Hydrogen-bond-mediated structural variation of metal guanidinium formate hybrid perovskites with unit cell volume.

The hybrid perovskites are coordination frameworks with the same topology as the inorganic perovskites, but with properties driven by different chemistry, including host-framework hydrogen bonding. Like the inorganic perovskites, these materials exhibit many different phases, including structures with potentially exploitable functionality. However, far less is known about their behaviour under pressure. We have studied the structures of of manganese and cobalt guanidinium formate under pressure using single-crystal X-ray and powder neutron diffraction. Remarkably, when pressure reduces these materials' volume, they transform to a phase isostructural to cadmium guanidinium formate, which has an larger volume. Using DFT calculations, we show that this counterintuitive behaviour depends on the hydrogen-bonded network of guanidinium ions, which act as struts protecting the metal formate framework against compression. Our results demonstrate more generally that engineering desirable crystal structures in the hybrid perovskites will depend on achieving suitable host-guest hydrogen-bonding geometries.