A review on the structural dependent optical properties and energy transfer of Mn4+ and multiple ion-codoped complex oxide phosphors

The tetravalent manganese Mn4+ ions with a 3d3 electron configuration as luminescence centers in solid-state inorganic compounds have been widely investigated because they emit bright light in the red to far-red region when they are excited by light with a wavelength in the UV to blue light region. Herein, we present an overview of the recent developments of Mn4+ and multiple ion such as Bi3+ and rare earth ion Dy3+, Nd3+, Yb3+, Er3+, Ho3+, and Tm3+ codoped complex oxide phosphors. Most of the specified host lattices of these complex oxide phosphors possess multiple metallic cations, which provide possible substitutions with different codopants and form various luminescence centers with diverse spectra. The luminescence of Mn4+ and multiple ion-codoped materials spans almost the whole visible light to near infrared (NIR) region. The crystal structures of complex oxide phosphors, the spectroscopic properties of Mn4+, and the energy transfer between Mn4+ and multiple ions are introduced and summarized in detail with regard to their practical applications. This review provides an insight into the optical properties of Mn4+ and the energy transfer process in multiple ion-codoped luminescence materials, which will be helpful in the development of novel excellent materials for applications in the lighting industry.


Introduction
The optical properties based on the structures of host lattices and the energy transfer between Mn 4+ and multiple ioncodoped complex oxide phosphors described in this review make the identied luminescence materials promising for application in solar energy cells, white light-emitting diodes (WLEDs), indoor lighting for plant cultivation, and temperature sensors, as illustrated in Fig. 1.
Solar energy cells and WLEDs are considered as alternative approaches to relieve the energy crisis with the increasing global energy consumption. Solar energy cells using crystalline silicon solar cells have occupied majority of the solar cell market owing to their well-developed techniques and low cost; however, their conversion efficiency should be improved further for their wide commercial applications. It is well known that most of the energy of the solar spectrum is concentrated at wavelengths beyond 900 nm including UV-visible (UV-vis) and NIR light, which cannot be absorbed by the current crystalline silicon solar cells with high efficiency. [1][2][3][4] Converting the energy of the solar spectrum at wavelengths beyond 900 nm into the range located 900-1100 nm, which matches the maximum spectral response of the absorption of crystalline silicon, is an important alternative approach to improve the energy conversion efficiency of crystalline silicon solar cells. Recently, much attention has been paid to developing Mn 4+ -doped phosphors because Mn 4+ usually shows sharp line emissions in the red-infrared (IR) region due to its unique 3d 3 electron congurations. It has been observed that Mn 4+ shows red to far-red photoluminescence, which is assigned to the spin-forbidden 2 E g / 4 A 2g transition under the excitation of UV or blue light owing to its high effective positive charge and the inuence of a strong local crystal-eld. [5][6][7][8][9][10][11][12] The reversible conversion of UV-vis into NIR, and NIR into visible light with dual-mode luminescence can be realized by codoping multiple ions such as Nd 3+ /Er 3+ /Yb 3+ into Mn 4+ ion-doped luminescence materials. The red emission of Mn 4+ can be obtained when it is excited by 980 nm due to the energy transfer from Nd 3+ /Er 3+ /Yb 3+ to Mn 4+ ions. 13 The NIR photoluminescence maxima at 1064, 1537, and 980 nm originating from Nd 3+ /Er 3+ /Yb 3+ ions can be sensitized by Mn 4+ with excitation in the UV-vis region (200-500 nm). [13][14][15][16] The conversion of UV-vis light into NIR light at about 1064 nm through energy transfer from Mn 4+ to multiple ions is desirable to improve the conversion efficiency of solar cells by coating the phosphor layer on the surface of a crystalline Si layer.
WLEDs have received extensive attention due to their high energy efficiency, long lifetime, and environmental friendliness.
The WLEDs fabricated with blue semiconductor GaN chips and yellow phosphor Y 3 Al 5 O 12 :Ce 3+ (YAG:Ce) can produce cold white light because the red component in their spectra is weak. To meet the requirement for indoor illumination, warm white light with a high color rendering index (CRI > 80) and a low correlated color temperature (CCT < 4000 K) is necessary. 17,18 Accordingly, phosphors with strong absorption in the blue light region and intense emission in the red light region should be co-coated on blue semiconductor GaN chips to produce warm white light. Mn 4+ ions located at octahedral crystallographic sites are favorable luminescent centers and promising for blue GaNexcited warm WLED applications because they have narrowband red emissions, broad-band blue excitations, and no reabsorption in white light, while being free of expensive rare earth metals. [5][6][7][8][9][10][11][12] Thus, much attention has been paid to developing red phosphors to provide alternatives to the commercial nitride phosphors. In particular, the interest in Mn 4+ -doped inorganic phosphors has increased because the Mn 4+ luminescence center usually shows sharp line emissions in the red region with high color purity due to the sharp feature of its emission spectrum. [5][6][7][8][9][10][11][12] Recently, indoor plant cultivation has attracted considerable attention because this advanced technology can exclude the unfavorable inuence of the climate and natural damage. To meet the requirement in lighting for indoor plant cultivation, blue-violet light in wavelength range of 420-500 nm is indispensable for chlorophyll A and chlorophyll B, and red-far red light in wavelength range of 640-750 nm is indispensable for phytochrome PR and phytochrome PFR. [19][20][21][22][23][24] The fabrication of red Mn 4+ -doped phosphors in blue LED chip results in a superior performance in lighting for indoor plant cultivation due to the blue light from LED chips and red light from Mn 4+ -doped luminescence materials excited by blue light. This type of light device is a promising light source for large scale industrial application because of the energy saving and long working time of LEDs, and low cost of Mn 4+ -doped luminescence materials. Bi 3+ and Mn 4+ codoped oxide phosphors, which emit dual blue and red light upon excitation by near UV (NUV) LEDs, are alternative candidates for application in the agricultural industry to improve the efficiency of photosynthesis. To maintain the electroneutrality of the compound, excess metal ion vacancies and O 2À ions in the lattices of complex oxides may be formed for charge compensation. [25][26][27][28][29][30] The upconverted NIR luminescence of Mn 4+ was realized with the aid of the efficient energy transfer of Yb 3+ / Ln 3+ / Mn 4+ in the specially prepared Yb 3+ /Ln 3+ /Mn 4+ (Ln ¼ Er, Ho, Tm) codoped YAlO 3 and its energy transfer efficiency was systematically claried by its steady-state and time-resolved upconverted emission spectra. 31 The dual emission based on Mn 4+ and multiple ion (such as Yb 3+ , Ln 3+ , and Mn 4+ ) codoped phosphors is promising for accurate temperature sensors due to the fact that the thermal quenching mechanisms of Mn 4+ and Ln 3+ are different. 32 Fig. 2 presents a summary of the energy transfer between Mn 4+ and multiple ions, the emission wavelengths, and corresponding electronic transitions of both the donor and acceptor. The octahedral environment-coordinated Mn 4+ ions emit red to far-red emissions in the region of 600 to 700 nm. Thus, tunable spectral emissions from the visible to NIR region can be realized by codoping Mn 4+ and multiple ions.
