Novel Pressure-Induced Transformations of PbTiO3

Abstract

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.

Figures (11)

• Figure 1: (a) Schematic of the DAC laser-heating XRD experiment. (b) XRD patterns of PbTiO$_3$ collected from 17 to 100 GPa at 300 K, with laser heating performed at 87 GPa followed by additional compression. The crystal structure of tetragonal (P4mm) PbTiO$_3$ is shown within the plot for reference.
• Figure 2: (a) XRD pattern of a laser-heated PbTiO$_3$ sample at 87 GPa with Le Bail refinement. (b) XRD pattern of the same sample after decompression to near-ambient pressure. In both plots, the black, red, and blue lines represent the experimental data, fitted profile, and residuals, respectively. Tick marks indicate Bragg reflection positions corresponding to PbTiO$_3$, PbO, and cotunnite-type TiO$_2$ phases.
• Figure 3: (a) $P$–$V$ relations for PbTiO$_3$ and its dissociation products. Curves represent DFT-calculated PbTiO$_3$ phases (I4/mcm, predicted P2$_1$/m), two high-pressure PbO phases (P4/nmm), and cotunnite-type TiO$_2$ (Pnma). Symbols denote experimental data showing coexistence of PbTiO$_3$ with dissociation products. (b) Relative enthalpy as a function of pressure. The P2$_1$/m phase becomes marginally more stable near 84 GPa, and dissociation into $\delta$-PbO and TiO$_2$ becomes energetically favored above $\sim$65 GPa.
• Figure 4: Crystal structures of the three PbO polymorphs. (a), (c), and (e) show the $\alpha$-, $\beta$-, and $\delta$-PbO unit cells, whereas (b), (d), and (f) present the corresponding (010) projections that highlight their contrasting bonding topologies. $\alpha$-PbO forms a three-dimensional framework, $\beta$-PbO exhibits puckered layers, and $\delta$-PbO displays a two-dimensional layered structure. At 87 GPa, Pb–Pb distances are 2.871, 2.925, and 2.996 Å for $\alpha$-, $\beta$-, and $\delta$-PbO, respectively. In $\alpha$-PbO, the Pb–Pb distance (2.871 Å) represents the nearest interatomic spacing within its 3D network, whereas $\beta$-PbO and $\delta$-PbO possess interlayer separations of $\sim$3.5 Å and $\sim$4.2 Å, respectively, between adjacent Pb–O layers.
• Figure 5: Band gaps of $\alpha$-, $\beta$- and $\delta$-PbO from 50–100 GPa, calculated with the DFT-r$^2$SCAN functional. $\alpha$-PbO metallizes near 70 GPa, whereas $\beta$- and $\delta$-PbO remain semiconducting.