Thermal expansion in Ruddlesden-Popper and perovskite sulfides

The thermal expansion of materials affects the capability, performance, and lifetime of materials operating in variable thermal conditions. Previously ferroelectric Ca₃Ti₂O₇ with the layered Ruddlesden-Popper (RP) structure (n=2 member of A_n+1B_nO_3n+1) was shown to exhibit pressure-tunable negative thermal expansion (NTE) and a pressure-independent softening of the bulk modulus due to a quasi-two-dimensional vibration [Huang et al, Phys. Rev. Lett. 117, 115901 (2016)], whereas nonpolar RP strontium titanates did not exhibit NTE [Huang et al, Chem. Mater. 30, 7100 (2018)]. Here, we analyze structural lattice dynamical, and thermodynamic properties of d⁰ Ruddlesden-Popper and perovskite sulfides using the self-consistent quasi-harmonic approximation with density functional theory. We relate the thermal expansion coefficient and bulk modulus to the number of octahedral units and type of octahedral connectivity present in the respective materials. This local structure understanding may help design thermal expansion property in other chemistries.