Preparation,Crystal Structure And Luminescence Properties Of Fe Doped MgO·nAl₂O₃ Fluorescent Materials

Magnesia-aluminum spinel crystal and transparent ceramic materials have attracted much attention due to the excellent optical properties.There are two types of tetra-and hexa-coordinated polyhedral environments formed by O^2-ion close-packed in the lattice of magnesia-aluminum spinel,and when transition metal or rare earth ions occupy these coordination environments,strong photoluminescence is produced due to energy level transitions.Magnesia-aluminum spinel has a wide range of solid solution and can be written MgO·nAl₂O₃.When n>1,excess Al³⁺is incorporated into the lattice,creating more point defects such as cation vacancies under the constraint of electrovalent equilibrium.The photoluminescence properties of Fe³⁺are highly dependent on local symmetry,as the involved transitions are spin-forbidden d-d transitions.When Fe³⁺ions occupy tetrahedral and octahedral sites in the spinel lattice,the excitation peaks of photoluminescence generally originate from the charge transfer（CT）transition and the d-d transition of Fe³⁺ion in the distorted octahedral（or tetrahedral）crystal field.The blue-green light emission of Fe³⁺ions in the tetrahedral site is attributed to the ⁴T₁→⁶A₁ transition.In contrast,the ⁴T_1g→⁶A_1g transition of octahedral coordinated Fe³⁺ion produces red or near-infrared light.In this study,Fe doped MgO·nAl₂O₃ phosphors were synthesized using a high-temperature steady-state reaction method to broaden the application area of magnesium-aluminum spinel transparent ceramics.In contrast to the phosphor materials prepared in containing carbon atmosphere and vacuum,the powder only in air is a spinel-type single-phase solid solution containing only Fe³⁺ions,and its powder properties are evaluated,the crystal structure is resolved by EPR,NMR,XRD Rietveld and the fluorescence mechanism is analyzed in connection with the fluorescence properties.Fe doped Mg Al₂O₄ and MgO·nAl₂O₃ transparent ceramics were prepared using the method of PS in air atmosphere+HIP.Mg_0.752Al_2.165-xO₄:xFe³⁺produced near-infrared luminescence at 770 nm and the fluorescence intensity reached its maximum at x=0.010,with a critical transfer distance（R₀）of 23.15?for Fe³⁺and fluorescence quenching mainly arising from multipolar coupling interaction.x=0.006 phosphor with high fluorescence intensity and low ionic interactions were selected to test the variable temperature fluorescence performance.When the temperature rises to 100°C,the fluorescence intensity reaches its maximum and the crystal structure shows that the bond length between the metal-ligand decreases with increasing doping,leading to a gradual red shift of the emission peak with increasing doping.For Fe³⁺doped MgO·nAl₂O₃,the fluorescence spectra produced significant changes with increasing n values.The n value is higher,the intensity of the new emission peak at 320 nm is stronger and the emission spectrum appears significantly red-shifted.Combined with crystal structure refinement results such as bond lengths and cationic inversion,it is ultimately suggested that the fluorescence mechanism of Fe³⁺in aluminum-rich spinel is altered.When the stoichiometric ratio n value increases,the surroundings of the octahedral position in which the Fe³⁺ion is located are more disordered and have lower the local structure symmetry,leading to the involvement of the doublet state energy level of Fe³⁺in the d-d energy level transitions,thus causing a change in the photoluminescence spectra.Fe²⁺doped Mg Al₂O₄ and Mg O·1.44Al₂O₃ transparent ceramics with transmittance at 750 nm of about 60%and 50%,respectively,were prepared PS at1550°C using a muffle furnace in air and HIP at 1800°C.After studying the optical properties of the synthesized transparent ceramics,the fluorescence properties of Fe³⁺were significantly diminished and replaced by the appearance of mid-infrared absorption peaks of Fe²⁺in the tetrahedra.It is difficult to synthesize high transmittance MgO·nAl₂O₃ fluorescent transparent ceramics containing only Fe³⁺,due to the constraints of the high temperature equipment atmosphere.The higher the value of n,the higher the preparation temperature will be,which makes the preparation of transparent ceramics even more difficult to achieve.The final prepared transparent ceramics are a more excellent Fe²⁺doped spinel-type fluorescent material with strong absorption and fluorescence properties at 200～300 nm and 1500～3000 nm,and have potential applications for shielding deep-UV and mid-IR lasers.