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 ammonolysis of mixtures of Mn acetate tetrahydrate and an Nb xerogel at 600–900°C. Observed phases were characterized by powder X-ray diffraction, scanning and transmission electron microscopy, combustion analysis, and thermogravimetric analysis. An Nb4N5-related phase, with a metal content of ≤15 at% Mn, was obtained pure at 650–700°C. A new hexagonal phase MnNb2(N,O)3 was obtained at 750–900°C. Its structure is related to, but not isotypic with, that 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 signifying metal vacancies), with Nb4N5- and Nb5N6-type structures, respectively, were synthesized, to make possible structural comparisons with the Mn-containing phases. A black trigonal phase Mn3.66(6)Nb2.34(6)(O,N)9 with a structure closely related to that of Mn4Nb2O9 was obtained at 700°C.