Tuning the Mechanical Properties of Molecular Perovskites by Controlling Framework Distortions via A-site Substitution

Molecular perovskites are important materials in the area of barocalorics, improper ferroelectrics and ferroelastics, where the search for principles that link composition, structure and mechanical properties is a key challenge. Herein, we report the synthesis of a new series of dicyanamide-based molecular perovskites [A]Ni(C2N3)3, where A+ is a range of alkylated piperidinium cations. We use this new family to explore how A+ cations determine their mechanical response by measuring the bulk modulus (B) – using high-pressure powder X-ray diffraction. Within the series, we find a positive correlation between the network distortions of the pseudocubic [Ni(C2N3)3]– network and B. Furthermore, we show that we can tune framework distortions, and therefore B, by synthesising A-site solid solutions. The applied methodology is a blueprint for linking framework distortions and mechanical properties in network materials and guides us toward principles for designing macroscopic properties via systematic compositional changes in molecular perovskites.