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

In this research, we have found that layered perovskite titanate Sr2TiO4 doped with Mn4+ exhibits photoluminescence even at room temperature despite no luminescence from Mn4+-doped SrTiO3 with a three-dimensional bulky perovskite structure. The relative position of t2g orbital of Mn to the valence band is a key factor for appearance of Mn4+-emission in Sr2TiO4:Mn. This result suggested usefulness of layered perovskite-type materials as hosts for Mn4+-activated phosphors than the bulky perovskite-type materials. Our investigation into photoluminescence of Mn4+-doped layered perovskite compounds has revealed that strontium scandium oxyfluoride Sr2ScO3F activated with Mn4+ exhibits Mn4+-emission with a peak at 697 nm under excitation at 300–600 nm and its emission intensity is much stronger than that of Sr2TiO4:Mn. The internal and external quantum yields of Sr2ScO3F:Mn were determined to be 50.5 and 43.5% under excitation at 345 nm, respectively.


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 Y 3 Al 5 O 12 :Ce, respectively. Such white light is inevitably cool white with high color temperature due to poor emission strength of Y 3 Al 5 O 12 :Ce in red region. Efficient redemitting phosphors are added to achieve artificial warm white light by tuning color temperature. Nitride phosphors activated with Eu 2+ such as (Sr,Ca)AlSiN 3 :Eu 2+ and M 2 Si 5 N 8 :Eu 2+ (M = Ca, Sr, and Ba) are extensively studied and commercially used as the red-emitting phosphors (Li et al., 2006(Li et al., , 2009Uheda 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 yellowto red-emitting phosphors activated with Eu 2+ , 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., 2012Daicho et al., , 2018Kim et al., 2013;Sato et al., 2014;Wen et al., 2016). Besides, phosphors activated with Mn 4+ 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., 2015Takeda et al., , 2017Zhou 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 6fold coordination sites are preferred for substitution of Mn 4+ ions. Fluorides and aluminates are paid much attention as hosts of Mn 4+ -activated phosphors from the viewpoints of their insulating nature and octahedral sites. Besides, titanates having semiconducting nature are also available for hosts of Mn 4+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 La 2 MTiO 6 (M: Mg and Zn) are available as host materials of Mn 4+ -activated phosphors although a representative perovskite-type titanate SrTiO 3 doped with Mn 4+ 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 Mn 4+ -activated phosphors to Sr 2 TiO 4 possessing a K 2 MgF 4 type layered perovskite structure. Both SrTiO 3 and Sr 2 TiO 4 are members in a perovskite family composed of the same constituent elements. SrTiO 3 of the representative perovskite-type compound is composed of TiO 6 octahedra sharing corners infinitely, building threedimensional bulky structure, while Sr 2 TiO 4 has layers of the two-dimensional perovskite slab with a single TiO 6 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 SrTiO 3 :Mn and Sr 2 TiO 4 :Mn gives important information to understand the relationship between structural dimension and photoluminescence properties with Mn 4+activation.
In this research, we investigated photoluminescence properties of Mn 4+ -activated layered perovskite compounds. The differences in photoluminescence properties especially thermal quenching properties between SrTiO 3 :Mn and Sr 2 TiO 4 :Mn are discussed from features in crystal structures. In addition, we also investigated into photoluminescence properties of Sr 2 ScO 3 F:Mn possessing the K 2 MgF 4 type structure as well as Sr 2 TiO 4 :Mn.

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., 1996Perdew et al., , 1997. The cutoff energies were set to 300 eV. The electron configurations of the atoms were O: 2s 2 2p 4 , F: 2s 2 2p 5 , Sc: 3s 2 3p 6 3d 1 4s 2 , Ti: 3s 2 3p 6 3d 2 4s 2 , Mn: 3d 5 4s 2 , and Sr: 4s 2 4p 6 5s 2 . Super cells of Sr 16 Ti 7 MnO 32 and Sr 16 Sc 7 MnO 25 F 7 were employed for models of Sr 2 TiO 4 :Mn and Sr 2 ScO 3 F:Mn, respectively. Where, one F atom was also replaced with an O atom accompanied by the FIGURE 2 | Luminescence spectra of SrTiO 3 :Mn and Sr 2 TiO 4 :Mn and corresponding excitation spectra at room temperature. Excitation and monitored wavelengths were 380 and 725 nm, respectively. substitution of Mn for Sc to maintain the charge balance in the Sr 2 ScO 3 F: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 Sr 2 TiO 4 :Mn Figure 2 shows photoluminescence spectra of SrTiO 3 :Mn and Sr 2 TiO 4 :Mn with corresponding excitation spectra at room temperature. Sr 2 TiO 4 :Mn showed deep-red emission with a peak at 725 nm attributed to 2 E g → 4 A 2g transition of Mn 4+ 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 SrTiO 3 :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; Sr 2 TiO 4 has a twodimensional layered structure whereas SrTiO 3 has a threedimensional bulky one. Therefore, the appearance of Mn 4+emission in Sr 2 TiO 4 :Mn may reflect advantage of the layered perovskite structure in the band structure than bulky one. The further discussion about Sr 2 TiO 4 :Mn is described later.

