Synthesis and physical properties study on mixed metal oxynitrides

Abstract

Mixed metal oxynitrides have attracted attention due to their interesting chemical and physical properties in the past twenty years. In this thesis, four series of mixed metal oxynitrides have been investigated. The samples have been synthesized by both thermal ammonolysis and high pressure high temperature methods. The structural exploration covers perovskite, scheelite and pyrochlore types. The structural studies were carried out using powder X-ray and neutron diffraction, and magnetic and conducting properties have been explored. A series of new RZrO2N (R = Pr, Nd and Sm) perovskites were synthesized using high pressure high temperature methods (HPHT) via a direct solid state reaction of R2O3 with Zr2ON2. All three new phases crystallize in the orthorhombic Pnma perovskite superstructure, and the structural distortion increases with decreasing R3+ ionic radius. RZrO2N contains both R3+ and d0 Zr4+ and thus shows a potential for multiferroic properties. EuWO1-xN2+x perovskites with a wide range of nitrogen contents (-0.16 ≤ x ≤ 0.46) were synthesized by thermal ammonolysis of an oxide precursor Eu2W2O9. Ferromagnetic ordering below a Curie temperature TC =12 ± 1 K and negative colossal magnetoresistances (CMR) have been discovered in these samples. In particular, for the lowest doped sample, EuWO0.96N2.04, CMR ≥ 99.7% was observed at 7 K. The possibility of tuning the physical properties by altering the chemical composition has been demonstrated. A linear relationship between the lattice parameter and nitrogen content of EuWO1+xN2-x was observed. An investigation has been made of the Eu-Mo-O-N system. A new pyrochlore oxynitride series Eu2Mo2O6-xN2+2x/3 (0.20 ≤ x ≤ 2.25) was synthesized by ammonolysis of Eu2Mo2O7. A ferrimagnetic ordering and semiconducting behavior has been observed in these samples. A detailed structural study of SrMO2N (M = Nb, Ta) has been performed using variable temperature neutron and electron diffraction. Partial anion order has been observed in both samples up to 750 oC. It is consistent with cis-ordering of the two nitrides in each MO4N2 octahedron. At low temperatures, this order directs the tilting of the octahedron to form a pseudo-tetragonal superstructure. It creates zig-zag MN chains in two or three dimensions within the lattice. This principle can be used to predict the local structures of perovskite-related oxynitrides AMO3-xNx.