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Siyuan Li, Chenyu Zhang, Qi Zhu, [Ji-Guang Li](https://orcid.org/0000-0002-5625-7361)

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[Cationic pair substitution in LaAlO3:Mn4+ for octahedral-tilting-dependent zero-phonon line](https://mdr.nims.go.jp/datasets/567ff9f5-979a-4d60-ae60-6d58869a7489)

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1  Cationic pair substitution in LaAlO3:Mn4+ for octahedral-tilting dependent zero-phonon line    Siyuan Li,a Chenyu Zhang,a Qi Zhu*a and Ji-Guang Li*b     aKey Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, China bResearch Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan  *Corresponding authors: Dr. Qi Zhu Tel: +86-24-8367-2700 E-mail: zhuq@smm.neu.edu.cn  Dr. Ji-Guang Li Tel: +81-29-860-4394 E-mail: LI.Jiguang@nims.go.jp       mailto:zhuq@smm.neu.edu.cnmailto:LI.Jiguang@nims.go.jp2  Abstract Zero-phonon line (ZPL) emission of Mn4+, without participation of phonon, is tightly related to the host crystal structure. However, the intensity of the intrinsic ZPL is much weaker than that of Stokes and anti-Stokes vibrational bands, and it always leads to a discontinuous emission peak. Regulating ZPL of Mn4+ for a charming emission is very important but remains a challenge for perovskite type oxides. Here, novel La1-xBaxAl1-xTixO3:0.001Mn4+ (LBAT:0.001Mn4+, x=0-0.2) and La1-yYyAl1-yGayO3:0.001Mn4+ (LYAG:0.001Mn4+, y=0-0.2) samples were successfully synthesized through a high-temperature solid-state reaction, and tunable ZPL of Mn4+ was found by cationic pair substitution of Ba2+-Ti4+ and Y3+-Ga3+ for La3+-Al3+ in LaAlO3:Mn4+. The ZPL intensity is related to the local symmetry around Mn4+ and the ZPL energy corresponds to the Mn-O bond distance and the O-Mn-O bond distortion. Through co-doping Ba2+-Ti4+, the ZPL at 710 nm is enhanced and the intensity increases continuously with increasing the x value, due to the local symmetric degree of Mn4+ decreases slowly. However, the Y3+-Ga3+ co-doping induces linear and quick increase of the intensity of ZPL at 704 nm with increasing y value, due to the local symmetric degree of Mn4+ decreases quickly. The octahedral tilting distortion is very important for the local symmetry. Ba2+-Ti4+ co-doping reduces octahedral tilting distortion, but Y3+-Ga3+ co-doping induces a serious octahedral tilting distortion. Consequently, the ZPL emission exhibits an octahedral-tilting dependent behavior. Mainly due to the larger distortion of O-Mn-O bond, the energy of ZPL for LYAG:0.001Mn4+ is higher than that for LBAT:0.001Mn4+. The outcomes of this work 3  provide a promising way to regulate the ZPL intensity and energy by tuning the local structure around Mn4+, and they may have wide implication for Mn4+-doped phosphors and solid state lighting.  1. Introduction The ABO3 perovskite oxides present a multitude of functional properties and are widely renowned for their potential in various types of applications, owing to their low cost and high abundance.1,2 For the simple ABO3 perovskite, A cation connects to 12 oxygen atoms to form a AO12 dodecahedron, and B cation links with 6 oxygen atoms to form a BO6 octahedron. BO6 is connected each other by O vertices to form the three-dimensional framework structure, and A cations are located in the cavities surrounded by eight BO6 octahedra.3,4 The most common distortion for ABO3 perovskite oxide is octahedral tilting, which is related to the rigid BO6 rations while keeping their corner-sharing connectivity.5,6 Controlling the degree of octahedral tilting is of great interest, owing to its significant effects on the physical and chemical properties.  LaAlO3 has rhombohedral symmetry with a pseudo-cubic cell parameters a=b=c=3.79 Å and =, and it is one of the most important ABO3 perovskite oxides.7,8 It is widely known that the phase transition from cubic to rhombohedral takes place at Tc ~817 K through AlO6 octahedron rotating along <-11-1> with half of the polyhedrons tilting clockwise and half tilting anticlockwise.9 During the rotation, the local structure changes a lot, which can be determined by the analysis of cell 4  parameter variation and Raman spectra. Furthermore, the structural changes from rhombohedral to cubic with increasing hydrostatic pressure are also observed, which are induced by decreasing AlO6 octahedron tilting.10 Not only the temperature and pressure can alter the tilting degree of AlO6, but also the replacement of La3+ or/and Al3+ by other cations.11,12 For example, when Nd3+ cations substitute La3+ in LaAlO3, the AlO6 octahedron tilting increases gradually.11 The octahedron tilting distortions would induce huge variations for local structure and produce an impact on properties of the functional materials.7 LaAlO3 