Spectroscopy of Mn4+ in Double Perovskites, La2LiSbO6 and La2MgTiO6: Deep Red Photon Generators for Agriculture LEDs

aGE Global Research, One Research Circle, Niskayuna, New York 12309, USA bCollege of Sciences, Chongqing University of Posts and Telecommunications, Chongqing 400065, People’s Republic of China cInstitute of Physics, University of Tartu, Tartu 50411, Estonia dInstitute of Physics, Jan Dlugosz University, Armii Krajowej 13/15, PL-42200 Czestochowa, Poland eCurrent & Lighting, Current Powered by GE, Cleveland, Ohio 44112, USA fLighting Enabled Systems & Applications ERC, Rensselaer Polytechnic Institute, Troy, New York 12180, USA

A phosphor emitting at wavelengths greater than 650 nm is not an efficient red photon generator for use in general illumination. This is because the spectrum of such phosphors will make a poor match with the human eye response (luminosity response function) resulting in low brightness even when the quantum efficiency of the phosphor is high. However, the far-red (FR) region (700-740 nm) of the spectrum generated by the deep red emitting phosphors has significant implications for plants. An important family of plant photoreceptors are the phytochromes (PHYs) that sense and signal changes in the red (660-670 nm) and FR (725-735 nm) ratios in a given spectrum. 1 Exposure of the plant to red light produces the biologically active PHY photoisomer (Pfr) while reversion to the inactive form (Pr) occurs under FR light. 2 They act as rapid and reversible molecular switches that can be used to control a wide variety of plant characteristics. 3 The balance of red to FR light is critical in nature and controlled environment agriculture as it regulates processes ranging from seed germination, height, leaf expansion, branching, plant immunity, circadian rhythm, leaf chlorophyll concentration to freezing tolerance. 4 Additionally, FR light plays a role in photosynthesis. The two photosystems (PSI and PSII) work electrochemically in series to generate the chemical energy required by the plant for growth and development. They absorb different regions of the spectrum with the quantum yields for PSII being greatest < 680 nm and is greatest between 680 nm and 720 nm for PSI. 5 In short, phosphors emitting wavelengths between 700-740 nm can be used to induce or inhibit photomorphogenesis (light mediated development) through PHYs in addition to optimizing photosynthesis through the balance of energy partitioning between the spectrally different photosystems.
A survey of the recent literature has indicated that research is being undertaken to device Mn 4+ (3d 3 electronic configuration) based phosphors which in a phosphor coated LED lamp would emit deep red light for plant growth. Examples include Mn 4+ activated La 2 MgTiO 6 , 6,7 La 2 LiTaO 6 , 8 La 2 MgGeO 6 , 9 Li 2 MgZrO 4 10 and Ca 14 Zn 6 (Al,Ga) 10 O 35 . 11 The compounds La 2 MgTiO 6 , La 2 LiTaO 6 and La 2 MgGeO 6 belong to the double perovskite family of compounds. We have extensively examined the optical properties of Mn 4+ in ABO 3 21 The goal of our program is to develop a set of guidelines for tuning the wavelength of the 2 E g → 4 A 2g emission transition, which per the Tanabe-Sugano diagram for d 3 ions, depends chiefly on the "Mn 4+ligand" bonding covalence. Analysis of data presented in the archival literature reveals that the 2 E g → 4 A 2g transition energy changes little in fluoride compounds and varies greatly in oxides. 22,23 This is because variation in covalence is smaller in fluorides than in oxides. Recently, we have presented how the "Mn 4+ -ligand" covalent mixing is influenced by connectivity of the octahedral moieties and deviation of O-Mn-O bond angle from the ideal value of 90 • in oxides. 24 We examined cases where the octahedral groups share corners (as in the ABO 3 and A 2 B ' B '' O 6 perovskites) or edges and faces as in α-LiAlO 2 , Sr 4 Al 14 O 25 and CaAl 12 O 19 . 24 It was concluded that a highly symmetrical octahedral moiety increases "Mn 4+ -ligand" bonding covalence. The resulting emission spectrum is in the deep red because of the lowering of the 2 E g → 4 A 2g emission transition energy.
Thus, a strategy for generating deep red photons in the perovskite structure for plant growth is to substitute Mn 4+ in sites with near ideal O h symmetry that maximizes "Mn 4+ -ligand" bonding covalence. This can be achieved by choosing a perovskite lattice in which the corner linked octahedral groups with equal M-L bond lengths and O-M-O bond angles that do not deviate from the ideal values.
In this paper, we report the synthesis and spectroscopic properties of the double perovskite La 2 LiSbO 6 activated with Mn 4+ . For comparative purposes, we also evaluate the spectroscopic data of the double perovskite, La 2 MgTiO 6 :Mn 4+ , which has been reported in the literature. 6,7 For both compounds, the energy position of the Mn 4+ 2 E g → 4 A 2g emission transition is identified and electronic energy levels calculated using the exchange charge model of crystal-field theory. This work is consistent with our goal of connecting the Mn 4+ emission wavelength with structural peculiarities that control the covalence of "Mn 4+ -ligand" bonding.

