The new compounds RE4X5B4 (RE = La, Ce, Pr, Gd and X = Br, I) and RE4I
5B2C (RE = La, Ce) are prepared via the reaction of RE metal, REX3 and
B, or B and C at temperatures 1670 greater than or equal to T greater
than or equal to 1270 K in welded tantalum ampoules. Chains of interc
onnected B-4 rhomboids are the characteristic features of the crystal
structures. According to band structure calculations, the compounds ar
e one-dimensional metals which undergo a gradual metal-to-semiconducto
r transition at low temperature as experimentally indicated by the ste
ep inrease of electrical resistivity.