Christopher S. Reuter, Lars Robben, M. Mangir Murshed, Thorsten M. Gesing

Journal of Solid State Chemistry 359 (2026): 126008

https://doi.org/10.1016/j.jssc.2026.126008

Each member of the mullite-type Pb₂(Pb_1-xSn_x)O₄ solid-solution can be described with the schafarzikite structure type. The endmember Pb₂PbO₄ (x = 0) crystallizes in space group P4₂/mbc at ambient conditions, whereas the orthorhombic subgroup Pbam is needed for the structural description of the phases for x ≥ 0.3. On cooling, Pb₂PbO₄ undergoes a P4₂/mbc → Pbam phase transition between 171(1) K and 174(4) K confirmed by FTIR and Raman data analysis, respectively. On heating, the other endmember Pb₂SnO₄ (x = 1) shows the reverse Pbam → P4₂/mbc phase transition at 1221(19) K, confirmed by a Landau-type fit of the lattice parameter difference ∆_ab . During thermogravimetric analysis Pb₂PbO₄ decomposes into PbO and oxygen. However, samples with 0.3 < x < 0.8 undergo an exsolution of PbO along with an enrichment of the Sn-content in the respective mullite-type phases. Due to slow decomposition kinetics several exsolution steps are visible in the temperature-dependent unit-cell volumes for different stochiometric x-values. For all phases, the lattice thermal expansion was modeled using the Debye-Einstein-Anharmonicity-Gliding (DEAG) approach. A single Debye term well describes the internal energy contribution followed by different phase-change properties. The initial phase-change temperatures can be derived from the respective fits of the gliding components. The stoichiometries of the intermediate phases are estimated from the calculated 0K unit-cell volume of the DEAG model. Based on the structure analysis a polyhedral distortion sum was calculated from the weighted octahedral distortion of Pb⁴⁺O₆ and the Wang-Liebau-eccentricity parameter of Pb²⁺O₄E nido-pyramids. Whereas the latter one does not play an important role for the temperature-dependent behavior, the structural distortion originating from the size difference of the cations on the octahedra position is found to be the main reason for the miscibility-gap of the solid solution for 0.0 < x < 0.3, as can be derived from the sum distortion parameters relative to their structural average.