PHASES IN THE MN-NB-N-O SYSTEM, FORMED BY AMMONOLYSIS OF MIXTURES OF MN ACETATE TETRAHYDRATE AND A NB XEROGEL

Phase formation in the system Mn-Nb-O-N has been studied by ammonolysi s of mixtures of Mn acetate tetrahydrate and an Nb xerogel at 600-900 degrees C. Observed phases were characterized by powder X-ray diffract ion, scanning and transmission electron microscopy, combustion analysi s, and thermogravimetric analysis. An Nb4N5-related phase, with a meta l content of less than or equal to 15 at% Mn, was obtained pure at 650 -700 degrees C. A new hexagonal phase MnNb2(N,O)(3) was obtained at 75 0-900 degrees C, Its structure is related to, but not isotypic with, t hat of Nb5N6. In addition, the preparations contained cubic NaCl-type (Mn, Nb)(N,O) phases. The Mn-free Nb4-y(N,O)(5) and Nb5-y(N,O)(6) (y s ignifying metal vacancies), with Nb4N5- and Nb5N6-type structures, res pectively, were synthesized, to make possible structural comparisons w ith the Mn-containing phases. A black trigonal phase Mn-3.66(6)Nb-2.34 (6)(O,N)(9) with a structure closely related to that of Mn4Nb2O9 was o btained at 700 degrees C. (C) 1998 Elsevier Science Ltd.