Frustration-induced valence-bond ordering in a new quantum triangular antiferromagnet based on [Pd(dmit)2]

The temperature dependence of magnetic susceptibility, chi(T), of the P2(1)/m crystal of a novel [Pd(dmit)(2)] salt, (C2H5)(CH3)(3)P[Pd(Pdmit)(2)](2), has been measured. The observed chi(T) above 30 K is reproduced by the model of a spin-1/2 Heisenberg antiferromagnet on a triangular lattice with the exchange coupling, J/k(B) = 250 K. A phase transition at 25 K to a spin-gapped state has been found. As well as a spin-Peierls transition, this transition breaks the translational symmetry, leading to the formation of a columnar arrangement of the spin-singlet pairs. A significant role of the lattice structure in stabilizing the spin-singlet ordering is pointed out. To our knowledge, this is the first report of a two-dimensional (2D) spin system exhibiting a spin-Peierls-like transition.