RIETVELD REFINEMENT STUDIES OF NB4N5-RELATED AND NB5N6-RELATED PHASESIN THE (MN)-NB-O-N SYSTEM

The structures of two Nb and two Mn-Nb (oxy)nitrides have been refined from Cu K alpha(1) powder diffractometer data by the Rietveld method and have been characterised by electron diffraction. The phases Nb3.49 N4.56O0.44 and Mn0.54Nb3.07N4.40O0.60 have structures related to that of Nb4N5, an NaCl type with ordered metal vacancies. Refinements in sp ace group I4/m, with R-F=1.3% and 1.1%, respectively, have shown that they are disordered and that metal atoms partially occupy both the nom inally filled 8h position and the nominally empty 2a position in the N b4N5 type. The observed bond lengths indicate that the 2a position is occupied only by Mn atoms in Mn0.54Nb3.07N4.40O0.60. The Nb5N6 type st ructure of Nb-5(N,O)(6) and the related structure of MnNb2(N,O)(3) has been refined in space group P6(3)/mcm to R-F=1.5% and 2.9%, respectiv ely, in the latter, the trigonal prism sites 6g are found to be occupi ed only by Nb, the octahedral 4d sites by approximately equal amounts Mn and Nb and the octahedral 2a site, empty in the Nb5N6 type structur e, only by Mn. The Nb (oxy)nitride structures exhibit short Nb-Nb dist ances which indicates localised bonding between Nb atoms. Magnetic sus ceptibility and electrical conductivity measurements of the tetragonal phases at 15-300 K show, respectively, small magnetic moments which a n coupled in an antiferromagnetic manner at low temperatures and poor, nearly temperature-independent, conductivities. (C) 1998 Elsevier Sci ence S.A. All rights reserved.