Solid state vibrational spectroscopy of anhydrous lithium hexafluorophosphate (LiPF[subscript 6])

Abstract

Raman and infrared studies of solid anhydrous lithium hexafluorophosphate (LiPF[subscript 6]) have been carried out. The studies were complemented by X-ray powder diffraction (XRD) and thermogravimetric (TG) analysis techniques. The results indicate that when solid LiPF [subscript 6] is studied in a strictly anhydrous environment, more consistent thermal stability data can be obtained. TG analysis, using a scan rate of 10 C min 1, indicate the onset of thermal decomposition of the anhydrous LiPF[subscript 6] occurring at about 134.84 C while the partially hydrolysed compound starts at 114.46 C. The Raman spectra of anhydrous MPF6 (M = Li+, Na+ and K+) are best interpreted in terms of a cubic space group Fm3m ðOs hÞ, (ZB = 1), giving rise to 21 vibrational modes ðAðRÞ 1g þ EðRÞ g þ T1g þ TðRÞ 2g þ 3Tð1RÞ 1u þ T2uÞ and as such, LiPF[subscript 6] may be considered isostructural with NaPF6 and KPF6. Crystal symmetry distortions in the anhydrous LiPF[subscript 6] give rise additional bands in the Raman spectrum due to T1u infrared active modes and the m1 (A1g) Raman band appears in the infrared spectrum in violation of the mutual exclusion selection rule for centro-symmetric sites. When these observations are considered, the Raman spectrum of LiPF[subscript 6] is similar to those of NaPF6 and KPF6, with observations of the expected shifts due to cation size and/or electronegativity effects.