In all the host lattice of complex oxides, as summarized in Table 1, Mn 4+ ions perfectly substitute the sites in the centers of the octahedral environment coordinated with six oxygen atoms due to their similar radius and valence, such as Ga 3+ , Al 3+ , Ti 4+ , Ta 5+ , and Mg 2+ -Te 6+ pairs, and Nb 5+ ions. Multiple cation sites in the complex oxide host lattice provide the possibility for codoping Mn 4+ ions and Bi 3+ or trivalent rare earth ions. The optical characteristics of Mn 4+ and other ions are strongly dependent on the structural symmetry of the host materials. This review aims to comprehensively present the structuraldependent optical properties based on the energy transfer between Mn 4+ and multiple ions in codoped complex oxide phosphors for potential applications in high-efficient solar energy cells, warm WLEDs, indoor plant cultivation, and temperature sensors.
Mn 4+ is isoelectronic with Cr 3+ , but the crystal eld at the higher charged Mn 4+ ions is stronger than that of Cr 3+ and the vibronic emission 2 E g / 4 A 2g of Mn 4+ is more intense than that of Cr 3+ . The Tanabe-Sugano energy diagram presents the energy splitting of the Mn 4+ ion with an octahedral coordination dependent on the crystal eld strength (Fig. 3a). [5][6][7][8][9][10][11][12]38,40 The Stokes shi and the features of the photoluminescence emission and excitation (PL and PLE) spectra of Mn 4+ ions are known to be tunable by changing the crystal-eld of the host. The   Fig. 4a shows that when viewed from the 100 plane, the unit cells for the crystal structure of Ca 14 Zn 6 Ga 10 O 35 (CZGO) possess a cubic structure with the space group F23 (196) and lattice parameters a ¼ 15.0794Å and V ¼ 3428.88Å 3 . According to Pauling's rules, one of these empty containers is lled with octahedral (Ga,Zn)O 6 À , while the others are half occupied by four corner-linked tetrahedral ZnO 4 sharing a common oxygen atom. 45 All the edges are shared by various Ca polyhedra. Thus, there are three independent Ca 2+ sites in CZGO, where two of them have an octahedral geometry and the third is in a seven-coordinated polyhedron. Moreover, the effective ionic radii of the six-coordinated Ga 3+ , Zn 2+ , and Ca 2+ ions are 0.62, 0.74, and 1.00Å, respectively. The specic crystal structure of CZGO makes doping multiple ions and forming tunable color luminescence centers possible. 46 Based on the effective ionic radii of cations with different coordination numbers (CN), 47 trivalent rare earth ions are expected to randomly occupy six-and seven-coordinated Ca 2+ (CN ¼ 6, r ¼ 1.00Å and CN ¼ 7, r ¼ 1.06Å) sites, and Mn 4+ (CN ¼ 6, r ¼ 0.53Å) ions are preferentially accommodated at the Ga 3+ (CN ¼ 5, r ¼ 0.62Å) sites with an octahedral coordination in the crystal structure. 41 Electroneutrality in the Mn 4+ and multiple ion-codoped CAZO phosphors can be easily achieved due to some defects such as the formation of Ca 2+ vacancies and excess O 2À ligands for charge compensation. 48 3.1.2 Dual mode energy transfer between Mn 4+ and Nd 3+ / Er 3+ /Yb 3+ in CZGO. The energy transfer efficiency depends on the matching of the energy levels between the excitation wavelength of the acceptor and donor emission frequency. 50,51 Fig. 5a-c depict the spectral overlap between the emission spectrum of Mn 4+ and the excitation spectra of Nd 3+ /Er 3+ /Yb 3+ , which demonstrates that the Mn 4+ ion has a strong possibility of being an effective sensitizer for NIR emission of Nd 3+ /Er 3+ / Yb 3+ through a non-radiative resonant energy transfer process. 49 Fig. 5e-g show the emission spectra of Mn 4+ and multiple ions Nd 3+ /Er 3+ /Yb 3+ codoped CZGO with different doping concentrations of Ln 3+ ions, respectively. Upon excitation at 313 nm, the NIR emissions of Nd 3+ /Er 3+ /Yb 3+ such as the emission peaks at 900 and 1075 nm are assigned to the 4 F 3/2 / 4 I 9/2 and 4 F 3/2 / 4 I 11/2 transitions of Nd 3+ , that at 978 and 1537 nm are ascribed to the 4 I 11/2 / 4 I 15/2 and 4 I 13/2 / 4 I 15/2 transitions of Er 3+ , and that at 980 nm is caused by the 2 F 5/2 / 2 F 7/2 transition of Yb 3+ . The emission intensity of Mn 4+ monotonously decreases with an increase in the content of Ln 3+ , which indicates energy transfer occurs from Mn 4+ to Nd 3+ / Er 3+ /Yb 3+ .
