High-pressure phases of SF6 up to 32GPa from X-ray diffraction and Raman spectroscopy

Abstract

X-ray diffraction and Raman spectroscopy were employed to study the phase diagram of SF 6 at pressures up to 32 GPa. The known phase transformation of SF 6 phase I to phase II at around 2 GPa was confirmed. Discontinuities in the pressure dependence of the Raman frequencies and positions of X-ray reflections indicate further phase transitions at 10(1) and 19(1) GPa.The crystal structures of phases I and II were determined using single crystal X-ray diffraction. The structure of phase I corresponds to the "plastic" body-centred cubic low-temperature structure and the structure of phase II to the ordered monoclinic low-temperature structure. The bulk moduli of phase I and II were determined to be B 0,I = 6.3(2) GPa with B ′ I = 4 (fixed) and B 0,II = 8.5(8) GPa with B ′ II = 7.4(9), respectively. The linear compressibilities of phase II showed a slight anisotropy, which can be rationalised by consideration of the packing of the SF 6 molecules.

Toward an Understanding of the Pressure Effect on the Intramolecular Vibrational Frequencies of Sulfur Hexafluoride

“…The SF 6 phase diagram as a function of pressure has been explored by X-ray diffraction up to 32 GPa. 23 A phase transition to the same crystal structure of the low temperature phase described by Cockcroft and Fitch 31 has been observed at 2 GPa (phase II). Further transitions have been observed to occur at 12 GPa (phase III) and 24 GPa (phase IV) as evidenced by the appearance of new lines in the diffraction pattern and changes in the Raman spectrum.…”

“…It can be noted that the scaled calculated frequencies agree quite satisfactorily with the crystal frequencies. 22 , 23 This may imply that the XP-PCM approach somehow accounts for the static field acting on the molecule albeit not (obviously) for the intermolecular coupling giving rise to the Davydov splitting.…”

“…The high symmetry of the SF 6 molecule ( O h ) and the limited number of atoms allow to analyze the effect of pressure on vibrational properties at high level of theory and to compare the results with the available experimental measurements. 22 , 23 In fact, several IR and Raman experiments 22 − 26 are available on the molecular crystal for different phases both at ambient and high pressures. 23 , 26 …”

“… 22 , 23 In fact, several IR and Raman experiments 22 − 26 are available on the molecular crystal for different phases both at ambient and high pressures. 23 , 26 …”

Study of the reaction products of SF6 and C in the laser heated diamond anvil cell by pair distribution function analysis and micro-Raman spectroscopy

“…The high-pressure behaviour of SF 6 has been studied by Sasaki et al [10] up to 10 GPa by Raman spectroscopy who found two different crystalline phases, denoted as phase I and phase II. We recently explored the phase diagram of SF 6 by means of X-ray diffraction and micro-Raman spectroscopy up to 32 GPa and observed two additional phase transformations at 10 GPa and 19 GPa, respectively [11]. The SF 6 molecule was stable up to the highest applied pressure at ambient temperature.…”