High Pressure Crystal Structure and Electrical Properties of a Single Component Molecular Crystal [Ni(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate).

Hengbo Cui,Chloe S Coates,Reizo Kato,Takao Tsumuraya,Hamish H.-M Yeung,Mark R Warren

doi:10.3390/molecules24101843

Abstract

Single-component molecular conductors form an important class of materials showing exotic quantum phenomena, owing to the range of behavior they exhibit under physical stimuli. We report the effect of high pressure on the electrical properties and crystal structure of the single-component crystal [Ni(dddt)2] (where dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate). The system is isoelectronic and isostructural with [Pd(dddt)2], which is the first example of a single-component molecular crystal that exhibits nodal line semimetallic behavior under high pressure. Systematic high pressure four-probe electrical resistivity measurements were performed up to 21.6 GPa, using a Diamond Anvil Cell (DAC), and high pressure single crystal synchrotron X-ray diffraction was performed up to 11.2 GPa. We found that [Ni(dddt)2] initially exhibits a decrease of resistivity upon increasing pressure but, unlike [Pd(dddt)2], it shows pressure-independent semiconductivity above 9.5 GPa. This correlates with decreasing changes in the unit cell parameters and intermolecular interactions, most notably the π-π stacking distance within chains of [Ni(dddt)2] molecules. Using first-principles density functional theory (DFT) calculations, based on the experimentally-determined crystal structures, we confirm that the band gap decreases with increasing pressure. Thus, we have been able to rationalize the electrical behavior of [Ni(dddt)2] in the pressure-dependent regime, and suggest possible explanations for its pressure-independent behavior at higher pressures.

Highlights

In the development of single-component molecular conductors, metal dithiolene complexes, with a small energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), have formed an important category
The results indicated that anisotropic Dirac cones emerge by a closing of the HOMO and LUMO
It has previously been reported that a single crystal of [Ni(dddt)2 ] could be obtained by a recrystallization of powder from benzene, which was prepared by I2 oxidation of

Summary

Introduction

In the development of single-component molecular conductors, metal dithiolene complexes, with a small energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), have formed an important category. The conductivity of such complexes can be greatly enhanced by changing the HOMO-LUMO gap, via increased π conjugation and flatness of the ligand part. Most neutral single-component metal dithiolene crystals are semiconducting, or electrically insulating. The application of pressure to insulating phases is an efficient way to reduce intermolecular distances, without altering intramolecular geometries substantially, leading to the discovery of new single-component molecular metals.

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Conclusion