While the introduction of large, bulky substituents such as tert-butyl, -SiMe3, or -Si(isopropyl)3 has been used recently to control the solid state structures and charge mobility of organic semiconductors, this crystal engineering strategy is usually avoided in molecular metals where a maximized overlap is sought. In order to investigate such steric effects in single component conductors, the ethyl group of the known [Au(Et-thiazdt)2] radical complex has been replaced by an isopropyl one to give a novel single component molecular conductor denoted [Au(iPr-thiazdt)2] (iPr-thiazdt: N-isopropyl-1,3-thiazoline-2-thione-4,5-dithiolate). It exhibits a very original stacked structure of crisscross molecules interacting laterally to give a truly three-dimensional network. This system is weakly conducting at ambient pressure (5 S·cm(-1)), and both transport and optical measurements evidence a slowly decreasing energy gap under applied pressure with a regime change around 1.5 GPa. In contrast with other conducting systems amenable to a metallic state under physical or chemical pressure, the Mott insulating state is stable here up to 4 GPa, a consequence of its peculiar electronic structure.

The lattice dynamics of the GeV 4 S 8 compound has been investigated using both density functional calculations and Raman/ infrared (IR) measurements. While the accordance between the computed and the experimental data is very good in the lowtemperature, ferroelectric phase (25K, Imm2), this is not the case for the high-temperature, paraelectric one within the F4̅ 3m group. Using group theory and first-principles calculations, we show that the IR/ Raman phonon modes are, however, compatible with the I4̅ m2 space group. Analysis of the different modes at the ferroelectric transition shows that simultaneous weakening/strengthening of two symmetryrelated bonds within the V 4 S 4 cluster is a direct consequence of the orbital-order instability driving the ferroelectric transition. The softening of modes associated with such distortions call for strong orbital occupation fluctuations above T c . These fluctuations, associated with the discrepancy between the X-ray scattering F4̅ 3m group and the lattice dynamics I4̅ m2 group, can be interpreted within a dynamical Jahn−Teller distortion model for the paraelectric phase.

International audienceResistive random-access memory (ReRAM) made of organic materials has recently received much attention for application in flexible devices. In the latter, resistive switching is usually obtained thanks to electrochemical effects or charge trapping. This work shows that, under electric pulses, the crystalline molecular Mott insulator [Au(Et-thiazdt) 2 ] exhibits a resistive switching based on an insulator-to-metal transition (IMT). Electric pulses exceeding a threshold electric field of a few kilovolts/centimeter induce a volatile transition, due to an intrinsic, purely electronic effect related to an avalanche phenomenon. Moreover, the application of electric pulses of higher amplitude induces a nonvolatile and reversible resistive switching. Both two-level and multilevel switching between resistances R on and R off are observed. At room temperature, a R off /R on ratio >100 is obtained, which is very high for this kind of switching mechanism and quite promising for ReRAM applications. [Au(Et-thiazdt) 2 ] appears as the first molecular member of a new class of ReRAM called Mott memories. ■ INTRODUCTION The demand for organic-based electronic devices has recently increased due to their potential advantages over conventional inorganic electronics, such as their low fabrication cost, high mechanical flexibility, light weight, and ease of fabrication. 1,2 Flexible organic memories that would enable data processing, storage, and communication are essential components to achieve flexible electronics. 3−9 In that context, resistive random-access memory (ReRAM) made of organic materials has recently received much attention. 10−14 In ReRAM, information storage is enabled by a nonvolatile resistive switching between two different resistance states achieved by the application of short electric pulses. In organic ReRAM, resistive switching mechanisms generally involve electro-chemical effects associated with anion or cation migrations or charge trapping in metallic nanoparticles. 15−20 A new type of resistive switching was recently reported in several chalcogenide inorganic narrow-gap Mott insulators with formulation AM 4 Q 8 (A = Ga, Ge; M = V, Nb, Ta; Q = S, Se). 21−24 The application of short electric pulses on these compounds induces a new phenomenon of volatile and nonvolatile resistive switching related to the collapse of the Mott insulating state. The volatile transition appears above threshold electric fields of a few kilovolts/centimeter and is based on an electronic avalanche process. 22,23 For electric fields much higher than the threshold avalanche breakdown field (E th), the resistive switching turns nonvolatile. 21 The switching mechanism is closely related with their Mott insulator character, as the avalanche breakdown induces the collapse of the Mott insulating state at the local scale and triggers the formation of a granular conductive filament formed by metallic and superinsulating domains. 21−23,25 First identified in inorganic compounds, the generalization of this purely electronic in...

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