is an appropriate host for Mn4+ doping to generate near-infrared emission.12 Mn4+ with 3d3 electronic configuration is an excellent luminescent center.13,14 Usually, Mn4+ prefer to occupy the octahedral sites to generate the luminescence, which depends on the covalence and local coordination symmetry around Mn4+ in the host. The emission peak of Mn4+ corresponded to 2Eg-4A2g usually consists of the zero-phonon line (ZPL) transition and the vibrational sidebands of ZPL with phonon assistance (Stokes and anti-Stokes vibronic band) ranging from 600-800 nm.15 Compared with phonon sidebands, the energy and intensity of ZPL are much more sensitive to the local coordinated environment. The reason is that any phonon does not participate in the ZPL emission process, which is only dependent on the host structure.16,17 Generally, the intensity of ZPL is related to the structure symmetry around Mn4+. According to the Tanabe-Sugano diagram, the ZPL emission energy remains the same with changing the crystal field splitting. In fact, the energy of Mn4+ emission is based on Mn-ligand hybridization.18,19 Decreasing hybridization should 5  lead to higher 2Eg energies and increase the emission energy. Furthermore, weaker hybridization can be induced via the increased Mn-ligand distance and/or distorted ligand-Mn-ligand bond angles.18 Mn4+ doped phosphors are usually used as the red component in LED devices. Interestingly, when the ZPL is located at the wavelength smaller than 700 nm, the color rendering index of LED devices can be further improved with enhancing the ZPL emission, which is very important to obtain the high-quality white LED devices.14 Because ZPL of the LaAlO3:Mn4+ phosphor is located at the wavelength larger than 700 nm, enhancing ZPL can not improve the color rendering index of the white LED device. However, the LaAlO3:Mn4+ phosphors can be used as LED devices, which are benefit for plant growing.13 At the same time, higher ZPL intensity can induce the divided two emission peaks (Stokes and anti-Stokes) become a broad band, and then make the emission spectra better match with the PFR region of plant absorption spectrum. Consequently, the luminous efficiency of phosphor with continued and broad emission band is higher than the phosphor with divided emission band.12 Therefore, exploring the changes of ZPL energy and intensity along with the local structure is vital for designing high efficiency deep-red or near-infrared phosphors.  In our previous work,12 zero-phonon line (ZPL) emission of Mn4+ was generated by substituting Mg2+-Ge4+for Al3+-Al3+ in LaAlO3:Mn4+, which resulted in a significant octahedral tilting distortion and a mismatched cation size distortion. The local structure of Mn4+ is influenced by MgO6 and GeO6, which link to MnO6 through O vertex. In this way, the symmetry degree of Mn4+ decreases gradually along with 6  increasing Mg2+-Ge4+ content, which induce an improvement of ZPL emission. However, the energy of ZPL emission is not discussed in detail in previous work, due to insufficient impact on the local structure around Mn4+. In order to further reveal the relationship between the local coordinated environment around Mn4+ and the ZPL behavior (energy and intensity), a cationic pair substitution way to tune the local structure was proposed here through replacing La3+-Al3+ rather than Al3+-Al3+ by Ba2+-Ti4+ and Y3+-Ga3+ in LaAlO3:0.001Mn4+. BaO12 and YO12 polyhedrons connect with MnO6 by sharing the same plane and edge, and GaO6 and TiO6 octahedra link to MnO6 through O vertex. So, they would show more serious effect on local structure of Mn4+, comparing to that only sharing the same O vertex by Mg2+-Ge4+ substitution for Al3+-Al3+. Thus, in this work, La1-xBaxAl1-xTixO3:0.001Mn4+ (LBAT:0.001Mn4+, x=0-0.2) and La1-yYyAl1-yGayO3:0.001Mn4+ (LYAG:0.001Mn4+, y=0-0.2) samples were successfully synthesized through the high-temperature solid-state reaction in atmospheric environment. The variations of the local structure were detected by the XRD Rietveld refinement, Raman spectra, SEM and TEM analysis. The luminescence properties were obtained by the diffuse reflectance spectra, PLE spectra, PL spectra, lifetime decay curves and temperature-dependent PL spectra. Through above analysis, the relationship between the local structure of Mn4+ and ZPL behavior (energy and intensity) was discussed in detail. 