Raman Spectroscopy Studies on the Barocaloric Hybrid Perovskite [(CH₃)₄N][Cd(N₃)₃]

[Abstract] Temperature-dependent Raman scattering and differential scanning calorimetry were applied to the study of the hybrid organic-inorganic azide-perovskite [(CH₃)₄N][Cd(N₃)₃], a compound with multiple structural phase transitions as a function of temperature. A significant entropy variation was observed associated to such phase transitions, |∆S| ~ 62.09 J·kg⁻¹ K⁻¹, together with both a positive high barocaloric (BC) coefficient |δTt/δP| ~ 12.39 K kbar⁻¹ and an inverse barocaloric (BC) coefficient |δTt/δP| ~ −6.52 kbar⁻¹, features that render this compound interesting for barocaloric applications. As for the obtained Raman spectra, they revealed that molecular vibrations associated to the NC₄, N₃⁻ and CH₃ molecular groups exhibit clear anomalies during the phase transitions, which include splits and discontinuity in the phonon wavenumber and lifetime. Furthermore, variation of the TMA⁺ and N₃⁻ modes with temperature revealed that while some modes follow the conventional red shift upon heating, others exhibit an unconventional blue shift, a result which was related to the weakening of the intermolecular interactions between the TMA (tetramethylammonium) cations and the azide ligands and the concomitant strengthening of the intramolecular bondings. Therefore, these studies show that Raman spectroscopy is a powerful tool to gain information about phase transitions, structures and intermolecular interactions between the A-cation and the framework, even in complex hybrid organic-inorganic perovskites with highly disordered phases.Conselho Nacional de Desenvolvimento Científico e Tecnológico (Brasil); 431943/2016-8Fundação de Amparo à Pesquisa e ao Desenvolvimento Científico e Tecnológico do Maranhão (Brasil); COOPI-07771/17Ministerio de Economía y Competitividad; MAT2017-86453-RXunta de Galicia; ED431G/0

Near-Room-Temperature Reversible Giant Barocaloric Effects in [(CH₃)₄N]Mn[N₃]₃ Hybrid Perovskite

[Abstract] We report giant reversible barocaloric effects in [(CH₃)₄N]Mn[N₃]₃ hybrid organic–inorganic perovskite, near its first-order cubic-monoclinic structural phase transition at ₀ ∼ 305 K. When driving the transition thermally at atmospheric pressure, the transition displays a large change in entropy of ∼80 J K⁻¹ kg⁻¹ and a small thermal hysteresis of ∼7 K, as well as a large change in volume of ∼1.5%. When driving the transition with pressure near room temperature, the transition displays large changes in entropy of ∼70 J K⁻¹ kg⁻¹, which represent a giant barocaloric response. Hybrid perovskites with similar barocaloric response and lower operating temperatures may find applications in environmentally friendly cooling.The authors are grateful for financial support from Ministerio de Economía y Competitividad MINECO and EU-FEDER (MAT2017-86453-R), Xunta de Galicia (ED431G/09), FAMEPA (COOPI-07771/17), and ERC Starting Grant no. 680032Xunta de Galicia; ED431G/0