The green and red emission centered at 551 (561) and 661 nm can be ascribed to the transitions of 2 H 11/2 / 4 I 15/2 ( 4 S 3/2 / 4 I 15/2 ) and 4 F 9/2 / 4 I 15/2 of Er 3+ , respectively, via the multiple non-radiative multiphonon relaxations from the 4 F 7/2 to H 11/2 , 4 S 3/2 and 4 F 9/2 levels. 53 The deep red emission ascribed to the transition of 2 E g / 4 A 2g of Mn 4+ is attributed to the energy transfer from Er 3+ to Mn 4+ , as illustrated in the corresponding mechanism diagram in Fig. 6c. The spectral overlap observed between the emission spectrum of Er 3+ and the excitation spectrum of Mn 4+ makes the reversal energy transfer from Er 3+ to Mn 4+ possible, as presented in Fig. 6d Fig. 7 shows the unit cell structure and the coordination environment of the cation sites of a typical Ca 14 -Zn 6 Al 10 O 35 (CZAO) compound. CZAO has a cubic structure with the space group F23. In the crystal structure of CZAO, Ca 2+ has three different coordination environments, where two of them are coordinated to six oxygen atoms, forming a distorted octahedron, while the third is in a seven-coordinated polyhedron and the average Ca-O distance is equal to 2.498Å. 54,55 In addition, four of the ve independent positions occupied by Zn and Al are in the tetrahedral coordination, with the average Zn-O distance of 1.951Å and average Al-O distances of 1.719, 1.794 and 1.891Å, respectively. The positions are in an octahedron coordination, and the one-h positions occupied by Al and Zn are octahedral coordinations. [56][57][58][59] The Ca 2+ site is likely to be replaced by a small amount of Nd 3+ /Yb 3+ ions without signicant structural changes due to the similar ion radii between Ca 2+ and Nd 3+ /Yb 3+ (Ca 2+ : r ¼ 0.100 nm; Nd 3+ : r ¼ 0.098 nm; and Yb 3+ : r ¼ 0.086 nm).
3.2.2 Energy transfer between Mn 4+ and Nd 3+ /Er 3+ /Yb 3+ in CZAO. Under excitation by UV to visible light from 250 to 550 nm, intense NIR emissions are produced at 900 and 1060 nm originating from the Nd 3+ : 4 F 3/2 / 4 I 9/2 and Nd 3+ : 4 F 3/2 / 4 I 11/2 in Mn 4+ and Nd 3+ -codoped phosphors. The emission at 980 nm in the Mn 4+ , Yb 3+ codoped samples is ascribed to the Yb 3+ : 2 F 5/2 / 2 F 7/2 transitions. 60 The energy transfer based on the strong absorption of Mn 4+ and spin-allowed transitions of Nd 3+ /Yb 3+ through dipoledipole interaction is illustrated in Fig. 8. The shapes of the PLE spectra of both the Mn 4+ /Nd 3+ and Mn 4+ /Yb 3+ codoped samples monitored at 1060 nm and 980 nm, respectively, are quite similar to that of the Mn 4+ single-doped sample ( Fig. 8a-c). Only weak and discrete PLE peaks in the visible region caused by the f-f transitions of Nd 3+ appear in the Nd 3+ single-doped sample and no PLE peak in the visible region is observed in the Yb 3+ singledoped sample ( Fig. 8a and b), respectively. Thus, the characteristics of the above PLE spectra demonstrate that the NIR luminescence of Nd 3+ /Yb 3+ in Mn 4+ and multiple ion Nd 3+ /Yb 3+ codoped CZAO is generated by the energy transfer from Mn 4+ to Nd 3+ /Yb 3+ ions. [61][62][63] The energy transfer efficiency depends on the spectral matching of the excitation of the acceptor and emission spectra of the donor. As shown in Fig. 8d, good spectral overlap can be observed between the 2 E g emission of Mn 4+ and the 4 F 9/2 , 4 F 7/2 , and 4 S 3/2 excitations of Nd 3+ . It can be seen from Fig. 8e that although there is a relatively large energy gap between the excited state 2 E g of Mn 4+ and 2 F 5/2 of Yb 3+ , an efficient energy transfer from Mn 4+ to Yb 3+ can still occur in the Mn 4+ and Yb 3+ codoped samples with strong electron-phonon coupling. 64 Therefore, the NIR luminescence of Yb 3+ may be mainly generated by phonon-assisted energy transfer from Mn 4+ to Yb 3+ . The excitation/emission and energy transfer pathways for the Mn 4+ and codoped Nd 3+ /Yb 3+ ion couples in CZAO are quite similar to that in the host lattice of CZGO. 14,49 NIR emissions from Nd 3+ /Yb 3+ have been observed in Mn 4+ and Nd 3+ /Yb 3+ codoped CZAO phosphors. The intensity of the NIR emissions of Nd 3+ /Yb 3+ increases initially with an increase in the content of rare earth ions Nd 3+ /Yb 3+ , and then decreases gradually as a result of concentration quenching. 62 The NIR luminescence intensity is enhanced by 338 times at 1060 nm for Ca 13 Fig. 9a and b illustrate the excitation spectra of Mn 4+ and emission spectra of Er 3+ in Mn 4+ and/or Er 3+ codoped samples with various doping concentrations. The two broad and intense excitation bands (monitored at Mn 4+ 710 nm emission) correspond to the spin-allowed transitions 4 A 2g / 4 T 1g and 4 A 2g / 4 T 2g of Mn 4+ (Fig. 9a). The weak and discrete excitation peaks (monitored at Er 3+ 1540 nm emission) are ascribed to the transitions from 4 I 15/2 to 4 G 11/2 , 4 F 5/2 , 4 F 7/2 , 2 H 11/2 , and 4 S 3/2 of Er 3+ . From the excitation spectrum of the Mn 4+ and Er 3+ codoped sample monitored at the 1540 nm emission of Er 3+ in Fig. 9b, it can be seen that not only broad and intense excitation bands ascribed to Mn 4+ ions but also the superimposed excitation peaks assigned to the 4 I 15/2 to 2 H 11/2 and 4 S 3/2 transitions of Er 3+ appear, indicating the energy transfer from Mn 4+ to Er 3+ .

