Probing Phase Transitions in Metal-Organic Framework Multiferroics Using Solid-State NMR

This dissertation is focused on NMR studies of phase transitions in perovskite-like Metal-Organic Framework Multiferroics. The materials studied are [(CH₃)₂NH₂]Mg(HCOO)₃ and [(CH₃)₂NH₂]Mn(HCOO)₃. Dimethylammonium magnesium formate, [(CH₃)₂NH₂]Mg(HCOO)₃] shows a dielectric transition around (T = 270 K), the mechanism behind this phase transition is explored in detail using high-resolution solid-state NMR on ¹³C (γ = 10.7054 MHz/T, I = 1/2), ¹⁵N (γ = 4.3143 MHz/T, I = 1/2), ²⁵Mg (γ = 2.6055 MHz/T, I = 5/2) found in the compound. The chemical shift and spectral changes with varying temperatures indicate the onset of phase transition which agrees with the previous dielectric constant measurements. We found evidence for a mixed behavior of order-disorder and displacive type phase transition by analyzing the chemical shift of the ¹³C and ¹⁵N in the compound. In addition, we quantified the timescale of the dynamics of nitrogen and dimethylammonium cation in the cavity by carrying out spin-lattice relaxation (T1) measurements and fit it to BPP theory to calculate the correlation time (τc). Near the phase transition, we also observed a co-existence of high-temperature and low-temperature phase peaks evident from the ¹³C spectra. Dimethylammonium manganese formate, [(CH₃)₂NH₂]Mn(HCOO)₃] (Mn-MOF) is one of the first materials in the class of metal-organic framework multiferroics. It shows a ferroelectric transition at (T = 185 K) and an antiferromagnetic phase transition at (TN = 8.5 K). We carried out wideband NMR measurements on ¹H (γ = 42.5774 MHz/T, I = 1/2) and ¹⁴N (γ = 3.0752 MHz/T, I = 1) to explore both the magnetic and electric ordering using a single characterization technique. The measurements were carried out on 12 T and 17 T wideband NMR instruments with a homemade probe. The ¹H spectra, ¹⁴N spectra, T₁, and T₂ relaxation measurements indicated both antiferromagnetic and ferroelectric phase transition, the values obtained for Tc (Ferroelectric) and TN (Antiferromagnetic) using NMR agree with the previously reported values. Angular dependence measurements were used to label the protons present in the compound. The nitrogen hopping mechanism above and below the phase transition temperature is observed with the help of ¹⁴N NMR. T₁ relaxation for the central methyl proton exhibited an anomalous behavior with double exponential curves of T1 for the same temperature. We also found that the interaction between the Mn ions is stronger than the mean-field theory using the critical scaling fit. The dissertation also includes a project on using a triple pulse scheme to eliminate background signals and acoustic ringing in both spin 1/2 and quadrupolar nuclei. With the appropriate changing of pulse phase in subsequent scans, one can suppress the acoustic ringing effects and by introducing an additional flip-angle dependent scaling the background signal coming from outside the probe is eliminated. We have successfully shown a new pulse technique to obtain a clean spectrum for nuclei with low NMR sensitivity and with high acoustic ringing.