Tuneable mechanical and dynamical properties in the ferroelectric perovskite solid solution [NH3NH2](1-x)[ NH3OH](x)Zn(HCOO)(3)

We report how mechanical and dynamical properties in formate-based perovskites can be manipulated by the preparation of an A-site solid-solution. In the series [NH3NH2](1-x)[NH3OH](x)Zn(HCOO)(3) with x(max) = 0.48, the substitution of [NH3NH2](+) by [NH3OH](+) is accompanied by a series of complex changes in crystal chemistry which are analysed using PXRD, SCXRD, H-1 solid state NMR, DSC and nanoindentation. NMR shows increased motion of [NH3NH2](+) in [NH3NH2](0.52)[NH3OH](0.48)Zn(HCOO)(3), which results in a shift of the ferroelectric-to-paraelectric phase transition temperature from T-c = 352 K ( x = 0) to T-c = 324 K (x = 0.48). Additionally, the loss of hydrogen bonds directly influences the mechanical response of the framework; the elastic moduli and hardnesses decrease by around 25% from E-110 = 24.6 GPa and H-110 = 1.25 GPa for x = 0, to E-110 = 19.0 GPa and H-110 = 0.97 GPa for x = 0.48. Our results give an in-depth insight into the crystal chemistry of ABX(3) formate perovskites and highlight the important role of hydrogen bonding and dynamics.