Lim Amber, Fredrickson Daniel C
Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States.
Inorg Chem. 2024 Jun 24;63(25):11726-11736. doi: 10.1021/acs.inorgchem.4c01226. Epub 2024 Jun 12.
The 18- electron-counting rule provides structural guidelines for electronically feasible transition metal (T)-main group (E) phases, contributing toward the goal of material design. However, the availability of numerous potential structure types at any electron count creates a challenge for the prediction of the preferred structures of specific compounds, as is illustrated by the concept of 18-+ isomerism. In this Article, we explore the driving forces stabilizing one 18-+ isomer over another with an analysis of the structure of PdSn, a layered intergrowth of the fluorite and CuAl structure types. The DFT-reversed approximation Molecular Orbital (DFT-raMO) method reveals that PdSn and its hypothetical parent structures all adhere to bonding schemes approximating the electronic configurations expected from the 18- rule, with various degrees of isolobal Pd-Pd bonding and Sn clustering. However, partial electron transfer between the Pd 5p orbitals to the Sn 5s orbitals contributes to the absence of convincing electronic pseudogaps near their Fermi energies. As such, there is no clear electronically driven preference among the structure types. This situation allows for atomic packing effects to prevail: DFT-Chemical Pressure (DFT-CP) analysis illustrates that in the fluorite-type parent structure, positive Pd-Sn CPs lead to overcompression of the Pd atoms and a stretching of the relatively open Sn sublattice. In contrast, in the CuAl-type parent structure, Sn atoms cluster into tetrahedra, opening space for an expanded Pd environment and the formation of Pd-Pd interactions. However, the tetrahedral packing of the Sn atoms here leads to frustration between negative and positive Sn-Sn CPs. Through the development of the angular CP correlation function (CP) as a tool to quantify frustration among interatomic interactions, we demonstrate how the observed PdSn structure balances these effects by tuning the degree of Sn-Sn clustering and expansion of the Pd environment. These observations point to generalizations for most 18-+ isomers, where increased main group ligand clustering (+) and isolobal bonds (+) can accommodate compositions with different T and E atomic sizes.
18电子计数规则为电子可行的过渡金属（T）-主族（E）相提供了结构指导方针，有助于实现材料设计的目标。然而，在任何电子数下都存在大量潜在的结构类型，这给预测特定化合物的优选结构带来了挑战，18- +异构现象的概念就说明了这一点。在本文中，我们通过分析萤石和CuAl结构类型的层状共生体PdSn的结构，探讨了使一种18- +异构体比另一种更稳定的驱动力。密度泛函理论反向近似分子轨道（DFT-raMO）方法表明，PdSn及其假设的母体结构都遵循近似18电子规则预期电子构型的键合方案，具有不同程度的等瓣Pd-Pd键合和Sn簇集。然而，Pd 5p轨道与Sn 5s轨道之间的部分电子转移导致它们的费米能附近缺乏令人信服的电子赝能隙。因此，在这些结构类型之间没有明显的电子驱动偏好。这种情况使得原子堆积效应占主导：密度泛函理论-化学压力（DFT-CP）分析表明，在萤石型母体结构中，正的Pd-Sn化学压力导致Pd原子过度压缩和相对开放的Sn亚晶格拉伸。相反，在CuAl型母体结构中，Sn原子聚集成四面体，为扩展的Pd环境和Pd-Pd相互作用的形成留出空间。然而，这里Sn原子的四面体堆积导致负的和正的Sn-Sn化学压力之间的矛盾。通过开发角向化学压力相关函数（CP）作为量化原子间相互作用矛盾的工具，我们展示了观察到的PdSn结构如何通过调节Sn-Sn簇集程度和Pd环境的扩展来平衡这些效应。这些观察结果指出了大多数18- +异构体的一般规律，即主族配体簇集增加（+）和等瓣键（+）可以容纳具有不同T和E原子尺寸的组成。
