STRUCTURAL STUDIES ON PRCU6, NDCU6, SMCU6 AT LOW-TEMPERATURES

Intermetallic compounds RCu6 (R=Pr,Nd and Sm) have phase transitions from the structures having orthorhombic symmetries to those of monoclinic symmetries at low temperatures. The monoclinic angles, γ's, for these compounds have been measured as functions of temperature down to 1.5 K. The γ's for PrCu6 and NdCu6 have monotonic temperature variations that could be reproduced by a simple mean field theory for the order parameter. The γ for SmCu6 was unusual in that it has a maximum at about 20 K, suggesting the coupling between order parameter and the electronic states. The temperature dependence of the unit cell volume of SmCu6 has been found to exhibit a significant decrease below about 120 K. These results have been analyzed on the basis of a coupling of the lattice to a two level system with a level separation of about 23 K. © 1990, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.