One major step towards the design of controlled thermal expansion materials

Thermal expansion is critical in many technological applications and its control represents a challenge for the material design. An international team of researchers from China, Italy, United Kingdom and United States has developed a method to control the thermal expansion in framework materials through a redox intercalation process. The study, conducted in part at the XAFS beamline of Elettra, is reported in Nature Communications.

The control of thermal expansion may be possible through the use of the unconventional negative thermal expansion (NTE) materials, i.e., materials contract upon heating. NTE is known to arise from electronic and magnetic mechanisms, and, in open framework materials (open structures of corner-sharing polyhedral units), from the presence of low-energy transverse vibrations of the central linking atoms. However, this does not lead to straightforward control of thermal expansion, and composites of negative and positive thermal expansion materials may fail after repeated cycling, so direct control of thermal expansion within a single homogenous phase is desirable.

By starting from the fact that the introduction of small molecules such as water into large pore materials is known to have dramatic effects on the thermal expansion, the research team has conceived a method for varying thermal expansion based on intercalation chemistry, where small cations such as Li⁺ are inserted into the empty space of the framework structures, as much used in Li-battery chemistry. The intercalated cations are expected to sterically hinder or reduce the transverse vibrations and, accordingly, the negative thermal expansion.

The method has been tested using scandium fluoride (ScF₃), one of the most recently discovered material that displays strong NTE over a wide temperature range. Insertion of Li ions into ScF₃ has been achieved by reductive lithiation after partial substitution of Sc by a reducible species (Fe³⁺) since Sc³⁺ is not easily reducible. The solid solution (Sc_0.9Fe_0.1)F₃ was thus synthesized and then lithiated. Structure, composition and thermal expansion of the lithiated Li_x(Sc_0.9Fe_0.1)F₃ products (x=0.02, 0.04, 0.06) have been determined by joint studies based on synchrotron X-ray diffraction, X-ray Absorption Fine Structure spectroscopy, neutron powder diffraction and spherical aberration-corrected STEM. The results show that the thermal expansion is tuned from negative to zero or positive as Li content increases (see Figure 1).

But how Li intercalation tunes the thermal expansion of ScF₃? The vibrational analysis reveals that the thermal vibrations of fluorine ions perpendicular to the Sc-F-Sc linkages, leading to NTE in ScF₃, are strongly perturbed by the inserted Li ions. Specifically, the direction of the perpendicular vibrations for the closest fluorine ions are deviated to an angle of ~50° thus inhibiting NTE. Even a small concentration of Li (x~0.04) is sufficient to suppress the overall NTE behavior.

The experiments have therefore demonstrated that the redox intercalation of guest cations into the voids of a framework material can hinder the transverse atomic vibrations responsible for NTE, thus providing an effective and general method for tuning the thermal expansion in NTE framework materials. 

Figure 1. The effect of Li ion interaction on the thermal expansion of ScF₃ based compounds, i.e., Li_x(Sc_0.9Fe_0.1)F₃ for x=0, 0.02, 0.04, 0.06. The inserted Li ions reduce the transverse vibrations of fluorine atoms and, accordingly, inhibits the overall negative thermal expansion.

This research was conducted by the following research team:

J. Chen¹, Q. Gao¹, A. Sanson², X. Jiang³, Q. Huang⁴, A. Carnera², C. Guglieri Rodriguez⁵, L. Olivi⁵, L. Wang⁶, L. Hu¹, K. Lin¹, Y. Ren⁷, Z. Lin³, C. Wang⁶, L. Gu⁸, J. Deng¹, J. P. Attfield⁹, and X. Xing¹

¹ Department of Physical Chemistry, University of Science and Technology Beijing, Beijing, China
² Department of Physics and Astronomy, University of Padova, Padova, Italy
³ Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, China
⁴ NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland, USA
⁵ Elettra – Sincrotrone Trieste, Trieste, Italy
⁶ Center for Condensed Matter and Materials Physics, Department of Physics, Beihang University, Beijing, China
⁷ Argonne National Laboratory, X-Ray Science Division, Argonne, Illinois, USA
⁸ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, China
⁹ Centre for Science at Extreme Conditions and School of Chemistry, University of Edinburgh, Edinburgh, UK

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