Spectroscopic Study of Structural Phase Transition and Dynamic Effects in a [(CH₃)₂NH₂][Cd(N₃)₃] Hybrid Perovskite Framework

We present a combined study of a [(CH 3 ) 2 NH 2 ][Cd(N 3 ) 3 ] hybrid perovskite dense metal–organic framework by infrared (IR), Raman, continuous-wave electron paramagnetic resonance (CW EPR), phase-transition-induced current, and dielectric measurements. The Raman and IR studies have shown that the phase transition observed near 175 K is associated with a pronounced narrowing and intensity increase of bands related to the NH 2 group of dimethylammonium (DMA + ) cations located in the cavities of the framework. This observation has proved that the phase transition is associated with an ordering of DMA + cations. The dielectric response of the [(CH 3 ) 2 NH 2 ][Cd(N 3 ) 3 ] single crystal has revealed a weakly expressed anomalous behavior of the dielectric constant at the phase transition point. In the low-temperature phase, a pronounced dispersion in the kilohertz frequency range has been observed, which might be assigned to the twofold reorientation dynamics of the DMA + cations. The phase-transition-induced current measurements of the DMACd(N 3 ) 3 single crystal reveal a small anomaly at the phase transition temperature, demonstrating a nonferroelectric behavior. The CW EPR experiments of manganese-doped DMACd(N 3 ) 3 disclose that the paramagnetic Mn 2+ probe ions have been successfully incorporated in the crystal structure and form MnN 6 octahedra. The temperature evolution of the CW EPR spectrum reveals that these centers are sensitive to the structural changes of the cadmium-azide framework occurring during the phase transition. The temperature evolution of the spin Hamiltonian parameters and CW EPR linewidth of the Mn 2+ ions have allowed the detection of the effects of the DMA + cation dynamics and characterization of the type of the phase transition.