Cerium intermetallic compounds with two inequivalent crystallographic sites

The low-temperature properties of five cerium intermetallic compounds, Ce(2)Sn(5), CePtGe(2), Ce(5)Ni(6)In(11) and Ce(4-x)Ru(4)Ge(12+x) (with x = 0 and 1), which have two inequivalent cerium lattice sites, have been measured. The first three behave as trivalent systems at high temperatures (> 50 K) but at low temperatures they exhibit some unusual properties because of the different kinds of cerium-cerium interactions, including heavy fermion, intermediate valence and magnetic order. The Ce(4-x)Ru(4)Ge(12+x) system is unusual and displays a variety of behaviors such as heavy fermion, magnetic and non-magnetic atom disorder (MAD/NMAD) spin glass, and both tri- and tetravalency. The existence of independent trivalent and tetravalent cerium atoms in an intermetallic phase is unique and as far as we are aware this is the first time such a distinct valence situation has been reported for a cerium compound. Other cerium systems with multi-cerium sites reported by others are also briefly discussed.