2. Experimental section Sample preparation A series of La1-xBaxAl1-xTixO3:0.001Mn4+ (LBAT:0.001Mn4+, x=0-0.25) and La1-yYyAl1-yGayO3:0.001Mn4+ (LYAG:0.001Mn4+, y=0-0.25) samples were 7  synthesized by the traditional high-temperature solid-phase reaction technique. La2O3, Al2O3, MnCO3, BaCO3, Y2O3, TiO2 and Ga2O3 were used as raw materials. La2O3 (99.99%) and Y2O3 (99.999%) were purchased from Huizhou Ruier Rare-Chem. Hi-Tech. Co. Ltd (Huizhou, China). Al2O3 (99.99%), MnCO3 (99.95%), BaCO3 (99.99%), TiO2 (99.8%), and Ga2O3 (99.99%) were purchased from Aladdin Chemical Reagent Co. Ltd (Shanghai, China). La2O3 was heated at 1000 oC for 2 h before use for removing adsorbed water. The raw materials were accurately weighed according to the stoichiometric molar ratio of the designed compositions. Then, these raw materials were placed in agate mortar and grinded for 30 min to be mixed thoroughly. After that, the mixtures were preheated at 1000 oC for 5 h and then sintered at 1500 oC for 8 h in air. The final powders were obtained to further measurement after cooling down to room temperature naturally.  Characterization methods The phase compositions of the samples were obtained by X-ray powder diffraction (XRD, Model SmartLab, Rigaku, Tokyo, Japan) with the use of nickel-filtered Cu Kα radiation (=0.15406 nm) and 40 kV, 200 mA operating conditions. The scanning range was 10-90 o and the scanning rate was 6 o per minute. The XRD patterns for Rietveld refinement were acquired in the range of 10-110o at a step size of 0.02 o with a step-scan mode of 2.5 s per step. The product morphology, microstructure and element mapping were analyzed through field emission scanning electron microscopy (FE-SEM, Model JSM-7001F, JEOL, Tokyo) and transmission electron microscopy (TEM, Model JEM-2000FX, JEOL, Tokyo). Raman spectra were collected via a Raman microscope (Model R-XploRA Plus, Horiba, Paris, France) 8  with the use of a 638 nm laser. The diffuse reflectance spectra were conducted with a Model UV-3600 Plus instrument (Shimadzu, Kyoto, Japan). Photoluminescence and fluorescence decays of the phosphors were recorded on a FP-8600 fluorospectrophotometer (JSACO, Tokyo) with a temperature controller (HPC-836, JSACO) and a liquid nitrogen cooling unit (PMU-830, JSACO). The internal quantum yield (IQY) of the samples were measured using the integrated sphere on the same FP-8600 instrument. 3. Results and discussion 3.1 Crystal structure and microstructure Fig. 1a shows a detailed crystal structure of LaAlO3 and the coordination environment of the cations. AlO6 octahedra are connected with each other by corner-sharing to form the basic framework of LaAlO3. The La3+ ion is coordinated with twelve oxygen atoms and located in the cavities of eight AlO6 octahedra. According to the similar ionic radii between Mn4+ (0.53 Å, CN=6) and Al3+ (0.535 Å, CN=6), the luminescence centers Mn4+ ions would occupy the crystallographic sites of Al3+ in AlO6 units. Therefore, from the consideration of ionic radii, Ti4+ (0.605 Å, CN=6) and Ga3+ (0.62 Å, CN=6) are expected to substitute Al3+ (0.535 Å, CN=6). Moreover, Ba2+ (1.61 Å, CN=12) and Y3+ (1.075 Å, CN=9) would replace La3+ (1.36 Å, CN=12). Figs. 1b and 1c show the XRD patterns of the LBAT:0.001Mn4+ (x=0-0.25) and LYAG:0.001Mn4+ (y=0-0.25) samples. Obviously, most of the samples fit well with the standard crystal diffraction data (JCPDS No. 85-0848) of LaAlO3 compound. However, when the x value and y value exceed 0.2, the impurity of 9  BaTiO3 (JCPDS No. 75-2117)20 and Y4Al2O9 (JCPDS No. 83-0935)21 are yielded, respectively. Therefore, later discussion is based on the x value and y value no more than 0.2. As it is seen in Figs. 1b and 1c, all the diffraction peaks shift to lower angles gradually with increasing x and y values. To analyze the shifts of diffraction peak with increasing x and y values, enlarged XRD patterns in the range of 32-34o are shown in Figs. 1b and 1c. The strongest diffraction peak at 33.4o shifts toward lower angles with increasing x and/or y value. But, the LBAT:0.001Mn4+(x=0-0.2) samples show a larger shift than that for LYAG:0.001Mn4+ (y=0-0.2) samples. Typically, the average ionic radii of Ba2+-Ti4+ and Y3+-Ga3+ pair are both larger than that of La3+-Al3+ pair, and Ba2+-Ti4+ is the largest among the three pairs. Modulating the Ba2+-Ti4+ and Y3+-Ga3+ content in LaAlO3:0.001Mn4+ could lead to the lattice volume expansion, and the results were in accordance with the experimental data. The dopants entered the crystal structure of LaAlO3:0.001Mn4+ successfully, and the homogeneous solid solutions were formed. In addition, the diffraction peaks broaden evidently with increasing x and/or y value from 0 to 0.2, demonstrating the crystal lattice becomes imperfect and the distortion degree of the crystal structures increases.22   10   Fig. 1 (a) Crystal structure of LaAlO3. XRD patterns of (b) LBAT:0.001Mn4+ (x=0-0.25) and (c) LYAG:0.001Mn4+ (y=0-0.25) in 2 range of 10-90o and enlarged XRD patterns in range of 32-34 o. In order to further observe the variation of the crystal lattice of LaAlO3:0.001Mn4+ after incorporation of Ba2+-Ti4+ and Y3+-Ga3+ pairs, the Rietveld refinements were conducted (Fig. S1). The refinement results for LBAT:0.001Mn4+ (x=0-0.2) and LYAG:0.001Mn4+ (y=0-0.2) samples were summarized in Tables S1 and S2, respectively. As can be seen, the residual factors (Rwp, Rp and ) for all the samples converged to low levels, showing that these refinement results are reliable. The lattice constant and the M-O bond length in MO6 octahedra increase linearly with increasing the doping content of Ba2+-Ti4+ and Y3+-Ga3+ (Figs. 2a and 2b). The O-M-O bond angle shows a decreasing trend with the increase of x value, while it shows a remarkably increasing trend with increasing y value (Fig. 2c). The octahedral tilting can be reflected by the octahedral tilting angle  between the Al-O bonding and the coordinated axis, which is the angle deviated 90.00o.7 The increased  value shows serious octahedral tilting distortion with Y3+-Ga3+ co-doping, but the value decreases with increasing x value, implying that the octahedral tilting distortion is suppressed (Fig. 2d). These variations of the samples co-doped Ba2+-Ti4+ and Y3+-Ga3+ in lattice constant, bond length, bond angle and tilting angle are different from each other, implying that co-doping Ba2+-Ti4+ and Y3+-Ga3+ pairs may result in 11  various distortions of the crystal lattice, owing to the mismatch in ionic radii of doping cations and the resultant octahedral tilting degrees.23,24 Consequently, the distortions of LBAT:0.001Mn4+(x=0.05-0.2) samples are caused by the competition between the mismatched ionic radii and the suppression of octahedral tilting. On the contrary, the distortions of LYAG:0.001Mn4+ (x=0.05-0.2) are composed of the mismatched ionic radii and the increased octahedral tilting.   Fig. 2 The variations of (a) cell parameter, (b) bond length, (c) O-M-O bond angle in a MO6 octahedron and (d) octahedral tilting angle versus various Ba2+-Ti4+ (x=0-0.2) and Y3+-Ga3+ (y=0-0.2) content doped LaAlO3:0.001Mn4+ phosphors based on Rietveld refinement results. Raman spectra is a useful tool to detect the distortion of LaAlO3.7,25 Figs. 3a and 3b display the Raman spectra of LBAT:0.001Mn4+ (x=0-0.2) samples and LYAG:0.001Mn4+ (y=0-0.2) samples, respectively. For LaAlO3:0.001Mn4+ phosphor, two strong peaks at 121 and 152 cm-1 are observed, which are assigned to A1g mode 12  (rotation of the oxygen octahedra) and Eg mode (pure La vibration), respectively. The wavenumber of A1g mode mainly correlates with the tilting angle of AlO6 octahedron.11 A blue shift of the A1g wavenumber from 121 to 109 cm-1 is detected with increasing the x value from 0 to 0.2, while a red shift from 121 to 195 cm-1 is observed with increasing the y value from 0 to 0.2. These phenomena explain that introducing Ba2+-Ti4+ reduces the octahedral tilting, while co-doping Y3+-Ga3+ pair increases the octahedral tilting. The results are consistent with the variations of tilting angle  (Fig. 2d) At the same time, the Eg wavenumber at 152 cm-1 keeps in the same position with increasing the x or y value. Because the defects in the crystal structure may contribute to the broadening of the vibrational bands, the full width at half-maximum (FWHM) can reflect the amount of defects.25 The FWHM value of the A1g mode is 15 cm-1 for LaAlO3:0.001Mn4+, and the value shows a continuous increase with increasing the Ba2+-Ti4+ and Y3+-Ga3+ contents. Furthermore, the FWHM value of LYAG:0.001Mn4+ (y=0.05-2) is always higher than that LBAT:0.001Mn4+ (x=0.05-0.2), implying that introducing Y3+-Ga3+ would generate more serious defects in the crystal structure.  Fig. 3 Raman spectra of (a) LBAT:0.001Mn4+ (x=0-0.20) and (b) LYAG:0.001Mn4+ (y=0-0.20). 13  The SEM images, TEM images, HR-TEM lattice fringes and element distributions for LaAlO3:0.001Mn4+, x=0.2 and y=0.2 samples are displayed in Fig. 4. The particles are irregular in shape and the sizes are in the range of 1.5-6 μm, which are similar to the particles prepared at higher sintering temperatures.26,27 Their surface morphology does not change significantly depending on the different co-doping amounts. The distinct interplanar space fingers indicate high crystallinity of the samples. The d values correspondent to (100) and (011) planes are determined to be ~0.377 and ~0.264 nm for LaAlO3:0.001Mn4+ (Fig. 4c). The d value correspondent to (100) plane is calculated to be ~0.383 nm for x=0.2 sample, and the d value correspondent to (011) plane is estimated to be ~0.270 nm for y=0.2 sample. Due to the Ba2+-Ti4+ and Y3+-Ga3+ doping contribution to lattice expansion, an increase of the interplanar spacing is found with increasing the x and y value. The element mapping of a single particle indicates a uniform distribution of La, Al, O, Mn, Ba and Ti (Y and Ga) for x=0.2 (y=0.2) sample (Figs. 4g and 4k). After the comparative analysis, the optimal phosphor samples LBAT:0.001Mn4+ and LYAG:0.001Mn4+ were successfully synthesized.  