High-pressure infrared transmission measurements up to 9.5 GPa were carried out on the rotor-stator molecular cocrystal C 60 ÁC 8 H 8 . Helium served as pressure transmitting medium, which intercalates into the C 60 ÁC 8 H 8 lattice. Thus, we investigated the pressure effects and effect of intercalation of helium into the C 60 ÁC 8 H 8 lattice. The pressure-induced shift of the vibrational modes of C 60 ÁC 8 H 8 shows an anomaly around 3 GPa. This anomaly can be interpreted in terms of the orientational ordering transition of fullerene molecules accompanied by a change in the crystal symmetry, which causes the splitting of the vibrational modes. We compare the value of the critical pressure to that obtained earlier [Thirunavukkuarasu et al.,

High-pressure infrared transmission measurements on (Ph4P)2IC60 were performed up to 9 GPa over a broad frequency range (200 -20000 cm −1 ) to monitor the vibrational and electronic/vibronic excitations under pressure. The four fundamental T1u modes of C − 60 are split into doublets already at the lowest applied pressure and harden with increasing pressure. Several cation modes and fullerene-related modes split into doublets at around 2 GPa, the most prominent one being the G1u mode. The splitting of the vibrational modes can be attributed to the transition from the dynamic to static Jahn-Teller effect, caused by steric crowding at high pressure. Four absorption bands are observed in the NIR-VIS frequency range. They are discussed in terms of transitions between LUMO electronic states in C − 60 , which are split because of the Jahn-Teller distortion and can be coupled with vibrational modes. Various distortions and the corresponding symmetry lowering are discussed. The observed redshift of the absorption bands indicates that the splitting of the LUMO electronic states is reduced upon pressure application.

We report the Jahn-Teller (JT) dynamics of the C À 60 monoanion under pressure in (Ph 4 P) 2 IC 60 . (Ph 4 P) 2 IC 60 was synthesized by electrochemical crystallization and investigated by infrared transmission measurements up to 5 GPa. The behavior of two T 1u vibrational modes of C À 60 , which exhibit signatures of the dynamic JT effect at ambient conditions, was followed under pressure. The pressureinduced transition from the dynamic to the static JT state was observed at $2 GPa. Due to symmetry lowering related to the dynamic JT effect, the G 1u mode is activated and shows a splitting at the pressure-induced dynamic-to-static JT distortion. 1 Introduction C 60 -fullerene, the highly symmetric system in nature has kindled interest among the scientific community, to investigate its intriguing properties. The neutral C 60 molecule crystallizes in face-centered cubic (fcc) structure at room temperature and has large enough interstitial space for doping with alkali metals. Several fullerene-related compounds were synthesized and have been investigated. With appropriate doping, the properties of C 60 can be tuned from insulating to superconducting state or even magnetic. Doping C 60 with electrons induces interesting properties in the corresponding compounds: metallic superconducting phases A 3 C 60 (where A stands for alkali metal), semiconducting A 4 C 60 or ferromagnetic TDAE-C 60 which are governed by various physical phenomena [1,2].(Ph 4 P) 2 IC 60 (C 60 -tetraphenylphosphoniumiodide) is one of the charge transfer salts, synthesized by electrochemical method [3]. This fully ionic (Ph 4 P) 2 IC 60 is air stable. (Ph 4 P) 2 IC 60 is tetragonal with space group I 4/m . The I 4/m space group sustains two geometrically equivalent orientations of the C

Abstract. Size dependent compositional variations for the ordered L1 2 -structure gamma prime (γ ) precipitates in the commercial Ni-based superalloy RR1000 have been investigated using scanning transmission electron microscope (STEM) imaging combined with energy-dispersive X-ray (EDX) spectroscopy. To address the problem of quantitative compositional determination for nanoscale particles within a metal matrix we have applied a novel electrochemical method to extract individual precipitates. The use of a high-efficiency EDX detector enabled compositional measurements to be obtained for particles with diameters as small as 20 nm with acquisition times of the order of a few minutes. We have studied compositional variations across the different size families of γ precipitates within a microstructure generated by slow cooling. Our results demonstrate the importance of kinetic factors for determining the precipitates compositions. In particular, we provide new evidence for the role of aluminium antisite atoms on the low-temperature growth kinetics of fine scale γ precipitates. Our findings provide valuable structural data towards improving the accuracy of predicting the microstructural evolution in Ni-based superalloys.

The alkali fulleride Rb4C60 was investigated by infrared spectroscopy under high pressure, to study the vibrational and electronic excitations. The material exhibits a bad metal behavior in the measured pressure range (0.8–8 GPa). With increasing pressure the Drude term in the far‐infrared region increases, which signals an increasing metallic character. By following the optical excitations under pressure, we could attribute the underlying mechanism of the pressure‐induced insulator‐to‐metal transition in Rb4C60 to the closing of the charge gap, and the nanoscale phase separation scenario discussed in the literature seems to be unlikely.