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)

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.

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)

“…The single crystal of [Pd(dddt) 2 ] used was from the same batch of crystals that had been synthesized for high-pressure electrical property measurements. Unlike previously reported highpressure single-crystal structure measurements of the isostructural system [Ni(dddt) 2 ] [29], crystals of [Pd(dddt) 2 ] suffered from twinning even at pressures below 1 GPa. To avoid twinning of [Pd(dddt) 2 ] crystals, the crystal was encapsulated with layer of Araldite epoxy resin, made thin to reduce background scattering.…”

“…However, our experimental results show that the central metal substitution in [M(dddt) 2 ] (M = Ni, Pd), directly effects the high-pressure properties and structure. In contrast to the high-pressure structure of the Ni analogue [29], the Pd-Pd distance in [Pd(dddt) 2 ] decreases smoothly from 4.7281 Å at 1 bar to 4.117 Å at 10.6 GPa. Unlike [Ni(dddt) 2 ], which does not exhibit a structural transition up to 11.2 GPa [29], [Pd(dddt) 2 ] undergoes an order-disorder transition, which may be the cause of the increase in resistivity and coexistence of the Dirac cone state with semiconductivity in the higher-pressure region.…”

Donor-Type Nickel–Dithiolene Complexes Fused with Bulky Cycloalkane Substituents and Their Application in Molecular Conductors

“…All the solvents were of analytical grade and were used without further puri All the reactions were performed in a nitrogen atmosphere. Oligo(1,3-dithiole-2,4 one) ((C3S5)x) [70] and (Bu4N)[Ni(dddt)2] [56] were prepared using a previously r method.…”

“…All the reactions were performed in a nitrogen atmosphere. Oligo(1,3-dithiole-2,4,5trithione) ((C 3 S 5 ) x ) [70] and (Bu 4 N)[Ni(dddt) 2 ] [56] were prepared using a previously reported method.…”

“…These structures induce crossing HOMO-LUMO band inversions through metal-metal interactions in the solid state, which provide various conducting, magnetic, and other physical properties by combining with closed-shell cations [47,[50][51][52]. The electron-donor-type metal bis-dithiolene complexes [M(dddt) 2 ] (M = Ni, Pd, Pt, and Au; dddt = 5,6-dihydro-1,4-dithiine-2,3-ditholate) are candidate materials for the development of molecular conductors due to their orbital symmetries, which are similar to those of BEDT-TTF [53][54][55][56]. A cation radical crystal, [Ni(dddt) 2 ] 3 (AuBr 2 ) 2 , is the first metal based on the donor-type metal-dithiolene complexes exhibiting metallic behavior down to at least 1.3 K [57].…”