Revisiting Solid–Solid Phase Transitions in Sodium and Potassium Tetrafluoroborate for Thermal Energy Storage

Sumit Konar*, Gertruda Zieniute, Elliot Lascelles, Beth Wild, Andreas Hermann, Yi Wang, Robert J. Quinn, Jan-Willem G. Bos, Andrew Fitch

*Corresponding author for this work

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Abstract

In situ synchrotron powder X-ray diffraction (PXRD) study was conducted on sodium and potassium tetrafluoroborate (NaBF4 and KBF4) to elucidate structural changes across solid–solid phase transitions over multiple heating–cooling cycles. The phase transition temperatures from diffraction measurements are consistent with the differential scanning calorimetry data (∼240 °C for NaBF4 and ∼290 °C for KBF4). The crystal structure of the high-temperature (HT) NaBF4 phase was determined from synchrotron PXRD data. The HT disordered phase of NaBF4 crystallizes in the hexagonal, space group P63/mmc (no. 194) with a = 4.98936(2) Å, c = 7.73464(4) Å, V = 166.748(2) Å3, and Z = 2 at 250 °C. Density functional theory molecular dynamics (MD) calculations imply that the P63/mmc is indeed a stable structure for rotational NaBF4. MD simulations reproduce the experimental phase sequence upon heating and indicate that F atoms are markedly more mobile than K and B atoms in the disordered state. Thermal expansion coefficients for both phases were determined from high-precision lattice parameters at elevated temperatures, as obtained from Rietveld refinement of the PXRD data. Interestingly, for the HT-phase of NaBF4, the structure (upon heating) contracts slightly in the a–b plane but expands in the c direction such that overall thermal expansion is positive. Thermal conductivities at room temperature were measured, and the values are 0.8–1.0 W m–1 K–1 for NaBF4 and 0.55–0.65 W m–1 K–1 for KBF4. The thermal conductivity and diffusivity showed a gradual decrease up to the transition temperature and then rose slightly. Both materials show good thermal and structural stabilities over multiple heating/cooling cycles.
Original languageEnglish
Pages (from-to)1238-1248
Number of pages11
JournalChemistry of Materials
Volume36
Issue number3
Early online date31 Jan 2024
DOIs
Publication statusPublished - 13 Feb 2024

Bibliographical note

Acknowledgments:
This work was funded by the Network+ for the Decarbonisation of Heating and Cooling, (EP/T022906/1), which is funded by the Engineering and Physical Sciences Research Council (EPSRC) as part of the UK Research and Innovation (UKRI) Energy Programme. All powder diffraction data were collected on the ID22 beamline at the European Synchrotron Facility (ESRF). Computational resources provided by the UK’s National Supercomputer Service through the United Kingdom Car–Parrinello HEC consortium (EP/X035891/1) and by the United Kingdom Materials and Molecular Modelling Hub (EP/P020194) are gratefully acknowledged. R.J.Q. and J.-W.G.B. acknowledge the Leverhulme Trust (RPG-2020-177). S.K. acknowledges Prof. Colin Pulham at the University of Edinburgh for his initial encouragement on this project.

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