Laboratoire PhLAM, Université Lille 1-Sciences et Technologies, CNRS UMR 8523, CERLA, CNRS FR 2416, Bât P5, Cité Scientifique, F-59655 Villeneuve d'Ascq Cedex, France.
J Chem Phys. 2009 Nov 21;131(19):194501. doi: 10.1063/1.3259048.
New ab initio embedded-cluster calculations devoted to simulating the electronic spectroscopy of Bi(3+) impurities in Y(2)O(3) sesquioxide for substitutions in either S(6) or C(2) cationic sites have been carried out taking special care of the quality of the environment. A considerable quantitative improvement with respect to previous studies [F. Real et al. J. Chem. Phys. 125, 174709 (2006); F. Real et al. J. Chem. Phys. 127, 104705 (2007)] is brought by using environments of the impurities obtained via supercell techniques that allow the whole (pseudo) crystal to relax (WCR geometries) instead of environments obtained from local relaxation of the first coordination shell only (FSR geometries) within the embedded cluster approach, as was done previously. In particular the uniform 0.4 eV discrepancy of absorption energies found previously with FSR environments disappears completely when the new WCR environments of the impurities are employed. Moreover emission energies and hence Stokes shifts are in much better agreement with experiment. These decisive improvements are mainly due to a lowering of the local point-group symmetry (S(6)-->C(3) and C(2)-->C(1)) when relaxing the geometry of the emitting (lowest) triplet state. This symmetry lowering was not observed in FSR embedded cluster relaxations because the crystal field of the embedding frozen at the genuine pure crystal positions seems to be a more important driving force than the interactions within the cluster, thus constraining the overall symmetry of the system. Variations of the doping rate are found to have negligible influence on the spectra. In conclusion, the use of WCR environments may be crucial to render the structural distortions occurring in a doped crystal and it may help to significantly improve the embedded-cluster methodology to reach the quantitative accuracy necessary to interpret and predict luminescence properties of doped materials of this type.
新的从头算嵌入簇计算专门用于模拟 Bi(3+)杂质在 Y(2)O(3) sesquioxide 中的电子光谱，用于替代 S(6)或 C(2)阳离子位置，特别注意环境的质量。与以前的研究相比，使用通过超晶胞技术获得的杂质环境会带来相当大的定量改进，这允许整个（伪）晶体弛豫（WCR 几何形状），而不是以前在嵌入簇方法中仅从第一配位壳层的局部弛豫获得的环境（FSR 几何形状）。特别是，当使用新的 WCR 杂质环境时，以前使用 FSR 环境发现的吸收能的 0.4 eV 均匀差异完全消失。此外，发射能因此斯托克斯位移与实验更吻合。这些决定性的改进主要归因于发射（最低）三重态几何弛豫时局部点群对称性的降低（S(6)-->C(3)和 C(2)-->C(1)）。在 FSR 嵌入簇弛豫中没有观察到这种对称性降低，因为嵌入的晶体场在真正的纯晶体位置被冻结，似乎比簇内的相互作用更重要，从而限制了系统的整体对称性。掺杂率的变化被发现对光谱几乎没有影响。总之，使用 WCR 环境可能对呈现掺杂晶体中发生的结构畸变至关重要，并可能有助于显著改进嵌入簇方法，以达到解释和预测此类掺杂材料发光性质所需的定量准确性。
