Vibrational Properties and DFT Calculations of Perovskite-Type Methylhydrazinium Manganese Hypophosphite

Raman study of pressure-induced phase transitions in imidazolium manganese- hypophosphite hybrid perovskite

By using Raman spectroscopy, we demonstrate that [IM]Mn(H₂POO)₃ is a highly compressible material that undergoes three pressure-induced phase transitions. Using a diamond anvil cell we performed high-pressure experiments up to 7.1 GPa, using paraffin oil as the compression medium. The first phase transition, which occurs near 2.9 GPa, leads to very pronounced changes in the Raman spectra. This behavior indicates that this transition is associated with very large reconstruction of the inorganic framework and collapse of the perovskite cages. The second phase transition, which occurs near 4.9 GPa, is associated with subtle structural changes. The last transition takes place near 5.9 GPa and it leads to further significant distortion of the anionic framework. In contrast to the anionic framework, the phase transitions have weak impact on the imidazolium cation. Pressure dependence of Raman modes proves that compressibility of the high-pressure phases is significantly lower compared to the ambient pressure phase. It also indicates that the contraction of the MnO₆ octahedra prevails over that of the imidazolium cations and hypophosphite linkers. However, compressibility of MnO₆ strongly decreases in the highest pressure phase. Pressure-induced phase transitions are reversible.

Hybrid Chlorides with Methylhydrazinium Cation: [CH3NH2NH2]CdCl3 and Jahn-Teller Distorted [CH3NH2NH2]CuCl3

The synthesis, structural, phonon, optical, and magnetic properties of two hybrid organic-inorganic chlorides with monoprotonated methylhydrazinium cations (CH₃NH₂NH₂⁺, MHy⁺), [CH₃NH₂NH₂]CdCl₃ (MHyCdCl₃), and [CH₃NH₂NH₂]CuCl₃ (MHyCuCl₃), are reported. In contrast to previously reported MHyM^IICl₃ (M^II = Mn²⁺, Ni²⁺, and Co²⁺) analogues, neither compound undergoes phase transitions. The MHyCuCl₃ has a crystal structure familiar to previous crystals composed of edge-shared 1D chains of the [CuCl₅N] octahedra. MHyCuCl₃ crystallizes in monoclinic P2₁/c symmetry with MHy⁺ cations directly linked to the Cu²⁺ ions. The MHyCdCl₃ analogue crystallizes in lower triclinic symmetry with zig-zag chains of the edge-shared [CdCl₆] octahedra. The absence of phase transitions is investigated and discussed. It is connected with slightly stronger hydrogen bonding between cations and the copper-chloride chains in MHyCuCl₃ due to the strong Jahn-Teller effect causing the octahedra to elongate, resulting in a better fit of cations in the accessible space between chains. The absence of structural transformation in MHyCdCl₃ is due to intermolecular hydrogen bonding between two neighboring MHy⁺ cations, which has never been reported for MHy⁺-based hybrid halides. Optical investigations revealed that the bandgaps in Cu²⁺ and Cd²⁺ analogues are 2.62 and 5.57 eV, respectively. Magnetic tests indicated that MHyCuCl₃ has smeared antiferromagnetic ordering at 4.8 K.

Luminescence and Dielectric Switchable Properties of a 1D (1,1,1-Trimethylhydrazinium)PbI3 Hybrid Perovskitoid

The synthesis and investigation of the physicochemical properties of a novel one-dimensional (1D) hybrid organic-inorganic perovskitoid templated by the 1,1,1-trimethylhydrazinium (Me₃Hy⁺) cation are reported. (Me₃Hy)[PbI₃] crystallizes in the hexagonal P6₃/m symmetry and undergoes two phase transitions (PTs) during heating (cooling) at 322 (320) and 207 (202) K. X-ray diffraction data and temperature-dependent vibrational studies show that the second-order PT to the high-temperature hexagonal P6₃/mmc phase is associated with a weak change in entropy and is related to weak structural changes and different confinement of cations in the available space. The second PT to the low-temperature orthorhombic Pbca phase that corresponds to the high change in entropy and dielectric switching is associated with an ordering of the trimethylhydrazinium cations, re-arrangement and strengthening of hydrogen bonds, and slightly shifted lead-iodide octahedral chains. The high-pressure Raman data revealed two additional PTs, one between 2.8 and 3.2 GPa, related to the symmetry decrease, ordering of the cations, and inorganic chain distortion, and the other in the 6.4-6.8 GPa range related to the partial and reversible amorphization. Optical studies revealed that (Me₃Hy)[PbI₃] has a wide band gap (3.20 eV) and emits reddish-orange excitonic emission at low temperatures with an activation energy of 65 meV.

