Exotic phenomenon in un-conventional Perovskites
Ferroelectricity and related multi-ferro order, lattice modes coupling in conventional inorganic perovskites is an old topic that have been studied for nearly a century. When the atoms in perovskite are replaced by some organic molecules, it forms the so-called in-organic-organic hybrid perovskite. It is a new group of materials that attract intensive attend mainly due to its high performance of photo-voltage effect. The introduction of molecules into the conventional inorganic perovskite frame work breaks the original symmetry and, definitely, brings additional degrees of freedom.
Several new degrees of freedom are relevant, i> the molecular itself e.g. dipoles, quadrupole, hexapole and even five-folded symmetry that is excluded in the conventional inorganic crystal, ii> the symmetry of the molecular's sub-lattice which can be either central symmetric or not. iii> orders of the in-organic perovskite frame work, e.g. displacive polar distortion and octahedral tilting. It can be expected that the coorperation between these degree of freedom will bring exotic physics in materials.