Experimental
The compound La 2 Li(Sb 0.998 Mn 0.002 )O 6 was synthesized by a conventional solid-state reaction technique. Prior to its use as the starting material, La 2 O 3 (Alfa Aesar, 99.99%) was heated to 1000 • C under nitrogen atmosphere. An excess (50 mole% over the stoichiometric proportion) of the starting material Li 2 CO 3 (Alfa Aesar, 99.998%) was added to compensate for any loss of lithium occurring because of the high vapor pressure at high temperatures during synthesis. These starting materials were blended with Sb 2 O 3 (Alfa Aesar, 99.9%) and Mn 2 O 3 (Aldrich 99.999%) and heated in air at 700 • C for one hour. The resulted powder was ball milled and heated to 1000 • C for 10 h to form the final product.
Luminescence measurements were performed as previously described. 12 The spectra were corrected for the wavelength dependent variations in the Xe-lamp intensity and the photomultiplier response.

Crystal Structure of La 2 LiSbO 6 and La 2 MgTiO 6 and Method of Calculations
Structural studies on La 2 LiSbO 6 are presented in References 25,26. The compound crystallizes in the P2 1 /C space group (No. 14) with two formula units per unit cell. The lattice constants are (in Å): a = 5.6226, b = 5.7199, c = 7.9689, β = 89.796 • In Reference 6, the XRD pattern of La 2 MgTiO 6 is indexed on a cubic cell. However, the monoclinc P2 1 /n space group is better suited to explain the diffraction pattern of this perovskite. 27 For our analysis, the data presented in Reference 27 where the XRD pattern is indexed on a monoclinc cell is used. The double perovskite crystallizes in the P2 1 /C space group (No. 14) with four formula units per unit cell. The lattice constants are (in Å): a = 5.5467, b = 5.5616, c = 7.8426, β = 89.959 • . 27 In these perovskites, the Mn 4+ ions substitute for the six-fold coordinated Sb 5+ and Ti 4+ ions, respectively. Obviously, in La 2 LiSbO 6 charge-compensation is required when Mn 4+ replaces Sb 5+ in the lattice. The La 3+ ions in both cases are eight-fold coordinated, and the Li + , Mg 2+ ions are six-fold coordinated (Figure 1). Enlarged views of the TiO 6 and SbO 6 octahedral complexes along with bond lengths and bond angles are shown in Fig. 2.
Details pertaining to the calculations of the Mn 4+ energy level by the exchange charge mode of crystal field theory has been presented before. 29,30,31 The reliability and vitality of the ECM is confirmed by its success in calculating the energy level of the transition metal and rare earth ions. 30,31 and references therein In the following we will only provide the results of our calculations that are relevant to the interpretation of the La 2 LiSbO 6 :Mn 4+ and La 2 MgTiO 6 :Mn 4+ experimental spectra.     The spectroscopic properties of Mn 4+ in the double perovskite La 2 MgTiO 6 were taken from Reference 6. The low temperature (T = 10 K) emission spectrum locates the 2 E g → 4 A 2g emission transition at 690 nm (14 493 cm −1 ). 7 We wish to point out that the emission spectra of the two perovskites (Fig. 3) are in the 700-740 nm wavelength range that is important for the plant growth application (see Introduction).

