Department of Chemistry , University of Wisconsin-Madison , 1101 University Avenue , Madison , Wisconsin 53706 , United States.
Materials Science and Engineering , University of Wisconsin-Madison , 1509 University Avenue , Madison , Wisconsin 53706 , United States.
Inorg Chem. 2018 Nov 5;57(21):13880-13894. doi: 10.1021/acs.inorgchem.8b02353. Epub 2018 Oct 18.
The structures and properties of intermetallic phases are intimately connected to electron count; unfavorable electron counts can result in structural rearrangements or new electrical or magnetic behavior when no such transformation is available. The compound PtGa appears to teeter on the border between these two scenarios with its two polymorphs: a cubic fluorite type form (c-PtGa) and a complex tetragonal superstructure (t-PtGa) whose Pt-Pt connectivity aligns with the 18- n electron counting rule. Here, we investigate the factors underlying this polymorphism. Electronic structure calculations show that the transition to t-PtGa opens a pseudogap at the Fermi energy that can be traced to Pt-Pt isolobal bond formation, in line with the 18- n bonding scheme. Conversely, DFT-chemical pressure (CP) analysis reveals a network of positive local pressures along Pt-Ga contacts, requiring that the c-PtGa to t-PtGa transition follows tightly concerted atomic motions. Experimentally, a series of samples with varying Pt:Ga ratios were synthesized to examine the stability ranges of the polymorphs. Ga-poor samples yield exclusively the cubic polymorph over the full range of temperatures studied, which can be correlated to the enhanced incorporation of interstitial Pt atoms (at points of negative pressure in the CP scheme). At more Ga-rich compositions, however, t-PtGa emerges as a low-temperature form. In these samples, the t-PtGa to c-PtGa transition is found to be reversible, but with a large hysteresis that in single crystals can exceed 100 °C. Together, the theoretical and experimental results indicate that the c-PtGa phase is buttressed at its unfavorable electron count by the interstitial atoms and networks of positive CPs that restrict atomic motion, suggesting more general strategies for achieving exotic electronic structures in intermetallic materials.
金属间化合物的结构和性质与电子计数密切相关；当没有这种转变时，不利的电子计数可能导致结构重排或新的电或磁行为。化合物 PtGa 似乎在这两种情况之间摇摆不定，它有两种多晶型物：立方萤石型（c-PtGa）和复杂的四方超结构（t-PtGa），其 Pt-Pt 连接与 18-n 电子计数规则一致。在这里，我们研究了这种多晶型的基础因素。电子结构计算表明，向 t-PtGa 的转变在费米能级处打开了一个赝隙，可以追溯到 Pt-Pt 等电子键的形成，符合 18-n 键合方案。相反，DFT-化学压力（CP）分析揭示了沿 Pt-Ga 接触的一系列正局部压力网络，要求 c-PtGa 到 t-PtGa 的转变遵循紧密协调的原子运动。实验上，合成了一系列具有不同 Pt:Ga 比的样品，以检查多晶型的稳定范围。贫 Ga 样品在研究的整个温度范围内仅产生立方多晶型，这可以与间隙 Pt 原子（在 CP 方案中的负压点）的增强掺入相关联。然而，在更富 Ga 的组成下，t-PtGa 作为低温形式出现。在这些样品中，发现 t-PtGa 到 c-PtGa 的转变是可逆的，但具有较大的滞后，在单晶中可以超过 100°C。理论和实验结果表明，c-PtGa 相通过间隙原子和限制原子运动的正 CP 网络来支撑其不利的电子计数，这表明在金属间材料中实现奇异电子结构的更一般策略。
