Tuneable mechanical and dynamical properties in the ferroelectric perovskite solid solution [NH₃NH₂]_1- [NH₃OH] ...

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- [NH₃OH] 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, ¹H solid state NMR, DSC and nanoindentation. NMR shows increased motion of [NH₃NH₂]⁺ in [NH₃NH₂]_0.52[NH₃OH]_0.48Zn(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 E₁₁₀ = 24.6 GPa and H₁₁₀ = 1.25 GPa for x = 0, to E₁₁₀ = 19.0 GPa and H₁₁₀ = 0.97 GPa for x = 0.48. Our results give an in-depth insight into the crystal chemistry of ABX₃ formate perovskites and highlight the important role of hydrogen bonding and dynamics.