Th.M. Gesing

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.

Chlorine-free, monolithic lanthanide series rare earth oxide aerogels via epoxide-assisted sol-gel method

AbstractSynthesis of chlorine-free, rare earth oxide aerogels from the lanthanide series was achieved using a modified epoxide-assisted sol-gel method. An ethanolic solution of the hydrated metal nitrate, propylene oxide, and ammonium carbonate was found to gel upon heating to 333 K. Critical point drying of the wet gel in CO2 yielded monolithic aerogels. Most of the aerogels were amorphous as-prepared, but became nano-crystalline after calcination at 923 K in air. The aerogels had high surface areas (up to 150 m2/g), low densities (40–225 mg/cm3), and were photoluminescent. HighlightsRare earth oxide aerogels were prepared by epoxide-assisted sol-gel route.Rare earth oxide aerogels are monolithic, chlorine-free, and possess large surface areas.Calcination at 923 K results in nano-crystalline aerogels, with particles less than 25 nm in diameter.Characterization of these aerogels includes photoluminescence spectroscopy, Rietveld refinements, and electron microscopy.