The nonpareil R-5(SixGe1-x)(4) phases

The R-5(SixGe1-x)(4) materials have been known for over 30 years, but only in the last few years has their uniqueness become evident. Their exotic beh aviors include a simultaneous first-order crystallographic-magnetic phase t ransition, the giant magnetocaloric effect, enhanced Curie temperature due to the existence of (Si,Ge)-(Si,Ge) bond pairs, and the absence of the magn etic contribution to the heat capacity below 10 K of ferrimagnetic Nd5Ge4, which orders at 55 K. These behaviors were observed in our studies of the p hase relationships, crystallography, and the electrical, magnetic and therm al properties of the pseudo-binary 5:4 compounds with R=La, Nd, Gd, Dy and Lu. (C) 2000 Elsevier Science S.A. All rights reserved.