Complex hexoxides as host lattices for Mn 4+ and multiple ion codoping
As shown in Fig. 10a, 65 Gd 2 ZnTiO 6 (GZT) crystallizes in a doubleperovskite monoclinic structure with the space group P21/n, with the cell parameters of a ¼ 5.3664(9)Å, b ¼ 5.6631(9)Å, c ¼  7.6847(9)Å and b ¼ 90.294 (2) . In the crystal structure of GZT, the Zn 2+ and Ti 4+ ion centers are at two slantwise octahedral sites surrounded by six oxygen atoms, and the Gd 3+ ion occupies the decahedron site coordinated with twelve oxygen atoms. La 2 LiTaO 6 is built up of alternating strands of LiO 6 and slightly disordered TaO 6 with La 3+ located in the cavities of the interconnected network of octahedral sites, as shown in Fig. 10b. [67][68][69][70] According to the doping rule that with a similar radius and the same valence of the dopants and host cationic ions, Mn 4+ ions perfectly enter the centers of the octahedral environment coordinated with six oxygen atoms and the trivalent rare earth ions can occupy the Gd 3+ and/or La 3+ sites in the host lattices of complex hexoxides, respectively. NaMgLaTeO 6 crystallizes in a monoclinic system with the P12 1 /m1(11) space group, as depicted in Fig. 10c. 16,71,72 Both Mg 2+ and Te 6+ are located at the six-fold sites to form MgO 6 and TeO 6 octahedra with a shared oxygen atom, respectively. Moreover, the La/Gd and Na/K atoms are coordinated with twelve oxygen atoms to form polyhedral La/GdO 12 and Na/KO 12 . These four types of polyhedra connect closely to construct the space framework of this crystal structure. 73 The Mg 2+ and Te 6+ sites at the centers of the octahedra are expected to be substituted by Mn 4+ ions and red luminescence centers of Mn 4+ are formed. In the Mn 4+ and Er 3+ coped GZT sample, efficient energy transfer from Mn 4+ to Er 3+ was observed, and the mechanism is quite similar to that in Mn 4+ and Er 3+ codoped CZAO. 14 It can be seen from Fig. 11a that the emission spectrum of Gd 2 ZnTiO 6 :yMn 4+ ,0.02Er 3+ (y ¼ 0, 0.002) is excited at 335 nm, corresponding to the 4 A 2g / 4 T 1g of Mn 4+ , and in that of Gd 2 -ZnTiO 6 :0.002Mn 4+ ,2xEr 3+ (x ¼ 0, 0.005) are excited at 379 nm, corresponding to 4 A 2g / 4 T 1g of Mn 4+ and 4 I 15/2 / 4 G 11/2 of Er 3+ . 75 Only the characteristic emission peaks ( 2 E g ) of Mn 4+ can be observed and no characteristic visible emission peaks ( 2 H 11/2 / 4 S 3/2 ) of Er 3+ for the GZT:0.002Mn 4+ ,0.02Er 3+ sample in the emission excited at 335 nm. Spectral overlap exists between the emission for Er 3+ ( 2 H 11/2 / 4 S 3/2 ) and the absorption for Mn 4+ ( 4 A 2g ), which provides a possible energy transfer pathway from Mn 4+ to Er 3+ . 76 The emission intensity of 2 E g of Mn 4+ upon the codoping of Er 3+ in GZT is much stronger than that of Mn 4+ single-doped GZT under the common excitation wavelength of 379 nm, which indicates that energy back transfer occurs from Er 3+ ( 2 H 11/2 / 4 S 3/2 ) to Mn 4+ ( 4 A 2 ) under the common excitation wavelength of 379 nm (see Fig. 11b).
The IR emission at 1529 nm is ascribed to the 4 F 9/2 ( 4 I 9/2 ) / 4 I 13/2 transition of Er 3+ through energy transfer from Mn 4+ in the Mn 4+ and Er 3+ codoped GZT phosphor and the corresponding mechanism is illustrated in Fig. 12a. The Mn 4+ ions are excited into their excite states under irradiation by short-wavelength light in the region of 250-550 nm, and then the energy transfer of 2 E (Mn 4+ ) / 4 F 9/2 , 4 I 9/2 (Er 3+ ) happens between the Mn 4+ and Er 3+ ions to populate the 4 F 9/2 and 4 I 9/2 levels of Er 3+ followed by nonradiative relaxation to 4 I 13/2 . Finally, IR emission at 1529 nm is produced by radiative transition from 4 I 13/2 to 4 I 15/2 of Er 3+ .
Far-red (FR) and near-infrared (NIR) double-wavelength emissions have been observed in the Mn 4+ and Yb 3+ codoped GZT phosphor, which are expected to application in LEDs towards plant cultivation. [77][78][79] The PLE and PL spectra of the Mn 4+ and Yb 3+ codoped samples are shown in Fig. 12b-d. The shapes and positions of both PLE spectra (Fig. 12b) monitored at emission 704 nm from the 2 E g / 4 A 2g transition of Mn 4+ and that at 980 nm from the Yb 3+ transition 2 F 5/2 / 2 F 7/2 are similar to that of Mn 4+ singly doped GZT, which indicates that energy transfer between Mn 4+ and Yb 3+ occurs in the codoping systems. Under the excitation of 365 nm light, both FR emission from Mn 4+ and NIR emission from Yb 3+ are observed in Fig. 12c and d. The FR emission intensity of Mn 4+ gradually decreases with an increase in the content of Yb 3+ , whereas the NIR emission intensity rst increases and then decreases due to the   Fig. 13b and c exhibit the emission spectra of the La 2Àx -MgTi 1Ày O 6 :Mn y ,Yb x and La 2Àx MgTi 1Ày O 6 :Mn y ,Yb x samples pumped by 460 nm light. The NIR emission band with the highest peak at 990 nm is from the 2 F 5/2 / 2 F 7/2 transition of Yb 3+ ions and its emission is strongly dependent the concentrations of Yb 3+ ions. 35,82,83 The integrated intensity of the NIR emission band centered at 990 nm increases initially with an increase in the concentration of Yb 3+ ions.
