitution of Mn for Sc to maintain the charge balance in the Sr₂ScO₃F:Mn system. 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. Geometry optimization was carried out with respect to all atomic coordinates.

Results and Discussion

Luminescence of Sr₂TiO₄:Mn

Figure 2 shows photoluminescence spectra of SrTiO₃:Mn and Sr₂TiO₄:Mn with corresponding excitation spectra at room temperature. Sr₂TiO₄:Mn showed deep-red emission with a peak at 725 nm attributed to²E_g→⁴A_2g transition of Mn⁴⁺ under excitation at 300–580 nm. Although the emission intensity is not high, this is an interesting result taking into consideration of the fact that SrTiO₃:Mn shows no emission at room temperature due to significant thermal quenching. Although both strontium titanates are composed of the same elements and are members of the perovskite family, a remarkable difference is present with regard to the structural dimension; Sr₂TiO₄ has a two-dimensional layered structure whereas SrTiO₃ has a three-dimensional bulky one. Therefore, the appearance of Mn⁴⁺-emission in Sr₂TiO₄:Mn may reflect advantage of the layered perovskite structure in the band structure than bulky one. The further discussion about Sr₂TiO₄:Mn is described later.

FIGURE 2

Figure 2. Luminescence spectra of SrTiO₃:Mn and Sr₂TiO₄:Mn and corresponding excitation spectra at room temperature. Excitation and monitored wavelengths were 380 and 725 nm, respectively.

Comparison of Luminescence Properties Between Sr₂ScO₃F:Mn and Sr₂TiO₄:Mn

Although, as shown in Figure 3A, Sr₂TiO₄ has a wider band gap (3.46 eV) than SrTiO₃ (3.21 eV) as reported in literature (Reyes-Lillo et al., 2016), other layered perovskite compounds possessing wider band gaps are preferred for efficient Mn⁴⁺-emission because of less probability of the electron transfer between Mn 3d and the valence and/or conduction band. Strontium scandium oxyfluoride Sr₂ScO₃F with a K₂MgF₄ type structure as well as Sr₂TiO₄, which has been recently discovered (Wang et al., 2015), was thought to be a good candidate because its octahedral building unit ScO₅F based on the optically inert rare earth element was expected to give a wider energy gap in comparison with TiO₆. XRD confirmed that Sr₂ScO₃F:Mn was obtained as the almost pure phase of Sr₂ScO₃F although it contained tiny amounts of SrF₂ and SrSc₂O₄ as impurities whereas Sr₂TiO₄:Mn was obtained as a pure phase without any impurities (Figure 3B). Relative intensities of diffraction peaks of Sr₂ScO₃F:Mn at 14.1, 28.3, 43.0, and 58.5 degrees corresponding to reflections from (002), (004), (006), and (008), respectively, were remarkably strong in comparison with the standard ones due to orientation of crystals in (00l). The band gap of Sr₂ScO₃F has been discovered to be 5.38 eV, being wider than that of Sr₂TiO₄ (Figure 3A). Figure 4 shows emission and excitation spectra of Sr₂ScO₃F:Mn and Sr₂TiO₄:Mn at room temperature. Sr₂ScO₃F:Mn showed deep-red emission owing to transition of Mn⁴⁺ giving a peak at 697 nm. Obvious two excitation bands in 300–460 nm and in 480–580 nm are attributed to spin-allow⁴A_2g→⁴T_1g and⁴A_2g→⁴T_2g transition of Mn⁴⁺ ions, respectively, while a weak excitation band owing to spin-forbidden⁴A_2g→²T_2g transition is difficult to distinguish and it may be embedded in the tail of the⁴A_2g→⁴T_1g band as observed in other titanates and tantalates (Sasaki et al., 2014; Wang et al., 2014; Takeda et al., 2015, 2017). Interestingly, the emission from Sr₂ScO₃F:Mn was much stronger than that from Sr₂TiO₄:Mn; the internal and external quantum yields of Sr₂ScO₃F:Mn excited at 345 nm at room temperature (50.5 and 43.5%) were much higher than those of Sr₂TiO₄:Mn excited at 380 nm (3.4 and 2.5%). The Mn⁴⁺-emission from fluoride hosts consists of some very sharp lines while that from oxide hosts is broad (Zhou et al., 2016; Adachi, 2018). The emission from Sr₂ScO₃F:Mn is broad as well as Mn⁴⁺-activated oxide phosphors despite presence of the Sc-F bond. This means that influences of F upon the photoluminescence property of Sr₂ScO₃F:Mn are not significant. In Sr₂ScO₃F:Mn, it is preferred from the charge compensation that one fluorine is replaced with one oxygen when Mn⁴⁺ is substituted for Sc³⁺. Such co-substitution results in the formation of MnO₆ octahedra which give broad Mn⁴⁺-emission. The spectra of CaSiAlN₃:Eu, which is the representative red-emitting phosphor activated with Eu²⁺, are also shown in Figure 4. The Sr₂ScO₃F:Mn emission is sharper and stronger than the CaSiAlN₃:Eu emission however the wavelength of the Sr₂ScO₃F:Mn emission is excessively long, that is, almost the half portion of emission is located in the invisible region (λ > 700 nm). CaAlSiN₃:Eu can be excited by blue-LEDs (λ = 450–470 nm) more efficiently than Sr₂ScO₃F:Mn while Sr₂ScO₃F:Mn can be excited by near ultraviolet LEDs (λ = 350–400 nm) more efficiently than CaAlSiN₃:Eu.

