Christopher S. Reuter, M. Mangir Murshed, Michael Fischer, Thorsten. M. Gesing 

Zeitschrift für Kristallographie - Crystalline Materials (2024)

https://doi.org/10.1515/zkri-2024-0088

The crystal structures of both Pb₂PbO₄ (Pb₃O₄) and Pb₂SnO₄ at room temperature can be described using mullite-type setting in the space groups P4₂/mbc and Pbam, respectively. At what chemical extend the crystal structure prefers either of the space groups would be an excellent playground in the Pb₂(Pb_1−xSn _x )O₄ solid solution. Members of the solid solutions have been prepared by solid-state reactions carried out in sealed quartz tubes. Each sample has been found to be phase pure confirmed by X-ray powder diffraction data Rietveld refinement. Samples with higher tin content require higher synthesis temperatures, and controlled decomposition of Pb₃O₄ serves as the source for both Pb²⁺ and Pb⁴⁺ cations. Since the Pb⁴⁺ cation is larger than Sn⁴⁺, the MO₆ polyhedral volume decreases with increasing Sn-content. As such, each metric parameter shows a linear trend following Vegard’s rule. The concomitant contraction of the MO₆ octahedra and the high stereo-chemical activity of the 6s² lone electron pairs of lead in the Pb²⁺O₄ distorted pyramid results in symmetry reduction. DFT suggests dynamical instability of the tetragonal Pb₃O₄ while Pb₂SnO₄ keeps orthorhombic symmetry at low temperatures, which agrees well with the experimental findings. The global blue shift of the vibrational mode frequencies is explained by the quasi-harmonic approach. The indirect band-gap linearly increases from 2.1(1) eV (x = 0) to a maximum value of 2.5(1) eV for x = 0.8 followed by a sharp drop towards Pb₂SnO₄. Thermogravimetric analysis demonstrates higher thermal stability with increasing Sn-content, which is explained in terms of higher bond strength of Sn–O than that of Pb–O in the MO₆ octahedra.