First principles investigation of ferroelectricity in epitaxially strained Pb$_2$TiO$_4$

TL;DR

This study uses density-functional theory to explore the structure and ferroelectric properties of the hypothetical Pb$_2$TiO$_4$ compound under epitaxial strain, revealing multiple ferroelectric phases.

Contribution

It provides the first theoretical investigation of ferroelectricity in Pb$_2$TiO$_4$ and predicts strain-induced phase transitions.

Findings

Unstable phonon leads to ferroelectric $I2mm$ phase.

Epitaxial strain induces additional ferroelectric phases.

Multiple ferroelectric phases are predicted under different strains.

Abstract

The structure and polarization of the as-yet hypothetical Ruddlesden-Popper compound Pb$_2$TiO$_4$ are investigated within density-functional theory. Zone enter phonons of the high-symmetry K$_2$NiF$_4$-type reference structure, space group $I4/mmm$, were calculated. At the theoretical ground-state lattice constants, there is one unstable infrared-active phonon. This phonon freezes in to give the $I2mm$ ferroelectric state. As a function of epitaxial strain, two additional ferroelectric phases are found, with space groups $I4mm$ and $F2mm$ at compressive and tensile strains, respectively.