FIGURE 3

Figure 3. (A) Diffuse reflectance spectra of non-doped Sr₂TiO₄, Sr₂ScO₃F, and SrTiO₃ and (B) XRD patterns of Sr₂TiO₄:Mn and Sr₂ScO₃F:Mn with reference patterns.

FIGURE 4

Figure 4. Luminescence spectra of Sr₂TiO₄:Mn, Sr₂ScO₃F:Mn, and CaAlSiN₃:Eu with corresponding excitation spectra at room temperature. Excitation and monitored wavelengths were 380 and 725 nm for Sr₂TiO₄:Mn, 345 and 697 nm for Sr₂ScO₃F:Mn, and 340 and 630 nm for CaAlSiN₃:Eu.

Measurements of thermal quenching were performed at low (80–300 K) and high temperature ranges (298–473 K). Both samples suffered temperature quenching even in the low temperature range especially higher than 200 K as shown in Figures 5A–C. The maximum peak intensity decreased as measurement temperature rose while the emission of anti-Stokes sidebands, which were observed in regions shorter than 710 and 675 nm in Sr₂TiO₄:Mn and Sr₂ScO₃F:Mn, respectively, was enhanced due to transition of excited electrons to upper vibration states by thermal energy (Wu et al., 2017; Adachi, 2018). It results in the non-obvious decreases in the integrated emission intensity up to 200 K. Sr₂ScO₃F:Mn exhibited stronger emission than Sr₂TiO₄:Mn at all temperatures, moreover, the intensity of Sr₂TiO₄:Mn at 80 K was lower than that of Sr₂ScO₃F:Mn at 300 K. Thus, Sr₂TiO₄:Mn exhibited more remarkable thermal quenching in comparison with Sr₂ScO₃F:Mn. In the high temperature range (298–473 K), significant thermal quenching occurred in both samples as shown in Figure 5D, however Sr₂ScO₃F:Mn showed lesser thermal quenching than Sr₂TiO₄:Mn. At 373 K, Sr₂ScO₃F:Mn showed 20% of emission intensity in comparison with that at 298 K whereas emission from Sr₂TiO₄:Mn was completely quenched.

FIGURE 5

Figure 5. Emission spectra of (A) Sr₂TiO₄:Mn and (B) Sr₂ScO₃F:Mn at low temperature (80–300 K), (C) relative integrated emission intensity of them, and (D) normalized integrated emission intensity of Sr₂TiO₄:Mn and Sr₂ScO₃F:Mn at high temperature (298–473 K). Excitation wavelengths were 380 and 340–345 nm for Sr₂TiO₄:Mn and Sr₂ScO₃F:Mn, respectively.

