Novel Transformations of PbTiO3 with Pressure and Temperature

We investigated the behavior of lead titanate (PbTiO3) up to 100 GPa, both at room temperature and upon laser heating, using synchrotron X ray diffraction combined with density functional theory (DFT) computations. At the high pressure temperature (PT) conditions produced in laser heated diamond anvil cells, PbTiO3 dissociates into PbO and TiO2, consistent with our DFT computations showing that decomposition becomes enthalpically favored above 65 GPa. In contrast, on room temperature compression, PbTiO3 persists in the tetragonal I4mcm phase up to at least 100 GPa. Laser heating produces distinct PbO phases: a compressed form of alpha PbO and a previously unreported delta PbO polymorph, both of which transform to beta PbO on decompression. The calculations predict that alpha PbO undergoes pressure-induced band gap closure, metallizing above 70 GPa, whereas the delta and beta phases remain semiconducting with a band gap above 1 eV even at megabar pressures. The experimental and confirming theoretical results reveal an unanticipated dimension of the behavior of PbTiO3, showing that distinct equilibrium and metastable phases can be stabilized along different PT synthesis paths.

💡 Research Summary

This study investigates the structural and chemical response of lead titanate (PbTiO₃) under extreme pressure and temperature conditions, combining synchrotron X‑ray diffraction (XRD) experiments in diamond‑anvil cells (DACs) with density functional theory (DFT) calculations. Experiments were performed at the European Synchrotron Radiation Facility (ESRF) and the Advanced Photon Source (APS), covering a pressure range from 17 GPa to 100 GPa. Two distinct pathways were explored: (i) cold compression at room temperature and (ii) laser heating to approximately 1400 K at 87 GPa followed by rapid quenching.

In the cold‑compression series, PbTiO₃ retains a non‑polar tetragonal I4/mcm perovskite structure throughout the entire pressure range, showing no evidence of the previously predicted transition to a monoclinic P2₁/m phase. Lattice parameters evolve smoothly with pressure and match the equation of state derived from DFT calculations using the meta‑GGA r²SCAN functional. This persistence indicates that, in the absence of thermal activation, the perovskite framework of PbTiO₃ is remarkably robust up to megabar pressures.

In contrast, the laser‑heated experiments reveal a fundamentally different behavior. After heating at 87 GPa, the XRD patterns display additional reflections that cannot be indexed to the parent perovskite. Le Bail refinements identify two lead‑oxide (PbO) polymorphs co‑existing with unreacted PbTiO₃: (a) a highly compressed form of the known α‑PbO (litharge, space group P4/nmm) with a c/a ratio less than unity, and (b) a previously unreported δ‑PbO phase, also P4/nmm, but with oxygen occupying the 2a Wyckoff position, generating a two‑dimensional layered network. Upon decompression to near‑ambient pressure, both α‑PbO and δ‑PbO transform into the orthorhombic β‑PbO (massicot, Pbcm). No distinct TiO₂ diffraction peaks are observed; theoretical calculations predict cotunnite‑type TiO₂ (Pnma) as the most stable high‑pressure polymorph, but its absence is attributed to amorphization or peak overlap with the PbO reflections.

Thermodynamic analysis incorporating the enthalpies of PbTiO₃, α‑PbO, δ‑PbO, and cotunnite‑TiO₂ shows that the decomposition reaction

PbTiO₃ → PbO + TiO₂

becomes energetically favorable above ~65 GPa. This pressure threshold aligns with the experimental observation that decomposition only occurs when the sample is heated, underscoring the kinetic role of temperature in enabling diffusion and chemical reaction. The previously proposed P2₁/m post‑perovskite phase is only marginally more stable than I4/mcm near 84 GPa, but the decomposition pathway overtakes it energetically, explaining why the transition is not observed experimentally under the studied conditions.

Electronic‑structure calculations reveal distinct pressure‑dependent band‑gap behaviors among the PbO polymorphs. The compressed α‑PbO undergoes a rapid band‑gap closure, metallizing near 70 GPa due to enhanced Pb–Pb interactions within its three‑dimensional framework. In contrast, δ‑PbO and β‑PbO retain semiconducting gaps of 1.4–1.9 eV even at 100 GPa, reflecting their more open, layered or puckered structures that keep Pb–Pb distances larger and electronic overlap weaker. These findings illustrate how subtle changes in crystal topology can dramatically affect electronic properties under pressure.

The authors place their results in the broader context of perovskite oxides. While many perovskites (e.g., KNbO₃, CaTiO₃, MgSiO₃) either undergo a series of symmetry‑lowering transitions or transform into post‑perovskite structures at high pressure, PbTiO₃ displays a bifurcated response: a temperature‑independent pathway that preserves the non‑polar perovskite up to 100 GPa, and a temperature‑activated pathway that leads to chemical disproportionation into simple oxides, producing a novel high‑pressure PbO polymorph. The discovery of δ‑PbO adds a new member to the family of lead‑oxide phases and suggests potential high‑pressure semiconductor applications, while the metallization of α‑PbO at modest pressures could be relevant for designing pressure‑tunable conductors.

In summary, the work demonstrates that (1) PbTiO₃ remains in the I4/mcm phase under cold compression to 100 GPa, (2) laser heating above ~65 GPa triggers decomposition into PbO and TiO₂, (3) the resulting PbO exhibits three polymorphs (α, β, δ) with distinct structural and electronic characteristics, and (4) temperature is a decisive control parameter that can switch the system between a metastable high‑pressure perovskite and an equilibrium decomposition pathway. These insights broaden our understanding of oxide perovskite chemistry under extreme conditions and highlight the importance of pressure‑temperature pathways in synthesizing new high‑pressure materials.