Dielectric Spectroscopy of Water Dynamics in Functionalized UiO-66 Metal-Organic Frameworks.

We present a dielectric spectroscopy study of dipolar dynamics in the hydrated UiO-66(Zr) type metal-organic frameworks (MOFs) functionalized with -NH2 and -F groups. Experiments are performed in a broad temperature and frequency ranges allowing us to probe several dipolar relaxations. For both samples at temperature below 220 K, we observe confined supercooled water dynamics, which can be described by the Arrhenius law. At slightly higher temperature, a second less pronounced dipolar relaxation is identified, and its origin is discussed. At even higher temperature, the dielectric permittivity exhibits anomalous increase with increasing temperature due to the proton conductivity. Upon further heating, the permittivity shows a sudden decrease indicating a reversible removal of water molecules. Measurements of the dehydrated samples reveal absence of all three dipolar processes.

Structural phase transition in perovskite metal-formate frameworks: a Potts-type model with dipolar interactions.

We propose a combined experimental and numerical study to describe an order-disorder structural phase transition in perovskite-based [(CH3)2NH2][M(HCOO)3] (M = Zn(2+), Mn(2+), Fe(2+), Co(2+) and Ni(2+)) dense metal-organic frameworks (MOFs). The three-fold degenerate orientation of the molecular (CH3)2NH2(+) (DMA(+)) cation implies a selection of the statistical three-state model of the Potts type. It is constructed on a simple cubic lattice where each lattice point can be occupied by a DMA(+) cation in one of the available states. In our model the main interaction is the nearest-neighbor Potts-type interaction, which effectively accounts for the H-bonding between DMA(+) cations and M(HCOO)3(-) cages. The model is modified by accounting for the dipolar interactions which are evaluated for the real monoclinic lattice using density functional theory. We employ the Monte Carlo method to numerically study the model. The calculations are supplemented with the experimental measurements of electric polarization. The obtained results indicate that the three-state Potts model correctly describes the phase transition order in these MOFs, while dipolar interactions are necessary to obtain better agreement with the experimental polarization. We show that in our model with substantial dipolar interactions the ground state changes from uniform to the layers with alternating polarization directions.

Temperature- and pressure-dependent studies of niccolite-type formate frameworks of [NH3(CH2)4NH3][M2(HCOO)6] (M = Zn, Co, Fe).

We report temperature-dependent electric, IR and Raman studies of niccolite-type formate frameworks templated by protonated 1,4-diaminobutane. Our results show that the zinc-analogue exhibits a first-order phase transition close to 240 K. Single-crystal dielectric data show a much stronger anomaly at the phase transition for ε' along the a-direction compared to the c-direction. They also reveal that dipole relaxation exists in bnZn. Pronounced temperature-dependence observed for bending and torsion modes of the NH3+ groups proves that ordering of protonated amine plays a major role in the phase transition mechanism. The ordering is associated with distortion of the zinc formate framework but the number of observed vibrational modes is much smaller than expected assuming 36-fold multiplication of the unit cell below TC. It is also much smaller than reported for the Mn-analogue, which exhibits only a two-fold increase of the unit cell below TC. We discuss the origin of this behavior. Our results also show that the Co-analogue exhibits a similar phase transition to its Zn-counterpart. However, the observed narrowing and splitting of the corresponding bands is significantly smaller, suggesting weaker distortion of the framework and the presence of some disorder for this compound even at 5 K. The Raman and IR spectra of the Fe-analogue show weak narrowing of bands upon cooling, indicative of statistical freezing of the protonated amine at low temperatures. We also report high-pressure Raman scattering studies of the zinc-analogue. This study revealed a pressure-induced reversible phase transition between 3.4 and 4.1 GPa. Large shifts and splitting of modes corresponding to the vibrations of HCOO- ions associated with weak changes of the protonated amine prove that the major contribution to the phase transition mechanism comes from distortion of the zinc formate framework.

Suppression of phase transitions and glass phase signatures in mixed cation halide perovskites.

Cation engineering provides a route to control the structure and properties of hybrid halide perovskites, which has resulted in the highest performance solar cells based on mixtures of Cs, methylammonium, and formamidinium. Here, we present a multi-technique experimental and theoretical study of structural phase transitions, structural phases and dipolar dynamics in the mixed methylammonium/dimethylammonium MA1-xDMAxPbBr3 hybrid perovskites (0 ≤ x ≤ 1). Our results demonstrate a significant suppression of the structural phase transitions, enhanced disorder and stabilization of the cubic phase even for a small amount of dimethylammonium cations. As the dimethylammonium concentration approaches the solubility limit in MAPbBr3, we observe the disappearance of the structural phase transitions and indications of a glassy dipolar phase. We also reveal a significant tunability of the dielectric permittivity upon mixing of the molecular cations that arises from frustrated electric dipoles.