负热膨胀是自然界中一种比较罕见的反常物理现象，它超出了传统晶格动力学预期，是对传统晶格动力学概念的一个挑战。目前人们对负热膨胀这一反常物理现象的认识还非常有限，尤其是最近在一些新型三元氧化物中发现的热缩现象更是无法用现有的热缩机制进行解释，新的热缩机制预示着将有一批新的负热膨胀化合物被发现或被设计出来。本项目拟以新型负热膨胀三元氧化物为主要对象，结合多种原位声子谱学方法和理论计算揭示自旋序或电子序与晶格序自由度之间的相互作用及对负热膨胀的贡献，阐明其热缩的新机制；建立氧空位与晶格相互作用的理论模型，阐明氧空位引起（增强）负热膨胀的物理机制，并将其推广到其它氧化物体系；实现对该类化合物热膨胀温区和热膨胀系数的调控，并揭示调控机制。本项目研究不仅将拓展人们对负热膨胀机制的认识，同时也将为基于负热膨胀材料的器件应用奠定坚实的理论和实验基础。

Negative thermal expansion (NTE) is an abnormal phenomenon rarely seen in nature, which is beyond and challenges the conventional notions of lattice dynamics. Up to date, the knowledge about NTE is still very limited. Particularly, the NTEs found recently in some ternary oxides cannot be explained using the existing thermal contraction mechanisms. Novel NTE mechanisms imply that a numerous of new NTE compounds could be discovered or designed. In this proposal, we choose the ternary oxides as target compounds to investigate their novel mechanisms of NTE by multi-in-situ phonon spectroscopies and theoretical calculations. It includes: to reveal the interplay of the spin ordering, electron ordering and lattice ordering degrees of freedom, and clarify each contribution to NTE; build up a theoretical model for oxygen vacancy-lattice interactions and to reveal the physical origins of NTE induced by oxygen vacancy; and tailor the NTE properties of the compounds and manifest the tailoring mechanisms. This investigation will not only present new knowledge on NTE, but also provide solid theoretical and experimental foundations for applications of these compounds.