Comparison of Luminescence Properties Between Sr 2 ScO 3 F:Mn and Sr 2 TiO 4 :Mn
Although, as shown in Figure 3A, Sr 2 TiO 4 has a wider band gap (3.46 eV) than SrTiO 3 (3.21 eV) as reported in literature (Reyes-Lillo et al., 2016), other layered perovskite compounds possessing wider band gaps are preferred for efficient Mn 4+ -emission because of less probability of the electron transfer between Mn 3d and the valence and/or conduction band. Strontium scandium oxyfluoride Sr 2 ScO 3 F with a K 2 MgF 4 type structure as well as Sr 2 TiO 4 , which has been recently discovered (Wang et al., 2015), was thought to be a good candidate because its octahedral building unit ScO 5 F based on the optically inert rare earth element was expected to give a wider energy gap in comparison with TiO 6 . XRD confirmed that Sr 2 ScO 3 F:Mn was obtained as the almost pure phase of Sr 2 ScO 3 F although it contained tiny amounts of SrF 2 and SrSc 2 O 4 as impurities whereas Sr 2 TiO 4 :Mn was obtained as a pure phase without any impurities ( Figure 3B). Relative intensities of diffraction peaks of Sr 2 ScO 3 F: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 Sr 2 ScO 3 F has been discovered to be 5.38 eV, being wider than that of Sr 2 TiO 4 ( Figure 3A). Figure 4 shows emission and excitation spectra of Sr 2 ScO 3 F:Mn and Sr 2 TiO 4 :Mn at room temperature. Sr 2 ScO 3 F:Mn showed deep-red emission owing to transition of Mn 4+ giving a peak at 697 nm. Obvious two excitation bands in 300-460 nm and in 480-580 nm are attributed to spin-allow 4 A 2g → 4 T 1g and 4 A 2g → 4 T 2g transition of Mn 4+ ions, respectively, while a weak excitation band owing to spin-forbidden 4 A 2g → 2 T 2g transition is difficult to distinguish and it may be embedded in the tail of the 4 A 2g → 4 T 1g band as observed in other titanates and tantalates (Sasaki et al., 2014;Wang et al., 2014;Takeda et al., 2015Takeda et al., , 2017. Interestingly, the emission from Sr 2 ScO 3 F:Mn was much stronger than that from Sr 2 TiO 4 :Mn; the internal and external quantum yields of Sr 2 ScO 3 F:Mn excited at 345 nm at room temperature (50.5 and 43.5%) were much higher than those of Sr 2 TiO 4 :Mn excited at 380 nm (3.4 and 2.5%). The Mn 4+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 Sr 2 ScO 3 F:Mn is broad as well as Mn 4+activated oxide phosphors despite presence of the Sc-F bond. This means that influences of F upon the photoluminescence property of Sr 2 ScO 3 F:Mn are not significant. In Sr 2 ScO 3 F:Mn, it is preferred from the charge compensation that one fluorine is replaced with one oxygen when Mn 4+ is substituted for Sc 3+ . Such co-substitution results in the formation of MnO 6 octahedra which give broad Mn 4+ -emission. The spectra of CaSiAlN 3 :Eu, which is the representative red-emitting phosphor activated with Eu 2+ , are also shown in Figure 4. The Sr 2 ScO 3 F:Mn emission is sharper and stronger than the CaSiAlN 3 :Eu emission however the wavelength of the Sr 2 ScO 3 F:Mn emission is excessively long, that is, almost the half portion of emission is located in the invisible region (λ > 700 nm). CaAlSiN 3 :Eu can be excited by blue-LEDs (λ = 450-470 nm) more efficiently than Sr 2 ScO 3 F:Mn while Sr 2 ScO 3 F:Mn can be excited by near ultraviolet LEDs (λ = 350-400 nm) more efficiently than CaAlSiN 3 :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 Sr 2 TiO 4 :Mn and Sr 2 ScO 3 F: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. Sr 2 ScO 3 F:Mn exhibited stronger emission than Sr 2 TiO 4 :Mn at all temperatures, moreover, the intensity of Sr 2 TiO 4 :Mn at 80 K was lower than that of Sr 2 ScO 3 F:Mn at 300 K. Thus, Sr 2 TiO 4 :Mn exhibited more remarkable thermal quenching in comparison with  Sr 2 ScO 3 F:Mn. In the high temperature range (298-473 K), significant thermal quenching occurred in both samples as shown in Figure 5D, however Sr 2 ScO 3 F:Mn showed lesser thermal quenching than Sr 2 TiO 4 :Mn. At 373 K, Sr 2 ScO 3 F:Mn showed 20% of emission intensity in comparison with that at 298 K whereas emission from Sr 2 TiO 4 :Mn was completely quenched. Band Structures of Sr 2 TiO 4 :Mn and Sr 2 ScO 3 F:Mn As described above, Sr 2 ScO 3 F:Mn showed superior characteristics, that is, higher emission intensity and lesser thermal quenching, to Sr 2 TiO 4 :Mn. Relative position of Mn 3d orbitals to the valence and conduction bands of host materials is an important factor for Mn 4+ -activated phosphors as we have reported previously (Takeda et al., 2015(Takeda et al., , 2017. Therefore, band structures of Sr 2 TiO 4 :Mn and Sr 2 ScO 3 F:Mn were