Kilduff Brandon J, Fredrickson Daniel C
Department of Chemistry, University of Wisconsin-Madison , 1101 University Avenue, Madison, Wisconsin 53706, United States.
Inorg Chem. 2016 Jul 5;55(13):6781-93. doi: 10.1021/acs.inorgchem.6b01124. Epub 2016 Jun 21.
While composition and pressure are generally considered orthogonal parameters in the synthesis and optimization of solid state materials, their distinctness is blurred by the concept of chemical pressure (CP): microscopic pressure arising from lattice constraints rather than an externally applied force. In this article, we describe the first cycle of an iterative theoretical/experimental investigation into this connection. We begin by theoretically probing the ability of physical pressure to promote structural transitions in CaCu5-type phases that are driven by CP in other systems. Our results point to the instability of the reported CaCu5-type CaPd5 phase to such a transition even at ambient pressure, suggesting that new structural chemistry should arise at only modest pressures. We thus attempted to synthesize CaPd5 as a starting material for high-pressure experiments. However, rather than obtaining the expected CaCu5-type phase, we encountered crystals of an incommensurately modulated variant CaPd5+q/2, whose composition is related to its satellite spacing, q = qbbasic* with q ≈ 0.44. Its structure was solved and refined in the (3 + 1)D superspace group Cmcm(0β0)s00, revealing CaCu5-type slabs separated by distorted Pd hexagonal nets with an incommensurate periodicity. DFT-CP analysis on a commensurate model for CaPd5+q/2 indicates that the new Pd nets serve to relieve intense negative CPs that the Ca atoms would experience in a CaCu5-type CaPd5 phase but suffer from a desire to contract relative to the rest of the structure. In this way, both the Pd layer substitution and incommensurability in CaPd5+q/2 are anticipated by the CP schemes of simpler model systems, with CP quadrupoles tracing the paths of the favorable atomic motions. This picture offers predictions for how elemental substitution and physical pressure should affect these structural motifs, which could be applicable to the magnetic phase Zr2Co11 whose previously proposed structures show close parallels to CaPd5+q/2.
虽然在固态材料的合成和优化中，组成和压力通常被视为正交参数，但化学压力（CP）的概念模糊了它们的区别：化学压力是由晶格约束而非外部施加的力产生的微观压力。在本文中，我们描述了对这种联系进行的迭代理论/实验研究的第一个周期。我们首先从理论上探究物理压力促进CaCu5型相结构转变的能力，在其他系统中这种转变是由化学压力驱动的。我们的结果表明，即使在常压下，报道的CaCu5型CaPd5相对于这种转变也是不稳定的，这表明新的结构化学应该只在适度压力下出现。因此，我们试图合成CaPd5作为高压实验的起始材料。然而，我们没有得到预期的CaCu5型相，而是遇到了一种非公度调制变体CaPd5+q/2的晶体，其组成与其卫星间距q = qbbasic* 相关，其中q ≈ 0.44。其结构在（3 + 1）D超空间群Cmcm(0β0)s00中得到解析和精修，揭示出由扭曲的Pd六边形网隔开的CaCu5型板，具有非公度周期性。对CaPd5+q/2的一个 commensurate 模型进行的DFT - CP分析表明，新的Pd网有助于缓解Ca原子在CaCu5型CaPd5相中会经历的强烈负化学压力，但相对于结构的其余部分有收缩的趋势。通过这种方式，更简单模型系统的化学压力方案预测了CaPd5+q/2中的Pd层取代和非公度性，化学压力四极子追踪有利原子运动的路径。这幅图景为元素取代和物理压力如何影响这些结构基序提供了预测，这可能适用于磁性相Zr2Co11，其先前提出的结构与CaPd5+q/2有密切相似之处。
