Structure Phase Transition and Electrical Properties of a Single-component
Molecular Crystal [Ni(ddds)2](ddds=5,6-dihydro-1,4-dithiin-2,3-diselenolate)under High PressureHengbo Cui1, Yeung Hamish2, Coates Chloe2, Reizo Kato1
1

Condensed Molecular Materials Laboratory, RIKEN
2

Oxford Univ.

In the development of single-component molecular conductors, metal dithiolene complexes, with asmall energy gap (~0.5 eV) between HOMO and LUMO, have formed an important category [1,2]. On theother hand, molecular crystals have a soft lattice, and thus an application of pressure to insulating
single-component molecular crystals is very efficient way to discover new single-component molecular
metals/superconductors. Indeed, we have modified the four-probe resistivity measurement technique by
using a Diamond Anvil Cell (DAC), and successfully found that insulating [Ni(ptdt)2] (ptdt =propylenedithiotetrathiafulvalenedithiolate) [3], [Cu(dmdt)2] (dmdt = dimethyltetrathiafulvalenedithiolate)[4]

turned metallic at 19.4 GPa and 4.7 GPa, respectively. Recently, we also found that a single-component

molecularsuperconductor

basedonmetal

dithiolene

complex[Ni(hfdt)2](hfdt=bis(trifluoromethyl)tetrathiafulvalenedithiolate) at 8 GPa (Tc= 5.5 K) [6]. However, not only large-sized
single-component molecular crystals, but also small-sized single-component molecular crystals could begood candidates for single-component molecular metals. Recently, Lorcy and coworkers found that thesmall-sized gold dithiolene complex [Au(Me-thiazdt)2] (N-Methyl-1,3-thiazoline-2-thione-4,5-dithiolate) [6]showed metallic behavior at ambient pressure. Its analogs [Au(Et-thiazdt)2] with the ethyl substitution [7],[Au(Et-thiazds)2] with the Se substitution [8], and the most basic metal dithiolene complex [Ni(dmit)2] (dmit= 1,3-dithiole-2-thione-4,5-dithiolate) [9] are reported to become metallic at 1.3 GPa, 0.6 GPa, and15.9 GPa,respectively.

Moreover,insingle-componet

molecular conductors that meet several conditions,the HOMO band and LUMO band can touch at asingle point to form a Dirac fermion system. We
have employed ab initio density functional theory(DFT) calculations for single-component molecular
crystal[Pd(dddt)2](dddt=bis(5,6-dihydro-1,4-dithiin-2,3-dithiolate) that exhibits temperatureindependnt resistivity at 12.6 GPa. The result
indicates that Dirac cones emerge in [Pd(dddt)2]
under high pressure. Here, we report the crystal
structure and the pressure dependent resistivity of[Ni(ddds)2] ( ddds = 5,6-dihydro-1,4-dithiin-2,3diselenolate ).
Blackneedle-shaped

singlecrystalsof[Ni(ddds)2] were prepared by electrochemical
oxidation of [(n-C4H9)4N][Ni(ddds)2] in acetonitrile

Figure 1

Molecular and crystals structure oftrimer structure [Ni(ddds)].

with tetrabutylamonium perchlorate as a supporting electrolyte. As shown in Fig. 1, two Ni(ddds)2 molecules
are strongly dimerized, and are stacking along the a axis. Within the dimer, the central Ni atom is connectedto the Se atom in the adjacent molecule with a bond length of 2.496 Å. The dimers are packed perpendicular
to each other and make a checkerboard type arrangement.
Figure 2 shows the temperature dependent of resistivity under various pressures for [Ni(ddds)2]. The
system was insulating at 1.9 GPa, and room-temperature conductivity (σrt) and activation energy (Ea) were 8× 10-4 S cm-1 and 203 meV, respectively. With increasing pressure, resistivity and Ea rapidly decreased. At
10.8 GPa, σrt reached up to 3 S cm-1 and the temperature dependence of resistivity was semiconducting with
Ea of 20 meV. However, at higher pressure, both resistivity and Ea remained almost pressure-independent,and σrt and Ea were 10 S cm-1, and 19 meV, respectively. The single crystal structure analysis under highpressure was performed in synchrotron radiation facility. Interestingly, a structural transition was found atHigh pressure crystal structure transition: [Ni(ddds) ]2 werehigh pressure, and the intermolecular Ni-Se bond at ambient pressure disappeared and new Ni-Ni bonds
formed (see Fig. 3).

6.9 GPa

1 bar

c

Pressurea = 6.623
ÅTemperatureand pressureFigure 2b = 14.800 Åc = 15.448 Å
β = 90.842 °

dependences of the resistivity of [Ni(ddds)2].
References0b

Monoclinic
a = 6.0715 ÅFigure
3 High pressure crystal structure of [Ni(ddds)2].
P2/n
b = 17.7565 Å
1c = 10.593 Å
β = 94.973 °

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