Phase transition and lattice distortion in the proposed spin-liquid system κ-(BEDT-TTF)2Cu2(CN)3

We report measurements of the uniaxial coefficients of thermal expansion alpha(1) and the specific heat C on the spin-liquid candidate kappa-(BEDT - TTF)(2)Cu-2(CN)(3) We observe anomalous and strongly anisotropic in-plane expansivities implying (i) distinct T-induced b-c lattice distortions and (ii) an increase in the ratio of the hopping amplitudes t'/t upon cooling into the low-temperature regime Most importantly the alpha(t) data reveal clear evidence for a second-order phase transition around 6 K accompanied by distinct lattice effects By using a Gruneisen-scaling Ansatz we are able to extract the corresponding anomaly to the specific heat Estimates of the entropy indicate that spin degrees of freedom alone cannot account for the phase transition anomaly suggesting that charge degrees of freedom are involved (C) 2009 Elsevier B V All rights reserved