The glass transition in twinned ((phenyl)₄As)₂C₆₀Cl crystals

Abstract

We have studied the crystal structure of ((phenyl)

Jahn-Teller distortion ofC60−in(Ph4As)2Long1, Schundler2, Adams3 et al. 2007Phys. Rev. BSelf Cite91200View full textAdd to dashboardCite

“…We studied the detailed temperature dependence of the FIR C 60 − modes in ͑Ph 4 As͒ 2 ClC 60 in order to look for an ordering transition reported in previous vibrational, 55 structural, 36,42 and ESR studies 33,40,41,43,47 of the ͑Ph 4 X͒ 2 YC 60 materials. Figures 3͑a͒-3͑d͒ show how the intensity and splitting of C 60 − modes develop with decreasing T. The insets trace the frequency changes with temperature of the inverted peak minima.…”

“…38 Since the fourfold crystallographic symmetry is not replicated in the I h point group, 39 there are two equivalent perpendicular orientations of the C 60 − cage that are randomly occupied. 34,36,37 Although both the disorder and the JT effect are reported to be static at low temperature, 36,40,41 no distortion of C 60 − in ͑Ph 4 X͒ 2 YC 60 has been detected by x-ray diffraction [34][35][36][37] or diffuse x-ray scattering, 42 and an upper bound of 0.01 Å has been set on deviations from an I h symmetry cage. 36 In addition to the structural studies, other experimental methods have been employed to clarify the properties of the C 60 monoanion in the ͑Ph 4 X͒ 2 YC 60 set of materials.…”

“…34,36,37 Although both the disorder and the JT effect are reported to be static at low temperature, 36,40,41 no distortion of C 60 − in ͑Ph 4 X͒ 2 YC 60 has been detected by x-ray diffraction [34][35][36][37] or diffuse x-ray scattering, 42 and an upper bound of 0.01 Å has been set on deviations from an I h symmetry cage. 36 In addition to the structural studies, other experimental methods have been employed to clarify the properties of the C 60 monoanion in the ͑Ph 4 X͒ 2 YC 60 set of materials. 33,[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58] Of particular relevance for this paper is a previous vibrational study of ͑Ph 4 P͒ 2 IC 60 which found splitting of the T 1u C 60 modes and activation of silent vibrations, consistent with a D 5d or D 3d JT distortion of the cage.…”

“…34,35 The crystal lattice of these materials is I4/m, with C 60 − and halide anions each at the center of a tetragonally distorted cube of Ph 4 X cations. 33,34,36,37 A nearly cubic symmetry embrace of phenyl rings encapsulates each C 60 − , 35 with offset face-to-face or edge-to-face attractive interactions between Ph rings and C 60 hexagons. 38 Since the fourfold crystallographic symmetry is not replicated in the I h point group, 39 there are two equivalent perpendicular orientations of the C 60 − cage that are randomly occupied.…”

“…The description of the low-temperature state, the nature of a transition to that state, and the transition temperature have also been the subject of disagreement. 33,36,40,42,55 Vibrational properties are highly sensitive to molecular symmetry. Recent combined experimental-theoretical vibrational studies of C 60 polymers found considerable mixing of the isolated neutral I h C 60 modes ͑mode mixing͒ and surprisingly large mode splittings in spectra of the bonded distorted cages.…”