Mariano Curti

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.

Liebau density vector: a new approach to characterize lone electron pairs in mullite-type materials

Abstract The bismuth 6s2 lone electron pair (LEP) in mullite-type Bi2M4O9/10 (where M = Al, Fe, or Mn) was characterized by means of several parameters derived from experimental and theoretical calculations. The Wang-Liebau eccentricity (WLE) parameter proved to be very useful to quantify the stereochemical activity of the LEPs. Calculations of electronic distributions (three-dimensional charge density difference isosurfaces) were used as independent measurements, which validated the relevance of the WLE parameter for the characterization and quantification of LEPs. The distribution of the Bi 6s2 electrons around the nucleus was evaluated and the maximum of electron density calculated. The spatial orientation of this electron density with respect to the nucleus is expressed as “Liebau density vector”. Therefore, this vector is ascribed to be a key result of this work as a proof that the purely geometrically defined Wang-Liebau vector indeed points towards the maximum electron density of the LEP. The LEP stereochemical activity was studied in terms of the type of structure (Bi2M4O9 or Bi2M4O10) and the nature of M. The effect of exchanging bismuth by lanthanum as well as the relative stabilities of Bi2M4O9 or Bi2M4O10 structures were calculated and discussed.

CHAPTER 3:Current Issues Concerning the Mechanism of Pristine TiO2 Photocatalysis and the Effects on Photonic Crystal Nanostructures

Titanium dioxide (TiO2) photocatalysis is undoubtedly a vast field for fundamental research and technological development. While many attempts are to be commended for enhancing and improving the efficiency, photonic photocatalysis emerges as an innovative method that introduces structural but not chemical modifications. A critical overview of various currently discussed mechanisms in TiO2 photocatalysis is presented here, pointing out numerous open questions and the effects beneficial or not they may have on TiO2 photonic systems.