2k(F) CDW transition in beta-(BEDT-TTF)(2)PF6 family salts

In an attempt to clarify the nature of the metal insulator transition of be ta-(BEDT-TTF)(2)PF6 family salts, temperature dependence of the static magn etic susceptibility, resistivity under various pressures, lattice parameter s and the intensity of X-ray superlattice reflections have been measured fo r beta-(BEDT-TTF)(2)PF6 and beta-(BEDT-TTF)(2)AsF6. The transition temperat ure (T-MI) Of these salts is shifted to higher temperatures with an increas e of pressure. A sharp drop of the activation energy is observed at 5 kbar for the PF6 Salt and 6 kbar for the AsF6 salt. Above these pressures, resis tivity becomes almost flat down to low temperatures though the resistivity again increases below 50 K. Phase diagrams were constructed for both materi als. Static magnetic susceptibilities of both salts exhibit abrupt drops be low T-MI A noticeable drop of the lattice parameters, lattice volumes and t he X-ray intensities around T-MI indicate that the M-I transition is associ ated with the structural transition. The existence of the 2-fold structural changes is established by observing the superlattice reflections correspon ding to (a, b, 2c) below T-MI.