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

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) ų, 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.