Synthesis, Photoluminescence and Vibrational Properties of Aziridinium Lead Halide Perovskites

Three-dimensional lead halide perovskites are known for their excellent optoelectronic properties, making them suitable for photovoltaic and light-emitting applications. Here, we report for the first time the Raman spectra and photoluminescent (PL) properties of recently discovered three-dimensional aziridinium lead halide perovskites (AZPbX₃, X = Cl, Br, I), as well as assignment of vibrational modes. We also report diffuse reflection data, which revealed an extended absorption of light of AZPbX₃ compared to the MA and FA counterparts and are beneficial for solar cell application. We demonstrated that this behavior is correlated with the size of the organic cation, i.e., the energy band gap of the cubic lead halide perovskites decreases with the increasing size of the organic cation. All compounds show intense PL, which weakens on heating and shifts toward higher energies. This PL is red shifted compared to the FA and MA counterparts. An analysis of the PL data revealed the small exciton binding energy of AZPbX₃ compounds (29-56 meV). Overall, the properties of AZPbX₃ are very similar to those of the well-known MAPbX₃ and FAPbX₃ perovskites, indicating that the aziridinium analogues are also attractive materials for light-emitting and solar cell applications.

Metal-free enantiomorphic perovskite [dabcoH2]2+[H3O]+Br- 3 and its one-dimensional polar polymorph

The structure and stoichiometry of a new metal-free and ammonium-free compound [dabcoH₂]²⁺H₃O⁺Br^- ₃ (where [dabcoH₂]²⁺ = 1,4-di-aza-bicyclo-[2.2.2]octane dication) correspond to the general formula ABX ₃ characteristic of perovskites. In enantiomorphic trigonal polymorph α of [dabcoH₂]²⁺H₃O⁺Br^- ₃, the corner-sharing [H₃O]Br₆ octahedra combine into a 3D framework embedding [dabcoH₂]²⁺ dications in pseudo-cubic cages. In the more dense polymorph β, the face-sharing [H₃O]Br₆ octahedra form 1D polyanionic columns separated by [dabcoH₂]²⁺ dications. These different topologies correlate with different crystal fields around the cations and their different disorder types: orientational disorders of [dabcoH₂]²⁺ dications and H₃O⁺ cations in polymorph α and positional disorder of [H₃O]⁺ cations in polymorph β. The orientational disorder increases the lengths of OH⋯Br hydrogen bonds in polymorph α, but NH⋯Br distances of ordered dabcoH₂ dications are longer in polymorph β. The presence of polar [H₃O]⁺ cations in [dabcoH₂]²⁺H₃O⁺Br^- ₃ polymorphs offers additional polarizability of the centres compared with analogous metal-free [dabcoH₂]²⁺[NH₄]⁺Br^- ₃ perovskite.

Structural, magnetic and photoluminescent properties of new hybrid hypophosphites: discovery of the first noncentrosymmetric and two cobalt-based members

Hybrid organic-inorganic perovskites comprising hypophosphite ligands are emerging functional materials exhibiting magnetic, photoluminescence, negative thermal expansion and negative linear compressibility behaviour. This work reports five novel hypophosphite perovskites, [A]M(H2POO)3 (A=...

Mechanism of Unusual Isosymmetric Order-Disorder Phase Transition in [Dimethylhydrazinium]Mn(HCOO)3 Hybrid Perovskite Probed by Vibrational Spectroscopy

[DMHy]Mn(HCOO)₃ (DMHy⁺ = dimethylhydrazinium cation) is an example of an organic–inorganic hybrid adopting perovskite-like architecture with the largest organic cation used so far in the synthesis of formate-based hybrids. This compound undergoes an unusual isosymmetric phase transition at 240 K on heating. The mechanism of this phase transition has a complex nature and is mainly driven by the ordering of DMHy⁺ cations and accompanied by a significant distortion of the metal–formate framework in the low temperature (LT) phase. In this work, the Density Functional Theory (DFT) calculations and factor group analysis are combined with experimental temperature-dependent IR and Raman studies to unequivocally assign the observed vibrational modes and shed light on the details of the occurring structural changes. The spectroscopic data show that this first-order phase transition has a highly dynamic nature, which is a result of balanced interplay combining re-arrangement of the hydrogen bonds and ordering of DMHy⁺ cations. The tight confinement of organic cations forces simultaneous steric deformation of formate ions and the MnO₆ octahedra.