The magnetic excitations in Ce(Cu,Al)4 solid solutions-An analysis of heat capacity and the role of atomic disorder

The tetragonal CeCuxAl4-x solid solutions show unusual inelastic neutron spectra containing additional excitations that cannot be explained by a standard crystalline electric field model assuming a single Ce trivalent ion. This observation was ascribed to a strong magnetoelastic coupling which leads to a formation of the vibron quasibound state. In this work, we present analysis of the specific heat data of CeCuxAl4-x (x ranging from 0.75 to 1.1) compounds. We show that the magnetic contribution in the paramagnetic region cannot be fitted within the standard crystalline electric field model using a single set of energy levels, in agreement with neutron scattering results. To describe the specific heat data, we introduce a model which assumes at least two different local surroundings of the cerium ions in the sample. It can serve as complementary or alternative view on the unusual excitation spectrum of this system and allows us to describe behavior of Ce(Cu,Al)4 compounds without introducing a vibron model. The analysis is confronted with previous structural studies and results of neutron scattering experiments.