Temperature-dependent properties and phase changes of the mullite-type Pb2(Pb1-xSnx)O4 solid solution

Each member of the mullite-type Pb 2 (Pb 1- x Sn x )O 4 solid-solution can be described with the schafarzikite structure type. The endmember Pb 2 PbO 4 ( x = 0) crystallizes in space group at ambient conditions, whereas the orthorhombic subgroup is needed for the structural description of the phases for x ≥ 0.3. On cooling, Pb 2 PbO 4 undergoes a → phase transition between 171(1) K and 174(4) K confirmed by FTIR and Raman data analysis, respectively. On heating, the other endmember Pb 2 SnO 4 ( x = 1) shows the reverse → phase transition at 1221(19) K, confirmed by a Landau-type fit of the lattice parameter difference . During thermogravimetric analysis Pb 2 PbO 4 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 0 K unit-cell volume of the DEAG model. Based on the structure analysis a polyhedral distortion sum was calculated from the weighted octahedral distortion of Pb 4+ O 6 and the Wang-Liebau-eccentricity parameter of Pb 2+ O 4 E 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. • Phase transformation due to PbO exsolution out of Pb 2 (Pb 1- x Sn x )O 4 solid-solution • Thermal expansion DEAG modeling of phase transformation using G-type anharmonicity • Miscibility gap explained by combination of distortion parameters