Band Structures of Sr₂TiO₄:Mn and Sr₂ScO₃F:Mn

As described above, Sr₂ScO₃F:Mn showed superior characteristics, that is, higher emission intensity and lesser thermal quenching, to Sr₂TiO₄:Mn. Relative position of Mn 3d orbitals to the valence and conduction bands of host materials is an important factor for Mn⁴⁺-activated phosphors as we have reported previously (Takeda et al., 2015, 2017). Therefore, band structures of Sr₂TiO₄:Mn and Sr₂ScO₃F:Mn were investigated by the DFT method. Figure 6 depicts projected density of states (PDOS) near the band gap of Sr₁₆Ti₇MnO₃₂ and Sr₁₆Sc₇MnO₂₅F₇ corresponding to Sr₂TiO₄:Mn and Sr₂ScO₃F:Mn. In Figure 6, positive and negative values in DOS represent DOS for up-spin (α) and down-spin (β) electrons, respectively, and 0 eV of energy represents the Fermi level. In Sr₂TiO₄:Mn, the valence and conduction bands of host are composed of O 2p and Ti 3d orbitals, respectively, like SrTiO₃:Mn. In PDOS of Sr₂TiO₄:Mn, the t_2g(α) orbitals of Mn look to be located slightly higher position than the valence band however a tail of t_2g(α) is embedded in the valence band. The DFT calculation reveals that Sr₂TiO₄:Mn has absolutely different feature in the relative position of Mn 3d orbitals to the valence band from SrTiO₃:Mn, in which the t_2g(α) orbitals are deeply embedded in the valence band (Takeda et al., 2015). Although a part of t_2g(α) orbitals is located in positive energy region, it doesn't indicate the presence of empty t_2g(α) orbitals. The total numbers of electrons calculated for both Sr₁₆Ti₇MnO₃₂ and Sr₁₆Sc₇MnO₂₅F₇ models were 443. In the quintet state, the numbers of occupied orbitals should be 223 and 220 for α- and β-electrons, respectively. If the top of t_2g(α) orbital of Mn 3d is empty, the lowest unoccupied molecular orbital for α-electron (#224 α-orbital) should be Mn 3d located near 0 eV. However, #224 α-orbital is not Mn 3d located around 0 eV but Mn3d orbital below the conduction band [indicated as e_g(α) in Figure 6] in both models. The small portion of the occupied orbitals beyond the Fermi level observed in PDOS is due to broadening of energy widths of orbitals by the smearing treatment in the process of PDOS creation. Thus, it has been confirmed that all Mn 3d orbitals around 0 eV are occupied ones. The PDOS of Sr₂ScO₃F:Mn shows that the t_2g(α) orbitals of Mn is located slightly higher position than the valence band without tailing portion at a lower energy side. The electron density contour maps for top four occupied molecular orbitals including the highest occupied molecular orbital (HOMO) are compared to see details of the differences between Sr₂TiO₄:Mn and Sr₂ScO₃F:Mn (Figure 7). In Sr₂ScO₃F:Mn, contribution of the occupied Mn 3d orbitals is seen only in the top three occupied orbitals (from HOMO to HOMO−2) and the forth highest occupied orbital (HOMO−3) is composed of only O 2p orbital. On the other hand, small contribution of the Mn 3d orbital is also seen in HOMO−3 of Sr₂TiO₄:Mn although Mn 3d orbitals mainly contribute to top three occupied orbitals. If hybridization between O 2p and t_2g(α) of Mn 3d is small, the occupied Mn 3d orbitals appear in only three orbitals. The appearance of Mn 3d in four orbitals in Sr₂TiO₄:Mn (from HOMO to HOMO−3) indicates stronger hybridization between O 2p and Mn 3d than Sr₂ScO₃F:Mn. It is also noticed in PDOS that energy gap between e_g(α) of Mn 3d and the bottom of conduction band is larger in Sr₂ScO₃F:Mn than Sr₂TiO₄:Mn. It reflects the remarkably wider band gap of Sr₂ScO₃F than Sr₂TiO₄.

FIGURE 6

Figure 6. PDOS of Sr₁₆Ti₇MnO₃₂ and Sr₁₆Sc₇MnO₂₅F₇.

FIGURE 7

Figure 7. Electron density contour maps of top four occupied orbitals for (A) Sr₁₆Ti₇MnO₃₂ and (B) Sr₁₆Sc₇MnO₂₅F₇.

