Abstract
By combining two methods, Raman spectroscopy and high-pressure single-crystal X-ray diffraction, we demonstrate that the disordered, ambient pressure phase α of [NH2CHNH2]Mn(H2POO)3 is less compressible than the methylhydrazinium analog due to the formation of strong hydrogen bonds between the formamidinium cations and the framework. Above 4.0 GPa, the ordering of formamidinium cations and the collapse of perovskite cages lead to triclinic phase γ of space group P1̅. Phase γ exhibits a rare and important mechanical property, i.e., a negative linear compressibility of −7.82 ± 0.6 TPa–1 along the c axis. Raman spectroscopy indicates the presence of other phase transitions to phases δ and ε above 4.7 and 6.0 GPa, respectively. These transformations occur due to the cumulative distortions of the chemical bonds and structural components of the manganese-hypophosphite framework.
Highlights
Pressure is emerging as another important parameter that can be used for tuning crystal structures and physicochemical properties of hybrid perovskites
Hypophosphite-based perovskites were synthesized for the first time in the year 2017, and up to now, only a few such compounds are known.[16,27−31] Interestingly, despite some chemical and geometrical similarities between H2POO− and HCOO−linkers, the structures and properties of hypophosphite and formate analogs are significantly different.[16,27−31] For instance, while in formate frameworks, the organic cations are located at the center of perovskite cavities, in hypophosphites, they are usually shifted away from the center.[16,27,29,30]
We show that [FA]Mn(H2POO)[3] undergoes several pressure-driven phase transitions that are associated with a strong deformation of the manganese-hypophosphite framework and ordering of FA+ cations, which may lead to various technologically important properties such as second-harmonic generation (SHG) or ferro, pyro, and piezoelectricity
Summary
X = halide, CN−, N3−, received considerable attention in recent years due to their interesting properties that make them suitable for many applications.[1−16] For instance, halides are promising materials for solar cell and light-emitting applications[1−4] and dicyanamides exhibit interesting optical, barocaloric, and dielectric properties,[5−8] while broad interest in ABX3 formates is related to their multiferroic properties.[9−11] In these compounds, the organic cations occupy cavities of the framework, interacting with the framework via a hydrogen bond (HB) and Coulomb interactions. We show that [FA]Mn(H2POO)[3] undergoes several pressure-driven phase transitions that are associated with a strong deformation of the manganese-hypophosphite framework and ordering of FA+ cations, which may lead to various technologically important properties such as second-harmonic generation (SHG) or ferro-, pyro-, and piezoelectricity
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