Structural and thermodynamic study of the system Th-C-N in the presence of excess graphite; the existence of a new hexagonal phase β 'ThCN' stable at high temperature

The progressive reaction of nitrogen on the 'dicarbide' of thorium in the presence of excess graphite has been studied using X-ray diffractometry at high temperature (T <= 19000C) under controlled pressure (10-3 <= psub(N2)(Torr) <= 760). Hence the different fields are successively determined between 1450 and 19000C using measurements of crystalline parameters, equilibrium pressures and free enthalpies of standard formation of the various carbonitrides and nitrides observed. It is notably shown that the 'dicarbide' in stable at psub(N2)'s considerably weaker than those stated by Benz and Froxel, the nitrogen content of the γ Th C2' cubic phase increasing furthermore with temperature. The new β 'ThCN' phase which does not quench crystallizes in the hexagonal system and in reattatched to the group space with P31 m, the various contractions of tis crystalline parameter Csub(β) is interpreted as a closing of the double bond of the C2 pairs in this structure. The temperature of 11250C is attributed to the new polymorphic transformation: β 'ThCN' hexagonal reversible α 'ThCN' monoclinic which appear to be of the martensitic type, the crystalline parameters of α'ThCN' being furthermore measured from 20 to 11250C. (orig.)