SYNTHESIS, STRUCTURE AND MAGNETIC-SUSCEPTIBILITY OF THE OXYNITRIDE SPINEL MN-2(MNTA3)N6-DELTA-O2+DELTA, O-LESS-THAN-OR-EQUAL-TO-DELTA-LESS-THAN-OR-EQUAL-TO-1

The oxynitride spinel Mn-2(MnTa3)N6-deltaO2+delta, with 0 less than or equal to delta less than or equal to 1, has been synthesized at 1175 K by ammonolysis of a mixture of a Ta-containing xerogel and Mn(OAc)(2) . 4H(2)O. The N content was determined by combustion analysis and thermogravimetric oxidation, yielding a composition confined between Mn-2(MnTa3)N6O2 (delta = 0) and Mn-2(MnTa3)N5O3 (delta = 1). The structure is cubic, with space group Fd3m and a = 8.8353(3) Angstrom. It was refined using the Rietveld technique and neutron powder diffraction data collected at room temperature and 15 K, to R(F) = 2.9 and 3.8%, respectively. The tetrahedral sites are occupied only by Mn atoms and the octahedral sites statistically by 25% Mn and 75% Ta atoms. The N and O atoms are randomly distributed over the anion sites. The magnetic susceptibility exhibits a maximum at 29 K and a Curie-Weiss behavior at higher temperatures with theta(a) = -250(20) K and mu(eff) = 5.7(2) Bohr magnetons per Mn atom. The neutron powder diffraction data collected at 15 K showed no evidence of magnetic ordering. A NaCl-type phase with a = 4.4382(2) Angstrom and tentative composition Mn0.8Ta0.2(O,N) was observed in preparations at 1175 K. A hexagonal Mn4Ta2(O,N)(x) phase with cell dimensions a = 5.3024(4) Angstrom, c = 14.493(2) Angstrom was obtained at 973 K. (C) 1995 Academic Press, Inc.