npj: 畸变稳定的有序结构—混合烧绿石中

2019 年 3 月 9 日 知社学术圈

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多组分氧化物通常具有增强的闪烁性、铁电性、压电性、高温稳定性和催化响应性。烧绿石氧化物（A₂BBʹO₇ ）在很多技术领域都得到了应用，包括耐辐射损伤和离子导体等。混合烧绿石（含两个A和/或两个B位阳离子）由于在该化学空间中可获得的化学和构型自由度不同，为定制性能提供了更大的灵活性。然而，混合烧绿石中的阳离子即使形成固溶体的情况下，它们还可能形成局部有序结构，而不是传统的无序固溶。这些有序结构可表现出与随机固溶体不同的输运行为。因此，在建立混合烧绿石的结构-性质关系的研究前，人们必须对混合烧绿石的原子排布特征作充分了解。

来自美国洛斯阿拉莫斯国家实验室的Ghanshyam Pilania和Blas P.Uberuaga等，使用密度泛函理论（DFT），以Zr含量x的作为变量，研究了一种双烧绿石Gd₂(Zr_xTi_1−x)₂O₇中不同阳离子排序的相对稳定性。他们的结果表明，在某些特定的局部阳离子排列下，由于氧弛豫位移大，该系统中的某些阳离子有序成分高度稳定，从而形成有序的“双”烧绿石结构。这些异常氧弛豫的起源可追溯到局部阳离子对称性和Zr原子对采用7配位环境的强化学偏好，而不是常规烧绿石结构中的6配位构型。混合烧绿石中潜在的有序结构开辟了定制这些系统性质的新途径，扩展了这类化合物的功能空间。双烧绿石这一研究可能与其组成离子的电导率有关，并与之前报道的实验结果有关。

该文近期发表于npj Computational Materials 5: 7 (2019)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。

Distortion-stabilized ordered structures in A₂BB’O₇ mixed pyrochlores

Ghanshyam Pilania, Brian Puchala & Blas P. Uberuaga

Pyrochlore oxides (A₂B₂O₇) are interesting for a number of technological applications, including radiation damage tolerance and as ionic conductors. Mixed pyrochlores—containing two A and/or two B site cations—provide even more flexibility for tailoring properties owing to the diverse chemical and configurational degrees of freedom accessible within this chemical space. Here, we examine relative stability of different cation orderings in one model double pyrochlore Gd₂(Zr_xTi_1−x)₂O₇, as a function of Zr content x. Our results show that, in the presence of some very specific local cation arrangements, certain cation-ordered compositions in this system are highly stabilized as a result of large oxygen relaxation displacements, leading to the formation of an ordered ‘double’ pyrochlore structure. The origins of these anomalous oxygen relaxations are traced back to both the local cation symmetry and a strong chemical preference of Zr atoms towards adopting a 7-fold coordination environment, as opposed to a 6-fold coordination available in a regular pyrochlore structure. Subsequently, we examine the stability of this type of ordering in 131 other pyrochlore compositions. Implications of our findings are discussed in relation to the observed composition-dependent ionic conductivity in these systems and connections with previously reported experimental findings are made.