Kristina Lipinska

Structural properties of mullite-type Pb(Al1–xMnx)BO4

Abstract We report on the structural characterization of the mullite-type PbAl1-xMnxBO4 series using neutron, synchrotron and in-house X-ray powder diffraction, Raman spectroscopy and density functional theory (DFT) calculations. The planar geometry of the BO3 group changes only slightly over the whole composition range. The rigid BO3 group plays the dominant roles in the thermal contraction in the a-direction followed by expansion in the b- and c-directions, leading to a correlation a · b/c ~ unity. The unit-cell volume at zero-pressure and 0 K was obtained, as well evaluated as the isothermal bulk-modulus from pressure dependent synchrotron X-ray diffraction using a diamond anvil cell as well as DFT calculations. Thermal expansion of the metric parameters was modeled using a first-order Grüneisen approximation for the zero-pressure equation of state. We used the double-Debye-double-Einstein- Anharmonicity model to calculate the temperature-dependent internal energy of the crystalline end members. The simulation helped to understand the anisotropic thermal expansion and together with the experimental and calculated bulk moduli to approximate the thermodynamic Grüneisen parameters.

New pressure induced phase transitions in mullite-type Bi2(Fe4-xMnx)O10-δ complex oxides

Abstract Single phased mullite-type Bi2Fe4–xMnxO10–δ mixed crystals (0.25 ≤ x ≤ 3.125) and the end-member Bi2Fe4O9, synthesized from the oxides by reaction sintering up to 825°C, were studied at high-pressures in order to probe their high-pressure behavior and any possible structural phase transitions. In-situ synchrotron radiation-based powder X-ray diffraction was carried out in a diamond anvil cell, under quasi-hydrostatic conditions, up to a pressure of about 20 GPa at room temperature for each sample. A pressure-induced phase transition was found in all samples. The transition appeared spread over a pressure range and was not completed at the top investigated pressure. This is the first report of a pressure-induced phase transition in Bi2Fe4–xMnxO10–δ mixed crystals.