Fakultät für Physik and Center for Computational Materials Science, Universität Wien, Sensengasse 8/12, A-1090 Wien, Austria.
J Chem Phys. 2012 Feb 14;136(6):064501. doi: 10.1063/1.3676408.
The structural and energetic properties of purely siliceous, proton-, and Cu- and Co-exchanged chabazite have been studied using periodic density-functional (DFT) calculations with both conventional gradient-corrected exchange-correlation functionals and hybrid functionals mixing exact (i.e., Hartree-Fock) and DFT exchange. Spin-polarized and fixed-moment calculations have been performed to determine the equilibrium and excited spin-configurations of the metal-exchanged chabazites. For the purely siliceous chabazite, hybrid functionals predict a slightly more accurate cell volume and lattice geometry. For isolated Al/Si substitution sites, gradient-corrected functionals predict that the lattice distortion induced by the substitution preserves the local tetrahedral symmetry, whereas hybrid functionals lead to a distorted Al coordination with two short and two long Al-O bonds. Hybrid functionals yield a stronger cation-framework binding that conventional functionals in metal-exchanged zeolites, they favor shorter cation-oxygen bonds and eventually also a higher coordination of the cation. Both types of functionals predict the same spin in the ground-state. The structural optimization of the excited spin-states shows that the formation of a high-spin configuration leads to a strong lattice relaxation and a weaker cation-framework bonding. For both Cu- and Co-exchanged chabazite, the prediction of a preferred location of the cation in a six-membered ring of the zeolite agrees with experiment, but the energy differences between possible cation locations and the lattice distortion induced by the Al/Si substitution and the bonding of the cation depends quite significantly on the choice of the functional. All functionals predict similar energy differences for excited spin states. Spin-excitations are shown to be accompanied by significant changes in the cation coordination, which are more pronounced with hybrid functionals. The consequences of electronic spectra and chemical reactivity are analyzed in the following papers.
纯硅、质子、Cu 和 Co 交换丝光沸石的结构和能量特性已使用周期性密度泛函（DFT）计算进行了研究，这些计算同时使用了传统梯度校正的交换相关泛函和混合泛函，这些混合泛函混合了精确（即哈特ree-fock）和 DFT 交换。进行了自旋极化和固定矩计算，以确定金属交换丝光沸石的平衡和激发自旋构型。对于纯硅丝光沸石，混合泛函预测出稍微更准确的细胞体积和晶格几何形状。对于孤立的 Al/Si 取代位点，梯度校正泛函预测取代所诱导的晶格畸变保留了局部四面体对称性，而混合泛函导致扭曲的 Al 配位，具有两个短和两个长 Al-O 键。混合泛函在金属交换沸石中产生比传统泛函更强的阳离子-骨架结合，它们有利于阳离子-氧键更短，最终也有利于阳离子的更高配位。这两种类型的泛函都预测出在基态中的相同自旋。激发自旋态的结构优化表明，高自旋构型的形成导致晶格松弛和阳离子-骨架键合变弱。对于 Cu 和 Co 交换丝光沸石，预测阳离子在沸石的六元环中的优先位置与实验一致，但阳离子可能位置之间的能量差异以及 Al/Si 取代和阳离子键合引起的晶格畸变取决于功能的选择相当显著。所有泛函都预测出激发自旋态的相似能量差异。自旋激发伴随着阳离子配位的显著变化，混合泛函更为明显。电子光谱和化学反应性的后果在以下论文中进行了分析。
