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.