investigated by the DFT method. Figure 6 depicts projected density of states (PDOS) near the band gap of Sr 16 Ti 7 MnO 32 and Sr 16 Sc 7 MnO 25 F 7 corresponding to Sr 2 TiO 4 :Mn and Sr 2 ScO 3 F: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 Sr 2 TiO 4 :Mn, the valence and conduction bands of host are composed of O 2p and Ti 3d orbitals, respectively, like SrTiO 3 :Mn. In PDOS of Sr 2 TiO 4 :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 Sr 2 TiO 4 :Mn has absolutely different feature in the relative position of Mn 3d orbitals to the valence band from SrTiO 3 :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 Sr 16 Ti 7 MnO 32 and Sr 16 Sc 7 MnO 25 F 7 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 Sr 2 ScO 3 F: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 Sr 2 TiO 4 :Mn and Sr 2 ScO 3 F:Mn (Figure 7). In Sr 2 ScO 3 F: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 Sr 2 TiO 4 :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 Sr 2 TiO 4 :Mn (from HOMO to HOMO−3) indicates stronger hybridization between O 2p and Mn 3d than Sr 2 ScO 3 F: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 Sr 2 ScO 3 F:Mn than Sr 2 TiO 4 :Mn. It reflects the remarkably wider band gap of Sr 2 ScO 3 F than Sr 2 TiO 4 . Figure 8 illustrates proposed mechanism based on photoluminescence measurements and band structure calculations for Mn 4+ -activated SrTiO 3 , Sr 2 TiO 4 , and Sr 2 ScO 3 F. The most significant difference in photoluminescence property between Sr 2 TiO 4 :Mn and SrTiO 3 :Mn is the appearance of Mn 4+emission in Sr 2 TiO 4 :Mn at room temperature. The difference in the relative position of t 2g (α) orbitals of Mn between SrTiO 3 :Mn and Sr 2 TiO 4 :Mn is of importance in explanation for the appearance of Mn 4+ -emission in Sr 2 TiO 4 :Mn. In SrTiO 3 :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 state 2 E g , resulting in low quenching temperature, ∼100 K (Takeda et al., 2015). In contrast to SrTiO 3 :Mn, t 2g (α) in Sr 2 TiO 4 :Mn is located slightly higher position than the valence band. Therefore, Sr 2 TiO 4 :Mn shows Mn 4+ -emission even at room temperature. Interaction between TiO 6 octahedra may affect the relative position of t 2g (α) orbitals of Mn. Each TiO 6 octahedron connects to six TiO 6 octahedra in SrTiO 3 while TiO 6 connects to four TiO 6 octahedra in the perovskite slab in Sr 2 TiO 4 as shown in Figure 1, indicating that degree of energy delocalization is higher in three-dimensional SrTiO 3 than two-dimensional Sr 2 TiO 4 . The smaller interaction between TiO 6 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 Mn 4+ -activated phosphors can be explained by the smaller interaction of MO 6 octahedra. Such discussion can be applied to another Mn 4+ -activated titanate phosphor La 2 MgTiO 6 :Mn with a B-site ordered double perovskite structure, which is an efficient Mn 4+ -activated phosphor with 58.7% of an internal quantum yield (Takeda et al., 2015). In La 2 MgTiO 6 , each TiO 6 octahedron is surrounded by six MgO 6 octahedra, meaning that TiO 6 is isolated from other TiO 6 octahedra even though the perovskite-type structure (Lee et al., 2000). Thus, the structure of La 2 MgTiO 6 can be regarded as a quasi-zero-dimensional structure with respect to the connection between TiO 6 octahedra. The less interaction of TiO 6 in La 2 MgTiO 6 :Mn leads the larger energy gap between the valence band and t 2g (α) of Mn, resulting in the superior photoluminescence efficiency to Sr 2 TiO 4 :Mn. In Sr 2 ScO 3 F:Mn, the t 2g (α) orbitals of Mn 3d are located above the valence band with a slightly larger energy gap than Sr 2 TiO 4 :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 Sr 2 ScO 3 F: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 Sr 2 ScO 3 F:Mn than Sr 2 TiO 4 :Mn, resulting in the stronger emission.

CONCLUSIONS
Photoluminescence properties of Mn 4+ -activated strontium titanates, SrTiO 3 :Mn with three-dimensional bulky perovskite structure and Sr 2 TiO 4 :Mn with two-dimensional layered perovskite structure, have been compared in this research. Sr 2 TiO 4 :Mn shows Mn 4+ -emission even at room temperature despite no emission from SrTiO 3 :Mn. In addition, the results in our systematic research suggest that the less interaction between MO 6 octahedra of B-site cation in the perovskite family provides positive influences in Mn 4+ -emission. Comparison between Sr 2 TiO 4 :Mn and Sr 2 ScO 3 F:Mn indicates that ScO 5 F octahedra are preferable constituents to TiO 6 ones for the Mn 4+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 Mn 4+ -activated phosphors.