Covalency is Frustrating: La ₂ Sn ₂ O ₇ and the Nature of Bonding in Pyrochlores under High Pressure-Temperature Conditions

Natural specimens of the pyrochlore (A ₂ B ₂ O ₇ ) compounds have been found to retain foreign actinide impurities within their parent framework, undergoing metamictization to a fully amorphous state. The response to radionuclide decay identifies pyrochlore systems with having high radiation tolerance and tailored use in radioactive waste applications and radionuclide sequestration. High pressure is a powerful pathway to high density states and amorphization with parallels to radiation-induced processes. Here, La ₂ Sn ₂ O ₇ is evaluated under extreme conditions via the combination of laser heating in a diamond anvil cell with X-ray diffraction and Raman spectroscopy. The measurements are supported by ab initio random structure searching and molecular dynamics calculations. A new ground state at 70 GPa is revealed, and high temperature annealing is fundamental to access its crystalline ground state and fully determine the structure. This crystalline phase (P2 ₁ /c) retains its structural integrity during decompression and is fully recoverable to ambient conditions. The final state of the system is shown to be highly pathway dependent due to the covalent nature of the Sn-O bonding. The Tc pyrochlore, La ₂ Tc ₂ O ₇ , is analyzed for similarities in the bonding to determine the likelihood of an analogous pathway dependency to a final state.