Neutron diffraction and micro-Raman scattering studies on rare-earth carbide halides

Neutron-diffraction experiments on powder samples and micro-Raman scattering investigations on single crystals of the layered compounds R(2)C(x)Hal(2), (R=Y, Gd, x= 1,2, and Hal= Br, I) have been performed in order to study their static and dynamic lattice properties. For the superconductors Y2C2I2 (T-c=9.97 K) and Y2C2Br2 (T-c=5.04 K), the C-C atomic;distances were obtained with high accuracy from neutron-diffraction experiments between T= 1.5 and 270 K. The expected Raman-active phonons were determined from a factor-group analysis of the crystal structures. In the monocarbide Y2CBr2, the Raman-active phonons of the heavy-ion sublattices have been observed. In the dicarbide compounds R(2)C(2)Hal(2), additionally, the stretching and tilting modes of the dimeric C-2 units were clearly identified by analyzing spectra from C-nat and C-13 substituted samples. The influence of the quasimolecular C-2 Unit On the electronic properties in the R(2)C(2)Hal(2) compounds and its interaction with the surrounding metal atom octahedra is discussed. [S0163-1829(98)03345-1].