Rietveld refinement studies of Nb 4N 5- and Nb 5N 6-related phases in the (Mn)–Nb–O–N system

Abstract

The structures of two Nb and two Mn–Nb (oxy)nitrides have been refined from Cu Kα 1 powder diffractometer data by the Rietveld method and have been characterised by electron diffraction. The phases Nb 3.49N 4.56O 0.44 and Mn 0.54Nb 3.07N 4.40O 0.60 have structures related to that of Nb 4N 5, an NaCl type with ordered metal vacancies. Refinements in space 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 nominally filled 8h position and the nominally empty 2a position in the Nb 4N 5 type. The observed bond lengths indicate that the 2a position is occupied only by Mn atoms in Mn 0.54Nb 3.07N 4.40O 0.60. The Nb 5N 6 type structure of Nb 5(N,O) 6 and the related structure of MnNb 2(N,O) 3 has been refined in space group P6 3/ mcm to R F=1.5% and 2.9%, respectively. In the latter, the trigonal prism sites 6g are found to be occupied only by Nb, the octahedral 4d sites by approximately equal amounts Mn and Nb and the octahedral 2a site, empty in the Nb 5N 6 type structure, only by Mn. The Nb (oxy)nitride structures exhibit short Nb–Nb distances which indicates localised bonding between Nb atoms. Magnetic susceptibility and electrical conductivity measurements of the tetragonal phases at 15–300 K show, respectively, small magnetic moments which are coupled in an antiferromagnetic manner at low temperatures and poor, nearly temperature-independent, conductivities.