Computer modeling of molecular perovskites [(C3H7)3(CH3)N]M(C2N3)3 (M = transition metal): tilt-and-shift polymorphism and vibrational/mechanical properties

Molecular perovskites have recently gained attention in the field of ferroelectrics, multiferroics and mechanocalorics. The incorporation of molecular building blocks in the 3D ReO3-type network of the perovskite structure leads to new geometric degrees of freedom, enabling the formation of polymorphic perovskite phases with different tilt and shift systems that are close in energy, i.e. tilt and shift polymorphs. We discuss the series of molecular perovskites [(C3H7)3(CH3)N]M(C2N3)3, where different polymorphs crystallise in the perovskite structure but with different tilt systems depending on the synthetic conditions. Our computer simulations show that for the whole stability temperature range the rhombohedral polymorph is the thermodynamically more stable phase than the orthorhombic phase. The absence of imaginary modes in both phases suggests that the transformation goes through a high-energy transition state as underlying mechanism of the irreversible phase transition. Given that the bulk moduli (B) characterises the suitability of these materials for barocaloric applications, we calculated B for a wider range of compositions (M=Mn, Co, Fe, Ni, Zn, Cd, Ba, Sr, Ca, Hg, or Mg), and discuss the geometric factors determining the mechanical properties.