Ian D. Fawcett

Synthesis, structure and properties of the oxynitrides SrWO2N and SrMoO2.5N0.5

Two oxynitrides, SrWO2N and SrMoO2.5N0.5, were synthesized by the ammonolysis of SrMO4 (M = W, Mo) at 900 and 800 °C, respectively. Both oxynitrides adopt the cubic perovskite structure (space group Pm3m) with a = 3.97951(9) A for SrWO2N and a = 3.9773(1) A for SrMoO2.5N0.5. The M(O,N) and Sr(O,N) bond lengths, determined from the Rietveld refinement of the powder X-ray diffraction data, are in good agreement with those reported for other oxynitrides with the perovskite structure. Nearly temperature independent electrical resistivities were observed for both compounds. The magnetic susceptibility measurements indicated Pauli-paramagnetic behavior in both cases. A sharp anomaly observed in the magnetization data of SrMoO2.5N0.5 at ~50 K has been attributed to a spin-glass-like transition.

Large magnetoresistance in double perovskite Sr2Cr1.2Mo0.8O6-δ

Abstract Sr 2 Cr 1.2 Mo 0.8 O 6-δ (δ=0, 0.2) with a double perovskite structure was prepared by solid state reaction in evacuated quartz tubes. Cr and Mo are partially ordered on the B site. Oxygen vacancies decrease the ordering, but increase the lattice parameters. X-ray absorption spectroscopy results are consistent with Cr being 3+, and Mo being close to 5+ for δ = 0.2 and 5.5 for δ = 0. The spin of Cr 3+ (d 3 ) and Mo 5+ (d 1 ) order in an anti-parallel arrangement by superexchange interaction, and lead to ferrimagnetic ordering below 465 K. Both compounds are n-type narrow gap semiconductors. Large magnetoresistance (-43%) is observed in Sr 2 Cr 1.2 Mo 0.8 O 6 . The MR behavior is attributed to an intra-grain tunneling mechanism.

Properties of the perovskites, SrMn1−xFexO3−δ (x=1/3, 1/2, 2/3)

Abstract The SrMn1−xFexO3−δ (x=1/3, 1/2, 2/3) phases have been prepared and are shown by powder X-ray and neutron (for x=1/2) diffraction to adopt an ideal cubic perovskite structure with a disordered distribution of transition-metal cations over the six-coordinate B-site. Due to synthesis in air, the phases are oxygen deficient and formally contain both Fe3+ and Fe4+. Magnetic susceptibility data show an antiferromagnetic transition at 180 and 140 K for x=1/3 and 1/2, respectively and a spin-glass transition at 5, 25, 45 K for x=1/3, 1/2 and 2/3, respectively. The magnetic properties are explained in terms of super-exchange interactions between Mn4+, Fe(4+δ)+ and Fe(3+e)+. The XAS results for the Mn-sites in these compounds indicate small Mn-valence changes, however, the Mn-pre-edge spectra indicate increased localization of the Mn-eg orbitals with Fe substitution. The Mossbauer results show the distinct two-site Fe(3+e)+/Fe(4+δ)+ disproportionation in the Mn- substituted materials with strong covalency effects at both sites. This disproportionation is a very concrete reflection of a localization of the Fe-d states due to the Mn-substitution.

Investigations of Sr3Fe2−xMnxO7−δ the n=2 Ruddlesden–Popper phases with d3/d4 interactions

Abstract We have prepared a series of Mn substituted strontium ferrates, Sr 3 Fe 2− x Mn x O 7− δ where x =2/3, 1 and 4/3, and investigated their properties with X-ray diffraction, X-ray absorption near edge spectroscopy (XANES), Mossbauer spectroscopy, variable-temperature resistivity and magnetic susceptibility. These compounds are metastable and have to be prepared at high temperatures, >1250°C, with solid state techniques followed by quenching in air to room temperature. As a consequence the compounds are oxygen deficient and contain some Fe 3+ . They are insulators and exhibit spin-glass like behavior at low temperatures due to frustrated magnetic interactions between the disordered array of transition metal ions on the B site. Mossbauer and XANES spectra show that the B cations are not fully oxidized and that the Fe 4+ in these compounds are charge-disproportionated into Fe 3+ and Fe 5+ at low temperature.

Magnetic and electrical properties of Eu2VO4

Abstract Eu 2 VO 4 with the K 2 NiF 4 -type structure was prepared by heating Eu 2 O 3 , V 2 O 5 , and V metal at 1473 K in an evacuated silica ampoule. Rietveld refinement of the powder X-ray diffraction (XRD) data confirmed the presence of layers of corner-shared VO 6 octahedra with V–O bond distances of 1.928(1) A (× 4) and 2.04(2) A (× 2) perpendicular to the tetragonal c axis. X-ray absorption near-edge spectroscopy (XANES) indicated that the Eu is mixed-valent [Eu(II)/Eu(III)], while vanadium exists exclusively in the trivalent state. These valence assignments were supported by the results of temperature-dependent magnetic susceptibility and Mossbauer spectroscopy data. Variable temperature electrical resistivity measurements indicated that the sample was insulating (ρ RT ≈ 10 4 Ω-cm). The compound undergoes a charge-ordering transition associated with the Eu sublattice at 445 K, as evidenced by Mossbauer spectroscopy, electrical resistivity, and in situ high temperature X-ray diffraction studies.