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Research Progress On Mechanism Of Correlation Between Fine Crystal Structure And Magnetic Properties Of Refrigeration Materials
May 14, 2018

The La(Fe,Si)13 compound was discovered by Shen Baogen et al., an academician of the Chinese Academy of Sciences. In the past 20 years, this giant magnetocaloric effect phase change material has attracted wide attention in related fields, especially the introduction of interstitial hydrogen atoms in its crystal lattice, which can extend the Curie temperature of the material to above room temperature and maintain the intrinsic characteristics of the material. Magnetic entropy change performance. This broadens the cooling temperature range and application range of the material, making the hydride gradually become a practical working fluid favored by magnetic refrigeration prototypes. However, there are no unanimous conclusions about the structural relationship between factors such as hydrogen atom occupancy, hydrogen atom acceptability, and Curie temperature.

Recently, Shao Yanyan, a Ph.D. student at the Institute of Rare Earth Magnetic Functional Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, used hard X-rays from a Shanghai synchrotron radiation source to study the crystal structure of lanthanide magnetocaloric compounds and their hydrides. For the first time, the intrinsic relationship between local fine structure and magnetic properties was established in the material.

The researchers first analyzed the expanded X-ray fine structure EXAFS spectrum, transformed the Fourier and inverse Fourier relationships, and fitted the oscillating function to precisely optimize the lattice parameters and constructed a more accurate lattice model. Hydrogen atoms occupy the 24d position of the 2 FeI and 4 FeII/Si centers. The hydrogen atom content in the unit cell determines the transition temperature of the magnetic volumetric phase transition, ie the Curie temperature, but the unit cell can accommodate the maximum amount of saturated hydrogen atoms. Previous studies have suggested that the content of saturated hydrogen is related to the size of the lattice volume. Some scholars have also found that the dependency of Curie temperature on pressure becomes smaller after hydrogen is charged, that is, there is a valence electron transfer phenomenon between the hydrogen atoms and the surrounding chemical environment in the hydride. In this study, by observing the X-ray absorption near-edge XANES spectrum of La, it is observed that the white line peak is significantly reduced after hydrogen charging, and it is directly proved that the local environment of La atoms influences the valence electron transfer between La and hydrogen atoms and determines the volume Hydrogen capacity size, lattice volume on the role of hydrogen capacity second. The determinant of the Curie temperature of La(Fe,Si)13 is generally considered to be caused by the interaction between the lattice volume and the d electrons of Fe and the s-electrons of Si. Leading role. This study uses the X-edge XANES peak of the Fe element to directly characterize the strength of the hybridization. In compounds with high Fe content, the K-edge front of the Fe element is more intense, indicating that there are more empty bands in the 3d orbit. Most of the 3d electrons transit from the local state to the paradox state. The reduction of the local electrons weakens the Fe-Si hybridization and brings about a decrease of the Curie temperature, thus revealing that the d electrons of Fe and the s-electron hybridization of Si greatly affect the degree of Curie temperature. For hydrides, the basic overlap of the front peaks indicates that the Fe and Si hybrids are similar, and therefore the hydride is dominated by the lattice volume factor rather than by the orbital hybridization to determine the Curie temperature.

The electronic level-based fine structure characterization technology was first applied to the study of LaFeSi magnetocaloric materials and has a strong enlightening effect on the understanding of the mechanism of giant magnetic entropy. It can provide engineering guidance for atomic doping or element substitution to control the Curie temperature. The role to provide research ideas for the research and mining of new systems and new functions of magnetocaloric materials.

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The relevant research results were published on Acta Materialia. The study was funded by the national key R&D program.