Effect of short and long range order on crystal structure interpretation: Raman and powder X-ray diffraction of LiPF₆

The structure of LiPF6 has been probed using Raman scattering as well as pXRD and the results are compared and contrasted. The conventional Bragg angle scattering pXRD determines that dry LiPF6 crystallizes in a trigonal structure (Space Group R-3 (148)), while Raman data suggests that the observed structure is close to cubic (Space Group Fm-3m (225)), similar to NaPF6 and KPF6. DFT heat capacity calculations using ab-initio methods for both R-3 and Fm-3m structures show better correlation with the Raman observations. The differences between Raman and pXRD data in LiPF6 appear to be in common with that observed in other materials that are close to phase transitions at the temperature where order/disorder and phase transition processes are known to occur. In these circumstances, Raman spectroscopy emerges as a more sensitive probe of local structural changes than pXRD, which is known to probe the overall average long range order of crystalline materials. The results are interpreted through the relationship between the local symmetry, internal energy and the heat capacity.