BEDT-TTF SALTS WITH ALPHA-KEGGIN POLYOXOMETALATES - ELECTRICAL, MAGNETIC, AND OPTICAL-PROPERTIES OF (BEDT-TTF)(8)[PMO12O40] AND (BEDT-TTF)(8)[SIW12O40] AND X-RAY CRYSTAL-STRUCTURE OF (BEDT-TTF)(8)[PMO12O40]CENTER-DOT((CH3CN-CENTER-DOT-H2O)(2))

We report the electrical, magnetic, and optical properties of two isomorphous salts of BEDT-TTF, bis(ethylenedithio)tetrathiafulvalene, with alpha-Keggin anions, viz., (BEDT-TTF)(8)[PMo12O40]. {(CH3CN . H2O)(2)} and (BEDT-TTF)(8)[SiW12O40]. The former crystallizes in the monoclinic space group I2 with a = 14.086(6), b = 43.26(1), c = 14.062(6) Angstrom, beta = 107.84(4)degrees, Z = 2. The structure consists of alternate layers of BEDT-TTF, organized as a pseudo-alpha-phase, and of the alpha-Keggin inorganic anions and solvents of crystallization, along the b axis. For the PMo12O40 Salt sigma(RT) = 10(-1) S cm(-1), E(A) = 0.088 eV for T < 150 K and 0.177 eV for T > 200 K and for the SiW12O40 Salt sigma(RT) = 0.4 S cm(-1) and E(A) = 0.098 eV. The conductivity anisotropy is 0.1:0.05:0.009 and 0.4:0.13, respectively. The magnetic properties are the sum of those of the BEDT-TTF layers, which is a combination of a uniform 1D linear chain antiferromagnet (alpha = 1) with J/k(B) = -27 K for SiW12O40 and -43 K PMo12O40 and a dimerized chain antiferromagnet (alpha = 0) with Delta/k(B) = -96.5 and -145 K, respectively, and of the anions which is diamagnetic for the SiW12O40 Salt and Curie paramagnetic (S = 1/2) for PMo12O40. The EPR of the SiW12O40 Salt is dominated by the resonance from the BEDT-TTF moiety. The PMo12O40 Salt shows one resonance, which comprises a combination of both the anion and the BEDT-TTF radical, above 150 K. As the temperature of the sample is lowered, it splits into two components; the one at lower g value grows at the expense of the other. From the temperature dependence of the intensities, we estimate an energy difference of 1:2(8) K. Both components show weak fine structures that originate from Mo-95 and Mo-97 (each with I = 5/2). The optical data show a polarized intraband transition at similar to 2000 cm(-1) (0.25 eV) consistent with the electrical measurements. The two compounds presented here are on the borderline of the Mott-Hubbard transition.