Metal-Insulator Transition in (TMTTF)₂BF₄ under Pressure

抄録

The pressure-temperature phase diagram of bis-tetramethyltetrathiafulvalenetetrafluoroborate, (TMTTF)₂BF₄, is determined through the electrical conductivity measurement along the a-axis. Two distinct metal-insulator transitions are observed at T₁=200 K (P<5 kbar) and at T₂=ca. 50 K (P>5 kbar). We conjecture that the T₁ transition is associated with an anion-orientational ordering, while T₂ is with the spin-Peierls ordering. The large difference in the transition temperatures for the resistivity at 200 K and for both the magnetic susceptibility and the specific heat at 41 K in the early reports is reconciled by taking account of the two types of transitions.