Figure 8 illustrates proposed mechanism based on photoluminescence measurements and band structure calculations for Mn⁴⁺-activated SrTiO₃, Sr₂TiO₄, and Sr₂ScO₃F. The most significant difference in photoluminescence property between Sr₂TiO₄:Mn and SrTiO₃:Mn is the appearance of Mn⁴⁺-emission in Sr₂TiO₄:Mn at room temperature. The difference in the relative position of t_2g(α) orbitals of Mn between SrTiO₃:Mn and Sr₂TiO₄:Mn is of importance in explanation for the appearance of Mn⁴⁺-emission in Sr₂TiO₄:Mn. In SrTiO₃:Mn, the t_2g(α) orbitals embedded in the valence band facilitate thermal quenching via the electron transfer from the valence band to the empty t_2g in the excited state²E_g, resulting in low quenching temperature, ~100 K (Takeda et al., 2015). In contrast to SrTiO₃:Mn, t_2g(α) in Sr₂TiO₄:Mn is located slightly higher position than the valence band. Therefore, Sr₂TiO₄:Mn shows Mn⁴⁺-emission even at room temperature. Interaction between TiO₆ octahedra may affect the relative position of t_2g(α) orbitals of Mn. Each TiO₆ octahedron connects to six TiO₆ octahedra in SrTiO₃ while TiO₆ connects to four TiO₆ octahedra in the perovskite slab in Sr₂TiO₄ as shown in Figure 1, indicating that degree of energy delocalization is higher in three-dimensional SrTiO₃ than two-dimensional Sr₂TiO₄. The smaller interaction between TiO₆ octahedra may cause less interaction between O 2p and occupied of Mn 3d orbitals, that is, t_2g(α) orbitals. Thus, the advantage of two-dimensional layered perovskite structure for Mn⁴⁺-activated phosphors can be explained by the smaller interaction of MO₆ octahedra. Such discussion can be applied to another Mn⁴⁺-activated titanate phosphor La₂MgTiO₆:Mn with a B-site ordered double perovskite structure, which is an efficient Mn⁴⁺-activated phosphor with 58.7% of an internal quantum yield (Takeda et al., 2015). In La₂MgTiO₆, each TiO₆ octahedron is surrounded by six MgO₆ octahedra, meaning that TiO₆ is isolated from other TiO₆ octahedra even though the perovskite-type structure (Lee et al., 2000). Thus, the structure of La₂MgTiO₆ can be regarded as a quasi-zero-dimensional structure with respect to the connection between TiO₆ octahedra. The less interaction of TiO₆ in La₂MgTiO₆:Mn leads the larger energy gap between the valence band and t_2g(α) of Mn, resulting in the superior photoluminescence efficiency to Sr₂TiO₄:Mn. In Sr₂ScO₃F:Mn, the t_2g(α) orbitals of Mn 3d are located above the valence band with a slightly larger energy gap than Sr₂TiO₄:Mn due to the small hybridization between O 2p and Mn 3d as described above. On the other hand, the larger energy gap between e_g(α) and the bottom of conduction band in Sr₂ScO₃F:Mn suppresses quenching via photoionization, in which an electron in e_g(α) orbitals in the excited state is transferred to the conduction band and then is relaxed without emission (Takeda et al., 2017). Thus, the two factors, the less interaction between t_2g(α) and the valence band and the large energy gap between e_g(α) and the bottom of conduction band, positively affect the smaller thermal quenching in Sr₂ScO₃F:Mn than Sr₂TiO₄:Mn, resulting in the stronger emission.

FIGURE 8

Figure 8. Proposed mechanism for SrTiO₃:Mn, Sr₂TiO₄:Mn, and Sr₂ScO₃F:Mn.

Conclusions

Photoluminescence properties of Mn⁴⁺-activated strontium titanates, SrTiO₃:Mn with three-dimensional bulky perovskite structure and Sr₂TiO₄:Mn with two-dimensional layered perovskite structure, have been compared in this research. Sr₂TiO₄:Mn shows Mn⁴⁺-emission even at room temperature despite no emission from SrTiO₃:Mn. In addition, the results in our systematic research suggest that the less interaction between MO₆ octahedra of B-site cation in the perovskite family provides positive influences in Mn⁴⁺-emission. Comparison between Sr₂TiO₄:Mn and Sr₂ScO₃F:Mn indicates that ScO₅F octahedra are preferable constituents to TiO₆ ones for the Mn⁴⁺-activated phosphors. Thus, the present research demonstrates that scandium, which is one of optically inert rare earth elements, is a useful element as a major constituent for design of Mn⁴⁺-activated phosphors.

Author Contributions

HidK managed all experiments and wrote the manuscript. YT performed experiments on synthesis of samples and evaluation of photoluminescence properties of them. HisK performed band structure calculations. MakK and MasK planned experiments and made discussion about the results.

Funding

This research was partly supported by KAKENHI Grant Numbers JP16H06438 and JP16H06439 in Scientific Research on Innovative Area Mixed Anion and JP16H02391 in Scientific Research (A) and Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials in Network Joint Research Center for Materials and Devices.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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