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.
Highlights
Compounds that combine simultaneously organic and inorganic chemical groups are of great interest since they enlarge the range of structural possibilities that allow the coexistence and modulation of fundamental physical properties, increasing their multifunctional potential [1]
In these relatively new compounds, the A site is usually occupied by an organic cation (A = protonated amine), the B site may be occupied by a divalent transition metal cation (e.g., B = Mn2+, Co2+, Ni2+, Cd2+, etc.) and the X site by a monatomic anion, mainly a halide, such as Cl−, Br− and I, or anionic polyatomic bridge ligands—for instance, azides (N3 – ), cyanides (CN– ) and formates (HCOO– ) [1,9,10,11]
Crystals of the azide compound [N(CH3 )4 ][Cd(N3 )3 ] belonging to the hybrid organic-inorganic perovskite family were obtained by the slow evaporation method
Summary
Compounds that combine simultaneously organic and inorganic chemical groups are of great interest since they enlarge the range of structural possibilities that allow the coexistence and modulation of fundamental physical properties, increasing their multifunctional potential [1]. Molecules 2020, 25, 4754 inorganic-organic compounds have attracted great attention in the last years in view of their high technological potential in the areas of optoelectronics [2], photovoltaics [3], ferroelectrics [4], multiferroics [5,6], and, very recently, in the field of barocaloric materials for solid-state cooling [7,8] In this context, the ABX3 perovskite family—which, for decades, has played a prominent role in the evolution and application of inorganic materials—has experienced a large expansion by incorporating organic blocks in its structure, giving rise to very outstanding materials known as hybrid perovskites.
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