基于中子衍射分析的稀土Laves相(R,R’)Co2体系的反常热膨胀和近零滞后低场巨磁热效应的相关机理研究

稀土Laves相RCo2化合物是最受瞩目的稀土基功能磁性材料之一。申请人前期通过化学掺杂在(R,R’)Co2化合物中分别获得近零膨胀和近零滞后低场巨磁热效应，但二者的相关机理尚不明确。目前人们对该类化合物相变及物性研究主要关注其单胞尺度的平均表观结构和宏观物性的演变规律。本项目基于中子粉末衍射(NPD)和中子全散射原子对分布函数(nPDF)方法，引入“真实键长”作为新的相变影响因素。以(R,R’)Co2化合物为研究对象，结合NPD和nPDF研究化学掺杂对原子占位、局域结构、表观结构及磁结构演化规律的影响，重点探究R-R/R-Co/Co-Co的“表观键长”及“真实键长”的变化与相变、负热膨胀、滞后和磁热效应的内在关联。利用第一性原理计算研究电子结构对相变及滞后大小的影响，建立相变势垒与滞后大小的对应关系，提取一级相变滞后的核心调控参数。该项目对于推动新型稀土基功能材料设计和优化具有重要意义。

The Laves phase RCo2 compound is one of the most noticed rare-earth-based functional magnetic materials. We obtained near-zero thermal expansion and near-zero hysteresis low-field giant magnetocaloric effects by chemical doping in the (R,R’)Co2 compounds in the early stage, but the related mechanism of the two are not clear yet. At present, the research on phase transition and physical properties of these compounds mainly focuses on the evolution law of the average apparent structure of the unit cell scale and macroscopic physical properties. This project introduces the “true bond length” as a new factor affecting phase transition, based on neutron powder diffraction (NPD) and neutron total scattering and atom pair distribution function methods(nPDF). We take the (R,R’)Co2 compounds as the research objects, combined with NPD and nPDF to study the influence of chemical doping on the evolution of atomic occupancy, local structure, apparent structure, and magnetic structure, focusing on the inner relationship between the changes of R-R/R-Co/Co-Co "apparent bond length" and "real bond length" and phase transition, negative thermal expansion, hysteresis and magnetocaloric effects of (R,R’)Co2 compounds. Then, we use first-principles calculations to study the influence of the change of the electronic structure on the first-order phase transition and the magnitude of the hysteresis from a theoretical perspective, and establish the corresponding relationship between the phase change barrier and the hysteresis，and then extract the core control parameters of the first-order phase transition hysteresis. This project has important scientific significance for promoting the design and optimization of new rare-earth-based functional materials.