B and B-C as interstitials in reduced rare earth halides

In this review we describe a variety of rare earth metal boride halides and boride carbide halides. In boride halides boron atoms occur either as disc rete atoms octahedrally coordinated by rare earth metal atoms or as chains of interconnected B-4 rhomboids in rods of fused metal atom bisphenoids. Ch aracteristic building units in boride carbide halides are quasi-molecular B -C, C-B-C, C-B-E-C, and C-2-B-B-C-2 entities. In these compounds B centers trigonal prisms of rare earth metal atoms, and C is located in tetragonal p yramids. Owing to the electropositive character of rare earth metals, B ato ms, B-4 units, and CnBm groups tend to be anions. Band structure calculatio ns as well as electrical and magnetic measurements have been performed. The compounds, show semiconducting or metallic behavior, depending on whether excess electrons according to the Zintl-Klemm formalism are localized or de localized. La9Br5(CBC)(3) is superconducting with T-c similar to 6 K. Ce an d Gd compounds are paramagnetic. In most cases anti-ferromagnetic ordering is observed at low temperature. (C) 1999 Elsevier Science S.A. All rights r eserved.