目前, 在Cr3+掺杂的近红外长余辉材料中, 主要研究工作是关于强晶体场和中等晶体场格位的发光中心与长余辉特性（发射波长、 余辉时间和光存储性质）的依赖关系, 但缺乏对处于弱晶体场格位的Cr3+与长余辉特性关系的研究。 采用自蔓延燃烧法制备了不同铬离子掺杂浓度的钆铝酸钙(CaGdAlO4∶x%Cr3+)近红外长余辉发光材料。 采用X射线衍射、 扫描电子显微镜、 激发和发射光谱等技术手段研究了离子掺杂浓度和热处理条件对粉末样品的微结构、 形貌、 粒子尺寸及发光性能的影响。 结果表明： 在0.1%～2.0%的掺杂浓度范围内, 由于具有相同配位数的Cr3+和Al3+半径相近, Cr3+取代了CaGdAlO4中的Al3+格位。 从样品的激发谱中可以发现, 240, 373及592 nm的激发峰分别归属于Cr3+的4A2→4T1(4P), 4A2→4T1(4F)和4A2→4T2(4F)的跃迁, 对应于276和313 nm的激发峰则来源于基质中的Gd3+ 8S2/7→6Ij和6P2/7→8S2/7的跃迁； 在红光（592 nm）的激发下, 650～850 nm范围内出现了极大值位于744 nm的近红外宽带发射, 并叠加有若干窄带近红外发射。 近红外发光强度随着Cr3+的掺杂浓度的增加呈先增加后下降的趋势, 最佳掺杂浓度为1%。 对上述优化浓度的样品经真空气氛800 ℃热处理后, 发现样品的平均晶粒尺寸由417 nm增大到843 nm, 发光强度增强了2倍。 实验发现, 在CaGdAlO4基质中, Cr3+取代了处于弱晶体场环境的Al3+格位。 通过晶场参数计算和光谱分析, 指认了样品光致发光的起源。 通过激发光谱数据的计算, 发现晶体场强度Dq/B=1.54<2.3, 理论计算表明Cr3+处于较弱的晶体场格位环境, 与实验研究结果相符合。 发射光谱中670 nm宽带发射可归属为4T2→4A2的零声子线, 744和756 nm宽带发射对应于4T2→4A2的声子边带跃迁。 热处理后的样品, 余辉时间超过了60 s。 尤其, 与Cr3+处于中等和强晶体场格位的情形相比（近红外发射峰极大值位于697 nm）, 处于弱晶体场环境的铬离子近红外发射峰的极大值移动到744 nm, 更接近于第一生物窗口的中央, 这将更有利于生物医学成像的应用。 该研究对发现新型长余辉发光材料和拓展其应用具有重要的实际意义。

At present, for near infrared (NIR) long afterglow materials doped with Cr3+, the efforts have been focused on the dependence of luminescent centers in the cases of strong and medium crystal fields on the afterglow properties (emission wavelength, afterglow time and storage properties), but the research on the relationship between Cr3+ ions around weak crystal field environments and the afterglow properties is lacking. This studyis very essential for developing novel long afterglow luminescent materials and exploring their applications. In this paper, Cr3+ doped CaGdAlO4 (CaGdAlO4∶x%Cr3+) powders with different Cr3+ doping concentrations were prepared by a self-propagating combustion method. The influences of Cr3+ doping and heat-treatment conditions on the microstructure, morphology, particle size and luminescent properties of the samples were studied by means of X-ray diffraction, scanning electron microscopy, excitation and emission spectra. It was found that, in the concentration range of 0.1%～2.0%, Cr3+ions substitute for Al3+ions in CaGdAlO4 because of their similar radii. From the excitation spectra, it could be found that the excitation peaks at 240, 373 and 592 nm are attributed to 4A2→4T1(4P), 4A2→4T1(4F) and 4A2→4T2(4F) transitions of Cr3+ ions, respectively, and the excitation peaks corresponding to 276 and 313 nm originate from the 8S2/7→6Ij and 6P2/7→8S2/7 transitions of Gd3+ ionsin the matrix. Under 592 nm excitation, NIR broadband emission peak with a maximum value of 744 nm appears in the range of 650～850 nm and several narrowband peaks overlap with it. The NIR emission intensity exhibits an initialrise and a subsequent decrease with the increase of Cr3+ doping concentration and the optimum doping concentration is ~1%. After the heat-treatment at 800 ℃ in vacuum, the average grain size increases from 417 to 843 nm and the luminescent intensity increases by 2 times. It was found that Cr3+ replaces Al3+ site in the weak crystal field environment in CaGdAlO4 host. The origin of NIR emission peaksof the samples was identified by the calculation of crystal field parameters and spectral analysis. It was found that the crystal field strength is ~1.54 and smaller than 2.3, i. e. Dq/B=1.54<2.3, indicating Cr3+ ions around a weak crystal field environment, which is consistent with the experimental result. The 670 nm broadband emission can be attributed to the zero phonon line (4T2→4A2) and 744 and 756 nm broadband emissions correspond to the phonon sideband transition 4T2→4A2. After the heat treatment, the afterglow time of the samples exceeds 60 seconds. In particular, compared with the case of Cr3+ ions around medium and strong crystal field environments (the maximum value of NIR emission peak at 697 nm), the maximum value of NIR emission peak of Cr3+ ions around the weak crystal field environment moves to 744 nm, which is closer to the center of the first biological window, suggesting that Cr3+ doped CaGdAlO4 has potential application in bioimaging.