Mechanism of Pressure-Induced Phase Transitions and Structure–Property Relations in Methylhydrazinium Manganese Hypophosphite Perovskites

A series of pressure-induced phase transitions between [MHy]Mn(H₂POO)₃ (MHy⁺ = methylhydrazinium) phases reveals the structural mechanism behind the elastic properties of this hypophosphite perovskite. In the ambient pressure phase α, NH···O hydrogen bonds to H₂POO– linkers stabilize the MHy⁺ cations outside the perovskite cages. When pressure increases to 1.1 GPa, the MHy⁺ cations are pushed into the perovskite cages, but the manganese-hypophosphite framework of this new phase β is similar to that of the phase α. This type of phase transition was not reported for related formate perovskites. Another phase transition to phase γ is observed at 1.2 GPa. This phase transition leads to collapse of the perovskite cages, but the conformation and positions of MHy⁺ cations in the cages hardly change. The phase transitions are equitranslational (zellengleichen), with the symmetry space group changing from Pnma (phase α), Pcmn (phase β), and P112₁/n (phase γ). The space group type of phases α and β is the same, but the crystal directions [x] and [z] are exchanged. Owing to the hierarchy of interactions, the sequence of volume drops is rather unusual: it is smaller for the lower-pressure phase transition from phase α to β than for the subsequent phase transition to phase γ. Raman data give evidence for yet another transformation to phase δ between 4.2 and 4.7 GPa. Crystal structure of this phase could not be solved, but very pronounced changes in the Raman spectra indicate that the phase transition to phase δ is associated with very large reconstruction of the manganese-hypophosphite framework.