Synthesis, characterization and structure-property relations in mullite-type Pb2(Pb1−x Sn x )O4 solid solution

Abstract

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(Pb1−x Sn 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 6s 2 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.