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 alpha, NH center dot center dot center dot 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 beta is similar to that of the phase alpha. This type of phase transition was not reported for related formate perovskites. Another phase transition to phase gamma 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 alpha), Pcmn (phase beta), and P112(1)/n (phase gamma). The space group type of phases alpha and beta 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 alpha to beta than for the subsequent phase transition to phase gamma. Raman data give evidence for yet another transformation to phase delta 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 delta is associated with very large reconstruction of the manganese-hypophosphite framework.