Concerning the first-order transition in the ?-phase (BEDT-TTF)4PtCl6 . C6H5CN

Abstract

BEDT-TTF )jPtcl<, CoH~CN is a tt-pha~e exhibiting a fir~t-order tran~ition ai 250 K. The crystal structure of this sali contains two different donor loyers before the transition but only one after the tran~ition. Every layer contains two different BEDT-TTF donor molecules. Our tight binding band structure study suggests that this i~ the first BEDT-TTF Salt containing BEDT-TTF in three different oxtdation states j0, +1/2 and + Ii. One of the layer~ j.-i) contains (BEDT-TTF'' and (BEDT-TTF)~ dimers whereas the other layer l'A contains two different )BEDT-TTF~ '~)j dimers. We suggest that the first-order transition i~ msociated with the disproportionation of the two (BEDT-TTF''~)~ dimers of layer SA in )BEDT-TTF'' )_ and (BEDT-TTF)~ dimers. Thi~ process is associated with a conformational change in one of the dimers which apparently optimizes the donor-accepter interaction~ in thi~ x-phase. According to the present results ii would be worthwhile to reexamine the transport properties of this sait. Among the many charge transfer salis presently known, the sa called «-phases hâve not only led to the highest superconducting transition temperatures Il (excluding fullerenes) but have also given a seemingly endless serres of electronic surprises. The transport properties of these softs seem to be extremely dependent on small changes in pressure, temperature, disorder or chemical environment (2, 3). For instance, whereas at ambient pressure «-BEDT-TTF~CU(N(CN()Br (4) and «-BEDT-TTF~CU(N(CN)~)Cljj~Brjj~ (5) (where BEDT-TTF means bis(ethylenedithio)tetrathiafulvalene, 1) are superconductors, 1 ~ ~ ~ ~ ~s~~ ~~s~ BEDT-TIF