Nonmetal to metal crossover and ethylene ordering in the organic superconductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br

Phase transformations in organic superconductor kappa-(BEDT-TTF)(2)Cu[N(CN) (2)]Br and ita deuterated analog have been studied by resistance measuremen ts of structural relaxation kinetics under isothermal conditions. The nonme tallic to metallic resistance temperature dependence crossover observed in the salts and anomaly of spin susceptibility temperature dependence at 60 K are shown to be due to first-order phase transitions caused by ordering of the terminal ethylene groups. The experimentally determined transformation sequence is in good agreement with the microscopic mean field model of eth ylene ordering, taking into account two possible configurations of ethylene groups and their interactions with anions. [S0163-1829(99)01905-0].