SYNTHESIS AND STRUCTURE OF SOME MIXED-VALENCY TERNARY TANTALUM NITRIDES WITH LITHIUM AND MAGNESIUM

Li2-xTa2+xN4 (ca. 0.2 < x < ca. 1) and Mg2.6-xTa1.3+xN4 (ca. 0.3 < x < ca. 1) were synthesized by reaction of Li3N, or either Mg3N2 or Mg(NH2)2, with Ta3N5. The structure of Li2-xTa2+xN4 was determined by single-crystal and that of Mg2.6-xTa1.3+xN4 by powder X-ray diffraction. The crystals of both compounds are cubic (Fm3m; Z = 1; a almost-equal-to 4.31 angstrom (Li2-xTa2+xN4) or a almost-equal-to 4.36 angstrom (Mg2.6-xTa1.3+xN4)) with an NaCl structure (cations disordered). Li2-xTa2+xN4 (x = 0.4) can be described as a disordered high temperature phase of LiTa3N5. Experiments aimed at attaining the tantalum-rich phase boundary for Li2-xTa2+xN4 with x = 1 (LiTa3N4) and for Mg2.6-xTa1.3+xN4 with x = 1.6BAR (MgTa3N4) produced the new hexagonal phases Li1-xTa3+xN4 (ca. 0.0 less-than-or-equal-to x less-than-or-equal-to ca. 0.02; P6(3)/mcm; Z = 3 a = 5.180 angstrom; c = 10.343 angstrom (for x = 0.02)) and Mg1-xTa2+xN3 (ca. 0.0 less-than-or-equal-to x less-than-or-equal-to ca. 0.06; P6(3)/mcm; Z = 4; a = 5.205 angstrom; c = 10.425 angstrom (for x = 0.06)). Powder X-ray and neutron diffraction data were used for the structure determination. The structures are of a filled up 2H-MoS2 type and contain (TaN2)3- layers between which Li+ and Ta5+ or Mg2+ and Ta5+ occupy octahedral holes surrounded by nitrogen.