Negative thermal expansion (NTE) is an intriguing physical phenomenon. Layered Ca₂RuO₄ exhibits giant NTE over a wide temperature range from 200 to 400 K, which makes it attractive for fundamental research and industrial applications. However, a clear physical understanding is lacking for the appearance of NTE over such a wide temperature range and the oxygen-content-dependent switch from NTE to positive thermal expansion (PTE). Herein, we present insights into the average crystal structure, local structure, and electronic and orbital states of Ca₂RuO₄. Surprisingly, a previously overlooked monoclinic distortion is identified by electron diffraction and synchrotron X-ray diffraction (SXRD). X-ray absorption fine structure (XAFS) and synchrotron X-ray pair distribution function (PDF) analyses show large local distortions in monoclinic Ca₂RuO₄. Moreover, local stress on Ru cations is confirmed by the existence of over-bonding states, which relaxes along with NTE. Theoretical calculations indicate that d_xy orbital ordering and disordering in the monoclinic structure are the origins of NTE. Moreover, interstitial oxygen plays a critical role in stabilizing elongated RuO₆ and locally breaks the d_xy orbital ordering, facilitating the occurrence of PTE. This work elucidates the electronic and orbital states in NTE materials with defective lattices and provides a different route to designing unconventional NTE materials.

中文翻译：

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还原和氧化 Ca2RuO4 中巨大负热膨胀的起源和不存在

负热膨胀 (NTE) 是一种有趣的物理现象。层状 Ca ₂ RuO ₄在 200 至 400 K 的宽温度范围内表现出巨大的 NTE，这使其对基础研究和工业应用具有吸引力。然而，对于在如此宽的温度范围内出现 NTE 以及从 NTE 到正热膨胀 (PTE) 的氧含量依赖性转换，缺乏明确的物理理解。在此，我们提出了对 Ca ₂ RuO ₄的平均晶体结构、局部结构以及电子和轨道状态的见解. 令人惊讶的是，以前被忽视的单斜畸变通过电子衍射和同步加速器 X 射线衍射 (SXRD) 确定。X 射线吸收精细结构 (XAFS) 和同步加速器 X 射线对分布函数 (PDF) 分析显示单斜 Ca ₂ RuO _{4 中存在}较大的局部畸变。此外，过键合态的存在证实了 Ru 阳离子的局部应力，该态随 NTE 一起松弛。理论计算表明，单斜结构中的d _xy轨道有序和无序是 NTE 的起源。此外，间隙氧在稳定拉长的 RuO ₆和局部破坏d _xy方面起着关键作用轨道排序，促进 PTE 的发生。这项工作阐明了具有缺陷晶格的 NTE 材料中的电子和轨道状态，并为设计非常规 NTE 材料提供了不同的途径。