Role of dimensionality on thermodynamic properties in layered calcium titanates

Negative thermal expansion of materials is important for applications such as the fabrication and operation of miniature electronic devices and optical systems. Previously, it was shown that Ca₃Ti₂O₇ with the layered Rudlesden-Popper (RP) structure (n=2 member of A_n+1B_nO_3n+1) exhibits pressure-tunable negative thermal expansion and pressure-independent softening of the bulk modulus due to a quasi-two-dimensional vibration [Huang et al, Phys. Rev. Lett. 117, 115901 (2016)]. Here, we evaluate the structural, lattice dynamical, and thermodynamic properties with layer thickness n=1,2,3, and ∞, in the calcium titanate family using the self-consistent quasi-harmonic approximation within density functional theory. This allows us to formulate layer thickness dependent models of the anharmonic lattice properties that may be used for property design in other chemistries.