Energy transfer from Mn 4+ to Yb 3+ occurs in the Mn 4+ and Yb 3+ codoped Ba 2 LaNbO 6 (BLNO) samples, as illustrated in Fig. 14. 35 The spectral shapes and positions of the excitation spectra monitored at 677 nm (Mn 4+ emission) and 998 nm (Yb 3+ emission) remain the same, but their intensities are different, which indicates that energy transfer from Mn 4+ to Yb 3+ occurs in the Mn 4+ and Yb 3+ codoped BLNO, as show in Fig. 14a and b. The emission centered at 998 nm is consistent with the infrared light needed for bacterial chlorophyll. 22,84 The intensity of the Mn 4+ emission at 677 nm decreases, while that of the Yb 3+ emission at 998 nm increases due to the transfer of energy from Mn 4+ to Yb 3+ . Fig. 14c shows the decay lifetimes of BLNO:0.003Mn 4+ ,yYb 3+ , which decrease with an increase in the Yb 3+ concentration, thus proving the occurrence of energy transfer from Mn 4+ to Yb 3+ in the phosphor. According to the mechanism of energy transfer of Mn 4+ and Yb 3+ based on Fig. 14d, 35,85,86 the Mn 4+ ions are excited from the ground state ( 4 A 2g ) to excited states ( 4 T 1g , 2T 2g , and 4 T 2g ) under UV light excitation, and then relax to the 2 E g state.
The energy can be transferred from the 2 E g state of Mn 4+ to the 2 F 5/2 level of Yb 3+ through nonradiative transition, thereby producing the NIR emission observed at 998 nm.
The energy transfer from Mn 4+ to Nd 3+ occurs in the Mn 4+ and Nd 3+ codoped (Na,K)Mg(La,Gd)TeO 6 samples, as illustrated in Fig. 15. 16 Upon excitation at 365 nm UV, both emissions from Mn 4+ and Nd 3+ are observed, and the Mn 4+ emission intensity and the corresponding decay time of Mn 4+ at 705 nm decrease monotonously with an increase in Nd 3+ concentration, which strongly conrms the efficient energy transfer from the Mn 4+ to Nd 3+ ions in these samples. 87,88 The energy transfer processes of Mn 4+ / Nd 3+ / Yb 3+ occurring in the Mn 4+ , Nd 3+ and Yb 3+ codoped NaMgLaTeO 6 (NMLTO) samples are illustrated in Fig. 16. 16 The emission spectra of NML:0.02Mn 4+ ,0.30Yb 3+ excited at 365 nm contains both the Mn 4+ emission band at around 705 nm due to the Mn 4+ 2 E g / 4 A 2g transition, and the Yb 3+ emission band with a maximum at around 1003 nm attributed to the Yb 3+ 2 F 5/2 / 2 F 7/2 transition. The excitation spectrum (200-900 nm) monitored at 1003 nm clearly contains the Mn 4+ absorption band, suggesting energy transfer from Mn 4+ to Yb 3+ ions. [89][90][91] In the Mn 4+ , Nd 3+ , and Yb 3+ codoped NMLTO sample, the emission spectra of the obviously present bands from all three ions Mn 4+ , Nd 3+ , and Yb 3+ in the range of 600-1300 nm upon 365 nm UV excitation. 92,93 The emission intensity of Nd 3+ decreases monotonously with an increase in Yb 3+ concentration, which   illustrates the possibility of energy transfer from the Nd 3+ to Yb 3+ ions as shown in Fig. 16a-c. Fig. 16d shows an overview of the partial electronic energy level diagram of Mn 4+ , Nd 3+ , and Yb 3+ in NMLTO and a schematic diagram illustrating the possible energy transfer processes occurring in Mn 4+ , Nd 3+ , and Yb 3+ codoped NMLTO. 16 The energy at the Mn 4+ excited state 2 E g can be transferred to the Nd 3+ levels 4 F 7/2 and 4 S 3/2 via the Forster resonant energy transfer process to produce the emissions at 910 and 1072 nm. 90,94 The NIR emissions of Nd 3+ at 910 and 1072 nm from the 4 F 7/2 and 4 S 3/2 levels, respectively, increases and the red emission of Mn 4+ at 705 nm from the 2 E g excited state decreases with an increase in the concentration of Mn 4+ , which indicates the energy transfer from Mn 4+ to Nd 3+ . 68,95 Then the excited 4 F 7/2 and 4 S 3/2 energy levels of Nd 3+ can relax nonradiatively to the 4 F 5/2 and 2 H 9/2 Nd 3+ energy levels, and transfer the energy to the 2 F 5/2 Yb 3+ excited state and enhance the Yb 3+ emission.
As can be seen in Fig. 17, the excitation spectra of the Mn 4+ , Nd 3+ and Yb 3+ codoped NMLTO samples match well with the solar spectrum in the UV and visible regions, and the emission bands are located at the ideal 930-1100 nm region for excellent response for crystal silicon solar energy cells. 68,96 Thus, the Mn 4+ , Nd 3+ and Yb 3+ codoped NMLTO sample has potential for the effective broadband spectral conversion of UV/visible light to the NIR band utilizing the energy transfer processes of Mn 4+ / Nd 3+ / Yb 3+ .

Energy transfer between Dy 3+ and Mn 4+
As displayed in Fig. 18a, the PLE spectrum of the Ca 13.88 Al 10 -Zn 6 O 35 :0.12Dy 3+ phosphor monitored at 576 nm consists of a series of sharp peaks with the strongest absorption at 351 nm due to the 6 H 15/2 / 6 P 7/2 transition of Dy 3+ . Under excitation at 351 nm, the PL spectrum consists of two dominant peaks at around 482 nm (blue) and 576 nm (yellow), corresponding to the 4 F 9/2 / 6 H 15/2 and 4 F 9/2 / 6 H 13/2 transitions of Dy 3+ , respectively. [97][98][99] As shown in Fig. 18b, signicant spectral overlap was observed between the PLE of Mn 4+ and PL of Dy 3+ , indicating that effective energy transfer from Dy 3+ to Mn 4+ is expected.
