Structure Dependent Phase Stability and Thermal Expansion of Ruddlesden-Popper Strontium Titanates

Ruddlesden-Popper strontium titanates with composition Sr(n+1)Ti(n)O3(n+1),RP-n-STO, n = 1, 2, ..., infinity, are prototypical layered perovskites from which many crystal-chemistry principles can be assessed. In addition, their phase stabilities and lattice thermal expansions are essential factors for their usage in realistic devices. In this work, we use first-principles electronic structure methods to determine the n-layer dependent phase stabilities, including thermodynamic energies and pressure-critical temperature (P-T-c) phase diagrams and coefficients of thermal expansion. We correlate these properties with the perovskite (SrTiO3)(n).SrO block thickness, n, to show that a change in materials behavior occurs at a critical thickness n(c) = 3, which we ascribe to the presence (absence) of octahedral layers exhibiting antiferrodistortions at n >= 3 (n < 3). Last, we attribute the absence of quasi-two-dimensional lattice anharmonicity in the layered RP titanates to the stiff (SrTiO3)(n) intrablock Ti-O bonds and strong interblock bonding. The layer dependent properties studied here will be useful for the design and application of related quasi-2D materials and devices.