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

Ruddlesden–Popper strontium titanates with composition Srn+1TinO3n+1, RP-n-STO, n = 1, 2, ..., ∞, 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–Tc) 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 nc = 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.