High-Pressure Crystal Structure and Unusual Magnetoresistance of a Single-Component Molecular Conductor [Pd(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate)

Hengbo Cui,Reizo Kato,Hamish H.-M Yeung,Yoshitaka Kawasugi,Takaaki Minamidate,Lucy K Saunders

doi:10.3390/cryst11050534

Hengbo Cui, Reizo Kato + Show 4 more

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https://doi.org/10.3390/cryst11050534

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Abstract

A single-component molecular crystal [Pd(dddt)2] has been shown to exhibit almost temperature-independent resistivity under high pressure, leading theoretical studies to propose it as a three-dimensional (3D) Dirac electron system. To obtain more experimental information about the high-pressure electronic states, detailed resistivity measurements were performed, which show temperature-independent behavior at 13 GPa and then an upturn in the low temperature region at higher pressures. High-pressure single-crystal structure analysis was also performed for the first time, revealing the presence of pressure-induced structural disorder, which is possibly related to the changes in resistivity in the higher-pressure region. Calculations based on the disordered structure reveal that the Dirac cone state and semiconducting state coexist, indicating that the electronic state at high pressure is not a simple Dirac electron system as previously believed. Finally, the first measurements of magnetoresistance on [Pd(dddt)2] under high pressure are reported, revealing unusual behavior that seems to originate from the Dirac electron state.

Highlights

Most single-component molecular crystals are semiconductors or insulators because of large HOMO-LUMO energy band gaps (HOMO = Highest Occupied Molecular Orbital, LUMO = Lowest Unoccupied Molecular Orbital)
We have found that a single-component molecular crystal, [Pd(dddt)2 ], forms a Dirac electron system under high pressure
We report more detailed highpressure resistivity measurements, the first experimental structure determinations performed at high pressure using single-crystal synchrotron X-ray diffraction, tight binding calculations performed using these data, and the first high-pressure MR measurements performed on single crystals of [Pd(dddt)2 ]

Summary

Introduction

Are, considered to be important candidates in the search for conducting single-component molecular systems. Through the last two decades of research, it has been revealed that chemical modifications, such as increasing π-conjugation and ligand planarity, can reduce the HOMO-LUMO band gap. This method, reduces the solubility of source materials and makes crystal growth very difficult. Only three single-component molecular metals, [M(tmdt)2 ] (M = Ni, Au; tmdt = trimethylenetetrathiafulvalenedithiolate) [1,2] and [Au(Me-thiazdt)2 ] (Me-thiazdt = N-methyl-1,3-thiazoline-2thione-4,5-dithiolate) [3], and one single-component Dirac electron candidate [Pt(dmdt)2 ]. Molecular crystals have soft lattices and so the bandwidths of HOMO and LUMO bands may be enlarged by the application of external pressure

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