A research team from the Institute of Solid State Chemistry, School of Chemistry and Biological Engineering, University of Science and Technology Beijing, in collaboration with partners, has discovered the anomalous phenomenon of negative thermal expansion in amorphous solids. This significant finding was published in the international physics journal Physical Review Letters.

Negative thermal expansion is a rare yet highly valuable physical phenomenon, referring to the volume contraction of materials during heating. It can effectively compensate for the positive thermal expansion of conventional materials, addressing issues such as deformation and failure of devices caused by temperature fluctuations, and finds extensive applications in key fields like quantum technology, aerospace, and energy conversion. Previously, negative thermal expansion was mainly reported in crystalline materials, with clear structure-property relationships. Due to the lack of long-range periodic lattices and highly disordered structures, amorphous materials were long considered to exhibit positive expansion as they were thought to lack the structural elements required for negative thermal expansion.

The team, along with collaborators, observed anomalous negative thermal expansion in the amorphous Fe-Y-Zr-B (a-FYZB) alloy with no periodic atomic arrangement for the first time. Within the temperature range of 200-375 K, a-FYZB exhibits intrinsic negative thermal expansion behavior, with an average linear expansion coefficient of -6.7 ppm/K. Moreover, its negative thermal expansion behavior remains fully reversible after 100 thermal cycles below the crystallization temperature (TX). In contrast, the crystalline alloy of the same composition (c-FYZB), which has a body-centered cubic structure, shows normal positive expansion (11.7 ppm/K), highlighting the unique regulatory role of the "short-range order, long-range disorder" structure of amorphous alloys in negative thermal expansion.

Figure 1 Structure and thermal expansivity of amorphous Fe-Y-Zr-B (a-FYZB) alloy

Through synchrotron X-ray total scattering experiments, the team calculated the intrinsic volume expansion coefficient, which is highly consistent with the dilatometer results, confirming that the negative thermal expansion originates from the amorphous structure itself rather than external interference. To reveal the atomic origin of negative thermal expansion, the team combined techniques such as extended X-ray absorption fine structure (EXAFS) and X-ray pair distribution function (X-PDF), and found strong thermal contraction between local Fe-Fe atomic pairs and significant heterogeneity in local thermal expansion. Further analysis of magnetism and local structure led to the proposal of a new physical mechanism for negative thermal expansion in amorphous alloys, involving "local structure-magnetic interaction-thermal relaxation". As a new class of negative thermal expansion material systems, the mechanism of negative thermal expansion in amorphous alloys not only breaks through the long-range ordered structure framework of traditional crystalline materials but also its behavior may be comprehensively influenced by various thermodynamic and kinetic factors such as cooling rate, element ratio, and energy barriers, which require further in-depth exploration and systematic understanding.

Figure 2 X-ray PDF Study on Local Structure and Mechanism of Negative Thermal Expansion

This work was published recently in Physical Review Letters under the title "Emergent Negative Thermal Expansion in Amorphous Fe-Y-Zr-B Alloys". The co-first authors of the paper are Gao Ming, a master's student in chemistry, and Xu Hankun, a doctoral student in metallurgical engineering. The corresponding authors are Prof. Kun Lin and Prof. Xianran Xing from USTB. Collaborating units include Professor Andrea Sanson from the University of Padua, Italy, the Japan Synchrotron Radiation Research Institute (JASRI/SPring-8), and the European Synchrotron Radiation Facility (ESRF) in France. The research was supported by the National Natural Science Foundation of China, the Key Research and Development Program of the Ministry of Science and Technology, and the Excellent Youth Team Cultivation Project of Central Universities, as well as technical support from synchrotron radiation facilities such as the Diamond Light Source in the UK, SPring-8 in Japan, and ESRF in France. Paper link: https://doi.org/10.1103/2g25-bdjx.