Dariusz Wardecki

Crystal and Magnetic Structure in Co-Substituted BiFeO3

Ultra-high-resolution neutron diffraction studies of BiFe(0.8)Co(0.2)O3 show a transition from a cycloidal space modulated spin structure at T = 10 K to a collinear G-type antiferromagnetic structure at T = 120 K. The model of antiparallel directions of Fe(3+) and Co(3+) magnetic moments at the shared Wyckoff position describes well the observed neutron diffraction intensities. On heating above RT, the crystal structure of BiFe(0.8)Co(0.2)O3 changes from a rhombohedral R3c to a monoclinic Cm. At 573 K only the Cm phase is present. The collinear C-type antiferromagnetic structure is present in the Cm phase of BiFe(0.8)Co(0.2)O3 at RT after annealing.

Structure Characterization and Properties of K-Containing Copper Hexacyanoferrate

Copper hexacyanoferrate, Cu(II)[Fe(III)(CN)6]2/3·nH2O, was synthesized, and varied amounts of K(+) ions were inserted via reduction by K2S2O3 (aq). Ideally, the reaction can be written as Cu(II)[Fe(III)(CN)6]2/3·nH2O + 2x/3K(+) + 2x/3e(-) ↔ K2x/3Cu(II)[Fe(II)xFe(III)1-x(CN)6]2/3·nH2O. Infrared, Raman, and Mössbauer spectroscopy studies show that Fe(III) is continuously reduced to Fe(II) with increasing x, accompanied by a decrease of the a-axis of the cubic Fm3̅m unit cell. Elemental analysis of K by inductively coupled plasma shows that the insertion only begins when a significant fraction, ∼20% of the Fe(III), has already been reduced. Thermogravimetric analysis shows a fast exchange of water with ambient atmosphere and a total weight loss of ∼26 wt % upon heating to 180 °C, above which the structure starts to decompose. The crystal structures of Cu(II)[Fe(III)(CN)6]2/3·nH2O and K2/3Cu[Fe(CN)6]2/3·nH2O were refined using synchrotron X-ray powder diffraction data. In both, one-third of the Fe(CN)6 groups are vacant, and the octahedron around Cu(II) is completed by water molecules. In the two structures, difference Fourier maps reveal three additional zeolitic water sites (8c, 32f, and 48g) in the center of the cavities formed by the -Cu-N-C-Fe- framework. The K-containing compound shows an increased electron density at two of these sites (32f and 48g), indicating them to be the preferred positions for the K(+) ions.

Internal pressure in annealed biogenic aragonite

The thermal expansion of biomineral aragonite (CaCO3) was studied by in-situsynchrotron radiation (SR) diffraction in both pulverized and non-pulverized samples. The thermal expansion of biogenic aragonite depends on the sample form. Pulverized biomineral aragonite samples show the same thermal expansion as the reference geological aragonite. Biomineral aragonite samples studied en-bloc (non-pulverized) show smaller thermal expansion coefficients as compared with the reference geological aragonite. The observed difference is attributed to the pressure exerted by the heated organic inclusions trapped in the lattice of the biomineral samples studied en-bloc. The estimated pressure values are of the order of 200–350 MPa at temperatures ranging from 400 to 500 °C. The pressure values are different for the main crystallographic directions. The structural phase transition from aragonite to calcite is observed at different temperatures for pulverized biomineral 280 °C, biomineral en-bloc 360 °C and geological 450 °C aragonite samples.

Magnetization of Polycrystalline BiFeO3 in High Magnetic Fields

The magnetization of polycrystalline ceramic BiFeO 3 has been studied with pulsed magnetic fields up to 58 T. We observe a magnetic transition in both, increasing and decreasing magnetic fields. Th...

Dilemma on the crystal structure of CaCu3Ti4O12

The crystal structure of CaCu3Ti4O12 has been studied using high resolution synchrotron radiation based x-ray powder diffraction. The observed x-ray diffraction patterns show pronounced Bragg peak asymmetries which should not be present assuming the commonly accepted cubic crystal structure of CaCu3Ti4O12 described by the space group Im-3. Several structural models are discussed. The first model assumes a coexistence of two phases with the cubic symmetry (both space group Im-3) and different lattice constants. The next models are based on subgroups of the cubic space group Im-3. The best agreement with the experimental data is found for the two-phase cubic model.

Magnetic properties of Ruddlesden-Popper phases Sr3-a : A combined experimental and theoretical investigation

We present a comprehensive study of the magnetic properties of Sr3-xYx(Fe1.25Ni0.75)O-7(-delta )(0 <= x <= 0.75). Experimentally, the magnetic properties are investigated using superconductin ...