The phase transition in sodium azide

The mechanism of phase transitions and luminescence properties of azide perovskites

The temperature-dependent IR and Raman spectroscopy has been used to study the phase transitions in Na/K-chromium-azide frameworks with tetramethylammonium (TeMA⁺) cations, [(CH₃)₄N]₂[NaCr(N₃)₆] and [(CH₃)₄N]₂[KCr(N₃)₆], which crystallize in a perovskite-like structure. The phase transition occurring within 310-315 K in both compounds leads to a symmetry change from Pa-3 to Fm-3m and its major contributor is an order-disorder process of the organic cations followed by an adjustment of the azide bridges. The luminescence properties arise from the optically active Cr³⁺ ions and the studies reveal some admixture of low-symmetry component of the octahedral crystal environment of the Cr³⁺ sites.

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

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 |δT_t/δP| ~ 12.39 K kbar^-1 and an inverse barocaloric (BC) coefficient |δT_t/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 NC_4, 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.

The mechanism of phase transitions in azide perovskites probed by vibrational spectroscopy

The temperature-dependent IR and Raman spectroscopy has been used to study the phase transitions in manganese-azide frameworks with either dimethylammonium (DMA⁺) or tetramethylammonium (TMA⁺) cations which adopt a perovskite-like crystal structure. The phase transition in DMA-analogue seems to be associated with cooperative tilting of MnN₆ octahedra and order-disorder of hydrogen bonds while in TMA-analogue it is more complex and composed of several processes, including the motions of both manganese-azide framework and tetramethylammonium cations and their possible coupling. Our results are in agreement with the data received from crystallographic and dielectric measurements in the case of TMA-analogue and we have demonstrated the importance of order-disorder of hydrogen bonds in the case of DMA-analogue which previously has not been taken into account.