SYNTHESIS AND CRYSTAL-STRUCTURE OF A MIXED-VALENT LITHIUM-TANTALUM NITRIDE LI2TA3N5

Lithium, lithium nitride or lithium amide (Li, Li3N, LiNH2) reacts with tantalum(V)-nitride (Ta3N5) to form Li2Ta3N5. The pure compound is obtained when the reaction between LiNH2 and Ta3N, is carried out in a high pressure autoclave (550-degrees-C; p(NH3) = 0.6 GPa; 5 days). A temperature gradient within the autoclave causes the separation of an excess of well soluble LiNH2 from sparingly soluble Li2Ta3N5 in supercritical ammonia. Powder X-ray and neutron diffraction data showed that the structure is monoclinic (C2/m; Z = 4; a = 9.812(2) angstrom; b = 4.201(1) angstrom; c = 9.640(2) angstrom; beta = 90.93(1)degrees) and of an ordered sodium chloride type. The Ta-N part of the structure resembles the room temperature modification of Ti3O5. The short interatomic distances d(Ta-Ta) of 2.90 angstrom and 2.96 angstrom and the diamagnetism of the compound suggest the presence of Ta-Ta bonds.