On the stability of calcium and cadmium based Ruddlesden-Popper and double perovskite structures

This paper presents ab initio calculations on stability properties and phase diagrams of Ca- and Cd-related hybrid improper ferroelectric materials, with the Ruddlesden-Popper (RP) and double perovskite (DP) structures. The results show that enthalpy relations favor the decomposition of Cd-based RP compounds, indicating that experimental synthesis may be nontrivial. Thus, pure Cd-related RP compounds are not stable. The RP Ca3B2O7 (B = Ti, Mn) systems, however, admit the incorporation of certain percentages of Cd replacing Ca, providing a possible route for tuning ferroelectric properties through chemical substitution. In addition, the stability results indicate that the Ca2TiO4 RP is not stable, in agreement with previous investigations. Searching for alternative stable phases, we found that the forsterite structure might be stable at room temperature and a new structure for this material. Moreover, the results reveal that the Cd-based DP CaCdTi2O6 is dynamically stable, therefore, a possible new hybrid improper ferroelectric.