Preparation And Luminescence Properties Of Cr³⁺-Doped Mg₂BeAl₆O₁₂ Luminescent Materials

Luminescent materials are a new type of green,environmentally friendly energy-saving materials,which are widely used in the area of the transportation,military,interior decoration,anti-counterfeiting,lighting and information storage.Up to now,the research on the blue and yellow-green luminescent materials are significantly more than that of the red luminescent materials.Most of the luminescent materials use the rare earth element as the activator and the luminescent efficiency is generally low.To improve the luminescent efficiency,it is necessary to use a high-purity rare earth compound having a relatively high market price.The transition metal element Cr³⁺is relatively inexpensive compared with the rare earth element,so Cr³⁺has a relatively obvious price advantage as a luminescent center.The Cr³⁺doped aluminate-based luminescent material has good stability,and Cr³⁺is relatively strong in crystal field strength,so its doping into the aluminate can emit a strong red emission.The crystal structure of Mg₂BeAl₆O₁₂2 contains a large number of tetrahedral and octahedral gaps,which is a good doping matrix material for the transition group-activated ion of Cr³⁺.In this paper,Cr³⁺doped Mg₂BeAl₆O₁₂2 luminescent materials were prepared by high temperature solid state method and modified sol gel method.The phase structure,excitation and emission spectra,and fluorescence lifetime of the prepared samples were characterized by X-ray diffraction?XRD?and photoluminescence?PL?.The color coordinates of the sample were plotted using CIE1931 colorimetric coordinate software.The results show the effects of Cr³⁺doping concentration,burning temperature and holding time on the structure and luminescence properties of the samples were investigated.Concluded as follow:The phase of Mg₂BeAl₆O₁₂:Cr³⁺luminescent materials prepared by two methods was characterized by X-ray diffractometer.The XRD patterns of Cr³⁺doped luminescent materials were obtained.By comparison of the diffraction peaks,no other peaks other than the Mg₂BeAl₆O₁₂2 diffraction peak were observed,and the peak position information was completely consistent.It indicates that Cr³⁺has been well doped into the Mg₂BeAl₆O₁₂2 lattice,and no new phase is introduced by Cr³⁺doping.In the crystal structure of Mg₂BeAl₆O₁₂,the ionic radii of Cr³⁺and Al³⁺are similar,which not only have a matching lattice ion size,but also do not change the lattice structure of the matrix.Therefore,the doped Cr³⁺will enter the lattice position of Al³⁺in the Mg₂BeAl₆O₁₂2 crystal.Mg₂BeAl₆O₁₂:Cr³⁺is prepared by two methods under different conditions.At the emission wavelength of 681 nm,the excitation spectrum of the sample has two excitation bands of 350 to 480 nm and 500 to 650 nm.Among them,the excitation wavelength of the 360-480 nm excitation band is 423 nm,which can be attributed to the ⁴A₂?⁴T₁ transition between the ground states allowed by the electron spin of Cr³⁺.The excitation wavelength of the excitation band at 500-650 nm is 591 nm,and the excitation wavelength of the sample prepared by high-temperature calcination is red-shifted to 593 nm.All of them are attributed to the ⁴A₂?⁴T₂ transition between the ground states allowed by the electron spin of Cr³⁺.The position of the excitation peak hardly changed with the increase of Cr³⁺doping concentration,burning temperature and holding time,but the peak intensity of the excitation peak mostly increased first and then weakened.When the Cr³⁺doping concentration is 1%,the excitation peak intensity is the strongest.From it,we can find the 1%doping concentration is optimum.The sample prepared by high temperature solid state method showed a gradual weakening of the peak intensity of the excitation peak with the increase of the burning temperature,but the attenuation decreased gradually.The sample with modified sol gel method has the highest excitation peak intensity at1300?,which is the best burning temperature.The best holding time of the samples prepared by the two methods is 3h,and the excitation peak intensity is the strongest.The samples prepared by the two methods are excited by their respective excitation wavelengths,both can emit red and near-infrared light with a wide range of660-800 nm,there are two narrow emission peaks at 688 nm and 681 nm in the red region,and a strong emission peak at 681 nm.The luminescence band in the range of660-800 nm is mainly attributed to the⁴T₂?⁴A₂ transition of Cr³⁺,The emission peak at 681 nm can be attributed to the R-line fluorescence generated by the electron spin-inhibited ²E?⁴A₂ transition of Cr³⁺.The R-line is consisted of two emission peaks of 681 nm and 679 nm.The emission peak at 688 nm belongs to the Stokes line of the R-line single photon.With the increase of Cr³⁺doping concentration,burning temperature and holding time,the position of the emission peak of the sample did not change,and the intensity of the emission peak of the sample first increased and then weakened.When the Cr³⁺doping concentration is 1%,the emission peak intensity reaches the strongest,which is the optimal doping concentration of Cr³⁺ions,that is,the fluorescence quenching concentration;the optimum burning temperature of the samples prepared by high temperature solid state method and modified sol gel method is the strongest at the 1500?and 1300?respectively;The samples prepared by the two methods have the strongest emission peak intensity at the incubation time of 3h,which is the best holding time.By using CIE1931 colorimetric coordinate software,the CIE color coordinates of different concentrations of Cr³⁺doped Mg₂BeAl₆O₁₂:Cr³⁺luminescent materials prepared by high temperature solid state method and modified sol gel method under excitation excitation at 423 nm were calculated.With the increase of Cr³⁺doping concentration,the x coordinate value first increases and then decreases,reaching the maximum value at 1%,and the y coordinate value is opposite to the minimum at 1%,which is related to the fluorescence intensity with the Cr³⁺doping concentration.The law of change is consistent.These points are located in the red region on the color coordinate map,corresponding to the red emission peak at 681 nm in the emission spectrum.The fluorescence lifetime decay curves of Mg₂BeAl₆O₁₂:Cr³⁺luminescent materials prepared at different firing temperatures were plotted.The fluorescence lifetime of the samples was obtained by curve fitting the lifetime diagrams.The samples prepared by high temperature solid state method and modified sol gel method were used in their respective samples,the maximum fluorescence lifetime values at the excitation wavelength of the center appeared at 1500?and 1300?respectively.