Heavy fermion like behavior in layered Ce-based oxysulfides

We have succeeded to obtain an almost single phase of layered oxysulfide CeOCuS and found a heavy fermion like behavior in this compound. It crystallizes in tetragonal LaCAgS-type structure and the lattice parameters a and c are 0.3923 and 0.8333 nm, respectively. They are smaller than those expected for trivalent Cc ion. This suggests that Ce ions exist in a mixed valence state. The temperature dependence of the specific heat, C shows an upturn below about 7 K. It is expressed by the following relation, C = gamma T + beta f(3) + delta T-3 In T. The value of gamma is about 324 mJ/mol K-2, which is comparable with that of conventional heavy fermion compounds. The temperature dependence of the electrical resistivity is metallic and obeys the following relation, p = p(0) + p(1) T-2 + P2T2 In T, below 100 K. The temperature dependence of the magnetic susceptibility is expressed by the sum of Pauli paramagnetic and Curie Weiss terms. The Pauli paramagnetic term has a large value, which corresponds to about 10% that estimated from gamma. The effective magnetic moment mu(eff), 1.62 mu(B), is smaller than the free ion value of trivalent Ce and indicates about 64% of Ce ions is trivalent. (c) 2005 Elsevier B.V. All rights reserved.