A NEW CONDUCTING TERNARY NITRIDE - NAXTA3N5 LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1.4)

Topotactic insertion of sodium into the channel structure of Ta3N5 has been accomplished by the reaction between Na and Ta3N5 at temperature s between 450 acid 600 degrees C in sealed Ni tubes. The limiting stoi chiometry 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 star ting 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 f ramework, The weak temperature dependence of the magnetic susceptibili ty 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 meta llic conductor, This is supported by the moderate conduction of an uns intered 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 bet ween 600 and 800 degrees C and a partly characterized hexagonal phase that results from nitrogen loss, Na1-xTa3+xN4 (x approximate to 0), wh ich forms at temperatures above 800 degrees C. The latter compound cry stallizes 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.