Tuning thermal expansion properties by insertion of guest molecules

Materials are expected to expand with increasing the temperature. Surprisingly, few of them show an opposite trend. Materials with negative thermal expansion (NTE) properties have been attracting great technological interest due to their potential use in mechanical and optical applications. Typically, they are combined with materials with positive thermal expansion in order to engineer composites with very low or zero thermal expansion. This strategy is not always viable and other approaches are therefore needed. Here we show how a material could have its NTE properties tuned thanks to the insertion of guest molecules into its structure.

Porous aluminophosphate AlPO₄-17 with a hexagonal erionite structure is the oxide which exhibits the highest coefficient of negative thermal expansion. The NTE mechanism in this material is rationalized by a structural contraction around the empty pores and cages that occurs with increasing temperature. The insertion of small molecules into the empty cages of AlPO₄-17 offers a means to modify its thermal expansion properties. In the present study, high quality, single-crystal and powder x-ray diffraction (XRD) measurements were used to monitor the NTE in AlPO₄-17. The insertion of oxygen molecules at high pressure into the empty cages of AlPO₄-17 was found to tune its very strong NTE. The experiments were performed at the Xpress beamline of Elettra, using a diamond anvil cell (DAC) in a closed-cycle He-cryostat with very precise control of the pressure and temperature.

Dehydrated AlPO₄-17 (with empty pores) was loaded with liquid oxygen cryogenically and, then, compressed by using a DAC to promote the insertion of molecular oxygen at the structural pores. Ruby, samarium-doped strontium borate and gold were loaded along with the sample to monitor in situ the pressure-temperature dependence (P-T calibrants). The structure of the oxygen-filled material was determined in situ at high-pressure by single-crystal XRD (see Fig. 1-a). Powder XRD allowed us to determine the thermal expansion coefficient upon cooling at a pressure of 0.38 GPa, by using a closed-cycle He-cryostat. Fig. 1-b shows the measured powder XRD data at 0.38 GPa and 127.5 K. Whereas the volumetric thermal expansion only exhibits a small change with respect to AlPO4-17 (with empty pores) at ambient pressure, the thermal expansion for the oxygen-filled material along the two main crystallographic directions are surprisingly different. While the thermal expansion along the a and b directions decreases almost to zero upon molecular insertion, the negative expansion along c becomes 7 times (!) larger (namely, -2.2·10^-5 K^-1), see Fig. 1-c. Such highly anisotropic thermal expansion properties are of great interest for mechanical and optical applications.

Figure 1: (a) Views of the crystal structure of O₂–filled AlPO₄-17 at 0.5 GPa obtained from single-crystal diffraction. The crystallographic directions a and b are equivalent in this case. (b) Experimental (black), calculated (red) and difference (blue) XRD profiles (λ=0.4958 Å) for oxygen-filled AlPO4-17 powder at 0.38 GPa and 127.5 K. Upper (green) and lower (orange) vertical bars indicate the calculated positions of the Bragg reflections of AlPO₄-17 and Au (P-T calibrant), respectively. The broad signal around 9.6° (see experimental-calculated curves) is due to fluid O₂ and the other broad features are due to the Kapton windows of the cryostat. (c) Unit cell parameters (blue square symbols) of oxygen-filled AlPO₄-17 powder at 0.38 GPa as a function of temperature. Data (red dashed line) for empty AlPO₄-17 at ambient pressure are from Attfield and Sleight, Chem. Mater. 10, 2013-2019 (1998).

Molecular guest insertion is thus a very powerful tool for tuning the thermal expansion properties of porous materials with close to zero thermal expansion. High-pressure, variable temperature XRD is the technique of choice to determine the effect of guest content on the thermal expansion properties of these porous materials. The insertion of non-volatile guest species or the polymerization of a guest molecule under pressure can be used as a strategy to recover composites (after high pressure processing) for potential applications.

This research was conducted by the following research team:

Frederico Alabarse¹, Benoît Baptiste², Boby Joseph¹, and Julien Haines³

¹ Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
² Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, (IMPMC), CNRS – Sorbonne Université – IRD – MNHN, Paris, France
³ ICGM, CNRS, Université de Montpellier, ENSCM, Montpellier, France.

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