Abid Berghout

The local structure of apatite-type lanthanum silicates of general formula La_9.33+x(SiO₄)₆O_2+3x/2 has been investigated by combining the atomic pair distribution function (PDF) method, conventional X-ray and neutron powder diffraction (NPD) data and density functional theory (DFT) calculations. DFT was used to build structure models with stable positions of excess oxide ions within the conduction channel. Two stable interstitial positions were obtained in accordance with literature, the first one located at the very periphery of the conduction channel, neighbouring the SiO₄ tetrahedral units, and the second one closer to the channel axis. The corresponding PDFs and average structures were then calculated and tested against experimental PDFs obtained by X-ray total scattering and NPD Rietveld refinements results gathered from literature. It was shown that of the two stable interstitial positions obtained with DFT only the second one located within the channel is consistent with experimental data. This result consolidates one of the two main conduction mechanisms along the c-axis reported in the literature, namely the one involving cooperative movement of O4 and Oi ions.

We successfully prepared the polycrystalline lanthanum germanate oxyapatite (LGO) by the reactive diffusion between solid La₂GeO₅ and gases [GeO + 1/2O₂] during isothermal heating at 1573–1723 K for 2–50 h. The reaction products were investigated by polarizing-microscopy for microtexture characterization, Raman spectroscopy for phase identification, and X-ray diffractometry for determination of the {00l}_LGO texture fractions. The LGO polycrystals were highly c*-axis-oriented, and hence the Lotgering factor f_00l ranged, regardless of annealing temperatures, between 0.94 and 0.99. The thickness (l/m) of LGO layer steadily increased with the increase of annealing time (t/s) in accordance with a parabolic relationship of l = [exp(−1.502 × 10⁴/T − 5.233)] × t^1/2. The growth of LGO layer was predominantly controlled by volume diffusion of GeO₂ component, with the activation energy being ca. 125 kJ mol⁻¹.

This article derives the interatomic potential for the TeO₂ system and presents the first results of molecular dynamics simulations of the pure TeO₂ structure.