The energy transfer process from Dy 3+ to Mn 4+ is elucidated according to the schematic energy level diagram in Fig. Fig. 18c. In the cross-relaxation processes, the Dy 3+ ions at the 4 F 9/2 level can be de-excited to the 6 F 9/2 / 6 H 7/2 , 6 H 9/2 / 6 F 11/2 , or 6 F 1/2 level, while the ions at the 6 H 15/2 ground state will accept the energies excited simultaneously to the 6 F 3/2 , 6 F 5/2 , and 6 H 9/2 / 6 F 11/2 levels. Although the energy level 4 F 9/2 of Dy 3+ (20 747 cm À1 ) is higher than the 2 E g energy level of Mn 4+ (14 025 cm À1 ), the energy transfer from the 4 F 9/2 level of Dy 3+ to the 2 E level of Mn 4+ may be realized via the assistance of phonons. 97,[100][101][102][103][104] The PL spectra of Ca 13.88 Al 10Ày Zn 6 O 35 :0.12Dy 3+ ,yMn 4+ (y ¼ 0, 0.01, 0.05, 0.10, 0.15, 0.20, and 0.25) upon excitation at 351 nm and the change in the emission intensities of Dy 3+ and Mn 4+ with the concentration of Mn 4+ are presented in Fig. 19. 97 The emissions at 482 and 576 nm are due to the 4 F 9/2 / 6 H J/2 (J ¼ 15, 13) transitions of Dy 3+ , and the red emission with a multi-peak structure in the wavelength range of 650 to 750 nm corresponds to the vibronic emission 2 E g / 4 A 2g of Mn 4+ . The emission intensity of Mn 4+ increases, whereas that of Dy 3+ is simultaneously found to decrease monotonically with an increase in concentration of Mn 4+ , indicating that the energy transfer from Dy 3+ to Mn 4+ is efficient. 105-108

Tunable dual emissions for Bi 3+ and Mn 4+ codoped phosphors
It was found that both the blue light from Bi 3+ and red light from Mn 4+ are produced in all the Bi 3+ and Mn 4+ codoped CZAO samples, as illustrated in Fig. 20a. The emission band from 400 nm to 550 nm with a maximum at 410 nm is ascribed to the 3 P 1 / 1 S 0 transition of the Bi 3+ ions, while that from 650 nm to 750 nm is ascribed to the 2 E g / 4 A 2g emission of the Mn 4+ ions. 25,109 The intensity of the blue emission decreases and that of the red emission increases with an increase in the Mn 4+ concentration, as shown in Fig. 20b, which indicates the occurrence of energy transfer from Bi 3+ to Mn 4+ . The dualemission color can be tuned by changing the Bi 3+ /Mn 4+ ratio.
A similar energy transfer from Bi 3+ to Mn 4+ was also observed in the Bi 3+ and Mn 4+ codoped CZAO phosphor due to the  spectral overlap in the PLE of Mn 4+ and PL of CZAO:0.008Bi 3+ , as shown in Fig. 20c. Under the same excitation source, Bi 3+ and Mn 4+ codoped CZAO phosphors show dual emissions, where the blue-violet emission is mainly from the 3 P 1 / 1 S 0 transition of Bi 3+ and the far red emission is attributed to the 2 E g / 4 A 2g transition of Mn 4+ . 26,110,111 As presented in Fig. 20d and e, the blue emission of Bi 3+ matches the absorption spectra of chlorophyll A and chlorophyll B, while the red emission from Mn 4+ matches the absorption spectra of phytochrome PR and phytochrome PFR, which indicate that the phosphor has potential for application in plant growth LED lighting.
The energy transfer process from Bi 3+ to Mn 4+ realized in Bi 3+ and Mn 4+ codoped La 2 MgTiO 6 (LMTO) phosphors is illustrated in Fig. 21. The absorption bands from 275 to 375 nm in the PLE spectra for LMT:0.005Bi 3+ in Fig. 21a are ascribed to the 1 S 0 / 1 P 1 and 1 S 0 / 3 P 1 transitions of Bi 3+ . A blue emission (375-500 nm) with a maximum at 417 nm of Bi 3+ is detected, which is due to the 3 P 1 / 1 S 0 transitions. The strong red emission band from 650 to 750 nm with an emission peak at 710 nm is observed owing to the 2 E g / 4 A 2g transition of Mn 4+ . The spectral overlap between the emission spectrum of Bi 3+ and the excitation spectra of Mn 4+ provides strong evidence for the energy transfer between Bi 3+ and Mn 4+ . The emission intensity of Bi 3+ gradually decreases and that of Mn 4+ presents a monotonous increase with an increase in the Mn 4+ doping concentration, which indicates that energy transfer occurs in the Bi 3+ and Mn 4+ codoped LMTO phosphors, as shown in Fig. 21b.
The electronic transitions and the energy transfer process in the Bi 3+ and Mn 4+ codoped phosphors are illustrated the schematic energy level diagram shown in Fig. 21c. 26 The Bi 3+ ions are initially excited from the ground state 1 S 0 to the excited state 3 P 1 , 3 P 2 , and 1 P 1 or even the conduction bands under the irradiation of UV light. Then, the Bi 3+ ions relax to the lowest excited state of 3 P 1 and return to the 1 S 0 ground state through radiative transition and yield blue emission. Simultaneously, the Bi 3+ ions in the 3 P 1 state can also transfer their energy to the adjacent Mn 4+ ions and promote the Mn 4+ ions from the 4 A 2g ground state to the 4 T 2g , 2 T 2g , and 4 T 1g energy levels and relax to the 2 E g level through a nonradiative transition and then produce red emission when they return to the 4 A 2g ground state. [113][114][115] The energy transfer occurring between Bi 3+ and Mn 4+ eventually lead to an enhancement in the far-red emission of Mn 4+ .

