Tolerance Factor Control of Uniaxial Negative Thermal Expansion in a Layered Perovskite,Chemistry of Materials

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Tolerance Factor Control of Uniaxial Negative Thermal Expansion in a Layered Perovskite
Chemistry of Materials ( IF 7.2 ) Pub Date : 2020-01-06 , DOI: 10.1021/acs.chemmater.9b04512
Chris Ablitt ₁ , Harriet McCay ₂ , Sarah Craddock ₂ , Lauren Cooper ₁ , Emily Reynolds ₂ , Arash A. Mostofi , Nicholas C. Bristowe ₃ , Claire A. Murray ₄ , Mark S. Senn _{1,

2}

Affiliation

By tuning the tolerance factor, t, of the Ruddlesden–Popper oxide Ca₂MnO₄ through isovalent substitutions, we show that the uniaxial coefficient of linear thermal expansion (CLTE) of these systems can be systematically changed through large negative to positive values. High-resolution X-ray diffraction measurements show that the magnitude of uniaxial negative thermal expansion (NTE) increases as t decreases across the stability window of the NTE phase. Transitions to phases with positive thermal expansion (PTE) are found to occur at both the high-t and low-t limits of stability. First-principles calculations demonstrate that reducing t enhances the contribution to thermal expansion from the lowest frequency phonons, which have the characteristics of octahedral tilts and have negative mode Grüneisen parameter components along the NTE axis. By tuning t to the lower edge of the NTE phase stability window, we are hence able to maximize the amplitudes of these vibrations and thereby maximize NTE with a CLTE of −8.1 ppm/K at 125 K. We also illustrate, at the other end of the phase diagram, that an enhancement in compliance of these materials associated with the rotational instability provides another mechanism by which NTE could be yet further enhanced in this and related systems.

中文翻译：

层状钙钛矿中单轴负热膨胀的公差因子控制

通过通过等价取代来调节Ruddlesden–Popper氧化物Ca ₂ MnO ₄的公差因子t，我们表明这些系统的单轴线性热膨胀系数（CLTE）可以通过从大的负值到正值来系统地改变。高分辨率X射线衍射测量表明，在整个NTE相的稳定性窗口内，随着t的减小，单轴负热膨胀（NTE）的幅度会增加。已发现在高t和低t稳定性极限处都会发生具有正热膨胀（PTE）的相变。第一性原理计算表明减少t增强了最低频率声子对热膨胀的贡献，该声子具有八面体倾斜的特性，并且沿NTE轴具有负模Grüneisen参数分量。通过将t调整到NTE相稳定性窗口的下边缘，我们能够最大化这些振动的幅度，从而在125 K时的CLTE为-8.1 ppm / K时最大化NTE。从相图的角度来看，与旋转不稳定性相关的这些材料的顺应性的增强提供了另一种机制，通过该机制可以在该系统和相关系统中进一步增强NTE。

更新日期：2020-01-07

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