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

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

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.

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.

The spectroscopic study of phase transitions in the series of cyanide perovskites

The series of MeA₂KFe(CN)₆, where MeA = CH₃NH₃⁺, (CH₃)₂NH₂⁺, (CH₃)₃NH⁺ and (CH₃)₄N⁺, has been studied by IR and Raman spectroscopy as the function of temperature in order to elucidate the mechanisms of the phase transitions. The order-disorder process has been confirmed in all cases. Different models have been proposed based on the dynamic effects observed in the spectra. The crystal containing (CH₃)₂NH₂⁺ cations constitutes a model with melt-like thermal behavior and strongly temperature-influenced hydrogen bonding. In the case of sample with (CH₃)₄N⁺ an unperturbed rotation of these cations is observed, while in the crystals with methyl- and trimethylammonium cations the hydrogen bonds acting as positional stabilizers prevent the organic cation from a completely free motion. Additionally, the statistical disorder of dimethyl- and trimethylammonium cations has been confirmed by the thermal evolution of the FWHM of the related bands.

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.

Pyrrolidinium-Based Cyanides: Unusual Architecture and Dielectric Switchability Triggered by Order-Disorder Process

Two three-dimensional metal–organic compounds of the formula Pyr₂KM(CN)₆, where M = Co, Fe and Pyr = pyrrolidinium ((CH₂)₄NH₂⁺), have been found to crystallize at room temperature in a monoclinic structure, space group P2₁/c. They are cyano-bridged compounds with an unprecedented type of architecture containing pyrrolidinium cations in the voids. The materials have been investigated by X-ray diffraction, dielectric, and spectroscopic methods as a function of temperature in order to determine their properties and the mechanism of the reversible phase transitions occurring at ca. 345–370 K. The phase transitions in both crystals are first order and are associated with a symmetry increase to a rhombohedral structure (space group R3®m) as well as a significant disorder of organic cations above T_c. On the basis of Raman scattering and IR spectroscopy it has been assumed that the phase transition in both crystals is triggered by thermally induced pseudorotation of the organic cation and large out-of-plane motions of its atoms followed by a “click-in” of the cyanide bridges. The materials have been proposed as possible switchable dielectrics due to their respective high differences in dielectric permittivities across the phase transition.

Two pyrrolidinium-based cyano-bridged compounds are described with reference to their dielectric properties, which can be switched by temperature. In addition, the mechanisms of the phase transitions are elucidated by X-ray diffraction and vibrational spectroscopy.

Phase transition in the extreme: a cubic-to-triclinic symmetry change in dielectrically switchable cyanide perovskites

Two novel three-dimensional metal–organic compounds of formula FA₂KM(CN)₆, where M = Co, Fe and FA = formamidinium (CH(NH₂)₂⁺), have been found to crystallize in a perovskite-like architecture.