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

Abstract

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.

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

“…Similarly to the recently reported pressure-induced transformation in [MHy]­Mn­(H 2 POO) 3 , the origin of the pressure-induced phase transition in [FA]­Mn­(H 2 POO) 3 can be attributed to the shear strain of the crystal, leading to the deformation of the cages. However, the phase transition associated with the collapse of the cages occurs at a much higher pressure for [FA]­Mn­(H 2 POO) 3 (4.0 GPa) than for [MHy]­Mn­(H 2 POO) 3 (1.2 GPa) . This behavior is consistent with higher stiffness of [FA]­Mn­(H 2 POO) 3 compared to [MHy]­Mn­(H 2 POO) 3 .…”

“…The bulk modulus of phase α, calculated using a second-order Birch–Murnaghan equation of state, is 24.5 ± 1.1 GPa, and this phase exhibits positive compressibility (Table S2). It is worth noting that the bulk modulus is significantly larger for [FA]­Mn­(H 2 POO) 3 than for [MHy]­Mn­(H 2 POO) 3 (11.02 ± 0.16 GPa) . This behavior indicates that [FA]­Mn­(H 2 POO) 3 is much less compressible than [MHy]­Mn­(H 2 POO) 3 .…”

“…The free space around the FA + cation, when it orders and its volume is reduced in phase γ, is reflected in the compressibility of phase γ, which is much smaller than that of phase α (Tables S2 and S3 and Figure ). Similarly to the recently reported pressure-induced transformation in [MHy]­Mn­(H 2 POO) 3 , the origin of the pressure-induced phase transition in [FA]­Mn­(H 2 POO) 3 can be attributed to the shear strain of the crystal, leading to the deformation of the cages. However, the phase transition associated with the collapse of the cages occurs at a much higher pressure for [FA]­Mn­(H 2 POO) 3 (4.0 GPa) than for [MHy]­Mn­(H 2 POO) 3 (1.2 GPa) .…”

“…However, the application of high pressure conditions to tune and study the structural and functional properties of hybrid materials, including hypophosphite-based perovskites, is vaguely neglected. Up to now, high-pressure studies were performed for M­(H 2 POO) 3 (MV, Al, and Ga) and [MHy]­Mn­(H 2 POO) 3 (MHy = methylhydrazinium) compounds only . The latter study revealed a rich sequence of pressure-induced phase transitions and significant differences in the high-pressure behavior of [MHy]­Mn­(H 2 POO) 3 and related formate frameworks …”

“…Up to now, high-pressure studies were performed for M­(H 2 POO) 3 (MV, Al, and Ga) and [MHy]­Mn­(H 2 POO) 3 (MHy = methylhydrazinium) compounds only . The latter study revealed a rich sequence of pressure-induced phase transitions and significant differences in the high-pressure behavior of [MHy]­Mn­(H 2 POO) 3 and related formate frameworks …”