Departamento de Ciencias de la Tierra y Física de la Materia Condensada, Universidad de Cantabria , Avenida de los Castros s/n, 39005 Santander, Spain.
J Phys Chem A. 2011 Mar 3;115(8):1423-32. doi: 10.1021/jp110586e. Epub 2011 Feb 4.
The origin of the spectrochemical series and the different dependence of crystal-field splitting (10Dq) and Racah parameters on the metal-ligand distance, R, is explored through ab initio calculations on Cr(3+)-doped K2NaScF6, Cs2NaYCl6, Cs2NaYBr6, and Cs2NaYI6 lattices. For this purpose both periodic and cluster calculations have been performed. An analysis of ab initio results proves that 10Dq values mostly come from the small admixture of deep nLs ligand orbitals present in the antibonding eg(∼ x(2)-y(2),3z(2)-r(2)) level and not from the dominant covalency with valence nLp ligand orbitals, which is actually responsible for the reduction of Racah parameters. This study thus reveals the microscopic origin of the stronger dependence upon R of 10Dq when compared to that observed for Racah parameters, thus explaining why electronic transitions which are 10Dq-independent give rise to sharp optical bands. As a salient feature, while the covalency with nLp levels increases significantly on passing from CrF6(3-) to CrI6(3-), the nLs admixture in eg is found to be practically unmodified. This fact helps to understand the progressive decrease of 10Dq through the series of CrF6(3-), CrCl6(3-), CrBr6(3-), and CrI6(3-) complexes embedded in the corresponding host lattices when compared at the corresponding equilibrium distance at zero pressure. The growing importance of the nLs admixture is well-depicted using deformation density diagrams on passing from the ground state (4)A2(t2g(3)) to the (4)T2(t2g(2)eg) excited state depicted at several R values.
通过对 Cr(3+)掺杂的 K2NaScF6、Cs2NaYCl6、Cs2NaYBr6 和 Cs2NaYI6 晶格进行从头算计算，探索了光谱化学系列的起源以及晶体场分裂（10Dq）和 Racah 参数对金属-配体距离 R 的不同依赖性。为此，进行了周期性和团簇计算。从头算结果的分析证明，10Dq 值主要来自于深 nLs 配体轨道的小混合，这些轨道存在于反键 eg（∼x(2)-y(2)、3z(2)-r(2))能级中，而不是来自于与价 nLp 配体轨道的主要共价键，这实际上导致 Racah 参数的降低。因此，这项研究揭示了 10Dq 对 R 的依赖性比 Racah 参数更强的微观起源，从而解释了为什么与 10Dq 无关的电子跃迁会导致尖锐的光学带。作为一个显著的特点，虽然与 nLp 能级的共价键在从 CrF6(3-)到 CrI6(3-)时显著增加，但发现 eg 中的 nLs 混合实际上没有改变。这一事实有助于理解在零压下比较相应的平衡距离时，嵌入相应主体晶格中的 CrF6(3-)、CrCl6(3-)、CrBr6(3-)和 CrI6(3-)配合物系列中 10Dq 的逐渐减小。通过从基态（4）A2(t2g(3))到（4）T2(t2g(2)eg)激发态的变形密度图，在几个 R 值上描绘了 nLs 混合的重要性增加。
