Preparation And Luminescence Properties Of Mn⁴⁺ Doped Double Perovskite Red Oxide Phosphors

It is well known that the growth and development of plants cannot be performed without sunlight.The light not only provides the energy needed for plants photosynthesis,but also regulates their life activities,such as seed germination and stem and leaf growth.With the rapid development of indoor crop production,agricultural lighting technology has become an essential part.White light-emitting diodes（LEDs）have many advantages,such as long life,small size,no pollution,and the ability to adjust the spectral composition by overcoating different types of phosphors so that they can emit the spectrum required to match the photosynthesis of plants.There are two main pigments affecting plant photosynthesis,the red photosensitive pigment P_rand the far-red photosensitive pigment P_fr,which mainly absorb red light near 660 nm and 730 nm.The Mn⁴⁺-doped perovskite type red phosphors have promising applications in plant lighting because of their red light emission in this range.In this paper,the double perovskite-type red oxide phosphors Ca₂YNb O₆:x Mn⁴⁺（x=0-0.9%）and Ba_3-yRe_yWO₆:x Mn⁴⁺（Re=La,Gd,Y,Lu,x=0-0.7%,y=0-1.3）were prepared by the high temperature solid phase method.In order to analyze the effects of reaction temperature,concentration of doped Mn⁴⁺ions,ratio of Ba to rare earth ions on the substrate and luminescence properties,the samples were characterized by the X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,fluorescence spectra and thermal stability.For the Ca₂YNb O₆:x Mn⁴⁺red phosphor,the reaction temperature and Mn concentration had little effect on the structure of Ca₂YNb O₆with space group P2_1/c.Mn exists at+4valent state in the crystal lattice.The comparison of XRD and fluorescence spectra determined that the optimal reaction temperature was 1500°C and the optimal doping concentration of Mn⁴⁺ions was 0.4 mol%.The photoluminescence spectra showed that the optimal excitation wavelength of the sample was in the ultraviolet region,and the emission peak（680 nm）belonged to the ²E_g→⁴A_2gcharacteristic transition of Mn⁴⁺.The concentration quenching mechanism was ascribed to the dipole-dipole interaction.For the Ba_3-yRe_yWO₆:x Mn⁴⁺system with the structure of space group Fm-3m,the rare earth doping had little effect on the structure of the sample at a lower concentration.However,only the doping of Gd³⁺could promote the luminescence enhancement of Mn⁴⁺ion.The optimal doping concentration of Mn⁴⁺ions and the optimal molar ratio of Ba:Gd were 0.3 mol%and 2.1:0.9,the maximum at 695 nm.respectively.The calculations show that Mn⁴⁺is located in the stronger crystal field.The fluorescence properties and thermal stability of the two oxide phosphor materials were compared,which confirmed that Ba_3-yGd_yWO₆:x Mn⁴⁺phosphors have more superior luminescence performances.In this work,the emission peaks（680-695 nm）of the two Mn⁴⁺-doped double perovskite-type red oxide phosphors prepared by the high temperature solid state reaction are in good agreement with the positions of the absorption peaks of P_rand P_fr,which are expected to be used in the plant lighting white LEDs.