Enhanced red emission of Mn 4+ by codoping rare earth ions
The Dy 3+ and Mn 4+ codoped Ca 14 Ga 10Àm Al m Zn 6 O 35 (CGAZO:-Dy 3+ ,Mn 4+ ) phosphor can exhibit strong far-red emission, which has potential application for plant growth LED lighting. 44 As shown in Fig. 22a, the three absorption bands A (200-290 nm), B (290-420 nm), and C (420-550 nm) of the phosphors in the  UV-vis absorption spectra can be attributed to the host lattice absorption, charge transfer transition of Mn 4+ -O 2-, and spinallowed transitions 4 A 2g / 4 T 1g and 4 A 2g / 4 T 2g of the Mn 4+ ions, respectively. 116 The absorption intensity of bands A and C decrease, but that of band B is enhanced with elevated Al 3+ concentrations, which indicates that the absorption intensity of the phosphor powder is enhanced in the ultraviolet light range, but reduced slightly in the blue light range.
As shown in Fig. 22b, the PL intensity of the Mn 4+ activator increased, whereas that of the Dy 3+ sensitizer simultaneously decreased monotonically with an increase in the concentration of Mn 4+ ions, which demonstrates that energy transfer from Dy 3+ to Mn 4+ occurred in the Dy 3+ and Mn 4+ -co-activated CGAZO, as described using Fig. 22c. The Dy 3+ ions are excited to their 6 P 7/2 or 6 P 5/2 or 4 I 13/2 excited states or conduction band under irradiation of near UV light and nonradiatively relax to their 4 F 9/2 state. The energy transfer process between the Dy 3+ and Mn 4+ ions occurs via 4 F 9/2 (Dy 3+ ) / 2 E g (Mn 4+ ) and the Mn 4+ ions return from the lowest excited level 2 E g (Mn 4+ ) to the 4 A 2g ground state (Mn 4+ ) through a radiative transition, which produces the far-red light emission at 715 nm.

Enhanced red emission of Mn 4+ by codoping Bi 3+
The red emission of the Mn 4+ ions in the phosphors based on the CaAl 12 O 19 , 117,118 Mg 2 TiO 4 , 37,119-121 and La 2 ATiO 6 (ref. 112) (A ¼ Mg, Zn) host lattices can be dramatically enhanced by the incorporation of Bi 3+ codopant. The spectral proles of the excitation and emission spectra of Mn 4+ with or without codoping Bi 3+ ions in these Mn 4+ doped phosphors are quite similar. Therefore, it can be speculated that the synergetic effect of codoping Bi 3+ plays a key role in the modication of the crystal structure and the luminescence efficiency of Mn 4+ . Thus, the strategy for enhancing the luminescence performance of Mn 4+ plays a pivotal role in the development of highly efficient red-emitting phosphors. [122][123][124][125] 6. Luminescent thermometers based on Mn 4+ and multiple ion-doped materials By employing the highly temperature-sensitive Mn 4+ luminescence as the temperature detecting signal, while the temperature-insensitive rare earth ion (Eu 3+ , Tb 3+ or Dy 3+ ) emission was used as a reference signal, Mn 4+ and multiple rare earth ion codoped phosphors exhibited an excellent temperature sensing performance with absolute and relative sensitivities as high as 0.114-0.441 K À1 and 2.32-4.81% K À1 , respectively, which indicate their potential application in luminescent thermometers. [126][127][128] In Fig. 23a and b, the bright red luminescence of the Eu 3+ and Mn 4+ codoped YAG samples originated from both the  transitions 5 D 0 / 7 F J of Eu 3+ and 2 E g / 4 A 2g of Mn 4+ . With an increase in temperature, the luminescence of Mn 4+ weakens quickly, whereas that of Eu 3+ exhibits a slight decrease. As shown in Fig. 23c, the remarkable change in I Mn /I Eu with a variation in temperature measured on the cycling process of heating-cooling can almost be restored to the original states aer the heating-cooling cycle. [130][131][132][133][134] As conrmed in Fig. 23d, this temperature-dependent I Mn /I Eu is repeatable and reversible aer several cycling experiments. Therefore, a highly sensitive temperature determination can be expected if the Mn 4+ emission is employed as the detection signal of temperature, while the Eu 3+ emission is used as the reference signal.
The photon generation and energy transfer between Mn 4+ and rare earth ions in Mn 3+ , Mn 4+ , and Nd 3+ codoped YAG nanocrystals can be illustrated by an energy level diagram, as presented in Fig. 23e. The Mn 4+ ions are excited from the 4 A 2g ground state to the 4 T 2 excited state, followed by nonradiative multiphonon relaxation, leading to population of the 2 E g state, and then emit red emission at 670 nm, which is ascribed to the radiative electronic 2 E g / 4 A 2g transition of Mn 4+ . The appearance of an intersection point between the 4 T 2 parabola and the 4 A 2g parabola at DE (activation energy, in this case DE 1 ¼ 2506 cm À1 ) is due to the strong electron-phonon coupling. The value of DE 1 is associated with the distortion of the Mn 4+ energy states, which is strongly dependent on the crystal eld.
