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

• Main Authors: Jan Albert Zienkiewicz, Edyta Kucharska, Maciej Ptak
• Format: Article
• Language: English
• Published: MDPI AG 2021-07-01
• Series: Materials
• Subjects: hybrid perovskite; phase transition; order-disorder; dimethylhydrazinium cation
• Online Access: https://www.mdpi.com/1996-1944/14/14/3984
• ISSN: 1996-1944

[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.