Novel Transformations of PbTiO3 with Pressure and Temperature

Technical Summary: Novel Transformations of PbTiO3 with Pressure and Temperature

Problem Statement
The stability and structural evolution of lead titanate (PbTiO3), a prototypical ferroelectric perovskite, under combined high-pressure and high-temperature (P–T) conditions have remained poorly constrained. While extensive research has mapped its behavior under cold compression—identifying transitions from the polar $P4mm$ phase to nonpolar $I4/mcm$, and potentially to a centrosymmetric monoclinic $P21/m$ phase above ~80 GPa—the response of PbTiO3 to simultaneous thermal activation at high pressures was unresolved. Previous theoretical models did not account for the possibility of chemical dissociation, assuming instead that the compound would stabilize into dense post-perovskite structures. The study addresses the unresolved question of whether PbTiO3 remains a stable perovskite or undergoes disproportionation under extreme P–T conditions typical of laser-heated diamond-anvil cell (DAC) experiments.

Methodology
The authors employed a combined experimental and theoretical approach to investigate PbTiO3 up to 100 GPa.

• Experimental: Polycrystalline PbTiO3 samples were subjected to synchrotron X-ray diffraction (XRD) at the European Synchrotron Radiation Facility (ESRF) and the Advanced Photon Source (APS). Experiments included room-temperature compression and laser heating within diamond-anvil cells. A specific protocol involved laser-heating the sample at 87 GPa, followed by XRD characterization after quenching to identify stable products.
• Theoretical: Density Functional Theory (DFT) calculations were performed using the VASP code with the meta-GGA r2SCAN functional. These computations included thermodynamic analysis of competing phases, including potential dissociation products (PbO and TiO2), and electronic structure calculations to determine band gaps and metallization pressures. Crystal structure predictions were assisted by XtalOpt.

Key Results

1. Divergent Pathways based on Temperature:

   • Room-Temperature Compression: PbTiO3 persists in the tetragonal $I4/mcm$ phase up to at least 100 GPa, consistent with previous reports of a nonpolar perovskite configuration. No dissociation was observed even above 65 GPa.
   • High P–T Conditions: Upon laser heating at 87 GPa, PbTiO3 dissociates into PbO and TiO2. Thermodynamic calculations confirm that this decomposition becomes enthalpically favored above ~65 GPa. The unreacted PbTiO3 remaining in the sample (due to one-sided heating) persists in the $I4/mcm$ phase.

2. Discovery of Novel PbO Polymorphs:
   The decomposition products revealed distinct PbO phases:

   • $\alpha$-PbO: A highly compressed form of the known litharge structure ($P4/nmm$) with a $c/a$ ratio inverted from ambient conditions ($c/a < 1$).
   • $\delta$-PbO: A previously unreported polymorph, also in the $P4/nmm$ space group but with a different atomic arrangement (layered configuration) compared to $\alpha$-PbO.
   • Decompression Behavior: Upon releasing pressure, both $\alpha$-PbO and $\delta$-PbO transform into the ambient-pressure stable $\beta$-PbO (massicot) phase.
   • TiO2: No diffraction peaks for TiO2 were detected, likely due to amorphization during the laser-heated decomposition, a phenomenon consistent with prior observations in nanoscale TiO2.

3. Electronic Properties:
   First-principles calculations predict distinct electronic behaviors for the PbO polymorphs under pressure:

   • $\alpha$-PbO: Undergoes pressure-induced band-gap closure, metallizing above ~70 GPa.
   • $\delta$-PbO and $\beta$-PbO: Remain semiconducting with band gaps above 1 eV even at megabar pressures. The persistence of the gap in $\delta$-PbO is attributed to its larger Pb–Pb separations and layered framework compared to the denser 3D network of $\alpha$-PbO.

Significance and Claims
The paper claims to reveal an "unanticipated dimension" of PbTiO3 behavior, demonstrating that distinct equilibrium and metastable phases can be stabilized along different P–T synthesis paths.

• Thermodynamic vs. Kinetic Control: The study highlights that while PbTiO3 is kinetically stable as a perovskite under cold compression, high temperature enables diffusion and reaction, driving the system toward thermodynamic equilibrium via disproportionation into simple oxides.
• Novelty of $\delta$-PbO: The identification of the $\delta$-PbO polymorph expands the known structural diversity of lead oxide, showing pressure-induced polytypism and structural flexibility.
• Paradigm Shift: The findings challenge the modern view that complex oxides universally stabilize into dense perovskite or post-perovskite structures under high pressure. Instead, PbTiO3 follows an earlier paradigm where complex oxides disproportionate into simple close-packed oxide phases under sufficiently high P–T conditions.
• Synthesis Control: The results underscore the utility of temperature as a control parameter for accessing specific kinetic pathways, allowing for the synthesis of novel phases (like $\delta$-PbO) that are not accessible through cold compression alone.

The authors conclude that these results expand the understanding of the chemistry and diverse properties of oxide perovskites, specifically illustrating how temperature dictates whether a system follows a metastable structural transition pathway or an equilibrium chemical decomposition pathway.