THE PHASE DIAGRAM OF NEW ORGANIC SUPERCONDUCTOR BETS SALTS WITH A MIXED HALIDE GALLIUM ANION

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In 1996, crystal structures, electrical resistivities and magnetic susceptibilities of a series of organic superconductors with tetrahedral mixed halide gallium anions, lambda-BETS2GaXzY4-z (X,Y=F,Cl, Br ; 0<z<4) having triclinic unit cells and fourfold quasi-stacking arrangement of BETS (=bis (ethylene-dithio) tetraselenafulvalene) molecules were examined. The resistivity behavior dependedon the size of anion. The superconducting behavior could be examined systematically by exchanging the halogen atoms (X,Y) and the z-value. The magnetic susceptibilities were studied on lambda-BETS_2GaCl_3F and lambda-BETS_2GaBr_zCl_<4-z> (0<z<1.5). Although the resistivity behaviors of lambda-BETS_2GaX_zY_<4-z> resemble closely those of so-called 10K-class kappa-type BEDT-TTF superconductors, the insulating ground state near the superconducting state is non-magnetic. Theoretically this non-magnetic phase is pointed out that it has the possibility of exsisting on the borderline of spin-gap phase and antiferromagnetic phase. This year this non-magnetic phase was examined precisely by ^1H-NMR.^1H-NMR did not exhibit the abrupt increase of 2nd moment, which was observed in kappa-(ET)_2[CuN(CN)_2]Cl. In lambda-(BETS)_2GaCl_<2.5>Br_<1.5> the divergence of 1/T_1 which is characteristicin forming the magnetic orderness was not observed within the temperature range from 25K to 2K.So this phase is considered not as antiferromagnetic phase but as non-magnetic phase. lambda'-BETS3GaBr4 has a modified lambda-type structure and it shows narrow-gap semiconducting behavior down to 50K.lambda'-BETS3GaBr4 transforms to the insulating phase below 50 K.The decrease of magnetic susceptibilities with the anisotropies parallel to H and perpendicular to H was observed below 60 K.This phase transition seems to be antiferromagnetic. Such a high temperature antiferromagnetic phase transition of organic molecules is very rare and very interesting, compared with high-temperature inorganic superconductors.