With an increase in the temperature, the population of higher vibrational states gradually increases up to the moment when the provided thermal energy is sufficiently high to overcome the intersection point (DE 1 ), above which electrons from the 2 E g level are transferred through 4 T 2g to the 4 A 2g ground state via nonradiative multiphonon relaxation. In contrast, rare earth ions are expected to be less affected by luminescence temperature quenching because their energy diagram usually consist of numerous f energy states due to low electron-phonon coupling. Therefore, Mn 4+ and rare earth ions codoped in a single host lattice can be applied in a luminescent thermometer. [135][136][137][138] The structural coordinate diagram in Fig. 23e proposes a possible mechanism for elucidating the high temperature sensitivity of Mn 4+ and rare earth ion (such as Eu 3+ /Tb 3+ and Dy 3+ ) codoped samples. The Mn 4+ luminescence is easily thermally quenched through an energy-level crossing relaxation (ELCR) between the 4 T 2g excited state and the 4 A 2g ground state due to the role of strong electron-phonon coupling. The thermal quenching of rare earth ions is completely different to that of Mn 4+ since there is no crossing point between the excited states and the ground state of rare earth ions because their 4f orbitals are shielded from the surroundings by the lled 5 S 2 and 5 P 6 orbitals, 139,140 and consequently the multi-phonon deexcitation (MPD) mode is the dominant mechanism responsible for the thermal-quenching of rare earth ions. The thermalquenching probability of Eu 3+ , Tb 3+ , and Dy 3+ luminescence is quite low because the required phonon numbers to bridge the energy gaps of Eu 3+ , Tb 3+ and Dy 3+ are 16, 21 and 10, respectively.
The representative thermal evolution of the emission spectra of Y 3 Al 5 O 12 :Mn 3+ , Mn 4+ , Nd 3+ nanocrystals presented in Fig. 24a indicates that the emission intensity of both the 2 E g / 4 A 2g emission band of Mn 4+ and the 4 F 3/2 / 4 I 9/2 band of Nd 3+ decreases with an increase in temperature. In contrast, the 5 T 2 / 5 E 00 emission of Mn 3+ exhibits different behavior. The upper lying 5 T 2 state of Mn 3+ can be populated via phonon-assisted energy transfer with the phonon absorption. The probability of this process increases with temperature according to the Miyakawa-Dexter theory. 31,141,142 On the other hand, Kuck et al. explained the increase in the Mn 3+ emission at elevated temperatures in terms of the thermal population from the 3 T 1 state. 143 The thermal evolution of LIR 1 for the series of 20 nm nanocrystals with different manganese concentrations is presented in Fig. 24b. In the low temperature range, LIR 1 increases with temperature, reaching the maximum at T ¼ 400 K. A further increase in temperature causes a reduction in the value of LIR 1 . In the low temperature range (below 350 K), the LIR 2 value is thermally independent, which is related to the high thermal stability of the Mn 4+ luminescence at low temperatures. This shows that the emission intensity of the 5 T 2 state of Mn 3+ increases at low temperatures, while that of the 2 E g state of Mn 4+ becomes stable.
The thermoluminescence glow curves of the all the Mn 4+ and rare earth ion (La 3+ , Gd 3+ , Dy 3+ , and Ho 3+ ) codoped MgAl 2 Si 2 O 8 host phosphors recorded aer band a-irradiation are shown in Fig. 25. 39 All the phosphors exhibit one main peak at about 261 AE 3 C for b-irradiation and many satellite peaks in the low temperature range up to 200 C. Furthermore, the a-irradiated phosphors had one main peak at about 245-252 C and the same satellite peaks. The addition of La 3+ , Gd 3+ , Dy 3+ , and Ho 3+ dopants in the MgAl 2 Si 2 O 8 :Mn 4+ phosphor did not cause any new TL peaks, but the peak intensities changed. In addition, the Dy 3+ and Gd 3+ co-doped phosphors had relatively high peak intensities compared with the other phosphors. The main peaks were shied towards the lower temperature region when the phosphors were exposed to a-irradiation. [144][145][146][147] The TL curves of the band a-irradiated phosphors exhibited substantial changes, which can be associated with the type of radiation. Therefore, the TL peak positions of MgAl 2 Si 2 O 8 :Mn 4+ with codoping La 3+ , Gd 3+ , Dy 3+ , and Ho 3+ activators did not change for aand b-irradiation.
Similar behavior was observed in the Mn 4+ and Tb 3+ codoped Sr 4 Al 14 O 25 nanocrystalline phosphor. The intense red emission associated with the 2 E / 4 A 2 electronic transition of Mn 4+ ions was drastically quenched, while the 5 D 4 / 7 F 5 emission of Tb 3+ remained almost thermally independent above 100 C. The combination of the thermally quenched luminescence from the Mn 4+ ions to the almost temperature-independent emission from Tb 3+ provided a sensitive luminescent thermometer (SR ¼ 2.8%/ C at 150 C) with strong emission color variability. Thus, the developed thermochromic luminescent nanomaterials based on codoped Mn 4+ and Tb 3+ possess the high application potential for thermal sensing and mapping. 153

Challenges and perspectives
Mn 4+ and multiple ion-codoped complex oxide phosphors have high stability, abundant starting materials, simple synthetic technology (solid state sintering), and tunable luminescence spectra covering the full visible light region from blue to red, and extending to the NIR region. The challenges and perspectives of the future work focusing on Mn 4+ and multiple ioncodoped materials are proposed as follows: (1) Applying the developed Mn 4+ and multiple ion-codoped phosphors for the fabrication of WLED devices, solar energy cells, etc.
(2) Enhancement of the luminescence efficiency of Mn 4+ by optimizing the synthetic parameters including codoping some content of multiple ions.
(3) Discovery of novel host lattice materials with multiple crystals sites to accommodate various dopants and luminescence centers in a single host lattice.
(4) Improvement of the efficiency of energy transfer between Mn 4+ and multiple ion-codoped phosphors to obtain tunable luminescence spectra.

Conclusions
This review summarized the recent research progress of Mn 4+ and multiple ion such as Bi 3+ and rare earth ions Dy 3+ /Nd 3+ /  Yb 3+ /Er 3+ /Ho 3+ /Tm 3+ codoped phosphors in the complex oxide host lattice, including their structural-dependent optical properties, energy transfer mechanism, and potential optical applications. Thus, these Mn 4+ -and multiple ion-codoped phosphors are potential candidates for application in the elds of solar energy cells, WLEDs, indoor plant cultivation, and temperature sensors. This review provides extensive insight for developing novel Mn 4+ -doped phosphors with desirable functional properties from an application point of view and helps to reveal the underlying energy transfer mechanism between Mn 4+ and multiple ions.

Conflicts of interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to inuence the work reported in this paper.