A new conducting ternary nitride: NaxTa3N5 (0<=x<=1.4)

Topotactic insertion of sodium into the channel structure of Ta3N5 has been accomplished by the reaction between Na and Ta3N5 at temperatures between 450 acid 600 degrees C in sealed Ni tubes. The limiting stoichiometry of NaxTa3N5 obtained is x approximate to 1.4. The structure of Na1.0Ta3N5 has been refined using Rietveld full-profile refinement of X-ray powder diffraction data using the Ta3N5 structure as the starting model, The space group remains Cmcm (No. 63) with parameters a = 3.995(1), b = 10.197(2), and c = 10.331(2) Angstrom. The Na atoms are shown to occupy 7-coordinate sites in the holes present in the Ta3N5 framework, The weak temperature dependence of the magnetic susceptibility between 100 and 320 K of Na1.0Ta3N5 and its small value (chi(mol) (300 K) = 6.5 x 10(-4) emu mol(-1)) suggest that the compound is a metallic conductor, This is supported by the moderate conduction of an unsintered pellet of Na1.0Ta3N5 powder, We also report the identification of the poorly crystalline disordered rock salt phase Na2Ta3N5 (a = 4.47(4) Angstrom) which is obtained when the reaction temperature is between 600 and 800 degrees C and a partly characterized hexagonal phase that results from nitrogen loss, Na1-xTa3+xN4 (x approximate to 0), which forms at temperatures above 800 degrees C. The latter compound crystallizes in P6(3)/mcm (No. 193) with a = 5.1784(1) and c = 10.3650(3) Angstrom and is analogous to previously reported Li and Mg compounds. (C) 1997 Academic Press.