Order-disorder phase transition and molecular dynamics in the hybrid perovskite [(CH3)3NH][Mn(N3)3]

R X Silva; R R Hora; A Nonato; Alberto García-Fernández; Jorge Salgado-Beceiro; María Antonia Señarís-Rodríguez; Manuel Sánchez Andújar; A P Ayala; C W A Paschoal

doi:10.1016/j.saa.2022.122198

Order-disorder phase transition and molecular dynamics in the hybrid perovskite [(CH₃)₃NH][Mn(N₃)₃]

Spectrochim Acta A Mol Biomol Spectrosc. 2023 Mar 15:289:122198. doi: 10.1016/j.saa.2022.122198. Epub 2022 Nov 29.

Authors

R X Silva¹, R R Hora², A Nonato³, Alberto García-Fernández⁴, Jorge Salgado-Beceiro⁴, María Antonia Señarís-Rodríguez⁴, Manuel Sánchez Andújar⁴, A P Ayala², C W A Paschoal²

Affiliations

¹ Departamento de Física, Universidade Federal do Ceará, Campus do Pici, 65455-900 Fortaleza, CE, Brazil; Coordenação de Ciências Naturais, Universidade Federal do Maranhão, Campus de Codó, 65400-000 Codó, MA, Brazil. Electronic address: rosivaldo.xs@ufma.br.
² Departamento de Física, Universidade Federal do Ceará, Campus do Pici, 65455-900 Fortaleza, CE, Brazil.
³ Coordenação de Ciências Naturais, Universidade Federal do Maranhão, Campus de Bacabal, 65700-000 Bacabal, MA, Brazil.
⁴ Grupo QuiMolMat, Departamento de Química Fundamental, Facultad de Ciencias y CICA, Universidade da Coruña, Campus A Coruña, 15071 A Coruña, Spain.

PMID: 36502746
DOI: 10.1016/j.saa.2022.122198

Abstract

We present a temperature-dependent Raman scattering study of a [(CH₃)₃NH][Mn(N₃)₃] hybrid organic-inorganic azide-perovskite, in which we have analysed in detail the wavenumber and full width at half-maximum (FWHM) of lattice modes and internal modes of the NC₃ skeleton, N₃^- and CH₃ molecular groups. In general, the modes exhibited unusual behaviour during the phase transitions, including discontinuity in the phonon wavenumber, bandwidth, and unconventional shifts upon temperature variation. Spectral features on heating reveal the absence of significant distortions in the NC₃ skeleton and a relatively restricted order-disorder process of the TrMA⁺ cations. On the other hand, linewidth anomalies of the δNC₃ and ν_asNC₃ modes have been attributed to the molecular dynamics of encapsulated cations. The unconventional blue shift of the symmetric stretching modes of azide ligands indicates the weakening of the intermolecular interactions between the TrMA⁺ cations and azido-bridges, and the strengthening of the intramolecular bonds. Additionally, we have used differential scanning calorimetry to confirm the subtle monoclinic to monoclinic (P2₁/c → C2/c) phase transition at around 330 K; and the phase transition to trigonal structure (R3¯m) above 359 K, whose associated entropy variation turns to be |ΔS| ∼ 22.3 J·kg^-1 K^-1 and displays a barocaloric (BC) tunability |δTt/δP| ∼ 3.17 K kbar^-1, according to our estimations using the Clausius-Clapeyron method. Although the obtained values of entropy change and BC tunability are very close to those reported on formate-perovskites and other important caloric materials, those parameters are much lower than the giant entropy change of ∼80 Jkg^-1 K^-1 and large BC tunability ∼12 K kbar^-1 observed for the analogue azide-perovskite [(CH₃)₄N][Mn(N₃)]₃ (TMAMnN₃). Very interestingly, our combined study shed light to understand such different behaviour, as they reveal that the hydrogen bonds created between the TrMA⁺ cations and the framework prevent an extensive order-disorder process that is needed to obtain large entropy changes and large BC coefficients as it occurs in the case of related azide-perovskites with no H-bonds between the A cations (for example TMA) and the framework.

Keywords: Azido frameworks; Hybrid perovskite; Phase transition; Raman spectroscopy.