14   Fig. 4 SEM (a, d, h), TEM images (b, e, i), HR-TEM lattice fringes (c, f, j) and element distribution (g, k) for LaAlO3:0.001Mn4+ (a, b, c), x=0.2 (d, e, f, g) and y=0.2 (h, i, g, k).   3.2 Photoluminescence of LBAT:0.001Mn4+ and LYAG:0.001Mn4+ samples Fig. 5a reveals the diffuse reflection spectra of LaAlO3:0.001Mn4+, x=0.2 and y=0.2 samples. All the samples exhibit three strong and wide absorption bands from 240 to 550 nm, which correspond to the strong spin-allowed transition of Mn4+ from 4A2g to the excited levels 4T1g, 2T2g and 4T2g. But a strong band at ~280 nm related to the Mn4+-O2- charge-transfer band is only observed for LaAlO3:0.001Mn4+ and 15  LYAG:0.001Mn4+ samples. For the purpose of gain more information about the band structures of above materials, the optical band gaps (Eg) were determined for LaAlO3, La0.8Ba0.2Al0.8Ti0.2O3 and La0.8Y0.2Al0.8Ga0.2O3 hosts. The corresponding equation was used as follows:28-30 )(A gn Ehvhv −=）（          (1) Where  h, v, and A represent the absorption coefficient, Planck constant, frequency and constant, respectively. n stands the direct or indirect transition while it is equal to 2 or ½, respectively. According to the previous report,31 the band structure of LaAlO3 is indirect. As shown in Fig. 5b, based on the plot of (hv)1/2 vs hv, the Eg for LaAlO3, La0.8Ba0.2Al0.8Ti0.2O3 and La0.8Y0.2Al0.8Ga0.2O3 is estimated to be 5.6, 3.9 and 5.7 eV, respectively. The Ba2+-Ti4+ doping induces the large decrease of Eg value. For further analysis the changes of Eg, the band structure and the partial densities of states (DOS) for La0.8Ba0.2Al0.8Ti0.2O3 were analyzed by density functional theory (DFT) calculation (Fig. S2). The band structure and DOS of LaAlO3 were mentioned in our previous work, and the Eg value is 5.0 eV.31 Furthermore, the La orbital levels dominantly contribute to the conduction band minimum (CBM), while the O orbital levels correspond to the valence band maximum (VBM). However, for the La0.8Ba0.2Al0.8Ti0.2O3 host, the Eg value is 3.0 eV (Fig. S2a), which is much smaller than LaAlO3. This is mainly due to that the electronic structure of CBM originates from Ti orbital levels predominantly, with the VBM still arising from the O orbital levels (Fig. S2b). It is noteworthy that there is a distinct smaller Eg of 3.0 eV for La0.8Ba0.2Al0.8Ti0.2O3, confirming that Ba2+-Ti4+ doping reduces the bandgap. The 16  results match well with the experimental data. Therefore, co-doping Ba2+-Ti4+ in LaAlO3 lead to the significant changes in Eg, and it may affect the luminescence properties of Mn4+.   Fig. 5 (a) The diffuse reflection spectra of LaAlO3:0.001Mn4+, x=0.2 and y=0.2 samples. (b) The band gap energy determination for the selected hosts.  To investigate the effects of Ba2+-Ti4+ and Y3+-Ga3+ chemical unit doping on the luminescence of Mn4+, the photoluminescence excitation (PLE) and photoluminescence (PL) spectra of LBAT:0.001Mn4+ (x=0-0.2) and LYAG:0.001Mn4+ (y=0-0.2) were measured and the results are shown in Fig. 6. There are two characteristic excitation bands appeared when monitored at 726 nm for LaAlO3:0.001Mn4+. The peak at 340 nm is assigned to the overlap of Mn4+-O2- charge-transfer band, 4A2g-4T1g and 4A2g-2T2g transitions, while the other peak at ~490 nm is related to the 4A2g-4T2g transition. There is an obvious red shift in the excitation spectra for LBAT:0.001Mn4+ (x=0-0.2) from ~333 to ~356 nm. The shift for LYAG:0.001Mn4+ (y=0-0.2) is from ~333 to ~358 nm. Furthermore, the peak at ~280 nm disappears for LBAT:0.001Mn4+ (x=0.05-0.2), while the PLE spectra of 17  LYAG:0.001Mn4+ (y=0.05-0.2) samples keeps the original shape. The appearance is due to the change of the optical band gap, which would be discussed in later. Under the 340 nm excitation, the PL spectrum of LaAlO3:0.001Mn4+ is composed of two bands, including the anti-Stokes phonon band at 698 nm and Stoke phonon band at 726 nm of the 2Eg-4A2g transition. The zero-phonon line (ZPL) emission is too low in intensity to be detected. With the introduction of Ba2+-Ti4+ and Y3+-Ga3+, both peaks keep at the same position and they persist the shape. However, a big difference between the anti-Stokes phonon band and the Stoke phonon band is found. A new peak at 710 or 704 nm was generated by co-doping Ba2+-Ti4+ and Y3+-Ga3+ in LaAlO3:0.001Mn4+, and, accordingly, the divided two peaks become a broad band. The new peak is related to the ZPL emission. Through the co-substitution of Ba2+-Ti4+, the new peak at 710 nm is enhanced slowly, while an unusual improvement of the peak at 704 nm is obtained by increasing Y3+-Ga3+ content. Thus, due to the different ZPL energies and intensities, the PL shape of these phosphors shows a huge difference by co-doping different cation pairs.   