POLYMORPHISM AND ALLOYS IN THE BIS(ETHYLENEDITHIO)TETRATHIAFULVALENE/M(CN)(4)(2-) RADICAL-CATION SALTS - STRUCTURAL AND PHYSICAL-PROPERTIES OF ALPHA,DELTA-(ET)(4)[NI(CN)(4)](X)[PT(CN)(4)](1-X)(H2O)(N)

The synthesis and structural and physical properties of some radical cation salts based on bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and tetracyanometallates M(CN)(4)(2-)(M = Pt(II) and Ni(II)) are reported. The salts (ET)(4)[Ni(CN)(4)](r)[Pt(CN)(4)](1-x)(H2O)(n) (X = 0-1; n = 0-4) have been isolated with different principal packing modes, in particular the a and delta types. In both cases the structures consist of alternating layers of organic cations and inorganic anions, and two different kinds of packing, namely, alpha(1) and alpha(2), have been observed in the case of the alpha type structure. The organic sublattice in the alpha(1) salts is built of two independent chains, while only one type of dimerized chain is observed in the alpha(2) modification. The delta phase is originated from an independent and strongly dimerized one-dimensional stack. All materials are conductors at ambient temperature and exhibit a semiconducting behavior upon cooling. For the alpha(1) phase the spin susceptibility and the ESR line widths decrease with temperature, giving rise to two anomalies respectively at about 200 and 20 K. The alpha(2) type shows a classical Currie-Weiss low-T dependence of the spin susceptibility with a Weiss temperature theta = -25 K, characteristic of weak antiferromagnetic interactions. In the case of the delta type a strong decrease of the spin susceptibility at about 180 K reminiscent of a Peierls-type transition is observed. The band calculations (EHT model) agree with the observed ambient temperature conducting behavior for both alpha and delta phases. The nesting of the Fermi surface for the latter type might be related to the occurrence of a structural phase transition. In the absence of nesting for the alpha(1) type, an alternative mechanism based on the alternation of electronic densities on the different intermolecular bonds (bond order wave) is proposed to explain the observed electronic localization at low temperature. Finally a general description of the polymorphism encountered with their related structural organization is proposed.