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Local Structure and Luminescence Tuning in Phosphors
Published in Ru-Shi Liu, Xiao-Jun Wang, Phosphor Handbook, 2022
The structures of nitridosilicates and oxysilicates usually consist of [SiN4] or [SiO4] tetrahedra. We introduce the inductive effect factor −μΔχ, which characterizes the electronegativity difference between constituent metal elements and silicon. Through statistics over 100 compounds, we set up a linear relationship between average Si–N/Si–O bond length and the inductive factor [43]. It reveals the relevance between chemical composition and the local structure. Besides that, there also exists a positive correlation between the inductive factor and the centroid shift of 5d-levels of Ce3+, as shown in Figure 4.7. With the increase of inductive factor, the anionic N/O atoms tend to seize electron from M instead of Si, leading to extended Si–N/O bonds but shortened M–N bonds. Therefore, it means the [SiN4] or [SiO4] tetrahedra framework takes up more space than that of M. After Ce enters the host, the binding between Ce and its ligands is changed due to the inductive effect, i.e., the covalency increases, which increases the centroid shift of 5d-levels in Ce3+.
Topological Analysis of Local Structure in Atomic Systems
Published in Jeffrey P. Simmons, Lawrence F. Drummy, Charles A. Bouman, Marc De Graef, Statistical Methods for Materials Science, 2019
Emanuel A. Lazar, David J. Srolovitz
The characterization considered in this chapter is a form of local structure analysis, as its objective is to describe the local environment around each atom by considering how its neighbors are arranged relative to it. Only after local structure is characterized can larger-scale structure be determined.
Ab initio studies of propane dehydrogenation to propene with graphene
Published in Molecular Physics, 2020
Although several extrinsic factors can influence the surface properties of a given catalyst and its performance during the propane to propene dehydrogenation reaction, this work uses first principles calculations to identify the active sites at which the reaction is most likely to happen on activated graphene ribbons. Because graphene has good thermal stability [24–26], among other interesting physical and chemical properties, the ability of defect-functionalised graphene sheets to catalyse the dehydrogenation of propane to propene is studied as a model, in order to identify the origin of the catalytic activity, and unravel its relationship with the intrinsic nature of the active sites. Pristine graphene has the semi-metallic electronic structure, which is characterised by coexistence of electron and hole states at the Brillouin zone corners. Its electronic structure is uniformly confined to give zero band gap [27,28], and this has far-reaching consequences for its catalytic properties. The introduction of patterned edges overcomes the lateral quantum confinement of the electronic structure in graphene nanoribbons [29]. Other methods of modifying the local structure include introduction of defects, strain, dopants, or adsorption of atoms, which can be used to tune the electronic structure. For instance, introduction of non-metallic dopants has been demonstrated to increase the density of defect-induced active sites [30], and therefore enhancing the catalytic activity of graphene [31].
Atomic structure insight into crystallization of undercooled liquid metal Zr during isothermal relaxation processes
Published in Philosophical Magazine, 2019
Dadong Wen, Yonghe Deng, Xiongying Dai, Zean Tian, Ping Peng
To establish the correlation of the atomic structures with the crystallization processes, the largest standard cluster analysis (LSCA) [28] is used to analyse the local atomic structures and their evolutions of undercooled liquid metal Zr during the isothermal relaxation processes. In LSC, the basic local structure is called a cluster that is composed of one central atom and its all neighbours. Around a given atom, the largest cluster that satisfies a topological criterion is unique and called as the largest standard cluster (LSC). In an LSC (see Figure 4), a reference-pair (composed of one neighbour and the centre) and their common near neighbours (CNNs) comprise a common neighbour sub-cluster (CNS). LSCA can characterise all kinds of local clusters beyond the nearest neighbour independent with any pre-set parameters [28,29]. The details about the topological criterion and an implementing algorithm can be found in Refs. [28,29]. Figure 4(a) shows an HCP LSC around atom O that comprises six S421 and eight S422 with the 12 neighbours. One S421 in this HCP LSC is depicted in Figure 4(b), which is composed of a reference-pair of O-A2 and their 4 CNNs, labelled A1, A3, B2 and B3; and one S422 in Figure 4(d), composed of the reference-pair of O-B2 and their four CNNs of A2, A5, B1 and B3. Thus, the compact denotations for HCP LSC is [6/421, 6/422]. Similarly, [12/421] means a perfect FCC composed of 13 atoms associated with twelve S421. A BCC composed of 15 atoms associated with 6 S444 and 8 S666 can be expressed as [6/444, 8/666].