18   Fig. 6 PLE (a,c) and PL (b,d) spectra for (a,b) LBAT:0.001Mn4+ (x=0-0.2) and (c,d) LYAG:0.001Mn4+ (y=0-0.2) at room temperature.  As generally known, the emission of Mn4+ is mainly dominated by the vibration of the side bands. In order to observe the variations of ZPL in detail, the vibration of the side bands should be suppressed. Here, the high-resolution PLE and PL spectra of these samples were recorded at low temperature (77 K), as shown in Fig. 7. The excitation spectra are similar to the spectra measured at room temperature for the LBAT:0.001Mn4+ (x=0-0.2) and LYAG:0.001Mn4+ (y=0-0.2) samples. The PLE spectrum of LaAlO3:0.001Mn4+ is fitted into four bands located at 316, 347, 410 and 490 nm, which are corresponding to the Mn4+-O2- charge-transfer band, 4A2g-4T1g and 4A2g-2T2g and 4A2g-4T2g transitions of Mn4+. For the LBAT:0.001Mn4+ (x=0.05-0.2) phosphors, the PLE spectra can be well divided into three sub-bands, which are assigned to the 4A2g-4T1g and 4A2g-2T2g and 4A2g-4T2g transitions of Mn4+. However, the PLE spectra of LYAG:0.001Mn4+ (y=0.05-0.2) phosphors are well-decomposed into four peaks, which are similar to LaAlO3:0.001Mn4+ sample. Both 4A2g-4T1g and 4A2g-4T2g transition peaks show red shifts for LBAT:0.001Mn4+ (x=0-0.2) and LYAG:0.001Mn4+ (y=0-0.2) phosphors. Notably, under the excitation of 340 nm, the 19  Stokes peak is still at 726 nm and the anti-Stokes peak disappears at 77 K. According to the previous work, the red shift of Mn4+ excitation peaks is mainly due to the change of the crystal field strength (Dq). Dq could be estimated by the following equation:16 5426RrZeDq =         (2) Where Z stands the charge or valence of the anion, r and R represent the radius of the d wave function and the bond length between the Mn4+ and its ligands, respectively. In LBAT:0.001Mn4+ (x=0.05-0.2) and LYAG:0.001Mn4+ (y=0.05-0.2) phosphors, the average bond length of Mn-O is larger than that in LaAlO3:0.001Mn4+ (Fig. 2b). Therefore, larger R can lead to a smaller Dq value, and it results in red shift of the excitation peak with increasing the x and y values. According to the Tanabe-Sugano energy level diagram, it can be seen that 2Eg and 4A2g energy levels are almost parallel to horizontal ordinate.12 Thus, 2Eg and 4A2g energy levels are not affected by changing Dq, further implying that the emission energy is basically independent on the Dq.    20   Fig. 7 PLE (a,c) and PL (b,d) spectra for (a,b) LBAT:0.001Mn4+ (x=0-0.2) and (c,d) LYAG:0.001Mn4+ (y=0-0.2) at 77 K.  Notably, the ZPL emission peak appears by co-doping Ba2+-Ti4+ and Y3+-Ga3+, but the positions and the intensities of the ZPL band are significantly different. For LaAlO3:0.001Mn4+ sample, the ZPL emission peak is too low to be observed. Based on previous work, the ZPL in LaAlO3:Mn4+ is located at ~710 nm.15 Introducing Ba2+-Ti4+ to replace La3+-Al3+ results in appearance of the ZPL at 710 nm, which intensity increases slowly with x increase. However, for the LYAG:0.001Mn4+ (y=0.05-0.2) phosphors, the position of ZPL is at 704 nm that displays a blue shift compared with that for LaAlO3:Mn4+ sample, and the ZPL intensity increases with a further increase in y value. Fig. 8a shows the intensity radio of the ZPL to the total emission. The intensity and energy of the ZPL, which is related to the emission without the participation of any phonons, are only dependent on the host lattice. Compared with the phonon sidebands, ZPL is much more sensitive to the local coordination structure. The intensity is determined by the local symmetry around Mn4+.32 With the substitution of Ba2+-Ti4+ for La3+-Al3+, the local symmetry of Mn4+ changes a little due to the competition of the increase of bond distances and the 21  suppression of octahedral tilting, and, thus, the intensity of ZPL shows a slow increase. However, when co-doping Y3+-Ga3+, owing to the increase of bond distances and octahedral tilting, the local symmetry of Mn4+ in LYAG:0.001Mn4+ (y=0.05-0.2) decreases gradually, which thus contributes to an enhanced ZPL intensity. In fact, the emission energy of Mn4+ corresponds to the hybridization, which is related to Mn-O bond length and O-Mn-O bond angle.18 Longer Mn-O bond length and larger O-Mn-O bond angle distortion generally lead to a smaller Mn-O hybridization, and then contribute to higher emission energies.18,19 According to the XRD Rietveld refinement information, LYAG:0.001Mn4+ (y=0.05-0.2) samples show shorter Mn-O bond length compared to that for LBAT:0.001Mn4+ (x=0.05-0.2), but they exhibit larger O-Mn-O bond angle distortion. Actually, the ZPL emission energies for LYAG:0.001Mn4+ samples are higher than those of LBAT:0.001Mn4+. Thus, through considering the influence of two aspects, the results show that octahedral tilting distortion, which affects the serious O-Mn-O bond angle distortion, is more important for the ZPL energy of Mn4+ in perovskite oxides.  