A NEUTRON-DIFFRACTION STUDY OF THE TETRAGONAL MONOCLINIC CRYSTAL-STRUCTURES OF SOME URANIUM-THORIUM DICARBIDES

Crystal-structure line profile refinements have been made for neutron powder diffraction data collected at room temperature from the ternary uranium-thorium dicarbide U//0//. //1Th//0//. //9C//2 and the two binary dicarbides UC//2 and ThC//2. U//0//. //1Th//0//. //9C//2 is monoclinic, with unit cell dimensions a equals 6. 630(2), b equals 4. 183(1), c equals 6. 690(2) Angstrom, beta equals 103. 86(1) degree , isostructural with ThC//2, a equals 6. 684(2), b equals 4. 220(1), c equals 6. 735(2) Angstrom, beta equals 103. 91(1) degree . The tetragonal structure for UC//2 is confirmed, with a equals 3. 522(1), c equals 5. 988(1) Angstrom. U//0//. //1Th//0//. //9C//2 contains discrete C-C groups 1. 297(8) Angstrom long; more precise values for C-C lengths have been determined for UC//2 (1. 322(4) Angstrom) and ThC//2 (1. 304(6) Angstrom).