High-Pressure (up to 10.7 GPa) Crystal Structure of Single-Component Molecular Metal [Au(tmdt)_2]

摘要

The crystal structure of the single-component molecular metal [Au(tmdt)_2] was examined at pressures up to 10.7 GPa in order to examine whether the high-pressure structure reflects the crystal's metallic nature. Crystal structure analyses were performed at 0.2, 0.8,1.3, 3.0, 5.5, and 10.7 GPa on the basis of the powder X-ray diffraction data obtained by using the synchrotron radiation source SPring-8. The unit cell volume at 10.7 GPa was ～75% of the initial volume, indicating that [Au(tmdt)2] is a 'soft material' like a typical molecular crystal in spite of its metallic nature. The pressure dependences of the bond lengths of the Au(tmdt)_2 molecule were found to be ~1 order of magnitude smaller than those of the intermolecular atomic distances. These results seem to justify the commonly accepted conjecture that the molecule usually behaves almost like a rigid body up to a fairly high pressure. It was found that the anisotropy of the lattice compression of the insulating I_2 crystal below 20 GPa can be essentially interpreted on the basis of a very simple 'interatomic repulsion model, which assumes that the molecules in the crystal are packed such that as far as possible, an increase in the interatomic repulsions between neighboring molecules is avoided. However, the maximum decrease in the intermolecular distance in [Au(tmdt)_2] was observed along the a direction although there were many intermolecular S…S contacts shorter than the van der Waals distance (3.70 Ǻ) along this direction. The shortest intermolecular S…S distance was 2.73 A at 10.7 GPa, which is ～1 Ǻ shorter than the S…S van der Waals distance (3.70 Ǻ). The crystal lattice of [Au(tmdt)_2] is considered to be stabilized by the enhancement of the intermolecular overlapping of the conduction molecular orbitals having large amplitudes on peripheral S atoms. Although the crystal is composed of 'isolated molecules' like a typical insulating molecular crystal, its compressibility behavior seems to reflect its metallic nature.