22   Fig. 8 (a) Related intensity ratio between ZPL and total emission. (b) The variation trend of lifetime at room temperature and 77 K with increasing x and y value. (c) Structural distortion trend of the MnO6 octahedron with different doping amounts.  Generally, a long decay time is obtained of luminescence centers in a symmetrical surrounding, while a short decay time is observed when the luminescence centers are located in the distorted site.33 Figs. S3 and S4 show the fluorescence decay curves corresponding to Mn4+ in the LBAT:0.001Mn4+ (x=0-0.2) and LYAG:0.001Mn4+ (y=0-0.2) phosphors under the excitation at 340 nm, as monitored at 726 nm at room temperature and at 77 K. All decay curves are found to be well-fitted by a single exponential function, suggesting the single site occupancy of the Mn4+ in host lattice. It is expressed as following:34 )(tAexpII 0t−+=            (3) Where It and I0 are the luminescence intensities at time t and 0. A refers to a constant and  represents the exponential component of the decay time. The lifetime decreases with the addition of Ba2+-Ti4+ and Y3+-Ga3+. Furthermore, the decrease trend is rapid for LYAG:0.001Mn4+ (y=0-0.2) phosphors whether at room temperature or at 77 K, compared to that for LBAT:0.001Mn4+ (x=0-0.2) samples (Fig. 8b). These results are identical with the ZPL intensity variations. 23  Through above analysis, the energy and intensity of ZPL emission peak is tightly related to the Mn4+ local structure environment, as same as the decay time. Fig. 8c shows the structural distortion trend of the MnO6 octahedron with different doping types and amounts. When Mn4+ is incorporated into the LaAlO3 host, Mn4+ is surrounded by six coordinated O2- ions to form a MnO6 octahedron. The MnO6 octahedron connects with AlO6 octahedra and LaO12 polyhedrons, and Mn4+ is located in a high symmetry surrounding environment, leading to a low ZPL intensity and the longest lifetime among all the phosphors. With Ba2+-Ti4+ substitution, some O2- vertexes of the MnO6 octahedron would link to Ba2+ and Ti4+, then inducing the change of Mn-O bond length due to the mismatched cation size. However, reducing octahedral tilting also takes place with increasing the x value, which makes the MnO6 be more symmetric. The Mn-O bond lengths and O-M-O bond angles are determined by the two factors. Higher level substitution of Ba2+-Ti4+ contributed to a growing number of BaO12 and TiO6 distributed around MnO6, but the degree of Mn4+ symmetry is almost persistent. Thus, the intensity of ZPL increases slowly and the lifetime decrease slowly, owing to the little change of the local symmetry of Mn4+. On the contrary, with the chemical unit co-substitution of Y3+-Ga3+ for La3+-Al3+, the distortions are consisted of the cation size mismatch and the increased octahedral tilting. The increased octahedral tilting would make Mn4+ to be in a more asymmetrical local environment with increasing the y value. Therefore, the ZPL intensity increases quickly and the lifetime decreases linearly with increasing the y value. Notably, when a small amount is added, the ZPL intensity for LBAT:0.001Mn4+ 24  (x=0.05, 0.1) is higher than that for LYAG:0.001Mn4+ (y=0.05, 0.1) and the lifetime for LBAT:0.001Mn4+ (x=0.05, 0.1) is shorter than that for LYAG:0.001Mn4+ (y=0.05, 0.1). This is because the average ionic radius of Ba2+-Ti4+ is larger than that of Y3+-Ga3+ pair, and, so, the distortions arising from cation size mismatch were initially dominant. But the LYAG:0.001Mn4+ (y=0.15, 0.2) phosphors show higher ZPL intensity and shorter decay time, compared to LBAT:0.001Mn4+ (x=0.15, 0.2) phosphors, because the octahedral tilting takes the dominate role at higher doping contents.  Fig. 9 Mechanisms of luminescence in LaAlO3:0.001Mn4+, x=0.2 and y=0.2 samples.  Fig. 9 displays the mechanisms in LaAlO3:0.001Mn4+, x=0.2 and y=0.2 samples for clear description of the energy transition process. For LaAlO3:0.001Mn4+ and y=0.2, the electron is excited from 4A2g energy level to 4T1g, 2T2g ,4T2g and even the Mn4+-O2- charge-transfer band. However, the observation is different from that for x=0.2 sample. Because the conduction band bottom shifts down and then drowns the Mn4+-O2- level, the excited electron jumps to the conduction band directly, rather than the Mn4+-O2- level. So, the LBAT:0.001Mn4+ (x=0.05-0.2) phosphors does not show Mn4+-O2- charge transfer band in excitation spectra. Then, the excited electrons relax 25  to the 2Eg level by non-radiative transition process and finally transfer to 4A2g energy level with the output of near-infrared emission. According to the PLE spectra (Figs. 6a and 6c), the samples co-doped by Ba2+-Ti4+ and Y3+-Ga3+ have lower energy in the 4T1g, 4T2g states, which result in red shift of the excitation peak. At the same time, the Stokes and anti-Stokes emissions are kept at same positions, but only the energy of ZPL emission changes, which is clearly described in Fig. 9.  