Evidence of Pressure-Induced Phase Transitions and Negative Linear Compressibility in Formamidinium Manganese-Hypophosphite Hybrid Perovskite

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.