CRYSTAL-STRUCTURE OF THE SPIN-PEIERLS COMPOUND ALPHA'-(BEDT-TTF)(2)AG(CN)(2) AT HIGH-PRESSURE AND LOW-TEMPERATURE

The crystal structure of alpha'-(BEDT-TTF)(2)Ag(CN)(2) [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene], which exhibits a spin-Peierls (at 7 K) and a semiconductor-semiconductor transition, was determined above and below the latter phase transition at 8 kbar (295 K) or 260 K (1 bar) by single crystal X-ray diffraction. The structural transition has been studied both by increasing pressure at 298 K and by lowering temperature at ambient pressure. It is characterized by the doubling of the b parameter and it is accompanied by a charge transfer from molecule to molecule, resulting in nonequivalent BEDT-TTF molecules alternating along both a- and b-axes. The crystal has the same isothermal compressibility above and below the transition with a limiting bulk modulus (K-v(-1)) of ca. 190 kbar. The structural behaviour of this salt is similar to that of the isomorphous alpha'-(BEDT-TTF)(2)AuBr2, which also shows a spin-Peierls transition.