Moreover, the internal quantum yield (IQY) for LaAlO3:0.001Mn4+ have been estimated to be 72.5%. For Ba2+-Ti4+ co-doping, the IQY of the series LBAT:0.001Mn4+ (x=0.05-0.2) phosphors were analyzed to be 66.7% (x=0.05), 62.4% (x=0.1), 59.3% (x=0.15) and 55.8% (x=0.2), respectively. For Y3+-Ga3+ co-doping, the IQY of the series LYAG:0.001Mn4+ (y=0.05-0.2) were analyzed to be 70.0% (y=0.05), 67.1% (y=0.1), 63.9% (x=0.15) and 61.3% (x=0.2), respectively. The decreased IQY with increasing x or y values is mainly due to the lattice expansion, which induces stronger photon-phonon interaction.35 The temperature-dependent luminescence quenching property is essential to evaluate the thermal stability of phosphors.36-38 The temperature luminescent properties were studied in the temperature range of 298-473 K. Temperature-dependent PL spectra of LaAlO3:0.001Mn4+, x=0.2 and y=0.2 phosphors are demonstrated in Fig. 10a-c. The relative integral emission intensity of the above three samples is plotted in Fig. 10d. The integral intensity of all the samples decreases monotonically. However, the three samples show different degrees of luminescence quenching with increasing the temperature. For better understanding of the thermal 26  quenching behavior, the Arrhenius equation is used to estimate the activation energy (Ea):39 ）（kTEAIIat−+=exp10      （4）          Where I0 and It are the emission intensities at the initial temperature and working temperature T, respectively. A is a constant and k represents the Boltzmann constant (8.617×10-5 eV/K). Linear fitting is conducted and the slope is the value of Ea (Fig. S5). The values are ~0.598, ~0.498 and ~0.583 eV for the LaAlO3:0.001Mn4+, x=0.2 and y=0.2 samples, falling into the range of the reported values of Mn4+ doped oxides.16    Fig. 10 Temperature-dependent PL spectra of (a) LaAlO3:0.001Mn4+, (b) x=0.2 and (c) y=0.2 phosphors. (d) The relative integrated intensity of the three samples.    27  4. Conclusion  A series of La1-xBaxAl1-xTixO3:0.001Mn4+ (LBAT:0.001Mn4+, x=0-0.25) and La1-yYyAl1-yGayO3:0.001Mn4+ (LYAG:0.001Mn4+, y=0-0.25) samples were successfully synthesized. The substitution of Ba2+-Ti4+ and Y3+-Ga3+ for La3+-Al3+ contributed to the increase of lattice constant and M-O bond length. Furthermore, co-doping Ba2+-Ti4+ resulted in the suppression of octahedral tilting. However, more serious octahedral tilting was found for the Y3+-Ga3+ substitution. Thus, the distortions of LBAT:0.001Mn4+(x=0.05-0.2) samples are caused by the competition between the mismatched ionic radii and the suppression of octahedral tilting. On the contrary, the distortions of LYAG:0.001Mn4+ (x=0.05-0.2) are composed of the mismatched ionic radii and the increased octahedral tilting. The distortions have a significant influence on the Mn4+ luminescence. Longer Mn-O bond length induces weaker excitation energy through decreasing Dq for LBAT:0.001Mn4+ and LYAG:0.001Mn4+ phosphors. But the various Dq could not change the position of emission peak. In fact, the emission shape and energy, particularly the ZPL emission, exhibited big changes by co-doping Ba2+-Ti4+ and Y3+-Ga3+. The ZPL intensity increases with substituting Ba2+-Ti4+ and Y3+-Ga3+ for La3+-Al3+. Because the symmetry around Mn4+ decreases slower by co-doping Ba2+-Ti4+ than that by co-doping Y3+-Ga3+, the intensity of ZPL for LBAT:0.001Mn4+ increases slower than that in LYAG:0.001Mn4+. At the same time, the ZPL energy for LBAT:0.001Mn4+ is higher than that for LYAG:0.001Mn4+, owing to larger distortion of the O-Mn-O bond angle. 28  Conflicts of interest There are no conflicts to declare.  Acknowledgements This work was supported in part by the Natural Science Foundation of Liaoning Province (Grant 2020-MS-081), and National Natural Science Foundation of China (Grant 51302032, 51972047, 52172112).  References 1 Y. N. Zheng, R. Z. Zhang, L. Zhang, Q. F. Gu and Z.-A. Qiao, A resol-assisted cationic coordinative co-assembly approach to mesoporous ABO3 perovskite oxides with rich oxygen vacancy for enhanced hydrogenation of furfural to furfuryl alcohol, Angew. Chem. Int. Ed., 2021, 60, 4774-4781. 2 J. F. Zhao, J. C. Gao, W. M. Li, Y. T. Qian, X. D. Shen, X. Wang, X. Shen, Z.W. Hu, C. Dong, Q. Z. Huang, L. P. Cao, Z. Li, J. Zhang, C. W. Ren, L. Duan, Q. Q. Liu, R. C. Yu, Y. Ren, S.-C. Weng, H.-J. Lin, C.-T. Chen, L.-H. Tjeng, Y. Long, Z. Deng, J. L. Zhu, X. C. Wang, H. M. 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