Results of energy level calculations.-
The octahedral moieties in both La 2 LiSbO 6 and La 2 MgTiO 6 deviate considerably from the ideal O h group. As exhibited in Fig. 2, both octahedral clusters are characterized by the presence of three pairs of different interatomic distances. Further, the characteristic octahedral bond angles are also deviating from the ideal values. Therefore, all orbitally degenerated energy levels of the Mn 4+ ion in such clusters will be split.
The crystal structural data were used to calculate the non-zero parameters of crystal field, which are listed in Table I. The values of the Racah parameters B and C, which were chosen from the best agreement with experimental data, are also given in Table I. We should point out that the value of the second Racah parameter C is found from adjusting the calculated position of the 2 E g ( 2 G) state to its experimental value determined from the emission spectrum. Thus, it is imperative to correctly identify the energy position of the 2 E g → 4 A 2g , zero phonon transition.
The crystal field splitting of all 8 LS terms of the Mn 4+ ion was calculated by diagonalizing the crystal field Hamiltonian with all crystal field parameters. The lowest calculated energy levels, which are located in the spectral range corresponding to the experimental excitation/emission spectra, are listed in Table II. The low symmetry of the crystal lattice sites is confirmed by a complete removal of the orbital degeneracy of the triplet and doublet states. There is good agreement between these calculated energy levels and corresponding experimental spectra (Fig. 3). Note that the splitting of the orbital triplets in La 2 MgTiO 6 is greater than in La 2 LiSbO 6 , which indicates considerably lower crystal field symmetry in the former case.

Influence of MnO 6 distortions on the energy of the 2 E g state in double
perovskites.-In this section, we present a comparative study of the spectroscopic properties of Mn 4+ ion in double perovskite compounds with the formulation A 2 BB ' O 6 ( Table III). The study establishes a relationship between the energy position of the 2 E g state and octahedral site distortion. The greater is the octahedral distortion, the smaller is the overlap between the Mn and O wave functions and higher is the energy position of the 2 E g state.
In Table III compares the values for the Racah parameters B and C, peak emission energy of the 2 E g → 4 A 2g transition and the value of the non-dimensional quantity The data in Table III are arranged in the order of decreasing octahedral distortions as deduced from the crystallographic data of each compound. We start with the double perovskite, Ba 2 LaNbO 6 in which the 2 E g state is located at the highest energy (14 992  In the family of double perovskites, the lowest energy of the 2 E g state is found in La 2 LiSbO 6 (Table III). In this perovskite, the SbO 6 octahedral group is comparatively very symmetrical; the linear O-Sb-O bond angle is 180 • and the deviation from 90 • is <1 • . The increased Mn-O covalent interaction results in the 2 E g state being located at lower energy, 14 388 cm −1 .
Finally, for comparative purposes, we consider the SrTiO 3 perovskite in which the 2 E g state is located at very low energy (13 792 cm −1 ). In this perovskite, the octahedral moieties are perfectly symmetrical: O-Mn-O bond angles are the ideal 90 • and 180 • and all Ti-O bond lengths are equal (1.951 Å). Therefore, there is a strong covalent bonding between the Mn and the six oxygen neighbors which decreases the energy of the 2 E g state. Figure 4 shows that energy of the 2 E g → 4 A 2g emission transition is linearly related to the β 1 parameter. Further, the β 1 parameter reflect correctly the bonding covalence variation in these perovskites as

LED Horticultural Market
The WinterGreen Research "LED Grow Light Market Shares, Strategies, and Forecast Worldwide" report projects the horticutural market at $3.6 billion by 2020. 33 A key factor driving the large-scale adoption of this technology is energy savings and the use of LEDs to control and improve crop production. Such market data will continue to motivate the search for phosphors that enable spectrally tailored LEDs for horticultural applications.

Conclusions
The energy levels of Mn 4+ in the double perovskites La 2 LiSbO 6 and La 2 MgTiO 6 are calculated by the exchange charge model of crystal field that allows to calculate the crystal field parameters and splitting of the orbitally degenerated energy levels. Agreement between the calculated and experimental results was shown to be good. Our study shows that octahedral site distortions decreases Mn-O covalent interaction which increases the energy of the 2 E g state. We, therefore, conclude that the development of deep red emitting phosphors for plant growth application in the perovskite based materials requires minimization of the octahedral site distortion. This results in stronger Mn-O covalent bonding interaction which decreases the energy of the Mn 4+ 2 E g → 4 A 2g , zero phonon transition.